[chbot] Pushing a trolley with a robot

[Wes Edwards]

Do you have the ability to modify the hitch of the picker-upper?  Can you
control the angle of the joint with a motor?  I'm thinking along the lines
of a wheeled loader such as used in earthworks or mining.  The tractor unit
has two wheels, the front end carrying the bucket also has two wheels and
all the steering is done by articulating the central joint.

Wes

-----Original Message-----
From: chchrobotics-bounces at lists.linuxnut.co.nz
[mailto:chchrobotics-bounces at lists.linuxnut.co.nz] On Behalf Of Charles
Manning
Sent: Thursday, 21 August 2014 7:45 a.m.
To: Christchurch Robotics
Subject: Re: [chbot] Pushing a trolley with a robot

I spent a lot of time at Trimble figuring out steering algorithms for
tractors etc. http://www.trimble.com/agriculture/ez-steer.aspx?dtID=overview

One of the hardest problems I solved was that of steering an implement on a
movable coupling. This involved steering both the coupling and the tractor
to keep both in sync. It is, in essence, the same problem as you are trying
to solve here (just with the extra wrinkle of avoiding jackknifing).


There are really two parts to this:

The coupling/steering mechanism mechanicals + feedback sensors.

The control software. Steering control is hard enough to just keep things on
line and stable when you're driving forward, when you also have to
"pre-steer" to push something along, it gets even harder.

If you are an inexperience and unskilled  trailer-reverser such as myself,
you will know how easy it is to jackknife etc.

The only way I can think of making this task mush easier is to make the
hitch moveable like your lead screw idea. That is used in movable hitches on
tractors to keep the implement in line when the tractor is offline.

The big problem you are trying to avoid is keeping the lead screw thing from
running out of room and then being unable to avoid a jackknife.

The way to do this is to use the lead screw to actually control the steering
of the whole machine. The trailing part is then steered to keep the position
of the lead screw near centre.

ie. you have two PID loops:

The steering PID loop (fast) that uses the offline of the front trolley as
input and controls the lead screw. It controls to minimise offline.
(Approximately, there are better ways of doing this to prevent oscillation
etc)

The correction PID loop (slow) that takes the position of the lead screw as
input. It then steers the truck to recentre the screw.

The reason the correction PID must be slower is that we don't want its
changes to propagate through to the steering. The screw PID must be able to
wind fast enough to swallow up those changes.

Hope that helps.




On Wed, Aug 20, 2014 at 5:56 PM, Andy Gardner <ceo at andygardner.com> wrote:
>
> How about having the trailer coupling mounted forward of the trailer
wheels (above the scoop system), then a bar coming out the front of the
truck, up higher than the height of the trailer going over the top of the
trailer, then attaching to the coupling.
>
> If you get what I mean. It might look a little silly, but it would work
and would be a lot easier, software-wise.
>
>
> On 20/08/2014, at 4:40 PM, Robin Gilks wrote:
>
>> Greetings all
>>
>> I've hit an interesting problem with my walnut harvester.
>>
>> The actual nut picker-upper (the trolley in the diagram) is a push 
>> device, obviously you can't pull it along as the wheels of the drive 
>> unit will either crush the nuts to be harvested or push them into the 
>> grass out of reach.
>
>
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