Stuttering Servo

I am testing a DT Instant interrupt routine which is supposed to set the length of time that two servos are on or off. Everything works OK except the servos runs very rough. The average position of the servos is correct, but the twitching movement is excessive, making it unuseable. The timing is based on the Elapsed Timer1 Program, from routines found on this Forum. For comparison purposes I have another routine that does not use interrupts, but relies on cycle counting. With this program the servos are smooth and quiet. The cycle counting demo follows here:

Code:

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'device is PIC 16F684

'set registers
        OPTION_REG.7 = 0    'enable porta pullups   
        CMCON0 = 7          'comparators off
        ANSEL = 0            'porta pins all digital
        TRISA = %111111      'set porta pins as input
        TRISC = %001101      'set portc pins 0,2,3 as input, all others output
        OSCCON = %1110000    'set internal osc. at 8 mhz, wait for stable freq.
        OSCTUNE = 0          'internal osc. running at factory calibration

        DEFINE OSC 8        '8 Mhz oscillator

        while OSCCON.2 = 0  'wait for osc.to be stable
        wend

'declare variables
        i      var word        'couting variable
        led    var portc.3      'led output on portc.3
        dummy  var byte        'variable needed to end porta mismatch
        throttlepos var word    'servo1 pulse width in .01 seconds
        servopos    var word    'servo2 pulse width
        pulsecnt    var byte 
start:
        low led      'flash red LED for 0.5 sec to show circuit is functioning
        pause 500
        high led 

        pause 2000  'delay timer start 2 seconds
       
        portc.1 = 0              'turn off servo
        portc.4 = 0              'initialize servo pulse low
        portc.5 = 0              'initialize throttle pulse low
        throttlepos = 400        '2*(200)  pulse width adjusted for 8 mhz osc
        servopos = 400            '2*(200)  pulse width adjusted for 8 mhz osc

        low Led                    'turn on led
        portc.1 = 1                'turn on servos
        for i = 1 to 500          'run servo for 10 seconds (500 cycles)
        pulsout portc.5,throttlepos               
        pulsout portc.4,servopos
        pause 16
        next

        high Led                  'turn off led
        throttlepos = 200        '2*(100)  pulse width adjusted for 8 mhz osc
        servopos = 400            '2*(200)  pulse width adjusted for 8 mhz osc

        for i = 1 to 75          'run servos for 1.5 seconds (75 cycles}
        pulsout portc.5,throttlepos               
        pulsout portc.4,servopos
        pause 16
        next

        low Led                  ' turn on led

        for i = 1 to 750          'run servos for 15 seconds 750 cycles)
        pulsout portc.5,throttlepos               
        pulsout portc.4,servopos
        pauseus 16100
        next

        high led              'turn off led

DT:    high portc.3          'turn off led
        servopos = 200        'DT release position 2*(100)
        pulsecnt = 75        'hold for 1.5 second
        gosub pulse          'this section uses a subroutine to run the servo
        servopos = 400        'home  position
        pulsecnt = 50        'hold for 1 second
        gosub pulse          'return to home position
        goto halt

'subroutine pulse runs the servo 
pulse:  portc.4 = 0                'initialize servo pulse low   
        for i = 1 to pulsecnt
          pulsout portc.4,servopos
          pause 18
        next
        return
halt:
        portc.1 = 0              'turn off servo
        end

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The demo interrupt program is follows here. Two interrupts are used in this case, but it makes no difference; the servos runs rough even when the interrupt on change is taken out. The twitching is still there even if only one servo is used. Any ideas on why the rough running?

Code:

---

'device is PIC 16F684

'set registers
        OPTION_REG.7 = 0    'enable porta pullups   
        CMCON0 = 7          'comparators off
        ANSEL = 0            'porta pins all digital
        TRISA = %111111      'set porta pins as input
        TRISC = %001101      'set portc pins 0,2,3 as input, all others output
        OSCCON = %1110000    'set internal osc. at 8 mhz, wait for stable freq.
        OSCTUNE = 0          'internal osc. running at factory calibration

        DEFINE OSC 8        '8 Mhz oscillator

        while OSCCON.2 = 0  'wait for osc.to be stable
        wend

'declare variables
        i      var byte
        led    var portc.3      'led output on portc.3
        MTtime  var word        'target motor runtime in tenths of seconds
        BKtime  var word        'target brake time in tenths of seconds
        DTtime  var word        'target DT time in tenths of seconds
        dummy  var byte        'variable needed to end porta mismatch
        throttlepos var word
        servopos    var word
        pulsecnt    var byte 
start:
        low led      'flash red LED for 0.5 sec to show circuit is functioning
        pause 500
        high led 

        pause 2000  'delay timer start 2 seconds
       
INCLUDE "DT_INTS-14 for F684.bas" 'Base interrupt with wsave selected for 16F684
INCLUDE "ReEnterPBP.bas"          'Using PBP interrupts
INCLUDE "Elapsed_INT_RI2.bas" 'Elasped timer counting seconds and tenths seconds

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler  RAC_INT,  _RDT,          PBP,  yes
        INT_Handler  TMR1_INT,  _ClockCount,  PBP,  yes
    endm
    INT_CREATE            ; Creates the interrupt processor
ENDASM

goto overint

'-------Interrupt Handler for RDT------------
RDT:
  high led                      'turn led off
  dummy = porta                'needed to end mismatch for porta
  IOCA.4 = 0                    'disable porta.4 interrupt
@ INT_RETURN
'--------------------------------------------

overint:

    portc.1 = 0              'turn off servo
    portc.4 = 0              'initialize servo pulse low
    portc.5 = 0              'initialize throttle pulse low
    throttlepos = 400        '2*(200)
    servopos = 400            '2*(200)
    dummy = porta            ' initialize dummy variable
    IOCA.4 = 1                ' enable porta pin 4 for interrupt on change
    MTtime = 100              ' 10.0 second motor run time
    BKtime = 15              ' 1.5 second brake time
    DTtime = 150              ' 15 second glide time
    gosub ResetTime          ' Reset Time to  0d-00:00:00.00
    gosub StartTimer          ' Start the Elapsed Timer

@    INT_ENABLE  RAC_INT      ; Enable PortA Change Interrupt
@    INT_ENABLE  TMR1_INT      ; Enable Timer 1 Interrupts 

        low Led                    'turn on led
        portc.1 = 1                'turn on servo
main: 
            pulsout portc.5,throttlepos               
            pulsout portc.4,servopos
            pause 17
        if IOCA.4 = 0 then
        goto newtime1              'jump to end-time on interrupt
        endif
        if sum => MTtime then      'stop when target end time is reached
        high led                  'turn off led
        goto newtime1
        endif
goto main                      'loop until endtime is reached
   
newtime1:
        gosub StopTimer          ' Stop Timer1
        gosub ResetTime          ' Reset Time to  0d-00:00:00.00
        gosub StartTimer          ' Start the Elapsed Timer
        throttlepos = 200        ' 2*(100)
        high Led                  'turn off led
        portc.5 = 0              'initialize throttle pulse low
   
brake: 
            pulsout portc.5,throttlepos               
            pulsout portc.4,servopos
            pause 17
        if IOCA.4 = 0 then
        goto newtime2              'jump to end-time on interrupt
        endif
        if sum => BKtime then      'stop if elapsed time = target end-time
        low led                    'turn on led
        goto newtime2
        endif
goto brake
   
newtime2:
        gosub StopTimer   
        gosub ResetTime          ' Reset Time to  0d-00:00:00.00
        gosub StartTimer          ' Start the Elapsed Timer
        IOCA.4 = 1                ' re-enable porta pin 4 for interrupt on change
        low Led                  ' turn on led
        portc.5 = 0              ' initialize throttle pulse low
        portc.4 = 0              ' initialize servo pulse low 
   
glide:
            pulsout portc.5,throttlepos               
            pulsout portc.4,servopos
            pause 17
        if IOCA.4 = 0 then
        goto DT                  'jump to end-time on interrupt
        endif
        if sum => DTtime then    'stop if elapsed time = target end-time
        high led                  'turn off led
        goto DT
        endif
goto glide

@    INT_DISABLE  RAC_INT      ; Disable PortA Change Interrupt
@    INT_DISABLE  TMR1_INT      ; Disable Timer 1 Interrupts 

DT:    high portc.3          'turn off led
        servopos = 200        'DT release position
        pulsecnt = 75        'hold for 1.5 second
        gosub pulse
        servopos = 400        'home  position
        pulsecnt = 50        'hold for 1 second
        gosub pulse          'return to home position
        goto halt

'subroutine pulse runs the servo 
pulse:  portc.4 = 0                'initialize servo pulse low   
        for i = 1 to pulsecnt
          pulsout portc.4,servopos
          pause 18
        next
        return

halt:
    portc.1 = 0              'turn off servo
    end

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What does the pulse train look like on an oscilloscope?

Good suggestion, but alas, I do not have a scope.

Could be anything from a weak power supply to weak code ;)
Try a simple code to run the servos without the "fancy" stuff to help narrow it down.

Hi,

@ first look the problem comes because PULSOUT commands might not be " interrupted " ... and maybe also because time between two pulse must be kept, say, between 15 and 30 ms ...

Alain

Hello all,

I have tried a minimalist aproach. With only one servo, holding at one position, and no interrupt on change ... just the timer1 interrupt keeping track of elapsed time. The one servo is still very twitchy to the point of not being useable.

BTW, the accuracy of elapsed time is very good when checked with a stopwatch..

Ok
Show us the current test code and tell us about the hardware ( power supply, servo models, how it is all connected, etc )

As mentioned above, the servo runs smooth and quiet when driven by the non-interrupt routine. So, I think the servo and circuit are OK.

However, I now realize that timer1 interrupts are occuring every 10 ms, while the servo pulses are only 1 to 2 ms long. Just guessing, but it seems that timer interrupts that periodically occur during the pulse on-time can signifigantly disrupt the pulse period and cause jittering. Therefore, I think my interrupt-timed approach will not work.

Experiment over.

Quote:

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Originally Posted by Dick Ivers

Good suggestion, but alas, I do not have a scope.

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Not a problem! Capture the signal train on your sound card, and see it thru' a sound editor like Audacity as described on Dave Houston's website. Or alternately, for these frequencies a sound card based scope like the one at http://www.zeitnitz.de/Christian/scope_en (my favourite) would work fine. WYSIWYG!

Just remember to scale down the logic levels by 10:1 or more.

Regards,

Anand