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Pneumatic Control Circuit |
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It was from Eric Brok's website that I first learnt about the use of pneumatics for control logic besides their use as actuators. The pneumatic control circuit opens up such wide and interesting applications that it warrants some discussion here. |
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Basic Pneumatic Control Loop |
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| The basic pneumatic control circuit comprises a pair of cylinders and their associated valves connected as shown in the following diagram: | |||
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| Controlling Pneumatic Models | |||
| The basic pneumatic control loop can be used to control any pneumatic model by connecting the cylinder/s in the model in parallel to one or other of the cylinders in the control circuit. | |||
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Integrating Control Circuit into Model |
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| The pneumatic control circuit may often be integrated in the model itself which will save on components. That is, when a cylinder moves some part of a model, it will also switch a valve which would activate another cylinder. This cylinder in turn will switch another valve controlling the first cylinder. In this way the moving part of the model will operate automatically. | |||
| This is done in the alternate model of the Technic 8868 set which is a garbage truck with grabber arm. The 8868 is an interesting set. It is the only Technic pneumatic set that comes with a motorised air compressor (but no blue air tank). It is also the only set to incorporate pneumatic control to operate the model automatically. | |||
| The pneumatic control circuit in this model is
shown in the following diagram: |
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| The cylinder to the right is purely a control block. The two cylinders in the middle which raise and lower the grabber are acting as both actuators and control. The cylinder at the left is purely an actuator to open and close the grabber arms. It is connected in parallel to the control block which also raises and lowers the grabber arms. | |||
| The sequence of pneumatic action is as follows: | |||
| 1. At its lowered position, the grabber arms move
from open to closed position. 2. The grabber is raised and tipped over to the back of the truck. 3. The grabber arms are opened. 4. The grabber arms are moved over the cab and lowered to the front of the truck. The cycle is repeated as long as the air compressor is running. |
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| It should be noted that the grabber arms are first closed before the grabber is lifted up. Also, the grabber arms are opened before the grabber is lowered back to the ground. This is because when cylinders are connected in parallel, the one with the lightest load will be activated first. This is a useful property to get additional actions in sequence. | |||
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| Extension of the Pneumatic Control Circuit | |||
| The pneumatic control circuit can be extended to
3 or more control blocks as shown in the following diagram. The control blocks are
connected to give positive feed-forward except the last, which is connected to give
negative feedback to the first control block. In this way, you can have a battery of
control blocks to perform a sequence of pneumatic actions automatically. |
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