User Manual

Table Of Contents
Torque Mode Limitations
Advanced Digital Motor Controller User Manual 173
Speed Limiting
All Controllers provide a way of smoothly limiting the speed in torque mode to prevent
motor runaways. The method for limiting the speed is based on PID speed over-ride con-
trol which provides very smooth motor output but requires PID tuning.
Therefore for the torque loop we use the Torque Proportional Gain (KPF) and the Torque In-
tegral Gain (KIF), in Closed Loop Torque parameters, and the speed limit is tuned using the
Speed Proportional Gain (KPG), the Speed Integral Gain (KIG) and the Speed Differential
Gain (KDG), in Closed Loop Speed parameters. The speed loop PID tuning can either be
done in “Closed Loop Torque Mode” at the speed limit or in “ Closed Loop Speed Mode
by looking at the response time.
Torque Mode Limitations
The torque mode uses the Motor Amps and not the Battery Amps. See “Battery Current
vs. Motor Current”on page 28. In some Roboteq controllers, Battery Amps is measured
and Motor Amps is estimated. The estimation is fairly accurate at power level of 20% and
higher. Its accuracy drops below 20% of PWM output and no motor current is measured
at all when the power output level is 0%, even though current may be flowing in the
motor, as it would be the case if the motor is pushed. The torque mode will therefore not
operate with good precision at low power output levels.
Furthermore the resolution of the amps capture is limited to around 0.5% of the full range.
On high current controller models, for example, amps are measured with 500mA incre-
ments. If the amps limit is set to 100A, this means the torque will be adjustable with a 0.5%
resolution. If on the same large controller the amps limit is changed to 10A, the torque will
be adjustable with the same 500mA granularity which will result in 5% resolution. For best
results use an amps limit that is at least 50% than the controller’s max rating. On newer
Brushless motor controllers, amps sensors are placed at the motor output and motor amps
are measured directly. Torque mode will work effectively on these models.