The motor controller is a cascaded style position, velocity and current control loop, as per the diagram below. When the control mode is set to position control, the whole loop runs. When running in velocity control mode, the position control part is removed and the velocity command is fed directly in to the second stage input. In current control mode, only the current controller is used.
Each stage of the control loop is a variation on a PID controller. A PID controller is a mathematical model that can be adapted to control a wide variety of systems. This flexibility is essential as it allows the ODrive to be used to control all kinds of mechanical systems.
The position controller is a P loop with a single proportional gain.
pos_error = pos_setpoint - pos_feedback vel_cmd = pos_error * pos_gain + vel_feedforward
The velocity controller is a PI loop.
vel_error = vel_cmd - vel_feedback current_integral += vel_error * vel_integrator_gain current_cmd = vel_error * vel_gain + current_integral + current_feedforward
The current controller is a PI loop.
current_error = current_cmd - current_fb voltage_integral += current_error * current_integrator_gain voltage_cmd = current_error * current_gain + voltage_integral (+ voltage_feedforward when we have motor model)
For more detail refer to controller.cpp.
Tuning the motor controller is an essential step to unlock the full potential of the ODrive. Tuning allows for the controller to quickly respond to disturbances or changes in the system (such as an external force being applied or a change in the setpoint) without becoming unstable. Correctly setting the three tuning parameters (called gains) ensures that ODrive can control your motors in the most effective way possible. The three values are:
<axis>.controller.config.pos_gain = 20.0[(counts/s) / counts]
<axis>.controller.config.vel_gain = 5.0 / 10000.0[A/(counts/s)]
<axis>.controller.config.vel_integrator_gain = 10.0 / 10000.0[A/((counts/s) * s)]
An upcoming feature will enable automatic tuning. Until then, here is a rough tuning procedure:
vel_gainby around 30% per iteration until the motor exhibits some vibration.
vel_gainto 50% of the vibrating value.
pos_gainby around 30% per iteration until you see some overshoot.
pos_gainuntil you do not have overshoot anymore.
0.5 * bandwidth * vel_gain, where
bandwidthis the overall resulting tracking bandwidth of your system. Say your tuning made it track commands with a settling time of 100ms (the time from when the setpoint changes to when the system arrives at the new setpoint); this means the bandwidth was 1/(100ms) = 1/(0.1s) = 10hz. In this case you should set the
vel_integrator_gain = 0.5 * 10 * vel_gain.
The liveplotter tool can be immensely helpful in dialing in these values. To display a graph that plots the position setpoint vs the measured position value run the following in the ODrive tool: