Ascension driver issue

[cnc_works]

Has anyone with an Ascension box had problems with unexplained driver faults?

My offside X driver will very occasionally fault out, cause the bot to lose steps then successfully reset after turning the box off and on. Until the next time. It can be months between faults.

Opened the box the last time and could detect no temperature differences, besides, according to the manufacturer, Gecko 202's do not have a hi temp sensor to fault them anyway.

Seems to always occur in the five foot middle range of the X axis, but not nearly in the same spot, and I think during rapids. Which are set very conservatively.

[Brady Watson]

Donn,
I've had the 202s flake out on occasion. Make sure all the little terminal screws and blocks are seated well onto the pins. If you do this and it still happens, consider upgrading to the v203 ones, which should offer some improved performance when jogging. I had a couple 202s that would be hit or miss. They got chucked for 203s, which seem to be a much better drive.

-B

[richards]

I use the G202 on my test bench. The blue terminal block used by Geckodrive on the G202 has been replaced on other stepper drivers with a black terminal block. The blue terminal block was discovered to have spring contacts that did not always return to their optimal position after the block was reinserted.

As Brady said, the G203v is the current "top" model. I have used that stepper driver extensively and prefer it; however, the G202 uses a +5V for "common" and the G203v uses GROUND for "common". In other words, the G202 requires signals from your break-out-board to be active LOW and the G203v requires signals from your break-out-board to be active HIGH. Check your break-out-board to see if it has a jumper to change the polarity of the signals. The PMDX-122 board that I used can be jumpered for either polarity, but all drives must use the same polarity.

The other problem is the inductance of the stepper motors. I'm assuming that you're using the original 3.6:1 geared motors that came with your machine. Those motors have four "leads" that only allow the motor to be wired bipolar series. If you look at Oriental Motor's PK296A1A-SG3.6 motor (which is electrically identical to the motor used by Shopbot), you'll see that it has 30.8mH inductance. According to Geckodrive, that motor would require 177VDC for optimum performance. Of course the G202 is limited to 80VDC maximum, so that drive is not suited to drive that motor. The PK296A2A-SG3.6 motor has 1.5mH inductance when wired half-coil (Black/Yellow, Red/White). That motor has optimum performance when connected to a 35V to 40V power supply. Of course the SG motors have gearboxes that are inferior to the TH gearboxes used on the motors supplied by Shopbot.

It's a real shame that the Shopbot motors don't have all six leads exiting the motors. If they did, they could be wired half-coil which would give them 7.7mH of inductance. Then they could be driven with an 80VDC power supply.

I'm suspecting that your main problem is that a motor is missing steps because it is being driven too fast. The torque charts for that motor shows that when wired bipolar series with a 48VDC power supply, that the torque drops off rapidly at 75 RPM. The math shows that 75 RPM is 1.25 revolutions per second, so with a 1.25" (25-tooth) spur gear and a 3.6:1 gearbox, you'll only be able to drive that motor at 1.09" inches per second before it reaches the shoulder on the torque curve. When a stepper motor is driven past its "shoulder point", it loses torque rapidly which means that it will more likely lose steps.

The torque charts for the PK296-01AA motor (which is the same motor as the PK296A1A-SG3.6), when wired half-coil, it still has the same torque at 450 RPM as your motor has at 75 RPM. The PK296-03AA motor (which is the same motor as the PK296A2A-SG3.6) can run at 950 RPM and produce the same torque as your motor has at 75 RPM.

This is just a long-winded explanation. It doesn't solve your immediate problem. Call Geckdrive and ask them to send you some black terminal blocks for your G202 stepper drivers. I think that they cost $1.50 per block. Next, try slowing down your jog speed by 25% and see how that works.

If that doesn't work, then consider replacing the motors with the PK296A2A-SG3.6 and replacing your power supply with a 35VDC unit lenier unit with a 15,000 uf (minimum) capacitor. Be aware that the SG gearbox might have a little backlash. (The four PK296A2B-SG3.6 motors that I have are tight without backlash.) If you also replace the G202 stepper drivers with the G203v model and replace your break-out-board with a PMDX-122, you'll be spending about $1,900 for the motors, the drivers, the break-out-board and the power supply. That's a lot of money, but those components work very well together.

[Brady Watson]

It's a real shame that the Shopbot motors don't have all six leads exiting the motors.

Mike,
Nice to see you chime in. All PR, PRT and PRS Standard motors in fact do have 6 leads coming off of them if you peel back the protective wrap between the Wago and the strain relief on the motor. So you can certainly wire them up in half coil if one felt inclined to do so.

-B

[richards]

Thanks Brady. I didn't know that. By wiring the motor half-coil, it will be much more responsive.

So, with that new information, here's a possible way to get the best out of an Ascension.

Change the power supply to a linear supply that outputs 70 to 80 (maximum) VDC. (Due to the way stepper motors draw power, a switching power supply is not the best choice, even if you attach an external 15,000 uf capacitor.) Geckodrive uses the formula: 32 X SQRT(inductance) = MAX voltage. The original motors, wired half-coil, have 7.7mH inductance, so 32 X SQRT(7.7) = 89 volts. The G202 or G203 is limited to 80V. I recommend 70 to 75 just in case the power company lets the line voltage rise a little too much.

Wire the motors half-coil (Black/Yellow, Red/White). If the motor turns in the wrong direction, switch the Back/Yellow to Yellow/Black OR switch the Red/White to White/Red. Wiring half-coil reduces the torque to 70% of what you had with bipolar series, but the speed range will be about 4X greater before torque drops off. (Look at the torque charts for the Oriental Motor PK296-01AA motor.)

Change the current limit resistors on the G202 to 18K (20K works fine).

Change the terminal blocks on the G202 stepper drivers to the better Black terminal blocks.

Total cost for a toroidal linear power supply and terminal blocks should be less than $150.

If the Shopbot has been well maintained so that there is no binding, and with proper acceleration values in Mach, you should be jog speeds in the 8-ips range.