Electronic Instrumentation
Chapter 4
•
Introduction
•
Position, Displacement and Level
•
Position, Displacement and Level
•
Strain and force
•
In this chapter we will focus on the measurement of
magnitudes of interest in the industrial/aeronautical
environment.
•
Classical electronic sensors will be addressed although some
optoelectronic and advanced sensors will be described.
optoelectronic and advanced sensors will be described.
•
IMPORTANT: It is
impossible
to cover ALL the sensors and
instrumentation systems currently being used. This is an open,
high-speed evolving field and the enumeration that follows
isn’t by any means complete.
(linear or angular) with respect to a selected reference. Displacement
involves movement from one position to another (the original position of the object acts as the reference). Level is used when a liquid is
involved.
• Position/displacement/level sensors are often part of more complex
• Position/displacement/level sensors are often part of more complex instrumentation systems as other physical magnitudes can be derived from this measurement (pressure, velocity,….)
• Types of sensors (some of them):
• Potentiometric Sensors
•
Based on the linear dependence between total impedance
and conductor length.
•
Several technologies: wire bound, conductive plastic, mixed,…
•
Both for linear and angular displacement measurements.
•
Both for linear and angular displacement measurements.
R0
L0 l
•
Angular and rotary Sensor:
Serial 891000
•
Angular and rotary Sensor:
Serial 891000
• Based on the dependence on Area, distance and dielectric constant of a parallel-plate capacitor.
d
A
ε
• Different sensors architectures can be envisaged based on changes in the distance between plates, active area and dielectric constant between plates.
•
Proximity Sensor:
E2K-C
•
Proximity Sensor:
E2K-C
• Based on the use of magnetic fields and the related currents and
voltages induced, with the many advantages associated to the fact that magnetic field can penetrate non-magnetic materials with almost no losses.
losses.
• Several types of sensors:
• LVDT (Linear Variable Differential Transformer)
• Linear position Inductive Sensors
• Transverse Inductive Sensors
• Hall Effect sensors
• Based on the use of one static transformers and two secondary
windings connected in series opposition so the two induced voltages are opposite in sign.
• Based on the change of the inductance of a coil with a mobile core attached to the target.
• Based on the change of the inductance of a coil in the presence of a ferromagnetic material (target), that crosses the magnetic field lines.
• Based on the creation of a transverse hall potential difference in a conductor where a dc current is applied in the presence of a magnetic field.
B
r
B
I
r
•
LVDT:
Solartron Technologies
Linear position Inductive Sensor:
HBM
•
Linear position Inductive Sensor:
HBM
•
Transverse Inductive Sensor:
Pepper+Fulch
• Based on the use of ultrasonic energy sent towards a target and reflected back. Range is obtained measuring the time difference between the sent pulse and the received signal. Such ultrasonic waves are mechanical acoustic waves covering the frequency range beyond the capabilities of human audition (usually 40kHz- 80kHz) .
• Optical Sensors are gaining presence due to their advantages as their simplicity, the absence of the loading effect and long operating
distances.
• Usually require at least three essential components: a light source, a
• Usually require at least three essential components: a light source, a photodetector and light guidance devices.
• Several types of sensors:
• Optical Range finders
• Grating Sensors
• PSD’s
•
As the ultrasonic systems, range is obtained measuring the
time difference between the sent pulse and the received
signal. Pulsed and amplitude modulation systems are used.
Transmitter Laser diode
• An optical displacement transducer can be fabricated with two overlapping gratings which serve as light-intensity modulator. They can be used as
proximity sensors or position encoders.
Photo-detector
Light emitter
• A PSD is a differential current device that gives an output related to the
position of a collimated beam on the device surface. It provides one and two-dimensional position. Target Laser/LED Diode D a PSD
• Force is one of the fundamental quantities to be measured in mechanical, aeronautical and civil engineering. Also, whenever pressure is measured, it requires the measurement of force.
• There are several methods to measure force. Amongst the most important are:
important are:
• Force-to-displacement conversion (using a spring , for example)
• Force-to-strain conversion (measuring the deformation of an elastic element).
• In this sense, strain (unit deformation) measurements are a very
common and useful in instrumentation not only for the importance of strain measurements themselves (e.g. structure deformation) but also because a lot of secondary magnitudes can be converted to
• Force and pressure can be measured using a system like the one shown in the figure where a spring converts the applied force to a displacement (x).
• The impedance of a metal wire changes when the material is
mechanically deformed. This is called the piezoresistive effect and is the origin for a widely used sensors known as strain gauges.
• In these sensors, the unitary change in resistance is proportional to the
• In these sensors, the unitary change in resistance is proportional to the elongation (strain) through a parameter known as gauge factor (K)
• K typically ranges between 2 and 6 for Constatan (a copper/nickel alloy) gauges. For semiconductor strain gauges, K is bigger but their behavior with temperature is much worst.
• The Strain Gauges are designed to be affected only by elongations in
• The Strain Gauges are designed to be affected only by elongations in the desired directions. In this sense there are many types of gauges.
R1 R4
Force
• They are almost always mounted on a Wheatstone Bridge, taking advantage of the null signal conditioning circuit to cancel temperature variations.
R2 R3
• Load Cells consist of a mechanical arrangement that suffers a deformation when a force is applied. The deformation is sensed using strain gauges usually in half or full bridge configuration.
• They are the most common transducer for weight measurements as well as vibrations and dynamic tests on structures
•
vibrations and dynamic tests on structures
• There are several types of load cells (double ended shear beam, single ended shear beam, single column, multi -column ) and for a wide range of
• The Piezoelectric effect is the generation of electric charge by a crystalline material upon subjecting it to stress. The effect exist in natural crystals as quartz, but currently man-made ceramics (PZT) and polymers are used.
• Under any stress a charge proportional to the Force applied is generated that results in a voltage across the crystal due to its capacitance:
•
results in a voltage across the crystal due to its capacitance:
• These sensors are widely used for “Tactile sensors” of great applications in Robotics and other industrial and Aeronautical applications.
• Although one may think that velocity and acceleration can be
extracted from a position measurement, taking derivatives, specially in noisy environments, may result in high errors. For this reason, they are not derived by position measurements but special sensors.
• Acceleration (Accelerometers) are often related to vibration
measurements and the use of a seismic mass. In this sense they are very important in shock analysis, structures characterization and other applications.
• Capacitive Accelerometers: The movement of the seismic mass is measured with a capacitive displacement transducer to detect the mass displacement with respect to the accelerometer housing (LIS2L02AL).
• Piezoresistive Accelerometers: In this case strain gauges are responsible for measuring the strain associated to the mass displacement. They can be used measuring the strain associated to the mass displacement. They can be used in a broad frequency range.
• Piezoelectric Accelerometers: In this case the piezo-electric effect is
responsible for the direct conversion of mechanical energy (strain) to voltage. They present good off-axis noise rejection, high linearity and wide operating temperature range.
• Thermal Accelerometers: The seismic mass is heated and the position calculated through the temperature distribution in the accelerometer housing. This principle can be integrated in an IC using gas as the seismic mass (MXD6125Q)
• They rely on the principle of conservation of angular momentum.
• Rotor Gyroscope: The classic system using a massive disk free to rotate about a spin axis. If the gyro platforms rotate around the input axis, the gyro develops a torque around a perpendicular axis that can be the gyro develops a torque around a perpendicular axis that can be measured.
• Monolithic Silicon Gyroscopes: They are based on MEMS
(Micro-electromechanical systems) technology and the concept of vibrating gyro based on the Coriolis acceleration.
industrial environments.
• Taking a temperature essentially requires the transmission of a small portion of the object’s thermal energy to the sensor. In this sense, the influence of the measurement in the object’s temperature is
sometimes of relevance. sometimes of relevance.
• Many physical and chemical phenomena are found to be functions of temperature, and thus many physical principles can be used to this measurement.
• Types of sensors (some of them):
• Thermoresistive sensors (RTD, Thermistors)
• Thermocouples
• Semiconductor PN Junction Sensors and IC’s
• Optical Sensors (Pyrometers)
• Based on the temperature dependence of resistivity of all metals and alloys.
• Although virtually all metals can be employed, platinum is used almost exclusively: predictable response, long-term stability and durability.
• All RTD’s have positive temperature coefficients.
• High accuracy and expensive sensors, they are standard used for most of the • High accuracy and expensive sensors, they are standard used for most of the
• According to the International Temperature Scale (ITS-90), precision temperature
instruments should be calibrated at reproducible equilibrium states of some materials. From the value of the resistance at those points, the coefficients for the interpolation polynomial are calculated:
• Attention should be paid to the self-heating of the sensor!! • Example: HEL-775
• They are also thermoresistive sensors but fabricated with metal-oxide materials that behave like semiconductors.
• They usually present a negative temperature coefficient (NTC), although PTC (Positive temperature coefficient) thermistors are also available.
• Low-accuracy and low stability sensors, but low cost, they present an exponential-like • Low-accuracy and low stability sensors, but low cost, they present an exponential-like
• Same way as RTD, the parameter for the curve-fitting (in this case an exponential) is done using reproducible equilibrium states of some materials .
• Attention should also be paid to the self-heating of the sensor!!
different thermoelectric properties of such metals produce a EMF (electromagnetic force) when two junctions are kept at different temperatures (Seebeck effect).
Thermoelectric voltage Thermocouple measurement
Thermocouple Thermocouple measurement
(µV/ºC) (ANSI) Pt (6%) / Rh – Pt (30%) / Rh 38 – 1800 7.7 B W (5%) / Re – W (26%) / Re 0 – 2300 16 C Chromel – Constantan 0 – 982 76 E Chromel – Constantan 0 – 982 76 E Iron - Constantan 0 – 760 55 E Chromel – Alumel -184 – 1260 39 K Pt (13%) / Rh – Pt 0 – 1593 11.7 R Pt (10%) / Rh – Pt 0 – 1538 10.4 S Cu – Constantan -184 – 400 45 T
Materials: Platinum (Pt), Rhodium (Rh), Rhenium (Re), Tungsten (W), Chromel (Ni-Cr), Alumel, Constantan
• Based on the dependence of the semiconductor band-gap voltage with temperature. • Easy integration sensors, they can be embedded in any electronic circuits.
• Schemes like the one described are usually integrated in monolithic Integrated Circuits (IC) for low-cost temperature sensors.
• They are also used for cold-junction compensation in thermocouple-based temperature sensors
• These instruments provide a no-touch means of estimating surface temperatures
• Based on blackbody radiation Laws:
• As the temperature of the Blackbody increases, the peak of maximum spectral emittance shifts systematically to shorter wavelengths.
• As the temperature increases, the area under the spectral emittance curves increases • As the temperature increases, the area under the spectral emittance curves increases
Wien’s displacement Law
•
In this chapter we have described some of the most
important sensors for magnitudes of relevance in the
industrial/aeronautical fields.
•
This brief revision has allow us to realize the magnitude
of this open, high-speed evolving field.
•
of this open, high-speed evolving field.
•
The selection of the proper sensor depends heavily on
the application and other design constrains (budget,
environmental conditions). Time should be spent on
choosing the best transducer for our applications. That
will probably save us a lot of time and money.
