Servo troubles.

[Glenn]

Hi, this is my first message, and I hope my problems isnt too stupid..

Anyway, I'm building something with RC-servos and stumbled upon PBP wich looked like an very easy way for me to control them, however, my first little test aint working, and I have no idea why, I given up with my original code and hammered it down to just some testroutine that, in my opinion SHOULD work..

Here is the testcode:

@ device INTRC_OSC_NOCLKOUT,MCLR_OFF,LVP_OFF,WDT_OFF,PROTECT_OFF
'@ device HS_OSC,MCLR_OFF,LVP_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 4

ct var byte

ct = 150

pause 250

low portb.1  ' force the status-LED off.

pulsout portb.0, ct   ' this should place the servo in middle position.

pause 1000  ' Wait a while..

loop:

for ct = 1 to 255 step 1

pulsout portb.0, ct  ' put the value of the counter to the servo.

Next ct

high portb.1  ' turn the status-LED on.

pause 1000 ' Wait..

low portb.1  ' Turn the status-LED off again.

goto loop

..This little code would in my opinion first put my servo in middle position, wait 1000, and then start to step it from one end position to the other. ..Then the status-LED should light up, wait 1000, and then go black again, and after than it should start to move the same again.

..This is not what happens, the servo does NOT enters the middle position, its goes to a position very near one end, then it moves back and forward a few millimeters, the LED acts as excpected.

Why ? ..first I thought that it was the internal oscillator that was the problem so I put a xtal with two caps at pin 15 and 16, and changed the first line to use an external clock, no difference at all.

Any idea ?

..Even more strange is that if I try to set the servo to 150 permanently it goes to one endstop and stops there ?


..I have a feeling that everything kinda works, but are completly " off scale", and I have no idea why.

..Oh, forgot, the pic is an 16F628A and the xtal is 4MHz.

[skimask]

Servo's need to be continuously updated about 50 times per second.
Those pauses are screwing with your servo causing it to lose it's mind.

[Glenn]

Aha.. Hmm, but my original code didnt have those pauses, and still doesnt work..

But I'll change my testcode and remove the pauses and makes it update continuosly.

..Also, if I make a loop that just send 150 to the servo, it stands in one of the nedpositions.. shouldnt that (150) be the middle position ?

[skimask]

Aha.. Hmm, but my original code didnt have those pauses, and still doesnt work..
But I'll change my testcode and remove the pauses and makes it update continuosly.
..Also, if I make a loop that just send 150 to the servo, it stands in one of the nedpositions.. shouldnt that (150) be the middle position ?

If your original code was going too FAST, you might see the same results.
And a middle position of 150 (1.5ms) is almost an arbitrary number...might be 160, might be 140-ish. Depends on the servo and what day of the week it was made...

@ device INTRC_OSC_NOCLKOUT,MCLR_OFF,LVP_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 4
ct var byte : ct2 var byte : ct = 150 : low portb.1
main:
     for ct = 150 to 250 step 1    'sweep one way
          for ct2 = 1 to 50
               pulsout portb.0, ct
               pause 18
          next ct2
     next ct
     for ct2 = 1 to 50    'hang out there for a bit
          pulsout portb.0, ct
          pause 18
     next ct2
     for ct = 250 to 150 step -1     'sweep the other way
           for ct2 = 1 to 50
                pulsout portb.0, ct
                pause 18
           next ct2
     next ct
     for ct2 = 1 to 50     'hang out there for a bit
          pulsout portb.0, ct
          pause 18
     next ct2
goto main

[Glenn]

Hmm, well that works, sort of.. its not centering and its not full to the ends..

I tried my old "center the servo" code with slightly modifications too:

' servocenter
@ device INTRC_OSC_NOCLKOUT,MCLR_OFF,LVP_OFF,WDT_OFF,PROTECT_OFF
DEFINE OSC 4

start:
	pulsout portb.0, 150
	pause 18
	goto start

..Well, this is strange..

The first time I tried it it seemed to work.. but if i moved the servoarm (with power disconnected ofcourse) it gives very strange results.

If its in 180 degrees position, or close to that, it will work, it will center (more or less) the arm and hold it there.. HOWEVER.. if its in another position, it will just hold the current position ??

this is so strange.

[Glenn]

Ok, now I suspected the servo, so I changed the brand new chinese servo to an old one from my box of "old RC stuff", and with two different (working) hitec servos (different models) I get another result, they go to one endpoint and stay there, regardless of the original position ??