Cyphal / TelegaV1 Servo Motor Control

[P.Berthold]

Hey community,

we are switching from Telega V0 / Drone Can to Telega V1 / Cyphal in order to make use of the servo operation mode. During bench testing I run into the following questions with our setup:

• AmpDrive AD0505A/B “Myxa” ESC running Telega V1

• Zubax Babel CAN Adapter

• BLDC motor with propeller attached (not relevant to the question)

We are quite pleased to be able to switch between the rpm-control mode and servo control by publishing to different cyphal topics/channels. However, as of now the servo mode appears to drive the motor coils at a fixed current to achieve a constant moment, which leaves us with a few questions:

• By varying the maximum allowed phase current register, the servo current changed likewise. Is there another way to change to the servo current without altering the max phase current? I found a kinematics.force value that can be published, without much sucess at the moment.
• Which other parameters influence the servo operation (beyond the jerk, acceleration, and velocity named here Servo - Telega v1.0 Reference Manual )
• Are there any plans to incorporate feedback and or control? I know that Myxa is a sensorless drive, however methods like high frequency injection might enable absolute positioning in the absence of back emf (low rpm), which appears to be very promising.

I am looking forward to your thoughts.

Kind regards from Munich,
PBerthold

[pavel-kirienko]

@P.Berthold

By varying the maximum allowed phase current register, the servo current changed likewise. Is there another way to change to the servo current without altering the max phase current? I found a kinematics.force value that can be published, without much sucess at the moment.

The target.force field sets the torque limit, which informs the phase current using a simplified PMSM model:

    [[nodiscard]] dyshlo::task::stepper::Command convertCommand(const motor::ServoCommand::Setpoint& in) const
    {
        dyshlo::task::stepper::Command out{
            .mechanical        = in.target,
            .constraint        = {.velocity     = par_.default_motion_profile.velocity,
                                  .acceleration = par_.default_motion_profile.acceleration,
                                  .jerk         = par_.default_motion_profile.jerk},
            .current_amplitude = par_.current_max,
        };
        <...snip...>
        if (in.force_limit) {
            // Using a simplified torque model that is good enough for the servo mode.
            const auto mr = static_cast<F32>(par_.mechanical_ratio);
            out.current_amplitude =
                std::min(par_.current_max,
                         ((2.0F / 3.0F) * std::abs(in.force_limit.value())) / (mr * par_.flux_linkage));
        }
        return out;
    }

@P.Berthold

Which other parameters influence the servo operation (beyond the jerk, acceleration, and velocity named here Servo - Telega v1.0 Reference Manual )

The maximum phase current also.

@P.Berthold

Are there any plans to incorporate feedback and or control? I know that Myxa is a sensorless drive, however methods like high frequency injection might enable absolute positioning in the absence of back emf (low rpm), which appears to be very promising.

Yes! Yes this is in the pipeline.

[P.Berthold]

Here is a caveate I was unaware of when using torque control in the servo mode (publishing the target.force field):
Publishing a position setpoint in the servo mode works w.o. publishing velocity, acceleration, and jerk limits. If however, the torque is published as well, the servo control stops. To operate with torque control it is necessary to also publish velocity and its derivatives.

Kind regards,
PBerthold