Taig mill/Mach3 missing pulses cause limit switch activation

[dpekin]

Hello,

I have a desktop taig mill using the standard parallel port driven stepper motors. No encoders. The PC is a 1Ghz box, XP-SP2 with no extraneous software running in the background. I have Bob-CAD to generate g-code files for a part I'm trying to make. I have the acceleration & velocities on the 3 axis slowed way down - 25IPM. The Mach3 test program (can't recall the exact name) indicates the computer / pulse generator is fine. I'm running the latest Mach3 (V3.043). The slides are not binding and Mach3 can run full travel in manual mode at fast/slow/med speeds. I do not have backlash compensation on.

The problem is that when I run the g-code file, after some number of lines, I get "Limit switch hit" error. It is not reproducible. If I re-run the file, it fails at some different line. I understand the limit swich error is because the system is somehow losing track of where it is, losing a pulse or something like that. All the moves are relatively short. Some are 2 axis, some are single axis. This occurs without doing any cutting. I have the Z stage up and just want to see if all the moves will be completed.

Does anyone have any suggestions on how to remedy this? Is there a control of the acceptible error band for the stepper motors? Also, is there anything like a standard test g-code file that exercises system and verifies correct operation?

Thanks for any input.

- Dave

[awerby]

If your machine doesn't have encoders, then it shouldn't involve the limit switch function in Mach3. Taig does make a machine (the DSLS 3000) with encoders on the motors that uses this function to stop the mill when it gets lost, but that's not the case here. Since the Taig doesn't actually have any limit switches (unless you added some), the only thing I can think of that might be causing this is the Softlimits function in Mach3. That allows you to set up virtual limit switches at certain places, so you can avoid over-running an axis or hitting a clamp. Make sure that function is disabled, and your problem might go away.

Andrew Werby
ComputerSculpture.com — Home Page for Discount Hardware & Software

[cameraguy]

i would agree - this does not sound like a lost pulse issue. if you have no limit switches, check the soft limits. if you do have switches, check the shielding and cable runs to make sure noise isn't hitting a pin and being interpreted as a valid signal. might as well also check to see if vibration/resonance isn't falsely tripping a bad switch or shaking a loose connection.

[lsces]

Dave

Limit switch inputs will only work if you have them enabled. So do you ACTUALLY have limit switches? If not, then disable them. Where did the configuration you are using come from? As indicated, the DSLS3000 config file does have the limits set up, but uses them to return the fault signals. The effect you are getting sounds very much like that sort of problem

[dpekin]

OK, I just want to make sure system and error is described correctly:

The DSLS 3000 has encoders. The system tracks pulses sent to the motors and verifies it receives encoder pulses back, thus verifying that the motor has moved the commanded amount. It has an encoder error band of 200. If the encoder count differs from the motor step count by more than 200, it generates a "Limit Switch Error" in the Mach 3 software. So, I am getting the Limit Switch error due to the encoder count exceeding the 200 count error band, not due to actual limit switches.

I have looked at the motor step pulses w/ the O-scope and they look very good 0-4V and square, no noise.

Now, the most telling thing happended to me today. I was running a part and I Stopped the run to do some calculations at the desk. While I was doing the calculations, I heard a Beep from Mach3 and I saw that the Reset button was blinking and the "limit switch" error was generated. IT OCCURRED WHEN ALL AXIS WERE STOPPED!

This tells me that there is a problem with the encoder circuitry, either a bad cable (corrosion, floating ground or noise) or the controller has a problem in the encoder section.

I saw another post in this forum where the end result was that they sent the controller back to MicroMill and there was a power supply going bad...

I don't have the specs on the power supply so I don't know how to test it. I'll first examine and ring out the cables.

Anyone know how to test the PS in the DSLS 3000?

Thanks.

- Dave

[cameraguy]

sounds like you are on the right path. if all else fails you can just disable the encoders entirely and run open-loop, too.
i don't want to seem like i am saying anything bad about the DSLS 3000 specifically, but steppers shouldn't need encoders if the system is set up properly with motors and drivers properly matched.

servos use encoders because they have to, but they also work to keep things accurate by giving the computer the ability correct on the fly. that is useful. your stepper/encoder system doesn't have that ability, so technically it really isn't a closed-loop system anyway. its an open-loop system with a cutoff. a properly set up stepper system doesn't miss steps anyway, but if it does the DSLS system can't correct for it - it will only shut down the system. in other words, it doesn't help prevent any problems or lost workpieces , it only reports that there was a problem after one already happened. as a diagnostic tool it can be useful sometimes to pinpoint exactly where a failure was in a toolpath, but beyond that its not really very helpful.

you can just disable it if you are getting falsing you can't track down, and if you are having problems with real lost steps, only changing the driver and motors will help.

[lsces]

cameraguy ... you obviously don't understand the DSLS3000 setup ... it is not possible to 'disable' the encoders as they form an integral part of the controller circuit. The point of Kurt's design is to keep applying power to the motor until it actually moves. Having lived with my machine for some time now I do question the advantage, since a bipolar setup will give better performance, but the system does work.

dpekin ... Starting point is to check the fuse inside the box. The high voltage supply has an in-line fuse which if it blows does reduce the power output of the box. It will work, but only at a low speed. I presume you do have a DSLS3000? The only other failure will stop a channel working, although I have had a dodgy input PIC, but again no movement.

If there is a problem with the encoder cable the axis will not run at all as it requires the end of the pulse to trigger the next one. If it is running then try halving the maximum speed and see if that helps. I still get problems from time to time where a job I've run many times in the past simply stops. Usually just jogging the machine full travel a few times clears the problem but now and again it needs a strip and clean. The plastic of th boxes I mainly machine seems to leave a sticky residue sometimes.

[dpekin]

Thanks for the suggestions.

Once again, the fact that the controller will generate a Servo Error/Limit switch error when the system is completely stopped and just sitting there indicates to me that there is a problem on the encoder side of the system. I will verify the encoder cable integrity and connections. I guess a floating ground or corroded pin might be able to generate this problem.

I did check the voltages at the hi/low caps and they are good 57/8.8 respectively. I assume this means the HV fuse is intact. I will verify however.

What velocities & accelerations should this system be able to run at reliably?

- Dave

[lsces]

Thanks for the suggestions.
Once again, the fact that the controller will generate a Servo Error/Limit switch error when the system is completely stopped and just sitting there indicates to me that there is a problem on the encoder side of the system. I will verify the encoder cable integrity and connections. I guess a floating ground or corroded pin might be able to generate this problem.
- Dave

Are we talking one channel or all? I would expect an encoder problem to only affect one channel, and the error message will tell you which channel. I leave the A axis encoder and motor unplugged and it does not affect things!

I did check the voltages at the hi/low caps and they are good 57/8.8 respectively. I assume this means the HV fuse is intact. I will verify however.
- Dave

I had to check, but yes, the fuse is between transformer and bridge recifier, so as long as you have volts ... all should be OK. Although it might be worth just checking that the power is getting to the driver board(s)

What velocities & accelerations should this system be able to run at reliably?
- Dave

I'm configured for metric, so 1500 mm/min and 100mm/s/s Works out about 60 inch/min. Even cutting at that speed you get round corners as the driver tries to keep up with Mach3, so my normal limit while cutting is 1000 mm/min, about 40 inchs/min while the open loop setup with the same motors and drivers is limited to around 30 inchs/min.