Gear motor spinout with Mach 3

[waynec]

I'm upgrading my PR to use Mach 3 and an Ascension controller. I'm coming along thanks to help from folks here.

My question today is about the Shopbot PR motors on the X axis. These have a gearbox, and under the Mach/Ascension combo I'm finding that they can't go very fast before they 'spin out' and loose steps.

How can I tell if my motors are worn or defective? And what sort of performance can I expect from these geared motors using an Ascension controller and Mach 3?

Any help would be appreciated.

Wayne from White Salmon

[carlcnc]

Hi Wayne
steppers "spin out" when they don't have A:enough power [amps]for acceleration rate
B: enough voltage available for higher speeds.
what are the specs on your motors
amp, voltage?
what is the voltage output on the power supply?
I have used a lot of Geckos, and Mach3 [aka Master5,Mach1,Mach2]
you can email me direct,
Carl

[Brady Watson]

...and C) Communication speed.

-B

[richards]

Wayne,
If you can post the power supply voltage, the value of the current limiting resistors and the color of the wires that connect to the Gecko stepper drivers, I might be able to run some tests.

If you contact Matt Roylance or Jerry Hicks, they can tell you what speeds they're getting on their machines.

Also, if you contact anyone who is running a 4g, they can tell you what speeds they are running. (Even though the 4g and the Ascension 1000 are different, they both run Gecko stepper drivers, Oriental Motor stepper motors and they both have somewhat similar pulse rates from the controller card. In other words, I can't imagine either the 4g or the Ascension unit being significantly faster or slower than the other.)

[waynec]

Thanks all.
I'm working with Alan of Custom CNC to try to get more performance from the Ascension/Mach combo. My work hours and location make it tough to talk with Alan, so I come here since I know there are people with both PR's and Geckos. Its often faster to get pointed in the right direction. Hope that's OK.

I'll check my Gecko resistors and get the info and post it here as well as email Alan at Custom.

Thanks for the help.

[waynec]

A bit of clarification on my motor skipping issue.

First, the problem. When I run the motors at speeds over about 250ipm, the motors 'skip' and lose steps. This happens EVEN WHEN THE MOTORS ARE NOT ENGAGED WITH THE RACK AND ARE SPINNING IN THE OPEN. You can hear the motor spinning, but the pinion does not turn, or turns intermittently.

Second, some details. The motors are Oriental PK296A1A-SG3.6, rated at 1.5 amps and 2.2 Ohms. According to Alan at Custom CNC, the Ascension control box runs these motors as bipolar. They use Gecko 2.2 drivers with an 82Kohm resistor in place.

I hope this helps to clarify the situation.

Wayne from White Salmon

[richards]

Wayne,
I'm wondering if there is a decimal out of place? The G202 stepper driver uses an 8.2K resistor for a 1-A motor, such as the PK296A1A-SG3.6 motor wired Bipolar Series.

First, check and recheck the connections between the motors and the Geckos. If that doesn't help, then you can rewire the motors using half-coil. Half-coil will allow higher speeds, but it will reduce torque at lower speeds. It may be worth trying.

The PK296A1A-SG3.6 motors have six wire leads. Phase A has wires colored Black, Yellow and Green. Phase B has wires colored Red, White and Blue. When the motors are wired Bipolar Series, the Black and the Green wires are used for Phase A, and the Red and the Blue wires are used for Phase B. To wire the motor as half-coil, you would use EITHER the Black and the Yellow wire OR the Yellow and the Green wire for Phase A. You would use EITHER the Red and the White wire OR the White and the Blue wires for Phase B.

For half-coil connection, would should also replace the 8.2K resistor with the closest standard resistor to 12.8K. (A 10K and a 3K resistor in series would work perfectly if you don't mind soldering or tightly twisting two resistors together.)

If a motor turns backwards to the direction desired, exchange the two wires that are connected to Terminals 5 and 6 on the G202 (Phase B and Phase /B).

For 6-wire motors, I have wired the motors both as Bipolar Series and as Half-Coil. For my purposes, I've decided to use the Half-Coil wiring scheme as my first choice. It gives me higher speeds without too much loss of torque at low speeds. At higher speeds, it produces a lot more torque than Bipolar Series.

[richards]

Wayne,
Doing the math for 250-ipm shows that you only need about 6,400 steps per second to turn those motors about 190 RPM. (30 tooth spur gear) So far, I haven't tested one 'square type' motor that I couldn't run at least 1,000 RPM using either the G202 stepper driver or the G203v stepper driver. Of course, that is on the test bench and not running a CNC router. So, I'm just as puzzled as you are. Stepper motors don't normally wear out. The bearings may finally give out, but you'd be able to easily hear metal rubbing on metal if that were the case.

[waynec]

Thanks once again, Mike!

I've been working on the mechanicals today, and I did double check all my wiring and retested to make sure that I'm still getting the same problem.

I found that I am still getting the problem as described, with one motor 'skipping out' a bit sooner than the other two. But this happens when motors are disengaged, just spinning freely using the jog keys. That's still what's happening.

I think I'll try the half coil wiring and see what results I get. I'll check out the torque at the lower speeds and see if the compromise is OK. I can really save some time with a faster jog speed.

I'm wanting to cut some sheet good and pine, and less often sign foam and MDF. I have a Porter Cable router, so the cut speed is limited by this as much as by motor torque, I suspect. I generally cut 3/4 sheet goods in three passes, but I'm hoping the new controller will let me do it in 2 passes at around 2ips.

Is this a reasonable expectation from a half-coil configuration?

Thanks so much for sharing your expertise, Mike.

Wayne from White Salmon

[richards]

Wayne,
Unipolar or half-coil torque ratings are about 71% of Bipolar Series ratings. However, the torque table for the PK296-01AA motor, which is probably the motor that Oriental Motor uses for the PK296A1A-SGxx geared motor, shows that that motor using Bipolar Series wiring starts out with almost 400 oz*in of torque but that the torque drops very quickly to 150 oz*in at 100 RPM and continues to drop to 50 oz*in at slightly more than 200 RPM. The same motor wired Unipolar (half-coil) starts out at 300 oz*in, is at 150 oz*in at 500 RPM and doesn't drop to 50 oz*in until it reaches 1,000 RPM. The other factor that needs to be considered is that the geared motor is limited to 20 lb*in of torque, or 320 oz*in regardless of how it is wired so I don't think that using half-coil wiring will actually rob you of any usable torque through Oriental Motor's gearbox. Remember also that you can easily adjust the ramping of the motor to adjust for low-speed torque characteristics.

I use the PK296B2A-SG3.6 motors (dual shaft) which seem to be a better match to the high performance Gecko G202 or G203v stepper drivers. These particular motors would not have worked well with the stepper drivers that Shopbot used before introducing the 4g upgrade, so the motors that you have are really the best motors that could be obtained with your original controller. However, if you decide to replace the motors at some future point, the PK296A1A-SG3.6, a 25V to 35V power supply and either the G202 or the G203v stepper driver make an excellent package.

One other point for stepper motor puriests is that Unipolar is not the same as half-coil. A true unipolar connection uses all six wires, while a Gecko stepper driver only uses four wires. For years I've used Oriental Motor's CSK stepper drive package which includes both a motor and a stepper driver. The stepper driver is a Unipolar driver. When I run the same motor at the same voltage with a Gecko stepper driver, I get higher torque than I got with the CSK component. So, in summation, if I had an application that required a stepper motor to rotate at not more than 100 RPM, I would probably use the Bipolar Series wiring connection, but for CNC router use where higher RPM is desired (up to about 400 RPM), I would use half-coil connections.

EDITED: I didn't answer your question on speeds and feed rates because I don't have any experience with your motors. Maybe someone who has installed the 4g upgrade or who has the Ascension 1000 unit could post their findings.

[richards]

Wayne,
I just finished a test with mixed motors using Gecko G202 stepper drivers, a 35VDC power supply and Mach 3 software set to run at 45,000 pulses per second. The motor of interest is the PK296B2A-SG3.6. (The other motors were two PK268-02AA and one PK299-F4.5.) All motors were connected using half-coil connections except for the PK299-F4.5, which was connected using Bipolar Parallel connections.

Assuming that you would be using a 30-tooth spur gear on your machine, I computed 1,527.884 steps per inch. Circumference of the spur gear is PI(1.5) = 4.7123 inches. 4.7123 / 3.6 gear ratio = 1.3089 inches per stepper shaft revolution. 1 inch / 1.3089 = 0.764 stepper shaft revolution per inch of travel. 2000 steps per shaft revolution X 0.764 = 1,527.884 steps per inch.

Motor tuning in Mach 3 required the following:
Steps per unit = 1,527.887
Velocity = 1,750
Acceleration = 50

Those settings would allow a maximum of 1,750 inches per second of axial movement, which is 29.166 inches per second.

The test program consisted of the following:
G90
G00 X0.0 Y0.0 Z0.0
M98 P0100 L3
M30

O0100
G00 X0.0 Y0.0 Z0.0
G01 F1750
X100
Y100
Z10
X0
Y0
Z0
M99

The feed speed (i.e. F1750) was varied from F500 to F1750, which gave feed speeds ranging from 8.333-ips to 29.166-ips.

All of the motors ran all of the tests perfectly without any skipping. Of course, this was just a test bench test; each motor only had to spin its own shaft. It should show, however, that Gecko stepper drivers, when connected to almost any motor will give impressive performance. In this particular test, the 35VDC power supply was just about maximum for the PK296B2A-SG3.6 motor. I have run the same test with the same results on that motor with a 27VDC power supply.

[waynec]

Mike,

THanks yet again. I'm waiting for a nod from Alan/Custom CNC before I wire the motors as half coil. He suggested that I NOT use the white and yellow wires, which are necessary for the half coil wiring if I read your post correctly. Just to be sure, I'm going to check with him first.

That sort of performance from a Gecko/Vextra motor is indeed impressive, and way beyond what my mechanicals would likely handle. I'd be thrilled to have 350 ipm, let alone 1750. My Mach 3 runs at 25K, and my spur gear is 20 teeth. This might change things, but not that much.

I'm sorta stumped by this- I can't quite see how to nail down whether my motors are weak or there is some other setting or possibly a problem with the Ascension. I suppose I could reconnect the old PR controller and see if the same problem happens with this setup.

Meanwhile I'll work on installing limit switches and squaring up the table.

Thanks for your input and helping me understand more about steppers and controllers.

Wayne from White Salmon

[richards]

Wayne,
I'll certainly defer to Alan at Custom CNC about the best wiring connection to use with his box; however, I am extremely curious why he would take that position. If you download "Step Motor Basics" from the support area at www.geckodrive.com (http://www.geckodrive.com), and carefully read pages 7 & 8, I think you'll note that Mariss states that a half-coil connection is only 3% less efficient than a parallel connection. Page 2 of the paper explains the relationship between voltage and torque. One of the very interesting points that Mariss makes is: "Because torque is proportional to current, motor torque is constant from zero speed to the corner speed. Above the corner speed, motor current is limited by the motor's inductive reactance." I added the italics to emphasize the limitation caused by the motor's inductance. A motor wired full-coil has 4X more inductance than a motor wired half-coil (30.8mH vs 7.7mH). That is why a motor wired Bipolar Series looses torque so rapidly as it speeds up.

I'm wondering whether Alan meant that when a motor is wired Bipolar Series, that neither the Yellow wire nor the White wire is used. Those wires should be individually taped off and insulated from each other. When a motor is wired half-coil, then (normally) the Green wire and the Blue wire are not used. They are individually taped off and insulated from each other.

At any rate, after you hear from Alan, please let me know why he suggests using the full-coil connection method. There might be something obvious that I've overlooked.

EDITED: I just did a quick check to compute the maximum voltage that could be used with the PK296A1A-SG3.6 motor. It can take voltages up to 88V. Since the Gecko is limited to 80V, voltage is not a factor. Wiring the motors half-coil (and changing the current limiting resistor) will increase current consumption from 2.4A to 4A, but I can't imagine anyone selling a CNC control that couldn't handle at least 4A. Even my little 27V power supply puts out 13.8A - and that is a very small toroidal transformer. (You add the current ratings for all the motors together and then multiply that number by 0.66 to compute the actual current required from the power supply.)

[tommyr]

Wayne,

Please check your terminal connectors again, they may seem like they are seated properly (but if like in my case) they are not.

Believe me the multiple problems I was having drove Alan and me crazy for two weeks until I stumbled upon this.

Everything would be working fine then intermittent problems would appear.

Look closely at the back of your controller where the gray terminal blocks connect to the green blocks. If you see some of the bright red metal part of the board in the little square opening this is what was keeping my terminals from seating
completely.

Take a pair of snips and carefully cut of the ends of the locking tabs (this is what is hitting the red area of the board, the tabs don't serve any purpose) on both sides of your gray connectors coming from your motors.
Then reconnect the terminals.

Since I did this my Ascension 1000 has worked perfectly.

Good Luck,
Tom

[waynec]

Thomas

Thanks very much for that tip. I'll check that thoroughly when I work on it tomorrow. I have had some intermittent axis behavior that I thought I solved by rechecking my Wago connections. Your suggestion is one I didn't know to look for.

Mike, Your guess is probably right on Alan's reasoning. I'm almost certain that his admonition to not use white/yel wires is for the bipolar wiring configuration.

Autocad class tonite- no time to check the CNC.

THanks all for the help. I'll try to post a pic or two to show the progress.

Wayne from White Salmon