Quad_I is a hybrid pneumatic-and-motor-driven quadruped.
It can walk forwards, backwards, and turn in its own footprint
(it can also stop, of course). The step rate is about one step
every 7 seconds. All power and pumping -- a full load of six
AAs and everything you need to generate and control the
compressed air -- is carried onboard the quad.
There are four pneumatic pistons, one for each leg. The four
pneumatic pistons are arranged in two pairs -- one pair for
the left legs and one pair for the right legs. Pistons in a pair
are either both open or both shut, so each pair requires only
one control valve (total of two control valves).
For reference purposes we can number the legs 1 through 4.
Looking from the top, the legs are arranged like this...
1 ----- FRONT ----- 2
The weight of Quad_I is supported either on legs 1 and 4, or
on legs 2 and 3. The weight is shifted from one set of legs to
the other by a motor (originally I used pneumatics to shift
the weight, but this was slooooow). A single motor is coupled
to two identical shifting mechanisms: one for the front pair of
legs and one for rear pair.
3 ----- BA CK ----- 4
The transmission from the weight-shift motor is arranged so
that, when it is driven one way, the rear weight-shifter is fully
to the *left*, the one at the front is fully to the *right*. When
the weight-shift motor is run the other way, the shifters move
in opposite directions until the rear shifter is all the way to
the *right* and the front shifter is all the way to *left*. In
this way, when the shifters are at their limit of travel, the
weight is either on legs 1+4 or on legs 2+3.
Since Quad_I uses one motor to shift the weight, the other
two motors are used to work the pneumatic elements. Each
motor drives a Technic differential -- 8t gear on the motor
engages the 24t gear of the differential. There is one
differential for each of the two motors. Each differental drives
a pump *and* controls a pneumatic valve. The differentials
are oriented vertically in the thickness of the quad with the
motors at the bottom of the quad to try and keep the center
of gravity low.
As you know, differentials have two axles sticking out of
them and, since the differentials are oriented vertically, we
can distinguish between the top and bottom axles. -- the top
axle of each differential provides power to turn a large
pneumatic pump; the bottom axle drives a lever which
switches the pneumatic valve. The path of the lever has
stops at either end -- when the lever cannot travel any
further all the power is transferred via the differential to the
pumps. When we reverse direction, the pumps work until they
are near stalling and this provides enough resistance to
make the lever (on the lower end of the differential) push the
switch on the pneumatic valve to the required position. This
system works well, but it is tough on batteries!
Control is effected by 7 touch sensors. There is one touch
sensor for each of the legs (total of 4), two touch sensors
which detect the state of the weight-shifting mechanism, and
a final touch sensor to check the air pressure reserve in the
pneumatic air tank.
Each leg sensor detects when the leg is fully forwards. Leg
sensors for each side are wired in parallel to give an OR
result (if the front OR the back leg OR both legs on that side
are forward, the result is TRUE). The sensors for the two
sides are then wired to the input port using an AND
arrangement (the combined input is only TRUE if at least one
leg on the right side is all the way forwards AND at least one
leg on left side is all the way forwards).
The second input port is used to detect when the weight is
fully shifted from one side to another. The front shifting
mechanism has a touch sensor at the left and right extremes
of its limit of travel. These two sensors are wired back to a
single input port so that the robot knows when the weight is
fully on one set of legs or the other. (Quad_I knows *which*
side it has shifting to, because it knows which way the
weight-shifting motor is running.)
The final input is used to determine whether there is enough
air pressure. The air from the two large pumps is fed into a
single, blue, air tank on the right side of Quad_I. At the
output from the tank there is a branch tube to a small
pneumatic cylinder which is kept in a closed position by two
blue rubber bands. When there is enough air pressure in the
tank, the pressure is sufficient to extend the small piston
against the pull of the rubber bands and press a touch
and RCX are trademarks of the LEGO Group of companies which does not sponsor, authorize
or endorse this site. Read the full heavy
latin from LEGO for more details.
Text, graphics and downloads from http://jpbrown.i8.com copyright © JP
Brown, 2001-2006. All rights reserved.