Differences Between Open-Loop Control and Closed-Loop Control

Open-loop control and closed-loop control are two basic structures in automatic control systems. Their core difference lies in the presence of a feedback mechanism and whether the system adjusts the input in real time based on the output results. Below is a detailed introduction to these two control modes.

Open-Loop Control

Open-loop control refers to a control process where only a forward-acting connection exists between the control device and the controlled object, with no reverse feedback link. A system configured in this way is called an open-loop control system, which is characterized by the fact that the system output has no impact on its control action. Open-loop control systems can be constructed either in setpoint-controlled mode or disturbance-controlled mode.

1. 

Setpoint-Controlled Open-Loop SystemsThe control action of such systems is generated directly by the system input. A given input value corresponds to a specific output value, and the control accuracy depends entirely on the precision of the components used and their calibration. Therefore, this type of open-loop control lacks the ability to automatically correct deviations and has poor anti-interference performance. However, due to its simple structure, convenient adjustment, and low cost, it still has practical value in applications where precision requirements are low or disturbance impacts are minimal. Currently, many automation devices used across various national economic sectors—such as vending machines, automatic washing machines, automatic product assembly lines, CNC lathes, and traffic light switching systems—are typically open-loop control systems.

2.  

Disturbance-Controlled Open-Loop SystemsThis type of system utilizes measurable disturbance variables to generate a compensating effect, thereby reducing or offsetting the impact of disturbances on the output. This control mode is also known as feedforward control. For example, in a typical DC speed control system, the rotational speed tends to decrease as the load increases, and this speed drop is correlated with changes in armature current. If we can measure the current change caused by the load and generate an additional control action proportional to this change to compensate for the resulting speed reduction, we can construct a disturbance-controlled open-loop system. This control method extracts information directly from disturbances to modify the controlled variable, offering good anti-interference performance and high control accuracy. However, it is only applicable in scenarios where disturbances are measurable.

3. 

Closed-Loop Control

Closed-loop control is a fundamental and widely used control mode in electromechanical control systems. In a closed-loop control system, the control action exerted by the control device on the controlled object is derived from feedback information of the controlled variable. This feedback is used to continuously correct deviations in the controlled variable, thereby achieving the task of controlling the object. This is the core principle of closed-loop control.

In fact, all human activities embody the principle of closed-loop control—humans themselves are highly complex closed-loop control systems. Simple daily actions, such as picking up a book from a table by hand or a driver steering a car to maintain a steady path on the road, all operate on the closed-loop control principle. To illustrate this mechanism, let us analyze the process of picking up a book:As shown in Figure A, the position of the book serves as the command information (or reference input) for the hand’s movement. To pick up the book, a person first uses their eyes to continuously monitor the hand’s position relative to the book and transmits this positional feedback to the brain. The brain then calculates the distance (i.e., deviation) between the hand and the book, generates a deviation signal, and issues commands to move the arm based on the magnitude of this deviation, gradually reducing the gap. This process repeats until the deviation is eliminated and the book is grasped. It is evident that the brain’s control of the hand to pick up the book is a process of deviation-based control—the core principle underlying closed-loop control.

Block Diagram of the Feedback Control System for Human Book-PickingFigure A: Block Diagram of the Feedback Control System for Human Book-Picking

The process of sending the controlled variable back to the input end and comparing it with the input signal to generate a deviation signal is called feedback. If the feedback signal subtracts from the input signal, reducing the deviation progressively, this is called negative feedback; conversely, it is positive feedback. Closed-loop control is a process that adopts negative feedback and uses deviations to regulate the system. The introduction of controlled variable feedback forms a closed loop for the entire control process, hence the name “closed-loop control”.

Its key feature is that whenever the controlled variable deviates from the desired value due to any internal or external factors, a corresponding control action is generated to reduce or eliminate the deviation, bringing the controlled variable back in line with the desired value. In summary, closed-loop control systems have the ability to suppress the impact of all internal and external disturbances on the controlled variable and offer high control accuracy. However, these systems require more components, involve complex circuitry, and present greater challenges in performance analysis and design. Despite these drawbacks, closed-loop control remains an important and widely adopted control mode, and it is the primary research object of automatic control theory.

A typical application of closed-loop control is the function recorder, a general-purpose automatic recording instrument that can plot the functional relationship between two electrical quantities on a Cartesian coordinate system. It also includes a paper transport mechanism to record the variation of an electrical quantity over time.

A function recorder usually consists of a converter, measuring elements, amplifying elements, a servo motor-tachogenerator unit, a gear train, and a rope pulley. It operates based on the negative closed-loop control principle, as illustrated in Figure B. The system input is the voltage to be recorded, and the controlled object is the recording pen, whose displacement is the controlled variable. The system’s task is to control the displacement of the recording pen to plot the curve of the input voltage on the recording paper.

Schematic Diagram of the Function Recorder PrincipleFigure B: Schematic Diagram of the Function Recorder Principle

In Figure B, the measuring element is a bridge-type measurement circuit composed of potentiometers RQ and RM, with the recording pen connected to the sliding contact of the potentiometer. Thus, the output voltage up of the measurement circuit is proportional to the displacement of the recording pen. When a time-varying input voltage ur is applied, a deviation voltage Δu = ur - up is obtained at the input of the amplifying element. This deviation voltage is amplified to drive the servo motor, which then moves the recording pen via the gear train and rope pulley, while reducing the deviation voltage. When Δu = 0, the motor stops rotating, and the recording pen comes to a standstill. At this point, up = ur, indicating that the pen displacement corresponds to the input voltage. If the input voltage changes continuously over time, the recording pen traces a continuous curve reflecting this variation.

Structure Diagram of the Function RecorderFigure C: Structure Diagram of the Function Recorder

In Figure C, the signal fed back by the tachogenerator is a voltage proportional to the motor speed, which is used to increase system damping and improve overall performance.