AP_Scripting: Add thrust stand

[MattKear]

This is a fun lua script that I have been working on for the past few months. I wanted to add another tool to the AP arsenal in terms of component selection / vehicle design. So I have designed a static thrust stand that uses scripting to take advantage of AP, its libraries, and mavlink for using the GCS for control and live telemetry. All of the CAD for the hardware will be open source and the little python post-processing tool I am making (still needs a lot of work though! :-D )

About the Project

Here is the thrust stand I have made:

Here are some of the features of the script:

• Logging of throttle, thrust, torque, voltage, current, motor commutations (RPM), time stamp.
• If using a BL Heli ESC then logging of ESC current, ESC RPM, ESC temp, ESC Voltage is also available.
• Throttle ramp mode: Gradually raises throttle to maximum (set as a param), pausing at 10 increments to capture steady-state response.
• Throttle transient mode: Ramps throttle to 'hover throttle' and then proceeds to run through a table of step inputs about the the hover throttle to capture transient behaviour. (The implementation of this still needs work).
• Over-current protection: Set a parameter value for max current. When exceeded the throttle is zeroed and the system stops to prevent damaging ESCs and motors on test, with the screen and lights informing the user.
• Addressable LEDs used for throttle bar.
• OLED display used for on-screen feedback of throttle, thrust, torque, current, and voltage.
• System arming and disarming, arms 'copter' to make use of the motor safety behaviour already in AP, (just in-case the script dies).
• Pendant 'dead-mans' button for operating the motor.

There will be a blog post going up on the forum soon where I will get more of the details out.

The other thing that I am quite please about it there is a lot of flexibility in this. Not everyone will necessarily need/want torque data for example, so they can easily get one of the low cost thrust stands off of bang good and plumb the load cell from that into this system to get way more data and automation.

I think one of the biggest benefits to users will be the ability to more easily get thrust data for expo fitting on multi-copters and VTOL planes. The post-processing tool I am working on will help with this.

The data captured by this script is dumped to a *.CSV file so the bar to entry for processing the data is very accessible. For example, it is easy to use the output in the already existing google sheets expo fitting tool.

Here is an example of some of the data I am getting out: Throttle Ramp Tests:

Throttle Transients:

Here is what the post-processor currently looks like. As I say, needs a lot more work.

About the code

The top section of the code is a port of the C library from sparkfun into LUA, to be able to make use of the NAU7802 amplifiers on the I2C bus. This script therefore uses @IamPete1's I2C scripting PR (#14276).

The lower section of the script is then the actual thrust stand code.

This script does depend on a few other PRs for scripting bindings: #15857 #16741 #16742

Although the later two PR's are on hold to better PRs that would enable this functionality. So I will update the thrust stand to incorporate those PRs as and when they are merged.

Finally, this PR is marked as WIP at the moment as I am still chasing out one bug, which I don't believe is actually a fault of this lua script. Basically, I am using a Matek F765. The amplifiers have the same address and are therefore on two seperate I2C busses. For some reason the amplifier for the thrust load cell 'looses' data and LUA gets nil values in return. I do have the OLED screen on the same bus, however I have removed this and the problem still occurs. I have also ruled out EMI as both thrust and torque I2C buses are run next to each-other and the fault always occurs on thrust and not torque. Plus both lines are run through a grounded metal braid. Also, I have swapped the thrust and torque amplifiers to rule out a faulty amplifier. This is an intermittent problem so doesn't always cause issues. Turning down the sampling rate reduces the likely hood of the issue happening, but I am pretty sure thats just because it is reducing the traffic on the bus. I think it might be a race condition or DMA buffer issue, but I am getting well out of my area. So, if anyone has any ideas/pointers on how to debug this I would love to hear them.

[rmackay9]

Wow, fantastic!

[amilcarlucas]

@Gone4Dirt can you address the comments so that this can be merged?

[ryan99alero]

@Gone4Dirt I may just be blind. Do you have either blueprints or cad files for your thrust stand or a site where you list them to buy and download. I like your design. If tytorobotics IE rcbenchmark had any of their 1585 thrust stand logic boards in stock I build a replica of their stands. Whatever reason the board is like $150 but they want 1100 with stand and load cells. Massive markup.