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Chapter 4 Applications
40 CNT-APG002-EN
The challenge in staging applications is to find the correct proportional
bias. This value determines the output when the error is zero. The pro-
portional bias should have the same value as the point at which the first
stage turns on (see “Determining the staging points” on page 42). In this
case, the first stage turns on at an output of 63%, so the proportional bias
is set to 63%.
Figure 29 shows the output versus error when the proportional bias is
63%. This graph can help us determine the error deadband setting. We
know that the first-stage fan turns on when the error becomes negative.
The second stage should not turn on until the output reaches 90% or an
error of –4°F (–2.2°C). Following the procedure presented in “Adjusting
error deadband for staged outputs” on page 20, the error deadband is:
0.45 × 4°F (2.2°C) = 1.8°F (1°C)
We can round the error deadband to 2.0°F or 1.5°F. Either choice should
ensure that the second stage does not turn on until the error is relatively
large.
Figure 29: Controller output versus error: proportional bias = 63%
Error
90% point
Proportional bias = 63%
10% point
Controller output (%)