Laser Interface/Driver PCB

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Re: Laser Interface/Driver PCB

Postby bdring » Wed Apr 27, 2011 2:47 pm

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.
Bart
"If you didn't build it, you will never own it."
bdring
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Re: Laser Interface/Driver PCB

Postby bdring » Wed Apr 27, 2011 7:08 pm

FYI: If you even need to cut a pad free of a trace or ground plane use one of these.

Pad Cutter...

pad_cutter.JPG
Pad cutter
Bart
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Re: Laser Interface/Driver PCB

Postby dirktheeng » Wed Apr 27, 2011 9:03 pm

Bart,

If you are going to redo the board, I suggest adding a 4th axis so we can do z controll and a rotary aixis. At least include traces for the connections even if the components aren't offered in a kit so we can add the parts later if we don't want it now.

Thanks,
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Re: Laser Interface/Driver PCB

Postby dirktheeng » Wed Apr 27, 2011 9:06 pm

Bart,

the prototype has a water flow switch option. Do you have a specific piece of equipment in mind for this, or do we have to make one.
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Re: Laser Interface/Driver PCB

Postby bdring » Wed Apr 27, 2011 9:31 pm

It will work with anything that completes the circuit like a switch that is in the closed position with normal flow. I anticipate designing more complex circuits in the future, so I added a power source on that connector to power an external circuit. It could be as simple as 555 missing pulse detector of a complete flow monitor controller.
Bart
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