Alt-BEAM Archive

Message #08019



To: beam@sgiblab.sgi.com
From: Greg Powell gpowell@engr.uvic.ca
Date: Thu, 25 Nov 1999 15:28:15 -0800 (PST)
Subject: [alt-beam] Re: photowalker


Jim sure has problems with his cat :)



8020 Thu, 25 Nov 1999 17:44:59 -0800 [alt-beam] ChloroBug "'beam@sgiblab.sgi.com'" Wilf Rigter

Here is a new design which has the advantage of an adjustable turning "dead
band". After adjustment, the pot can be replaced with an equivalent fixed
resistor. The dead band means that, when the light on the PD bridge exceeds
a certain percent imbalance, the ChloroBug turns in the direction of the
brighter side but, once pointed to the light, the ChloroBug will move
straight towards it with both motors on. When going straight, this circuit
should be more efficient and twice faster than the previous photochlor SE
circuits which either "waggle" or use alternating PWM in which only one
motor is on at a time. Also note the optional 100pf cap for positive
feedback to make it lean a little more into the turns. A bumper feeler and
timer cap on each side of the pot could be added to steer away from
obstacles.

<>

Wilf Rigter mailto:wilf.rigter@powertech.bc.ca
tel: (604)590-7493
fax: (604)590-3411

> -----Original Message-----
> From: Maynard [SMTP:cybug@home.net]
> Sent: Tuesday, November 23, 1999 7:46 AM
> To: beam@sgiblab.sgi.com
> Subject: Re: Cholroplast thingy
>
> Hi Wilf:
>
> I can see a lot of strengths to your design! I'm anxious to try it out
> ASAP.
>
> Thanks
>
> Craig
>
> > Hi Craig,
> >
> > The "small" caps in my schematic also have a long time constant because
> I
> > use very low current green LEDs for photovoltaic cells. The small caps
> are
> a
> > lot cheaper than 1uf monolithic caps and much lower leakage current than
> > 1uf electrolytics. In your simple "feeler/tank" turning arrangement,
> the
> > turning time constant depends on the light level which may actually be
> > desirable since it tracks the solar cell charging rate. This only works
> if
> > you use small solar main storage cap ie 4700uf, where each SE trigger
> > results in a relatively small forward motion but which is repeated
> > frequently. If you use large super caps, the bot could turn a few
> circles
> > before the LED cap charges up again. (g)
> >
> > It is interesting to observe how placing the caps in parallel or in the
> > positive feedback path creates different behaviour. Putting the caps in
> the
> > feedback path is somewhat desirable since it forces clean switching of
> the
> > motor driver. I noticed that with the large parallel caps there can be a
> lag
> > in response and when light is equal on both PDs, both drivers come on.
> With
> > the smaller feedback caps, the bot is always waggling, and the motors
> are
> > pulsewidth modulated when the light is nearly equal. Attached is yet
> > another version of the PhotoChlorSE circuit which works very well, using
> the
> > venerable 2N3904s, good for 100ma motors. The total voltage drop on the
> > 2N3904 transistor is 120mV instead of the 2V drop of two series
> connected
> > MPSA12 darlington transistors which should give the motors a little more
> > Zap. Note the 0.6V clamped voltage feedback from the base of the
> transistors
> > was needed to avoid feedback from the sagging voltage on the opamp
> output
> as
> > the main cap discharges. I have, in the meantime, added a simple
> > feeler/turning circuit with a time out independent of light level which
> I
> > will show you next time.
> >
> > <>
> >
> >



Attachment: pcse2.gif

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