Manual for Theory

Training package "SENSORIC"

2.1.1 Basic Construction 2.1.2 Reduction Factor

2.1.3 Coil Size and Sensing Range 2.1.4 Installation Problems

2.1.4.1 Housing

2.1.4.2 Flush Mounting 2.1.5 Electronic Circuit

2.2 Types

2.21 Cylindrical and Rectangular proximity switches 2.2.1.1 Definitions

2.2.2 Slotted Types 2.2.3 Ring Types

2.2.4 Bistable Switches

2.2.5 Sensors for use in Welding Magnetic Fields

2.2.6 Sensors to distinguish between different materials 2.2.7 Inductive Analogue Sensors

2.3 Interfaces for Inductive Proximity Switches

2.3.1 Electrical Types and their Positive Effects 2.3.1.1 Direct Current Switches

2.3.1.2 Alternating and All voltage Switches 2.3.1.3 Sensors to DIN 19234 (NAMUR) 2.3.2 Protected and Safety Switches

2.3.2.1 Reverse Polarity and Over Voltage Protection 2.3.2.2 Overload Protection

2.3.2.3 Safety Circuits

2.3.3 Loads and their Characteristics 2.3.4 Bus Connection

2.4 Manufacturing Technology

3.1.3 Reduction Factor 3.2 Practical Model 3.2.1 RC Oscillator 3.2.2 Interference Suppression 3.2.2.1 Interference Effects 3.2.2.2 Contamination Compensation 3.2.2.3 Cutting out Interference Pulses 3.3.3 Models

3.3 Applications

4 Ultrasonic Sensors

4.1 Fundamental Principles

4.1.1 Propagation of Sound Waves in Air 4.1.2 Generation of Ultrasonic Waves 4.1.2.1 Electrostatic Converter 4.1.2.2 Bending Oscillator 4.1.2.3 Membrane Oscillator 4.1.2.4 L/4- Oscillator 4.2 P&F- Oscillator 4.3 Methods of Operation

4.4 Distance Measuring Ultrasonic Sensors

4.5 Ultrasonic Sensors in Through-Beam Mode

4.6 Possible Errors in distance measurements with Ultrasonic Sensors

4.7 Operating Conditions

4.8 Sensor Types

5.1.1.2 Solid State Laser Diodes 5.1.2 Receiver Element

5.1.2.1 Photodiodes 5.1.2.2 Phototransistors

5.1.2.3 Position Sensitive Diode

5.2 Methods of Operation of Photoelectric Sensors

5.2.1 Direct Detection Photoelectric Sensor 5.2.2 Reflex Photoelectric Sensor

5.2.3 Through-Beam Photoelectric Sensor

5.3 Signal Processing in Photoelectric Sensors

5.3.1 Interference with Photoelectric Sensors 5.3.2 Stages in the Interference Suppression

5.3.2.1 Interference Suppression using Optical Modulation 5.3.2.2 Interference Suppression with Band Pass

5.3.2.3 Interference Suppression using Blanking 5.3.2.4 Interference Suppression using Digital Filtering 5.3.3 Function Reserve

5.3.3.1 Static Function Reserve 5.3.3.2 Dynamic Function Reserve

5.3.4 Protection against Mutual Interaction

5.4 Types

5.4.1 Reflex Photoelectric Sensor with Polarising Filter 5.4.1.1 Polarising Filter

5.4.1.2 Retro-Reflector

5.4.1.3 Through-Beam Detection

5.4.2 Direct Detection Photoelectric Sensor with Background Screening 5.4.3 Direct Detection Photoelectric Sensor with Light Guides

5.4.3.1 Light Guides

5.4.3.1.1 Principle of Operation 5.4.3.1.2 Glass Fibre Light Guides 5.4.3.1.3 Plastic Light Guides 5.4.3.2 Sensors with Light Guides

5.4.4 Output Stage of Photoelectric Sensors

5.5 Triangulation Sensors

5.6 Phase Correlation Sensors

6.2.1 Hall Effect Sensors

6.2.2 Magnetic Resistive Sensors

6.3 Saturated Core Probes

6.3.1 Construction and Mode of Operation 6.3.2 Function and Measurement Circuit 6.3.2.1 Evaluation using an Oscillator 6.3.2.2 Evaluation using Impulse Current

6.3.2.3 Evaluation using Impedance Measurements

- Collection of Experiments

- Solutions and Evaluation of Results - Data Sheets

- Folio Set SENSORIK

- Video „New Photoelectric Sensors“ - PLC programs

- CBT "Industrial Sensors 1.0"

The training case is the central part of the training pack; with this set of demonstrations and exercises, experiments with different levels of difficulties, which demonstrate the function, specific characteristics, parameters and typical application for each sensor type can be performed.

The theory required for the training pack is contained in the handbook covering: inductive, capacitive, photoelectric, ultrasonic and magnetic sensors.

The documentation is not only to aid the further education programme, but is also suitable for self study. The theory presented covering the fundamental physical principles, method of operation, type and possible uses of the sensors has been designed for use with the training case but could be used independently to study the application of sensors in automatic control.

materials handling or administration. The following are the main aims in doing this: * Improvement in product quality

* Savings in energy and raw materials * Increase in productivity

* Reduction in damage to the environment * Humanization of the work place

Usually the required control engineering is achieved using a computer or an PLC as the central element. In the end the system can only fulfill the required tasks if it is supplied with reliable process information.

This is achieved with the use of sensors, which operate according to the most widely different physical principles. These sensors convert non-electrical process measurements such as distance , angle, position, level, temperature or pressure into electrical signals in order that the controller or regulator can operate.

At the present time over 100 physical, chemical and biological effects are known for which „technical feelers“ are on the market or under development. The sensors, because of their different operating principles, are only suitable for specific range of applications. This must be taken into account during the planning stage of an installation.

Diagram 1.1: Survey of signal conversion with sensors p l v ω pH % T B,H γ non-electrical signals electrical signal 2 1 3 4 5 E R Q ∆∆∆∆∆t C E ∆∆∆∆∆t U E U R U R U W U = Voltage R = Resistance

Q = Quality factor of a resonant circuit ∆t = Time interval C = Capacitance E = Electric field W = Electrical energy 1 Ultrasonic sensor 2 Inductive sensor 3 Capacitive sensor 4 Magnetic sensor 5 Photoelectric sensor p = Pressure l = Distance, Gap v = Speed ω = Angular velocity, Speed of rotation pH = Ion concentration % = Volume % Gas concentration T = Temperature B = Flux density H = Field strength γ = photon

allowing the output circuit to operate.

Sensors, which operate without physical contact, have a number of advantages over mechanical contacts:

- no power required, no feedback and no contact bounce

- Greater number of switching operations and high switching frequency - No contact wear

- Maintenance free

- Resistant to harsh environments

Subsequently explanation of some terms:

Sensor: other names are primary element, detector, measuring transformer, measuring transducer, pick-up

Initiator: Referred to as proximity switches

Sensor element: the part of the sensor, which detects the quantity to be measured, but cannot operate alone as the signal processing element and the connectors are also required.

Example: Coil of the saturated core of a magnetic sensor,or the transducer of an ultrasonic sensor.

Multi-Sensor System:A sensor system in which a number of the same type of sensors or a number of different types of sensors are used together to complete the required task. Due to the concentration the analysis of individual elements is achieved electronically, by the use of logic or mathematics. Example: The combination of a number of initiators to distinguish between production parts of different shapes and materials or a combination of gas analyses sensors; where the operating ranges of the sensors overlap and the total of their measurements by intelligent analysis gives more information than that obtained from individual sensors.

- magnetic objects - sensing range up to 60 mm - switching frequency up to 1 kHz - up to 70 °C - up to IP 67

- high noise immunity - DIN 19234 (NAMUR)

Capacitive Sensor:

- metallic, non metallic objects,

solids and fluids

- sensing range up to 50 mm - switching frequency up to 100 Hz - up to 70 °C - up to IP 68 - DIN 19234 (NAMUR) Ultrasonic Sensor:

- objects which reflect or absorb sound

- sensing range up to 15 m - reaction time > 50 ms - up to 70 °C

- up to IP 67

- lower noise immunity - color independent - not sensitive to dirt

Optical Sensor:

- objects which are light-reflecting or non-transparent - sensing range up to 100 m - switching frequency up to 1,5 kHz - up to 300 °C (fibre optic) - up to IP 67

- detect smallest objects (fibre optic)

- DIN 19234 (NAMUR) - fibre optic, adaptierbar - metal objects

- sensing range up to 50 mm - switching frequency up to 5 kHz - up to 250 °C

- up to IP 68

- high noise immunity - DIN 19234 (NAMUR)

incorporated in and are ideal for the sensor development.

It is only a matter of time before an integrated circuit and miniature sensor element can be produced on a piece of silicon, gallium arsenide or another semiconductor material.

An interesting development is the use of enzymes, microbes or whole cells as sensor elements; known as „Bio-Sensors“.

By the end of this decade contact element, that is control elements, will also be come part of this development.

Finally, mechanical parts, such as pressure jets, switches or even motors with gears are already available based on the micro-electronic technology in mini-format.

Sensor materials • ceramic • amorphous metal • Fibre optic • Bio-components Technology

• Surface mount, hybrid • IC design technology • Laser alignment • micro-machining

NEW

SENSORS

2 Inductive Sensors

2.1 Fundamental Principles

2.1.1 Basic Construction 2.1.2 Reduction Factor

2.1.3 Coil Size and Sensing Range 2.1.4 Installation Problems

2.1.4.1 Housing

2.1.4.2 Flush Mounting 2.1.5 Electronic Circuit

2.2 Types

2.21 Cylindrical and Rectangular proximity switches 2.2.1.1 Definitions

2.2.2 Slotted Types 2.2.3 Ring Types

2.2.4 Bistable Switches

2.2.5 Sensors for use in Welding Magnetic Fields

2.2.6 Sensors to distinguish between different materials 2.2.7 Inductive Analogue Sensors

2.3 Interfaces for Inductive Proximity Switches

2.3.1 Electrical Types and their Positive Effects 2.3.1.1 Direct Current Switches

2.3.1.2 Alternating and All voltage Switches 2.3.1.3 Sensors to DIN 19234 (NAMUR) 2.3.2 Protected and Safety Switches

2.3.2.1 Reverse Polarity and Over Voltage Protection 2.3.2.2 Overload Protection

2.3.2.3 Safety Circuits

2.3.3 Loads and their Characteristics 2.3.4 Bus Connection

2.4 Manufacturing Technology

2.1 Fundamental Principles

Inductive sensors, in particular in the form of inductive proximity switches, also known as initiators, are widely used in automation and the process industry.

2.1.1 Basic Construction

The active elements of an inductive sensor are the coil and ferrite core (see diagram 2.1). an alternating current is passed through the coil producing a magnetic field, which passes through the core in such away that the field only leaves the core on one side; this the active face of the proximity switch. When an metallic or magnetic object is near to the active face the magnetic field is deformed. An exact picture of the magnetic field can be obtained from computer simulation (see diagram 2.2). The effect on the magnetic field of a conducting material can be seen, in this case a steel plate. The change in the magnetic field due to the steel plate, also produces a change in the coil so that it’s impedance changes.

This change in impedance is evaluated by the integrated sensor electronic and converted to a switch signal. Eddy currents are induced in electrically conducting materials present in the alternating magnetic field. The damping plate may be considered as short circuited winding, and the arrangement of damping material and sensor coil can be considered as a transformer.

Ferrite core coin

damping plate

Diagram 2.2: Diagram showing the lines of force of the magnetic field of an inductive sensor with and without damping plate made from ST37

The sensor coil forms the primary winding and the metal plate the short circuited secondary winding, see diagram 2.3. Because of the inductive coupling, represented by the mutual inductance M12 the current flowing in the secondary circuit i2 is reflected in the primary circuit. This manifests itself in the change of the coil impedance Z. This can easily derived from a comparison with the ideal transformer.

Primary side: u

1 = (R1+j·w·L1)·i1 + j·w·M12·i2

Secondary side: 0 = u₂ = (R₂+j·w·L₂)·i₂ + j·w·M₁₂·i₁. From the above we have:

It can be seen that in the presence of a conducting material the real part of Z is increased above the resistance of the coil R

1 the increase is dependent on R2, L2, M12 and w.

Experience shows that the imaginary part of Z only shows a measurable change with very small separation between the coil and the metal plate; it is only necessary to draw on the change in the real part of Z to detect an object made of conducting material.

U 2= O R ₁ i ₁ R 2

i

2.1.2 Reduction Factor

The increase in the real part of Z by the damping piece is largely dependent on the distance between the plate and the coil assembly and the material from which the plate is made, in particular from the material conductivity and permeability u. The largest change is obtained with damping pieces manufactured from mild steel (St37). The sensing range s of various materials

is standardised against sn, which is the sensing range obtained with St37 and define a reduction factor, also known as correction factor: reduction factor = s/s_n.

Diagram 2.4 shows the dependence of reduction factor on the quotient of electrical conductivity divided by the relative permeability of the test piece; the example is for a proximity switch with 5mm sensing range ( no account is taken of the hysteresis loss of the test piece). The curve varies for each type of proximity switch, however it always has the same tendency.

Diagram 2.4: Reduction factor of a proximity switch as a function of the quotient electrical conductivity / permeability of damping piece.

2.1.3 Coil size and Sensing range

Diagram 2.2 shows that the magnetic field only extends over a limited distance, which in the end determines the maximum possible sensing range of an inductive proximity switch. It is evident that the extension of the field and the sensing range sn increase with increasing coil diameter. There is a moderate increase in the sensing range with increase in core diameter for proximity switches with standard sensing range (diagram 2.5).

Diagram 2.5: Nominal sensing range s_n for an inductive proximity switch, with standard sensing range, as a function of core diameter d.

2.1.4 Installation Problems

The surroundings of the coil system, of inductive proximity switches, which includes conducting material outside of the active area creates a problem, in that this also has an effect on the shape of the magnetic field and therefore the impedance of the coil.

2.1.4.1 Housing

Where a stainless steel housing is used for a proximity switch, the induced eddy currents. in the housing, causes an initial damping in the coil system and the oscillator, which in turn reduces the maximum sensing range. The effect can be reduced by mounting a copper ring in the steel housing; the magnetic field in the housing is reduced in this way (see diagram 2.6). The eddy currents which now flow in the copper ring instead of the housing produce a lower loss, because the electrical conductivity of copper is approximately 40 times that of usual housing material V2A (see also diagram 2.4). The pre-damping is reduced to such a degree that it is possible that the sensing range is increased.

Diagram 2.6: Lines of force of the magnetic field of an inductive sensor with integrated copper screening

coil shell core

shell core copper ring V2A-Housing

2.1.4.2 Flush mounting

Further undesirable losses are produced when a sensor is flush mounted in a conducting material, e.g. machine parts made from steel. The sensing range is reduced due to the additional pre-damping of the sensor magnetic field. In unfavourable cases the initiator may switch by the installation. In this situation the screening produced by the copper ring has a positive effect in that the eddy currents produced in the installation material are reduced. Sensors with increased sensing range, for flush mounting are normally provided with copper ring screening. The effect of the screening however is reduced with sensors with larger diameters, so that the flush mounting of larger sensors remains a problem. A possible solution for the future could be that the proximity switch senses it’s surroundings, this will require an increased technical effort in both the construction and

the control circuit of the sensor.

2.1.5 Electronic circuit

The coil system of the proximity switch together with a capacitor forms a parallel resonant circuit. A simplified equivalent circuit is shown in diagram 2.7, L represents coil inductance and R_v = Re (Z) the coil resistance, which is dependent on the damping piece (object sensed). C is the parallel capacitor considered as an ideal capacitor. The resistance R

v determines the Quality Factor Q of the resonant circuit.

Diagram 2.7: Simplified equivalent circuit for the resonant circuit of an inductive sensor.

A block diagram of an inductive proximity switch is shown in diagram 2.8. The resonant circuit is part of an oscillator and the quality factor of the resonant circuit

Q = wL/R

v determines the amplitude of the resulting HF oscillations. With the approach of

the damping piece the quality factor of the coil is reduced due to the increase in the loss resistance R_v and therefore a reduction in the amplitude of oscillation. When the amplitude falls below a preset value a comparator operates, which in turn operates the output circuit and the sensor switches.

Diagram 2.8: Block diagram of an inductive proximity switch

output stage

In diagram 2.9 the change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm is presented.

Diagram 2.10 Shows the relative change in quality factor ∆Q/Q for the same case, with reference to the undamped coil. The change in quality factor, which is taken as the switch point, is in the region of 10% to 50% (in this example 10%) for sensors with standard sensing range. In the case of initiators with double the sensing range only a change in quality factor of 1% to 6% is available, which demands a higher specification of the sensing electronic particularly with regard to temperature sensitivity.

Diagram 2.10: Relative change in quality factor

∆Q/Q, of the coil system of an inductive sensor with 10mm sensing range, as a function of the sensing distance s of the damping piece, with respect to the undamped system. Diagram 2.9: The change in quality factor Q, as a function of distance s from damping piece, for a coil system of a flush mounting proximity switch with a nominal sensing range of 10 mm.

A basic oscillator circuit is shown in diagram 2.11. The resonant circuit comprises L and C. Transistor T is connected in the common collector configuration and operates as an non-inverting amplifier with a voltage amplification less than 1; because of this the transformer feedback is necessary to produce the required voltage boost. The transformer is formed by tapping on to the coil. R_b and diode D determine the DC operating point of the transistor. Continuous oscillation of the oscillator is ensured by R_E, which is also used to adjust the switching point. In practice this circuit exhibits a number of disadvantages, in particular with reference to temperature stability; because of this a slightly modified version is used as shown in diagram 2.12.

Diagram 2.11: Principle of the oscillator circuit Diagram 2.12: Oscillator circuit

Here the diode is replaced by the base emitter of a second transistor. When both transistors are at the same temperature, which can be best obtained with a dual transistor, the temperature drift of one is compensated for by that of the other. The capacitor C in the resonant circuit is connected so that the inductance of both coil windings is used. In this way the required capacitance is reduced for a given frequency of oscillation f. This is given by : 1

f = ————. 2·p·(LC)½

Depending on the switch type this ranges from a few kHz to a few MHz and is to a large extent dependent on the size of the coil core and therefore the sensing distance sn (diagram 2.13). The current taken by the output of the oscillator is high in the undamped condition and low in the damped condition. Diagram 2.14 shows the current taken by the oscillator, for an initiator using this circuit, with 10 mm switching distance, as a function of the distance of the damping object.

The switch point lies in the area of the rapid change in current, which offers the greatest sensor sensitivity.

Various temperature effects cause unwanted drift in the coil quality factor. The ohmic resistance of the coil, which is wound with copper wire, increases with temperature. The hysteresis loss in the core, which increases with frequency, is also effected by temperature, this can be positive or negative depending on the ferrite material. These effects determine, together with other effects e.g. skin effect in the coil, frequency and temperature behaviour in the coil system. An attempt is made, by experiment, to determine a frequency at which the counter acting temperature effects cancel one another so that a constant quality factor of the coil is obtained. Diagram 2.15 shows the change in Q, of a coil system for an initiator with 10 mm switching distance, as a function of frequency for different temperatures. The curves are closest together at a frequency of approximately 550 kHz, therefore this the operating point at which the least temperature drift occurs.

Diagram 2.13: Oscillator frequency f, of inductive sensors, as a function of the rated sensing range s_n

Diagram 2.14: Current taken I of an inductive proximity switch, based on the circuit of diagram 2.12, with 10 mm switching distance as a function of the distance from the damping piece

Diagram 2.15: Quality factor Q of the undamped coil system, of an inductive proximity switch with 10 mm sensing range, as a function of frequency f, for different temperatures.

2.2 Types of Proximity Switches

2.2.1 Cylindrical and Rectangular Proximity Switches

The basic form of an initiator is the cylindrical form. One of the end faces of the cylinder being the active face. The same circuit is also supplied in a rectangular housing (Diagram 2.16).

Diagram 2.16: Examples of Cylindrical and Rectangular Sensors

Cylindrical proximity switches have a steel or plastic housing. The coil system with the ferrite core is mounted at the front active face and is protected by a plastic cap. Behind this is the electronic circuit mounted on a printed circuit board or as a thick film circuit assembly. An LED serves to indicate the sensor switched condition. The housing is sealed with an end cap, which holds the connecting cables. The whole of the inner space is filled with plastic encapsulating material (Diagram 2.17).

Plastic cap LED O- Ring-Seal

Holding Ring Housing

Component carrier Ferrite core Support IC

Covering paste

Encapsulating material

Diagram 2.17: Assembly principles of a cylindrical inductive sensor

2.2.1.1 Definitions

The terms used in the specification and classification of inductive proximity switches, as well as the methods used to measure the most important parameters are defined in DIN EN50010 and 50032. A standard test plate is required made from a square piece of 1mm thick mild steel grade 37; the side length is dependent on the nominal sensing range s_n of the initiator. The nominal sensing range is a theoretical characteristic, which is used to classify proximity switches, without taking account of tolerances in the devices (diagram 2.18). The actual sensing ranges s

r is determined with rated voltage and at an ambient temperature of 20 °C.

A deviation of + 10% from the nominal sensing range s_n is permitted. 0,9·s

n < sr < 1,1·sn.

The effective sensing range s_u is the useful sensing range, which can be set, within the specified temperature and voltage range. It must not deviate more than + 10% from the actual sensing range:

0,9·s_r < s_u < 1,1·s_r.

The operational sensing range s

a is the sensing range within which the sensor operates under

the permissible operating conditions. The value lies between 0 and the smallest value of the effective sensing range.

0 < s a < 0,81·sn. s_u max s_r max s_n s_r min s_u min = s_a +10 % +10 % -10 % -10 % s_n Test Plate

s_n = Nominal sensing range s_r = Actual sensing range s_u = Effective sensing range s_a = Operational sensing range

Sensing range s_a

1,21 s_n 1,1 s_n 0,81 s_n 0,9 sn

2.2.2 Slotted Initiators

The slotted initiator consists of two coil systems facing each other, this forms a transformer with a large air gap and and poor flux linkage (diagram 2.19). The two coils each represent a winding of the transformer in the oscillator circuit diagram shown in diagram 2.12. In the undamped condition the coupling between the two coils is sufficient to allow the oscillator to oscillate. The inductive coupling is reduced when a metal object is placed in the slot between the two coils. At a particular depth of the object in the slot the feedback in the oscillator reaches a critical value and the oscillations cease, the initiator then switches. Due to it’s construction the slot initiator is insensitive to changes in position of the metal object in the direction of the core axis, so that in this direction the system is inexact. The sensitive

direction is perpendicular to the core axis.

In this type of sensor it is mainly the change in coupling between the two coils which is evaluated; the increase in resistive loss is of little importance. For this reason the material parameter of the damping object has much less effect on the switching point, as compared to the case of cylindrical initiators.

2.2.3 Ring Initiators

Ring initiators have a toroidal core instead of a pot core, which is mounted cylindrically around the coil (diagram 2.20). It shields the magnetic field from the surroundings, so that the active area lies within the coil. Here again the oscillator circuit of diagram 2.12 is used. The oscillator circuit is damped as soon as a metallic object enters the space inside the ring. One application is the recognition and counting of small metallic objects, which pass through the initiator. Ferrous and non-ferrous metals can be detected, as with the reduction factor for cylindrical proximity switches, the smallest non-ferrous object must be larger than the smallest ferrous object in order to initiate switching.

Housing Diagram 2.19: Construction of a Slotted initiator (principle)

Coil 1

Coil 2

Diagram 2.20: Coil system and section through a Ring Initiator

Metal Object

Coil Ferrite Ring

2.2.4 Bistable Switch

The bistable switch has two stable switched conditions, in which it can remain unchanged, even when the initiating object is removed. In principle this achieved with a bistable ring initiator; the coil system and schematic block diagram is shown in diagram 2.21. Two separate coils are mounted within a ferrite ring, each is connected to a separate oscillator. The two oscillators are linked to oppose each other so that only one can oscillate at a time. The circuit design ensures that on switching the supply on oscillator 1 operates. When a metal object approaches the initiator, from the left, coil 1 is damped and oscillator 1 stops oscillating and oscillator 2 commences oscillation; if the object enters coil 2 this will also be damped and oscillations will cease. As soon as the damping is removed from coil 1, by further movement of the object, the oscillations in oscillator 1 will recommence. When a conducting material passes from left to right through the bistable initiator, oscillator 1 will oscillate according to the initial stable condition; when the object passes from right to left through the initiator oscillator 2 oscillates, the second stable condition. It is therefore possible to use bistable initiators for direction detection. The oscillators are so designed that they require different operating currents, therefore the switched condition of the initiator can be detected from it’s operating current.

Diagram 2.21: Coil system and Block Diagram of a bistable Ring Initiator

Ferrite Ring Metal Object

Coil 1 Coil 2

Coil 1

2.2.5 Proximity Switches for use in A.C. and D.C. Welding Fields

When inductive proximity switches are used near to electric arc welding equipment, two detrimental effects occur. The strong alternating magnetic fields produced by the welding currents influence the magnetic core of the proximity switches, effecting the core up to the point of saturation or at least shifting the operating point, since the reversible permeability is noticeably reduced, thereby reducing the Q factor. In other words the coil system is damped, which may cause the sensor to switch. This can be remedied by using special cores made from sintered iron granules, which saturate at a flux density 2 or 3 times the saturating flux density of the conventional ferrite cores. However the cores have a lower permeability, so that the coil Q Factor is reduced. The second detrimental effect is that the alternating magnetic field produced by the welding equipment induces voltages in the sensor coil. These voltages effect the oscillator and may lead to an uncontrolled switching characteristic, which must be prevented by effective circuit design. Proximity switches for use in welding applications indicate the rough working conditions encountered particularly by their robust mechanical construction.

2.2.6 Sensors for distinguishing between different materials

Diagram 2.2 illustrates the principle of an inductive sensor, which is capable of distinguishing between ferrous and nonferrous metals. In addition to that shown in diagram 2.2 the sensor in this case has a closed metal ring is fitted around the core to ensure pre-damping.

Diagram 2.2: Principle of the Material distinguishing Sensor

The operating current of these sensors in the undamped condition can be reduced to approximately half of that of a standard sensor, by the choice of the ring material and it’s dimensions. If the sensor is damped by a ferrous material, e.g. mild steel St37, then the operating current falls to the minimum value.

Object Pre-damping

Core Coil

Object: Iron Object: Aluminium Object Type 2 Pre-damping Core Coil

Diagram 2.23: Current draw with and without pre-damping

When a nonferrous material enters the sensor field the current draw increases, with decreasing distance from the sensor, up to the maximum value. Technically this behaviour can be used in various applications. This switch is suitable for safety circuits, where the opposite switching characteristic is required to that of a standard sensor. In this case the damping material must be a nonferrous material e.g. aluminium. Another application is as a selective inductive sensor. Here the evaluation unit two switching thresholds are defined, one lies above and one below the current draw in the undamped condition. By including two independent outputs, which are controlled from the different evaluation units, the assigned output will operate depending on the damping material.

2.2.7 Inductive Analogue Sensors

The inductive analogue sensor occupies a special position among inductive sensors, because instead of a switch signal at a particular distance of the damping piece from the sensor, an

output signal is produced, which is proportional to the distance from the sensor. The output current, of the sensor, is proportional to the distance s of the object from the sensor over a definite operating range (see diagram 2.24). The mechanical construction and the coil system takes the form of a cylindrical proximity switch. The principle of operation is shown in diagram 2.25. The oscillator circuit supplies the resonant circuit with an alternating current of constant amplitude i. The following is true for the voltage u of the resonant circuit:

u ~(1 + Q2₎½ _.

For Q factor greater than 10 u is almost proportional to Q and within certain limits to the distance of the damping piece from the sensor. In some types of analogue sensors an additional linearisation circuit is included, which increases the useful upper end of the operating range; this is not necessary in other types. In the output circuit the sensing signal is converted to a current which is proportional to the distance s. As in the case of standard proximity switches the data for analogue sensors is with reference to a standard mild steel test plate. Where nonferrous materials are used the operating range shifts and is reduced correspondingly. The inductive analogue sensors, as well as being suitable for contactless distance measurement, are also suitable for the identification of different materials.

0 3

Diagram 2.24: Output current I_A of an inductive analogue sensor as a function of the distance s of the damping piece.

20 10 0 I a/mA s/mm with linearisation without linearisation Lineari-sation i = constant Output Signal proportional Output circuit

2.3 Inductive Proximity Switch Interfaces

Inductive proximity switches are divided into two basic groups, AC sensors and DC sensors. Two, three and four wire sensors are available. They may have normally open, normally closed or changeover functions. On the sensor side the interface is provided by the output stage of the sensor, which provides the link between the sensor and the customer interface (diagram 2.26) and fulfills numerous tasks:

- Energy supply of the sensor - Interpret the sensor signal

- Changing voltage level and amplification - Interference suppression (filter)

- Optical Indicator (LED)

- Protection against incorrect connection

- Suppression of erroneous signals (e.g. due to switch on impulse) - Drive different loads on different circuits.

2.3.1 Electrical Types and Effective area of operation

The standard DC switches are available for the operating voltage ranges 10-30 V and 10-60 V. AC Voltage switches work over the range 20 V to 250 V. All current sensors (AC/DC sensors) operate over the range 20-300 V with DC voltage and 20-250 V with AC supplies. Initiators with an interface to DIN 19234 (NAMUR) are a special case.

2.3.1.1 Direct Current Switches

DC switches are available in two, three-or four-wire versions (Diagram 2.27). Two wire switches are operated in series with the load and require only two connecting cables for this. They can be connected with reverse polarity and are therefore similar to a mechanical switch.

Diagram 2.26: Function of an Inductive Sensor Output Stage as

the link between the Sensor and the Customer Interface.

Sensor Output Stage Customer Circuit

Output

Load Output

Diagram 2.27: Principle of the output stage of a three wire DC Switch, positive and negative switching versions.

Load

2 - Wire - Technology 3 - Wire - Technology 4 - Wire - Technology

Load

Load

Load Load Load Load Load

p-switching p-switching

In order to supply the sensor itself with electrical energy a small residual current flows in the OFF condition through the load and in the ON condition a voltage drop is present. This must be taken into account in the selection of suitable loads. Three and four wire switches have separate supply connections and one or two outputs for the load; so the limits referred to above are removed. The decision to use positive or negative switching versions depends on whether the switch output connects the load to the positive or negative of the supply (diagram 2.28). Many two or three wire switches are available as normally closed or normally open switches. In the case of normally closed the load is switched off when the oscillator is in the damped condition, in the case of normally open the load is switched on. Four wire sensors have both functions, that is each has a normally closed and a normally open output.

2,1 1,2 3 ∆ I ∆ S Switch points 0 2 1 I/mA s/mm 2.3.1.2 AC and AC/DC Voltage Switches

AC and AC/DC voltage switches are available in two wire and three wire versions. What has been said above for the DC sensor types is also applicable for these sensor types.

2.3.1.3 Sensors to DIN 19234 (NAMUR)

These sensors are simple 2-wire DC sensors without the output stage. They contain only the oscillator as shown in diagram 2.12. DIN 19234 describes how the 2-wire sensor works together with a switching amplifier; it specifies the characteristics of the amplifier, the sensor and the switch point. The amplifier supplies the sensor with power, which in turn controls the amplifier due to a variable internal resistance which results in a variable current consumption. The operating values are kept so small that it is possible to install these proximity switches in potentially explosive atmospheres, taking account of the pertinent regulations and guiding principles for intrinsically safe apparatus in ignition protection zones.

The sensor produces a output signal which is proportional to the distance of the influencing object to the sensor. An example of a stable output characteristic is described in diagram 2.29.

Diagram 2.29: Relationship Distance/current according to DIN 19234 Difference in distance

of switch points Difference in current at

Here is defined that the characteristic curve must be identical regardless of the direction of movement. The switching point, which is determined by the switching amplifier, must lie in the range 1.2 mA to 2.1 mA and must exhibit a difference in switching current (hysteresis) of 0.2 mA (typical 1.6 mA and 1.8 mA). The required switching distances are dependent on the damping material and the nominal sensing range of the sensor. The two wire connecting cable represents a resistance between the sensor and the switching amplifier; this resistance should not exceed 50 ohms. When the sensor operates in potentially explosive areas the maximal cable length is limited by it’s inductance and capacitance. The power supply of the amplifier, which supplies the sensor usually has a linear output characteristic with an open circuit voltage of approximately 8.2 volts and a short circuit current of approximately 8.2 mA. The sensor design is such that the internal resistance of the sensor is approximately 10kohms in the damped condition and approximately 1kohm in the undamped condition, which results in a maximal current in this circuit of approximately 4.1 mA. If the sensor current falls below the above value by 0.15 mA, this will be taken as an indication that there is an open circuit in the wiring or that a fault has developed in the sensor. If the current demand of the sensor rises above 6.0 mA a short circuit will be diagnosed. Both faults can be recognised by the monitoring circuits built into the amplifier, indicated and further processing of the information prevented.

2.3.2 Protection and Safety Circuits

Various protection circuits are used to protect inductive proximity switches from damage from external sources by overloads or incorrect handling. The safety circuits guarantee that no incorrect signal appears at the sensor output, which could cause incorrect operation of the next stage.

2.3.2.1 Reverse polarity and Over voltage Protection

In the case of sensors with reverse polarity protection swapping the connecting cables at will does not lead to damaging the sensor. This is achieved by wiring protection diodes, or diode bridges, to the connections. An over voltage impulse on the supply voltage due to poor regulation of the power supply on switch on, or random disturbances, do not damage or cause incorrect operation of a switch with over voltage protection. Over voltage protection is achieved with a resistor and zener diode or by means of a varistor. From time to time in the electrical wiring of motor vehicles with generators (alternator) high voltages are produced, particularly where mechanical regulators are used. For example, if at maximum charging current the battery connection is loose causing an intermittent connection, which could result in a voltage transient on the supply of 100 to 200 volts approximately, due to the inertia of the regulator. Even in normal operating conditions over voltage transients can occur due to on and off switching of system components. Special circuit techniques such as a larger series resistor in the over voltage protection, higher voltage rating of the semiconductors and a higher rated over voltage protection element prevent damage to the switches intended for use in motor vehicles.

2.3.2.2 Overload Protection

Sensor with overload protection are not damaged when the load resistance reduces even to a short circuit, this is true for the complete specified voltage and temperature range of the device. The danger of overloading the output stage is the increased power loss in the output semiconductor and the increase in the device temperature above the maximum allowable temperature, which could result in damage to this component. The cheapest overload protection is the use of a thermistor with a positive temperature coefficient in series with the load; this had however certain disadvantages, a very high peak current flows in the case of a short circuit, the switch off current is very much dependent on the ambient temperature and this results in a high thermal load on the switch. This type of protection therefore can only be usefully applied with small load currents (Il < 100 mA) and low supply voltages (Us < 30 V). The recovery time after an operation is very long (approx. 1 minute).

The principle is robust and free from interference, because of it’s sluggish operation. Because of it’s inertia it can be used to switch large capacitive loads. Another method of overload protection is to limit the output current to a constant value. This is the cheapest electronic solution; this leads however to a large power dissipation, particularly in the case of a short circuit. For this reason it is only used for small load currents (Il < 10 mA) and low supply voltages (U_s < 30 V). It’s advantage is that it is immediately ready to operate once the overload is removed. In applications where large loads must be switched it is of particular importance that overloads are immediately detected and switched off. The switch off can be self-locking, that is after the removal of the fault the sensor does not automatically start to operate but requires a reset signal. This allows a simple localisation of the fault and in addition is desirable in relevant safety applications. In this case there is no thermal load on the sensor. The most flexible, also the most costly, solution is the pulsed overload protection. When an overload occurs the output switches off and after a short period (tp) switches on again, if the overload is still present the the current is limited to a value Ik and switch off again after a short time (9tk milliseconds), see diagram 2.30. The cycle is repeated as long as the fault is present. This produces an autonomous switch on after the fault is removed. The time required for the sensor to be again operational is the off period tp. There is only a small thermal load on the switch as the ratio of impulse (I = Ik) to off period (I = 0) can be small (t

k/tp » 1/100).

2.3.2.3 Safety Circuits

In the case of high resistance loads, that is without a definite On/Off signal level, e.g. measurements with a digital voltmeter, the reverse current of the semiconductor switching component in the output, which is approximately 10 µA, must be safely by-passed. In addition three and four wire switches for DC voltages have a base-load, which without an external load, cause a current of approximately 1 mA to flow through the conducting output stage; so that a break in various cables does not produce incorrect switching, undefined switching impulses are suppressed internally. During the starting period of the sensor circuit, after the supply voltage is switched on all outputs are suppressed in order to prevent any undefined output pulses. After the so called initialisation period, approximately 10 ms, the sensor is ready to operate.

2.3.3 Loads

Pure resistive loads add no special demands on the output stage of an inductive proximity switch. Neither over-currents or over-voltages occur during switch on or switch off. On the other hand inductive loads produce problems due to induced voltages. During switch off the load current I_L continues to flow, due to the inductance L, through the overvoltage protection components (e.g. zener diode, varistor), the current decreases exponentially. The energy transferred during this time is proportional to L and I_L2 _{so that the maximum allowable} inductance must be specified. If this is exceeded the overvoltage protection components will be damaged and therefore the switch will be damaged, independent of whether the output stage is protected against overload or not. For this reason to switch high inductance loads a free-wheel diode should be mounted in parallel with the load, however this increases the dropout time of the relay, or contactor, because the stored energy,

W = 0,5 × L × I_L2

is slowly dissipated in heat. The inductance can then be as large as required. The requirement for reverse polarity protection prevents the diode being mounted in the sensor. Relays behave as inductive loads however it should be taken into consideration that the inductance in the pull-in condition is different to that in the drop-out condition. Since switch off occurs in the pull-in condition this is the determining inductance. It should be noted where contactors are used as AC loads that the impedance in the Off condition is very much smaller than that in the On condition, since the inductive portion of the impedance is very much larger than the resistive part; this results in a pull-in current 5 to 8 times the rated current.

The pull-in time is in the region of 10 ms. If the power switch is a thyristor or a triac it is only possible to break the circuit in the region of zero current (I

L < 20 mA), because of the device

holding current; therefore at switch off only a small amount of energy must be dissipated in the overvoltage protection components. In practice the load inductance does not have to be taken into consideration. Special consideration must be given to capacitive loads. At switch on the capacitor load appears as a short circuit, the load current is only limited by the output stage design. Often in the case of switches without short circuit protection the short circuit current is not defined and in these cases only small capacitances in the region of 100 nF can be switched. By exceeding the maximum allowed capacitance causes the overload protected switch to revert to pulse operation and leads to damage in the case of switches without overload protection. Incandescent lamps also require special consideration. The data provided by the lamp manufacturer, rated current and wattage, refers to lamp in the illuminating condition. On switch on the tungsten filament is cold and the lamp draws 8 to 12 times the rated current in the case of vacuum lamps or gas filled lamps and in the case of Halogen lamps 10 to 15 times the rated current. The cold start current falls to about twice the rated current after 10 ms.

Example: Lamp: Rated voltage U

n = 24 V; Pn = 2 W.

From this: Rated current I_n = 83 mA, P_n/U_n = 83 mA Cold starting current I_k = 12 83mA » 1A

This means that the output stage in the switch must carry 1A for short period, without damage or change over into pulse mode operation. Output stages with overload protection,which are not designed especially for incandescent lights warm the lamp filament with a number of over current pulses, however analysing units such as relays, SPS or counters record these pulses. The effect is that relays oscillate, incorrect counter pulses generated etc..

2.3.4 Bus Connection

As automatic production systems are become increasingly complicated, the trend is more and more to decentralised systems. Thereby the communications requirements, at all levels, increases, down to the level where the sensors are established. On the other side components found at this level sensors, multiplexers etc. are increasingly supplied with digital electronics it would be advantageous to provide these with a serial Bus interface. This results in a number of advantages; one the system is clearer as with a star shaped individual wiring of all components; the system remains flexible, since modifications and extensions are possible without great expense.

In addition the bidirectional Bus system makes it possible to transmit additional information, such as configuration, initialisation and parameter data, status and fault messages. It is increasingly possible to perform functions, which are at present handled by the central control, at the sensor, examples are signal pre-processing, linearisation, temperature compensation, calculating the mean value and analogue to digital conversion. Lastly not to ignore the saving in cable. In the future these advantages, and the introduction of standardised Bus-systems for this level, will lead to basic sensors, such as inductive proximity switches or distance indicators having a bus interface available.

2.4 Manufacturing technology

The various rising electronic manufacturing technologies are to be found in proximity switches. Therefore parallel to standard printed circuit board technology are found surface mount technology, hybrid and integrated circuit design technology. The complexity and reliability of the circuits increases in the order of the technologies given above. In the past ,because of the limited space available in a proximity switch, only relatively simple circuits with few components could be produced. Today with Standard- or Custom ICs technology it is possible to have hundreds or thousands of transistors on one chip with a few millimeters edge length. To a large scale SMT- and hybrid technologies are used in the manufacture of inductive sensors today. For many years standard integrated circuits have also been available, which contain in addition to the oscillator the signal evaluation and conversion to a switching signal functions. in addition these ICs offer auxiliary functions such as voltage regulation, suppression of switch on transients, short circuit and overvoltage detection and processing, which enables a simple design of a high quality sensor. On the other hand the use of customer specific ICs for inductive proximity switches is in it’s infancy.

3

Capacitive Sensors

3.1 Basic principles

Capacitive sensors, as do inductive sensors, without touching, non-interacting and contactless. They add to the range of sensor applications, where the inductive operating principle is unsuitable. Capacitive sensors can also detect nonconducting materials. Capacitive sensors are mainly available as proximity switches, recently however analogue sensors have also become available, which give an output signal proportional to the separation.

3.1.1 Sensor Construction

The active component of a capacitive sensor is the arrangement of a disc shaped electrode inside a cup-shaped screen (Diagram 3.1).

These two electrodes form a capacitor with a basic capacitance C

g. When a target

approaches the sensor (distance s) the capacitance changes by an amount ∆C.

The capacitor is part of a RC oscillator, the output voltage of which is dependent on the effective capacitance C_a= C_g + ∆C between the sensor electrode and the screen potential. A block diagram of a capacitive proximity switch is given in diagram 3.2.

The oscillator output voltage is rectified, filtered and interference pulses suppressed. This forms a switch signal which is converted to an output signal in the output stage.

Target

Screening Sensor Electrode

In principle there are three different possible methods of operating a capacitive sensor: with a nonconducting target, an isolated conducting target or an earthed conducting target. A nonconducting target (e.g. glass or plastic) can only increase the capacitance C

a by

changing the dielectric in the field area of the capacitor. This increase in capacitance is very small and depends on the size and permittivity ε_r of the target. This enables only small switching distances. If an insulated conducting target (metal) approaches the sensor in addition to the basic capacitance C

g two series connected capacitors are formed, that is

between the target and sensor electrode and between the target and the screening. The increase in capacitance dC is greater than with the nonconducting target; this produces an average response sensitivity. The largest increase in capacitance, therefore the greatest sensing range, is obtained with an earthed metal target. The additional capacitance between the sensor electrode and the target is in parallel with the capacitance C

g.

s

Diagram 3.3: Methods of activating capacitive sensors

a) non conducting target b) isolated conducting target c) earthet conducting target

Sensor electrode Target

Screening

Diagram 3.2: Block diagram of a capacitive sensor. Target Screening Sensor electrode Sensor electrode Screen Screening Target Sensor electrode Sensor electrode Screen a) b) c) Target Screening Sensor electrode Sensor electrode Screen

3.1.2 Sensitivity

The sensitivity is found by determination of the change in capacitance ∆C

s, at which the

switch signal at the output of the sensor occurs. In order to have an impression of the order of magnitude of the change, we consider the case of an earthed conducting target. The problem is reduced to a plate capacitor with round plates of d= 30 mm diameter. The switch point for axial approaching targets should be s1 = 15 mm and the switch hysteresis h = 1. The switch point for targets moving away is then s2 = s1 + h = 16 mm.

The capacitance of a plate capacitor is calculated from:

A = Area of plate, s = distance apart of the plates, ε

r = 1.

From this the capacitance at the switch point s

1 is

At switch point s₂ the capacitance has a value of

The change in capacitance which produces a signal change at the output is therefore

∆C

s = C1 - C2 = 0,03 pF. Due to parasitic elements the basic capacitance Cg = 5pF

approximately, this gives a relative change in capacitance of :

ε₀× A C = _____ ; s ε₀⋅ π × d2 C₁ = ________ = 0,4 pF ; 4 × s ε₀⋅ π ⋅ d2 C₂ = ________ = 0,39 pF. 4 × s₂ ∆C_s ∆C_s 0,03 pF ____ × 100% = ________ × 100 % = _______ × 100 % = 0,5 %. C a Cs + C1 5,42 pF

3.1.3 Reduction Factor

Depending on the material of the non-conducting target, as shown in chapter 3.1, different changes in capacitance ∆C are produced.

This effect can be observed at the capacitive sensors output as a change in the switch point. A material dependent reduction factor is defined analogues to that of the inductive proximity switch. The factor indicates by how much the nominal switching distance sn, which is obtained using an earthed metal target, must be reduced for a given material.

In diagram 3.4 is shown this reduction factor = s/s_n,

as a function of the permittivity r, for various materials.

Where the permittivity is temperature dependent a drift in the switching distance must be taken into consideration. Some sensors have the facility of adjusting the sensor range in order to compensate for the different sensing ranges, resulting from the reduction factors of various materials. For reliable operation of the sensor care should be taken not to set the sensing range to too high a value, as in this condition the RC oscillator can become unstable. This condition would become noticeable through an increase in the hysteresis (h > 0,1·s).

Diagram 3.4: Reduction factor of a capacitive sensor as a function of the permittivity ε_r of the target.

Reduction factor

Glass

Ceramic

PVC

Ice

3.2 Practical Example

3.2.1 RC Oscillator

The circuit used is a two stage RC oscillator (Diagram 3.5). The amplification of the first stage is: U₁ Z₁ + Z₂ V 1 = ____ ₌ ______ _{; (U} 3 » U2). U₂ Z₂

The second stage, a common collector circuit, has an amplification of : U

a

V₂ = _____ _{= 1.}

U

1

Feedback of the output voltage is achieved via P and C

k; with P is the ratio:

U

2

A = —— adjusted. U

a

Setting the switch point with the potentiometer P, in the absence of a target, the following condition is produced:

Z

1 + Z2

V₁·V₂·A = ———— · A < 1 Z₂

This means the oscillator cannot oscillate. The approach of a target leads to a reduction of Z

2; with this V1 increases and the circuit amplification becomes V1

. _V

2

. _{A > 1.}

The oscillator starts to oscillate. The relationships are opposite to those of the inductive proximity switch, in which the oscillator without target oscillates and is damped by the approaching target. In the case of the capacitive sensor there is no oscillation without the target, in the presence of a target the system oscillates.

Diagram 3.5: Principle of the RC oscillator of a capacitive sensor.

3.2.2 Interference suppression 3.2.2.1 Interference Effects

Important interference factors are alternating electrical fields. These are coupled in the high resistance input circuit of the oscillator through the sensor electrode and can cause oscillation. The source of these interference fields are, for example, fluorescent lamps, solenoid valves, thyristor drives and radio transmitters. Continuous interference can only be eliminated by changing the oscillator frequency, providing the field is not too strong. Transient interference can be eliminated by the interference filter, which is described in 3.2.2.3 below, providing the pulse length lies within an adjustable time window. Another source of interference is the temperature effect. Changes in temperature effect the RC oscillator particularly. This effect can be minimised by setting a suitable operating point. Humidity, dust and other forms of contamination effect the sensor by changing the permittivity in the area of the active surface. Compensating for the contamination, described in 3.2.2.2 below, leads to a satisfactory improvement in many applications.

3.2.2.2 Contamination Compensation

The aim of the contamination compensation is to maintain a constant sensing range s, when the surface of the sensor is contaminated (e.g. by drops of water or a film of water). This is achieved through an additional cup shaped compensation electrode between the sensor electrode and the screen, which is connected to the oscillator output (diagram 3.6). The contamination increases the capacitance between the sensor electrode and the screen; this leads to an increase in the amplification V1. At the same time the capacitance between the sensor electrode and the compensation electrode increases. This effect reduces the circuit amplification V = V1. _V2 . _{A. The amplification V remains constant by suitable geometric} design of the sensor, compensation and screen electrodes, providing the contamination is homogeneous.

Diagram 3.6: Principle of contamination compensation

Housing Sensor electrode Compensation electrode probe screen Contamination

3.2.2.3 Interference Filter

As described in section 3.2.2 electric fields can lead to malfunction of the oscillator. Following the rectification and the low-pass filtering of the oscillator output signal it passes through an interference filter (see diagram 3.2); which suppresses interference pulses, by the use of nonlinear filter components providing these do not exceed a maximum, selectable, period of time. This has however has the disadvantage that required switching signals, which have a longer pulse width, can not be detected; this means that the maximum possible switching frequency of the capacitive sensor is reduced. Normally the frequency is in the range 1Hz to 10 Hz.

3.2.3 Models

Capacitive sensors are mainly available as cylindrical or rectangular proximity switches, with an active face at the front end (diagram 3.7). The construction principle of a cylindrical sensor is shown in diagram 3.8. There are however special forms , for example, flexible sensors, which can be glued to level or curved surfaces. The manufacture of sensor electrodes on sheets or flexible copper laminated foil offers a large choice in the sensor construction. All the familiar electrical interfaces of the inductive proximity switches can be used. Available are two, three and four wire models for DC and AC voltages with normally open , normally closed and changeover functions. Also models to DIN 19234 (NAMUR) are available. Detailed information about the different interfaces can be found in the chapter „ Inductive Sensors“.

Diagram 3.7: Examples of a cylindrical and a rectangular capacitive sensor.

Sensor

electrode _{Housing Case Diagram 3.8:}

Principle of the capacitive sensor construction.

3.3 Applications

4 ULTRASONIC SENSORS

4.1 Fundamental Principles

4.1.1 Propagation of Sound Waves in Air 4.1.2 Generation of Ultrasonic Waves 4.1.2.1 Electrostatic Transducer 4.1.2.2 Bending Oscillator 4.1.2.3 Membrane Oscillator 4.1.2.4 λ/4- Oscillator 4.2 P+F- Oscillator 4.3 Methods of Operation

4.4 Distance Measuring Ultrasonic Sensors

4.5 Ultrasonic Sensors in Through-Beam Mode

4.6 Possible Errors in distance measurements with Ultrasonic Sensors

4.7 Operating Conditions

4.8 Sensor Types

4.1 Fundamental Principles

4.1.1 Propagation of a sound wave in air

Ultrasonic waves denotes sound waves in the range above 20 kHz; the outside of the human hearing limit. As opposed to electro-magnetic waves sound waves can only be propagated through matter.

The sound wave is dependent on changes of density ρ in time and space, the pressure P and the temperature T of the medium and with local changes and changes of speed of the medium particles.

All the above values vary around a fixed average value.

A prerequisite for sound waves in a medium is it’s elastic properties. The propagation velocity for an ultrasonic wave in gas is given by:

c = (k · P/ρ)½ ₌ λ _{· f,}

P denotes the gas pressure and k is the adiabatic coefficient of the gas. For air the adiabatic coefficient k = 1.4 and the density r has the value of 1.29 Kg/m3 _{at an air pressure of 1013Pa.}

Since the density of a gas decreases with increasing temperature the velocity of sound is also temperature dependent.

For air the relationship is given by: c = c₀·(1+T/273)½_,

where c

0 = 331.6 m/s (velocity of sound at T = 0°C) and T is the temperature in degrees

centigrade.

The change in the velocity of sound per K at room temperature, from

this formula, is approx. 0.17%/K. The following table summarises the values of the velocity of sound against temperature.

T [°C] -20 0 20 40 60 80

c [m/s] 319,3 331,6 343,8 355,3 366,5 377,5

In addition to being temperature dependent the velocity of sound is also heavily dependent on the air pressure, such that the speed increases with increasing pressure. The relative change in the velocity of sound with the normal changes in the atmosphere is approximately

In addition to these dependencies, the velocity of sound is also dependent on the air mixture, for example, the percentage of CO

2in the air and on the relative humidity. The effect of relative

humidity is less than that of temperature and pressure and produces an additional change in the sound velocity of about 2% between dry and moisture saturated air.

4.1.2 Production of Ultrasonic sound in air

In ultrasonic sensor technology the majority of transducers use a

piezoelectric ceramic transducer. Ultrasonic transducers which use the magnetostriction effect are only used in ultrasonic welding technologyand therefore will not be further discussed here.

Apart from the piezo electric transducer the electrostatic transducer is widely used; because of this it will be briefly covered here.

Piezo-electric crystals have the property of changing there dimensions when a voltage is applied to the surface, also electrical energy can be converted to mechanical energy. Conversely when pressure is applied to the outer surface a charge is produced on the upper surface which can be measured as a voltage, which is typically in the order of 100V-. The materials used for these piezoelectric crystals are lead titanium oxide (PbTiO3) and lead zirconium trioxide.

Because the production technology to grow macro crystals is difficult piezo ceramics have found many applications.

pressure velocity of sound

Diagram 4.1

Effect of temperature and air pressure on the velocity of sound

Piezo ceramics are obtained from the sintering of piezoelectric crystals with additives (binding agent). The ceramic produced by the sinter process must be polarised by applying a high polarisation voltage at high temperature, since at first the dipole of the micro crystals are arranged in a random manner.

The elongation in the polarisation axis is a maximum due to the polarisation. Typical elongation in such ceramics, by the application of a few hundred volts, is dl/l = 10-4_{, during}

this the forces produced are in the region of 106 Pa.

The transition between the ultrasonic generator and the surrounding air is very important during the production of ultrasonic waves in air. To obtain efficient radiation of ultrasonic waves in air the ultrasonic generator must produce a large surface amplitude. An adaption mechanism is necessary, which transforms the high energy, but small amplitude, of the piezoelectric ceramic into a low energy, large amplitude movement.

In the following different adaption methods are compared:

4.1.2.1 Electrostatic Ultrasonic Transducer

The transducer (diagram 4.2) in principle consists of a thin metallized plastic foil and a grooved metal plate, which together form a capacitor. When voltage is applied an electrostatic force acts on the foil.

Foil and plate attract each other. An alternating voltage, which is superimposed on a DC voltage, causes the foil to oscillate at the same frequency. The DC voltage is necessary because the force on the foil is proportional to the square of the applied voltage, and with a pure AC voltage the frequency of oscillation would be twice the frequency of the applied voltage.

The foil is held under a constant pressure by means of a flat spring. A frequency tuning up to approx. 500 kHz is possible through the air cushion ,which is trapped between the foil and grooves in the metal plate.Characteristics:

Diagram 4.2 Schematic representation of a electrostatic ultrasonic transducer. Grooved Metal Plate Perforated Metal Plate Metallized Plastic Foil Metal support Flat Spring