Automating Manufacturing Systems - Process Control and ...

plc pid - 25.1
25. CONTINUOUS CONTROL
Topics:
• Feedback control of continuous systems
• Control of systems with logical actuators
• PID control with continuous actuators
• Analysis of PID controlled systems
• PID control with a PLC
• Design examples
Objectives:
• To understand the concepts behind continuous control
• Be able to control a system with logical actuators
• Be able to analyze and control system with a PID controller
25.1 INTRODUCTION
Continuous processes require continuous sensors and/or actuators. For example,
an oven temperature can be measured with a thermocouple. Simple decision-based control
schemes can use continuous sensor values to control logical outputs, such as a heating element.
Linear control equations can be used to examine continuous sensor values and set
outputs for continuous actuators, such as a variable position gas valve.
Two continuous control systems are shown in Figure 25.1. The water tank can be
controlled valves. In a simple control scheme, one of the valves is set by the process, but
we control the other to maximize some control object. If the water tank was actually a city
water tank, the outlet valve would be the domestic and industrial water users. The inlet
valve would be set to keep the tank level at maximum. If the level drops there will be a
reduced water pressure at the outlet, and if the tank becomes too full it could overflow.
The conveyor will move boxes between stations. Two common choices are to have it
move continuously, or to move the boxes between positions, and then stop. When starting
and stopping the boxes should be accelerated quickly, but not so quickly that they slip.
And, the conveyor should stop at precise positions. In both of these systems, a good control
system design will result in better performance.