Debugging CNC Drives

These are my notes on troubleshooting CNC drives. I use a checklist approach that follows the signals through the system to see where the fault lies.

0100 Motors don't move at all and don't make a sound

0100. Are the motors firm? If so, the are at least connected well enough that they are generating holding torque. Jump to 0102 and take a look at the Step/Dir signals.

0101. Check for DC voltage out of rectifier and capacitors? Is it the expected voltage? Is the polarity to the motor drive boards correct?

- Fix wiring polarity as needed

- If no DC voltage, check AC voltage out of transformer and into rectifier.

- Check fuses and circuit breakers!

- Got AC and no DC? Rectifier must be bad or wired incorrectly. Recheck wiring, replace rectifier if needed.

If there's DC voltage, check the step/dir signals from the breakout board to the Gecko drives.

0102. See if drivers are getting the dir signals from the breakout board. My GRex has a little LED status for every signal, so I can see when it gets step/dir. Mariss F. suggests the following procedure with a multimeter to check step/dir signals:

1) Set a multimeter to DC volts, 20V scale if not autoranging.

2) Place voltmeter ground to PC ground.

3) Place voltmeter probe on the drive's Direction input.

4) Cause the drive to jog CW. Write down the voltage.

5) Cause the drive to jog CCW. Write down the voltage again.

6) Measure the voltage on the drive's Common terminal. The voltages from (4) and (5) must be 0.5V or less and 4.5V or more to be within spec for the drive. The voltage from (6) must be 5VDC +/-0.25V. If the voltage is wrong, check your wiring, check your breakout board, make sure your PC has a parallel port capable of 5V operation and not 3.3.

7) You've now tested the connection from Mach 3 all the way to the Gecko Drive board, BTW, at least for the dir signals.

0103: Try disconnecting the step wire from the Gecko Drive and tapping it against ground. Each tap should move the motor a step or two. If it does, this verifies the problem is somewhere from the PC to Gecko Drive (i.e. the Step connection) and is not downstream. If this test fails, you have either:

Power supply problem (should've been checked over in 0101, but recheck the wiring again to make sure the Gecko is getting juice).

Motor wiring problem

Gecko Drive might be bad.

0104: If the step/dir signals are not comming through to your motor driver, its either a software problem or a breakout board problem.

Double check the pinout assignments in Mach 3. Also try the different polarities for step and direction.

Got step/dir signals and good DC voltage from motor supply? OK, move on to checking the wiring from the Gecko Drives to the motors.

0200 The motors are making chattering and jerking noises and don't really turn

0201. Check step and dir signal polarity in Mach 3. Is it right for the boards you're using? I had to reverse one but not the other to make my Xylotex board work properly when it was doing this.

0202. If test #1 is good, it probably means one of the motor coils isn't being energized for some reason. Check the wiring. I had to reverse 2 of the conductors because a cable was wired wrong one time when this happened.

0203. If the wiring to the motor coils checks out, you will need to tests your Gecko stepper drives as recommended by Mariss F.:

- Power down

- Measure resistance between pin 1 and each of the 4 motor control pins.

- Resistance should be 100K ohms or so else the H-bridge in the driver is damaged.

0300 Stepper motors are growling at low speeds, though they will turn

0301. Try the ADJUST trimpot on the Gecko Drive to see if it helps.

0302. If you have a NEMA42 motor, make sure the Gecko's internal NEMA42 jumper is set properly.

0303. Make sure the proper size current limiting resistor is in place on the Gecko Drive.

0400 Motors twitch sometimes and do not always perform commanded motions

0401. Check the Step/Dir signals to the Gecko Drives as described in 0102 above.

0402. You may have a noise problem. Check the Noise Problems page for ideas.

0500 My fuses keep blowing!

Mariss says drives that blow fuses usually have 1 or more damaged MOSFET power transistors. Here's how to check the health of the MOSFETs:

1) Pull the main connector block from the drive to expose the connector header pins.

2) Set your multimeter to Ohms. Set it to the '200K' range if the meter isn't auto-ranging.

3) Connect one probe lead to terminal 1 (GND) of the drive.

4) Use the other probe to measure the resistance to term. 3, 4, 5 and 6.

5) Now connect the first probe lead to terminal 2 (+SUPPLY).

6) Repeat step (4).

7) All 8 readings should be 100K Ohms or more for a good drive. A bad reading will be near zero Ohms and indicates a blown MOSFET.

0600 Other Tests

0601 Check the health of your power supply filter capacitor. Set a multimeter to AC volts and put it across your power supply when it is under load. The AC voltage must not read more than 3% of your DC voltage. If it does, your main filter cap is too small or is failing. This can destroy the drive because the cap is too small to absorb returned energy.