Thursday, April 28, 2011

Z-axis homing & opto-isolation board

It is always uncomfortable bringing logic-level signals outside of the "box" and into the real world.  Any excess voltage can wipe-out your stepper controller board - and since the inputs are fairly high impedance, these lines are very susceptible to induced noise - especially from the nasty spikes caused during the driving of the stepper motors.
I designed a 5-channel opto-isolator board to keep the ugly outside world away from my HobbyCNC controller board.  I designed the board to run from around 12 volts, but I added a constant-current source to each input line.  This ensures 20ma flowing through the opto isolator LED.  The added benefit is I can add extra LED's to the 'chain' without affecting the signal and I can safely run the limit switch wiring through the same wiring harness as the stepper motor cables with no fear of interference.
I added a yellow LED on the board for each of the 5 inputs - this allows me to verify an input is working without needing to grab a voltmeter.  The constant current source also allows me to put another LED out where the work is done.
To complete the board is a green power LED and a fuse.  In the images, the opto chips are actually surface-mounted on the underside of the board - the one lit yellow LED is for the limit switches - this tells me all 6 limit switches are closed (good).

video
I created a special rig to set the z-axis home.  It uses an alligator clip and a precision brass 1/2" square bar.  I added an LED in the bar so I can confirm all is good before I press "home".  Any problem with this setup and the system will drive the tool into the workpiece - a real big issue if using small drills for PCBs.  The short video shows the Z axis home setup being tested to confirm continuity, then the video shows the opto board during the same test.

4 comments:

  1. nice work. Do you have home switches in each end of axis?How your paralel port does look ? How many free pins do you left?

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  2. I have limit switches (not home switches) at the end of all 3 axis - these are far more important than I originally thought. Currently, I only use the software to home the z axis. I am noodling on a way to home all 3 axis'. It is super-sweet to hit "home" and the system does all the work. There are 5 input pins on the HobbyCNC board, so far I am using 2 - one for limit switches (all wired in series) and one for the z-home jig.

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  3. And how did you configure it in emc?

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  4. I posted the details here: http://woodworkerbcncrouterproject.blogspot.com/2011/05/configuring-emc2-for-limit-and-home.html

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