Custom Servo

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EpicentrE
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Custom Servo

Post by EpicentrE »

Something I'd really like to be able to purchase from somewhere would be a high-powered board for making a custom servo. For example, imagine something similar to a small single-channel ESC (such as a Fingertech TinyESC), but which had an extra 3 wires for connecting to a potentiometer. After you'd gone through an initial calibration, you could then use a Pololu HP motor with a separate potentiometer as a make-shift servo. This would have numerous advantages:

-You could run it on 3 lipo cells, which is higher than most servos can go, giving extra speed and power.
-The whole assembly would be lighter, as a Pololu HP motor (on 12v) worked into a servo outperforms even 30g+ servos. In many configurations it would also be smaller than a standard servo.
-You could use different types/sizes of motor/gearbox to suit the application, giving you options outside of the small range offered by normal servos. For example, you could use a 298:1 with external gearing to make a very slow, powerful crusher, or a 5:1/10:1 for very quick actuating of a pneumatic valve or a srimech which doesn't need much torque.

I've been researching this for a while, but haven't found anything that fits the bill as of yet. The simplest solution would be to use a Pololu Baby Orangutan to do this, which is something I've tried to do in the past, but it's much bigger than it needs to be and you'd only end up using one of the motor channels, which would be a waste. I'm also a bit wary of using them as I've had quite a few die on me. Another option would be to find a single-channel ESC with a spare pin on the micro-controller and reprogram it, but this is probably beyond my skill level. There's also something called OpenServo, but this would need to be reprogrammed as it normally only runs off serial, and I can't find anywhere that's selling them pre-made (excluding Sparkfun's version which is needlessly huge).

Has anyone experimented with anything like this before? Does anyone know of any other products which I've not mentioned which might be suitable (with or without modification) for doing something like this?

Thanks all!
Scott Fyfe-Jamieson, Captain of Epic Robotics. Champion of AWS38/41/42.
http://www.epicrobotics.co.uk

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peterwaller
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Re: Custom Servo

Post by peterwaller »

I did once start looking at a spring flipper using two HP motors but never really got as far as the electronics.
I decided that the gearboxes were not really up to the forces experienced when overunning the motors.
If you compare the size of the gear teeth with those of a powerful servo you get an idea of what they can do.
I was going to use one of the magnetic encoder chips I built into one of my wall followers that gives 256 step resolution outputing in the form of a PWM signal. (AS5030)

Rapidrory
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Re: Custom Servo

Post by Rapidrory »

I designed a circuit and program for a high powered servo board based on the N2HV ESCs. It would be pretty easy to do, but I would have to use a different microcontroller with analog inputs (The one used in the N2xVs have none) and print another PCB for it, which I could only do at uni. I decided it wasn't really worth going ahead with construction due to low demand compared to the development effort... Might give it a go one day though as my uni course is on feedback systems, which is basically all a servo is, so I could justify it as revision for uni :P Would only be a run of a few prototype boards though... I'll let you know If i ever put any together :L
Rory Mangles - Team Nuts

Robots: Nuts 2 and many more...

NanoTwo Motor Controllers: https://nutsandbots.co.uk/product/nanotwodualesc

EpicentrE
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Re: Custom Servo

Post by EpicentrE »

I've been investigating further options this morning, and was looking at bigger motors, but many of them specify that the gears are likely to strip if you go over 10kg/cm. Does anyone know how much force the Pololu gearboxes can actually take? Has anyone stripped one, and if so, do you know how much force was on it at the time?

I'm thinking the best option would be to use a big external gear reduction as the final stage using mod0.5 or mod1 gears to keep the strain off the (apparently) underspecced motor gearboxes.
Scott Fyfe-Jamieson, Captain of Epic Robotics. Champion of AWS38/41/42.
http://www.epicrobotics.co.uk

razerdave
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Re: Custom Servo

Post by razerdave »

I've not stripped any but the hole in the gearbox face holding one of the gear axles in place has worn out over repeated use (St Jimmy mostly).

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peterwaller
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Re: Custom Servo

Post by peterwaller »

I have manged to strip one but couldn't tell you the load.
On the pololu site there is this warning.
Note: The 1000:1 gearmotors can generate enough torque to damage themselves. The stall torque values given for those two motors are theoretical, and we strongly recommend you avoid stalling them.
If that can happen to the 1000:1 on normal voltage running the lower ratio ones at higher voltages is likely to have the same effect.
I have been trying to justify to myself buying one or the other of these but they aren't cheap.
http://www.hobbyking.com/hobbyking/stor ... c_50g.html
http://www.hobbyking.com/hobbyking/stor ... c_68g.html

Andrew_Hibberd
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Re: Custom Servo

Post by Andrew_Hibberd »

I have stripped a 298:1 HP motor on 2 cells. That was stalled but not for very long.
TEAM GEEK!

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