3. Sensor/transducers specifications

Transducers or measurement systemsare not perfect systems. Mechatronics design engineer must know the capability and shortcoming of a transducer or measurement system to properly assess its performance. These are a number of performance related parameters of a transducer or measurement system. These parameters are called as sensor specifications.

Sensor specifications inform the user about deviations from the ideal behavior of the sensors. Following are the various specifications of a sensor/transducer system.

3.1 Range

The range of a sensor indicates the limits between which the input can vary. Thus, for example, a thermocouple for the measurement of temperature might have a range of 25-225°C.

3.2 Span

The span is difference between the maximum and minimum values of the input. Thus, the above-mentioned thermocouple will have a span of 200°C.

3.3 Error

Error is the difference between the result of the measurement and the true value of the quantity being measured. A sensor might give a displacement reading of 29.8 mm, when the actual displacement had been 30 mm, then the error is - 0.2 mm.

3.4 Accuracy

The accuracy defines the closeness of the agreement between the actual measurement result and a true value of the measurand. It is often expressed as a percentage of the full range output or full–scale deflection. A piezoelectric transducer used to evaluate dynamic pressure phenomena associated with explosions, pulsations, or dynamic pressure conditions in motors, rocket engines, compressors, and other pressurized devices is capable to detect pressures between 0.1 and 10,000 psig (0.7 KPa to 70 MPa). If it is specified with the accuracy of about±1% full scale, then the reading given can be expected to be within ± 0.7 MPa.

3.5 Sensitivity

Sensitivity of a sensor is defined as the ratio of change in output value of a sensor to the per unit change in input value that causes the output change.For example, a general purpose thermocouple may have a sensitivity of 41 µV/°C.