Control Loops

The QuimDC has four control modes, one is an open loop and the others are closed loops. Default mode is PWM Open Loop control mode.

In open loop, ref_speed is used to control the motors, here is an example :

With ref_speed set to 0, the pwm value is 128.

With ref_speed set to 62, the pwm value is 190.

With ref_speed set to -18, the pwm value is 110.

The current control period is 1 ms.

This loop needs the current amount that passes through the motor. The QuimDC has a 12 bits ADC but has no clue about the current sign. We need to estimate the current sign using this formula :

Vbemf = ((60 * speed) / (ticks_per_turn_motor * 0.001)) / kv_motor
Voltage = nominal_power_voltage * ( pwmDuty - 128 ) / 128      with pwmDuty [0;255]
itemp = (Voltage - Vbemf) / resistance_motor

We use the itemp sign to estimate the current sign. By default we have the following value :

resistance_motor = 1.0
kv_motor = Inf
ticks_per_turn_motor = 1
nominal_power_voltage = 1.0

With this value, the current sign is estimated with the pwm sign. Resistance_motor, kv_motor, ticks_per_turn_motor and nominal_power_voltage are only used in this formula.

In this loop, we have two PID in cascade. The output of the speed PID is used as a reference for the second current PID. To ensure that the current doesn't exceed the ref_current, the first PID output is saturated by ref_current.

The speed is obtained by doing the difference between the encoder value at period n and period n-1 and then convert it into tick per second. We can change the period of the speed loop by setting the position_speed_period parameter.

If the quadrature encoder channel are swap, you can use the polarity to make them work (the polarity must be set to 1 or -1, any other value will not send error but will lead to unexpected results).

There are three PID in cascade, position, speed and current.