Drone Test Stand

If you've been following the Goliath Quadcopter project you've seen the test stand being used to test Goliath. It's built from 2x10s and is designed to support Goliath's relatively large mass (240 lbm). Up to now the stand has had no electronics, and the only requirement was to safely constrain the movement to safely test the vehicle, but allow 6 inches of vertical movement for hover testing. Goliath is supported from above with 8 lines, 8 more lines from the sides constrain any horizontal movement and another 4 lines are attached from below. All of the lines are taut with exception to the four below, which have 6 inches of slack in them.

While the setup is simple, I can only tell if the the vehicle is either not-hovering if the bottom lines have slack, or hovering if the bottom lines go taut. Recently the first test was conducted at full throttle and the bottom lines stayed slack.

Now there is a need for more data to determine what is going wrong. Two pieces of information needed are weight and thrust. RPM is also needed, but that will likely be computed on-board and be covered as part of another project.

To do this, the vertical supports will be modified to allow weight and thrust measurement with load cells (FX1901). As @zakqwy mentioned one of the downsides to load cells is that they can be sensitive to off-axis/shear loading, so there will need to be some mechanical design considerations which I'll cover at a later point.

Why the FX1901? Because they are relatively inexpensive and I have some Mouser credit I got during The Hackaday Prize. These specific ones measure up to 100 lbf and use a 5V supply voltage. I'll be using 4 of them together to measure the total weight, and then use them do deduce thrust as the vehicle is lifted. The other reasons I chose these load cells is because of a tutorial I found on how to wire this load cells up to an Arduino. The voltage output from the load cells are analog and low, so tutorial outlines how to use an instrumentation amplifier to amplify the signal for the Arduino.

Since the signal is analog, reading the sensor with a micro-controller requires an Analog to Digital Converter (ADC). I have a Raspberry Pi loaned to be from a friend not being used, but apparently they don't have an ADC built in. The Raspberry Pi seems a bit overkill for this anyways, so I'm going to get a 5V based Arduino Pro Mini or a Trinket Pro (Arduino Based).

Since I don't know much about either I browsed through some other projects on the site. I hadn't really followed the Trinket Every Day Carry (EDC) contest, but I've had a chance to look at some of them now and I have to say there were a lot of neat projects. One that seems somewhat relevant to what I'm doing is the Multi-Meter Remote Display. It uses a a pair of trinkets to remotely display the multi-meter readings. I'd like a similar function, but in my case I want the weight instead of a voltage reading.

It may also make sense to add temperature and pressure measurements, to record the values for each test. It may also make sense to add some sort of interface to tare the weight measurement

This will be my first arduino base project, so I've got a bit of learning to do. I've taken high school electronics and a circuit analysis class as part of my mechanical engineering degree, but it's been a while, so if anyone is looking at what I'm doing and sees something wrong, PLEASE let me know.