Controlling 3 stepper motors simultaneously

Question

I have three stepper motors connected to the drv8825 drivers to an arduino uno.

I am using the example code below (from here), which works fine for moving multiple stepper motors at the same time BUT they all move for the same number of steps and in the same direction. I would like to tweak the code so that all of them move at the same time but each one for a different number of steps and in different directions. The motor with the least steps would probably have to wait for the motor with the most steps to finish its movement before going again.

int dirPin = 8;
int stepperPin = 7;
int bdirPin = 6;
int bstepperPin = 5;

void setup() {
 pinMode(dirPin, OUTPUT);
 pinMode(stepperPin, OUTPUT);
 pinMode(bdirPin, OUTPUT);
 pinMode(bstepperPin, OUTPUT);
}
 void step(boolean dir,int steps){
 digitalWrite(dirPin,dir);
 digitalWrite(bdirPin,dir);
 delay(50);
 for(int i=0;i<steps;i++){
 digitalWrite(stepperPin, HIGH);
 digitalWrite(bstepperPin, HIGH);
 delayMicroseconds(800);
 digitalWrite(stepperPin, LOW);
 digitalWrite(bstepperPin, LOW);
 delayMicroseconds(800);
 }
}
void loop(){
 step(true,1600);
 delay(500);
 step(false,1600*5);
 delay(500);
}

Thank you in advance for your help!

Answer 1

You would first have to provide all the necessary information to your step() function: direction and number of steps for each motor. Then you just have to stop the loop at different points for each motor. You do this by running the loop up to the number of steps of whichever motor needs to run for longer, and making the steps of each motor conditional on the loop index, as follows:

void step(boolean dir0, int steps0,
          boolean dir1, int steps1,
          boolean dir2, int steps2) {
    digitalWrite(dir0Pin, dir0);
    digitalWrite(dir1Pin, dir1);
    digitalWrite(dir2Pin, dir2);
    delay(50);
    int max_steps = max(max(steps0, steps1), steps2);
    for (int i = 0; i < max_steps; i++) {
        if (i < steps0) digitalWrite(stepper0Pin, HIGH);
        if (i < steps1) digitalWrite(stepper1Pin, HIGH);
        if (i < steps2) digitalWrite(stepper2Pin, HIGH);
        delayMicroseconds(800);
        if (i < steps0) digitalWrite(stepper0Pin, LOW);
        if (i < steps1) digitalWrite(stepper1Pin, LOW);
        if (i < steps2) digitalWrite(stepper2Pin, LOW);
        delayMicroseconds(800);
    }
}

It would be more elegant to put the data for all the motors in an array, and you would probably want to do so if you have more than three motors.

Answer 2

You need a struct for each stepper with a timestamp, a position and rate of change and a loop that updates the postion based on rate which is the change in position over some time:

struct stepper {
  unsigned long timestamp;
  int position;
  int rate;
};

// init steppers array

while (keepGoing()) {
  unsigned long now = millis();
  int si;

  delay(10);

  for (si = 0; si < 3; si++) {
    struct stepper *stepper = &steppers[si];

    if (steppers[si].timestamp > now) {
       steppers[si].position += steppers[si].rate;
       steppers[si].timestamp = now + 100;
    }
  }

This is a back-of-a-napkin sketch of course. It is only supposed to illustrate how you can update any number of things concurrently.

The important thing is that you only call delay once. And presumably it would be only a very short delay compared to the unit of time for the rate. In the above example it would adjust the stepper position every 100ms. You would need to experiment with the timing to find a good behavior given the speed of the stepper, speed of the code and maximum permitted rate.

Beware that there is a lot to think about beyond this. For example, you should initialize the timstamps with different timestamps so that the motors are not all running at the same instant and thus try to minimize the total power consumed by the motors at any particular time.

Note that I have never played around with stepper motors but this type of loop is a common programming idiom for handling things concurrently.

Answer 3

I have two ideas:

• I don't have personal experience working with the AccelStepper Library, but I believe it implements a class that allows you to work with multiple steppers.
• I plan on controlling two steppers for a project of my own (a self-balancing robot implemented with an STM32 processor), and I plan on using hardware timer interrupts to control the speed of my motors. A quick google search shows that a similar approach is possible with Arduino. You can probably use Timer0, as 1ms is fairly fast for a single step. This has several benefits...
  • The refresh function will run on set intervals, (allowing you to control the speed very precisely.
  • It will run with the correct timing, even if there is a function in loop() that is taking a long time.

Note: do NOT put any delay statements in your interrupt handlers. If you do this, it will probably prevent other interrupts from occurring and cause some API functions (such as millis or micros) to misbehave. In addition, the execution of anything in loop is halted while the interrupt is being handled, meaning that you would effectively stop everything until the interrupt is done. See the caveat in Arduino reference page for delay.