Multi-turn servo.

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surfdabbler
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Joined: Sat Mar 14, 2009 12:29 pm

Multi-turn servo.

Post by surfdabbler » Sat Mar 14, 2009 12:43 pm

I'm in the process of hacking some standard servos and regearing them to give some multiturn servos. I've removed the pot and geared it down about 16 times, so where the standard servo used to do 90 degrees operation, it now does 4 full turns.

Problem is, the circuit is now wrong, and the motor slows down about 1 full turn before reaching the end point, which means it doesn't get enough power to push hard enough right to the end of it's range.

I've just found about OpenServo, and I have some questions...

1) Will OpenServo fix this for me, and allow my servos to drive all the way to the end?

2) Will Open Servo run on 6V? I saw something somewhere that said it needed a higher voltage supply than a standard servo, which is no good for my project.

It looks like all up, for 2 servos, OpenServo will probably cost around AUD$150 to buy the board and programming kit, so it's pretty steep, and I want to know if it will work for my application before committing.

jharvey
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Post by jharvey » Sun Mar 15, 2009 4:17 pm

I'm fairly new to Open Servo, but this is my understanding at this point in time.

Open Servo has PID parameters, that can be tuned specific to a Servo. These parameters are the problem your facing with now relative to how fast it gets to point B. In Open Servos wiki, search for PID. There are some notes there about how it's tunned.

About the voltage, I see Vbat goes to a couple chips, one being a Vreg chip. My suspicion is that it can handle normal voltages, but I'd have to look at the chips directly connected to Vbat before I could tell for sure.

ginge
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Post by ginge » Mon Mar 16, 2009 1:36 am

Hi Guys, and welcome to you both!
1) Will OpenServo fix this for me, and allow my servos to drive all the way to the end?
You can run the OpenServo without a pot by soldering 2 fixed reference pots into the holes. This allows you to run the motor continuously with a given speed.

Yes, PID is the correct thing to be looking for here. Correctly tuned PID variables will allow you to tune how much it slows before it finally stops, and also stop overshoot/jitter.
2) Will Open Servo run on 6V? I saw something somewhere that said it needed a higher voltage supply than a standard servo, which is no good for my project.
Yes and no. They will not run directly from 6v, as the regulator needs at least +5v + 1.2V to operate. You could remove the regulator and power directly from 5v, if you have that handy.
jharvey wrote: About the voltage, I see Vbat goes to a couple chips, one being a Vreg chip. My suspicion is that it can handle normal voltages, but I'd have to look at the chips directly connected to Vbat before I could tell for sure.
The VBat goes to the FET switching, the power monitoring, current sensing and the regulator. None of these are really dependant on any particular voltage input. I have run the v3 on 9v before now, and althoug it got a little toasty, it did work.
What you need to worry about is the regulator. You could try replacing it with a very low dropout type (0.7v) but you will be very close to browing out the circuit when the motor is loaded.

ginge

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