Cheap Idea for a DRO

[1113562]

Hi Irving,

That sounds like a good approach and one I would take re the balanced tranceiver at the read head. Is your small pic down there doing the timing and stripping the word off for transmission. I take it your I2c is bi directional for reset and imperial/metric mode selection as down-link commands?

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?

Have you mounted these readers on your own machined bar that replaces the caliper bar they come with? If so, was there any particular features that you had to accurately emulate to make the capacitive sensor work reliably and accurately. I'm still mystified as to how these things work as a sensor. When I first opened one up I was surprised when I did not find an optical encoder rolling on the bar or other optical solution. Its certainly a neat non contact sensor.

Cheers,

John

[irving2008]

That sounds like a good approach and one I would take re the balanced tranceiver at the read head. Is your small pic down there doing the timing and stripping the word off for transmission. I take it your I2c is bi directional for reset and imperial/metric mode selection as down-link commands?

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)

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?

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

Have you mounted these readers on your own machined bar that replaces the caliper bar they come with? If so, was there any particular features that you had to
accurately emulate to make the capacitive sensor work reliably and accurately. I'm still mystified as to how these things work as a sensor. When I first opened one up I was surprised when I did not find an optical encoder rolling on the bar or other optical solution. Its certainly a neat non contact sensor.

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...

[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...

[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.

[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.