Sensors AND Transducers

Sensors can be broadly classified in two categories: discrete event and continuous. Discrete event, or on/off sensor, changes its state based on the occurrence of some external event. These sensors typically only give knowledge of two states based on the condition being sensed. They are based on mechanical, electrical or optical technology. Continuous sensors provide information over the continuous range of operation of the process and are commonly used in continuous control applications, where the process is being regulated based on continuously sensed attribute data. They are based on electrical, optical and acoustical technologies.

ACTIVE AND PASSIVE SENSORS

The sensors can be classified as active and passive. A passive sensor has no power supply and all the energy it delivers to the next stage (the signal conditioning) is drawn from the measurand. Passive sensors are also known as self-generating sensors. An active sensor is a modulator and can therefore deliver more energy to the next stage than it draws from the measurand. If the power supply is dc, the output is modulated by the measurand, and has the same frequency. If the supply is ac, the output is the carrier frequency with sidebands at signal frequency.

BASIC REQUIREMENTS OF A SENSOR/TRANSDUCER

A transducer is normally designed to sense a specific measurand or to respond only to that
particular measurand. A complete knowledge of the electrical and mechanical
characteristics of the transducer is of great importance while choosing a transducer for a
particular application. Often, it is deemed essential to get details of these characteristics
during the selection of instrumentation for the experiment concerned. The basic
requirements are : 

Ruggedness

Ability to withstand overloads, with safety stops for overload protection.

 Linearity

Ability to reproduce input-output characteristics symmetrically and linearly.
Overall linearity is the main factor considered.

 Repeatability

Ability to reproduce the output signal exactly when the same measurand is
applied repeatedly under same environmental conditions.

 Convenient Instrumentation

Sufficiently high analog output signal with high signal to noise ratio; digital

TRANSDUCERS

A useful way to classify transducers is on the basis of the physical property the device is intended to measure. The important properties discussed in this section are :

 Position

 Velocity

 Force or Pressure

 Temperature

 Position Transducers

Position transducers are widely used in servomotors, linear position tables, and other applications where prices position is important. In this section we will discuss four analog position transducers (potentiometers, linear variable differential transformers, floats and resolvers) and two digital position transducers (the optical encoder and ultrasonic range sensor).

Velocity Transducers

Velocity transducers are used for speed control. We shall describe the digital (optical
encoder) and analog (DC tachometer) velocity transducers.

Force or Pressure Transducers
Force sensors are used extensively in automatic weighing operation in the process
industries and in robotic applications when it is necessary to control gripping pressure. In
this section we shall examine two analog transducers: the load cell and the strain gage.

Load Cells

A load cell is used in processes where precise weighing is required. It can be
implemented using a strain gage or a LVDT. Illustrates a load cell
implemented by using a LVDT and a spring with linear force displacement relation.