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CHAPTER 6 Printer: Yet to Come October 9, 2014 8:1 254mm×178mm

SENSORS

6.1 INTRODUCTION TO MEASUREMENT DEVICES

Measurements of variables are needed for monitoring and control purposes. Typical vari-
ables that need to be measured in a data acquisition and control system are:
1. position, velocity, acceleration,
2. force, torque, strain, pressure,
3. temperature,
4. flow rate,
5. humidity.
Figure 6.1 shows the basic concept of a measurement device. The measurement
device is called the sensor. We will discuss different types of sensors to measure the above
listed variables. Figure 6.2 shows an example of sensors used on a construction equipment.
A sensor is placed in the environment where a variable is to be measured. The sensor is
exposed to the effect of the measured variable. There are three basic phenomenon in effect
in any sensor operation:
1. The change in the measured physical variable (i.e., pressure, temperature, displace-
ment) is translated into a change in the property (resistance, capacitance, magnetic
coupling) of the sensor. This is called the transduction. The change of the measured
variable is converted to an equivalent property change in the sensor. The transduction
relationship, that is the relationship between the measured variable and the change in
the sensor material property, is the fundamental physical principle of the sensor oper-
ation. It is desirable that this relationship is repeatable and does not vary with other
environmental variables. For instance, a pressure sensor output voltage as a function
of pressure should not change much due to changes in the ambient temperature.
2. The change in the property of the sensor is translated into a low power level electrical
signal in the form of voltage or current.
3. This low power sensor signal is amplified, conditioned (filtered), and transmitted to
an intelligent device for processing, for example to a display for monitoring purposes
or use in a closed loop control algorithm.

Sensor types vary in the transduction stage in measuring a physical variable. In response
to the physical variable, a sensor may be designed to change its resistances, capacitance,
inductance, induced current, or induced voltage.
In any measurement system, accuracy is a major specification. Let us clarify
the terminology used regarding accuracy. Figure 6.3 shows the meaning of accuracy,

Mechatronics with Experiments, Second Edition. Sabri Cetinkunt.
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.
Companion Website: www.wiley.com/go/cetinkunt/mechatronics

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