I want to use pixhawk with the apm rover firmware to operate an agricultural tractor. I need to set up my tractor as an rc vehicle.  My plan is as follows: any feedback I can get is appreciated.  I plan to order the parts in the next couple days but if there is something I've overlooked or my plan has no chance of success let me know.

Throttle control: Linear servo (https://www.servocity.com/html/25_lbs__thrust_linear_actuator.html#...) I don't see too many problems here.

GPS: I plan to use the gps already installed in the tractor.  I am planning on buying a pololu 23201a to convert the serial nmea to something I can input to the pixhawk.

Steering: This is what I'm most worried about.  The tractor has an electric over hydraulic steering valve, so basically provide 12v to right side tractor turns right, provide 12v to left side tractor turns left. I am planning to put in a dual 12v relay controlled by pwm signals for the steering https://www.servocity.com/html/electronic_pwm_controlled_dual.html#...  Anybody have a gut feel on whether this will work or not?  The problem is the steering output from pixhawk needs to reach a certain threshold before steering valve will operate this is quite a bit differnt than how it is set up on my rc car where each pwm value corresponds to a specific steering angle.

Clutch: Much testing has to be done with a person in the cab to operate the clutch and some sort of ignition failsafe installed before I get the tractor to be totally autonomous but I would like to start thinking about the clutch, and start testing some setups with a person in the cab.  I could get a linear actuator that releases the clutch when throttle pwm reaches some predefined threshold? But I would really like a system that if the power is cut or signal is lost clutch is automatically depressed which will stop the vehicle. I could set the clutch up to be always depressed and use a linear actuator to engage it. Connect the actuator to the clutch with an electric solenoid so if power is lost actuator releases and clutch disengages. Anyone else have some good ideas?

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The parts are coming. I'll try and update this semi regularly with pics and project updates. Here's a picture of the tractor I'm going to work with.
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That's quite the RC vehicle you're going to have! Hope you've got a big red button in there just in case! 

I am looking at a self designed RC mule type vehicle rather than converting a full size product, but am keen to see what you come up with.

Hi Matthew - that is one big tractor.  Have you tried out the Rover software, configure etc on a small RC model first?  I would strongly recommend doing that before trying things out on that large tractor for the first time.  Ah, I can see you mention an RC car - great.

I've asked Philip Rowse to provide some advise on your steering interface.  As for the software it might be as simple as setting a large dead zone value eg RCx_DZ.

I'm sorry I can't help you with the mechanical engineering side.  Hopefully someone else here can.

Thanks, Grant

Hi Mathew
  I would be tempted to add feedback into the system, it may require a special driver to ensure that the system behaves well, and you will need to program Hysteresis into it so it does not hunt. There is some code already in the antenna tracker that copes with on / off style control, so I am sure that this is possible.  is there any feedback anywhere as to the position of the steering?  This may be an application where you may need to monitor the Tractors computer system to get this feedback.

Thanks for the tip on the antenna tracker code. I would have never thought to look there.

This tractor is all mechanical (even the engine timing) so no feedback on steering angle. I'd have to add on a potentiometer or something.

The after market GPS/autosteer works without feedback but a more modern tractor I have that uses feedback is much better able to negotiate curves. So I guess we will see.

Really cool project.  When using my rover in my yard, I've had so many drive aways using my APM 2.6 that I'm looking to use my dog's underground fence receiver collar as an independent geofence failsafe (before I put any additional hardware on the rover with significant mass), although I suspect that that is not an option for you given the size of your area.   

that's a great idea... might have to do that for my robo-mower when I get around to building one...

Cool project, I'll be watching. Keep us posted.

Parts have arrived and all day tomorrow I plan to install and hopefully begin some useful testing.  I've encountered a problem already though. My linear servo doesn't move when hooked to pixhawk.  It moves fine if I connect it directly to my rc reciever.  I suspect it is the pwm update rate needs to be increased. Anyone know how to do that?

I think I forgot to turn off emit heartbeat so when I went to change it with the terminal all I got was gibberish. Ooops

@Matthew,

The Pixhawk has to be armed for there to be an output from the servo output PWM channels.

Regards,

TCIII AVD

Thanks. I know I'm getting output because when I plug in a servo off my rc car it all works good but as soon as I try the linear servo i get nothing. I tried changing the pwm update rate using the terminal but I couldn't get the servo to move.  Haven't spent too much time at it yet have to try again in the A.M.

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