Re: Not So Dummy Opamp Walker was Re: [beam] Re: Reversing Gracefully
- Hi Ben,
The reference to the NSD walker in the Quadrature walker post was based on,
what I mistakenly remembered as, the NSD walker's use of crossed-over servo
Since I had not recently looked at nor ever really did an analysis of the
Dummy and Not So Dummy Walker circuits, so this seemed like a good time.
On closer examination it turns out the Dummy/NSD circuits are quite
different in their use of the feedback potentimeters and are really
unrelated to the Quadrature opamp design.
It is however quite possible to do a Opamp version of the NSD walker.
You are quite right about the NSD walker's "only one motor on at a time"
which gives the same stride as a microcore walker. If I had build the NSD
walker or had seen a video (hint) of it walking that would have avoided the
Attached are the waveforms for the Dummy Walker which also applies to the
NSD walker unless it encounters a stuck leg in which case the waveforms
depend on the RC delay as shown in the separate NSD walker circuit detail. I
might add that the choice of two 6.8M resistors may not be optimum as it may
not "unstuck" if the Schmitt hysteresis is Vcc/3 (unlikely). However the RC
delay can be much reduced by using a 6.8M feedback resistor with a 10M
resistor across the coupling capacitor which may reduce the element of
surprise but is also less wear and tear on the servo motors.
The Quadra walker evolved from the discussion of using feedback pots and
opamps to sense the midposition of a leg for synchronous reversing. That led
to the observation that in bicore walkers the midposition of one leg is the
end of travel for the other leg etc and therefore this accurate sensing of
midposition of one leg can be used to change the direction of the other leg.
Therefore the Quadra walker uses the feedback pot of the second servo as a
control signal for the first servo and vice versa. So if one leg gets
stuck, its motor will eventually reverse as long as the second leg is not
stuck since the stuck leg is controlled by the feedback pot of the second
leg. Since two legs can be stuck at the same time, an RC time constant will
be added to make Quadra walker smart about untangling itself from a sticky
The Quadrature opamp walker circuit, like the MS Bicore walker, generates an
efficient quadrature stride but uses true position feedback for springless
Speaking of historical references: The Dummy Walker principle of using a
Schmit trigger to sense limits of travel from a servo feedback pot in order
to reverse the direction of servo rotation was described in the uCrawler1.0
article of Oct 1999 on the old alt beam mailing list. The problem of
"stalled" legs and the solution of using an RC delay to free the legs and
using the same circuit for two motor walkers was also mentioned in the same
article reprinted here:
Those ideas were not explored any further until Ben posted his super simple
Dummy and NSD walker designs with their innovative use of a 1 1/2 h-bridge
motor driver, which by the way can be broadly applied to other two motor
reversing driver designs where both motors are never active at the same time
(ie a microcore).
----- Original Message -----
From: "nebbian" <beh01@...>
Sent: Sunday, July 27, 2003 11:40 PM
Subject: Not So Dummy Opamp Walker was Re: [beam] Re: Reversing Gracefully
> Hi Wilf,
> Your circuit could probably be better described as a "Dummy Opamp
> Walker", as I reserve "Not So Dummy" for a circuit that can free a
> leg that gets stuck. The two 6.8Meg resistors and 1 uF Capacitor on
> my schematic will automatically reverse a leg if it doesn't change
> state within several seconds, which to my mind is another form of
> true environment sensing. It's quite funny to watch people when I
> tell them to grab the legs and stop them moving -- they put some
> force on the legs, and then 5 seconds later when the legs try to go
> the other way the person usually shouts out with surprise and drops
> the robot :-)
> Nice adaptation though, I like the positive feedback hysteresis to
> simulate the schmitt trigger. Please note, however, that the two
> circuits differ in a key functionality -- in mine only one motor
> moves at any one time, whereas in yours both move at once. Six of
> one, half a dozen of the other, I think.
> --- In email@example.com, "Wilf Rigter" <wrigter@d...> wrote:
> > Speaking of Ben's "not so dummy" walker, here is the Not So Dummy
> > Walker. Note the use of the swapped servo pots to drive each motor
> which is
> > the key mechanism of this circuit.
> > Using opamps makes it easier to trim the center position of each
> motor when
> > the other motor should change direction. I added a small cap and
> > hysteresis to avoid false triggering from noisy servo pot wipers.
> > This walker maintains correct timing over a wide range of supply
> voltage (
> > 3V-32V) because of the ratiometric comparator action of
> the "core". How
> > about a 24V walker?
> > You can add all kinds of features to this circuit, such as dead
> > turning and reversing. Interfacing sensors be a snap because of the
> > accuracy of the opamps.
> > wilf
> > ----- Original Message -----
> > From: "J Wolfgang Goerlich" <jwgoerlich@h...>
> > To: <firstname.lastname@example.org>
> > Sent: Friday, July 25, 2003 3:00 AM
> > Subject: [beam] Re: Reversing Gracefully
> > > I have been following this thread with interest, and have what
> may be
> > > a question. It would seem that there are two approaches: adjusting
> > > the reverse based on the core process, or adjusting the reverse
> > > on the leg position by way of mechanical switch. Would it be
> > > to use a partially modified servo -- pot intact as in Hitchcock's
> > > Dummy Walker -- to produce a hybrid of these two approaches?
> > >
> > > J Wolfgang Goerlich
> > >
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