The Foxboro 43AP pneumatic controller is a versatile process instrument controller that can be used to control pressure, temperature , flow and level. As with all process controllers, the Foxboro 43AP pneumatic controller continuously detects the difference between a process measurement and its set point, and produces an output air signal that is a function of this difference and the type of control.
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| Controller Loop for a Pneumatic Controller |
The output signal is transmitted to a control valve or other control device. The process measurement, set point, and output signal are indicated on the controller.
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| Foxboro Pneumatic Controller with Proportional, Reset and Derivative Actions and Automatic Manual Transfer System. Photo Credit: Foxboro |
Principe of Operation of the 43AP Pneumatic Controller
The above schematic shows the basic internal parts of the mechanism of operation of a Foxboro 43AP pneumatic controller. The principle of operation of the device is explained below:
- A differential linkage measures difference between measurement pointer and setting index positions as shown in the schematic above. This error signal moves proportioning lever.
- The proportioning lever pivots at its center on the end of a flat spring.
- This motion of the proportioning lever changes flapper nozzle relationship, causing relay to establish an output pressure.
- This output pressure is fed back to the proportioning bellows, which acts through the proportioning lever to re-balance flapper nozzle.
As shown above, this particular controller model has reset bellows as well as a derivative tank. The reset bellows and tank assembly are used when measurement must be maintained exactly at control point -that is without “offset”. The derivative tank assembly is used to improve system response to a slow process.
