How to split tasks between interrupts and the main loop on a bare metal controller?

Question

I'm working on a robotics project where I have 3 services running. I have my sensor DAQ, my logic ISR (motor controller at 20kHz) and my EtherCAT slave controller.

DAQ and EtherCAT run in the idle and the logic runs during an interrupt. The logic does some calculations and controls the motor. The EtherCAT service (kinda like CANbus) runs together with my DAQ in the idle loop. I can not run the DAQ in the interrupt because that leaves me with less than 100ns for the EtherCAT service to run.

I'm not sure whether this is the right way to do this especially considering all the scary things i've read regarding data corruption when using interrupts.

Does anyone have some nice ideas on how to handle these services?

I'm running all my code on a Zynq 7020 (on the ARM Cortex) and it's written in C++.

Here is an example of my code:

/**
 * Get all sensor data
 */
void Supervisor::communication(void) {
    // Get all the sensors data
    dispatchComm.getData(&motorAngle, &motorVelocity, &jointAngle, &springAngle, &tuningParameter);

}

/**
 * Run all the logic
 */
void Supervisor::logic(void) {

    dispatchLogic.calculate(tuningParameter, motorAngle, motorVelocity, jointAngle, springAngle);

    dispatchLogic.getData(&angle, &magnitude);

    // Dispatch values to the motor drive
    dispatchComm.setMotorDriveSetpoint(angle, magnitude);
    dispatchComm.writeToPiggyback((uint32_t) (tuningParameter), motorAngle, motorVelocity);
}

Answer 1

Some very general advice.

My first impression from your question is that you are struggling with the fact that no hardware provides infinite resources. You can either slow down your control loops or look for better hardware (which may not exist).

If your control has nested loops, then run each one a slow as possible. Are you controlling position at 20kHz? Pull that out and run that at 500Hz (A little faster if your mechanical mechanism is designed for high performance). Is you DAQ measuring current for the controller? Try running your current control at 5kHz but commutating at 20kHz.

All those numbers are assuming a meso scale device using electro-magnetic actuators.

Regarding interrupts, the rule of thumb is to do as little as possible, as quickly as possible, inside the interrupt and everything else outside it. However it feels like you might be using a timer interrupt to substitute for a real-time task scheduler (nothing wrong with that), in which case, you will want to do everything needed for that cycle in the interrupt. Add a time-overrun check at the end so that if your interrupt is taking longer than 50us you know about it and can adjust your strategy or optimize code.

If you are writing interrupts then you need to go learn the details about data corruption issues. They are not scary and they are straightforward to manage on all modern processors. You can protect data in your idle loop by disabling interrupts in critical sections or using higher level synchronization constructs if you really need to.

A final thought; even though you are coding in C++, you should read the processor datasheet and instruction set so that you have an idea of the capabilities and limitations of the hardware if you have not already.

EDIT:

From the limited code you added it looks like high level control for a series elastic actuator. The spring will dominate the mechanical time constant of the actuator system and you probably do not need that loop to run at 20kHz in lockstep with your motor commutation.