Hello,


I have a project in which I am making a CNC machine.  I am using stepper
motors, however, I am having problems making the
acceleration/decleration timer values work.  My machine requires 3200
steps to move one inch.  Now, this is what I have done.

I am using Timer0 to handle acceleration and deceleration.  I am using
timer 1 to handle constant speed.  What I want to happen is that
initially the motion begins in timer 0 which is constantly adjusted
until timer 0 values are equal or greater that timer 1 values.  Then,
turn off timer 0 and turn on timer 1 for the constant maintained speed.
Then when the motion is about over, timer 1 will disable itself and set
a flag for timer 0 to indicate a deceleration and then enable timer 0.

First, my question is how can I determine the constant acceleration
value given an acceleration rate of 30"/sec^2.  I mean what value do I
add to the timer 0 registers on each interrupt to achieve a smooth
acceleration from 0" to say 5"/sec with a give acceleration rate?  I
can't seem to get this part to work at all.

Here is the code that I have tried.

;My variables are
ACCELRATE_L
ACCELRATE_H             ;This is the constant to add to the current
acceleration timer 0 values.
ACCELMOTION_L
ACCELMOTION_H         ;These are the current timer values each time
timer 0 interrupts.
MOTIONRATE_L
MOTIONRATE_H           ;These are the desired speed to accelerate up to.
These are use to set timer 1 for constant rate.

Now, lets say that I want to have a constant speed of 5"/sec that would
make my MOTIONRATE LOW AND HIGH =
(0xffff - (.10000000/(.5*3200)))  remembering that 1" inch is 3200
pulses.  This makes it equal to 0xFD8E.  Is this correct?

My device is running at 40MHz so Focs/4 = .10000000 as shown above.

So I set MOTIONRATE_L = 0X8E and MOTIONRATE_H = 0XFD.

For an acceleration I initially set ACCELMOTION_L and ACCELMOTION_H =
0x00 because we are starting from 0" /sec

QUESTION:  What should I set ACCELRATE_L and ACCELRATE_H to for an
acceleration rate of 30"/sec^2? How do I calculate this value?

Such that my timer 0 routine would look something like this:

BTFSS              ACCELFLAGS, ACCEL_ACTIVE             ; Are we
accelerating or decelerating?
BRA                 Decelerate
; Ok we are accelerating.

MOVF               ACCELRATE_L, W, BANKED
ADDWF            ACCELMOTION_L, F, BANKED
MOVF               ACCELRATE_H, W, BANKED
ADDWFC          ACCELMOTION_H, F, BANKED
; Check to see if Rate is >= to desired rate
MOVF               ACCELMOTION_L, W, BANKED
SUBWF            MOTIONRATE_L, W, BANKED
MOVF               ACCELMOTION_H, W, BANKED
SUBWFB          MOTIONRATE_H, W, BANKED
;Is new value greate than desired constant speed?
BTFSS              STATUS, C, A                           ; A low on C
indicates a borrow.
BRA                 AccComplete                             ;Yes, then
we are done accelerating.
; No, then load need rate into timer 0 and take motion set for desired
axis.
MOVFF             ACCELMOTION_H, TMR0H
MOVFF             ACCELMOTION_L, TMR0L
BRA                 TakeMotionSteps                       ; Branch to
step routine

I have tried using a constant like 0x04ff for the acceleration rate, but
it does not seem to work because for the first second of motion, the
motors appear to only move at a fixed slow rate, then just jump to full
speed.  It does appear however, that the deceleration routine works.????
It code is the following:

;Make sure we are in an active deceleration
BTFSS  ACCELFLAGS,DECEL_ACTIVE,BANKED
RETFIE FAST      ;This condition should not happen.
                                                  ;Timer 0 is for
accel/decel only.
MOVF               ACCELRATE_L,W,BANKED
SUBWF            ACCELMOTION_L,F,BANKED
MOVF               ACCELRATE_H,W,BANKED
SUBWFB          ACCELMOTION_H,F,BANKED
BTFSS              STATUS,C,A                             ;If result is
a borrow then we are done.
BRA                 DeccComplete
;Check for < than desired rate
MOVF               PREVIOUSRATE_L,W,BANKED
SUBWF            ACCELMOTION,W,BANKED
MOVF               PREVIOUSRATE_L,W,BANKED
SUBWFB          ACCELMOTION,W,BANKED
BTFSS              STATUS,C,A     ;If negative, then make equal.
BRA                 DeccComplete
;Check for equality
MOVF               PREVIOUSRATE_L, W, BANKED
CPFSEQ           ACCELMOTION_L, W, BANKED
BRA                 DecelContinue
;Not equal so continue.
MOVF               PREVIOUSRATE_H, W, BANKED
CPFSEQ           ACCELMOTION_H, W, BANKED
BRA                 DecelContinue
; Not equal so continue.
;Ok values are equal so complete the routine and turn off timer.
MOVF               A_ARG_L,W,BANKED
IORWF             A_ARG_H,BANKED                   ;Equal?
BTFSC              STATUS,Z,A
BRA                 DeccComplete

DecelContinue:
;Otherwise, move new rate into timer zero, then CALL step routine
MOVFF             ACCELMOTION_H,TMR3H
MOVFF             ACCELMOTION_L,TMR3L
BRA                 TakeMotionSteps

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