Speed regulator for sewing machine

[Kenwood]

Hope to get some help or advice.
I am new to programing and trying to learn. This is also my first post.
I am trying to design a speed regulator for a sewing machine used for quilting. The sewing machine is on an X, Y table and I need to keep a constant number of stitches regardless whether sewing machine moves X or Y or diagonally. My encoders on the tacks give me 55 pulses per inch of travel and I feed these signals into the inputs of the Pic. I take the pulse width of each encoder convert the value back to hertz. I took these values of X and Y and do a hypotenuse calculation on them and take the value and feed a PWM output. The Pic works on the bench using signal generators etc but in practice it has problems. There is far too much time delay during acceleration and deceleration. I.e. it takes .65 second to sense a 0 pulse in. This circuit uses a 4 MHz crystal. I tried 20 mHz and it improved but causes other problem. Thanks

' PULSIN Command
' 16F88
' Serial Pulse Width Meter
        Include "modedefs.bas"          ' Include serial modes
        
        DEFINE OSC 4
        
        
       
SO      VAR PORTB.5                       'Define serial out pin
FI      VAR PORTB.4                       'Define frequency input pin X1
FI2     VAR PORTB.2                       'DEFINE FREQUENCY INPUT PIN Y1
W0      var WORD
W1      VAR WORD
W2      VAR word
W3      VAR WORD
W4      VAR WORD
TRISB = %00010100
          #CONFIG __config _CONFIG1, _HS_OSC & _WDT_ON & _LVP_OFF & _CP_OFF & _CCPMX_RB3 & _CCP1_RB3
          #ENDCONFIG

 displayloop:
         Pulsin FI,1,W0               ' Measure pulse (in 10 uSec)
         Pulsin FI2,1,W1              ' Measure pulse (in 10 uSec)
 
         W2 = 50000 / W0
         W3 = 50000 / W1
         W4 = (W2 HYP W3)               'HYPOTENUSE CALCULATION
        
        If W4 = 0 Then disp             ' If non-zero, display
        Serout SO,N2400,[#W4]
disp:   Serout SO,N2400,[" Hertz",13,10]        ' Display trailer
 HPWM 1,W4,1500                          
        Goto DISPLAYLOOP                 ' Forever

[Demon]

What if you tweak PULSIN_MAX?

(See PBP manual for PULSIN)

If you can't speed up the oscillator, I'd decrease the maximum waiting time for a pulse.

Robert

[Archangel]

So you are using the encoders for speed calc instead of distance . . . have you tried using comparators to shape the pulses for you?

[Kenwood]

Hi Arhangel. Thanks for the reply. Maybe you might be right. the encoders that I have put out very good defined square wave pulses at 55 pules an inch. I expect my maximum travel speed to be 2 inches per second. As I accelerate the PWM output should increase smoothly and it isn't. I am not sure how using comparators would help me? Other people I've talked to have suggested using linear encoders i.e 0 to 5 volts out with increase in speed and feeding the output into the A to D convertors. Just anothe Idea for me to try. Thanks

[HenrikOlsson]

Hi,
I Think it's just a matter of time, so to speak....
If your top speed is 2"/sec I'm going to guess that a typical slow speed is something like 0.05"/sec. At that speed, with 55 pulses per inch, the signals period time is 1/(55*0.05)=363ms, then you're doing that on two "channels" (no, they are not measuring the two pulses simultanously).

There's really not much the software can do about that, it can't update the output of it doesn't get any input and the input is only updated ~1.5 times per second at such slow speed.

Using tachometers is better since they are continous output devices but you may need some preamplifier to boost the output voltage at slow speed - it all depends. And you're going to need a precision rectifier or an external ADC that can measure both positve and negative voltages since the polarity of the output changes with direction of rotation. Changing to higher resolution encoder is another option.

/Henrik.

[Kenwood]

Hi Henrik
Thanks for the feedback. I started this project a number of months ago and just getting back into it. The reason I picked 55 pulses per inch was that the maximum number that the hypotenuse calculation on X and Y was about 127 before it would go into overflow. I am using a 16F88 and I purchased an 18F2550 to give me a larger number to work with. I only have the student version of PBP so that’s why the choices. I haven’t tried the 18F2550 yet as I’m a beginner it looks very complicated.

I don’t know why there is not a smooth rise or fall on the output of the PWM but maybe using the PULSIN is not the way to go.
Using the ADC convertors may be faster responding.

I have lots of options to try. The encoders can put out a maximum of 2048 pulses per revolution or about 1500 pulses per inch. I can also feed them into a DAC chip to give me a voltage out.
You can buy these speed controllers commercially so it can be done but they are a bit too much money for me. May someone out there has one that they can reverse enginner for me?
Thank Jan

[Kenwood]

Hi Demon
Thanks for the reply. I have played with the PULSIN_MAX but as I decrease the value I loose slow speed response. As A beginner I realy don'y understand what is happening. I assume the PULSIN measures the first pulse width and puts that value into a var. I then divide it into 50000 and the do a hypotenuse calculation on the two values of X and Y. I'm guessing it should take 20 or so clock cycles for all this to happen. Does the PULSIN take the next pulse in or maybe the third pulse in and repeat the calculation? I have an LED across the PWM output and I can see a step in brightness as I increase the pulses in where it should be a smooth increase in brightness.
Thanks