Re: Autonomous ATV

"John Nagle" wrote

For Team Overbot, we used a Maxon servomotor with a planetary gearhead,
coupled directly to the steering shaft via a shock-absorbing coupling.
The steering wheel was removed. The motor had an HP encoder.


Hi John, I'm very happy you could respond to my post, since my project is
very similar to yours and you have certainly gone through the problems I'm
having and the ones that I will have.

I was looking at the Maxon line of products and although I particularly like
their products (very high quality), I couldn't find one that would provide
enough power. Well, I will elaborate more further, as there are some
interesting parts of your answer to be commented on. I had not though about
shock-absorbing coupling, could you explain what sort did you use?
I had selected US Digital encoders, because they are cheap (around $70). I
didn't know that HP manufactured encoders, where did you buy them from?


On the steering shaft, we mounted a circular through-shaft
pot/encoder of the type used on GM cars. This produced encoder
signals at 1 degree intervals, plus a signal from a pot for
absolute positioning.

I'm gonna look for it. I was planning on using the encoder on the gearhead
output, and since the pulleys are 1:1, that should do fine don't you think?


Control was provided by a Galil DMC-1416 motor controller,
which also read the encoders and pots, and the auto/man switch.
At power-up, the controller read the pots and centered the
steering.
The steering wheel became a purely electrical input device,
with an identical pot/encoder. This allowed manual driving
with electrical power steering.

Yes, we're going to remore the handlebar as well. The roboteq controller has
inputs for RS232, analog joystick and RC RF receiver, so we can implement
manual local control with a joystick.


Power came from a 3KW Generac generator driving Vicor
24VDC power supplies.

That's interesting, and I saw a couple of entrants on the last darpa gc
using generators. Why is that? Why not to use extra batteries and an
alternator to recharge the batteries?



Today, the Roboteq controller is a good choice. That
wasn't available when we started. Get
the one with the encoder input, and connect that to the
motor's encoder. You'll need a second sensor of some
kind to home the steering. A limit or home switch is
sufficient. A useful trick is to have a home switch
or sensor that will detect if the wheels are pointed
straight ahead, plus a second switch to tell you whether
the wheels are pointed right or left. Then you can
home to center under program control. We used the
through-shaft pot for that.

Yes, I was reading the roboteq user manual and they have a similar
suggestion


Incidentally, we steered the Overbot with less than
8A peak at 24VDC, and could turn the wheels without
difficulty even stopped on asphalt. This was a
Polaris Ranger, which is a big, manual-steering ATV.

I measured 45Nm torque at the steering column to turn from lock to lock on
asphalt, I wonder if that's because the polaris ranger has a rack and pinion
steering system, while in my case the steering column drives the steering
rods directly to the wheels. Well, I guess total power should still be less
than in your case, given that supposedly there is more friction to overcome.


Thanks for sharing your knowledge with us


Cheers

Padu


.

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