Alt-BEAM Archive

Message #04813



To: Wilf Rigter Wilf.Rigter@powertech.bc.ca
From: DENNLILL dennlill@buffnet.net
Date: Wed, 23 Jun 1999 04:02:33 -0400 (EDT)
Subject: [alt-beam] Re: Ah! Blessed Skepticism! (was: Feedback explained?)


Hey, I just happened to grab this post. Recently, I had built a stryder
type robot using bicores and bg micromotors. At first I had no buffering
and was driving the shindig directly. (thats changed) but I noticed what
your talking about. It may be helpfull, but in my robot I found that It
could lead to unhappy bicores. And some 'grumpy' ness from the bicores if
the legs were held. Basically I mean it led to seemingly less usefull
motions from the robot. But hey, just observations.

Dennison

On Tue, 22 Jun 1999, Wilf Rigter wrote:

> I was impressed by many of the previous comments on this thread (not
> including my own).
>
> Most of Mark T. comments were on microcores and >4Nv cores and some are very
> puzzling indeed. Take the following statement:
>
>
> What is surprising
> though is that a 4 neuron microcore with a single process in it can
> still be influenced by it's prior passing over a neuron 3 neurons
> ago. It can "see" itself and be repulsed.
>
> Try it. Dead short a cap without a process on it, remove quickly
> and watch the process rate change in a microcore. Processes live
> in a tenuous voltage space very susceptible to any charge
> influence.
>
> Neuron outputs trigger on the log decay of the coupling cap before
> the inverter input, which is a very shallow voltage curve and
> subject to any charge influences prior to it. Exact equations are
> pending, but as many of you know, a 5% variation in coupling
> capacitors can mean seconds in variation characteristics.
>
> But you didn't hear it from me, cause you won't believe it until
> you can use it.
>
> markt.
>
> This statement about a "memory effect" of previous processes in Nv cores
> must have been based on some theoretical work since these effects are
> probably caused by an average dc component of the process waveform shifting
> the trigger level which shows up as a term in equations that do not include
> the "dc restoration" effect of the commonly used HC logic family input
> protection diodes. However if this "memory effect" is desired or considered
> useful then the simple addition of resistors in series with the Nv bias
> point can re-introduce this "DC component term".
>
> <>
> Now I agree that garbage in equals garbage out no matter how much positive
> or negative feedback you put in the loop and all the mumbo jumbo aside, I
> did this bench test:
>
> To a standard suspended Bicore add a BG micro motor directly to the Bicore
> outputs. Observe the rotation and Bicore pulsewidth is about 90 degree each
> way and 1 second pulsewidth duration. Now load the output shaft with some
> finger pressure (not a complete stall) and note the rotation is about 30
> degree each way and the pulse width is 0.5 seconds. So there it is
> mechanical AND electronic feedback. Obviously the rotation is reduced by
> mechanical loading BUT the pulsewidth is also reduced. This appears to be
> caused by a combination of motor noise coupled through the Bicore timing
> caps pre-emptively triggering the Bicore and/or the effect of reduced Bicore
> output voltage swing coupled through the timing capacitors affecting the
> Bicore time constant.
>
> It seems to me that in a walker this MAY be a useful feedback by reducing
> wasted power in case of a stuck leg and it MAY be safer to change walking
> gait to a shorter shuffle on slope to avoid tipping over or on a high
> resistance surface it MAY produce a more efficient gait.
>
> If the Bicore output is buffered before connecting to the motor, the output
> pulse duration is actually slightly longer when the motor is loaded even if
> a small resistor is introduced in series with the powersupply to reduce
> supply voltage under load (so much for powersupply feedback).
> Next I will add some gain to this motor current feedback signal to see if
> these effects can be more precisely controlled.
>
> enjoy (although that's starting to sound like the Prisoner's "be seeing
> you")
>
> Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
> tel: (604)590-7493
> fax: (604)590-3411
>
> > -----Original Message-----
> > From: Bruce Robinson [SMTP:Bruce_Robinson@bc.sympatico.ca]
> > Sent: Tuesday, June 22, 1999 6:35 PM
> > To: beam@sgiblab.sgi.com
> > Subject: Re: Ah! Blessed Skepticism! (was: Re: Feedback explained?)
> >
> > James Wilson wrote:
> > >
> > > Here's my cents worth!
> > >
> > > When a motor is stalled (leg stuck) the inductor draws as much current
> > > as possible from the power source. The motor then happens to move if
> > > even a little bit and this will cause a back emf to increase the
> > > resistance of the circuit and thus influence the nervous net. Is this
> > > not apparent?
> >
> > and John A. deVries II wrote:
> >
> > > Ok, sure, people have -some- sort of qualitative observations indicating
> > > that a nervous net robot acts differently when a leg is stuck than when
> > > they the legs are free, but NO ONE seems to have made any repeatable
> > > experiments that either support or deny any particular hypothesis.
> >
> > So let's see. Ian brought up the feedback idea. I refuted it. Ben
> > pointed out a flaw in my argument. I mentioned that there COULD be some
> > feedback at certain points in the walking cycle IF the supply voltage
> > was sucked way down. Steve pointed out that we were going over old turf.
> > And John said no one's actually done a proper experiment to
> > prove/disprove it.
> >
> > After thinking about it some more, I realized that the main cause of
> > feedback to the Nv net would be when a motor was really stalled -- and
> > in that case the mechanical effects (motor barely moves) greatly
> > outweigh the Nv effects (Nv delay is shortened, reducing the amount the
> > motor will move).
> >
> > And the previous discussions that Steve so kindly sent us (thanks,
> > Steve!) seem to support this point of view. When I look at the walker as
> > a complete system, I return to my original assertion, that inherent
> > feedback to the Nv is bogus. When (if) it IS present, the
> > electro-mechanical conditions that cause it have a far greater effect.
> >
> > So to go back to John's statement, show me a repeatable experiment that
> > clearly demonstrates the effect, and I'll agree it's significant.
> > Otherwise I'm a confirmed skeptic.
> >
> > And I promise not to say any more about this for a long, long time .
> >
> > Take care,
> > Bruce
>


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