Shepard v1.1 Dev Log (DAQ) 04-02-13

A log of changes that are being made to the thrust sensing capability of the Shepard DAQ system, version 1.1.
Added by Jeremy Wright about 10 years ago

It occurred to me that the Shepard 1.0 DAQ software has not been milestoned and added to DMSF yet. I've already shipped the 1.0 DAQ shield for the Arduino to J for the Yuri's Night demos, so he's going to test the version in the repo out. Once it's confirmed that works, I'll zip the code up for upload to DMSF as the 1.0 milestone version. For now, I'll keep my changes local.

The new thrust measurement circuitry for the DAQ system has been prototyped, and seems to work well.

It uses a higher quality FSR ("Force Sensing Resistor" that costs about $25), and uses a variable resistor to set the effective range of the FSR (Flexiforce Sensor). The variable resistor's max is 100k-Ohm since this is the max specified in the FSR's documentation. In order to see and adjust this range, you can use the "Raw Value" field of the Calibration_Util_Processing.pde Processing app. The process for setting the max range is below, and is a rough cut at creating a max range setting process.

1. Add the amount of weight onto the FSR that represents the max that you ever want the sensor to be able to read (up to the 25 lb max).
2. Adjust the variable resistor until the "Raw Value" reading is maxed out at that weight (~1023 if possible).
3. Put a dot of clear silicone (RTV) on the side of the resistor where the "turntable" and the body meet. This will prevent the resistor from turning on its own during a rough shipping trip.

While working on this I was reminded that the default reference voltage for the Arduino's ADC is 5.0 volts. Since the FSR is being used in a voltage divider that's being fed from the Arduino's 5V pin, this means that we'll never "fill" the ADC's range, and thus we won't have the maximum resolution possible. This can be corrected by building a voltage divider and using it to feed the AREF (external ADC reference voltage) pin with the max voltage that we ever expect seeing from the FSR's voltage divider.


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