Stepper motor damping

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   I have an old futurelight 'Miracle' light, a fairly straightforward effects projector with a pair of gobo wheels, colour wheel, a 575 lamp and nice wide angle optics.

It has developped a fault with the rotating gobo wheel which is a 5 position wheel for D size gobos. Rotation is the standard set up with a free wheeling central gear mounted on the shaft of the stepper which selects gobo positions. this freewheeling gear is then driven by another gear mounted on the gobo rotation motor.

The problem is with the gobo select. initial position resetting is achieved by driving the motor into a mechanical stop and this works correctly.  gobos can be selected correctly if the motor is moved quickly but if the position is selected slowly (moving the control fader slowly) then the  motor intermitantly jumps and dithers and loses position. There is a mechanical damper fitted, The motor shaft extends from the rear of the motor and on this is mounted a disc with a grub screw through its face which then holds a spring loaded nylon (?) plunger which applys friction to the surface of the motor body. Adjusting the grub screw applys greater or lesser friction and this is the mechanism by which I believe this jittering should be controlled.

I have adjusted this friction brake, I have replaced the drive motor and control PCB. I have tried running the system without the gobo rotate driver gear in place. I have tried swapping the rotating wheel with the static wheel (smaller and much less mass) and still the situation persists (although to a lesser extent). I'm at a loss to know what else to try - Anyone have any ideas?

This is an old unit and it has had bits swapped around from other units over the years...

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imageoven Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 28 March 2007 Location: Scotland Status: Offline Points: 2165	Post Options Post Reply Quote imageoven Report Post Thanks(0) Quote Reply Topic: Stepper motor damping Posted: 01 March 2019 at 5:08pm
	I have an old futurelight 'Miracle' light, a fairly straightforward effects projector with a pair of gobo wheels, colour wheel, a 575 lamp and nice wide angle optics. It has developped a fault with the rotating gobo wheel which is a 5 position wheel for D size gobos. Rotation is the standard set up with a free wheeling central gear mounted on the shaft of the stepper which selects gobo positions. this freewheeling gear is then driven by another gear mounted on the gobo rotation motor. The problem is with the gobo select. initial position resetting is achieved by driving the motor into a mechanical stop and this works correctly. gobos can be selected correctly if the motor is moved quickly but if the position is selected slowly (moving the control fader slowly) then the motor intermitantly jumps and dithers and loses position. There is a mechanical damper fitted, The motor shaft extends from the rear of the motor and on this is mounted a disc with a grub screw through its face which then holds a spring loaded nylon (?) plunger which applys friction to the surface of the motor body. Adjusting the grub screw applys greater or lesser friction and this is the mechanism by which I believe this jittering should be controlled. I have adjusted this friction brake, I have replaced the drive motor and control PCB. I have tried running the system without the gobo rotate driver gear in place. I have tried swapping the rotating wheel with the static wheel (smaller and much less mass) and still the situation persists (although to a lesser extent). I'm at a loss to know what else to try - Anyone have any ideas? This is an old unit and it has had bits swapped around from other units over the years...
	Keep pushing on, things are gonna get better.

csg Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 17 September 2007 Location: bedford Status: Offline Points: 6047	Post Options Post Reply Quote csg Report Post Thanks(0) Quote Reply Posted: 10 March 2019 at 10:33pm
	have you checked each winding on the motor for continuity? sounds like you may have a coil out. If that is OK, may be worth checking that the stepper motor is able to move as freely as it should ( they always have a certain amount of resistance as i'm sure you know, but easy to cross check with a known good motor) - as it happens I'm in the process of servicing all our nova light novaflowers prior to sale - a couple of which had erratically behaving colour flags which turned out to be stepper motors that needed stripping and cleaning of the rotors to enable free rotation of the shaft.
	“The fact is this is about identifying what we do best and finding more ways of doing less of it better”

imageoven Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 28 March 2007 Location: Scotland Status: Offline Points: 2165	Post Options Post Reply Quote imageoven Report Post Thanks(0) Quote Reply Posted: 29 March 2019 at 9:39am
	Sorry Chris, missed your reply. Yes windings are ok, I measure 30 ohm on each winding right back to the PCB (I was worried one of the wires in the loom to the motor might be cracked as that would explain why swapping the motors out didnt solve it) The motors all feel good and I stripped them all down and rebuilt them just in case, nothing gritty or stiff about them. At some point in the past I swapped the gobo holding gears (with ones from a PSD 200 promotion scan) so I could use clip in gobos rather than the origional bolt in gobos. It all runs smoothly enough but the only thing I can think to try now is to swap the old gears back in again....
	Keep pushing on, things are gonna get better.