Difference between open-loop and closed-loop control systems

Controls and regulators have a very similar function, but they are not entirely the same.

The control system

A control system influences a machine without taking feedback into account.

The regulation

In a control system, it must be ensured that the setpoint is reached and remains constant. A control system measures the controlled variable, processes this signal and outputs a manipulated variable to an actuator. The controlled variable and manipulated variable are called control variables.

What are control variables

The actual and setpoint values are referred to as control variables. The actual value shows the current state or is the currently measured value (e.g. current room temperature). The setpoint is the value set by the user (e.g. preset room temperature). The control system always tries to adjust the actual value so that it equals the setpoint.

What is a Actuator

An actuator is a component that can convert electrical variables into physical ones.

The PLC as a control or regulation system

The abbreviation PLC stands for „Programmable Logic Controller“. The PLC is responsible for control and regulation tasks in large machines. Sequential function charts (SFCs) are used for programming. A distinction is made between text-based and graphical sequential function charts.

text-based Sequential Function Chart

Instruction list (IL) or structured text (ST) is used in text-based programming. IL is a universally applicable machine language which is very similar to assembler (programming language). Even today, it is still frequently used. ST has a similar structure to the high-level language Pascal. The great advantage is the compact formulation with which complex tasks can be realised.

Graphical Programming Language

Graphical programming can be thought of as a programming interface where you drag in components and building blocks and connect them with lines, similar to a circuit diagram. With this type of programming, there are two types: ladder diagram (LD) and function block diagram (FBD). In the LD, the functions (AND, OR, make contact or break contact) are represented using electrical engineering circuit symbols. When programming with FBD, the symbolism of digital electronics is used.

Faults in open-loop and closed-loop control systems

Control and regulation systems are extremely complex and versatile assemblies. As a result, it is impossible to list or know all faults. In the following section, you will find a few of the most common faults of these components.

Total failure

Nothing on the assembly is lit up anymore, and no actuators are being controlled either. On the PLC, both POWER and RUN are dark.

Incorrect control behaviour

The controller regulates even though there is no deviation. The actual value deviates very significantly from the setpoint. It is also possible that no setpoint can be set or the actual value can no longer be evaluated.

Control of single or multiple actuators not working

One or more actuators can no longer be controlled, even though the input signal is present and being evaluated.

signal input

If the signal input is damaged, the regulation or control system can no longer evaluate the value. As a result, the outputs are naturally no longer controlled properly either.

Communication error

The PLC or controller cannot communicate with the rest of the system.