EE060 Electro Hydraulic Controls TP Inst

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SRI LANKA INSTITUTE of ADVANCED TECHNOLOGICAL EDUCATION

Training Unit

Electro-Hydraulic Controls

Theory & Practice

ELECTRICAL and ELECTRONIC

ENGINEERING

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Training Unit

Electro-Hydraulic Controls

Theoretical & Practical Part

No.: EE 060

Edition: 2009

Editor: MCE Industrietechnik Linz GmbH & Co Education and Training Systems, DM-1

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ELECTRO-HYDRAULIC CONTROLS LIST OF CONTENT

CONTENTS Page

1 Safety regulation ...5

1.1 Safety regulations Electrics ...5

1.2 Safety regulations Hydraulics...7

2 Foreword to the trainer's manual ...8

2.1 Didactic notes...8

3 Basic principles of electro-hydraulics...10

3.1 General...10 3.2 Basics...10 3.2.1 Electric current ...10 3.2.2 Electric voltage ...12 3.2.3 Electric resistance ...12 3.2.4 Electric power ...13 3.3 Basic circuits ...13 3.3.1 Series connection ...13 3.3.2 Parallel connection ...14

3.4 Measuring of current I and voltage U ...16

3.4.1 Current measurement ...16

3.4.2 Voltage measurement ...16

3.5 Electro-hydraulic equipment...17

3.5.1 Power density ...17

3.5.2 Electro-hydraulic valves ...18

3.5.3 Direct operated directional valves ...21

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3.6.6 Symbols of the most important switching elements...31

3.6.7 Designation of contacts at contactor/relay...31

3.7 Electrical self-locking Suppression by means of a momentary-contact switch ...33

3.7.1 Signal storage ...34

3.7.2 Function chart...36

3.8 Momentary-contact limit switches ...39

3.8.1 Magnetic momentary-contact limit switches with reed contacts ...40

3.8.2 Optical momentary-contact limit switches ...40

3.8.3 Capacitive proximity switches ...41

3.8.4 Inductive proximity switches ...41

3.8.5 Example of connection of an inductive proximity switch...42

3.8.6 Momentary-contact/proximity limit switches ...42

3.8.7 Parallel and series connection of proximity switches ...44

3.9 Pressure switches ...45

3.10 Mechanical locking by means of momentary contacts ...48

3.11 Electrical locking by means of contactor/relay contacts ...49

3.12 Time relay...49

3.12.1 ON-delay ...50

3.12.2 OFF-delay...50

3.12.3 ON- and OFF-delay ...51

4 Experiment cross-reference...53

5 Required electrical components ...54

6 Required hydraulic components ...55

6.1 Component plates ...56

7 Symbols ...59

Experiment 1: Extension of a cylinder upon the Operation of a push-button...79

Experiment 2: Signal storage by electrical self-locking...86

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Experiment 5: Signal storage by means of electrical self-locking Resetting by means of

a proximity switch...102

Experiment 6: Rapid advance circuit ...107

Experiment 7: Pressure-dependent movement reversal ...113

Experiment 8: Pressure switches and proximity switches ...119

Experiment 9: Pressure-dependent sequence control...126

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1 Safety regulation

1.1 Safety regulations Electrics

For working with electrical systems and equipment, the regulations for the prevention of accidents "Elektrische Anlagen und Betriebsmittel" (electrical systems and equipment) (VBG4) issued by the industrial trade associations as well as the VDE regulations VDE 0105 part 1 and part 12 have to be observed. Electrical equipment means any apparatus which is used for applying electric power or for the transfer and processing of information. Electrical systems are formed by connecting electrical equipment.

The VBG 4 regulations are quite short and supplemented by procedure instructions of the regulations for the prevention of accidents. These are interesting since they explain the limiting conditions for working on live parts. An excerpt of this table is given in this manual. Three categories of qualifications have to be differentiated:

The electrics specialist, the instructed person and the layman. It must be noted that trainees are laymen. Even after the instruction according to this series of exercises, the trainees are laymen from an electro-technical point of view. They may only carry out work on systems and equipment with a nominal operating voltage of up to max. 25 V AC or 60 V DC.

The trainees must expressly be informed that they, even as skilled workers, must not connect equipment operating at voltages higher than the values given above, unless they become instructed personnel who may service certain systems as a result of having passed in company training seminars.

Working on electrical controls is only permitted if the source of danger of the system to be controlled has been secured beforehand. When working on an electrical control, one must be aware that this may trigger off machine movements which may represent a risk for man and machine.

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VDE 0105 part 1 describes:

"Betrieb von Starkstromanlagen - Allgemeine Bedingungen" (operation of high voltage systems - general conditions)

VDE 0105 part 12 describes:

"Besondere Festlegungen für das Experimentieren mit elektrischer Energie in Unterrichtsräumen" (special regulations for experimenting with electrical energy in classrooms).

THE FOLLOWING IS ALSO VALID FOR ELECTRICS:

SAFETY IS PARAMOUNT!

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1.2 Safety regulations Hydraulics

General guidelines for working with hydraulic systems

 Make sure that the training stand is easily accessible! (Minimum distance to walls and equipment at least 1 m)

 "Where" and "How" can the training stand be shut down in an emergency other than by actuating the "OFF" button ? (Disconnect electrical supply via connecting plug or mains switch).

 The electrics may only be serviced by a specialist !

 Protect adjacent equipment from oil contamination ! (Oil spills must not damage valuable equipment).

 Observe cleanliness, wash your hands frequently, wipe off oil drips! Some oils can be harmful, e.g. when they come in contact with the eye or the mouth! Apart from this, there is the risk of injury from slipping on oil spills.

Working with the training stand

 Set the master switch to "0" before and alter the experiment.

 Protect yourself by ensuring that nobody can switch on the pump during the experiment set-up and that the oil flow to the component carrier is interrupted.  Check all quick-release couplings for proper fit by pulling.

 Hoses must not be excessively bent or curved (risk of bursting).

 Check the condition of fittings and hoses from time for perfect condition.

THE FOLLOWING IS VALID FOR HYDRAULICS

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2 Foreword to the trainer's manual

The present trainer's manual is intended for trainers and instructors in the field of hydraulics as complementary manual for the Fluidprax hydraulic test stand.

The manual describes experiments that were developed in line with practical applications and are connected to hydraulics exercises prepared by the "Bundesinstitut für Berufliche Bildung" in Berlin (BIBB) (federal institute of vocational training in Berlin).

The experiments were carried out on the hydraulic training stand "Fluidprax".

Generally, it must be noted that the test results documented in this manual are intended to provide a guideline for the trainer and instructor and reflect the tendency of the individual experiment.

For explanations regarding the hydraulic training stand "Fluidprax"

please refer to the system manual "Fluidprax".

The short codes (such as DW3E, ZY1, DF14 etc.) refer to the complete training component with connecting plate, coupling connector or sleeve and fixing elements.

Descriptions and calculation principles for the individual components can be found in the 092-Hydraulic Controls.

Flow measurements can be taken with high precision using the DZ30 flow meter. However, it is also possible to use the measuring reservoir on the Fluidprax. For procedures on how to take the measurements with this device, please refer to the general notes.

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At the same time, this system allows both teamwork and individual training.

These new training methods were taken into account when this manual was developed.

Thanks to suitable operating symbols on directional valves, independent and individual working can be supported.

To promote cooperation, the method of questions and answers should also be applied.

Although these training methods were taken into account in the present manual, the concept is also suitable for autodidacts, as the general structure was developed "from easy to difficult tasks".

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3 Basic principles of electro-hydraulics

3.1 General

The exercises for the Electroprax are a continuation of the hydraulic exercises, which you have carried out with the Hydroprax HP3, HP4 or HP6. With these exercises you have deepened your knowledge of "mechanical" hydraulics and put it into practice. This electro-hydraulic series of experiments deals, of course, also with "mechanical" electro-hydraulics. Since you, alter having carried out exercises with the Hydroprax, are almost an expert, these exercises with the Electroprax do not go into details of hydraulics but focus on the electrical part (e.g. control etc.). Naturally, questions with regard to hydraulics have also to be clarified in connection with the Electroprax, e.g. the drawing up of hydraulic circuit diagrams. In any case, it is therefore recommended to clarify any arising questions or misunderstandings with the help of the 092-Hydraulic Controls or to make oneself again and again aware of the functional diagrams of hydraulic equipment. In this way you can deepen your knowledge further, forth Electroprax does not deal in detail with components which you have become familiar with in the course of exercises with the Hydroprax. As already mentioned, with the Electroprax, emphasis is put on the electro-technical part of hydraulics. Since you have "only" dealt with hydraulics so far, we want to repeat the most important electro-technical basic terms and principles before starting the series of experiments. Thus, the introduction into electro-hydraulics is to be simplified.

3.2 Basics

3.2.1 Electric current

Electric current can only flow in a closed electric current circuit. The simplest current circuit consists of a current source (e.g. battery), a consumer (e.g. bulb) and the line between the

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Fig. 1: Electric circuit

Electrical current has different effects: 1) Induction (solenoids)

2) Capacitive effect (capacitors) 3) Resistance heating (resistance wire) 4) Electrolytic effect (electroplating)

In view of electro-hydraulics, the magnetic effect of the current is to be mentioned specifically. The electric current I is measured using an ampere meter (see 3.4.1 current measurement). The unit of electric current is ampere (A).

In connection with electrical current, the following current types have to be differentiated:

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Comparison with hydraulics:

With certain restrictions, electric current (electrons) can be understood as flow (oil). In the first case, electrons flow through a conductor, in the other case oil through a pipe.

3.2.2 Electric voltage

Electric voltage is the actual reason for current. In line with the types of current, direct voltage and alternating voltage must be distinguished.

Electric voltage U is measured using a voltmeter (see 3.4.2 voltage measurement). The unit of electric voltage is volt (V).

3.2.3 Electric resistance

In an electric curcuit, the consumers as well as the lines create a resistance vis-à-vis the electric current. The unit of resistance R is ohm (S2). The correlation of current I, voltage U and resistance R is described by Ohm's Iaw. Either in electrics and hydraulics, it is valid that a resistance affects current (flow) and voltage (pressure).

Ohm's law is as follows:

Resistance = Voltage Current R (Ω) = U (V)

I (A)

Ohm's law can be used to calculate currents from a known voltage and a known resistance.

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3.2.4 Electric power

Power is indicated in the unit watt (W) or kilowatt (kW). With electrical equipment, the power is the product from current and voltage. Thus, the equation for electric power P is as follows:

Power = Current • Voltage P (W) = I • U

Here, we would like to refer to the comparison with hydraulic power. In principle, the following can be applied:

Power = Flow • Pressure P (W) = Q • p

3.3 Basic circuits

According to the arrangement of the consumers in an electrical circuit, we speak of series connection or parallel connection.

3.3.1 Series connection

With a series connection, the individual consumers (resistances) are connected in series one alter the other.

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With this type of circuit, the same current flows through the consumers (resistances). However, the voltage is subdivided into partial voltages according to Ohm's law. The individual consumers (resistances) can be added to one total resistance.

We already compared electrical engineering with fluid mechanics from time to time. The series connection of consumers also allows such a comparison.

In a line, in which two throttles (resistances) are connected in series, each throttle causes a pressure drop (voltage drop). The flow (current) through both throttles (resistances) remains unchanged. In a series connection, the current flowing through the circuit is always identical.

The sum of partial voltages equals the total

Ug = U1+ U2 + ...Un

The sum of the individual resistances equals the total resistance:

Rg = R1+R2 + ---Rn

Partial voltages behave like the related resistances:

U1 R1 U2 = R2

3.3.2 Parallel connection

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In contrast to the series connection, with a parallel connection the same voltage is applied to the consumers (resistances). The current divides into partial current according to the resistances.

Also here, the individual resistances can be voltage applied: added up to one total resistance.

1 1 1 1

Rg = R1 + R2 + … Rn

Comparison with hydraulics:

If two throttles are connected in parallel in a line, the same pressure (voltage) is applied to Partial voltages behave like the related each of them. However, the flow (current) resistances: subdivides depending and the resistances.

In a parallel circuit, the same voltage is applied to all resistances.

The sum of the partial currents corresponds to the total current:

Ig = I1 + I2 + … In

The partial currents operate in reverse to the related resistances.

I1 R2

I2

= R1

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3.4 Measuring of current I and voltage U

3.4.1 Current measurement

Current is measured using an ampere meter. For this, the ampere meter must be connected to the electric circuit in series.

Fig. 5: Current measurement

When taking current measurements take care that you connect the ampere meter in series. Otherwise, the measuring instrument can be destroyed.

3.4.2 Voltage measurement

The voltage is measured using a voltmeter. For this, the voltmeter must be connected in parallel to the consumer.

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When taking measurements using voltage or current measuring instruments always take care that the largest possible measuring range is selected in order to avoid damage to the measuring instrument.

3.5 Electro-hydraulic equipment

On the last few pages we repeated the general basic principles of electrical engineering in order to polish up your knowledge of this topic. Now let us focus an the tasks and basic principles of electrical engineering in electro-hydraulics:

In electro hydraulics the electrics assume mainly signalling and control tasks whereas the hydraulics, due to their high power density, assumes the power functions.

3.5.1 Power density

The power density is one of the essential features of hydraulics. By this, we understand for example the ratio of a power output by a motor (hydraulic motor) in relation to its weight or its size. Electric motors have, for example, a considerably lower power density. An electric motor, which provides the same output power as a hydraulic motor is by far heavier and larger. However, it must be mentioned here that the power density of electric motors has increased in the last few years and is expected to be further improved in the future.

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3.5.2 Electro-hydraulic valves

The interface between electrics and hydraulics is the solenoid operated valve.

Fig. 7: Electro-hydraulic circuit

The solenoid valve

The heart of a solenoid valve is the solenoid.

The operating principle of the solenoid is founded on the fact that a magnetic field is generated by a coil through which a current flows.

Due to this, a force acts on an iron rod (armature) immerged in this magnetic field.

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Fig. 8: Solenoid which performs a stroke movement when the coil is energized

Fig. 9: Classification of solendoids

“Wet pin” solenoids are “immersed” in the oil of the individual hydraulic system. These solenoids have the following features;

- less corrosion - less wear - softer switching - better dissipation of heat - no special sealing required

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Type of solenoid:

Depending on the type of excitation voltage, two types of solenoids are available: Direct current and alternating current solenoids.

Some differences between the two types of solenoids are listed below:

DC solenoid AC solenoid Advantages

- No burning through - Shorter switching times - High switching frequency - Less expensive - Softer switching

- Insensitive to overloading

Disadvantages

- Slower - Burning through

- Higher price - Lower switching frequency - Higher control expenditure

Solenoids in directional valves

The solenoid is fitted to the directional valve by means of screws in order to facilitate maintenance and interchange ability in the case of faults. Three connector pins protrude from the solenoids: 2 connector pins for the solenoid coils (opposite pins) and a ground pin. The latter can also connect the entire valve body to the ground potential, but is not connected in the case of a 24 V voltage.

Example of a nameplate for solenoid valves Voltage/ 220 V, 50 Hz or type of current: 24 V DC Voltage tolerance: ± 10% Power consumption: 16 VA

with alternating current, 26 W with direct current

Duty cycle: 100 % Temperature range: In general:

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3.5.3 Direct operated directional valves

Air-gap solenoids

In order to provide a better comparison, fig. 10 shows an air-gap alternating current solenoid (1) on the left and an air-gap direct current solenoid (2) on the right-hand side of. In this example, the valve has 2 spool positions, with the spool being not pushed into a certain Position by means of a spring. Here, we have a so called impulse or detented spool.

When the solenoid coil is energized, the armature moves the control spool via a plunger. Here, the AC solenoid (1) is energized and has pushed the spool into the right position. With air-gap solenoids, the armature chamber is sealed towards the tank channel by means of the seal in the bushings (3).

Here, the springs hold the bushings (3) in position.

Fig. 10: Air-gap solenoids

In this sectional drawing, the solenoids are fitted with a manual override (4). Thus, the control spool can be operated manually and externally. Thus, the solenoid's switching function can easily be verified.

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Wet-pin solenoids

Figure 11 shows a wet-pin direct current solenoid (1) on the left, and a wet-pin alternating current solenoid (2) on the right. Both armature chambers are connected to the tank. Here, we have a valve with 3 spool positions.

Fig. 11: Wet-pin solenoids

In this sectional drawing, the solenoids are fitted with a manual override (4). This can be used to operate the control spool manually and externally. Thus, the switching functions of the solenoid can easily be verified.

Each of the channels P, A and B is separated by means of segments in the housing. The Channel is not provided with this separation but is connected to the atmosphere and is only sealed by fitting the control element or a cover.

The springs (3) are supported on the solenoid housings and hold the piston via bushing and plate in the centred position.

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3.5.4 Pilot operated directional valves

Directional valves of larger nominal sizes, i.e. for larger hydraulic powers, are pilot operated. The reason for this is the operating forces required for moving the control spool and the related solenoid sizes (power density).

For an exact functional description, see 092-Hydraulic Controls.

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3.6 Extending a cylinder by pressing a push-button

Relays or contactors are electro-magnetic switches with spring return. The switches are electro-magnetically actuated or held in the switched position.

A relay or contactor consists of a coil, which attracts an armature when energized. Thus, one or several contact decks are opened and/ or closed. When the coil is de-energized, the armature and the contacts are returned to their initial position by means of the spring force (see figure 14).

Contactor coils can be energized using either alternating current or direct current, and, depending on the rating of the coils, different control voltages can be connected. We have to differentiate between primary contactors and auxiliary contactors.

Primary contactors are used for switching primary circuits for DC and AC actuators.

Auxiliary contactors are used for switching secondary power circuits. Since the switching ability of auxiliary contactors is limited, they are not suitable for primary power circuits with higher loads. Relays assume similar functions as auxiliary contactors. Basically they are suitable for lower excitation voltages and used almost exclusively for DX excitation. The permissible current loads range from the smallest current value up to approx 1.5 A.

Relays assume similar functions as auxiliary contactors. Basically they are suitable for lower excitation voltages and used almost exclusively for DC excitation. The permissible current loads range from the smallest current value up to approx. 1.5 A.

When selecting contactors, the switching conditions have to be taken into account since these have a major influence on the service life.

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3.6.2 Making procedure

When the coil is energized by the operating voltage, the armature starts to move against the return spring after the rise time tag and when the pickup current is achieved (fig. 15). The contacts usually start to move after the stroke time th which is determined by the mechanics. However, the response of the individual contacts can be delayed or priority can be assigned to them by mechanical means, e.g. spreading S (comparison tf). Although the contacts are closed with the stopping of the armature, the switching process is not yet terminated. The contacts vibrate at a natural frequency determined by their spring rate and mass but this vibration will decay according to a damping rate which is determined by friction. Thus the current is cut in and out several times.

This time is called chatter time tp .

Only after this time will the making process reach a stable condition.

3.6.3 Breaking procedure

With the breaking procedure, first the operating voltage is switched off, which results in a drop of the operating current. After the rise time tal has elapsed, the falling current is reached. This is far lower than the pickup current. Only then the larger force of the return spring starts to move the armature into its initial position. During the armature stroke, the contacts are switched earlier or later depending on their spread and arrangement. The breaking procedure is only terminated after the chatter time tp.

A more detailed description of these expressions is included in the standard DIN 41215. Operating voltage, operating current and/or coil resistance and load ability of the contacts are indicated on the nameplate of the relay.

Notes on the electrical connection of contactors and relays:

In practice, the supply voltage is often grounded to its 0 V potential. For safety reasons, contactors and relays should therefore always be connected with the electrical negative line before being connected to the plus line via a momentary-contact switch, proximity

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Theoretically it would, of course, also be possible to connect the relay to the plus pole (fig. 16).

Fig. 16: Electrical connection of contactor or relay to the plus wire

However, there is a risk that due to insulation defects or other reasons the minus terminal can be connected to the ground thus creating an electrical conductive connection which is not desirable (circuit fault).

Contactors are most often used to switch higher powers, whereas relays are used to interlink electrical signals in a control circuit.

Letter symbols

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3.6.4 Schematic diagram

Instead of a manual Operation of the valve, the solenoid fitted to the valve is energized and operates it.

The control function is illustrated in the schematic diagram. However, this does not provide any information and the wiring of the individual elements but only represent the principle procedure. In fact, the contacts of contactor K1 are for example not separated from the contactor but are integrated into its housing.

The schematic diagram is drawn up using standardized symbols. All the elements are arranged in parallel, vertical lines which are numbered. These lines are called circuit sec-tions and correspond to the current paths.

When drawing up a circuit diagram, the following rules must be observed:

a) Switches and relays are clearly arranged without taking into account the mechanical interrelation of the individual components.

b) The circuit is represented in the de-energized status. c) The equipment is drawn in the non-operated condition.

d) The theoretical direction of movement of the symbols must be in the plane of projection and as a standard always be illustrated from left to right.

3.6.5 Main circuit and control circuit

In practice, we have to differentiate between main and control circuit.

"Control circuit" means that it assumes only control tasks and therefore requires less power.

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The consumers in the main circuit most often require higher powers which are cut in and out via contactors. The power supply to the main and the control circuit can be a separate or a common supply.

a) Control directly at the main circuit b) Separation of main and control circuit b) Separation of main and control circuit

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3.6.6 Symbols of the most important switching elements

Fig. 19: Representation of contacts

The contact designation refers to the condition after Operation (figure 19).

3.6.7 Designation of contacts at contactor/relay

Fig. 20: Table of switching elements re fig. 21

The relation of contactor, coil and associated switches is made clear by proper identification (fig. 21). Thus, lines representing the effects between coil and contacts can be omitted.

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3.7 Electrical self-locking Suppression by means of a momentary-contact switch

In circuit engineering, signal storage via electrical self-locking is indispensable. By the use and the proper arrangement of switches and relays circuits can be realized which are capable of storing switching impulses. The principle behind signal storage can be made clearer using the following example.

You are riding on a bus and want to get out at the next Station. In order to Signal this to the driver you press one of the push-buttons "next stop", which are fitted in several locations on the bus. After pressing this push-button, a display lights up at the driver console even when you no longer press the "next stop" pushbutton. Your Signal is and continues to be applied until it is suppressed by another signal ("open doors").

a) Break contact Priority

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3.7.1 Signal storage

Signal storage or a self-locking circuit can be implemented by means of contactors and/or relays with different arrangements of normally closed and normally open contacts. With a break contact priority (fig. 22) the contactor or relay will not pick up when both push-buttons S1 and S2 are pressed simultaneously; however, with a make contact priority (fig. 23), the coil will be energized. For safety reasons, the dropout priority is in most cases preferred. Break contact circuits are often used as protection in the case of a power failure because then they return to their initial position (OFF). According to the relevant regulations, machines or parts of machines must not start up automatically when voltage is again applied after a power failure. This requirement can easily be met by means of contactor controls with self-locking circuits.

b) Make contact priority

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Signal storage ranges among the so-called logic operations. In logic circuitry, self-locking is designated as RS flipflop. Besides the one mentioned above, the most important operations are AND and OR operations (fig. 24).

R = Reset = OFF; S = Set = ON.

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3.7.2 Function chart

In control engineering, comprehensive controls are represented in function charts. These replace the circuit description.

Function charts should be drawn up taking into consideration the following aspects:

1. Arrangement of the individual components so that signal lines are short. 2. Arrows indicate the direction of the effect of the individual functions.

3. ON impulses are only represented as small squares. Such an ON impulse is illustrated in fig. 25.

4. Signal lines are drawn as thin lines.

Function charts do not always contain momentary-contact switch as individual elements. Especially in comprehensive machine controls, these switching elements are not drawn. They are replaced by the following symbols which are included in the signal flow of the relay or contactor.

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Example 1:

Circuit diagram with associated function chart

Fig. 26: Circuit diagram

For a detailed description of the illustration of function charts, see VDI guideline 3260. The type of illustration below should help you understand the structure of a function chart.

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Example 2: Signal elements

Fig. 28: Signal Element• Fig. 29: Signal element

From this, the following function chart can be derived:

Fig. 30: Function chart

This diagram could further be simplified by drawing the signal elements directly to the function line of the 4/2-way directional valve and to omit contactor K1.

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3.8 Momentary-contact limit switches

Momentary-contact limit switches range among so-called control switches. This group also includes momentary-contact switches. By control switches we understand switches which actuate control and auxiliary circuits.

While momentary-contact switches are switched off manually by the Operator, mechanical momentary-contact limit switches are actuated e.g. by a cylinder moving a roller lever.

In fig. 31 limit switches are grouped by the type of excitation on the sensor side. They can be classified by the type of contact output on the output side, i. e. via mechanical contacts or via semi-conductor switches, which means without contact. In the exercises and experiments we mainly use two types of limit switches.

Fig. 31: Categories - limit switches

Besides these mechanical momentary-contact limit switches, which have to be operated via direct contact, there are also so-called proximity switches. According to the design, proximity switches can be subdivided into inductive, capacitive and optical (light barriers) proximity switches.

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Independently of their design principle, commercially available limit switches are not only used to restrict dangerous movements. In the case of a component failure their contact less outputs can lead to active or passive malfunction. An additional momentary-contact limit switch with positively mechanically opening contacts must be provided to stop the movement.

3.8.1 Magnetic momentary-contact limit switches with reed contacts

The switching function of reed contacts is achieved by pre-loading the contacts via a small magnet. If a stronger magnet is brought near the contact members, the pre-load is overcome and the contact is made.

Fig. 32: Reed contact

3.8.2 Optical momentary-contact limit switches

are realized by means of light barriers. There are three possibilities:

One-way light barrier:

Sender and receiver are arranged on opposite sides. If the light beam to the receiver is interrupted, a switching procedure is initiated.

Reflexion light barrier:

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3.8.3 Capacitive proximity switches

The principle of a capacitive proximity switch is based on the change in capacitance of a capacitor. The capacitance C of a capacitor depends on the charged surface A, the plate distance I and the dielectric constant c; the following is valid :

If a dielectric with a higher dielectric constant (glass plate, PVC plate) is brought into the electric field of a capacitor, its capacitance increases, and with an AC voltage supply to the capacitor, a higher charging current is applied. If the dielectric is removed from the electric field, the process takes place in the opposite manner, i.e. the additional charging current decreases. This additional charging current triggers off the switching process via the elec-tronics of the proximity switch.

The reference dielectric of a capacitive proximity switch is air.

Note:

Since dirt also acts as dielectric, inadvertent incorrect operations are possible as a result of heavy dirt deposits.

Capacitive proximity switches look similar to inductive ones.

3.8.4 Inductive proximity switches

The function principle of an inductive proximity switch is based on the disturbance of an electromagnetic field. If a metal object is brought into the field, the latter will be changed. This change provoques a switching process in the electronics of the inductive proximity switch.

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Fig. 34: Switching distances with an inductive proximity switch

Many switches of this type signal the switching process via a light emitting diode (LED). This LED helps to determine the switching distance between the inductive proximity switch and the metal object. In practice you will quickly find out that objects made of steel have a larger switching distance than for example objects made of brass, aluminium or copper.

3.8.5 Example of connection of an inductive proximity switch

In general, each proximity switch comes with a connecting diagram. Nevertheless, we would like to illustrate such a schematic diagram here:

Fig. 35: Connecting diagram

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Momentary-contact limit switches often operate to the principle of a changeover switch. According to standards, momentary contact switches have the following symbol:

Fig 36: Mech. momentary contact limit switch

Fig 37: Proximity switch

Depending on the circuit and the intended use, momentary-contact limit switches/proximity switches can be normally closed or normally open. In our exercises we only use normally open contacts.

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3.8.7 Parallel and series connection of proximity switches

Fig. 38: Parallel connection Logic operation OR

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3.9 Pressure switches

Pressure switches are hydraulically operated switches which switch an electric circuit when a preset pressure has been reached.

The switching contact does not get in direct contact with the medium to be monitored such as water, oil, etc. A change in pressure causes a sensor element (diaphragm, gaiter, Bourdon spring (fig. 42), Bourdon tube, piston (fig. 41)) to move thereby actuating a plunger. The switching points of the upper and lower limiting value can be varied within predetermined ranges by adjusting the spring pretensioning rate.

In most cases, pressure switches are designed as changeover switches (fig. 40) capable of being operated either as normally open or normally closed depending an the pin allocation. The circuit diagram of a pressure switch is as follows:

Fig. 40: Circuit diagram - pressure switch

The sensor element determines the type and the designation of the pressure switch (e.g. piston type pressure switch, Bourdon tube pressure switch).

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Fig. 41: Piston type pressure switch

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A pressure switch assumes the function of a guard. When the upper pressure limiting value is exceeded or the pressure falls below the lower pressure limiting value, a main circuit or an auxiliary current path can be opened or closed. As a result of two switching points, limit switches are capable of monitoring e.g. temperatures, speeds and other variables within a limited range.

Pressure switches have a hysteretic, i.e. the signal of the pressure switch remains in a certain pressure range. This is caused by the spring travel in the switch as well as by frictional forces which occur of articulated joints and dynamically loaded sealing points.

Hysteretic is the continuation of an effect whose cause no longer exists. If hysteretic is lowered to less than a certain amount, the switching characteristics become instable. The pressure switch becomes "too sensitive".

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3.10 Mechanical locking by means of momentary contacts

By means of locking circuits, current paths can be prevented from cutting in. This means for example that a locking function avoids simultaneous switching of two or more contactors or the timely overlap of switching processes.

The following example (fig. 44) describes a locking function with the contacts of the switch.

With a simultaneous operation of the switches the energization of the contactors becomes impossible.

Fig. 44 Mechanical locking

The illustrated locking function is purely mechanical. The two contacts are usually integrated into a housing and are switched by means of a lever.

When using AC solenoid valves, switches S1 and S2 must be locked in order to avoid damage to the solenoid coil caused by simultaneous energization. With DC solenoid valves, locking should be provided for safety reasons.

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3.11 Electrical locking by means of contactor/relay contacts

Contactors can be locked electrically by mutual switching of normally closed contacts immediately before contactor coils. An interruption of non branched current paths provides a disconnection priority, i.e. a safe disconnection of the contactors.

Fig. 45 Electrical locking

With cross locking functions with normally closed contacts of the contactors as illustrated here, overlaps are possible. If the associated momentary-contact switches are operated simultaneously, both contactors are energized simultaneously and the contactor armatures pick up. All contacts switch for a short time (overlap).

This overlap can be excluded by providing additional mechanical locking of the switches.

3.12 Time relay

Time relays are switching relays with intended time characteristics (DIN IEC 255).

Time relays are fitted with a contact assembly which switches immediately after energization of the coil as with a relay or a contactor, and a contact assembly, which switches after a settable delay time. Both contact assemblies can consist of normally closed and normally open contacts.

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In the switching element table, the delayed normally open or normally closed contact is identified by the prefix v (v = delayed).

These switching elements are mainly used in auxiliary circuits of relay and contactor controls. By a "relay" we generally understand a switching element which switches one or several output signals with one input signal. With a time relay, the output signal is implemented with a certain time delay. The time delay can be subdivided into three types:

3.12.1 ON-delay (Fig. 46)

After an input signal is received, a delay mechanism is initiated. After the delay time has elapsed, the switching contacts are operated and held in this position until the input signal is reset. Then the contact returns to its rest Position.

Fig. 46

3.12.2 OFF-delay (Fig. 47)

The switching contacts are actuated by the application of an input signal. When the input signal is withdrawn, a delay mechanism is initiated. After the elapse of the delay time, the switching contacts return to their rest position.

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3.12.3 ON- and OFF-delay (Fig. 48)

Here, the two basic types of time delays are combined in one component.

•

Fig. 48 •

Fig. 49: Basic functions of time relays

All components for the generation of time functions have the intermediate storage of an auxiliary variable in common. The type of intermediate storage and the suitability of a time relay type for a certain application depends on the lengths of the time delay or interval to be achieved, on the required and achievable accuracy, on the possibility of a variation in the time delay or interval and the repetition frequency.

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Type of component Intermediate storage and auxiliary variable

Achievable time delay

Pneumatic time relay Air in a pneumatic damper 50 ms to 2 min. Thermal time relay Thermal capacitance of materials 2 s to 5 min. Motor-driven time relay Rotation of mechanical shafts 1 s to 50 h

Time relay with clockwork Clockwork 1 s up to several days Capacitor time relay Capacitance of capacitors 1 ms to 10 s

Magnetic time relay Inductance of reactors 5 ms to 100 ms Electronic time relay Capacitance of capacitors in electronic

circuits

1 ms to 20 min.

Micro-processor time relay Digital divider circuit 1 ms up to several days Fig. 50: Delay mechanisms

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4 Experiment cross-reference

No. Mannesmann Rexroth - Experiments Experiment BIBB (1st issue) Experiment no. 1 Extending a cylinder by operating a push-button 2 2 Signal storage by electrical self-locking

Setting and resetting using a momentary-contact switch

3

3 Mechanical locking by means of momentary-contact switch contacts 8 4 Electrical locking by means of contactor contacts 9 5 Signal storage by means of electrical self-locking Resetting by means of

a momentary-contact switch

4

6 Rapid advance circuit 11, 12

7 Pressure-dependent reversing 7

8 Pressure switches and proximity switches 7, 10

9 Pressure-dependent sequence control 10

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5 Required electrical components

Electro-hydraulics to BIBB

E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 E11 Total

Momentary-contact switch 1 nc/ 1 no 1 1 Momentary-contact switch 1nc 1 1 1 1 2 1 1 2 2 3 3 Momentary-contact switch1no 1 1 1 1 1 1 Maintained-contact switch 1 nc 1 1 1 1 1 1 1 1 1 1 1 Relay 4 x changeover 1 1 1 1 3 1 2 2 4 7 7 Lamp 24 DC 1 1 Momentary-contact

limit switch 1no 1 2 3 2 3

Momentary-contact

Iimit switch1 nc 1 1 1 1

Power supply 24 DC

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6 Required hydraulic components

Electro-hydraulics to BIBB

E1 E2 E3 E4 E5 E6 E7 E8 E9 E10 Total Pressure relief valve

(direct operated) DD1.X 1 1 1 1 1 1 1 2 1 1 2 Pressure switch DD6E 1 1 1 1 Throttle valve (adjustable) DF1.X 1 1 1

Throttle check valve

DF2.X 1 1 1 1 1 1 2 Check valve (pilot operated) DS1 1 1 4/2-way directional valve DW3E 1 1 1 1 1 (1) 2 1 2 (1) 4/3-way directional valve DW4E 1 1 1 (1) 1 1 1 Pressure gauge DZ1.X 1 1 1 1 1 1 2 1 2 Isolator valve DZ2.X 1 1 1 1 1 1 Distributor DZ4.X 2 1 3 2 2 3 3 Cylinder 1 1 1 1 1 1 Pressure hose 1000 mm with mini- mess connection DZ25

1 1 1 1 1 1 2 2 1 2

Loading unit DW12

(AZ + loading unit) 1 1 1 1 1 1 1

Cylinder Z1 (Z2)

+ load unit L1 (L2) * 1 1 1 1 1 1 1 1 1 1 1 *

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6.1 Component plates

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7 Symbols

Graphical symbols to DIN

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Experiment 1: Extension of a cylinder upon the Operation of a push-button

1. Description of the experiment

In this experiment, a double-acting cylinder is to extend and retract. The extending process is controlled by operating a pushbutton. When the pushbutton is released, the cylinder retracts automatically.

With this experiment, the following knowledge should be imparted:

a) How to complete a simple circuit diagram with two current circuits b) Illustration of the difference between a control circuit and a main circuit

c) Indication of types of switching contacts and their classification figure (normally closed, changeover switch, normally open)

d) How to draw a contact element chart

Example:

Upon the Operation of a push-button, the cylinder of a press brake extends. The work piece (sheet steel) is bent around an edge. As the push-button is released, the cylinder retracts automatically.

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2. Task

2.1 Hydraulic circuit

Supplement the circuit illustrated below so that a cylinder extends upon the operation of a push-button. When the push-button is released the directional valve returns to its initial position due to its spring centring and the cylinder retracts. In order to be able to vary the extension velocity, install a throttle and limit the system pressure by means of suitable valve.

Figure 1: Circuit diagram (hydraulic circuit)

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2.2 Electrical circuit

Supplement the electrical circuit diagram so that the solenoid coil Y1 of the 4/2-way directional valve is energized as soon as the push-button S1 is operated. Upon releasing the push-button, the solenoid coil is to be de-energized.

Figure 2: Circuit diagram (electrical circuit)

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3. Preparation of the experiment

The following components should be an hand for this experiment:

3.1 Hydraulic circuit:

1 Pressure relief valve DD1.X 1 Throttle check valve DF2.X 1 4/2-way directional valve with spring return DW3E

1 Pressure gauge with distribution DZ1 or DZ25 with DZ1 .X 1 Cylinder

Pressure hoses

3.2 Electrical circuit:

1 Push-button ON S1 1 Relay K1

1 Ampere meter, voltmeter Wander leads

Before starting the set-up of the experiment, please refer to the section "safety regula-tions", which can be found in chapter 1.

4. Experiment set-up 4.1 Hydraulic circuit

Set up the hydraulic circuit and follow the steps below:

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Now set up the electrical circuit following the steps below:

1. The Power supply is switched off, the system is de-energized.

2. Connect the individual components using the wander leads according to the electrical circuit diagram. Use red wander leads for positive electrical connections and black wander leads for negative connections. This simplifies trouble-shooting in the case of possible mistakes.

5. How to carry out the experiment

1. Check the set up circuits.

2. Check that the flow control valve at the Hydroprax to the training rig is set to 5 LJmin. This corresponds approximately to a setting of 1.24 on the scale of the flow control valve (only with Hydroprax 4).

3. Make sure that the connecting hoses fit properly (verify by pulling).

4. Make sure that all four EMERGENCY OFF push-buttons (only with Hydroprax 4) are connected to the Hydroprax, disengaged and are available at the trainings rigs.

5. Now switch the red main switch of the Hydroprax to I.

6. Switch on the power supply at the control panel by means of the key switch by turning the key clockwise.

7. Switch the pump of the Hydroprax on by operating the yellow push-button.

8. Take care that the isolator valves on the adjacent training rigs to the Hydroprax are closed (Hydroprax 4).

9. Open the isolator valve (only with Hydroprax 4) at the Hydroprax, to which your training rig is connected.

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6. Experiment

a) Set the system pressure to 30 bar at the pressure relief valve DD1.X and turn the throttle check valve to the central position.

b) Measure the following currents on the basis of the individual circuit diagram with acti-vated and released push-button:

Control current (figure 3) Valve current (figure 4) Total current (figure 5)

Enter the measured values into the experiment chart.

c) Measure the voltage with the push-button being activated and released. Complete the values in the chart with the measured values.

d) Close the isolator valve at the Hydroprax to your training rig (only with Hydroprax 4). e) Switch the power supply off.

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Figure 4: Valve current Figure 5: Total current

Note:

The total current is the sum of control current plus valve current. G = S + V (Ampre)

7. Evaluation

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Experiment 2: Signal storage by electrical self-locking

1. Description of the experiment

A cylinder is to be extended by means of a push-button impulse. When the push-button is released the cylinder is to continue to extend by means of signal storage until the end position is reached. The retraction of the cylinder may only be possible when the signal storage is reset by means of a second push-button.

The following knowledge is to be imparted:

a) Set-up of an electrical circuit with electrical signal storage b) How to draw up a circuit diagram for signal storage c) Supplementing and explaining a function chart

2. Task

2.1 Hydraulic circuit

Supplement this circuit so that the cylinder can be extended and retracted by means of the solenoid operated directional valve. The DF1.X fine throttle influences the retracting and extending velocity of the cylinder.

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Supplement the circuit diagram so that the solenoid coil Y1 of the 4/2-way directional valve is energized when push-button S1 is operated. When the push-button is released power should still be supplied to the solenoid coil. The cylinder retracts when the power supply to the solenoid coil is interrupted by the Operation of a second push-button S2.

Note:

Connect a normally open contact relay K1 in parallel to push-button S1. Storage of the signal from push-button S1 by K1.

Withdraw the signal provided by push-button S1 by means of push-button S2.

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1 Pressure relief valve DD1.X

1 4/2-way directional valve with spring return DW3E 1 Throttle DF1.X

1 Pressure gauge with distribution DZ1 or DZ25 with DZ1.X 1 Cylinder

Pressure hoses

3.2 Electric circuit:

1 Push-button ON (normally open) S1

1 Push-button OFF (normally closed) S2 1 Relay K1 Wander Leads

Before starting the set-up of the experiment, please refer to the section "safety regulations", which can be found in chapter 1.

4 Experiment set-up 4.1 Hydraulic circuit:

Set up the hydraulic circuit by following the steps below:

1. The isolator valve at the Hydroprax to your training rig is closed (only with Hydroprax 4).

2. Hang the various components onto the component carriers according to the experiment set-up and secure them.

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Now set up the electrical circuit by following the steps below:

1. The Power supply is switched off, the system is de-energized.

2. Connect the individual components using the wander Leads according to the electrical circuit diagram. Use red wander Leads for positive electrical connections and black wander Leads for negative connections. This simplifies trouble-shooting in the case of possible mistakes.

5 How to carry out the experiment:

2. Make sure that the flow control valve on the Hydroprax to the training rig is set to 51./m in. This corresponds approximately to a setting of 1.24 on the scale of the flow control valve (only with Hydroprax 4).

4. Make sure that all four EMERGENCY OFF push-buttons on the Hydroprax are con-nected and disengaged and are available at the training rigs (only with Hydroprax 4). 5. Now switch the red main switch of the Hydroprax to I.

7. Switch on the pump of the Hydroprax by operating the yellow push-button.

Take care that the isolator valves on the adjacent training rigs are closed (only with Hydroprax 4).

8. Open the isolator valve (only with Hydroprax 4) at the Hydroprax, to which your training rig is connected.

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6 Experiment:

a) Set the system pressure to 30 bar on the pressure relief valve DD1.X. Switch the throttle DF1.X to position 5.

b) Briefly press push-button ON Si. The cylinder extends and remains in its end position. Withdraw the self-locking signal by pressing push-button OFF S2. Relay K1 is now de-energized, the 4/2-way directional valve DW3E shifts and the cylinder retracts.

c) Describe the sequence of this circuit in the function chart.

d) Close the isolator valve on the Hydroprax to your training rig (only with Hydroprax 4). e) Turn the power supply off.

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Experiment 3: Mechanical locking by means of momentary-contact switch contacts

1 Description of the experiment:

In this experiment, the cylinder is to approach any intermediate position in the inching mode. By "inching mode" we understand an impulse circuit without self-locking. The cylinder is to be locked hydraulically in any intermediate position. This can be achieved by means of pilot operated check valve.

The following knowledge is to be imparted:

a) How to set up a hydraulic and electrical circuit with mechanical locking of the momentary-contact switches

b) Mechanical locking

2. Task

Supplement this circuit so that the cylinder retracts or extends when the 4/3-way directional valve DW4E is correspondingly controlled. During the retracting process, the check valve must be unlocked by energizing the solenoid coil of the 3/2-way directional valve DW3E. The pilot operated check valve DS1 (leak-free) positively isolates the cylinder in any intermediate position.

This is not possible with the directional valve DW4E (spool valve) alone since in contrast to poppet valves this valve is not leak-free. The cylinder would sag under external loading. it should be possible to regulate the retracting velocity, to set the system pressure and to monitor it via the pressure gauge.

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Figure 1: Circuit diagram (hydraulic circuit) mechanical locking

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Complete the electrical circuit diagram so that the cylinder retracts and extends in the inching mode by the operation of two momentary contact switches S5 and S9. Switch S9 should be locked. When both switches are operated simultaneously, the retracting direction should priority.

•

Figure 2

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1 4/3-way directional valve DW4E 1 4/2-way directional valve DW3E (A not connected by means of hoses)

1 Pilot operated check valve DS1 1 Pressure relief valve DD1.X 1 Throttle check valve DF2.X

1 Isolator valve DZ2.X

1 Loading unit »Via -(1DW12Yor AZ with loading unit 1 Cross unit DZ4.X

1 Pressure gauge DZ1 with distributor or DZ4.X with DZ1.X and DZ25 Pressure hoses

1 Push-button ON (normally open) S5

1 Push-button OFF (normally closed + normally open) S9 2 Relays K1, K2 Wander leads

Wander leaders

Before starting the set-up of the experiment, please refer to the section "safety regula-tions", which can be found in chapter 1

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4 Experiment set-up 4.1 Hydraulic circuit:

1. The isolator valve of the Hydroprax to your training rig is closed (only with Hydroprax 4).

3. Connect the individual components according to the circuit diagram using the pressure hoses.

Now set up the electrical circuit by following the steps below:

1. The power supply is switched off, the system is de-energized.

2. Connect the individual components using the wander leads according to the electric circuit diagram. Use red wander leads for positive electrical connections and black wander leads for negative connections. This simplifies trouble-shooting in the case of possible mistakes.

5. How to carry out the experiment:

2. Make sure that the flow control valve on the Hydroprax to the training rig is set to 5 Umin. This corresponds approximately to a setting of 1.24 on the scale of the flow control valve (only with Hydroprax 4).

4. Make sure that all four EMERGENCY OFF push-buttons on the Hydroprax are connected and disengaged and are available at the training rigs (only with Hydroprax 4).

5. Now switch the red main switch of the Hydroprax to 1.

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6. Experiment:

a) Limit the system pressure to 40 bar; for this, the isolator valve DZ2.X must be closed. After having set the pressure, open it.

b) Open the throttle check valve DF2.X.

c) Extend the cylinder. When retracting, stop at an intermediate position. d) Draw up a function chart.

e) Retract the cylinder completely and close the isolator valve on the Hydroprax to your training rig (only with Hydroprax 4).

f) Turn the power supply off.

7. Evaluation:

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Experiment 4: Electrical locking by means of contactor contacts

1. Description of the experiment:

In this experiment, the cylinder is to extend and retract upon the Operation of a push-button. The movement of the cylinder is to be controlled by means of three push-buttons "FORWARDS", "BACKWARDS" and "STOP". it is required that the cylinder can be stopped at any position during retracting or extending.

The following knowledge is to be imparted: a) Set-up of the hydraulic and electrical circuit b) Electrical locking

c) Combination of mechanical and electrical locking

2. Task

Work out a circuit with which the cylinder retracts and extends when a directional valve is accordingly controlled. It should be possible to regulate the extending velocity, to set the system pressure and to minitor it via the pressure gauge.

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2.2 Electrical circuit:

Supplement the electrical circuit diagram and the connection diagram so that the cylinder retracts and extends when the push-buttons S5 and S6, respectively, are operated. The individual signals are to be stored by means of self-locking. The locking contacts (normally closed) of the relays have to lock the relay that is not used for the respective motion sequence. By pressing push-button OFF S9, the entire power supply to the contactors is to be interrupted and the seif-locking function of the individual push-button must be withdrawn as long as this push-button is operated. After this, the cylinder can again be traversed when the corresponding push-button for the relevant direction of movement is operated. By pressing push-button OFF S9, the entire power supply to the contactors is to be interrupted and the self-locking function of the individual push-button must be withdrawn as long as this push-button is operated. After this, the cylinder can again be traversed when the corresponding push-button for the relevant direction of movement is operated.

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3. Preparation of the experiment:

1 4/3-way directional valve DW4E 1 Pressure relief valve DD1.X 1 Throttle check valve DF2.X 1 Isolator valve DZ2.X 1 Cylinder

1 Pressure gauge DZ1 with distributor or DZ4.X with DZ1.X and DZ25 Pressure hoses

2 Push-buttons ON (normally open) S5, S6 1 Push-buttons OFF (normally closed) S9 2 Relay K1

Wander leads

Before starting the set-up of the experiment, please refer to the section "safety regula-tions", which can be found in chapter 1.

4. Experiment set-up 4.1 Hydraulic circuit:

1. The isolator valve at the Hydroprax to your training rig is closed (only with Hydroprax 4).

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Now set up the electrical circuit by following the steps below: 1. The power supply is switched off, the system is de-energized.

2. Connect the individual components using the wander leads according to the electrical circuit diagram. Use red wander leads for positive electrical connections and black wander leads for negative connections. This simplifies trouble-shooting in the case of possible mistakes.

5. How to carry out the experiment:

2. Make sure that the flow control valve on the Hydroprax to the training rig is set to 5 Umin. This corresponds approximately to a setting of 1.24 on the scale of the flow control valve (only with Hydroprax 4).

4. Make sure that all four EMERGENCY OFF push-buttons on the Hydroprax are connected and disengaged and are available at the training rigs (only with Hydroprax 4).

5. Now switch the red circuit breaker of the Hydroprax to I.

Take care that the isolator valves on the adjacent training rigs are closed (only with Hydroprax 4).

8. Open the isolator valve (only with Hydro- prax 4) at the Hydroprax, to which your training rig is connected.

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6. Experiment:

a) Limit the system pressure to 40 bar by means of the pressure relief valve DD1.X. For this, the isolator valve DZ2.X must be closed. After having set the pressure, open the valve.

b) Bring the throttle check valve to the central position. c) Now extend the cylinder once.

d) Retract the cylinder and stop the cylinder movement by operating the push-button OFF S9. Take the electrical circuit diagram and try to find out how this is possible.

e) Work out a function diagram on the basis of the cylinder movement and the push-button positions for the extension movement of the cylinder with intermediate stop and for the retraction movement.

f) Close the isolator valve on the Hydroprax to your training rig (only with Hydroprax 4) g) Turn the power supply off.

7. Evaluation of the experiment:

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Experiment 5: Signal storage by means of electrical self-locking Resetting by means of a proximity switch

1. Description:

This experiment is to demonstrate the function of an inductive proximity switch in practice. The cylinder is to extend upon the Operation of a push-button and automatically return with the help of a proximity switch.

In addition, it should be possible during the retraction movement to re-extend the cylinder at any position by operating a push-button.

With this experiment, the following knowledge is to be imparted:

1. Set-up of an electrical and hydraulic control, in which electrical seif-locking is reset by means of a proximity switch.

2. Function and types of proximity switches (see chapter 3.8).

2. Task

Work out a circuit, with which the cylinder retracts and extends when a directional valve is accordingly controlled. it should be possible to regulate the extending velocity, to set the system pressure and monitor. it via the pressure gauge.

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Complete the electric circuit diagram so that the directional valve (Y1) shifts when push-button S5 is operated. This switching impulse is to be stored by electrical self-locking. The cylinder retracts automatically, when proximity switch B1 switches and thus resets the. self-locking function.

Figure 2: Circuit diagram (electrical circuit) proximity switch