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INDUSTRIAL CONTROL HANDBOOK - 1.1 QUALITY OF SENSORS

The best sensor for any job is one that has sufficient quality for the job, has adequate durability, yet isn't any more expensive than the job requires. Spec sheets (short for "specification sheets") use many terms to describe how good sensors are. The following section discusses what these terms mean.

 

1.1.1 Range and Span

The range, or "span," of a sensor describes the limits of the measured variable that a sensor is capable of sensing. A temperature transducer must have an output (e.g., electric current) that is proportional to temperature. There are upper and lower limits on the temperature range at which this relationship is reasonably proportional. Figure 1.1 shows the concept of span in describing the usefulness of a resistance-type temperature sensor. Current flow through the sensor varies with temperature between t1 and t4, but is only reasonably proportional to temperatures between t2 and t3 To be able to claim reasonable proportionality, the sensor's supplier would actually quote the more restrictive temperature span of from t2 to t3.

 

1.1.2 Error

The error between the ideal and the actual output of a sensor can be due to many sources. The types of error can be described as resolution error, linearity error, and repeatability error.

Fig. 1.1 Range and linearity error in a temperature sensorFig. 1.1 Range and linearity error in a temperature sensor

 

1.1.3 Resolution

The resolution value quoted for a sensor is the largest change in measured value that will not result in a change in the sensor's output. Put more simply, the measured value can change by the amount quoted as resolution, without the sensor's output changing. There are several reasons why resolution error may occur. Figure 1.2 demonstrates resolution error due to hysteresis. Small changes in measured value are insufficient to cause a change in the output of the analog sensor. Figure 1.3 demonstrates resolution error in a sensor with digital output. The digital sensor can output 256 different values for temperature. The sensor's total span must be divided into 256 temperature ranges. Temperature changes within one of these subranges cannot be detected. Sensors with more range often have less resolution.

Fig. 1.2 Hysteresis and resolution in a temperature sensorFig. 1.2 Hysteresis and resolution in a temperature sensor

Fig.1. 3 Resolution in a digital temperature sensor output numbers representing temperature step input (temperature)Fig.1. 3 Resolution in a digital temperature sensor output numbers representing temperature step input (temperature)

 

1.1.3 Repeatability

Values quote for a sensor's repeatability indicate the range of output values that the user can expect when the sensor measures the same input values several times. The temperature sensor in Figure 1.2, for example, might output any value from a voltage value slightly higher than V1 to a value slightly lower when the actual temperature is at temp 1. Repeatability does not necessarily mean that the output value accurately represents the sensed condition! Looseness in mechanical linkages are one source of repeatability error.

 

1.1.4 Linearity

The ideal transducer is one that has an output exactly proportional to the variable it measures (within the sensor's quoted range). No transducer's output is perfectly proportional to its input, however, so the sensor user must be aware of the extent of the failure to be linear. Linearity is often quoted on spec sheets as a +/- value for the sensor's output signal. Figure 1.4 demonstrates what a linearity specification of +/- 0.5 volts for a pressure sensor means.

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