64 SF01U
B.8 TUNING THE PI CONTROL
Once the PI control is configured properly, it needs to be tuned in order to maintain the process
setpoint. First, set the Integral Gain (Parameter 65) to zero, and increase the Proportional
Gain (Parameter 64) until the system becomes unstable, then lower the gain until the system
stabilizes again. Set the Proportional Gain about 15% less than that value that stabilizes
the system. If only Proportional Gain is used, and the system is operating in a steady-state
condition (setpoint is fixed and process variable has settled to a fixed value), there will always
be a certain amount of error in the system. This is called the steady-state error.
Integral Gain (Parameter 65) is used to force the steady-state error to zero by increasing the
output speed command with respect to time. Over time, the error will be forced to zero because
the Integral term will continue to change the speed command, even after the Proportional
term reaches steady state and no longer affects the speed command. The Integral Gain
affects the rate of rise of the output speed command from the Integral term. Small amounts
of Integral Gain can cause large changes in PI performance, so care must be taken when
adjusting Integral Gain. Too much Integral Gain will result in overshoots, especially if large
step changes in error occur.
The other parameter setting that affects the response of the PI control is Parameter 66 - PI
ACCEL/DECEL. This sets the acceleration and deceleration rate of the setpoint reference
into the PI unit. When the setpoint changes, this function will “filter” the input to the PI unit
by ramping the setpoint reference from the previous value to the new value. This will help
prevent overshoots that can occur when the PI control attempts to respond to step changes
in setpoint, resulting in smoother operation. If PI ACCEL/DECEL is set to 0.0 seconds, it is
effectively disabled.
B.9 ADDITIONAL STATUS DISPLAYS
The PI option contains additional messages that may appear on the display:
PI OPTION DISPLAYS
DISPLAY DESCRIPTION
ACP AUTO CONTROL PAD: The s and t buttons are used to adjust the setpoint in PI mode.
nCP
MANUAL CONTROL PAD: The s and t buttons are used for manual speed control when PI
mode is enabled but the drive is operating in open-loop mode.
FL
FOLLOWER LOST: If a 4-20 mA signal is used as a speed reference, PI setpoint reference,
or PI feedback, and the signal drops below 2 mA, the drive will trip and display “FL” to
indicate that the signal is lost.
SLP
SLEEP MODE: The drive is in Sleep Mode. The motor is at zero speed. The drive will wake
up when system demand requires a motor speed above SLEEP THRESHOLD (P75).
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