Shepard v1.1 Dev Log (DAQ) 04-03-13

A log of the work done on the thrust sensor circuitry for Shepard 1.1.
Added by Jeremy Wright about 11 years ago

The first thing following from yesterday's work was to determine the maximum voltage coming from the FSR's output at a target weight, so that I could build a voltage divider to feed that voltage to the AREF pin on the Arduino.

I used a multimeter and measured between the output (yellow wire in yesterday's pic) and ground (green) to get this voltage. The absolute max that I was able to get out of the FSR's voltage divider with the variable resistor set to approximately 100k-Ohm was about 4.0 volts (with 5 volt supply from the Arduino). This would represent the maximum force for the FSR, which is 25 lbs (110 N). The maximum thrust that we expect to see from the Estes motors used with Shepard will be 30 N (6.7 lbs). Assuming that we want to calibrate to at least 1.4 times the maximum expected value (good engineering practice), the FSR range needs to be set to max out somewhere around 41.8 N (9.4 lbs).

I don't have the means to add this amount of weight to the sensor right now, but I do have access to a gallon jug and water (about 8.3 to 8.5 lbs). This will give me a relatively known weight to experiment with.

The jug of water's weight gave me about 2.6 volts, and if I work that out based on a max of 4.0 volts at 25 lbs I get:

2.6 volts / x lbs = 4.0 volts / 25 lbs

which becomes:

2.6 volts * 25 lbs = 4.0 volts * x lbs

which becomes:

x = (2.6 volts * 25 lbs) / 4.0 volts


x = 16.25 lbs

Obviously, 16.25 lbs a little less than twice the value I should get. It's getting late where I'm at, so I'll figure out what I did wrong tomorrow evening.

For anyone reading this dev log in the meantime, I'd welcome your help. Did I make a simple mistake (very likely at this hour of the night)? Is there a fundamental flaw in the way I've built the circuit? Is there something else going on? Let me know in a comment to this news item.


Added by Jeremy Wright about 11 years ago

I think I figured out this morning what I did wrong. The maximum rating for FSRs (or the range if given) normally just specifies the range within which the FSR has the most linear response. When I was finding the maximum voltage last night, I was just pressing down on the FSR until it saturated (maxed out). There's a very good chance that the FSR was saturating at a much higher force than 25 lbs, which would have caused the high x value that I saw.

I've got a small C-clamp, and I'll try to find a hanging scale tonight so that I can get a more accurate view of the force the FSR is seeing.

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