Erratic Behaviour with Servo

Zackary

Problem: (See title)

Description: Servo behaves irratically. Sometimes it moves step, step, step, step, but then it goes crazy. Step, step, dies. (Long wait) Step, craziness etc.

Equipment
- PIC16F877A
- Parallax continuous servo
- Coding in assembly (with MPLab)
- 5 V on everything (except the microcontroller)

Could it be a faulty servo? Umm, I was new with servos, so I tapped the signal wire to power (5 V), to see what would happen. Could that have killed it? I also turned the servo by hand. Could that damage the unit?

I should mention my pseudocode:

Initialize
For inifinity
_____For 4 times
_________Pulse servo for 1.25 ms
_________Wait 18 ms
_____Pause for a second
_____For 4 times
_________Pulse servo for 1.75 ms
_________Wait 18 ms
_____Pause for a second
Repeat

Sorry about "hijacking" the other thread. Newbie.

Thanks to Mr. Baker. I'll try it out when I get home.
So if I keep pulsing it at say 1.25 ms with 20ms width, will it move to a constant position or will it rotate·with a constant speed?

Tom Walker

To answer your last question, it should rotate continuously in one direction. For a standard servo, it would move to a particular position, but with a continuous, it should just rotate at a constant rate.

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Truly Understand the Fundamentals and the Path will be so much easier...

rockin_rick

You should be sending a (say) 1.25ms long positive pulse once every ~20ms NON-STOP. If you are pausing for 1 sec between sending pulses then it won't work right. IOW - the servo doesn't latch the pulse width value.

With a modded servo (to continous rotation) then the speed of rotation is determined by the pulse width. A constant pulse width = constant speed. Larger PW = faster constant speed.

Turning it by hand shouldn't be a problem, if the power is off.

Rick

Zackary

Thanks for the responses.
I've tried the suggestions. The pseudocode reads

Loop
____Set servo pin high
____Delay 1.25 ms
____Set servo pin low
____Delay 18.75 ms
____Repeat

It still behaves erratically. (See above)

I find it hard to debug, because
a) I have never had a working servo before, so I don't know what to look for
b) I can't see a pulse that's 20 ms long (and only high for a fraction of that) [noparse]:)[/noparse]

Debugging Attempts
1) I connect the servo pin to an LED
- I know its brightness when at a constant 5 V
- so I assumes it's pulsing when the LED is slightly less bright

2) I also have a counter IC so I can verify that it is pulsing

What I know
- how a servo works (from what I read from http://forums.parallax.com/forums/default.aspx?f=6&m=104381 )
- that the servo I have "works" (i.e. if I tap the signal wire to power, I can make the servo move)

Zackary

Thanks for the responses.
I've tried the suggestions. The pseudocode reads

Loop
____Set servo pin high
____Delay 1.25 ms
____Set servo pin low
____Delay 18.75 ms
____Repeat

It still behaves erratically. (See above)

I find it hard to debug, because
a) I have never had a working servo before, so I don't know what to look for
b) I can't see a pulse that's 20 ms long (and only high for a fraction of that) [noparse]:)[/noparse]

Debugging Attempts
1) I connect the servo pin to an LED
- I know its brightness when at a constant 5 V
- so I assumes it's pulsing when the LED is slightly less bright

2) I also have a counter IC so I can verify that it is pulsing

What I know
- how a servo works (from what I read from http://forums.parallax.com/forums/default.aspx?f=6&m=104381 )
- that the servo I have "works" (i.e. if I tap the signal wire to power, I can make the servo move)

Zackary

If anyone knows PIC assembler, I have·attached·my code.

Thanks in advance!