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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Some more food for thought.  I need to review all of the safety features Matthew has incorporated but I had an idea that was a take off from the "row of  red panic" buttons that were contemplated but as far as I know, not implemented in this first incarnation.  What if we fabricated a couple of "whiskers" out of PVC or light weight Aluminum tubing that were sticking out of the front of the tractor in a Y configuration.  Then, between the ends, maybe with a couple of small pulleys or guides, we ran a wire or high strength fishing line across the ends.  Back at the mounting point on the tractor then all we would have to do is mount a single blade switch, or an insulator between two contacts that could be pulled out if anything interfered with the wire/string,  Maybe place the "sensor" at about knee height so it would interfere with most bumpers, wheels, fenders, and even a hand held up in the air by someone lying on the ground.  It should be pretty easy for the Arduino to monitor this single switch but you get a lot of coverage with only one line to monitor and if it was wide enough you could trip the wire without being directly in front of the tractor, hope it doesn't come to that!  Perhaps with some tweaking, the frame could pivot and point in the direction of the front wheel line of travel?  I Apologize for the crude sketch but something along the lines of this. (You would need enough lateral clearance to not interfere with the trucks/trailers during the unloading process)

Or with some side coverage as well, like this...

Jim, The generic ag leader kit you have referenced ( 200-0625-01 )has been replaced with PN: 4100175-01 which includes the following pieces:

Drawing of WHEEL ANGLE SENSOR KIT - GENERIC

If you wanted to save some money, you can get just PN 200-0468-01 by itself which includes the sensor, threaded rods, and joints.

This wheel angle sensor that Ag Leader uses has been used in many different vehicle applications and is very common. The only difference between different vehicle kits are mounting plate brackets for various machines. The sensor itself is the same. The generic threaded rods are the same between kits. The rods come in a large lengths and the user is asked to cut the rods to their specific machine requirements. 

Ag Leader (and also AutoFarm) has used this particular sensor in many many vehicle applications for many years. If you are ok with used sensors, I would suggest visiting with one of your local dealers and asking if they have any used wheel angle sensors available. As people trade tractors and switch guidance systems, often these kits are pulled off and thrown in a box as spare parts.

Ag Leader is also very good about posting vehicle install manuals for their steering kits for free on their website If you need ideas for your specific vehicle ( installing aftermarket hydraulic valve, installing wheel angle sensor, location of factory steering valve, etc). Go to the bottom of this page for a list of vehicle install manuals (blue links under 'Kit Install'): 

https://dealer.agleader.com/kbp/index.php?View=entry&EntryID=1589

James that's decent but you would have to consider the intertia of the PVC rods and that the distance ahead of the front axle would tend to make them wobble quite a lot at high speed. possibly causing triggering accidentally.

I maintain my earlier sentiment that a sonar sensebar is likely the best combination of safety and ease of application. It would likely take about 3 sonar sensors, with a beamwidth of somewhere between 20 and 30 degrees . If they are mounted about 5 feet off the ground ~ hood height~ish, and pointed forwards and down. They could be set to trigger if they see any extreme deviations in ground height, which would indicate that you are either about to run someone/something  over, or drive into a hole, which would also be bad. Such a circuit would be relatively easy to set up as analog, or with an arduino, which could set a flag to the main processor to trigger an E-stop, or a slowdown (or even obstacle avoidance) - which I'm pretty sure the pixhawk has code for in rover somewhere?

Can you describe what is involved in installing the Outback kit a bit?  The quote I received from a Texas dealer was $7500.  (e.g. Am I replacing any existing power steering components or only adding components?)  Am I basically adding a hydraulic control valve between the current steering and the steering pistons and then connecting an electronic control unit to the hydraulic control?  The pdf online is pretty vague.

First the dealer said, "The 5055 is a very small tractor it might can be done, but I would have to do some research since the tractor is not on the model matrix.  This tractor may lend itself better to the EDrive Esi electric steering wheel versus the hydraulic steering option....".  

After answering a few questions (e.g. that a JD5055 has open center hydraulics) he came back and said it would be $7500.

You don't replace anything. Basically just adding some hoses and a metal block that has some electric solenoids inside.  One opens and the tractor steers right other opens and tractor steers left. $7500 is a lot. He must have quoted you for an entire autosteer system including gps reciever, monitor in cab and edrive box. Everytime I've done it my tractor or sprayer has been on the list and has always come with really nice instructions (including pictures) that show you which hoses and fittings to replace. If your tractor is not on the list then you might need some custom fittings for some hose ends.  Since it is smaller than most tractors the hoses should all be long enough if you order the kit for the closest model to yours.  I would phone outback direct I have had great tech support from them in the past.

I have an EDrive VSI (electric steering wheel) in my swather and it works good.  I have not tried to control it with the pixhawk but it should be possible.  This would require a good amount of disassembly on your part.

Here's a schematic of my current set up. http://www.schematics.com/project/pixhawk-tractor-wiring-24753/

Thank you Matthew!

I have wanted this kind of schematic from you. I will look at this and if necessary update my elecric drawing. 

Thank you again.

Here's a very brief video showing the working prototype I have made for streaming engine data.  The code on the arduino is up on github. The code on the pixhawk and the modified mavlink libraries will be up eventually as well.

Here's a video of my tractor on a snowblower. Nothing of technical value here at all but I had a lot of fun.  Merry Christmas!

For those interested I just received my Wheel Angle Sensor Kit part no. 200-0625-01.  No time yet to install, but wanted to share photos of what came in the package.  Cost including shipping (in Texas) was $531.

Be aware there is no wiring harness included.  If you can open the MS One Drive link attached you will see the photos and notes  and some research I did as I am thinking I will have to craft my own mating connection.

One Drive link to notes

This is amazing!!  My brother and I are thinking maybe a grain cart could fit in our operation if we didn't need to hire a 4th guy to run it.. hmmm.....  Keep up the inspiring work!

Photo of the installation.  Brackets were made a fabrication shop nearby.  Cost $300.  I made the wiring harness with the parts from Mouser mentioned above, but I did not have it on when I took this photo.  The arc of travel is about 2500 ticks vs. the full 4095, but should be plenty for me.

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