Thread: Raspberry Pi
Oops, forgot the cutarc() subroutine
Let me know what else I forgot
Been playing around with interpolation routines but run into a snag... so need to go back and rethink the code I've written. Issue is this... in 2D to make it easy... want to move from 0,0 to 1000,1300 say (steps, not mm). Lets assume that the feed rate requires this to take 1 second... then the x rate is 1000steps/sec and the y rate is 1300 steps/sec. But I have a base rate of 5000 steps sec (10khz interrupt/2). so X is every 5th interrupt and y is every 3.85 interrupts.. but you cant do 3.85.. so i need a much higher interrupt rate and some interpolation... in this case 50000 and divisors of 38,38,39,38,38,39,38,38,38,39 per 5000 gives 1301 with a max error of 2 steps... So i need to get that kernel level interrupt working... in the meantime, i'll have to run it all at 1/10 speed :(
It is fun isn't it, the hours I spent making it work. Everytime I thought I had it figured I found the next little snag
I find the major axis which steps on every interrupt, and slave everything else to that. I reload the timer after every step from the acceleration table, stretching the delay by 1.5 (approx root 2) if both X and Y go together.
I can give you the working code that drives my mills but it may not help being written in Z80 assembler. It runs from a pre-digested buffer of straight lines without any multiplies or divides. On a slow CPU, 8MHz, the interrupt steps anyone set to step on entry then reconfigures the step and direction lines at exit.
I'm a bit of a dinosaur.
How fast is your interrupt and do you change the rate? It occured to me that one way to control feed rate would be to have a programmable interrupt timer using the Pi's PWM generator - but that wastes a GPIO output pin as I don't think you can programme it to interrupt if its not allocated to an output pin (and possibly needs to be fed back to an input).
I have a buffer of straight lines. Lines currently are 24 bits long max. With the acceleration/deceleration/directions included I can define a straight line, XYZ, in 12 bytes.
I pinched the idea, I won't say where from. It also makes jogging easy but the buffer needs canny attention after you hit pause because unexpected braking has to become unexpected acceleration when you unpause.
I put a tidemark at 800 lines, if the buffer drops below that it gets another 10 lines. Drip feeding from the host may not be the way you want to go.
I use 2 interrupts, one for the stepping, one for the comms.
I have one bug at the mo', if the host misses a byte from the status report it waits forever for it to arrive. I need to add a timeout/retry. I haven't lost any data going the other way, yet, touch wood.
Edit: You step at .05mm? Is that a typo? I step at .005mm.
Last edited by Robin Hewitt; 01-08-2012 at 11:24 AM.
yes it was a typo...
I now have a working 3D modified/optimised Bresenham run-length algorithm. I interpret the G-Code file (or will do soon) and write the run length data to a file. When you press 'go' it just reads the file and fills the buffer, then keeps it topped up. With over 3Gb of storage available on the SD card I dont need to worry about running out of memory! I added a second 'playback' buffer so once the interrupt handler outputs the steps to the stepper it puts what it just did in the playback buffer. This is then used by the visualisation routines which run at a low priority...
Still got to work on acceleration and deceleration approaches...