Sensor Fundamentals

Terminology

Range

Every sensor is designed to work over a specified range. The design ranges are usually fixed, and if exceeded, result in permanent damage to or destruction of a sensor. It is customary to use transducing elements over only the part of their range where they provide predictable performance and often enhanced linearity.

Zero

When making a measurement it is necessary to start at a known datum, and it is often convenient to adjust the output of the instrument to zero at the datum. It, therefore, is a value ascribed to some defined point in the measured range.

Zero Drift

The signal level may vary from its set zero value when the sensor works. This introduces an error into the measurement equal to the amount of variation, or drift as it is usually termed. Zero drift may result from changes of temperature, electronics stabilizing, or aging of the transducer or electronic components.

Sensitivity

Sensitivity of a sensor is defined as the change in output of the sensor per unit change in the parameter being measured. The factor may be constant over the range of the sensor (linear), or it may vary (nonlinear).

Resolution

Resolution is defined as the smallest change that can be detected by a sensor.

Response

The time taken by a sensor to approach its true output when subjected to a step input is sometimes referred to as its response time. It is more usual, however, to quote a sensor as having a flat response between specified limits of frequency. This is known as the frequency response, and it indicates that if the sensor is subjected to sinusoidally oscillating input of constant amplitude, the output will faithfully reproduce a signal proportional to the input.

Linearity

The most convenient sensor to use is one with a linear transfer function. That is an output that is directly proportional to input over its entire range, so that the slope of a graph of output versus input describes a straight line.

Hysteresis

Hysteresis refers to the characteristic that a transducer has in being unable to repeat faithfully, in the opposite direction of operation, the data that have been recorded in one direction (Figure 2).

Calibration

If a meaningful measurement is to be made, it is necessary to measure the output of a sensor in response to an accurately known input. This process is known as calibration, and the devices that produce the input are described as calibration standards.

Span (input)

A dynamic range of stimuli which may be converted by a sensor id called a span or an input full scale (FS). It represents the highest possible input value which can be applied to the sensor without causing unacceptably large inaccuracy (shown in Figure 3).

Full Scale Output

Full scale output (FSO) is the algebraic difference between the electrical output signals measured with maximum input stimulus and the lowest input stimulus applied. This must include all deviations from the ideal transfer function. For instance, the FSO output in Figure 3 is represented by SFS.

Accuracy

A very important characteristic of a sensor is accuracy which really means inaccuracy. Inaccuracy is measured as a ratio of the highest deviation of a value represented by the sensor to the ideal value. It may be represented in terms of measured value (Delta, shown in Figure 3)

Sensors in the Industrial Environment

In the laboratory the atmosphere is relatively free from contaminants, the temperature is relatively stable, the area is free from vibration, and personnel are specialized in handling the equipment they are using. The industrial measure is often made under reverse conditions. The produced signal must be capable of transmission to recording equipment which may be a considerable distance away. The wiring between the two may be induced much electrical noise.