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Author Topic: considering a custom servo power stage - suggestions, ideas, yay, nay?  (Read 2547 times)

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Offline izuaTopic starter

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I'm experiencing problems with hacking a servo. I wasn't really glad with the idea of hacking a device for another purpose, since I know hacks won't always work. And they didn't. As you read (or haven't :P) in my other post, I couldn't glue the pot right (or maybe i did), nor could get a center with the resistors and with a trimpot, well, same happens. So I believe that since the servo IC is heating, a resistance gets altered, and with it, the control signal. And that's why I can't get a fixed center - so after a bit of work, the 1.5 ms center (or any other value, for that reason) drifts away, making control unreliable. Done with the intro  :-X

So, I'm considering building a custom power level. That's an H-Bridge (4 transistors), a small pic (12f675 or 629), a quartz and probably a voltage stabilisier. The problem is, the pic won't work with more than 5 volts, and the source won't work under 6. So i don't want to add more accus, but I don't want to risk burning the PIC, either.

Now, here's a design problem: should it be inside the servo, or out? Or maybe, just the h-bridge should be in? I'm thinking PCB here, but testboard (vero) should do it, or even soldering components in the air (though it's a bit unsafe). What control method would you use? Would you do it?

Thoughts, suggestions, ideas, let me know.
« Last Edit: February 23, 2008, 02:20:28 PM by izua »
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Offline airman00

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #1 on: February 23, 2008, 07:48:06 PM »
PICs work up to 6 volts

If you are making your own H bridge for the servo then it should be outside it since it might not work, and it'll be easier to debug when it's outside.
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Offline izuaTopic starter

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #2 on: February 24, 2008, 03:53:56 AM »
yes, i was thinking that way, but more wires will be involved and dangling around.. :P

what about control? How would you send data to the motors?
I was thinking i2c, but it's hard to implement on some small PICs. serial might be ok, but it only between two devices, and will have to be adapted if i add more stuff, or i'll have to make several serial ports on the master.
sending a pulse width (like servos do) might be interesting, but i want it to control both motors.
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Offline Ro-Bot-X

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #3 on: February 24, 2008, 06:33:45 AM »
You can use a dual H-bridge circuit like the L293D or the more powerfull SN754410 (1 Amp per motor). In fact, you may be able to use both H bridges in paralel to controll one motor and install everything inside the servo. There is a device like this called "The secret motor driver" from Solarbotics. Check out the docs from here. If you look at the last schematic you will see that with the addition of a tranzistor to invert the logic you can use only one pin from the microcontroller. If you make a small PCB that will go inside the servo, add this tranzistor and a 7805L voltage regulator, you may keep the original 3 wire cable for the servo. And most importantly, you may use Locked Antiphase PWM on the command pin. This works like this: when the PWM duty cyle is 50%, the motor will be braked; when the PWM duty cycle is 0%, the motor will spin full speed in one direction and when the duty cycle is 100% the motor will spin full speed in the other direction.

Another important hack for your servo would be adding an encoder inside the servo. Check out this link and this link to see how can be done.
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Offline gamefreak

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #4 on: February 24, 2008, 11:30:06 AM »
What exactly is the purpose of that motor driver? You still have to open up and modify the servo.

Is it for more current flow? I see that they cut the 4 outputs into 2 in order to double the power output.

« Last Edit: February 24, 2008, 11:32:02 AM by gamefreak »
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Offline izuaTopic starter

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #5 on: February 24, 2008, 02:14:34 PM »
i've opened the servous at least 15 times now :P
yes, the purpose of the power stage is to eliminate the crappy mass produced one.
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Offline Admin

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #6 on: February 27, 2008, 08:06:19 PM »

Offline izuaTopic starter

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #7 on: February 28, 2008, 01:37:25 AM »
that's very cool!
but for the moment, i'm trying to build a simple moving box..
i think that's way too advanced for now - although i can see some uses in CNC robotic arms, with that kind of resolution
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Offline paulstreats

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #8 on: February 28, 2008, 07:16:17 AM »
Quote
What exactly is the purpose of that motor driver?

Its most likely an h-bridge that is tried and tested and works so you dont have to make your own and mess about too much

Offline izuaTopic starter

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Re: considering a custom servo power stage - suggestions, ideas, yay, nay?
« Reply #9 on: February 28, 2008, 09:40:27 AM »
exactly. i initially wanted to make it with 4 pwm-optimised mosfets, but that's why there are chips like l293D or l298
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