Alt-BEAM Archive
Message #05804
To: beam@corp.sgi.com
From: "Jason -" evenflow88@hotmail.com
Date: Fri, 20 Aug 1999 07:50:57 GMT
Subject: [alt-beam] Walker leg centralizing info
Hi all, recently the thing that made me dislike or bored of building bots is
the fact that how to centralize a leg in a two motor walker....i heard stuff
like mechanical stops and springs and etc. i am all tired of that and it can
make the ot reall uncool and messy....i would like to know how dave made his
two motor walker kit like scoutwalker which uses servo centralizing its own
leg.....manufaturer tolerence is one thing that is hard to avoid and so i
really hope wilf can whip out a schematic to centralize a leg
electronically...here is a reply i got from Mark T which how he used to
cetralize his:
One is an active stop where a button is pushed
and opens the motor connection, dynamic where an encoder wheel tells a
control circuit dynamically where to stop ahead of time, and passively
which is what I use.
and for all you beamers which never used springs and mechanical stops out
there and yet can manage to centralize those walker legs of yours...please
do state out a suggestion
jason
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5805 Fri, 20 Aug 1999 04:30:25 -0700 [alt-beam] Re: TMOSFET sensor madness (stubborn anyone?) beam@sgiblab.sgi.com Sean Rigter Hi Richard,
What exactly was the result of using your mosfet circuit with and
without the bias circuit.
Terry Newton's circuit and your detector both use "random leakage"
effects. The value of the gate voltage is the sum of all leakage
currents and totally random and unknown. By connecting a very high
value resistor between the gate and a voltage source, the gate voltage
can be "set" and the FET operation can be made more predictable. This
resistor should be an order of magnitude smaller than the stray leakage
"resistance" and as a result will stabilize operation at a reduction in
the sensitivity.
I expected a few questions about the function of the two back to back Si
diodes. They function as a very high resistance bias resistor since
their leakage current is very small at room temperature but is much
larger than the mosfet gate leakage current. The voltage at the wiper of
the pot will leak to the gate and set the operating point of the FET.
The alternative to Si diodes is to use a >1000M resistor which is not a
commonly available. To effectively use such high resistances, stray
leakage currents to other parts of the circuit also has to be
controlled, ideally using a Teflon standoff as the junction point for
the FET gate and antenna (and Si diode if used).
E-fields are invisible complex , fluctuating electrostatic fields and
would be interesting to map in 3D.
The predominant fluctuating (ac) E-field comes from the household
50/60Hz power circuit. The ac sensitivity of the sensor is proportional
to the stray capacitance to GND and the AC signal at the output signal
may actually go down as you bring your hand closer to the antenna
coupling stray capacitance between your body and GND. If were are
closely coupled to the power circuit (danger!) the signal would of
course increase. The dc field is the difference in static electron
charges but this is not as simple as it seems since charges (and
E-fields) are displaced (disturbed) by attraction or repulsion of other
charges. Charges are also carried by gas ions so the picture is quite
muddled.. Since the leakage and bias currents "leak" the induced gate
charge, the detector is not stable at detecting true dc E-field but
can detect slow changes in voltage as E-fields change.
One thing you should consider is that the sensor detects the charge
between itself and other objects equal to the difference in the charges.
So an object with zero charge can still be sensed if the sensor platform
itself has a charge with respect to other objects. If the charges get
large walking across a nylon rug on a dry day, think "fried" mosfet. If
the charges get very large think Lightning! Try to observe the output
of you detector during a thunder shower and you may get an indication
just before a lighting discharge of the very large e-field between earth
and clouds.
Place the E-field detector inside a metal screened box and note the
Faraday cage effect (all in the safety of your home of course).
Finally, one more thought: if a "delta e-phobic" bot approaches the edge
of a table there should be a large change in the e-field which may
(perhaps) be used to reverse and avoid "Geronimo"!
Anyway those are some of my observations. I like to hear more about the
result of your experiments and the phenomena you have observed.
regards
wilf
Richard Caudle wrote:
> -----------------------------------------------------------------------
> Evening all, I've been playing with using the 2N7000 as a E-field
> sensor.
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