Automating Manufacturing Systems - Process Control and ...

plc pid - 25.3
q 1
Main water
supply
q 2
x
For control add,
feedback
setpoint
system error
1. Feedback of hydrostatic pressure through a rubber tube.
2. This input slider adjusts the position of the bellows (can
be adjusted with a screwdriver).
3. Bellows expand/contract as pressure increases/decreases,
and move the rod that closes/opens the valve
4. The valve changes the flow into the tank, thus changing
the water height.
1. Some means of measuring the water height (system state)
2. Some input for desired control height
3. Some error compensation
4. An actuator to change the system input
Figure 25.2
A Feedback Controller
Continuous control systems typically need a target value, this is called a setpoint.
The controller should be designed with some objective in mind. Typical objectives are
listed below.
fastest response - reach the setpoint as fast as possible (e.g., hard drive speed)
smooth response - reduce acceleration and jerks (e.g., elevators)
energy efficient - minimize energy usage (e.g., industrial oven)
noise immunity - ignores disturbances in the system (e.g., variable wind gusts)
An engineer can design a controller mathematically when performance and stability
are important issues. A common industrial practice is to purchase a PID unit, connect it