Sunday, August 12, 2012

The Power Meter Origin

Once upon a time during my Master’s degree I was exposed to building a load cell. It was a very very small load cell. In reality this was just an exercise in strain gauging that didn’t go anywhere for the degree. It did give me some exposure to something interesting and useful. In fact, as a direct result of this my current company sent me away for professional training to ensure my skills were sufficient for a testing program.

I’m going to give the abbreviated version of this unfinished website.

This is what I’ve been referring to as V1. It proved a concept, but at the time my electrical experience with strain gauging wasn’t sufficient to see the problems. Mainly the prototyping board and loose wires plus the high gain give very noisy results. The cheap completion resistors don’t help either. Vishay has a training course that is 3 days long and about 1200 dollars. If you are serious about strain gauging I suggest you do it or get an experience person to run you through the basics of everything with a test piece or 3.

Strain Gauges from Vishay. General purpose gauges with a protective cover on the gauge face.

The FSA crank which I will be instrumenting

The Arduino and bridge completion resistors. *Don’t use these cheap resistors for bridge completion. Your project will be a failure… why? Drift. These resistors have 300-500ppm/degree drift. What you want is wirewound or preferably thin film. Vishay make them. They are around 5 to 15 dollars each. My V3 design will eliminate most of these costly resistors, the first step to eliminating them is a full bridge strain circuit. Most sensitive and can be lower cost – sometimes.

Tiny Axial load cell with poor soldering directly to the gauge from my Masters program.

Sanding time. Need to get down to the bare metal and prep the surface for the gauge application.

Okay bare metal time, sanding in stages

120 grit go!

Progressively sand. 320 grit then 600. There is no need to go beyond this. If you polish the surface the glue won’t have much to adhere to, but you don’t want it too lumpy or the glue line will be inconsistent.

Wipe clean and apply the gauge by laying it out on a clean piece of glass or plastic and picking it up with the tape. Place it down in the area which you want to apply (normally you would have alignment marks, but since it would be calibrated after the fact for torque and not surface strain, alignment isn’t critical).

Pull the tape back, apply the glue to the surface and press the gauge forward ensuring no air bubbles. Hold with thumb pressure for at least 1 minute. Check out Vishay Micromeasurements site for more instructions.

Solder on fine wires, usually 30 – 36 AWG wires to the pads. You’ll see my new version using bondable terminals as it’s easier to bond small wires to gauges and big wires to the terminals. It also provides strain relief.

121 Ohms which is about right.

121 ohms, good

Trying to apply load and take a picture after the two gauges are wired in series and measured against the mid point of a voltage divider creating a half wheatstone bridge arrangement.

Well, I don't want to break the gages so lets protect them. Epoxy, 5 min kind. Very flexible and soft compared to metal and the superglue glueline.


After mixing coat the gauges. Let set. This 5 minute stuff takes several hours to fully cure but is mostly cured in 5 – 20 minutes.


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