This invetion is an electro-pneumatic adding/subtracting
machine. It uses pneumatics to do the arithmetic, and electricity/motors to feed
bits into the pneumatics, and touch sensors to read the results from the
pneumatics. Why build a pneumatic adding machine when we can make the RCX do the
math much more easily and a lot faster? Good question! There are two reasons:
1. Because I perceive pneumatics as a way of getting around the I/O
constraints of the RCX.
2. Because our illustrious envcons made the bold statement that it could not
be done!
To use the calculator, you press the keys on the 16 key keyboard, when
neccessary the program feeds the numbers to be added or subrtacted to the
pneumatics, and the results are displayed on the RCX's LCD display.
There are two innovations in this invention; pneumatic OR gate, and a 16 key
keyboard.
The pneumatic OR gate is made up of two large pneumatic pistons, and a
pneumatic switch. I consider an expanded piston to be a 1 input into the gate,
and a contracted piston to be a 0 input. The mechanics of the gate are such that
if either or both of the pistons are expanded, then the pneumatic switch is
flipped one way. If neither piston is expanded, the pneumatic switch is flipped
the other way.
There are three OR gates used to make a half adder, which has two inputs
bits, a sum output and a carry output. In NQC terms the pneumatics calculate
like this:
int in0_bit, in1_bit; // the inputs to the adder
int sum_bit, carry_bit; // the outputs of the adder
carry_bit = in0_bit & in1_bit;
sum_bit = (in0_bit | in1_bit) & ! carry_bit;
There are four possible combinations of inputs the the half adder: Using DeMorgen's theorum we can convert an OR gate to an AND gate by
inverting (change a 1 to a 0 and a 0 to a 1) the inputs and inverting the
output. In my pneumatic logic, inversion is done by reversing the hookups to the
pneumatic cylinders.
When people add two numbers together in decimal, we start with the rightmost
decimal place, add the two digits, record the carry into the next decimal place,
and moce onto the next decimal place adding the carry and the two digits in that
column. PneumADDic does the same thing only in binary.
For each bit in the numbers (operands) you want added together, pneumADDic
must perform three pneumatic calculations:
1. Add the two bits of the operands, remembering the sum and the carry
results.
2. Add the sum bit from step 1 to the carry in (from the previous bit columns
result), record the sum bit and the carry bit.
3. Add together the carry bit from step 1 and the carry bit from step 2 (only
one can be on) and record the sum bit (as carry in for the next bit column).
The sum bit from step 2 is the result for this binary place in our operands.
The sum bit from step 3 is the carry out for this binary place, which will be
used as the carry in for the next binary place.
When all binary places are computed, pneumADDic displays the results on the
RCX's LCD display. PneumADDic can handle 3 decimal digits.
PneumADDic uses two motor controlled pneumatic switches to feed values into
the pneumatic half adder. It reads the results out of the pneumatic logic using
small pistons that when expanded press touch sensors.
The 16 key keypad is arranged in an array of four rows by four columns. Each
row and each column has a touch sensor to detect if a key in that row or column
was pressed. When you press down a key, two touch sensors get pressed: a row
touch sensor and a column touch sensor.
I hooked all of the row touch sensors together in a resistor ladder network
and hooked that to one sensor input. I did the same for the column sesnors and
ran that to a second sensor input. I made my own resistor blocks that when
hooked between a touch sensor and RCX, puts the resistor in series with the
touch sensor.
The touch sensor/resistor ladder combination acts as a digital to analog
converter. The RCX's analog to digital converter, plus reading the sensor in raw
mode enables the program to tell which row and which column touch sensors were
pressed.
PneumADDIc uses two compressors to charge up four air tanks that provide the
pressure for running the pneumatic logic. A large piston, rubber bands and a
touch sensor are used to create a pressure sensor. The program uses the pressure
switch to know when to run the comnpressors.
In the picture, the compressors, air tanks and pressure sensor are in the
back left. The three OR gates that make up the half adder are the large things
in the back. The sum and carry output sensors are to the right of the pneumatic
logic. Motor controlled pneumatic switches are in front of the sum and carry
outputs.
The sum and carry output sensors are hooked to the RCX as part of the row and
column resistor ladder networks.
The keyboard is in the front left. To its right are the resistor ladder
networks, and to the right of that is the RCX.
There are 11 touch sensors, four motors, two large pumps, 5 pneumatic
switches, 7 large and two small pistons in the design.
in0_bit in1_bit | carry sum
0 0 | 0 0
0 1 | 0 1
1 0 | 0 1
1 1 | 1 0
Those who are aware of boolean logic will recognize the sum_bit output as
an exclusive OR gate (XOR) output, and the carry_bit output as an AND gate
output.
