Getting Started with the NI 9502 and AKM Motors
24
ni.com
Motor Responds Poorly
The NI 9502 example programs are configured with basic gains that may not work well for your system.
This section contains some tuning best practices, but does not cover all aspects of tuning a system from
scratch.
Note
Go to
ni.com/info
and enter
MotorTuning
for more information about tuning principles.
Tuning Using the Trapezoidal Commutation Example
The
9502 Trapezoidal Commutation
example VI allows you to adjust the current and velocity
gains during execution to observe the effect of different gains on the system. The following steps will
help you get started with tuning your system.
1.
Connect a load to the motor.
2.
Set the
integral gain
in the
velocity gains
section to 0.
3.
Set the
proportional gain
in the
velocity gains
section to a low number, and slowly increase the
value until the velocity returned begins to oscillate around the velocity setpoint.
The proportional gain (Kp) determines the contribution of restoring force that is directly
proportional to the position error. This restoring force functions in much the same way as a spring
in a mechanical system. A system with too small a value of Kp is unable to hold position and is
very soft. Increasing Kp stiffens the axis and improves its disturbance torque rejection. However,
too large a value of Kp often results in instability.
4.
Keeping the proportional gain value set at the value determined in the previous step, set the
integral
gain
to a low number, and slowly increase until the velocity returned is the same as the velocity
setpoint, with little oscillation.
The integral gain (Ki) determines the contribution of restoring force that increases with time,
ensuring that the static position error in the servo loop is forced to zero. This restoring force works
against constant torque loads to help achieve zero position error when the axis is stopped.
In applications with small static torque loads, this value can be left at its default value of zero (0).
For systems having high static torque loads, this value should be tuned to minimize position error
when the axis is stopped.
Although non-zero values of Ki cause reduced static position error, they tend to cause increased
position error during acceleration and deceleration. This effect can be mitigated through the use of
the Integration Limit parameter. Too high a value of Ki often results in servo loop instability.
5.
If necessary, tune the current loop using the same tuning principles as for the velocity loop.
