Help me troubleshoot a simple robot

[hueydj]

Hello, I am in dire need of some help troubleshooting some odd behavior of a simple machine that I need to get working in order to get my pHd.  The machine was built about 5 years ago by someone who has since graduated and has been in at least operable condition until about 6 months ago.  I am a bioengineering student with very little background in machines, motors etc but I need to get this thing working soon.  The purpose of the device is to move, via a stepper motor with LVDT feedback, linearly over a range of about 1cm with resolution of around 1um.  The motor turns a threaded plastic sleeve with causes a threaded rod to move up and down

So with a little background out of the way I think I will start with some general questions regarding the issues I am having with it.  The problem I am having with it is that sometimes the motor will "bind up".  By that I mean instead of turning smoothly in one direction it will rapidly oscillate CW and CCW and not go anywhere.  If I gently turn the plastic sleeve I can feel that the motor is trying to turn but just doesn't seem able to.  What is the general cause of this problem (does it point to the motor or controller)?  The device is completely unloaded at the time and there is no way the linear motion is being physically hindered by anything. This problem, however, is intermittent and sometimes without me doing anything the device will begin to work properly for some amount of time before stopping again.

I have measured voltages coming out of the motor controller and when the device is working they range from 37-40V but when it is not working they range from 38-40V.
 
Thats all the info I'm going to bombard you with for now.  If you need any more info please ask.  Thank you so much for any help you can give.

[waltr]

My first though is there any lubricant on the plastic sleeve that has dried out?

It does sound more like a mechanical problem than an electronic one.
Does your school have a mechanical/electronic engineering dept that could help you?

[z.s.tar.gz]

Seeing as it is several years old and just suddenly changed, I'd agree with the lubricant theory.

[hueydj]

That was one of the questions I initially emailed to the guy who built the machine.  He said it should not require any lubrication, but I really think it would.  I will give it a try.  But my question would be if it wasn't properly lubricated why would it work sometimes and not other times?

I just started taking a look at some of the currents and voltages supplied to the motor and I noticed something odd.  Although the motor says 5.1V 1.0A, the voltage supplied to each of its leads is 37-40 V and current 0.3-0.8A.  That voltage seems way to high right?

One more observation, traditionally people that have used this machine have noticed the motor, when powered up, runs very hot.  I noticed this too until about 2 weeks ago and now it runs completely cool to the touch.  Any ideas.

Thanks again for the help.  I'll try the lubrication idea and let you know.

[z.s.tar.gz]

Things running hot and strange readings is usually a wiring problem, which can be very unpredictable in nature.

That would be the second thing I'd look into.

[SmAsH]

Well, with voltages 8x what they should be the motor would run insanely hot, maybe even melting the coils!
I really would think that is the problem in this...

[z.s.tar.gz]

Didn't think of that... Wiring is 3rd for testing then.

[Soeren]

Hi,

Take a look at the power supply - particularly the large capacitors, it sounds like they're the problem.

If they bulge at the top (or ends if they are mounted lying flat) they need to be replaced.
Even if they don't bulge, you should measure them for capacity and ESR and since you probably haven't got a clue as to what I'm talking about, perhaps you should just replace them anyway.

If the power supply is a separate item, test it without anything else connected.
Such large voltages might have damaged other components, but changing the caps will be the first line of defence here IMO.