Cheap Idea for a DRO

[ecat]

I love the idea of absolute position feedback, should all but eliminate concerns about backlash. I also love the sound of pneumatic drive, well if it sounds like the doors from StarTreck :)

How are you planning to address the problem of overshoot? The nice thing about steppers (and servos?) is that you can never overshoot by more than 1 step (or the error response time of the closed loop in the servo?). I know nothing about pneumatics but I imagine it to be a 'springy' sort of drive which, when coupled with the 20ms response time of the callipers could lead to problems?

One note on your circuit: If I understand your description correctly, the metal work of the callipers will be +1.5 Volts above USB ground. USB ground is of course at PC/PSU ground. I've just tested a spare PC PSU I have lying around and it appears the PSU ground is connected directly to mains earth, could be a problem if the bed is earthed and the callipers also make an electrical connection to the bed? You look to be using a laptop and I think the power brick used by most laptops usually float with respect to mains earth, I'm never sure if this is a good or a bad thing, so I guess you'll be ok for now.


Quick update on my 1.8v test:
All looks good though I'm still cautious of trying out the real scales. I do like the LED's so I'll investigate swapping in some high brightness red devices which appear to have a lower forward voltage drop. The only problem I did notice was the occasional jumping of the measured value, a 100uf capacitor fitted close to the calliper end of the cable fixed that nicely :)