Low cost / high speed / high torque stepper solution

[richards]

--- For Do-It-Yourselfers only ---

This morning while I was looking at an Oriental Motor PK299-02AA stepper motor and wondering whether I'd ever find a use for it, I FINALLY noticed that it is rated at 6mH Inductance and 3A. The proverbial light bulb turned on and I realized that I (possibly) had found the ideal motor/stepper driver combination if I paired it with a Geckodrive G540. It was one of those "golly-gee" moments. (That's what we say in Utah when we get really, really excited.)

Geckodrive sells a G540 combination unit that has four stepper drivers attached to a circuit board. The problem with that unit is that it is limited to 50VDC and 3.5A per motor. I've tested it extensively with PK296A2A-SG3.6 motors and with the (too small) PK268-02AA stepper motors (left over from the process control computers that I once designed). Although the "maximum" voltage for the PK299-02AA motor is 78VDC, it can be run at 50VDC with good results. Because it is rated at 3A, it seemed like a possible match with the G540. So I set up the test bench.

Power Supply = 48VDC (2 X 24V switching PS in series)
Current limit resistor = 3K 1/4W
Steps per inch = 1697.7928
Jog speed = 15 inch per second
Cut speed = 6 inch per second
Acceleration = 0.99G !!! (The acceleration was set that high to give me an indication of how the motor would handle on a CNC machine, i.e. would it miss steps - it didn't.)

The steps per inch is the number that would be required if a 4:1 belt-drive were attached to the motor and if a 30-tooth pinion were used (0.000589 inch per step = 1697.7928 steps per inch)

After running a test for two hours with a PK299-02AA motor on the X-axis and a PK268-02AA motor on the Y-axis, the 299 motor is at 46C which is warm but not hot enough to burn. The 268 motor is 58C, which is too hot to hold. The motors are rated at 100C with a maximum 80C temperature rise, so they are both well within their ratings. The G540's case temperature (bottom of case directly below the X-axis G250 stepper driver was 50C with the G540 just sitting on a piece of MDF. The G540 is rated with a maximum case temperature of 85C. The top of the G540 is 32C. Normally the G540 would be mounted to an aluminum case and a computer case fan would recirculate air inside the aluminum case. But the PK299-02A motor is not causing the G540 any grief.

The reason that this combination has got me all excited is that the PK299-02AA motor generates 620 oz*in of holding torque when wired half-coil. With a 4:1 belt-drive transmission, that is 2480 oz*in of torque or 155 lb*in, which is about 2X more torque than the 7.2:1 motors on my (upgraded) PRT-Alpha! The price of the PK299-02AA motor paying full retail from Oriental Motor is $205 each. The G540 sells for $299. A 48VDC toroidal based power supply (AnTek) is $95. A build-it-yourself belt-drive is about $150 per motor. If you bought Shopbot's V201 (or newer model) controller, you could build a complete control box, including the motors for about $2,000 that would have 2X the torque of my PRT-Alpha and all of the speed (jog speed possibly limited by the Shopbot controller's maximum pulse rate - I'm running the tests at 45,000 pulses per second maximum).

The smaller PK296-02AA motor ($137 each) is rated at 310 oz*in holding torque, which, with a 4:1 belt-drive, would give it 77 lb*in of torque, which is about the same as the 7.2:1 Alpha motor. That smaller motor has 3.5mH inductance, so it is a better match for a 50VDC power supply, but could be used with a 30-38VDC power supply to keep the heat down.

The beauty of the Geckodrive G540 is that it not only has four (replaceable) stepper drivers built in, but it also has four general-purpose opto-isolated inputs and two general purpose opto-isolated outputs. It also has a 0-10V opto-isolated output which can be used as a speed controller with the VFD of a spindle. Interfacing the G540 to a Shopbot controller would take some expertise (the G540 was designed to be plug-compatible with Mach3 - which would require us 'botters to totally rewrite all of our tool path files), but it would make a killer small system for those that only need the equivalent of the PRS-Standard features with the speed and torque of the PRS-Alpha.

[rb99]

Are there any plans for the belt drives available?

Thanks,

RIB

[richards]

Richard,

I've built several different models for my PRT-Alpha, including models using acrylic, Delrin and aluminum for the side plates. Today I built a 'quick and dirty' 6:1 unit using the smaller PK268-02AA motor. That model was built using aluminum. Until now, I've always thought that the size 23 motors would be too small, but it looks like I may have been wrong. That little unit jogs at 16-ips. After running it on the test bench for two hours, it was barely warm. Normally I run my tests using a Geckodrive G203v, but this one was attached to a G540 module.

I'll show that unit, plus some other electronics tomorrow at the Magna Camp (Salt Lake).

I can send you some generic drawings that you could adapt to your needs. I use AutoCAD LT, which creates DXF files. If you're interested, send me an email.

[rickwoody]

"Stepper drivers, G251 X 4 X $69 = $276
Stepper motors, PK299-02AA X 4 X $205 = $820
Power supply, Antek 1 X $95 = $95
Self-built transmissions, 4 X $150 = $600
Total = $1,791
Add about $100 - $200 for misc. wire, connectors, etc. and you've got an electronic package with 2X the torque of the Alpha and jog speeds of 10-ips to 15-ips, depending on your controller's pulse generating capacity."

Can these components be connected to the V3 controller?

Rick

[Gary Campbell]

Not to pi$$ on anyones parade here, but for Rick to say that his combo has twice the torque of the Alpha means that he hasnt seen my machine cut full depth thru 3/4" plywood as fast as his listed jog speeds. BTW, my machine is a stock PRSa (circa 2010)

It takes torque to gain speed. It takes torque to overcom inertia. It takes torque to push a bit hard thru thicker material. I will not argue that the earlier machines needed both speed and torque. Starting with a smaller motor and installing a gearbox adds lots of torque, but sacrifices speed.

Please sir, dont quote theoretical numbers or comparisons without real world data. As they say: "In theory, there is no difference between theory and real world. In the real world, no evidence has been found to support the theory"

[ken_rychlik]

Actually Rick didn't say it. He was quoting Mike.

To answer Rick's question, yes it is possible to hook this up to a shopbot control card. It is not a plug and play kind of thing though. It would take someone with some electrical knowldedge and some research on their part as well.