Electronics > Projects, Designs, and Technical Stuff

DIY Digital Caliper

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MitkoDyakov:
Hey!

I am trying to design a digital caliper. So far I have got my hands on a couple of cheap calipers and a few papers and patents. I have create footprints for both the scale and the sensor board and seem to be working quite well. You can follow the build log (link in the bottom) for more details, also the footprints are available if you want to check them out too. 

https://hackaday.io/project/194778-diy-digital-caliper

The difficulties I am having now are connected with conditioning of the signal I am receiving. 

I used this schema to get a reading with my scope



The result is as follows:



Now from what I gather I need to amplify or at least buffer this signal, then transform it to a sine wave and then detect the zero crossing. Compare the phase shift to a base sine frequency to determine the direction and displacement.

1. Are my assumption of how the thing works correct at the first place?
2. What should I be looking for when designing the amplification circuit?
3. How do I choose R/C values for creating my sine? 
4. How do I detect zero crossing when my sine wave does not go negative?

Cheers,
M

ali_asadzadeh:
Just following the project, I think you can either choose hardware zero cross detection, say you would make an op-amp comparator with the one side of op-amp set to mid VCC scale, which does not works perfectly or you could choose the second option which is to apply DSP algorithms to Detect Zero crossing, which needs reading the wave form with ADC and apply some math formulas to it, and extract zero crossing.

tszaboo:
For a quadrature encoder, which things are based on, you need two signals. So I'm going to assume you are only showing one of the signals. I think some of the issue comes from the 22 MOhm value that you have on the board. A smaller value would probably make the signal cleaner.
Then about how to amplify the signal: Comparators. You feed the output of the opamp into a comparator with hysteresis (feedback resistor from  the output to the DC input).

MitkoDyakov:

--- Quote from: tszaboo on April 22, 2024, 10:32:34 am ---For a quadrature encoder, which things are based on, you need two signals. So I'm going to assume you are only showing one of the signals. I think some of the issue comes from the 22 MOhm value that you have on the board. A smaller value would probably make the signal cleaner.
Then about how to amplify the signal: Comparators. You feed the output of the opamp into a comparator with hysteresis (feedback resistor from  the output to the DC input).

--- End quote ---



The dotted line on the first row is the resultant sine wave if sense pad is located on top of all excitation pads. I call this the base frequency, mind you that it does not exists physically (you can't measure it with a scope). 

And depending which way you shift the caliper head the resulting sine wave you measure on the sense pad get phase shifted to the right or to the left.

So I don't have two signal in the sense that a quadrature encoder has, but rather one signal to measure to determine the phase shift.

moffy:
You are talking about a sine wave but it looks like you are using a square wave for excitation is that the case?

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