Stepper detent, or holding torque for an indexer

[Art]

I need to increase the detent strength (or holding torque) on the A axis of my rotary indexer. I am running a 920 oz stepper with a 4.5:1 ratio achieverd by external gearing. As a side issue I suspect that I may replace the stepper with one that has 5:1 or 10:1 gearing to improve resolution.
Rotation and stop is fine but I need to increase the holding power when the work is stationary. I looked at using an electric brake but turning it on and off become a real barrel of snakes.
From my understanding of 2 phase steppers they are electricaly detented by appling voltage on both phases and then movement is acomplished by dropping voltage to one phase and increasein amperage to the other phase. It seems to me that I could acomplish what I want by increasing the amperage applied at the detented mode. How can I acomplishe this?

[Richards]

Art,
I think you're being bitten by the "reduced current" bug. Most stepper motor drivers reduce the motor's current when the motor is idle (about one second without receiving step pulses and that current reduction can be anywhere from 30% to 50% of the full current value). So, if the step pulses are continous, the driver would never enter the "reduced current" mode.

One way to do that is to pulse the motor one step CW, pause for about 1/4-second, then pulse the motor one step CCW, pause for about 1/4-second and repeat. I doubt that you would notice the back and forth motion with a 4.5:1 gear ratio, since that configuration would give you 9,000 steps per revolution.

Although the idea is simple, coordinating the "standby" pulses with the "active" pulses would probably require you to use a microprocessor inline with the step and direction signals. The program would be some kind of 'missing pulse detector'. A $5.00 Atmel or PIC brand processor, when programmed correctly, would certainly be sufficient to do the job.

The one big drawback in using this kind of solution would be much higher temperatures for the stepper motor and the stepper driver. (Lowering the current reduces the temperature significantly.)

[Gerald D]

Art, I suspect that your A-axis is never required to drive fast or hard - well, not as "hard" or as "fast" as your other axes. If you are using G201/202 drives, I would be tempted to change the "Automatic Standby Disable" jumper inside so that reduced current mode is switched off. If the motor gets too hot after that, then experiment with the external resistor to drop the current. This will mean more holding torque at standstill, but less torque at working speed.

[Richards]

Gerald, that is a much better solution than adding some fairly complex circuitry. I was almost going to suggest the same thing, but I assumed that with a 920 oz*in motor, that the current rating would be much higher than the 2A maximum allowed when by-passing the reduced current mode.

[Gerald D]

Mike, if he has the 203V drive then he doesn't have the jumper option - I don't think.

Electromagnetic brake gets my vote. At the simplest, drive it with an M-Code in the program (same as switching a dust collector on/off).

[Richards]

Gerald, it took me a little while to find some electromagnetic brakes for stepper motors. Here is one site that lists them:

www.anaheimautomation.com/brakes.htm

The model for a 34 frame size motor costs about $110 (1,152 oz*in model). Note that this model is ON (brake on) until power is applied. So it might also be a good unit to use on the Z-axis to keep the Z-axis from falling when power is off.

[Gerald D]

Art, remember that worm gearbox that I suggested? That will give you lots of "detent".

[smreish]

...and if you go high enough....40:1, it's "close to" self locking as long as it's a 90 degree worm gear. No brake should be necessary. On a similar note, I have used the WARN electrical face brakes on fincor servo systems and they work well. The only draw back I found was the brake had significant hsyterisis and often lagged about 30-80ms for ON times...off was very quick. Just my 2 cents.

Sean

[Gerald D]

Art wouldn't need a totally self-locking gearbox because the motor detent will still be there. Guess that his motor won't budge from about 20:1 and higher . . . ? Anyway, this is one of the reasons that real indexers mostly use worm gearboxes.

[Art]

The Gecko 202s that I am using have a simple solution. Under the cover there is a jumper called Automatic Standby Disable. Normal is with the jumper installed then the voltage is dropped to 30% 1 second after motor movement ends. Remove the #%@#@# jumper. The motor will get hotter jut not enough to harm anything per Gecko.

[Art]

I have had the machine powered up for 24 hr and the stepper is not too hot to hold and works great!

[Robert M]

Hi Art,
Hopping you having a ball !!?
I’m slowly progressing toward my goal of a MM with an indexer integrated
When I think indexer, I think ART !!
I would much appreciate an update on your holding torque dilemmas/situation.
Last we spoke, If I remember correctly, you mention a different drive could be a better solution ?
I’m not there yet, but soon to be and I’m still not quite certain witch venue I should take and how to cope with this indexer holding torque & drive situation ?
Thanks, Robert

[Art]

I went with a Chinese drive from Bob Campbell, http://campbelldesigns.net/. I don't know the brand name but it solved the problem.

[Robert M]

But, why this Chinese stuff if the Geco 202 was :

Originally Posted by Art

I have had the machine powered up for 24 hr and the stepper is not too hot to hold and works great!

I remember from our conversation you suggesting to go with this Chinese one, but can't remember why not the 202 or other solution evoked in this thread :

Originally Posted by Gerald D

Electromagnetic brake gets my vote. At the simplest, drive it with an M-Code in the program (same as switching a dust collector on/off).

Originally Posted by Richards

electromagnetic brakes for stepper motors. Here is one site that lists them:
www.anaheimautomation.com/brakes.htm

How do others w/indexer manages? Nothing against this other Chinese drives, just like to understand better & improve my knowledge.
Thanks for your input!

[Art]

Gecko apparently has problems with "large drives" per several posts on the Gecko forum. Bob offered the use of the Chinese drive as a test. Don't know, or particulary care about the details. Just know problem went away. Check general section, just posted a major find.