The Pololu PCB is working fine now. I tried a lot of things, but the one thing that really did the trick was to move the laser power supply away from the 24VDC power supply and the other electronic items.
While this solved the problem for me, I am concerned that is might still be a little sensitive to noise, so I want to redo the board. I talked to an EMI expert I work with. He had several suggestions, that I want to implement.
- Wrap the return tube (ground) line around the postive line on way back to the laser power supply. He thought this might get the biggest result. You should start this a few inches back from the positive electrode (so that arcing to the electrode is not an issue) and run it back to the power supply.
- A large electrolytic cap (220uF) should be placed on Vdd right near each stepper driver.
- A small RC filter circuit should be added to the step and direction lines and they should be run through a non inverting schmitt trigger. The filter needs to roll off well before 1uS. I am also going to add a 10K pull down resistor to prevent issues if the control cable is ever not connected.
- I may add a slower filter to the input lines for the cover and limit switches.
- Cables should be shieded wherever possible. Shield cable at one end only. Use the end where the connection will have the lowest impedance to ground.
- I was thinking about using a CAT6 connector for the laser power supply connection. This allows cheap cable with shielding plus color coded wire and a simple snap in connection.
- I might use a coaxial connector for the power input to the board. This would be the typical connector at the end of a wall wart.
- A small RC filter circuit should be added to the step and direction lines and they should be run through a non inverting schmitt trigger. The filter needs to roll off well before 1uS. I am also going to add a 10K pull down resistor to prevent issues if the control cable is ever not connected.
- Cables should be shieded wherever possible. I was thinking about using a CAT6 connector for the laser power supply connection. This allows cheap cable with shielding plus color coded wired and a simple snap in connection.
- I might use a coaxial connector for the power input to the board. This would be the typical connector at the end of a wall wart.
If anyone wants any of the current (raw) boards to play with I will sell them for just the cost of packing and shipping them. $10 will get one shipped to you. They do have some issues.
- The rectifier diode at the power input was added for an experimental non-Pololu use. It does not have the current capability for use when drivers are installed. Just solder a jumper wire across this.
- The silkscreen for setting the logic level is backwards. The board is fine, but if you want to use 5V logic, place the jumper in the 3.3 position, etc.
- The schematic had an error on the pinout of the Pololu drives so Vdd and Gnd are swapped on the board at the Pololu connectors. This means you need to cut the traces at these pads and add jumpers. Due to the way the board works it is only one jumper, I will cut and install it.
- The stepper motor were supposed to be in XYZ order. Two of them got swapped. It does not affecting anything, just be aware when you plug in the steppers. The designators are correct, so just check the schematic.