Page 56 - Instrumentation and Measurement

Some terminology that is used in describing controllers:
1. Range

The range is the two extreme values between which the system operates. A common controller
output range is 4-20 mA.
2. Span
The span is the difference between the two extreme values within which the system operates, e.g.
a temperature control system might operate between 0 ͦ C and 30 ͦ C and so have a span of 30 ͦ C.
3. Absolute deviation
The set point is compared to the measured value to give the error signal, this being generally
termed the deviation. The term absolute deviation is used when the deviation is just quoted as the
difference between the measured value and the set value, e.g. a temperature control system might
operate between 0 ͦ C and 30 ͦ C and have an absolute deviation of 3 ͦ C.
4. Fractional deviation
The deviation is often quoted as a fractional or percentage deviation, this being the absolute deviation
as a fraction or percentage of the span. Thus, a temperature control system operating between 0 ͦ C
and 30 ͦ C with an error of 3 ͦ C has a percentage deviation of (3/30)x100=10%. When there is no
deviation then the percentage deviation is 0% and when the deviation is the maximum permitted by
the span it is 100%.
Generally, with process controllers, the proportional gain is described in terms of its proportional
band (PB). The proportional band is the fractional or percentage deviation that will produce a 100%
change in controller output (Figure 4.10):

The 100% controller output might be a signal that fully opens a valve, the 0% being when it fully
closes it. A 50% proportional band means that a 50% error will produce a 100% change in controller
output; 100% proportional band means that a 100% error will produce a 100% change in controller
output.

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