Open-loop and closed-loop control

Having defined the term "controlled system" it only remains to give definitions
of closed-loop control as contained in standards. First it is useful
to fully understand

Open-loop control
German standard DIN 19 226 defines open-loop control as a process
taking place in a system where by one or more variables in the form of
input variables exert influence on other variables in the form of output
variables by reason of the laws which characterize the system.
The distinguishing feature of open-loop control is the open nature of its
action, that is, the output variable does not have any influence on the
input variable.

Closed-loop control
DIN 19 226 defines closed-loop control as a process where the controlled
variable is continuously monitored and compared with the reference
variable. Depending on the result of this comparison, the input variable
for the system is influenced to adjust the output variable to the desired
value despite any disturbing influences. This feedback results in a
closed-loop action.
This theoretical definition can be clarified using the example of volumetric
flow control.

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.
Control engineering deals with all problems that occur in this connection.
The controlled variable is first measured and an electrical signal is created
to allow an independent closed-loop controller to control the variable.
The measured value in the controller must then be compared with the
desired value or the desired-value curve. The result of this comparison
determines any action that needs to be taken.
Finally a suitable location must be found in the system where the controlled
variable can be influenced (for example the actuator of a heating
system). This requires knowledge of how the system behaves.
Closed-loop control technology attempts to be generic – that is, to be
applicable to various technologies. Most text books describe this with
the aid of higher mathematics. This chapter describes the fundamentals
of closed-loop control technology with minimum use of mathematics.

Reference variable
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
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
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

Controlled system
There are complex relationships between the manipulated variable and
the controlled variable. These relationships result from the physical interdependence
of the two variables. The part of the control that describes
the physical processes is called the controlled system.