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A project log for DIY Digital Caliper

I design my own digital caliper based on STM32F103 blue pill.

dimitarDimitar 02/10/2024 at 18:290 Comments

I was think that I need get my hands on a a caliper first and disassemble it. To be honest I did not get any super interesting information out of that. I just ruined perfectly good caliper. The t-scale PCB is 0.5mm thick and has copper only on one side with very thin solder mask. It was covered by a sticker. The PCB containing the MCU and the sensor is 2 sided but populate just on one side. The interesting thing about it is that it is machined to fit inside the caliper head. Also the it utilizes COB (Chip on Board). I tried to strip the black epoxy, what I found is the the silica was placed as close as possible to the via of the sensor pad. Later on I recreated that as well on my own design.

I tried to measure what is the width of the T scale and the electrodes on the MCU PCB, but with no luck. It was hard aligning the two PCBs and also the solder mask did not made it any easier. All I could do is to test for contact with a multimeter. 

It wasn't immediately clear to me how the caliper works, so I decided to do some digging

First is a primer about how capacitor sensor work. Capacitive Sensors by L. K. Baxter. When we start running experiments we will go back to this document to be able to understand the variables in play when dealing with cap sensors.

Second is a article from a site called Yardo The biggest gold from it is this beautiful oscilloscope screenshot

From here we could see that we need to generate 8 PWM signals. We can group 2 together as one is inversion of the other. We could see that those signal generate sin waves with periods shifted 90 degree. Also we see the signal generated by the movement of the caliper. 

 The next article is the US patent for Capacitive displacement measuring device with t-shaped scale. From this, we can draw two key insights. One is that there were a lot of smart people in 1991 and also this block diagram:

It will help us later to figure out how to process the signal we measure back from the sensor pad. 

So far so good, but we still don't know how to design our T-scale and transition group. We could find that in this article called A High Precision Capacitive Linear Displacement Sensor

In this paper they are using two pairs of capacitors S+ S- and C+ C-. The orange shape should be the vertical part of our T-scale. We could clearly see that it should span over S+ C+ and one gap spacing.  

Armed with this information I will be making my own footprints for the T-scale and capacitor groups. 

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