Strain gauge for exercise bike.

Discussion in 'The Projects Forum' started by Eddie Van Current, May 6, 2015.

  1. Eddie Van Current

    Thread Starter New Member

    May 6, 2015
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    Hi everyone, my first post :)

    I do a spin bike class every week and we use a static bike with a heavy flywheel and an instructor who shouts a lot. The resistance of the pedals (ie how hard you have to push) is governed by a threaded rod which screws down a pressure pad onto the top of the flywheel. We have to adjust the resistance setting on the instructors commands to simulate hill climbs or for sprints, he'll say quarter / half / full turn either way to make it harder for us. The trouble is, all the bikes are a bit different and we end up out of sync so each person is just setting the resistance to their own preference. This is ok if you are reasonably self motivated but it does make it easy to cheat.

    The pressure pad is connected to the bike frame with a thin, sprung steel, strip of metal approx 75mm long and 15mm wide. When the thumbwheel (threaded rod) is rotated counter clockwise, the pressure pad moves upwards (relieving the tension from the sprung steel strip) and vice versa. The overall movement of the steel strip is just a few mms.

    I was thinking, how simple could it be to glue a strain gauge onto the metal strip and connect it to a circuit that results in a 0 - 9 on a digital display or a row of leds.
    For me this would an extremely challenging project. Hence my post and humble request for help from you most knowledgeable people. I do have vero board and a soldering iron. I can transpose a circuit diagram to afformentioned vero board (but maybe not 100% first time). It's been a while.

    Calibration will have to be worked out, possibly with a feeler gauge for the zero and torque wrench for the nine. I see this as a couple of trimmers for high and low adjustment with, maybe, a 3 way centre off rocker switch for low calibrate - normal display - high calibrate. Some sort of display bearing in mind the room is fairly dark and we watch bike videos (whilst wearing padded lycra shorts - is this the right forum?) and a PP3 as a likely power source.
    The range of movement / output of the strain gauge is unknown until I make one to try, I realise this will have an effect on component values, some starting values would be a great help. The exact positioning of the gauge from bike to bike may also have an effect, as will variations in the sprung steel strip (each gauge is likely to have different resistance 'swings' (excuse me, I don't know the correct term) but 5 on one bike may be different to another bike even with 0 and 9 calibrated. I may be trying to think too far ahead as I won't know until I try it and it may not be a problem. Is it hysteresis?

    When you pedal faster (sprint) with a bit of resistance on, this may have the effect of pushing the pressure pad upwards a bit and the sprung strip with the strain gauge will also be affected. This will probably be a very small movement which won't be detrimental to the overall benefit of having a consistent measurement of effort.

    I'm really open to ideas and would appreciate any guidance offered.

    Thanks
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,026
    There are commercial solutions for measuring pressure on the pedals (or maybe crank strain), which is ultimately what the rider feels. I'm not keen on the spring strain idea because, while you might be able to get a good measure of that, it doesn't necessarily correlate to resistance to the rider. Something like the pull or push on the whole spring assembly might be closer. Could you supply a picture? I might have it wrong in my head.
     
  3. Eddie Van Current

    Thread Starter New Member

    May 6, 2015
    3
    0
    I understand, it was my initial thought for a means of measurement without involving too much mechanical engineering. The bikes are called Lemond Revmaster. [​IMG]
     
  4. wayneh

    Expert

    Sep 9, 2010
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    There should be a clever way to insert a load cell at the joint where the pad pivots on the yellow post, so that the tangential force on the pad (same as the force on the wheel) is measured.
     
  5. Eddie Van Current

    Thread Starter New Member

    May 6, 2015
    3
    0
    That would be ideal but would no doubt need a bit of chopping about. It doesn't need to be very accurate but there would need to be reasonable consistency . For example, if one bike was at '3' then you would expect another bike on '3' to be near enough the same, but the '3' wouldn't equate to any specific pad pressure. I don't know what sort of discrepancies may be adjustable within the electronics, I was more into fixing things and didn't get involved with design work after college, in 1982.
    If you think that using the spring will not get the right results then I'll probably give it a miss anyway, I need to keep it simple. I am grateful for your replies.
     
  6. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,026
    Maybe the solution is a sort of mechanical calibration, so that the dial position really does come close to the same applied pressure for each machine. For instance you could make sure that "0" on each dial is when the pad just touches the wheel, something like that.
     
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