Fundamentals of closed-loop control technology…

 

What is closed-loop control technology?

Variables such as pressure, temperature or flow-rate often have to be set on large machines or systems. This setting should not change when faults occur. Such tasks are undertaken by a closed-loop controller.

 

Reference variable

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In closed-loop control the task is to keep the controlled variable at the desired value or to follow the desired-value curve. This desired value is known as the reference variable.

 

Controlled variable

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This problem occurs in many systems and machines in various technologies. The variable that is subject to control is called the controlled variable. Examples of controlled variables are:

• Pressure in a pneumatic accumulator
• Pressure of a hydraulic press
• Temperature in a galvanizing bath
• Flow-rate of coolant in a heat exchanger
• Concentration of a chemical in a mixing vessel
• Feed speed of a machine tool with electrical drive

 

Manipulated variable

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The controlled variable in any system can be influenced. This influence allows the controlled variable to be changed to match the reference variable (desired value). The variable influenced in this way is called the manipulated variable. Examples of manipulated variable are:

• Position of the venting control valve of a air reservoir
• Position of a pneumatic pressure-control valve
• Voltage applied to the electrical heater of a galvanizing bath
• Position of the control valve in the coolant feed line
• Position of a valve in a chemical feed line
• Voltage on the armature of a DC motor

 

System

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The controlled system has an input variable and an output variable. Its response is described in terms of dependence of the output variable on the input variable. These responses between one or several variables can normally be described using mathematical equations based on physical laws. Such physical relationships can be determined by experimentation.

Example
A water bath is to be maintained at a constant temperature. The water bath is heated by a helical pipe through which steam flows. The flow rate of steam can be set by means of a control valve. Here the control system consists of positioning of the control valve and the temperature of the water bath. This result in a controlled system with the input variable "temperature of water bath" and the output variable "position of control valve".

 

The following sequences take place within the controlled system:

• The position of the control valve affects the flow rate of steam through the helical pipe.
• The steam flow-rate determines the amount of heat passed to the water bath.
• The temperature of the bath increases if the heat input is greater than the heat loss and drops if the heat input is less than the heat loss.
• These sequences give the relationship between the input and output variables.