Wednesday, August 29, 2012

Omega Strain Gauges

Parts are starting to arrive for the V3 Power Meter. The first item to arrive in are the new shear strain gauges. I’ve never used Omega Strain Gauges, only Vishay at this point. Both companies have very similar claims in terms of tolerances for resistance and sensitivity, however on receipt of the gauges something is very noticeable. (I should be picking up some backordered strain gauges on Friday from the Vishay dealer here in Ontario. I’ve had several calls from the HBM rep for work. They don’t currently make any gauges suitable for my application though.)
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What I immediately notice is that the Vishay packaging contains tested data from the batch.  You see the batch and foil is tracked, they provide thermal coefficients, and gauge factor and tolerances. Omega has gauge factor and Tolerance and batch data, but it’s not readily apparent what the tolerance is of (resistance or gauge factor). While I won’t be using the thermal coefficients I do believe that the Vishay is providing a greater amount of testing data which might give more confidence.

There is a big however; most of this data is relatively meaningless in terms of creating a load cell. The reason for this is relatively simple. The shear gauges are full bridge, so they are thermally compensated. My recent experience using strain gauges in research related to the Nuclear industry is that I’ve been able to get amazing accuracy out of half bridges. In some cases my results have been < 0.3% different from theoretical. During testing we found half bridges / full bridges were much more thermally stable than quarter bridge arrangements. For those who may need some background, check out Wikipedia on wheatstone bridges. So that takes care of the thermal coefficients.

The sensor is going to be calibrated against a known load and thus the gauge factor has little impact. The only factor of significance is the resistance tolerance with respect to each other gauge as this will affect any offsets present in the wheatstone bridge. My V2 setup uses a gain of 200 and a voltage of 3.3V supply. If for instance the wheatstone bridge has small mismatch of resistance causing a 0.5mv offset, this could become 0.1V after gain, or about 3% my entire voltage range. However since I’m only measuring torque in one direction this could potentially be 6% as I could lose have the available range if my reference is Vss/2.

On to the actual gauges.
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Small double shear gauge next to the 20 x 20 mm AP2 module.I think it’s about 8mm x 10mm. I was impressed by the quality of the grid. This is not even considering these gauges are about 10 dollars each, where the other single shear gauges I’ve used from Vishay is $35 each and the double bending bridge ones are about $22.

In order to make a power meter that measures each leg you will need two of these gauges. Total cost is about $20. Using my double bending bridge gauges you need 4 x 20 = $80. This would have a significant impact on cost.

For anyone who has read Hunter Allan’s Book he lists out how many gauges per power meter each has and it feels like it’s implying more gauges means more accurate. This is not the necessarily the case.
SRM and Quarq both need to use a decoupling algorithm. This means that the stiffness of the surrounding area affects how much each arm of the crank spider arm affects the torque. A professor at my university showed me how this works with a stuart gough load cell with 6 arms that only measure axial forces to determine torque and forces in all Cartesian directions.

The choice on how to instrument the crank determines how many gauges are used. Most commercial load cells are a single full bridge transducer. This is essentially my design as I do not require any decoupling algorithm. So in short my design will have 4 “gauges” on the one backing for each leg, totalling 8.

3 comments:

  1. Thanks for documenting this Keith. To be honest I've only discovered your blog yesterday and have begun to reading from post 1. Likewise, I used to follow Metrigear's development blog as well before they got taken over by Garmin. I'm a mechatronics engineer, I write a cycling blog and I hope to gain some insights from your development endeavor. Thanks!

    -Ron George

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  2. Yes great review, thanks. I've seen some great strain gauges here too: http://www.directindustry.com/industrial-manufacturer/strain-gauge-73369.html

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  3. The principle of operation of the strain gauge load cell is based on the fact that the resistance of the electrical conductor changes when its length changes due to stress. Cu Ni alloy is commonly used in strain gauge construction as the resistance change of the foil is virtually proportional to the applied strain.

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