User Manual

Closed Loop Relative and Tracking Position Modes
160 Advanced Digital Motor Controller User Manual V2.1 December 3, 2020
Beware not to use accelerations and max velocity that are beyond the motor’s physical
reach at full load. This would result in a loop error which will stop the system if growing
too large.
Operating in Closed Loop Tracking Mode
In this mode, the controller makes no effort to compute a smooth, millisecond by milli-
second position trajectory. Instead, the current feedback position is periodically compared
with the requested destination and power is applied to the motor using these two values
in a PID control loop.
This mode will work best if changes in the commands are smooth and not much faster
than what the motor can physically follow.
Position Mode Relative Control Loop Description
The controller performs the Relative Position mode using a full featured Proportional, In-
tegral and Differential (PID) algorithm (using the Position Gains). This technique has a long
history of usage in control systems and works on performing adjustments to the speed
command based on the difference measured between the desired position (set by the
user) and the actual position (captured by the position sensor).
Figure 10-4 shows a representation of the PID algorithm. Every 1 millisecond, the control-
ler measures the actual motor position and subtracts it from the desired position to com-
pute the position error.
The resulting error value is then multiplied by a user selectable Proportional Gain. The result-
ing value becomes one of the components used to command the motor. The effect of this
part of the algorithm is to calculate the speed command to the motor control that is pro-
portional with the distance between the current and desired positions: when far apart, high
speed command is generated, with the power being gradually reduced and stopped as the
motor moves to the final position. The Proportional feedback is the most important compo-
nent of the PID in Position mode.
If the PID of the speed PI controller is not configured, then the output of the position PID
corresponds to the applied output voltage (power). In this case, it is highly recommended
to configure only the flux FOC gains and set the torque FOC gains to zero.
A higher Proportional Gain will cause the algorithm to apply a higher level of speed/
voltage commands for a given measured error, thus making the motor move quicker. Be-
cause of inertia, however, a faster moving motor will have more difficulty stopping when
it reaches its desired position. It will therefore overshoot and possibly oscillate around
that end position.