Repairing Servos

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Andrew Cowan

Senior Member
I have a few (three) servos that have faulty circuit boards. I want to repair them by replacing the circuit board with a new (PICAXE) one.

I thought I would use an 08M reading the sinal via pulsin. It would then readadc the position of the potentiometer, then move the motor accordingly, via some driver. I would slow the motor using PWM when the potentiometer gets near its wanted position.

I thought I could also add features such as failsafes (when you first plug in the servo, the incoming signal is stored as the failsafe setting).

The whole project will be done in SMD components.

I would like feedback on:
- Any other features that would be useful (eg EPA?)
- A good H bridge driver for one motor that can provide the current needed (+1A?). Preferably the driver would be for one motor only, so I can keep both costs and size down.

Andrew
 
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Michael 2727

Senior Member
Servos are fairly cheap, why would you bother, unless just to prove it can be done,
it can, but at what cost/time etc. You may end up spending days/weeks in design,
testing and prototyping and still end up with a less efficient, aesthetic looking job.
I'd just put the servos in the spare parts box till another one strips a cog or blows a
motor or the dog mangles a case.
But if it's something you have to do, go for it, good luck :)
 
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BeanieBots

Moderator
I have to agree with Michael 2727.
A 'typical' servo does 90deg rotation in 0.1S. Your code will have to run very fast to be able to CONTROL the movement at that rate. It could spend as much as 20mS waiting for a pulse, then it has to get the pot value, do some complex (ideally PID) maths and then generate a PWM signal.
IMHO, not feasable (code wise) or cost justifiable.
I'd love to be proved wrong though.
 
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Andrew Cowan

Senior Member
Michael 2727 said:
Servos are fairly cheap, why would you bother, unless just to prove it can be done,
it can, but at what cost/time etc. You may end up spending days/weeks in design,
testing and prototyping and still end up with a less efficient, aesthetic looking job.
I'd just put the servos in the spare parts box till another one strips a cog or blows a
motor or the dog mangles a case.
But if it's something you have to do, go for it, good luck :)
Click to expand...
Electronics is what I do for a hobby - I would much rather spend the next few weeks making a PICAXE servo then buying a new one. Where's the fun in that?

Andrew
 
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slurp

Senior Member
Have you seen these?

http://www.solarbotics.com/products/k_smd/

ok, it's not quite the picaxe solution, alternatively you might want one of these...

SI9986 (VISHAY SILICONIX) Buffered H-Bridge 3.8-13.2V Motor Source, 1.0A (1.5A Peak)

MPC17511A (Freescale Semiconductor) H-BRIDGE 2.0-6.8V Motor Source, 1.0A (3.0A Peak)

I like the look of the Vishay device but can't say I've used either yet.

regards,
Colin
 
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boriz

Senior Member
Servo’s make useful geared DC motors when you remove the circuit and the ‘stopper’. Seems a pity to do it though coz servos are so useful. But if you already have a broken one…

I saw this recently (http://www.dealextreme.com/details.dx/sku.12932) and have ordered a couple with the intention of trying them out as geared motors. If they work ok, I don’t see where you can get cheaper. £1.50 each and free delivery. Woot!

If you wanna build a servo from scratch, these would be a good start, assuming they don’t have much backlash. I’ll let you know how well they work after they arrive.
 
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Jeremy Leach

Senior Member
Andrew Cowan said:
Electronics is what I do for a hobby - I would much rather spend the next few weeks making a PICAXE servo then buying a new one. Where's the fun in that?

Andrew
Click to expand...
I agree with those sentiments. I've thought of making a picaxe servo too ... just for the satisfaction and fun of creating a servo system myself ;)
 
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Rickharris

Senior Member
Jeremy Leach said:
I agree with those sentiments. I've thought of making a picaxe servo too ... just for the satisfaction and fun of creating a servo system myself ;)
Click to expand...
A thought I have had, but not yet tried, is to connect the motor to a screw thread - Run a nut on the thread so as the motor turns, the nut goes up and down the thread (a linear actuator). Connected to the nut an LED/Sensor in a tube to measure the light output so as the screw moves the led away the light intensity gets less so you have a feedback mechanism that the picaxe can resolve against the input position signal.

Cheap and mechanically easy to implement. suitable for either direct numerical control or PWM
 
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demonicpicaxeguy

Senior Member
Rickharris said:
A thought I have had, but not yet tried, is to connect the motor to a screw thread - Run a nut on the thread so as the motor turns, the nut goes up and down the thread (a linear actuator). Connected to the nut an LED/Sensor in a tube to measure the light output so as the screw moves the led away the light intensity gets less so you have a feedback mechanism that the picaxe can resolve against the input position signal.

Cheap and mechanically easy to implement. suitable for either direct numerical control or PWM
Click to expand...
not bad idea, works well if you can do a good pressfit of the threaded rod to the shaft, the trick is drilling the right size hole in the end of the threaded rod ,making it centered and a tight fit, all without a lathe,
unless you use one of those couplers

i like those high torque geared motors at jaycar for this
 
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Jeremy Leach

Senior Member
That's a good idea Rick. The sensor output is likely to be very non-linear versus distance, but as long as that could be catered for in a simple code lookup it might work. Not sure it would be very accurate though.

I've had more luck putting a gear onto a threaded rod and driving that gear, rather than directly couple the motor shaft to the rod. Drilling out the centre of a plastic gear a little, so that it can be screwed onto the rod has been easier because the drill bit automatically centres on the centre of the gear. I tried to couple a motor shaft directly with a threaded rod trying all sorts of methods ... I used flexible plastic insulation etc ....but always found the motor had problems turning the rod because of slight misalignment. The torque of the motor wasn't very high and it was quite a long rod. I made a prototype weighing machine that was a servo mechanism .... used slotted disc to count revs of rod, with 08M controller.
 
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Rickharris

Senior Member
Andrew Cowan said:
You mean one of these? http://www.mutr.co.uk/product_info.php?products_id=204

I have got one at home - might be worth experimenting with... Still, it's not very small.

Andrew
Click to expand...

Almost - I have used that product but generally make my own as it is fairly pricey for what you get and the bits/principle is simple.

To connect to motor Shaft I use a short length rubber/silicone tube as an adapter you get self alignment then with no need to drill the rod (although I also have a lathe) . MUTR (see above) also do an adapter plug that converts the "standard" 2 mm motor shaft to a 4 mm which also makes a good connection.

If accuracy were required a linear pot could replace the photo sensor set up. The nice thing here is relatively small and low cost systems can be experimented with, with a minimum of expert construction techniques such as precision gear boxes etc.

In my previous life we used a bulb/photo transistor set up through a rotating plate with a triangular slot in it to control the position of a tape at high speed. The tape loop controlled the plate position - this worked well.

I guess counting an interrupted beam is also an option.
 
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Rickharris

Senior Member
Andrew Cowan said:
You mean one of these? http://www.mutr.co.uk/product_info.php?products_id=204

I have got one at home - might be worth experimenting with... Still, it's not very small.

Andrew
Click to expand...
As far as small goes, I have made the actuator with an M3 bolt with the head cut off and a pager motor. - The compact the length I put a gear on the motor shaft and the screw. This allowed the screw to run along the length of the motor - it wasn't much bigger than the pager motor.
 
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