User Manual

Torque Mode Limitations
Advanced Digital Motor Controller User Manual 163
Speed Limiting in Brushless controllers
Brushless Controllers provide a way of smoothly limiting the speed in torque mode to pre-
vent motor runaways. The method for limiting the speed is based on PID speed over-ride
control 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
Integral Gain (KIF), in FOC parameters (for both sinusoidal and trapezoidal modes), and the
speed limit is tuned using the Proportional Gain (KP), the Integral Gain (KI) and the Differ-
ential Gain (KD), in Closed loop 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.
Torque Mode Using an External Amps Sensor
The limitations described above can be circumvented using an external amps sensor de-
vice such as the Allegro Microsystems ACS756 family of hall sensors. These inexpensive
devices can be inserted in series with one of the motor leads while connected to one of
the controller’s analog inputs. Since it is directly measuring the real motor amps, this sen-
sor will provide accurate current information in all load and regeneration conditions. This
technique only works for DC brushed motors. On brushless motors, the current in the mo-
tor wires is AC and therefore an external sensor cannot be used.