Hi, I’m keen to use a Sparkfun Openlog Artemis to log strain data from several individual strain gages alongside logging IMU data, on an engineering project I’m running. I haven’t bought any devices yet, as I’m uncertain they are suitable for this specific application, but it seems to be a great system.
My questions are:
Can the SparkFun Qwiic Scale - NAU7802 be used to deliver strain data rather than weight? I need to use individual strain gages for this project and not load cells. I gather using three precision resistors plus a strain gage in a full bridge configuration will satisfy the sensor input requirements for the Qwiic Scale - Am I correct about this approach?
Do the NAU7802 onboard settings allow a gage factor to be applied (to internally convert voltage to strain). Perhaps this can be done by setting a suitable weight conversion factor? I’d like strain to be recorded in the CSV file whilst logging.
Alternatively, would the Load Cell Amplifier HX711 be a better option? If so, can this readily connect to the OLA for simple data logging applications and multiplex with other sensors too.
The four gauge bridge load cell configuration has some big advantages. One gauge is in compression, one in extension, doubling the sensitivity. The other two gauges provide temperature compensation.
A single gauge and three resistors will ~work, but you lose sensitivity and the temperature sensitivity will be completely different. The resistors and gauge will have different coefficients.
Make sure the resistor resistance is the same as the gauge, otherwise you’ll have a huge common mode voltage to deal with.
The OpenLog Artemis (OLA) firmware is set up for weighing scale applications. It uses the NAU7802 Library getWeight function to read the load. For your application, getAverage would be a better choice as it doesn’t use the zero offset and calibration factor. But the OLA firmware doesn’t provide access to that. You could modify the firmware and recompile. You can use Docker (Docker Desktop) to ease the code compilation.
The OLA firmware does not support the HX711. Please be aware that the HX711 is not I2C. It has a much simpler clock and data interface. It’s not a good match for OLA.