Automating Manufacturing Systems - Process Control and ...

plc pid - 25.2
valve
a) Water Tank
q 1
q 2
valve
h
motor
controller
Vin
b) Motor Driven Conveyor
Figure 25.1
Continuous Systems
A mechanical control system is pictured in Figure 25.2 that could be used for the
water tank in Figure 25.1. This controller will adjust the valve position, therefore controlling
the flow rate into the tank. The height of the fluid in the tank will change the hydrostatic
pressure at the bottom of the tank. A pressure line is connected to a pressure cell. As
the pressure inside the cell changes, the cell will expand and contract, opening and closing
the valve. As the tank fills the pressure becomes higher, the cell expands, and the valve
closes, reducing the flow in. The desired height of the tank can be adjusted by sliding the
pressure cell up/down a distance x. In this example the height x is called the setpoint. The
control variable is the position of the valve, and, the feedback variable is the water pressure
from the tank. The controller is the pressure cell.