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august 10th, 2002.

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access to my Lego pages since creation.

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Copyright 1996, 2000, Denis Cousineau


  1. How to turn on/off a pneumatic.

To my knowledge, there are four methods to turn on/off a pneumatic:

  1. Manually
  2. Using electric pistons
  3. Mechanically
  4. Electrically


  1. Manually

     The manual method is obvious: you simply turn the switch yourself. This method is neat and simple, but is not very useful for robot design...

  2. Electric pistons

    Leo in his Leo’s Designs (see a link on my home page) has suggested this method. The idea is simply not to use pneumatics at all, but to use instead pseudo-pistons that are activated by a micro-motor. These pseudo-pistons have the same behavior as standard pistons, that is a forward/backward movement of the tip. Although elegant, this design suffers from two drawbacks: i) the micro-motors are not as powerful as the pistons (see my page on motors), and ii) if your replace the micro-motor with a regular motor, then the problem of weight is back again.

  3. Mechanically

    Controlling the switch mechanically is pretty easy. Various methods are possible. One difficulty is that these switches are very rigid, and require a large torque to be moved. Soh (see link on home page) describes one technique, illustrated below. Another difficulty is to stop the interruptor right in the middle to interrupt the piston. This can hardly be done without a sensor. The book by Mario Ferrari describe one such interruptor with a touch sensor (see references and links page).  This is the solution adopted for my Arm-v3

    pneumatic_station_switch.jpg (48024 bytes)

  4. Electrically

    Electrical control of the switch is the preferred method. This can be accomplished using a solenoid air valve. These are rather common electrical parts composed of a solenoid (an electromagnetic switch) connected to a valve. The Fischertechnik cie produces such a part (part no ??), roughly compatible with Lego parts. 

    This part is not a three way like the Lego switch. It works the following way:

    where the "vent in" occurs if the valve is not under current (not connected). If the valve receive 9v electrical input, it exits the pressure through the exit connection.

    Because they are not three-way, at least 2 such valves must be used to control a piston in or out, as in the following diagram:


    The following table shows what happen when A or B or both are turned ON:

       A       B       Result  
    0 0 piston free to move
    1 0 piston moving out
    0 1 piston moving in
    1 1 piston locked

    If the last condition is never used, one easy way to turn A or B on is to use an electronic parallel demultiplexor. Thus, only one output of the RCX can fully control a pneumatic. In addition, no need for a sensor is required. It is therefore the most economical solution.