Changing target during motion (with ramping)

[Axel Hösl]

At the moment I am trying to setup a prototype where one should be able to change the target of a motor motion dynamically (meaning while a motion from A to B is already executed). As far as my tests went with AccelStepper, I found that there is always a hard stop when changing the target (e.g. via a Serial command) and a new acceleration-motion-deceleration cycle is started afterwards. What I am trying to achieve is a change from a traget B to B' (in the easiest case B' is behind B).

I went into the source of AccelStepper but didn't find the part where the stop occurs. I am wondering whether this might be possible with AccelStepper at all or not. Do you think this doable with AccelStepper while not sacrificing the ramping? 

[gregor]

the stop probably occurs when reading the serial input.
i send my commands in patterns like "D motornumber target ", e.g. "D 1 100 ". without a whitespace at the end, my code stops for 1 second.

[Mike McCauley]

Hi,
AccelStepper is designed so that if you change the target position at any time
it should accel/decel to suit, not just stop.

Suspect the problem is in your code, which you have not posted so I can say no
more.

Cheers.

--
Mike McCauley VK4AMM mi...@airspayce.com
Airspayce Pty Ltd 9 Bulbul Place Currumbin Waters QLD 4223 Australia
http://www.airspayce.com
Phone +61 7 5598-7474

[Axel Hösl]

Thanks for the input the so far! I wasn't aware that the Serial command might trigger the stop. As my code became bigger over time due to the adding of features, I won't bug you with it and setup a new sketch where isolating that part, so that the issue can be looked at without the interferance of the other parts.

Thanks again!

[Christian Thomsen]

I think your troubles might be related to mine:

I want a two-wheeled differentially steered robot to not only perform some pre-programmed moves (AccelStepper is really good for that purpose!). I also want the motors to be able to change speed in mid-flow, so as to turn the robot left/right to steer away from obstacles.

For that purpose, i've setup a little test-sketch which reads a serial input from 0-9 (multiplied by 100) and sets the max-speed to that . It also sets a distant target, move(60000) to keep rolling...

void loop()

{

   if(Serial.available()) {

      // read the most recent byte (which will be from 0 to 255):

      int inSpeed = Serial.read()-48; // 48 is ascii '0'

      

      stepper1.setMaxSpeed(inSpeed*100);  

            if(inSpeed==0) { // '0': stop

         stepper1.stop();

      }

      else {

          stepper1.move(60000);

      }

   }

   stepper1.run();

}

This works nicely.

BUT:

AccelStepper can only accelerate, not decelerate in mid-flow. (is that why it's called "AccelStepper"?) in the function AccelStepper::computeNewSpeed() there's a feature to limit the top speed at the current _maxSpeed, and that causes a jolt when the speed suddenly changes, when you command a lower speed, not a smooth deceleration.

I made a copy of the library and implemented some changes which will allow this. The main changes were in AccelStepper::setMaxSpeed(float speed) and in AccelStepper::computeNewSpeed(), as well as adding a state-variable to indicate when a deceleration is in progress. Unfortunately, i've made quite some other changes to my version, as i only need a DRIVER-type stepper, and so i eliminated all code not related to driver-style interface.

These are my versions of those:

void AccelStepperDriver::setMaxSpeed(float speed) {

    if (_maxSpeed != speed) {

if(_direction == DIRECTION_CW) {

if(_speed > speed) { 

_isAccelerating = false;

}

else {

_isAccelerating = true;

}

}

else { // inverse if going ccw

if(-_speed > speed) { 

_isAccelerating = false;

}

else {

_isAccelerating = true;

}

}

_maxSpeed = speed;

_cmin = 1000000.0 / speed;

// Recompute _n from current speed and adjust speed if accelerating or cruising

if (_n > 0) {

_n = (long)((_speed * _speed) / (2.0 * _acceleration)); // Equation 16

if(_isAccelerating == false) {

_n = -_n;

}

computeNewSpeed();

}

    }

}

void AccelStepperDriver::computeNewSpeed() {

    long distanceTo = distanceToGo(); // +ve is clockwise from curent location

    long stepsToStop = (long)((_speed * _speed) / (2.0 * _acceleration)); // Equation 16

    if (distanceTo == 0 && stepsToStop <= 1) {

// We are at the target and its time to stop

_stepInterval = 0;

_speed = 0.0;

_n = 0;

return;

    }

    if (distanceTo > 0) {

// We are anticlockwise from the target

// Need to go clockwise from here, maybe decelerate now

if (_n > 0) {

// Currently accelerating, need to decel now? Or maybe going the wrong way?

if ((stepsToStop >= distanceTo) || _direction == DIRECTION_CCW) {

_n = -stepsToStop; // Start deceleration

}

}

else if (_n < 0) {

// Currently decelerating, need to accel again?

if(stepsToStop < distanceTo && _direction == DIRECTION_CW) {

long targetStepsToStop = (long)((_maxSpeed * _maxSpeed) / (2.0 * _acceleration)); // Equation 16

if(_isAccelerating || _n > -targetStepsToStop) { // braking to reach target speed complete

_n = -_n; // Start accceleration

_isAccelerating = true;

}

}

}

    }

    else if (distanceTo < 0) {

// We are clockwise from the target

// Need to go anticlockwise from here, maybe decelerate

if (_n > 0) {

// Currently accelerating, need to decel now? Or maybe going the wrong way?

if ((stepsToStop >= -distanceTo) || _direction == DIRECTION_CW) {

_n = -stepsToStop; // Start deceleration

}

}

else if (_n < 0) {

// Currently decelerating, need to accel again?

if ((stepsToStop < -distanceTo) && _direction == DIRECTION_CCW) {

long targetStepsToStop = (long)((_maxSpeed * _maxSpeed) / (2.0 * _acceleration)); // Equation 16

if(_isAccelerating || _n > -targetStepsToStop) { // braking to reach target speed complete

_n = -_n; // Start accceleration

_isAccelerating = true;

}

}

}

    }

    // Need to accelerate or decelerate

    if (_n == 0) {

// First step from stopped

_cn = _c0;

_direction = (distanceTo > 0) ? DIRECTION_CW : DIRECTION_CCW;

    }

    else {

// Subsequent step. Works for accel (n is +_ve) and decel (n is -ve).

_cn = _cn - ((2.0 * _cn) / ((4.0 * _n) + 1)); // Equation 13

if(_isAccelerating) { // allows deceleration to lower speeds

_cn = max(_cn, _cmin); 

}

    }

    _n++;

    _stepInterval = _cn;

    _speed = 1000000.0 / _cn;

    if (_direction == DIRECTION_CCW) {

_speed = -_speed;

}

}

I hope you can use these:

Cheers

Christian

[Nick Suttell]

So far this has worked nicely for me! Thank you very much for your adjustment, and big thanks to Mike for setting up the library!

Best Regards,

Nick