Alt-BEAM Archive

Message #04929



To: beam@sgiblab.sgi.com
From: Wouter Brok w.j.m.brok@stud.tue.nl
Date: Tue, 29 Jun 1999 23:25:52 +0200
Subject: [alt-beam] Re: Bicore Feedback Experiments


Hello Ivar,

Thank you for doing those experiments, I think you did them in a really
analytic way !!!

Well, for the explanation: when I read it I immediately had an idea in mind
and this got confirmed when I read more. Don't know if it is right though:

Why does the frequency get lower above some critical load: when you ask
current from the output of an HC240 inverter its voltage will get lower;
the more current, the lower the voltage. At a certain loading the
output-voltage of the inverter, which should be Vcc drops below Vcc/2, the
threshold of the other inverter. Well, imagine that when an inverter
changes state, say from low to high, its output voltage wants to get its
voltage up to Vcc, but this doesn't work, because so much current is asked.
Actually it can't get its output even above Vcc/2. How to explain it
further in text I don't know at the moment: have a look at the waveforms in
the suspended bicore article and probably you will figure it out for yourself.

I realise that this isn't much of an explanation, if correct in the first
place .... will have a better look at it tomorrow.

Goodnight,

Wouter Brok.


>Hey, all.
>
>I found the discussion on Bicore feedback facinating, so I pulled
>out the breadboard to do some more experimenting.
>
>I made a suspended Bicore, tripled the outputs for extra driving
>power and to use all the invertors, and connected it to a BG micro
>motor. Exactly like Wilf said, the frequency of occilation increased
>
>as I held the motor shaft. Shorter, faster movements of the legs.
>
>But I found I different state which I think Wilf may have
>accidentally looked over. If you hold the shaft nearly completely
>rigid (You need to attach legs so that you can get enough torque
>with these motors...), the length of the frequency drastically
>_decreases_, to perhaps half of the original frequency. It's really
>interesting. The bicore will accept a certain amount of load and
>just increase the frequency to a point, but when it gets really
>stuck it decreases the step speed so it has longer "thrashings".
>
>I think that seems to be an effective use of feedback, if you ask
>me. It gets a little stuck, and it shakes faster. It gets really
>stuck, and it thrashes violently with long strides until it is free.
>
>As a comparison, if you hold the motor shaft of a regular bicore
>(not suspended, but 2 resistors to ground), holding the legs does
>indeed increase the speed of the shaft, but the 'critical point'
>never happens. The frequency of the bicore just increases up to the
>stall point of the motor. This also holds true for the microcore and
>
>multi-neuron designs.
>
>Getting back to the suspended bicore bit, I ran a few more tests to
>see if I could determine why the suspended bicore does this and not
>the regular bicore, and under what conditions it did it. I'm using a
>
>74HC240 at 6volts, by the way.
>
>First, I simply connected the motor directly to the outputs of a
>suspended bicore. No buffers. Little driving power, mega feedback.
>It doesn't even act like a bicore, but is interesting just the same.
>
>The motor spins (weakly) one direction indefinitely, and stopping
>the turning of the leg with your hand causes it to press against
>your hand briefly and spin the other direction indefinately until it
>
>encounters a similar bump. After you apply a certain amount of
>resistance to the turning of the leg, however, it begins to occilate
>
>like a bicore normally does. The more pressure, the faster the
>occilation up until it reaches stall.
>
>Now is when the weird stuff started happening. Stick inverters (One
>for each wire going to the motor) in the circuit between the motor
>and the bicore to act as a buffer. Suddenly, it works like a bicore
>at no load. Presumably the inverter reduces the feedback to the
>bicore, but not totally. At a very light load, the occilation
>frequency increases like expected. But at moderate and heavy loads,
>the Bicore occilation _slows down_ a lot, maybe to half of the
>original frequency.
>
>Now, if you put another inverter in parallel with the buffer
>inverters (Pardon me if my elektrik(tm) speak is no good;-) the
>effect becomes easiest to notice. Light load, and the frequency
>increases. Heavy load, and it slows down below the original
>frequency.
>
>Put three inverters in parallel to use all of the inverters in a
>240, and the results are much the same as with two inverters, but
>you have to reach a higher load level before the frequency shifts to
>
>the slow frequency.
>
>My basic conclusion from one night of playing with these is that:
>
>
> -The bicore responds to feedback from the motors by
>increasing the frequency of occilation up to the shift point.
> -Further motor load after the shift point makes the motor
>slow considerably up until the motor stalls.
> -The shift point is determined by the amount of amperage the
>motor drivers can handle. The more power available, the higher the
>shift point and the harder the motors have to be loaded before the
>occilation shifts speeds.
>
>Now, I would really like somebody to explain why all this happens,
>or at least duplicate it on a breadboard so I know I'm not going
>insane. I have breadboarded this three or four times with similar
>results all the times. It may be a particular lucky combination of
>the BG Micro motors and the 74HC240, I am not sure and it's late at
>night so I may be making mistakes.
>
>Doing similar experiments with H-Bridges for motor drivers would be
>nice, too. How much feedback to different designs provide, does a
>feedbark resistor help, etc.
>
>So, assuming that the feedback tendancies of the suspended bicore is
>
>useful to a robot (Shake at light load, kick at high load), what
>about those robots with microcores, couldn't this be harnessed too?
>Enter the suspended microcore? I tried to figure out how this would
>work, but I was unable to because I have read (less than) one book
>on electronics theory, so circuit design is not my strong point...
>Tying all of the bias resistors to each other, or to the opposite
>pairs didn't work like I had hoped, although the experiments led to
>some other interesting circuits. If you make a four neuron
>'suspended' microcore on a HC240, you end up with a Self Healing
>Super Whoosh Bicore/Quadcore Thingy. You can remove two of the
>resistors opposite each other, even connect two to ground or
>positive, and it continues to run flawlessly. It can't go to a
>regular single pulse state, either. It 'heals' itself if one of the
>pulses are killed so it is always running at two. It is very
>difficult to kill all the pulses, as well. You have to kill two
>pulses simultaniously or it heals. Could be useful in very rugged
>bots or ones with bicore stability problems. I will post a schematic
>
>if nobody understands what I mean.
>
>Well, I better go to sleep if I want to get more than 4 hours...
>
>
>Ivar Thorson
>--
>www.welcome.to/beam/
>--
>
>
>
>

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