Cheap Idea for a DRO

[1113562]

The PIC will do all the decoding and sampling and filtering and will upload a value on request from the display processor. It is fronted by a dual comparator to do the level shift and generates a PWM output to drive the -1.5v charge pump. The output will be in native format (note that the reader always outputs in imperial, metric conversion is done by the display processor)


No, not looked but doubt you will find one...


The 'machined bar' is a PCB, under the graduated label... see here for more info. This makes it hard to extend, but easy to cut down...

Thanks for that Irvine,

Thats interesting. Re the reader units. Am I correct in thinking that the 4 bit BCD type readers do change the transmitted values when switching between metric and imperial? If that is the case it would be a lot easier to get the reader to do the conversion rather than the top end.

Thank's for the insight into the capacitive sensor operation. I was not aware the bar has a strip pc under the label. That's a bit of a nuisance as my plan was to buy cheap 6" calipers and fit longer bars. making long strip pcb's is not out of the question but I guess that the whole reader accuracy relies on the pitch accuracy if the T and comb? Do you know if the comb fingers are isolated from each other or linked together, its hard to see on the photo. It looks like the Ts are isolated from each other.

[irving2008]

Am I correct in thinking that the 4 bit BCD type readers do change the transmitted values when switching between metric and imperial? If that is the case it would be a lot easier to get the reader to do the conversion rather than the top end.

Cant say, I have never seen a BCD one, all 5 of mine are the 24bit type. My DRO processor/display will always display in mm as I dont use imperial for metalworking - but if i did it wouldn't be hard to add a in/mm button. Anyway how difficult is adding:

 
double reading_in_mm[3];
for(int I=0;i<2;i++)
  reading_in_mm[i]=reading_in_inch[i]*25.4;
display(readin_in_mm[]);

Do you know if the comb fingers are isolated from each other or linked together, its hard to see on the photo. It looks like the Ts are isolated from each other.

Not looked closely, but I'd say they were...

[tumutbound]

If all you will end up needing is the reader PCB assembly have you found an import source just to buy these boards on their own?

If you're referring to replacement read heads for the calipers/scales, then they are available.
I have some scales I'm removing the LCD from prior to mounting on a mill and bought a couple of spare read head 'just in case'. They were only $8 or so each. These are 24 bit, not BCD type.

[irving2008]

Yes I looked at this...here in the UK (when I looked about 6mo ago) it was actually cheaper to buy a cheap 4" or 6" caliper as a replacement head (about £7) than the replacement head itself. It seems the expensive bit tho is the stainless steel channel and the PCB. I'm wondering if these PCBs are machined rather than etched? It might be possible to make a new PCB/channel on a CNC machine and get the accuracy needed - it would be interesting to try. I've not found anyone selling the channel seperately, maybe because they are pretty indestructable.

[BillTodd]

If anyone has figured the serial code or has already solved this I would love to know, there is no point in reinventing the wheel.

Here's my version: http://www.cnczone.com/forums/showthread.php?t=62704

There's commented PIC code and circuits code buried in the thread.

[1113562]

Not looked closely, but I'd say they were...

I have had another look at the photo and it does look like the fingers are all linked together with a thin continuous strip of copper along the bottom edge. I will have to pull the graduated label of mine to check. This would make butting two strips together to make a longer length tricky as you would need to retain this electrical connection across the joint. Better obviously not to try and butt together and to make as one length. Etching a long PCB would be difficult due to need for long light box, long artworks and etching tank etc.

[1113562]

Cant say, I have never seen a BCD one, all 5 of mine are the 24bit type. My DRO processor/display will always display in mm as I dont use imperial for metalworking - but if i did it wouldn't be hard to add a in/mm button. Anyway how difficult is adding:

 
double reading_in_mm[3];
for(int I=0;i<2;i++)
reading_in_mm[i]=reading_in_inch[i]*25.4;
display(readin_in_mm[]);

Not looked closely, but I'd say they were...

Here's my version: http://www.cnczone.com/forums/showthread.php?t=62704

There's commented PIC code and circuits code buried in the thread.

I have had a quick look at your circuit (not your code yet). I use that size PIC a lot so I have lots in DIP and surface mount. With the way you drive the reader board do you not get problems with the grounds as the reader +1.5V is grounded to the SS metal parts of the caliper. I guess the PC 0V on the com port should not come into contact with your mill/lathe machine ground?

[John S]

update - it seems LittleMachineShop in the US have a source for the cables again... pricey tho... MEDW advertise them in the UK for £11 +P&P

Just got back from the Harrogate show and the Machine DRO guys have some cables for £7.50 each.

[BillTodd]

I have had a quick look at your circuit (not your code yet). I use that size PIC a lot so I have lots in DIP and surface mount. With the way you drive the reader board do you not get problems with the grounds as the reader +1.5V is grounded to the SS metal parts of the caliper. I guess the PC 0V on the com port should not come into contact with your mill/lathe machine ground?

If you look at the multi-scale reader, I use an external PSU (wall-wart) to power the reader and off-set the supply to +3v5 and -1v5, so that the PC and scale grounds can be connected together.


My code handles both BCD and Binary scales automatically by measuring the clock pulses at switch on. It passes the scale data to the PC in string form, the PC/VB app determines which format and unit the data is in, then converts it to Inches (which is the inherit unit of the binary scales). Objects within the app convert the unit as required. I find that displaying MM and Inch (decimal and fractions) simultaneously is VERY useful.

If you're planning a display device make sure it's got an X2 (for lathe use) and a reverse button (easy swithing from absolute and relative is also very handy).

[irving2008]

Just got back from the Harrogate show and the Machine DRO guys have some cables for £7.50 each.

The cables are available now for £8.75 inc P&P and will be on their website in the next week or so according to Tony Ward at Allendale.