Cheap Idea for a DRO

[BillTodd]

R? is selected to give about 1v5 across the red LED (probably in the region of 330R) this only necessary because the 1v5 scales have the cell +ve connected to the metal body, so to avoid grounding problems I've shifted the PICs Vss supply (and the scales own Vss) 1v5 below the system (PC and Machine) ground.

If your scale has a 3v lithium cell you may have to rethink the power supply, because I'm not sure if the 3v scales have their metal body connected to the +ve end of the cell or not (none of mine are 3v) .

The other LEDs in the outputs are just to limit the voltage to the scale when the PIC output commands.

[BillTodd]

Double post caused by this bloody snow!

[ecat]

k,

All callipers are 1.5v.
Maybe I have some wack LEDs, don't think I've measured less than 1.7v across the one in the power section and that was with >1k resistor, not good for maintaining voltage stability at those values. I thing the vf of my reds is around 2v

[BillTodd]

Try a couple of forward biased silicon diodes (e.g. 1n4148, or even 1n4000 series would do), they should give you about 0.7v each .

[ecat]

YaY

Progress of sorts.... Or maybe just chance!

43 48 3A 49 58 0C 30 0C 44 41 43 49 4D 41 4C 0C CH:IX.0.DACIMAL.
49 4E 43 48 3A 49 58 2C 30 2C 44 65 63 69 6D 61 INCH:IX,0,Decima
6C 2C 49 6E 63 68 3A 00 18 3A 03 18 3A 02 18 3A l,Inch:

May not look like much bit it makes me happy...

Had a dodgy scope and a very old Thurlby storage adaptor on it, it looked very much like the transistor was slow in turning off. The above is the first time I've seen anything approaching English, trick was to half the value of the 3k3 collector resistor.

Unfortunately the above was looking like a 1 off :(

Undeterred I added a 2nd and 3rd parallel 3k3 ending up with 825r collector resistance. heh. It's alive :)

49 58 0C 30 0C 44 45 43 49 4D 41 4C 0C 49 4E 43 IX.0.DECIMAL.INC
48 3A H:

My Prolific dongle is only giving about -4v on DTR, problem?

[BillTodd]

Hmm. -4v seems low, but if it works... I know I'm abusing the serial port, but I've used this circuit many times may be the prolific device is different to mine???

BTW are you driving it from a PC or laptop? (laptop USB ports seem to lack the ooomph required for most USB devices)


You could try sticking a 10u cap between the DTR line and Vss just to stiffen the -ve rail a bit.

Bill

[ecat]

More progress \o/

The sharp eyed would spot the problem in my previous post, I had no commas! That's right, every comma, 2Ch, was coming through as 0Ch which is form feed :( I tried reducing the collector resistance a little more and the comma appeared but so did the occasional funny in the rest of the message :( :(

A little reading showed me another trick, adding a resister between base and emitter is a common way to speed things up. 3k3 between b and e and things appear to be working beautifully :) :)

Hmm. -4v seems low, but if it works... I know I'm abusing the serial port, but I've used this circuit many times may be the prolific device is different to mine???

BTW are you driving it from a PC or laptop? (laptop USB ports seem to lack the ooomph required for most USB devices)


You could try sticking a 10u cap between the DTR line and Vss just to stiffen the -ve rail a bit.

Bill

Indeed Bill, -4v does look poor, not much margin for error. I'm connecting to a PC but via a powered hub. Another experiment perhaps.

The DTR line looks quite solid, at least it did before my latest changes, heh. A cap certainly wouldn't hurt, thanks for the tip.

As of now your PC app is mostly happy with the board. A couple of range errors when I simulate the calliper with a finger swipe but no data errors so far. Thanks again for your help :)

[ecat]

Not wanting to damage my actual scales I decided to risk my cheap but incredibly useful callipers at 1.88v. On the up side I know the meter I was using reads about 0.03v over actual, on the down side 1.85v is still around 20% above the voltage of typical fresh SR44 type battery . I'd normally consider 10% over to be a maximum safe(-ish) voltage but so far so good , I have the PC app working and I've set the scale to -6.66mm for luck. I'll leave it running for a couple of days and see how it goes.

Thanks again Bill, elegant circuit and nicely crafted software

[irving2008]

Gosh you are a busy bee Sean!...

[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 :)