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.

[richards]

I've received several emails asking how the G540 can be connected to a Shopbot. A better solution might be to buy the G251 modules that have a screw terminal block and a heat sink attached to the module, then, wiring the Geckodrive G251 would be as simple as connecting a G203v.

It's hard to imagine that a little stepper driver 1.675" X 1.560" could drive a large stepper motor, but it can and it works very, very well. Although I don't have a G251, it and the G250 modules are exactly the same except for the connection options. The G250 is one of the four modules used in the G540 unit. The G251 sells for $69 each, compared to $147 for the G203v. The G251 will drive a PK299-02AA motor just as fast and with just as much torque as the G203v at on 46% of the cost.

One possible problem is the limited pulse rate generated by some of the Shopbot controllers. The Geckodrive modules, G201, G202, G203v, G250 and G251 all generate 2,000 steps per revolution. That gives excellent resolution (2X better than the Alpha driver - as it is normally configured), but it also requires a lot of pulses to move an axis. If you built a belt-drive transmission with 4:1 reduction, and if you used a 30-tooth pinion gear, each pulse would move the axis 0.000589". Moving an axis 10 inches per second would require a pulse rate of almost 17,000 pulses per second. Moving an axis 15 inches per second would require a pulse rate of about 25,500 pulses per second. I don't know how fast the various Shopbot controllers can generate pulses. (I can't find my notes on the V201 that I'm using on my PRT-Alpha, but I think that I tested it up to 30,000 pulses per second - but that may just be the faulty memory of any old man.)

So, if you're looking at a possible do-it-yourself update for an older Shopbot, use the following prices to give you an idea of what to expect:

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.

If you use the smaller PK296-02AA motor, that cost $138 each, you would save $67 per motor, or $268. The smaller motor, when geared 4:1, gives 77 lb*in of torque, or almost exactly the same as the Alpha 7.2:1 motor. In addition, the 4:1 belt-drive would give better resolution (4 X 2000 compared to 7.2 X 1000) and there would be no possibility of backlash, although my 7.2:1 gearboxes don't have any apparent backlash.

[robert_cheal]

Hi, Mike
I would very much like to do something like that as soon as I am able to so.

I have been looking forward to seeing you at Magna.

Robert

[jon]

I recently upgraded to the 7.2 gearheads, and have noticed some slop in my cuts that I think might be attributed to backlash. The reason I say this is because the slop disappears when the old ASM911AA-2.65V steppers are subbed back out. Has anyone else noticed this? Or is this the trade off for increased resolution? The test part I use to cut is a square with slots cut into the flats. I'm seeing a slight curve or vector 'slop' occuring after 90 degree corners. All the axis' are tight, and I have a spindle.

[richards]

The SG (Spur Gear) gearboxes are advertised as having 1-degree to 2-degrees of backlash. In comparison, the TH (Tapered Hob) gearboxes are advertised to have as much as 43-arc minutes of backlash. A minute of arc is 1/60 of one degree. So 43 arc min = 0.71 degrees. So, worse case, an SG gearbox that has 2-degrees of backlash would have 2.7X more backlash than a TH gearbox.

Of the four SG 3.6 motors that I have, only one gearbox has any discernable backlash.

The Oriental Motor document that describes their gearboxes and the backlash for each type of gearbox is found at:

www.orientalmotor.com/products/ac-dc-step-motors/geared-step-motors.html

The PK296A2A-SGxx motors are compact and relatively inexpensive (about $260 each). Another alternative would be to buy the PK296-03A or PK296-F4.5A motor and add a belt drive (4:1 if you use an 18-tooth pulley on the motor and a 72-tooth pulley on the drive shaft). Building a belt-drive transmission will cost about $100 to $200 per motor, depending on the materials you use and whether you do the work yourself. I've just bought the parts to make four belt-drive transmissions out of aluminum. The total price of parts, without the motors is $484. The motors cost about $140 each, so my cost, per motor will be about $261. The belt-drives have no backlash. They have 4X the tourque of the motor (4 X 300 = 1,200 oz*in = 75 lb*in = Alpha torque) and with a 30-tooth pinion drive gear, they will have a resolution of 0.000589" per step (compared to 0.0006545" per step for the Alpha 7.2:1 motors - the Alpha motors have 1,000 steps per revolution, the Gecko stepper driver has 2,000 steps per revolution). Even driving those motors with a modest 20,000 steps per second pulse stream will jog an axis 11.78" per second.

The belt-drive transmission that I've described requires high-performance motors and high-performance motors require high-performance stepper drivers. I use the Geckodrive G203v (mounted on a slab of 3/8" thick aluminum for a heatsink), a 35VDC power supply and 100k 1/4w current limit resistors. The motors are wired half-coil, although the PK296-F4.5A could be wired bipolar parallel for even more torque but at a cost of substantial heat. When wired half-coil and driven hard for long periods of time, the motors will reach about 60C, which is well within their intended temperature range. The PK296-F4.5A motor, when wired bipolar parallel and run at 6A will hit 80C to 85C, which is still within its intended temperature range, but 80C is hot enough to cause blistering - too hot for me.

[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.