Performance Terminology

TRANSDUCERS

It is an element which is subjec ted to physical change experience a related cha nge. Example:

Tactile Sensors.

SENSORS:

It is an element which is not subjected to physical change experience a related change. Example:

LVDT

PERFORMANCE TERMINOLOGY:

Static Characteristics:

Range and Span:

The range of a transducer defines the limits between which the input can var y.

The difference between the limits (maximum value - minimum value) is kno wn as span.

For example a load cell is us ed to measure force. An input force can vary from 20 to 100 N. Then the range of load cell is 20 to 100 N. And the span of load cell is 80 N (i.e., 100-

20)

Error:

The algebraic difference betw een the indicated value and the true value of the measured parameter is termed as the error of the device.

Error = Indicated value — tru e value

For example, if the transduc er gives a temperature reading of 30°C when the actual temperature is 29° C, then the error is + 1°C. If the actual temperature is

3 1° C, then the error is — 1°C.

Accuracy:

Accuracy is defined as the ability of the instrument to respond to the true value of the measure variable under the reference c onditions.

For example, a thermocouple has an accuracy of ± 1° C. This means th at reading given by the thermocouple can be expecte d to lie within + 1°C (or) — 1°C of the true valu e.

Accuracy is also expressed as a percentage of the full range output (or) full scale deflection.

For example, a thermocouple can be specified as having an accuracy of ±4 % of full range output. Hence if the range of the thermocouple is 0 to 200°C, then the reading

given can be expected to be w ithin + 8°C (or) — 8°C of the true reading.

Sensitivity:

The sensitivity is the relation ship showing how much output we can get per u nit input.

sensitivity = Output / Input

Precision:

It is defined as the degree of exactness for which the instrument is intended t o perform.

Hysteresis error:

When a device is used to measure any parameter plot the graph of output Vs value of measured quantity.

First for increasing values of the measured quantity and then for decreasing values of the measured quantity.

The two output readings obtained usually differ from each other.

Repeatability:

The repeatability and reproducibility of a transducer are its ability to give the same output for repeated applications of the same input value.

Reliability:

The reliability of a system is defined as the possibility that it will perform its assigned functions for a specific period of time under given conditions.

Stability:

The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time.

Drift:

The term drift is the change in output that occurs over time.

Dead band:

There will be no output for certain range of input values. This is known as dead band. There will be no output until the input has reached a particular value.

Dead time:

It is the time required by a transducer to begin to respond to a change in input value.

Resolution:

Resolution is defined as the smallest increment in the measured value that can be detected.

The resolution is the smallest change in the input value which will produce an observable change in the input.

Backlash:

Backlash is defined as the maximum distance (or) angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part.

Backlash is an undesirable phenomenon and is important in the precision design of gear trains.