Introduction to Sensors and Actuators Micro- and NanosensorsSensors and actuators are two critical components of every closed loop control system. Such a system is
also called a
mechatronics system
. A typical mechatronics system as shown in Figure 9.1 consists of a
sensing unit, a controller, and an actuating unit. A sensing unit can be as simple as a single sensor or
can consist of additional components such as filters, amplifiers, modulators, and other signal conditioners.
The controller accepts the information from the sensing unit, makes decisions based on the control
algorithm, and outputs commands to the actuating unit. The actuating unit consists of an actuator and
optionally a power supply and a coupling mechanism.
9.1 Sensors
Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.)
produces a proportional output signal (electrical, mechanical, magnetic, etc.). The term transducer is
often used synonymously with sensors. However, ideally, a sensor is a device that responds to a change
in the physical phenomenon. On the other hand, a transducer is a device that converts one form of
energy into another form of energy. Sensors are transducers when they sense one form of energy input
and output in a different form of energy. For example, a thermocouple responds to a temperature change
(thermal energy) and outputs a proportional change in electromotive force (electrical energy). Therefore,
a thermocouple can be called a sensor and or transducer.
Linear and Rotational Sensors
Linear and rotational position sensors are two of the most fundamental of all measurements used in a
typical mechatronics system. The most common type position sensors are listed in Table 9.1. In general,
the position sensors produce an electrical output that is proportional to the displacement they experience.
There are contact type sensors such as strain gage, LVDT, RVDT, tachometer, etc. The noncontact type
includes encoders, hall effect, capacitance, inductance, and interferometer type. They can also be classified
based on the range of measurement. Usually the high-resolution type of sensors such as
hall effect,
fiber
optic inductance,
capacitance
, and
strain gage
are suitable for only very small range (typically from 0.1 mm
to 5 mm). The
differential transformers
on the other hand, have a much larger range with good resolution.
Interferometer
type sensors provide both very high resolution (in terms of microns) and large range of
measurements (typically up to a meter). However, interferometer type sensors are bulky, expensive, and
requires large set up time.
Among many linear displacement sensors, strain gage provides high
resolution at low noise level and is least expensive. A typical resistance
strain gage consists of resistive foil arranged the
seismic mass
type and the
piezoelectric
accelerometer. The seismic mass type accelerometer is based
on the relative motion between a mass and the supporting structure. The natural frequency of the seismic
mass limits its use to low to medium frequency applications. The piezoelectric accelerometer, however,
is compact and more suitable for high frequency applications.
Force, Torque, and Pressure Sensors
Among many type of force/torque sensors, the
strain gage
dynamometers
and
piezoelectric type
are most
common. Both are available to measure force and/or torque either in one axis or multiple axes. The dynamometers
make use of mechanical members that experiences elastic deflection when loaded. These types
of sensors are limited by their natural frequency. On the other hand, the piezoelectric sensors are
particularly suitable for dynamic loadings in a wide range of frequencies. They provide high stiffness,
high resolution over a wide measurement range, and are compact.
Flow Sensors
Flow sensing is relatively a difficult task. The fluid medium can be liquid, gas, or a mixture of the two.
Furthermore, the flow could be laminar or turbulent and can be a time-varying phenomenon. The
venturi
meter
and
orifice plate
restrict the flow and use the pressure difference to determine the flow rate. The
pitot
tube
pressure probe is another popular method of measuring flow rate. When positioned against the flow,
they measure the total and static pressures. The flow velocity and in turn the flow rate can then be determined.
The
rotameter
and the
turbine meters
when placed in the flow path, rotate at a speed proportional to the flow
rate. The
electromagnetic flow meters
use noncontact method. Magnetic field is applied in the transverse
direction of the flow and the fluid acts as the conductor to induce voltage proportional to the flow rate.
Ultrasonic flow meters
measure fluid velocity by passing high-frequency sound waves through fluid. A
schematic diagram of the ultrasonic flow meter is as shown in Figure 9.4. The transmitters (T) provide
the sound signal source. As the wave travels towards the receivers (R), its velocity is influenced by the
velocity of the fluid flow due to the doppler effect. The control circuit compares the time to interpret
the flow rate. This can be used for very high flow rates and can also be used for both upstream and
downstream flow. The other advantage is that it can be used for corrosive fluids, fluids with abrasive
particles, as it is like a noncontact sensor.
Temperature Sensors
A variety of devices are available to measure temperature, the most common of which are thermocouples,
thermisters, resistance temperature detectors (RTD), and infrared types.
Thermocouples
are the most versatile, inexpensive, and have a wide range (up to 1200
°
C typical). A
thermocouple simply consists of two dissimilar metal wires joined at the ends to create the sensing
junction. When used in conjunction with a reference junction, the temperature difference between the
reference junction and the actual temperature shows up as a voltage potential.
Thermisters
are semiconductor
devices whose resistance changes as the temperature changes. They are good for very high
sensitivity measurements in a limited range of up to 100
°
C. The relationship between the temperature
and the resistance is nonlinear. The
RTD
s use the phenomenon that the resistance of a metal changes
with temperature. They are, however, linear over a wide range and most stable.
Infrared type
sensors use the radiation heat to sense the temperature from a distance. These noncontact
sensors can also be used to sense a field of vision to generate a thermal map of a surface.
Proximity Sensors
They are used to sense the proximity of an object relative to another object. They usually provide a on
or off signal indicating the presence or absence of an object.
Inductance,
capacitance,
photoelectric
, and
hall effect
types are widely used as proximity sensors. Inductance proximity sensors consist of a coil wound
around a soft iron core. The inductance of the sensor changes when a ferrous object is in its proximity.
This change is converted to a voltage-triggered switch. Capacitance types are similar to inductance except
the proximity of an object changes the gap and affects the capacitance. Photoelectric sensors are normally
aligned with an infrared light source. The proximity of a moving object interrupts the light beam causing
the voltage level to change. Hall effect voltage is produced when a current-carrying conductor is exposed
to a transverse magnetic field. The voltage is proportional to transverse distance between the hall effect
sensor and an object in its proximity.
Light Sensors
Light intensity and full field vision are two important measurements used in many control applications.
Phototransistors,
photoresistors
, and
photodiodes
are some of the more common type of light intensity
sensors. A common photoresistor is made of cadmium sulphide whose resistance is maximum when the
sensor is in dark. When the photoresistor is exposed to light, its resistance drops in proportion to the
intensity of light. When interfaced with a circuit as shown in Figure 9.5 and balanced, the change in light
intensity will show up as change in voltage. These sensors are simple, reliable, and cheap, used widely
for measuring light intensity.
Smart Material Sensors
There are many new smart materials that are gaining more applications as sensors, especially in distributed
sensing circumstances. Of these,
optic fibers,
piezoelectric,
and
magnetostrictive
materials have found applications.
Within these, optic fibers are most used.
Optic fibers can be used to sense strain, liquid level, force, and temperature with very high resolution.
Since they are economical for use as
in situ
distributed sensors on large areas, they have found numerous
applications in smart structure applications such as damage sensors, vibration sensors, and cure-monitoring
sensors. These sensors use the inherent material (glass and silica) property of optical fiber to sense the
environment. Figure 9.6 illustrates the basic principle of operation of an embedded optic fiber used
to sense displacement, force, or temperature. The relative change in the transmitted intensity or spectrum
is proportional to the change in the sensed parameter.
Micro- and Nanosensors
Microsensors (sometimes also called MEMS) are the miniaturized version of the conventional macrosensors
with improved performance and reduced cost. Silicon micromachining technology has helped the
development of many microsensors and continues to be one of the most active research and development
topics in this area.
Vision microsensors have found applications in medical technology. A
fiberscope
of approximately 0.2 mm
in diameter has been developed to inspect flaws inside tubes. Another example is a
microtactile
sensor
,
which uses laser light to detect the contact between a catheter and the inner wall of blood vessels during
insertion that has sensitivity in the range of 1 mN. Similarly, the progress made in the area of nanotechnology
has fuelled the development of nanosensors. These are relatively new sensors that take one step
further in the direction of miniaturization and are expected to open new avenues for sensing applications.
Selection Criteria
A number of static and dynamic factors must be considered in selecting a suitable sensor to measure the
desired physical parameter. Following is a list of typical factors:
Range
—Difference between the maximum and minimum value of the sensed parameter
Resolution
—The smallest change the sensor can differentiate
Accuracy
—Difference between the measured value and the true value
Precision
—Ability to reproduce repeatedly with a given accuracy
Sensitivity
—Ratio of change in output to a unit change of the input
Zero offset
—A nonzero value output for no input
Linearity
—Percentage of deviation from the best-fit linear calibration curve
Zero Drift
—The departure of output from zero value over a period of time for no input
Response time
—The time lag between the input and output
Bandwidth
—Frequency at which the output magnitude drops by 3 dB
Resonance
—The frequency at which the output magnitude peak occurs
Operating temperature
—The range in which the sensor performs as specified
Deadband—The range of input for which there is no output
Signal-to-noise ratio—Ratio between the magnitudes of the signal and the noise at the output
Choosing a sensor that satisfies all the above to the desired specification is difficult, at best. For example,
finding a position sensor with micrometer resolution over a range of a meter eliminates most of the sensors.
Many times the lack of a cost-effective sensor necessitates redesigning the mechatronic system. It is, therefore,
advisable to take a system level approach when selecting a sensor and avoid choosing it in isolation.
Once the above-referred functional factors are satisfied, a short list of sensors can be generated. The
final selection will then depend upon the size, extent of signal conditioning, reliability, robustness,
maintainability, and cost.
Signal Conditioning
Normally, the output from a sensor requires post processing of the signals before they can be fed to the
controller. The sensor output may have to be demodulated, amplified, filtered, linearized, range quantized,
and isolated so that the signal can be accepted by a typical analog-to-digital converter of the controller.
Some sensors are available with integrated signal conditioners, such as the microsensors. All the electronics
are integrated into one microcircuit and can be directly interfaced with the controllers.
Calibration
The sensor manufacturer usually provides the calibration curves. If the sensors are stable with no drift,
there is no need to recalibrate. However, often the sensor may have to be recalibrated after integrating
it with a signal conditioning system. This essentially requires that a known input signal is provided to
the sensor and its output recorded to establish a correct output scale. This process proves the ability to
measure reliably and enhances the confidence.
If the sensor is used to measure a time-varying input, dynamic calibration becomes necessary. Use of
sinusoidal inputs is the most simple and reliable way of dynamic calibration. However, if generating
sinusoidal input becomes impractical (for example, temperature signals) then a step input can substitute
for the sinusoidal signal. The transient behavior of step response should yield sufficient information
about the dynamic response of the sensor.
9.2 Actuators
Actuators are basically the muscle behind a mechatronics system that accepts a control command (mostly
in the form of an electrical signal) and produces a change in the physical system by generating force,
motion, heat, flow, etc. Normally, the actuators are used in conjunction with the power supply and a
coupling mechanism as shown in Figure 9.7. The power unit provides either AC or DC power at the
rated voltage and current. The coupling mechanism acts as the interface between the actuator and the
physical system. Typical mechanisms include rack and pinion, gear drive, belt drive, lead screw and nut,
piston, and linkages.
Classification
Actuators can be classified based on the type of energy as listed in Table 9.2. The table, although not
exhaustive, lists all the basic types. They are essentially of electrical, electromechanical, electromagnetic,
hydraulic, or pneumatic type. The new generations of actuators include smart material actuators, microactuators,
and Nanoactuators.
Actuators can also be classified as binary and continuous based on the number of stable-state outputs.
A relay with two stable states is a good example of a binary actuator. Similarly, a stepper motor is a good
example of continuous actuator. When used for a position control, the stepper motor can provide stable
outputs with very small incremental motion.
Principle of Operation
Electrical Actuators
Electrical switches are the choice of actuators for most of the on-off type control action. Switching devices
such as diodes, transistors, triacs, MOSFET, and relays accept a low energy level command signal from
the controller and switch on or off electrical devices such as motors, valves, and heating elements. For
example, a MOSFET switch is shown in Figure 9.8. The gate terminal receives the low energy control
signal from the controller that makes or breaks the connection between the power supply and the actuator
load. When switches are used, the designer must make sure that switch bounce problem is eliminated
either by hardware or software.
Electromechanical Actuators
The most common electromechanical actuator is a motor that converts electrical energy to mechanical
motion. Motors are the principal means of converting electrical energy into mechanical energy in industry.
Broadly they can be classified as DC motors, AC motors, and stepper motors. DC motors operate on DC
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