Open Loop and Closed Loop Systems04/10/2018A control system that does not have a device to sense output and make corrections to the process is called an open-loop control system. For example, a DC separately excited motor has an inherent drooping characteristic and will decrease its speed as necessary to meet changing torque demands, but it will not run as a constant speed. More modern AC drive control systems have built in current sensing components to regulate the system, but are considered open-loop because of the lack of an external feedback device. A closed-loop system on the other hand, has an external component to monitor and sense the motor output, compares the output against a desired condition, and corrects the system to achieve the desired output. Referring to the DC separately excited motor, if a DC tachometer is mechanically attached to the motor shaft, a voltage proportional to the speed is produced. If the speed voltage is fed back and compared against an input voltage representing the desired or set speed, an error signal will be produced whenever there is a difference between the desired and actual speeds. In turn this error signal can be used to control the firing delay angle of a thyristor phase-controlled converter, which can return the shaft speed to its desired value. A servo drive is a prime example of a closed-loop control system. It uses a controller, an amplifier, an actuator or motor, and some type of feedback such as an encoder or a resolver. Control systems are typically visually represented by block diagrams such as those illustrated here. The blocks represent individual functions of the system with arrows indicating direction of information flow. Figure 1A and 1B illustrates the concept of an open-loop control system along with a closed-loop system respectively. The final measure of success of a closed-loop system would be the degree of closeness between the desired value and the measured value of the controlled variable, regardless of the frequency and magnitude of load changes or setpoint changes. The quality of the system determines the maximum error that must be present before corrective action can take place. Another extremely important characteristic is the speed of response or setting time, that is, the time interval between detection of the error and of the appropriate corrective action. The last, but by no means, least, characteristic is the offset residual error or steady state error. The residual error is the final difference between the desired value and the measured value of the controlled variable. These three characteristics of a good control system tend to be mutually exclusive. The residual error should respond to an increase in gain of the controller, but increasing the gain makes the system more sensitive, and as a result may increase the maximum value of the error as well as increasing the settling time. Another effect of increasing the controller gain is to change the type of damping of the system in response to the disturbances. See Figure 2A and 2B. The type of damping is best described in terms of the controller gain as follows: 1. Overdamped: Low gain; the dynamic or transient response is very slow and a large residual error may be present. 2. Critically damped: Low to medium gain; the least amount of damping that produces an output without any overshoot or oscillation. 3. Underdamped: Gain has been increased and the output overshoots and oscillates with a diminishing amplitude response. Any further increase in gain will result in the system becoming unstable. There are a number of stabilizing methods which may be used to increase the damping effect and at the same time permit an increase in the gain of the controller in order to reduce the settling time and amplitude of the residual or ready-state error. These methods depend on developing a force or signal to oppose changes in the controlled variable. Back To Blog
Open Loop and Closed Loop Systems04/10/2018A control system that does not have a device to sense output and make corrections to the process is called an open-loop control system. For example, a DC separately excited motor has an inherent drooping characteristic and will decrease its speed as necessary to meet changing torque demands, but it will not run as a constant speed. More modern AC drive control systems have built in current sensing components to regulate the system, but are considered open-loop because of the lack of an external feedback device. A closed-loop system on the other hand, has an external component to monitor and sense the motor output, compares the output against a desired condition, and corrects the system to achieve the desired output. Referring to the DC separately excited motor, if a DC tachometer is mechanically attached to the motor shaft, a voltage proportional to the speed is produced. If the speed voltage is fed back and compared against an input voltage representing the desired or set speed, an error signal will be produced whenever there is a difference between the desired and actual speeds. In turn this error signal can be used to control the firing delay angle of a thyristor phase-controlled converter, which can return the shaft speed to its desired value. A servo drive is a prime example of a closed-loop control system. It uses a controller, an amplifier, an actuator or motor, and some type of feedback such as an encoder or a resolver. Control systems are typically visually represented by block diagrams such as those illustrated here. The blocks represent individual functions of the system with arrows indicating direction of information flow. Figure 1A and 1B illustrates the concept of an open-loop control system along with a closed-loop system respectively. The final measure of success of a closed-loop system would be the degree of closeness between the desired value and the measured value of the controlled variable, regardless of the frequency and magnitude of load changes or setpoint changes. The quality of the system determines the maximum error that must be present before corrective action can take place. Another extremely important characteristic is the speed of response or setting time, that is, the time interval between detection of the error and of the appropriate corrective action. The last, but by no means, least, characteristic is the offset residual error or steady state error. The residual error is the final difference between the desired value and the measured value of the controlled variable. These three characteristics of a good control system tend to be mutually exclusive. The residual error should respond to an increase in gain of the controller, but increasing the gain makes the system more sensitive, and as a result may increase the maximum value of the error as well as increasing the settling time. Another effect of increasing the controller gain is to change the type of damping of the system in response to the disturbances. See Figure 2A and 2B. The type of damping is best described in terms of the controller gain as follows: 1. Overdamped: Low gain; the dynamic or transient response is very slow and a large residual error may be present. 2. Critically damped: Low to medium gain; the least amount of damping that produces an output without any overshoot or oscillation. 3. Underdamped: Gain has been increased and the output overshoots and oscillates with a diminishing amplitude response. Any further increase in gain will result in the system becoming unstable. There are a number of stabilizing methods which may be used to increase the damping effect and at the same time permit an increase in the gain of the controller in order to reduce the settling time and amplitude of the residual or ready-state error. These methods depend on developing a force or signal to oppose changes in the controlled variable.