Alt-BEAM Archive
Message #08021
To: beam@sgiblab.sgi.com
From: Wilf Rigter Wilf.Rigter@powertech.bc.ca
Date: Thu, 25 Nov 1999 20:46:56 -0700
Subject: ChloroBug
> 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.
> > >
> > > <>
> > >
> > >
>
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