Simple Servo Position Control Program - Is this right?

[marcusvm]

I have a pretty simple process that I need carried out via a 16F88 and a servo.

I have an arm that has a heating element on the tip of it. I need this heating element to be lowered onto a bed of grass for one second, then lifted out of the grass. I am testing the flammability of grass within a climate controlled wind tunnel. I want this to be triggered from a simple push of a NO button, then have it carry out the whole process from there. Problem is that I have not messed with a servo motor before and I am not sure if I am controlled it in the correct way. The button is on A0 and the servo control output would be on A1. What I have for code is

'Set internal clock settings for 16F88
Define OSC 8
OSCCON.4=1
OSCCON.5=1
OSCCON.6=1

'Turn off A/D conversion
ANSEL=0


'Define variables and pins
I	        VAR 	BYTE
BUTTON		VAR	PORTA.0
SERVO		VAR	PORTA.1

'Continuous loop until triggered by ignition button
buttonloop:

If (BUTTON=0) Then goto buttonloop
	Pause 10

If (BUTTON=1) Then goto positionloop

goto button loop

'Servo position control loop
positionloop:

Pulsout PORTA.1, 100		'pulse to servo for a length of 1 ms to rotate servo CCW certain number of degrees
For I=1 TO 100			'pulse to servo every 10 ms for a total of 1 second
	Pause 1000
NEXT

Pulsout PORTA.1, 150		'pulse to servo for a length of 1.5 ms to move to "zeroed" position

goto buttonloop

End 				'end of program never reached

Is this the correct way of doing it? Any help/suggestions would be great!

-Marcus

[HenrikOlsson]

Hi,
You need to provide the servo with pulses continously if you want it to "hold position", usually at a frequency of 50hz. In your application though it sounds like it would be enough to have it move to position and then "release" it. However, it's still not enough to just give it a single pulse.

There are a couple of problems with your code

Pulsout PORTA.1, 100		'pulse to servo for a length of 1 ms to rotate servo CCW certain number of degrees
For I=1 TO 100			'pulse to servo every 10 ms for a total of 1 second
	Pause 1000
NEXT

This will only send a single a pulse to servo because the Pulsout is outside of the for next-loop. On top of that you are pausing 1000ms (1 second) each time thru the loop. I'd try something like:

For I = 0 to 100
 Pulsout PortA.1, 100
 Pause 19              'Pause 19ms for a total of 20ms which is 50Hz
NEXT

You have the same problem when trying to move the servo back to "zero". You only send a single pulse when you need to send it several in order to move it to position (and keep sending them at 50Hz) to "guarentee" that it stays there.

/Henrik.

[marcusvm]

This will only send a single a pulse to servo because the Pulsout is outside of the for next-loop. On top of that you are pausing 1000ms (1 second) each time thru the loop. I'd try something like:
Code:

For I = 0 to 100
Pulsout PortA.1, 100
Pause 19 'Pause 19ms for a total of 20ms which is 50Hz
NEXT

Thanks for the help. That makes sense now that I think about it. The timing of it was probably the biggest problem I was having.

With moving the servo back to its "zeroed" position- I was thinking that if I send it one pulse to lift it up at the end of the one second test, so that it doesn't sit there until it decides to return to its "zeroed" position on its own. So, with the one pulse I send it at the end, I am helping the servo return to its natural "zeroed" position. Does that make sense?

I am going to do the programing and wiring here in a bit and play with the actual position of the servo in relation to the pulse width. Hopefully all goes smoothly.

-Marcus

[HenrikOlsson]

Does that make sense?

Yeah, kind of. But it won't work if the mechanical load on the servo doesn't happen to be exactly as much as is needed to "coast" back to "zero" with the help of a single pulse.....

I'm pretty sure the servo won't decide to move anywhere on its own. It might move due to external load on it if you don't "refresh" it continously though.

So, when you're moving it back to "zero", do something similar to when you're moving into position, ie. pulse it at least enough times to make sure it reaches position.

Good luck!

[marcusvm]

Ok. I wasn't thinking about the load on the servo. What I think I'll do then is put the pulse that controls the "zero" position in the loop that is waiting for the button to be pressed (so that it is "zeroed" when button is NO), then have the movement pulse in the second loop.

'Continuous loop until triggered by ignition button
buttonloop:
	
Pulsout PORTA.1, 150			'pulse to servo for a length of 1.5 ms to move to "zeroed" position

If (BUTTON=0) Then goto buttonloop
	Pause 10

If (BUTTON=1) Then goto positionloop

goto button loop

'Servo position control loop

positionloop:

		
FOR I=1 TO 100
	Pulsout PORTA.1, 100		'pulse to servo for a length of 1 ms to rotate servo CCW certain number of degrees		
	Pause 19			'pulse to servo every 10 ms for a total of 1 second
NEXT

goto buttonloop

End 					'end of program never reached

Thanks!

[Acetronics2]

Hi, Marcus

this one should work as desired ...

'****************************************************************
'*  Name    : GrassBurner.BAS                                   *
'*  Author  : Acetronics                                        *
'*  Notice  : Copyright (c) 2010                              *
'*          : All Rights Reserved                               *
'*  Date    : 23/12/2010                                        *
'*  Version : 1.0                                               *
'*  Notes   :                                                   *
'*          :                                                   *
'****************************************************************
'
@ __config _CONFIG1, _INTRC_IO & _WDT_ON & _LVP_OFF & _CP_OFF
' DEFINES
Define OSC 8
Define BUTTON_PAUSE 0
' Internal Osc 8Mhz
OSCCON = %01110010
' Turn off A/D conversion - Digital Inputs
ADCON0 = 0
ANSEL = 0
' Comparators Off
CMCON  = 7
 
'Define variables and pins
I         VAR BYTE
Position var WORD
Delay  var Byte
OldButton  var bit
PORTA  = 0
TRISA   = %11111101
BUTTON_  VAR PORTA.0
SERVO  VAR PORTA.1
 
buttonloop:     ' Button check loop
 OldButton = 0
 LOW Servo
 Pulsout SERVO, 300  ' pulse to place servo to neutral
 Pause 19
 
BUTTON BUTTON_,1,255,0,Delay,0,Buttonloop
 OldButton = 1       ' Launched ... We've pressed it !!!
'******************************************************************************
positionloop:  'Servo position control loop
 
I = 0       ' Reset time counter
While ( OldButton  AND ( I < 75 ))   '  if button has been pushed and time < 1.5s
 
 IF I >= 50 THEN     ' 1 second is ... 50 x 20 ms !    
  position = 300
 ELSE 
  POSITION = 200
 ENDIF 
 
 LOW Servo
 Pulsout SERVO, Position  ' pulse to servo 
 
 PAUSEUs 19813 - ( Position * 5 )        ' time between pulses
 
 I = I+1                         'Increase Time counter
Wend                                ' go for seeking a new button push ...
 
goto buttonloop
End         'end of program supposed never reached

may be you'll have to modify a bit the " @ __config ......" line if you use PM as an assembler.

Also correct " IF I >= 50 ..." to i.e. " IF I >= 55 " to take servo positionning time into account ... knowing 1 unit is 20 ms.

Alain