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192 Masterpiece 4 • PneumADDic II

Powering PneumADDic II

One of the problems with the original PneumADDic was that it was painfully slow. It
took about 10 minutes to add two three-digit numbers! This issue was attributed to the
fact that the gates required high pressure, however a second issue was that the compres-
sors (it had two) were not very strong.
PneumADDic II contains a single compressor comprised of two motors and four
large pumps. When compressing a pump, the first half of the compression stroke does not
require a lot of force.The closer the pump handle is to maximum stroke, the more force
it takes to move the pump handle. PneumADDic II’s compressor uses four pumps, but
only one of the pumps is being driven to maximum compression at a time. On a clock,
this is like making the first pump compress at 12:00, the second pump at 3:00, the third
pump at 6:00, and the fourth pump at 9:00.This means that the motors only need to
have enough force to compress one piston fully, and the next piston partially.
PneumADDic II’s compressor uses two motors to drive the pumps giving the
compressor twice the power to compress the pumps. By using four pumps and two
motors, PneumADDic II’s compressor is much more efficient than PneumADDic’s
two compressors.
PneumADDic II requires a total of three motorized switches.Two switches are for
feeding values to the inputs of the pneumatic adding circuit.The third motorized switch
controls when the Carry Memory remembers (or stores) the carry for a given binary
place.The RCX only has three motor output ports. Each output port can drive at least
two motors.The two motors in the PneumADDic II compressor can be driven by one
RCX output port. But that only leaves us two motor ports and the need for three
motorized switches.To solve this shortfall, I decided to combine one of the switches into
the compressor.
The compressor uses a handy gearing device called a differential.A differential can be
used many ways, and can be somewhat confusing to understand through reading, so I sug-
gest you assemble one as shown in Dual Motor Switch Step 9.A differential has a total
of four independent gears and two axles. PneumADDic II’s compressor motors turn the
case of the differential (one of the independent gears), which makes the two axles turn.The
two axles can rotate independent of each other, but are related in how they do so.
If you rotate the differential casing (the dark gray partial cylinder in added in Dual
Motor Switch Step 9), both axles turn at the same speed. If you prevent one axle from
turning, all the rotation at the differential case is applied to the free axle. If you restrict
the rotation of both axles using friction, the amount of power delivered to the differential
is split between each axle based on how much resistance each axle is experiencing.
In the case of the Dual Motor Switch sub-assembly for the compressor, one of the
axles is used to flip the switch handle from one position to the other.The other axle pro-
truding from the differential is used to compress the pumps.
When the dual motors start running, the differential automatically determines which
axle is requires less force to turn which in our case could be either the pumps in the