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Coordinated Stepper Motor motion factoring Aceel/Decel for arduino

timryder

Ok so I'm creating a machine for myself and its going to be like an X,Y CNC with a possible A axis rotary.

What I want to do with it is write the code from scratch to so it's lean and fast. I might not be able to achieve this but it's going to serve as both a learning tool and a functional product for me to use.

 

I've been doing some experimenting controlling stepper motors with an Arduino Mega 2560 and I've had some success with an Arduino Library called "AccelStepper".  This is a pretty handy library which allows me to control multiple steppers and many types of drivers or direct. What it lacks however and is the most important is a canned function  which would allow me to coordinate motion of multiple axis factoring in their Accel and Decel profiles. If you haven't dealt with this at all it's a huge pain in the but because stepper motors unlike a dc brush less motor are digital, that is they can't take an analog motion profile.  Given their digital nature the standard trapezoid model of accel/decel won't work as each step of the stepper motor will/may be greater than the distance over time desired to match the rise/run profile of the accel/decel.  So what I usually end up with is the axis with the shorter move finishes it's motion faster than the longer motion axis.

 

Does anyone out there have any experience solving this dilemma?  I've tried some simple equations all of which would work for an analog device but I'm not gaining any ground on this battle at the moment.

 

If anyone needs more information I can do a video or some sort of diagram.


Thanks!

  • 0

    If you are using arduino, then is shouldn´t be a problem.Try to write some examples first, to drive steppers, then try to write all logic that it needs to preform tasks.

    At the end, combine all those into one .ino file. Write a function for all axis and then try to combine all together. image

  • 0

    Wish I could help but If I need lean/decent stepper control on an Arduino I usually use grbl (Bengler: GRBL) which is very nice and very lean, even runs on a standard Arduino.

  • 0 in reply to ckraft

    Chris!

     

    You nailed it with GRBL, I took sections from it's AVR_C and made my own using it as inspiration. Very cool program.

    Now when you write a program with AVR_C and compile it, does it run faster than using one written through the Arduino IDE?

  • 0 in reply to timryder

    If you write the C-code yourself it has the potential to be faster than the Arduino code but it is not a guarantee. Ultimately the Arduino code at some point ends up compiled code so there is not an obvious difference in performance. Plus odds are that you and the Arduino code are compiling in/linking the same libraries from Atmel/AVR so there won't be much difference there.

     

    That said when you write the code yourself you have the ability for tighter control over what code is there and how it is structured. So if the Arduino tool is doing something stupid for the sake of consistency between platforms you can make your code more efficient.

     

    It is a great exercise to learn more about the platform. That's why I encourage people using the Arduino tool to dig under the surface and take a look at the code it generates to better understand what is going on.

  • 0

    The trick is to use the same method that is used by the bresenham line drawing algorithm.  Look it up on the web.  Consider using an add statement instead of the x2 multiply that you will generally find to boost motor max slew speed.  You will write 8 different subroutines per the algorithm (2 per quadrant).  This method will ensure that both motors start and stop at the correct locations at the same time.  The motors will track each other within about one step of error and you can do any angle.  Write your code to pace the entire algorithm at your desired accel, decel and slew rate.  The motor with the longest travel will be limited by this speed.  The motor with the shortest (or equal) travel distance will run at the same speed or slower.  I did this years ago and it worked fine.  Max motor slew speed is a bit slow due to the calculations involved but they are all integer math and compares.  No floating point required.