If you were Registered and logged in, you could reply and use other advanced thread options
|
Posted by Padu on February 22, 2006, 1:42 pm
"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
|
|
Posted by John Nagle on February 23, 2006, 12:26 am
Padu wrote:
> "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?
http://www.overbot.com/public/underhoodhardware.html
Take a look at this early picture. That big vertical cylinder is the Maxon
motor. The grey-green part of the cylinder is the motor, and the black
part is a planetary gearbox to gear it down. The aluminum sections
above and below the motor are mounting cylinders.
The encoder, the little black thing atop the motor, is an option from Maxon.
Below the motor is a spider coupling from Stock Drive Products.
This provides some rotational shock absorbtion and allows slight
misalignment, so that when a wheel hits a curb and applies forces big
enough to change the steering direction, the gears in the motor
drive aren't stripped by the shock load. Instead, some of the shock
squeezes the rubber spider in the coupling for the first few milliseconds,
allowing time for the gearbox to spin up the motor and force
it to move against motor power.
You need to calculate the stresses and inertias for this
to work right. Dr. Celia Oakley did that for us. When
we did hit a rock or a curb, nothing broke.
The motors you're looking at seem OK, as does the controller.
You have a belt drive, so you don't need the spider coupling.
We had to couple two steel shafts end to end, so unless
we put something in there, all shock loads would hit the motor.
>
>
>> 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?
The main encoder should be near the motor, not isolated from it
by anything with flexibility, or you'll have trouble stabilizing
the control loop.
>
>
>> 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.
Our experience was that manual driving with a joystick really sucked.
We tried. With a joystick, everybody seriously oversteers.
Just like R/C cars. We had to put in a steering wheel. Actually, we put
the original steering wheel back on, so everything looked
normal, but it was purely an electrical input device.
Our steering wheel had no force feedback; you could turn it fast
and wait for the motor drive to catch up. This took some getting
used to, but was tolerable.
>
>> 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?
Because the Polaris Ranger has a really wimpy alternator, and
it's built into the engine, so you can't upgrade it.
>> 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.
Our lock-to-lock steering time was four seconds. That's reasonable
for the Jeep-sized Ranger, but a lively little ATV might use a faster drive.
Or not; you might flip the thing.
Good luck. Feel free to contact me directly if you need more info.
Incidentally, the Overbot is now at U.C. Santa Cruz. We donated
everything to them.
John Nagle
|
|
Posted by Padu on February 24, 2006, 1:01 pm
"John Nagle" wrote:
> http://www.overbot.com/public/underhoodhardware.html
Thanks for the links. It was very fruitful for me. Very good work you guys
did.
> Because the Polaris Ranger has a really wimpy alternator, and
> it's built into the engine, so you can't upgrade it.
I was afraid you would say something like this. I'm even more afraid to look
at how electricity is generated by my ATV. With a Polaris like yours, it is
simple to mount a generator since there is enough room for that. On a small
110cc ATV, I can't see where to mount a generator, at least not the ones
that I know.
> Our lock-to-lock steering time was four seconds. That's reasonable
> for the Jeep-sized Ranger, but a lively little ATV might use a faster
> drive.
> Or not; you might flip the thing.
Interesting, that's almost one order of magnitude slower than what I'm
planning. I wonder if such fast steering is really necessary.
> Good luck. Feel free to contact me directly if you need more info.
Thanks John, believe me I will.
> Incidentally, the Overbot is now at U.C. Santa Cruz. We donated
> everything to them.
DAMMIT! Why didn't we get to be as lucky? :-)
By the way, where did you guys would test your bot?
Cheers
Padu
|
|
Posted by John Nagle on February 24, 2006, 11:17 pm
Padu wrote:
> "John Nagle" wrote:
Re power source:
>
>> Because the Polaris Ranger has a really wimpy alternator, and
>>it's built into the engine, so you can't upgrade it.
>
> I was afraid you would say something like this. I'm even more afraid to look
> at how electricity is generated by my ATV. With a Polaris like yours, it is
> simple to mount a generator since there is enough room for that. On a small
> 110cc ATV, I can't see where to mount a generator, at least not the ones
> that I know.
We had more electrical power than we needed. We installed a Generac
3KW AC generator intended for RVs, the smallest RV generator Generac
makes. That generator has an electrical fuel pump and will work
when it is tilted. Many of the little gravity-feed generators won't.
>> Our lock-to-lock steering time was four seconds. >
>
> Interesting, that's almost one order of magnitude slower than what I'm
> planning. I wonder if such fast steering is really necessary.
Probably not. We went for speed on the brake actuator, but not
the steering. If you're going to do super-fast steering, you're
going to have to have all the smarts to understand the vehicle
dynamics, or you will roll the vehicle.
>
>> Incidentally, the Overbot is now at U.C. Santa Cruz. We donated
>>everything to them.
>
>
> DAMMIT! Why didn't we get to be as lucky? :-)
> By the way, where did you guys would test your bot?
Testing was tough, here in Silicon Valley, which
has little suitable open space.
Team Overbot was in an industrial start-up space (1600 square
feet) in Redwood City, CA, in the Redwood Junction industrial park.
We went to considerable effort to rent a paved, fenced half-acre parking
lot, where we were sure that if something went seriously wrong, no one
would be injured.
Once things worked reliably in that small space, we arranged
to get the use of a Sun Microsystems parking lot intended for
some office buildings that were never built. That was about
ten acres, but it was full of lights and traffic islands.
Then we did off-road testing at some horse ranches nearby.
(I own a horse, so I have good connections in the horse
community).
We had an industrial-grade emergency stop radio,
an E-stop relay box which slammed on the brakes if
a 125ms timer wasn't reset by the computers or the
E-stop radio link was lost, and a general commercial
liability insurance policy for several million dollars.
John Nagle
|
Related Posts
Latest Posts
|
|
Yahoo! 
Google 
Windows Live 
del.icio.us 
digg 
Netscape 
XML site map
> coupled directly to the steering shaft via a shock-absorbing coupling.
> The steering wheel was removed. The motor had an HP encoder.