1. This forum section is a read-only archive which contains old newsgroup posts. If you wish to post a query, please do so in one of our main forum sections (here). This way you will get a faster, better response from the members on Motherboard Point.

Need absolute rotary position encoder

Discussion in 'Embedded' started by John Harlow, Nov 12, 2004.

  1. John Harlow

    John Harlow Guest

    I have a design which requires absolute position sensing in about 330
    degrees of rotation; quite similar to a potentiometer but needs to be rugged
    (optical?) and low cost. This will be a rather hostile environment think
    automotive) so I'm pretty sure a standard potentiometer wouldn't last long.
    The device should be sealed to withstand the equivalent of a marine
    environment.

    The output would be preferably digital, but analog could be made to work as
    well. Repeatability is the primary goal over accuracy.

    An ideal device would be one which reported shaft position in degrees or
    some other metric.

    Does anyone have any suggestions?
     
    John Harlow, Nov 12, 2004
    #1
    1. Advertisements

  2. On Thursday, in article <>
    How fast is this rotation? Max speed of rotation affects what will work.

    How often do you need to read results?

    Are there any positions (like end stops) that must be read precisely and
    repeatabile to greater accuracy than any other positions?
    How many readings Min and Max are required and can be processed?

    To what resolution and accuracy? Do you want to read every 5 degrees or
    down to 0.5 degrees?

    There are various shaft and rotation encoders around that either produce
    quadrature encoding or digital o/p (usuaully 256 values), but rarely have
    end stop information, the most crucial part everyone forgets on limited
    travel equipment driven by motors.

    Other various forms of position sensing (linear, displacement or rotational)
    are often done as potentiometers made of conductive plastics for long life
    and easy sealability.
    The first company I would look at for this sort of thing even sealed for
    marine (including submersible) components would be Penny and Giles, now
    part of Curtis Wright group.

    <http://www.pennyandgiles.com/>

    I have used them for faders, joysticks on studio video and remote control
    equipment and for other industrial applications over the years. They are
    damn good parts to use at the user interface and position sensing but most
    require A/D, which means less wires from point of sensing to capture point.
    This way you determine how accurate you want results.

    Digital O/P methods mean you have to keep up with the data stream of variable
    data rate and ensure you are reading a steady value (even in Grey coding).
    Some require to clock every value in which may be too much overhead
    depending on your system and interface. The generation of a clock may be
    a challenge in itself as some produce 8 bit outputs from the sensor and you
    have to work out changes and clocking methods, which at higher rotation
    speeds can be tricky.
     
    Paul Carpenter, Nov 12, 2004
    #2
    1. Advertisements

  3. John Harlow

    John Harlow Guest

    How fast is this rotation? Max speed of rotation affects what will
    It's quite slow; on the order of 100 rpm max.
    It will be polled a few times a second (perhaps 10).
    No, and I have an alternate means to detect end stops, so I'm not concerned
    about that.
    I'm not sure what you are asking. The position will be arbitrary between 0
    and ~330 degrees and should be encoded thusly.
    A degree or so would be plenty. A 256 code encoder would likely suffice.

    Not needed in this situation as I have another means to acquire end
    information, but good point.
    Yes, I remember them from my days of working in the audio post production
    industry; they make some top of the line moving fader controls.

    I am looking at the
    http://www.pennyandgiles.com/products/products.asp?strAreaNo=402_11&intElement=1202&intIndustry=2 -
    it seems to be an excellent fit depending on the price. I will have to go
    through the process of finding a disributor and getting pricing.

    Thanks for the info!
     
    John Harlow, Nov 12, 2004
    #3
  4. On Friday, in article <>
    If a new digital reading is presented every 3 degrees then at 100 rpm, you
    could get

    330/3 = 110 readings per 'revolution'

    110 * 100 = 11,000 readings per minute

    11000 / 60 = 183.3 readings per second
    If digital o/p not clocked you will have to take a SERIES of readings to
    ensure you are reading a stable value (not on point of change).

    .....
    What if you read 10 times a second, then the other 170 odd readings that
    are changing on the inputs and missed. How do you ensure you are reading
    valid data and not last value or change data?

    With missing data you do not know which bit is the one bit changing in a
    grey code. Grey code is only useful if you read ALL values to monitor the
    changing bit which changed.

    With analogue at 10Hz reading rate, you can filter it and determine your
    reading rate and average easier.
    Beware of missing values (by ytour sampling method) when using digital
    devices, unless you put something to buffer the values accurately. Clocked
    per change position of revolution.

    ......
    Main advantage is that on faders they give full potentiometer travel
    WITHOUT gearing on a small travel device.
    Glad to be of help.
     
    Paul Carpenter, Nov 12, 2004
    #4
  5. John Harlow

    Gary Pace Guest

    Gary Pace, Nov 13, 2004
    #5
  6. Not exactly. With Hiperface, quadrature [1] gives 1 bit (IIRC) more
    precision than the serial interface. Additional resolution is achieved by
    A/Ding the sin/cos signals and doing a bit of trig. It's important to sample
    both sin and cos at the same instant.

    [1] Achieved by feeding sin/cos into comparators, giving 4 states per
    sin/cos cycle.

    Steve
    http://www.fivetrees.com
     
    Steve at fivetrees, Nov 14, 2004
    #6
  7. John Harlow

    matt Guest

    Gary Pace wrote in message ...

    first time I've heard Heidenhein being recommended as low cost . Excellent
    products but orders of magnitude more expensive than the OP's target price.

    best regards,
    matt tudor
     
    matt, Nov 15, 2004
    #7
  8. John Harlow

    Paul Burke Guest

    Sounds like a job for a resolver. If you aren't too worried about speed,
    you can construct a simple phase tracker using a CPLD, otherwise Analog
    have low(ish) cost RDCs available (often on long leadtimes though.

    Paul Burke
     
    Paul Burke, Nov 15, 2004
    #8
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.