How the Foxboro 43AP Pneumatic Controller Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How the Foxboro 43AP Pneumatic Controller Works

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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. 
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.

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:

  1. A differential linkage measures difference between measurement pointer and setting index         positions as shown in the schematic above. This error signal moves proportioning lever.
  2. The proportioning lever pivots at its center on the end of a flat spring.
  3. This motion of the proportioning lever changes flapper nozzle relationship, causing relay to        establish an output pressure.
  4. 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.