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CNT-APG002-EN 51
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Chapter 6
Frequently asked questions
Why is the output of my PID loop always zero?
•
Maximum PID output may be set to zero.
• PID action setting may need to be changed.
• Setpoint may be zero or negative, driving the output to zero. Change
the setpoint to a reasonable value manually or add a limit block to the
PID loop to keep the setpoint within a reasonable range.
• Physical output may not have enough power to achieve the setpoint,
leaving the output at the low end of its range. This problem is known
as integral windup (see “Integral calculation” on page 4 for more
information).
• Measured-variable input may have failed and the fail-safe may be set
to zero.
• Fan status is off so the program has set the output to zero.
• Scaling may be inconsistent for setpoints and inputs.
Why is the output of my PID loop always at its maximum value?
•
Minimum PID output may be set to the maximum value.
• PID action setting may need to be changed.
• Setpoint may be high enough that the output stays at its maximum
output. Change the setpoint to a reasonable value or add a limit block
to the PID loop to keep the setpoint within a reasonable range.
• Physical output may not have enough power to achieve the setpoint,
leaving the output at the high end of its range. This problem is known
as integral windup (see “Integral calculation” on page 4 for more
information).
• Measured-variable input may have failed and the fail-safe may be set
to the maximum value.
• Scaling may be inconsistent for setpoints and inputs.
Why is the output of my PID loop oscillating?
•
PID action setting may be wrong. If the action is reverse when it
should be direct, the system may start oscillating as the output
changes around setpoint.
• Sampling frequency may be too slow or too fast. See “Sampling fre-
quency” on page 12 for instructions on setting an appropriate sam-
pling frequency.
• Proportional, integral, or derivative gains may be set too high. Reduce
the gains until the oscillation subsides.