Robotic light stepper motor fault. - Page 1
Forum section: electronics / projects

author: ceecrb1
author_rank: Contributor
timestamp: December 16, 2012, 09:54:36 pm
content: Ok, its time to bite the bullet and swallow my pride and as for help!!!I have a profesional robotic moving light on my bench that has been there for quite a while.... and its fault is really getting to me!!!The stepper motors (2) for the pan function are not working properly... It will lock itself in place when not ordered to move. Ie the brake function is working, but when ordered to move, be it during the reset stage or by the lighting desk etc... the motor just "goes limp". There is no physical resistance, no juddering no nothing.. Its as if it is powered down.I am trying to not give too much information incase I sway opinions and say "it cant be that". I am willing to admit I may have overlooked something or mis-read something.Where I am really lucky is that the tilt circuitry and motors are identical so I can move components between the two.. I also have a spare old and broken mother board (due to other faults)... which I have been using for replacements.. NOTHING I do is making any difference.There are 3 wires going to this motor... and one of these seems to be low (been a week or so since I last looked at it so cant remeber numbers) but there are no pulses showing on the scope as in with others.Ok comments, ideas etc.Doing this at home, after leaving it a few weeks to clear the hea. Tomorrow I can add more info, fotos etc

author: David_AVD
author_rank: Super Contributor
timestamp: December 17, 2012, 07:00:31 am
content: Broken (intermittent) wiring harnesses are reasonably common in moving lights.Also, faulty driver chips. Chips can die due to a sticking motor.What brand / model?

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2012, 09:39:51 am
content: Trace the stepper wires back to the driver and check its power supply.Braking is usually done by shorting the motor coils so it still works without power.

author: David_AVD
author_rank: Super Contributor
timestamp: December 17, 2012, 09:08:39 pm
content: Every moving light I've ever seen energises the winding to brake. Turning the unit off allows every axis to move freely in my experience.

author: ceecrb1
author_rank: Contributor
timestamp: December 17, 2012, 10:08:30 pm
content: Its a martin mac 2000 performance.The problem with this one is that the pan and tilt functions do not have quick-replacable drivers.. but a more complex circuit built directly on the PCB.Dont mean to sound cheaky.... but if it were the wiring loom or motor, I would have found that in minutes and it would be back in stock ready to use....I'm asking because this one has me really stumped and the oficial technical support being offered is "just buy a new motherboard".... and they cost over 1000 euros... and I work in Spain, where we are lucky if we get paid every month, so no cash free for that!This is a photo of the offending board.What you can see from the left are 3 vertical seccions, one for each phase of tilt... then to its right is the pan section..The most basic and first test was just swap the pan and tilt motor connectors over.. so I KNOW its the board.After going crazy I did repeat the test with a new motor and wire that I had loose on the bench.It originally entered my workshop with some components visibly blown.. all on that first "virtical seccion" on the far left. and all have been replaced using parts from a now unused board, and re-replaced incase those had a fault.

author: Things
author_rank: Regular Contributor
timestamp: December 18, 2012, 12:38:14 am
content: Well if you know all the wiring is fine, then do as people have said before and trace it back to the stepper driver chip. If there was already visible component damage from something else, then possibly the stepper drive chip is also cooked. Sometimes you can't tell just from looking at it.

author: daedalus
author_rank: Regular Contributor
timestamp: December 18, 2012, 12:05:35 pm
content: excuse my ignorance, but from the photo it appears that each motor drive section consists of 3 half bridges, with mosfet drivers above. Surely that would indicate that the motors are brushless, not steppers?1) check the supply voltage to the bridge circuit at the mosfets, in case there is a fault supplying power to the bridges.2) get the datasheet for the row of 3x 8pin soics below J6, these should be the mosfet drivers. Figure out where the input pins and put an oscilloscope on them to confirm that whatever microcontroller is being used is actually commutating the half-bridges. If not trace these lines back towards the micro and test everything along the way.

author: daedalus
author_rank: Regular Contributor
timestamp: December 18, 2012, 12:19:55 pm
content: well I guess you learn something new every day, never heard of 3 wire steppers before. Ignore my brushless comment.

author: ceecrb1
author_rank: Contributor
timestamp: December 18, 2012, 09:34:49 pm
content: Yeah thats why theres no just "swap out the driver" option..Like I said earlier... if it was an easy or obvious fix I would've found it by now.. this is what I normally/regularly do at work..Its just THIS one is causing me a lot of head scratching and the manufacturer is still just trying to sell me a whole new board, which just doesnt seem right to me.option 1, power to the mosfets! yes I may have actually missed that out!TBH I cant remeber because I have left this one on the back burner a couple of weeks incase I get a hit of inspiration.option 2. yeah I'd say that tactic makes senes. I started it but without the data sheet as a basic hunting around the general area... Guess its just time to really get my hands dirty and mind in gear.