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Constant output from measuring angular speed

  1. Jan 15, 2009 #1
    Hello all,

    Im new to the site so please dont hurt me :blushing: !

    My final goal is to measure linear speed of a pedal powered car, and send the speed wirelessly to my laptop which will record the speed profile.

    My problem is that I cant find a sensor that will measure angular velocity and output a constant voltage (which a speed can be calculated from) ! I can only find sensors that output pulses. The reason I need a constant value is due to sample rate issues with the wireless kit Im using to send the data, I can only sample at around 0.5Hz, which is fine if im sending actual speed values once/twice a second, but not fine if I have to process loaaadsss of pulses after the singal has been sent wirelessly. I need a sample rate of around 200Hz if I am to send a square wave to laptop.

    Any ideas of a sensor that can do such a thing ? I thought it would be easy to find, but I cant find it anywhere.

    Any thoughts would be very helpful
     
  2. jcsd
  3. Jan 15, 2009 #2
    Tell us a little more about the wireless kit and how you'll processing the data. What frequency? What type of modulation? Why does the sample rate have to be so low? I believe a sample rate of 0.5 Hz is once every two seconds, not one/twice a second.
     
  4. Jan 15, 2009 #3
    Use a sensor that outputs pulses and use a frequency to voltage convertor. Then you can send back at .5 hz or whatever you like.
     
  5. Jan 15, 2009 #4
    Yeah silly mistake, I meant once every two seconds.

    Im currently looking into using a frequency to voltage converter. If I know the voltage of each pulse, how easy is it to work out what components I would need to make a circuit such as this :

    http://www.national.com/mpf/LM/LM2907.html

    (the typical application diagram)

    Thanks
     
  6. Jan 15, 2009 #5

    berkeman

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    Staff: Mentor

    The voltage ofr the pulses doesn't matter, at least not if the average value is 0V, as shown in that figure (notice the comparator monitoring the pulses is comparing with respect to GND.

    Is the laptop going to be in the vehicle? Why does it need to be wireless, or maybe it doesn't but you want to incorporate it to learn more?
     
  7. Jan 15, 2009 #6

    berkeman

    User Avatar

    Staff: Mentor

    BTW, a DC motor will serve as a pretty good angular speed sensor.... Just couple something frrom the drive train to a small (low-drag!) DC motor, and measure its output voltage and make a calibration curve. You'll want to put a snubber circuit (low pass filter) on the output to keep brush noise from messing things up.
     
  8. Jan 15, 2009 #7
    Personally I prefer an optical encoder. You can easily create one on a computer and print it out on transparent plastic. Measuring the time from rising edge to rising edge is easy with a microcomputer and making it simple to calculate velocity.
     
  9. Jan 16, 2009 #8
    The vehicle runs on pedal power, battery power and solar powar. It needs to be efficient as possible in terms of weight and performance as it will be racing. A support vehicle will be nearby throughout the race, analysing and recording performance data - so that race strategy can be changed depending on road gradients/sunlight levels/battery power remaining etc etc

    Would this really be possible to setup causing negligible drag ? Efficiency is of utmost importance. Also, I need to find a solution as quickly/easily as possible (deadlines are approaching!!!), would it be easy to build a LPF ? My electronics knowledge is very limited. I can get components and build a circiut, but how difficult would it be to decide on the components etc?

    Is implementing a microcomputer easy ? My knowledge is massively limited when it comes to building circuits. If I managed it, would the microcomputer output a constant value (at a constant speed) or would it be a pulse/frequency (the output im trying to avoid).

    Ideally I want a single steady voltage or current output which corresponds to speed (or freq.) using electronics and signal processing as little as possible. So basically im searching for the simplest solution.


    [I hope im explaining my situation/problem well enough so that it makes sense...please tell me if otherwise]
     
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