Quad Op-amp Walker Circuits
A family of circuits by Wilf
In a November 2002 post,
Wilf came forward with a phototropic
walker design based loosely (see below) on Ben Hitchcock's
so dummy walker."
Wilf's writeup on the circuit
is as follows:
Speaking of Ben's "not so dummy" walker, here is
the Not So Dummy Opamp 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 some hysteresis
to avoid false triggering from noisy servo pot
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 band, turning and reversing. Interfacing
sensors would be a snap because of the inherent accuracy
of the opamps.
In a July 2003 post,
Wilf further developed this design, enhancing it with
reversing capability and the ability to track light.
Wilf's writeup on the circuit
is as follows:
Here is the Quadrature Opamp walker with
and reverser subsystems, the latter using some 74AC240
(which limits the supply choice) Four of the remaining
could be used as motor drivers for the 6V Servo motors.
The motors are driven with position feedback so sync
reversing is not needed.
All as yet untested but will probably work fine.
It was later pointed out to Wilf that the "Not so dummy
walker" worked differently than he remembered, and that he
had in fact invented a new type of walker circuit.
expands on this:
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
feedback pots. I had not recently looked at nor ever
really did an analysis of the Dummy and Not So Dummy
so this seemed good at the time.
On closer examination it turns out the Dummy / NSD
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
walker. If I had build the NSD walker or had seen a video
(hint) of it walking that would have avoided the
I might add that the choice of two 6.8M resistors
may not be optimum as it may not "unstuck" if the
/ 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 leg centering.
Speaking of historical references: The Dummy Walker
principle of using a Schmitt
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 (i.e., a microcore).