Simple Circuit Construction

Keywords: Lego, Pneumatic, Sequencing, Circuit, Cylinder

The notation and circuit used in this tutorial were based off ones presented in the Pneumatic Sequencing Tutorials by Kevin Clague.

The goal of this tutorial is to build on the progress students have made from the last three tutorials. In the previous three tutorials, students have learned how to build a motorized air compressor to create a consistent air flow and also how to build pneumatic switches to direct that air flow. Now students will combine that knowledge to build a circuit that does two very simple functions. As the circuit operates, one piston, piston A, will contract and expand. As it contracts and expands, it will change the direction of the air flow to another piston, piston B, which will follow the motions of piston A. An additional piston, piston C, will be connected so that it performs the operations inverse of piston B. Pistons B and C will be synchronized in motion, and piston A will have to wait until pistons B and C have reacted to its initial motion before it can move again.

When building pneumatic circuits, it is necessary that we have a way of describing the input and output relations of the system. For this reason, we will develop a method by which to notate the different ports of the system so as to describe the air flow using Boolean algebra.

The piston and cylinder pairs will be labeled alphabetically to keep track of how many pairs are used without the use of non-binary numbers. Our first objective, is to label the pressure ports on the piston itself. When air flows into the port at the base of the piston, it expands. For this reason, the lower port will be noted with an ‘x’ next to the pistons letter. When air flows into the pressure port located at the top of the piston, it contracts. For this reason that port will be noted with a ‘c’ next to the pairs name.

Similarly, we must devise a naming convention for how the switch directs the air flow. As the air flows in through the middle port, it can flow out through either the left or right ports depending on the position of the lever. When the lever is towards the right and air exits the left port, we will note this position with the letter of the pairs name. When the switch is reversed, a ‘~’ (pronounced “not”) will be placed before the letter.

The two figures below illustrate the naming convention. In the figure on the left, air is flowing into Ac, causing the piston to contract, which directs airflow in the switch to port ~A. In the figure on the right, air is flowing into Ax causing the piston to expand which directs the airflow in the switch towards the A port.

Step 1:
For the first step, students must complete tutorials I-III and repeat the first tutorial a second time to end up with the necessary components. Students should have one motorized air compressor, one pneumatic cylinder with two switches, and two pneumatic cylinders each with one switch.

Step 2:
Students should use their length of pneumatic hose and pneumatic T pieces to connect the pistons and switches as shown in the photos below. Air should flow from the compressor into the center ports of switches A and B. The initial state of the system should have pistons A and B contracted and piston C expanded.The left port of switch C1 and the right port of switch C2 are plugged using a small length of hose, and a minifigure hand. Without blocking air flow, air pressure will be relieved through those ports and the circuit will not operate correctly.

To derive the equations for the system, we must first decide how many individual equations we will have. Pneumatic pistons are boolen devices, meaning they only have two values; expanded, or contracted. For this reason, we must derive a single equation for each expansion and contraction of each piston. For the three pistons in the circuit, there will be a total of six equations. To derive the equations, we must trace the airflow, from each pressure port on the cylinder, all the way back to the initial input from the compressor, indicated by the orange hose. The flow to the expansion port will be noted by indicated by light green hoses, the flow to the contraction port will be indicated by the purple hoses. Piston A:
Air only flows into the expansion port when switch C2 is on the right side which gives us the equation Ax = C. However, switch C2 only receives airflow, if switch B is facing the left side. This gives us Ax = ~BC. For the contraction port of piston A, airflow is only present when piston C is contracted and switch C1 is towards the left. Airflow is only present in switch C1 when switch B is towards the left side making the algebra for this port Ac = B~C.

Piston B:
Airflow for both of the ports of piston B rely solely on the state of switch A. Switch A always has airflow coming straight from the compressor. When piston A is contracted and the switch is leaning towards the left, the air flows into the contraction port of piston B. Similarly, when piston A expands and the switch moves to the right side, air flows into the expansion port of piston B. From this we can see that the algebraic expression for piston B is Bc = ~A and Bx = A.

Piston C:
The output of the ports of switch A split into T joints and is sent to piston C as well. The hoses leading to the expansion and contraction ports are opposite relative to where they lead on piston B. The algebraic expression for for piston C is Cc = A and Cx = ~A

The tutorials objective was to give students an introduction to pneumatic sequencing. This one circuit introduces several concepts that will be necessary for further, more advanced pneumatic sequences. After the completion of this tutorial, students should be able to implement piston sequencing, synchronization, and inversion into future pneumatic projects.