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Servo Jitter Problem

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tommyr00

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I am having trouble with my servos jittering and dancing. I am a leaving cert student, and the problem is on my Technology project which is due for submission soon. I am using as PICAXE High Powered Board, which uses the PICAXE 18M2 chip. The board is running a futaba s3003 servo motor and an astrosyn stepper motor. As it is secondary school level, my teacher is fully sure of the solution as he is not educated in depth in electronics. The board is programmed using Flowchart on the PIC Logicator Software. The board is using two seperate power supplies, a 9v and a 4.5v. When the servo is in idle it seems to twitch and jump, and when It carries out its command it does not stay still in its position. When i initially turn the board on the servo seems the smack the ground in the opposite direction then goes back to its idle position. The servo also jumps and almost violently when the stepper motor activates.
I have tried researching online but i cant find a definite answer and I am unable to understand a lot of the information as i am relatively new to this.
Any help or advice would be greatly appreciated as this project is worth 50% of my final leaving certificate grade ( In Ireland the leaving certificate is the final set of exams which determine whether you get into college!)
 

gophert

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Most Helpful Member
I am having trouble with my servos jittering and dancing. I am a leaving cert student, and the problem is on my Technology project which is due for submission soon. I am using as PICAXE High Powered Board, which uses the PICAXE 18M2 chip. The board is running a futaba s3003 servo motor and an astrosyn stepper motor. As it is secondary school level, my teacher is fully sure of the solution as he is not educated in depth in electronics. The board is programmed using Flowchart on the PIC Logicator Software. The board is using two seperate power supplies, a 9v and a 4.5v. When the servo is in idle it seems to twitch and jump, and when It carries out its command it does not stay still in its position. When i initially turn the board on the servo seems the smack the ground in the opposite direction then goes back to its idle position. The servo also jumps and almost violently when the stepper motor activates.
I have tried researching online but i cant find a definite answer and I am unable to understand a lot of the information as i am relatively new to this.
Any help or advice would be greatly appreciated as this project is worth 50% of my final leaving certificate grade ( In Ireland the leaving certificate is the final set of exams which determine whether you get into college!)

I just worked with futaba servos and found they jump all over the place if you do not spend enough time thinking about how they work but, with care, they can be controlled.

First option, if you need the servo to move to a position and it does not have to hold anything in position once it gets there, that is the easiest case.

Here, you need to give the servo pulses between 1mSec (Full counterclockwise position) and 2mSec. (Full clockwise position) and about 20millisecond gap between pulses AND KEEP REPEATING THE PULSE until the servo reaches its destination position (about 10 pulses for small moves and 25 for full end-to-end moves.

Once it reaches the destination, it can be ignored -UNLESS - you have other pulses in your circuit that pass through to your servo (i.e interference/cross-talk).

To avoid cross talk, you can power the servo through a mosfet Transistor (use a pin from your PicAx (assuming you have a spare pin.
 

Pommie

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Most Helpful Member
Can you post your code? I'm not famiiar with the picaxe but have used pics with servos with no problem. Does PicAxe use hardware to produce the pulses?

Mike.
 

Mickster

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Here's the board it appears the OP is using:

This appears to be the programming environment:

As knowledgeable members above suggest, seeing the code is vital in order to help.
Time and time again, the most-relevant information has to be almost pried out of the clutched hands of those seeking help, as if it's some kind of state secret. If you want help, give us everything you have that relates to the problem.
 

Pommie

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That chip (PicAxe 18M2) is allegedly multitasking. It's a pic chip so the only way to multitask is by time slicing. I'd guess this time slicing is messing up the pulse timing.

Mike.
 

Beau Schwabe

Active Member
How far are the servo's from the micro? Over 12 inches and considering the noise in your environment (i.e. "The servo also jumps and almost violently when the stepper motor activates ") , you might need a diode/capacitor filter on the power supply as close as you can to the servo.
 

alec_t

Well-Known Member
Most Helpful Member
The servo also jumps and almost violently when the stepper motor activates.
One way that interference can get through to the MCU and mess things up is via the ground connection. Heavy current pulses caused e.g. by motor operation cause a voltage drop across any conductor to/from the motor. If this conductor is shared by the MCU then that drop may affect the MCU. A way round this is to use a "star ground" system. (Look that up if you are unfamiliar with the term).
 
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