Diaphragm actuator works on the principle of gauge pressure i e difference between input pressure signal and atmospheric pressure (Pressure = force per unit area)
Where, = Pressure difference (Pascal)
A = Diaphragm area ()
F = Force (N)
• Diaphragm actuator consists of a diaphragm made of rubber and is sandwiched between two circular steel discs. The diaphragm is mounted in upper chamber where the varying pressure from controller is applied.
• The bottom chamber contains a spring that forcing the diaphragm and connected to shaft down against the spring force. Fig. shows the construction of actuator diaphragm.
Fig. Pneumatic diaphragm actuator
• In upper chamber on applying the varying air pressure results in a varying force on the top of the diaphragm. Initially, with no air pressure, the spring (S) forces the diaphragm upward and holds the valve fully open. The pressure on opposite side of diaphragm is maintained at atmospheric pressure by open whole H.
• The effect of changes in input air pressure applies a force on the diaphragm this forces diaphragm down against the sprig force Fig shows the condition of diaphragm after applying pressure and maximum travel of shaft (high pressure state).
Fig Pneumatic diaphragm actuator under high pressure
• The pressure and force are linearly related by equation (1). As the compression of a spring is linearly related to force hence the shaft position is linearly related to applied control pressure. It is given by
Where, = Shaft travel (meters)
= Differential pressure (Pa)
A =Diaphragm area
K = Spring constant (N/m).