Mechatronics & Robotics : Sensors and actuators

By Apoorbo Roy|Updated : July 16th, 2021

Sensors and transducers

The term sensor is used for an element which produces a signal relating to the quantity being measured. Thus in the case of, say, an electrical resistance temperature element, the quantity being measured is temperature and the sensor transforms an input of temperature into a change in resistance. The term transducer is often used in place of the term sensor. Transducers are defined as elements that when subject to some physical change experience a related change. Thus sensors are transducers. However, a measurement system may use transducers, in addition to the sensor, in other parts of the system to convert signals in one form to another form.

                                                                  

 

Sensors and transducers

The term sensor is used for an element which produces a signal relating to the quantity being measured. Thus in the case of, say, an electrical resistance temperature element, the quantity being measured is temperature and the sensor transforms an input of temperature into a change in resistance. The term transducer is often used in place of the term sensor. Transducers are defined as elements that when subject to some physical change experience a related change. Thus sensors are transducers. However, a measurement system may use transducers, in addition to the sensor, in other parts of the system to convert signals in one form to another form.

Performance terminology

The following terms are used to define the performance of transducers, and often measurement systems as a whole.

Range and span: The range of a transducer defines the limits between which the input can vary. The span is the maximum value of the input minus the minimum value. Thus, for example, a load cell for the measurement of forces might have a range of 0 to 50 kN and a span of 50 kN.

Error: Error is the difference between the result of the measurement and the true value, of the quantity being measured.

Error = measured value – true value

Thus if a measurement system gives a temperature reading of 25°C when the actual temperature is 24 C, then the error is +1°C. If the actual temperature had been 26°C then the error would have been –1°C. A sensor might give a resistance change of 10.2 Ω when the true change should have been  10.5 Ω. The error is – 0.3 Ω.

Accuracy: Accuracy is the extent to which the value indicated by a measurement system might be wrong. It is thus the summation of all the possible errors that are likely to occur, as well as the accuracy to which the transducer has been calibrated.

Sensitivity: The sensitivity is the relationship indicating how much output you get per unit input, i.e. output / input.

Hysteresis error: Transducers can give different outputs from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change.

Non-linearity error: For many transducers a linear relationship between the input and output is assumed over the working range, i.e. a graph of output plotted against input is assumed to give a straight line.

Static and dynamic characteristics

The static characteristics are the values given when steady-state Conditions occur, i.e. the values given when the transducer has settled down after having received some, input. The terminology defined above refers to such a state. The dynamic characteristics refer to the behaviour between the time hat the input value changes and the time that the value given by the transducer settles down to the steady-state value. Dynamic characteristics are stated in terms of the response of the transducer to inputs in particular forms. For example, this might be a step input when the input is suddenly changed from, 0 to a constant value, or a ramp input when the input is changed at a steady rate, or a sinusoidal input of a specified frequency.

  1. Response time This is the time which elapses after a constant input, a step input, is applied to the transducer up to the point at which the transducer gives an output corresponding to some specified percentage. For example, if a mercury-in-glass thermometer is put into a hot liquid there can be quite an appreciable time lapse, perhaps as much as 100 s or more before the thermometer indicates 95% of the actual temperature of the liquid.
  2. Time constant This is the 63.2% response time. A thermocouple in air might have a time constant of perhaps 40 to 100 s. The time constant is a measure of the inertia of the sensor and so how fast it will react to changes in its input, the bigger the time constant the slower will be its reaction to a changing input signal. 
  3. Rise time This is the tine taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value.
  4. Settling time This is the time taken for the output to settle to within some percentage, e.g. 2%, of the steady-state value.

Pneumatic and hydraulic systems

Pneumatic signals are often used Lo control final control elements, even when the control system is otherwise electrical This is because such signals can be used to actuate large valves and other high power control devices and so move significant loads. The main drawback with pneumatic systems is, however the compressibility of air Hydraulic signals can be used for even higher power control devices but are more expensive than pneumatic systems and there are hazards associated with oil leaks which do not occur with air leaks.

Directional control valves

Pneumatic and hydraulic systems use directional control valves to direct the flow of fluid through a system. They are not intended to vary the rate of flow of fluid but are either completely open or completely closed, i.e. on/off devices. Such on/off valves are widely used to develop sequenced control systems. They might be activated to switch the fluid flow direction by means of mechanical, electrical or fluid pressure signals..

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