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  1. #1
    I've been looking ito the idea of a cheap DRO for X Y and Z axis, not essential I know for a CNC mill but a nice to have for reassurance and for manual operation. The cheap Lidl and Aldi Digital calipers all seem to use the same basic design of of display and linear position circuit board that works on some clever capacitive non contact arrangement with the bar it slides on. The small 150 mm calipers are not just limited to that length either as I have removed the display head and you can carry on sliding it along and the display keeps on going for at least 700 mm or more. The best thing though is they all have a gold plated edge connector which can be used to transmit the digital read-out signal to a remote display. The output is a 24 bit digital signal with a timing clock and the value is transmitted every 320 ms (about 3 per second). Negative values are supported as well. The value is transmitted in both imperial inches as well as metric to 0.01 mm resolution. The 4 pin connector connects to power (1.5 V and 0V on my workzone) and the other two pins are clock (timing) and data. The whole data word (all 24 bits) is transmitted in 100 uS in 7 bursts of 4 bits so actually there are 28 bits. I have yet to fully decode the bits but I assume they are a binary word according to the chinese converted to english manual

    When the unit switches off after a timeout (on my workzone) it is actaully still transmitting so just the local display switches off. So, no need to worry about this, but I think the powerfix version with the 3V CR2032 cell may fully switch off after a timeout of idle, I need to check.

    I need to work on figuring out the code of the transmitted value but once understood a small PIC processor could be developed to drive a large segment digital display in the correct units.

    More work is needed but at 8.99 a shot these calipers are worth modifying and fitting to your machine as a pretty darn accurate DRO.

    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.

    PS: I have tried to get hold of a connector and cable plus remote software for a PC but although this is hinted at being available I can't find a source. These are imported devices from China and the UK importers are completely useless.

    Some of you may be interested in the developments of this project for your own machines.

    john

  2. All the figuring out has been done...

    See here and here and here

    and here is my own breadboard with a ex-POS terminal VFD for the display...

    Click image for larger version. 

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    oh and BTW, you can't get the leads on their own any more.... or at least no one seems to be able to. I was working on making a connector - here is one I managed to get..
    .Click image for larger version. 

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    Last edited by irving2008; 06-05-2010 at 11:37 PM.

  3. #3
    Thanks Lad,

    I say that as it appears your about 4 years younger than me.

    I knew someone would have sussed this all out as these things have been around for ages.

    It appears I have a BCD version which transmits the seven packets of 4 bits.

    I had already sussed out the odd bias that earths the +1.5V butten cell to the metal parts and that the clock and data lines go negative from this reference.

    The links you provide will save me loads of time particularly the different protocols.

    I'm surprised no one is knocking out cheap displays and cables for these caliper scales? What has happened to your prototype? Has it stayed in that breadboard form or have you advanced its build state? I would be interested to know what processor/controller you used for the decode/display driver.

    I'm not too fussed about a connector to the gold contacts if they are not readily available as I would have just soldered wires onto the board. its not worth messing around for a 9 scale is it.

    Have you managed to successfully transfer the scale head of these things onto a longer bar for machine mounting? I'm not clear how accurate one needs to emulate the original caliper slide bar as far as materials and texture need to be. Your comments on this mechanical aspect would be appreciated.

    I will look at my other caliper recently purchased to see what remote display protocol it uses.

    Thanks for you help so its been so refreshing to get expert advice for once.

    Cheers,

    John

  4. You can pick up a simple 3 axis display for these cheap calipers/scales for about $US190. Try CDCO or Shars.
    I've a couple of the Shumatech units as mentioned in a previous post. The older model of these is still available as a kit from Model Engineers Digital Workshop in the UK
    Regards
    Geoff
    My home

  5. I'm still waiting to find time to make a proper PCB. Software is pretty much done. Its a PIC16F477 if my memory serves. The PCB is a generic board I am making that has a RS232 port (for the VFD display) a keyboard (4x4 matrix) port and a LCD display port (for HD4470 style 4bit LCD displays) and a general IO port.

    The way I am implementing my scales is a small PCB containing the level-shift comparators, a small PIC and a -1.5v charge pump that is attached to the reading head. It will communicate the reading over a 2-wire i2c link back to the display processor - the link will also be the +5v power to the PCB. Eventually I will build the read-head into a purpose-milled plastic case and remove the display.

  6. #6
    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

  7. #7
    $190 is not what I would call cheap in comparison to the read head scales especially for a home project. Thanks for the links though.

  8. #8

  9. Quote Originally Posted by 1113562 View Post
    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)

    Quote Originally Posted by 1113562 View Post
    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...

    Quote Originally Posted by 1113562 View Post
    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...

  10. 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
    Last edited by irving2008; 08-05-2010 at 11:31 AM.

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