[ODE] hexapod walking example

[Nate W]

On Thu, 27 Dec 2001, Adam Rotaru wrote:

>   Those who are interested in using ODE to simulate articulated
> creatures might be interested in my first little attempt, and, might
> be able to help me with some problems I'm having.

That's exactly what I'm using ODE for.  Or what I will be using ODE for in
a week or so, if all goes well.  Right now my articulated creatures have
no muscles.  They do collapse in a very believable fashion, though. :-)

> The desired position of each leg is modified according to the pattern,
> and torques are applied proprtional to the deviation from the desired
> position.

In a nutshell, something like this?

error = (desired_position - actual_position)
torque = gain * error

That feedback function is something I expect to be adding to my app in a
day or two.  If that doesn't work for my app, I am considering integrating
the error over time, so the feedback function will have a proportional
term and an integral term, like this:

prop_error = (desired_position - actual_position)
int_error = int_error + prop_error
torque = (prop_gain * prop_error) + (int_gain * int_error)

The "int_error" variable needs to be preserved across calls to the
function, and there needs to be a separate int_error for each joint.

The idea behind the integral term is to make the feedback increase over
time if the joint remains deviated from the desired position.  Tuning
feedback functions is basically a matter of trial and error, but the
integral gain should probably be about 1/10th of the proportional gain, to
start with.

> - I had to apply large torques to support the weight of the body.
> However, when a leg is unsupported, it moves very quickly due to the
> large torques.  Sometimes the bug falls apart, which I attribute to
> the large torques.  Smaller steps help here.

Stick with the proportional feedback fuction and try this... set two gains
for the feedback equation, a small and a large.  When applying torque in a
direction that would lift the body, use the large gain.  When applying
torque in a direction that would left the foot, use the small gain.  
Hopefully this will keep the legs from flailing too much when they're
being lifted, but you'll still have enough power behind them when they're
being pushed downward to lift the bug's "torso."

Like I said, I haven't gotten to this point in my own app yet.  I can only
tell you what I'm going to try, so I dunno if these will actually help.  
If all goes well I will be adding feedback control to the joints of my own
critters early next week.  If you get a chance to try these ideas sooner,
I'm VERY interested in hearing how they work for you.

-- 

Nate Waddoups
Redmond WA USA
http://www.natew.com