Newsgroups: bit.listserv.csg-l
Date:         Sat, 22 Aug 1992 20:22:51 EST
From:         Avery Andrews <andaling@FAC.ANU.EDU.AU>
X-To:         csg-l@vmd.cso.uiuc.edu

I've been thinking a bit about steering, & here's what I've come up
with so far.  The only direct and simple relationship is between
the absolute angular position of the wheel, and the curvature of
the car's path, but I think that people pay little, if any, attention
to either of these factos.  What probably does matter are such things as:

  a) the difference between where you are and where you want to be
      (transverse to the road), P*

  b) the rate at the car is drifting across the road, dP*.
      There may be serveral ways of picking this up (such as
      differentiating P*, or watching the motions of lines on
      the road w.r.t. the car hood, & I'm sure other things as well).

  c) the rotational velocity of the steering wheel.

  d) the angle between the orientation of the car and the tangent
      to the road.

  e) how far the wheel has turned since some time t0.

I'll only consider (a-c), tho (d-e) seem like they might play a role
too.

Observe that on a big lane change you don't normally try to get into
your new lane as quickly as possible, but drift transverse to the
road at a fairly steady rate.  And also, that when you get close to where
you want to be, the drift rate needs to be smoothly decreased to zero.
So I propose that P* is controlled by means of controlling dP*.  So dP*_ref
should be a function of P*, zero when P* is zero, small when P* is small,
but flattening off to a constant fairly quickly under normal
circumstances (I won't speculate about panic swerves).

Then dP* itself gets controlled by controlling the perceived rate
of motion of the steering wheel: when dP* is too big in the rightward
direction, crank the wheel counterclockwise, & vice versa (I suspect that
there are various ways in which the reference rate of wheel-rotation might be
determined, and that it might even be possible to devise experiments to
figure out which one(s) people were actuallay using).  Lower-level
systems take care of summoning up enough torque to get the wheel to
rotate at the right speed (and producing the hand-over-hand motion needed
to effect a large total rotation).

Hence there is a three-level hierarchy of controlled perceptions: relative
position, rate-of-change of position, and rate-of-rotation of the steering
wheel.  And of course there'd be some more levels involved in producing
the wheel movements.  Monitoring the rate of change of (transverse) position
makes it unnecessary to do the otherwise complex calculations that would be
needed to figure out how fast to turn the steering wheel when, and monitoring
the rate of rotation of the steering while makes it unnecessary to predict
how much force needs to be applied to the wheel to get the required curvature
of the path.

A further elaboration:  it seems plausible that drivers also assess the
curvature of the upcoming road, and anticipate how much turning of the
steering wheel will be needed to keep the car on course (it ought
to be possible to do a video-game experiment to see whether they
actually do this).    So there could be an anticipator circuit
assessing the appearance of the road ahead, and producing
reference levels for rate of wheel-turning.  The higher-level feedback
loop can then add an additional term to correct for the anticipator's errors.
Having control for a rate-of-turn of the steering wheel means that this
anticipator can be a lot simpler than it would otherwise have to be,
since it doesn't have to guess how much force ought to be applied to
the wheel (though there could be anticipators contributing the force
reference levels, they wouldn't have to be very precise).

Avery.Andrews@anu.edu.au
,
Comments: Gated by NETNEWS@AUVM.AMERICAN.EDU