Capacitive linear encoders

[bojan]

I felt that this encoder concept deserves the separate thread.
http://www.capsense.com/capsense-wp.pdf

The principle is not new - every digital calliper works that way.
The beauty of it is that it can be done with standard PCB layout process - and the achieved accuracy and resolution are impressive.
Also, it seems Mitutoyo sells the strip tape cut to size - when I found the source, I will post it here.

For example, if the encoder strip (standard resolution is 0.005mm) is attached to the rim of the 300mm diameter wheel, the angular resolution of such rotary encoder is 7arcsec!

[mswhin63]

Hi Bojan,

I have a couple of these on my desk now, I bought them ages ago and really haven't had the time to do anything with them.

I thought these would be good to work on due to low current consumption. This is the limiting factor on running a DSC on USB power only. I will get onto them one day although need to remember where I bought them from.

[snowyskiesau]

These sensor strips and read heads are available here (http://www.wixey.com/fence/spareparts/index.html).
I'm not sure if these models have a separate data port.

[bojan]

I know about this website (Wixey).. however on CNC forum someone mentioned they (or the new rep in Canada) are a bit slow in responding to small quantity enquires.. (see here) (http://www.cnczone.com/forums/benchtop_machines/115073-linear_encoder_can_you_buy-2.html)

The reading head is designed for linear movement measurements (the strip must be flat where the reading head is) but perhaps the small curvature of the strip can be tolerated :shrug:
However, U$35 (plus US29.99 for head) is not such a big loss in exchange for potentially excellent encoder (I can already see the direct application on small lathe or milling machine.. or focuser).

Attached are the dimensions of the strip (taken from before mentioned CNC forum)

Some Chinese suppliers may offer even a fraction of the above price for all this (Chinese calipers are using different data format at output port, it is not Digimatic standard protocol...
It is documented here (http://www.shumatech.com/support/chinese_scales.htm) and here )
(http://pcbheaven.com/exppages/Digital_Caliper_Protocol/)

[Poita]

I've used these for another application before - a digital plunger for a Video-Pinball table, like these:

http://www.hyperspin-fe.com/forum/showthread.php?p=18885

I haven't tried curving it at all, but the tape *is* flexible.