THORLABS KBD101 User Manual page 46

Chapter 6
Position Loop Settings
The motion processors within the KBD101 driver uses a position control loop to
determine the motor command output. The purpose of the position loop is to match
the actual motor position and the demanded position. This is achieved by comparing
the demanded position with the actual encoder position to create a position error,
which is then passed through a digital PID-type filter. The filtered value is the motor
command output.
The PID and other closed loop parameters must be set according to the stage
or actuator type associated with the driver, the load being positioned and the
speed/duty cycle of operation. Default values have already been optimized and
stored within the stage, and these are loaded into the controller on power up.
If problems are encountered (e.g. stability of the closed loop position control,
lost motion or incomplete moves) the position PID parameters should be
adjusted to tune the stage for the given application. Normally, only minor
adjustment of the Proportional, Integral and Derivative parameters should be
necessary, and some trial and error will be required before the ideal settings
for a specific application are achieved. In cases where further adjustment of
the control loop parameters is required, the following guidelines are provided
Proportional Gain - Increasing the proportional (Prop) term will increase the amount
of effective torque used to correct a given position error. Typically this is used to
minimise the amount of position error when an impulse event affects current target
position during motion. i.e. sticksion, vibration...etc .If the proportional term is too high
this can lead to overshoot and general instability. If this is too low it can result in a
sloppy response. It accepts values in the range 0 to 32767.
Integral Gain - Increasing the integral (Int) term minimises following error and final
position error. If Integral is too high this will typically lead to motion overshoot during
and at end of move. If the integral term is too low final position may take a long time
to reach, if at all. It accepts values in the range 0 to 32767.
Derivative Gain - Increasing the derivative (Deriv) term decreases the rate of change
of driver output. Typically this is used to reduce the overshoot from a given motion. If
derivative is too high it can become sensitive to noise from the measured position
error. If derivative is too low, velocity fluctuations may arise during motion. It accepts
values in the range 0 to 32767.
Integral limit - Limits the wind up limit for the integral term such that excessive
overshoots are prevented. Typically used to prevent runaway integral calculations
due to stiction and other such physical forms of random forces. Normally set as low
as possible, but high enough that with the given integral term final position can be
reached. It accepts values in the range 0 to 32767. If set to 0 then the integration term
in the PID loop is ignored.
46
Caution
in order to assist in the tuning process.
HA0364T Rev Ck Jan 2017