Apm Rover in tractor setup.

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#.VVOUCJNyP5U) 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#.VVTFmZNyP5U  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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    • Seems that I'm not using the latest firmware but an upgrade to the latest daily build should solve my problem.

    • Here's a log file which is just starting things up connecting via mission planner sending it to some point and then disconnecting mission planner and nothing happens.  I stop the tractor with my failsafe feature and unplug pixhawk


    • Well the cable works but it doesn't work with the 3dr radio. I think the 3dr radio and that cable use the same serial driver on my laptop and the other cable uses a different driver but I don't really know.

      I'm very curious about the different failsafe behaviors also. I will be doing lots of testing in the next few weeks also and will keep you posted.
  • If you want no-nonsense reliability with full compatibility and more range than you should check out an RFD900 pair. Range is like 20-30km and they are still pretty cheap (~AU$200 for a pair).

  • I saw at hobby king the circular polarized antennas for the radios, I couldn't buy them but I'm going to try to find here to test, for the moment I'm using dipole with little better results than stock ones, (but my radios are chinese).

  • 3dr radio range.

    I did a range test of the radios the other day.  They stoppeed working at 1/2 a mile but I had them (purposely) mounted in the worst spot I could think of and there were lots of trees and a house in the way of the two radios.  As soon as I moved the radios up and away from other electronics and metal the link signal went to about the 50% level which indicates that I should get a 1/2 mile reliably and probably with a little bit of tweaking a full mile.   A full mile is preferable but I can probably work with a 1/2 mile if I had to.  I can't believe that they worked this good actually it's real good bang for your buck.

    • Are you running the stock 3DR antennas?
    • Yes. Stock 3dr antenna with default settings.
  • Safety Feature Video

    I've spent a lot of time thinking about safety and I think I have finally come up with a solution that I think is going to be reliable enough and robust enough that I can start testing without a person in the cab.  My original plan was to kill the engine in an emergency, but this proved untenable.  When you shut the tractor off while it is moving the stop is so sudden that if you tried it at any speed over a slow crawl I am sure something would be wrecked. I'm not sure if the parking brake is automatically applied when the engine quits or if it has something to do with the CVT transmission needing hydraulic power or both, but if I want my tractor to keep working I can't stop it this way.

    There are two main ways to stop the tractor and a third that can slow it down to below a walking pace.  The first way is the shuttle shifter, this is connected to the throttle channel of apm rover.  This is the main way to start and stop the tractor. It can be operated off the rc transmitter but it will be mostly controlled by my GCS.  The second method is an air actuated cylinder attached to the brake pedal.  It has a spring return so that you must apply air for the brake to come off (much like the brakes on a semi truck).  This is connected to the throttle channel of the rc transmitter, the brake will be applied if the rc transmitter goes out of range or loses connection for any reason.  The brake can also be manually applied from the rc transmitter by moving the throttle stick to the low position.  When the brake is not applied there will be a flashing light on the roof to warn people not to approach the tractor.  I still need to mount a big bar across the front of the tractor that will trip a switch, causing the brakes to be applied if it comes into contact with any obstacle.  The brake is also applied anytime the pixhawk is disarmed.  Also if the pixhawk loses power the brakes will be applied.  The third option is that by moving the tractor speed selector to the very lowest setting the tractor drives slower than a walking pace so as a matter of last resort it would be possible to hop into the cab while the tractor was moving.  Obviously I never want to have to do this but it could be an option if everything else failed.

    • Developer

      Hi Matthew.  I'm not expert but I suspect its the compression lockup of your diesel engine that is causing the sudden stop when you turn the engine off.

      You approach sounds great.  Just remember to test every scenario you can with someone in the cab to make sure all the safety systems work.

      Here in Aus we use these radio's if we want better range.


      More expensive but not crazy prices.  Your 3DR radios are 100mW.  These are 1W.  Still legal here in Aus in the 915MHz band but you would need to check in Canada.

      Looking forward to hearing the results.

      Thanks, Grant.

      RFD 900+ Modem
      RFDesign designs and manufactures the Antennas, RF Electronics and the RFD900 radio modem
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