Closed loop control system is a type of control system.The specific content is to send part or all of the output of the control system back to the input end of the system through a certain method and device, then compare the feedback information with the original input information, and then apply the results of the comparison to the system for control to avoid the system from deviating from the intended goal.Closed loop control system usesnegative feedback。 It is composed of signal forward path and feedback pathClosed loopOfautomatic control system, also known asfeedback control system 。[1]
be based onFeedback principleEstablishedautomatic control system。The so-called feedback principle is to control according to the information of system output changes, that is, to obtain the expected system performance by comparing the deviation between the system behavior (output) and the expected behavior and eliminating the deviation.stayfeedback control system There is not only a forward signal path from input to output, but also a signal feedback path from output to input, both of which form a closed loop.Therefore, feedback control system is also called closed loop control system.Feedback control is the main form of automatic control.Automatic control systems are mostly feedback control systems.In engineering, the feedback control system that keeps the output consistent with the expected value in operation is often calledAutomatic regulation systemThe feedback control system used to precisely follow or realize a certain process is calledservo systemOr servo system.[2]
The feedback control system is composed of controller, controlled object and feedback path.The comparison link is used to subtract the input and output and give the deviation signal.This link may be collectively referred to as regulator together with the controller in a specific system.Taking furnace temperature control as an example, the controlled object is the furnace;The output variable is the actual furnace temperature;The input variable is the given constant temperature, which is generally expressed in voltage.The furnace temperature is measured by thermocouple. The thermoelectric EMF representing the furnace temperature is compared with the given voltage, and the difference voltage between the two is used to drive the corresponding actuator for control after power amplification.[2]
Basic composition
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Closed loop control system[3]
The basic composition of a typical feedback control system can be represented by the block diagram shown in Figure 1.8.The components of the system are divided into the following categories according to their functions in the system:
(1) Given element: give the input quantity corresponding to the expected output.
(2) Comparison element: calculate the deviation between input quantity and feedback quantity, usually usingIntegrated operational amplifier(referred to as integrated operational amplifier).
(3) Amplifying elements: because the deviation signal is generally small, it is not enough to drive the load, so it needs amplification elements, including voltage amplification and power amplification.
(4) Actuator: directly drive the controlled object to change the output.Commonly used are motors, regulating valves, hydraulic motors, etc.
(5) Measuring element: detect the controlled quantity and convert it into the required electrical signal.In the control systemTachometer generator, photoelectric encoding disk, etc;For position and angle detectionResolver, self-contained machine, etc;Transformer for current detection and thermocouple for temperature detection.These detection devices generally convert the detected physical quantities into corresponding continuous or discrete voltage or current signals.[3]
(6) Correction element: also called compensation element. It is an element whose structure and parameters are easy to adjust. It is connected to the system in series or feedback mode to complete the required calculation function to improve the performance of the system.According to different positions in the system, they can be called series correction elements and feedback correction elements respectively.[3]
difference
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Different from open-loop control system
withOpen loop control systemIn contrast, closed-loop control has a series of advantages.In the feedback control system, whatever the reason (external disturbance or internal change of the system), as long as the controlled quantity deviates from the specified value, corresponding control action will be generated to eliminate the deviation.Therefore, it has the ability to suppress interference, is insensitive to changes in component characteristics, and can improve the response characteristics of the system.However, the introduction of feedback loop increases the complexity of the system, and improper gain selection will cause instability of the system.In order to improve the control accuracy, when the disturbance variable can be measured, the disturbance based control (i.e. feedforward control) is also often used as a supplement to the feedback controlCompound control system。
Generally speaking, the open-loop control system is simple in structure and low in cost.The disadvantage of open-loop control system is that the control accuracy is not high, the ability to suppress interference is poor, and it is sensitive to the change of system parameters.It is generally used in occasions where external influences can be ignored or precision requirements are not high, such as washing machinesStepper motorControl device and water level regulation system, etc.[4]
In the closed-loop control system, whether it is the change of input signal, or the influence of interference, or the change of internal parameters of the system, as long as the controlled quantity deviates from the specified value, it will have a corresponding role to eliminate the deviation.Therefore, closed-loop control has strong ability to suppress interference.Compared with open-loop control, the system is insensitive to parameter changes, and less precise components can be selected to form a more precise control system to obtain satisfactory dynamic characteristics and control accuracy.However, the use of feedback devices requires the addition of components, which is costly and also increases the complexity of the system. If the structural parameters of the system are not properly selected, the control process may become very poor, or even unstable conditions such as oscillation or divergence may occur.Therefore, how to analyze the system, reasonably select the structure and parameters of the system, so as to obtain satisfactory system performance, is a problem that automatic control theory must study and solve.[4]
Control characteristics
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The closed-loop control system has the following characteristics in control: because the feedback quantity of the output signal is compared with the input quantity to produce a deviation signal, the deviation signal is used to control or adjust the output quantity, so the output quantity of the system can automatically track the input quantity, reduce the tracking error, improve the control accuracy, and suppress the influence of the disturbance signal.In addition, the closed-loop control system composed of negative feedback has other advantages: after the introduction of feedback path, the system is insensitive to the changes of element parameters in the forward path, so that the system has low requirements for the accuracy of element components in the forward path;Feedback can also make the whole system insensitive to some nonlinear effects.[2]
Application examples
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Closed loop control system
This is an automatic control system, which includes power amplification and feedback to make the value of the output variable respond to the value of the input variable.Numerical control deviceAfter the command pulse is sent, when the command value is sent to the position comparison circuit, if the worktable does not move at this time, that is, when there is no position feedback signal, the command value makes the servo drive motor rotate, which is driven by gear, ball screw nut pair and other transmission elementsMachine tool workbenchMove.The position measuring element installed on the workbench of the machine tool measures the actual displacement of the workbench, then feeds it back to the comparator of the numerical control device to compare with the command signal, and uses the difference value after comparison to control.If there is a difference between the two, amplify it after the amplifier, and then control the servo drive motor to rotate until the difference is zero, the workbench stops moving.This system is calledClosed-loop servo system。
