Indexer Unit Values

[richards]

I've just hooked up the prototype electronics for an indexer for my PRT-Alpha. Because I had a bunch of stuff on hand, I just kludged together the electronics from those parts. The basic parts list is:

-Gecko G203v stepper driver
-Oriental Motor PK296A2B-SG3.6 stepper motor
-27VDC power supply (19X rated voltage of the motor)

Hooking up the Common, Direction, and Step signals on the Gecko G203v directly to the corresponding pins on the Shopbot's 'B' channel worked perfectly. (The G203v uses a common ground signal with Active High pulses, which is what the PRT-Alpha controller card delivers.)

Now to the unit values for the B-axis. I used the formula (2000 / 360) X 3.6 to get 20.0 for the unit value. The G203v needs 2000 steps per revolution, so 2000 / 360 degrees = 5.5555 steps per degree. The stepper motor has a 3.6:1 gear box, so 5.5555 * 3.6 = 20.0.

The default value of 13.888 seems to be for a 2.5:1 drive reduction. Is that correct? I was under the impression that the Shopbot Indexer was 25:1 - ten times higher. That's the main reason for this post; to see if I'm foggy in my thinking.
Any advice?

[pfulghum]

Can't help you with the UV, BUT....

I have some questions on Power Supply and motor selection.

Doesn't the Gecko 203V want a 4 wire motor.
It is my understanding that the best way to give it 4 wires is with a "Series" motor (Oriental calls this BiPolar).

So in the case of a PK296A2B. The voltage is 2VDC (in Series). So using a 35VDC power supply would give you 17X rated voltage.


It looks like your calculations are based on Unipolar wiring. Why? Which wires?

-- pat

[Brady Watson]

Mike,
The Alpha-spec indexer uses a 10:1 ratio gearbox.

From ShopBot's Command Reference (VU):
Computing the Unit Value for an indexer would be done as follows, assuming you want to express the rotary motion in degrees. With an indexer having a 10:1 gearbox and with the alpha driver set at 500 steps/rev, we would have: (10 * 500) / 360 = 13.8889 [or, 13.8889 steps per degree].

Your setup would be as follows:
(3.6 * 2000) / 360 = 20 steps per degree

-B

[Brady Watson]

Pat,
You only need to use 4 wires from the PK296 to wire it up in parallel. Drivers are unipolar or bipolar, not motors. Incidentally, all PR, PRT & PRS Standard motors are 8-wire, but only 4 are used.

There is a general rule when powering steppers that says you shouldn't go over 25X the rated voltage on the nameplate. This is because more voltage will just cause unecessary heat. I personally don't go by these rules and have several machines running 75v+ with original ShopBot motors. Some of them have run for 35hrs straight in 120° heat with no failures, loss of position or other problems.

-B

[pfulghum]

Brady/Mike,

Is there an advantage running in unipolar or bipolor or does it really matter?

-- pat

[richards]

Brady,
Thanks for the information. I didn't realize that the stepper/driver for the indexer for the PRT-alpha was using the Oriental Motor's Alpha stepper/driver. (And, since Oriental Motor doesn't sell a 2.5:1 geared motor - to my knowledge - I spent a few minutes scratching my head.) By the way, the only way that I get 2482.8171 as the unit value for the newer 7.2:1 Alpha Stepper/Driver that are used with the PRS-Alpha and as an upgrade to the PRT-Alpha is if the steps per revolution = 325 (assuming a 30 tooth pinion gear). Is that correct? (It really doesn't matter from a performance viewpoint or from a resolution viewpoint, since the 7.2:1 gear box would still give a 0.002" step - and I could certainly live with a step that was about 1/2 the thickness of a piece of copy paper - I'm just trying to figure out how the magic number 2482.8171 came about.)

Pat,
I normally use half-coil (unipolar) wiring when I connect a six-wire motor to a G203. In this case, I use the Black wire and the Yellow wire for the A coil. I use the Red wire and the White wire for the B coil. The Green wire and the Blue wire are capped off and not used. In the half-coil configuration, the PK296A2B-SG3.6 motor has a voltage rating of 1.4V, so 19X 1.4V requires a power supply of about 27V. I've run steppers all the way from 10X rated voltage to 25X rated voltage. The higher the voltage, the faster I can spin the motor - but at the cost of much greater heat.

The PK296A1A-SG3.6 motor, which is probably very close in specifications to the motor shipped with a lot of the PRT machines, has a series voltage rating of 4.4V and a half-coil/unipolar voltage rating of 3.3V. In either case, a 50V toroidal transformer giving 70VDC after rectification and filtering with a large capacitor, is ideal for the motor. Whether the motor is wired with the coils in series or as a half-coil motor, the actual voltage supplied by that particular power supply is slightly less than the magic 25X number and also slightly less than the 80V limit of the G20x stepper drivers.

The testing that I've done makes me prefer the half-coil wiring scheme over the series wiring scheme, but in actual practice, either would work perfectly well. (The best performance is from a full parallel winding using eight wires, but when the motor only has six wires, going full parallel is not an option. One of my test motors, the 8-wire PK299-F4.5, does everything that I've ever expected a stepper motor to do.)

We need to keep in mind that test benches and real-world CNC routers are totally different animals. When I'm running a CNC router, I want to use an electronic configuration that will last forever. That usually means going a little conservative in the power supply and settling for a little less speed in favor of much higher reliability. Note that I'm talking only to the do-it-yourself Shopbotters. The PRS machines, either standard or Alpha, have been configured with excellent electronics that should last for years.