I am planning the build of a large scale rover. I have the following in mind:
4 wheel drive
4 wheel chair motors, 450W each
Combustion engine combined with alternator for charging battery bank, motor will also be used to drive power take off shaft for powering mower deck
Mid articulating chassis
For controlling the four motors I was planning on using two 2x60A Sabertooth motor controllers.
Does anyone has experience with controlling two motor controllers with a Pixhawk?
For steering I would like to use the skid steering function, the articulation hinge will not be actively powered it this scenario.
Another possibility would be to include 1, or 2, hydraulic cylinders in the hinge point to actively articulate the chassis during steering. However I am not sure how to integrate these hydraulic control valves with the motor controllers and autopilot.
Currently I am sourcing parts and designing the chassis, would like to hear your thoughts on the drive and steering system.
I never build a rover with articulated steering, but I do see some problems doing the articulation passiv by skid steering. You will have to dampen the joint somehow, otherwise you will get large steering reactions with small differences in wheel rpm. Or imagine one of the front wheels hitting a bump, it slows down, but the other wheels go on with their normal speed, folding the rover at the joint.
Getting the rover to drive straight without active control of the joint will be difficult or impossible. I would go for active articulation with a linear servo/hydraulics without any skid steering.
Connecting the controllers should not be a problem. You can assign as many outputs as you like to skidsteering or normal throttle, or use y-cables.
Thats a good point regarding the passive system. I am planning to install a small 24v hydraulic power unit to power one or two small hydraulic cylinders to actively pivot.
I guess this has to be done in combination with skid steering the E-motors using the two motor controllers.
For controlling the hydraulic cylinders I am thinking of using an proportional hydraulic valve. Any suggestions how to control this valve using the Pixhawk?
To clarify, I think it is nicer to have skid steering capabilities in combination with the active pivot to ensure smooth cornering, so inner wheels can turn at lower rpm then outer wheels. Correct?
Skid steering does not work well in combination with “conventional” steering methods. With articulated or ackermann steering, the steering input only moves the wheels/joint in a certain direction. Skid steering input causes the wheels to turn, even without throttle input. This means, in a standstill, full steering input to the left causes the joint to go to full deflection to the left, but combined with skid steering, it also causes the left wheels to go full throttle reverse, the right wheels to go full throttle forwards. I am trying to imagine what that might do to the rover, given that the wheels do not point straight forwards and backwards at this point.
I never used it, but there is support for lua scripts in ardurover 4.0. It might be possible to do custom mixing this way to simulate a differential gearbox: