[From Bill Powers (951103.0940 MST)]
Shannon Williams (951103) --
All I can say, Shannon, is that you'd better fall in love and marry some
real nice PCT person very soon. You are too dangerous to be floating
Whenever I hear that someone is a mechanical engineer, my eyes light up
because I have a problem that only a really really good mechanical
engineer can solve. I don't know whether you've come across my Little
Man version 2 simulation of pointing behavior. It's available on the CSG
Web page or from Dag Forssell, or direct from me. It's a model of an arm
with three degrees of freedom: pitch and yaw at the shoulder, pitch at
the elbow. It uses a fairly good muscle model, and the arm itself is
modeled using physical dynamics. The simple control systems work very
nicely, very stably, and without using any inverse calculations at all.
They mimick the spinal reflexes. The model (programmed in C) runs on a
486-33 in real time.
My problem is this. The only reason I was able to incorporate the
physical dynamics of the arm into the model is that Greg Williams, who
skulks in the background of CSG-L nowadays (and with whom I promise to
get in touch Real Soon Now about the book in gestation), is still
mechanical engineer enough to have looked up the equations in several
robotics textbooks, checked the derivations (finding some mistakes), and
put the equations into a form that I could use. But I want to do more
than model one 3-df arm or leg at a time. I want to model the whole body
This, unfortunately, requires writing the forward dynamics equations for
all the major body segments as well as their interactions with each
other, the ground, other objects, and applied forces like gravity and
wind. All we need is a model that will convert all the muscle-generated
torques and external forces into a resulting angular acceleration,
angular velocity, and angle around each joint or pivot point. We don't
need the inverse calculations because the PCT model doesn't use them.
You'd think that somewhere in the world, somebody must have done this,
but what with everybody going gung-ho after models that use inverse
calculations, and what with this being a somewhat difficult problem, I
haven't been able to come up with a source. And I sure as hell don't
know how to do this myself.
What I need is a test bed into which I can build control systems.
Without the control systems, the modeled body would lie on the ground
like a collapsed marionette. My problem (any anyone's else who wants to
get in on the programming, like Avery Andrews and our other PCT
programmers) is to build the control systems that will make this body
straighten out, wave its arms and legs, roll over, crawl, stand up, and
walk, in roughly that sequence. This may be slightly more than I can
hope to accomplish in the rest of my life, but I'd like to get it
started and show that it's interesting enough for someone else with more
time ahead to take over. It's sort of like having a baby.
Both Dag Forssell and Greg Williams have mechanical engineering
backgrounds. There may be others hanging around CSG-L, for all I know.
There may be some physicists with good dynamics backgrounds, too. It
seems to me that building the Forward Dynamics Model of the Body - FDMB
-- is a finite and ultimately doable project, and something that could
kick off a whole new approach to modeling physical behavior. With half a
dozen people working on it, how long could it take to come up with the
basic test bed?
Who knows, maybe someone on the net will have enough credentials to get
some money to do this, perhaps from NASA.
What do you think?