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-is controlled directly, and only, by an input signal. The basic units of this type consist only of an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.With this system, the input is a signal that is fed to the amplifier. The output of the amplifier is proportional to the amplitude of the input signal. The phase (ac system) and polarity (dc system) of the input signaldetermines the direction that the motor shaft will turn. After amplification, the input signal is fed to the motor, which moves the output shaft (load) in the direction that corresponds with the input signal. The motor will not stop driving the output shaft until the input signal is reduced to zero or removed. This system usually requires an operator who controls speed and direction of movement of the output by varying the input. The operator could be controlling the input by either a mechanical or an electrical linkage
- is one in which control action is dependent on the output.
- closed-loop control system is another name for a servo system. To be classified as a servo, a control system must be capable of the following:
1. Accepting an order that defines the desired result
2. Determining the present conditions by some method of feedback
3. Comparing the desired result with the present conditions and obtaining a difference or an error signal
4. Sending a correcting order (the error signal) that will properly change the existing conditions tothe desired result.
Although not technically accurate by definition, open-loop control systems are also often referred to in the Navy and many publications as servo systems even though they lack one of the basic requirements.
A closed-loop control system is one in which an input forcing function is determined in part by the system response. The measured response of a physical system is compared with a desired response. The difference between these two responses initiates actions that will result in the actual response of the system to approach the desired response. This in turn drives the difference signal toward zero. Typically the difference signal is processed by another physical system, which is called a compensator, a controller, or a filter for real-time control system applications.
- Closed-Loop system utilizes feedback to measure the actual system operating parameter being controlled such as
temperature, pressure, flow, level, or speed. This feedback signal is sent back to the controller where it is compared
with the desired system set point. The controller develops an error signal that initiates corrective action and drives the
final output device to the desired value.
All automatic control systems use negative feedback for controlling a physical parameter like position, velocity, torque etc. The parameter which has to be controlled is sensed by a suitable transducers and fed back to the input, for comparison with the reference value (see Figure 20.1. This subtraction of the sampled output signal with that of reference input is called as negative feedback. The difference signal, called the ``error" is then amplified to drive the system (referred to as actuation ) in such a manner that the output approaches the set reference value. In other words the system is designed to minimize the error signal.
All practical loads have inertia and spring constants due to which there is a delay in actuation. Hence, even though a system may be designed for negative feedback, due to inherent time lags, the feedback may turn into positive feedback at certain frequencies. If the loop gain is more than unity at some frequency at which the feedback is positive, the system will oscillate. Hence, in designing control systems great care has to be taken to avoid such situations.
experiment 39 [comm. 2]
click to download
just open "experiment_39.html"...
sagot sa feedback
Control systems are broadly classified as either CLOSED-LOOP or OPEN-LOOP.The reason for quicker response and greater accuracy of closed-loop control system is that an automatic feedback system informsthe input that the desired movement has taken place. Upon receipt of this feedback information, the system stops the motor, and motion of the load ceases until another movement is ordered by the input. This is similar to the system that controls heat in many homes. The thermostat (input) calls for heat. The furnace (output) produces heat and distributes it. Some of the heat is "fed back" to the thermostat. When this "feedback" raises the temperature of the room to that of the thermostat setting, the thermostat responds by shutting the system down until heat is again required. In such a system, the feedback path,input to output and back to input, forms what is called a "closed loop." This is a term you will hear anduse often in discussions of control systems. Because closed-loop control systems are automatic in nature,they are further classified by the function they serve (e.g., controlling the position, the velocity, or the acceleration of the load being driven).An open-loop control system is controlled directly, and only, by an input signal, without the benefitof feedback. The basic units of this system are an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job. Open-loop control systems are not as commonly used as closed-loop control systems because they are less accurate.
OPEN-LOOP CONTROL SYSTEM
-is controlled directly, and only, by an input signal. The basic units of this type consist only of an amplifier and a motor. The amplifier receives a low-level input signal and amplifies it enough to drive the motor to perform the desired job.With this system, the input is a signal that is fed to the amplifier. The output of the amplifier is proportional to the amplitude of the input signal. The phase (ac system) and polarity (dc system) of the input signaldetermines the direction that the motor shaft will turn. After amplification, the input signal is fed to the motor, which moves the output shaft (load) in the direction that corresponds with the input signal. The motor will not stop driving the output shaft until the input signal is reduced to zero or removed. This system usually requires an operator who controls speed and direction of movement of the output by varying the input. The operator could be controlling the input by either a mechanical or an electrical linkage
THE CLOSED-LOOP CONTROL SYSTEM
- is one in which control action is dependent on the output.
- closed-loop control system is another name for a servo system. To be classified as a servo, a control system must be capable of the following:
1. Accepting an order that defines the desired result
2. Determining the present conditions by some method of feedback
3. Comparing the desired result with the present conditions and obtaining a difference or an error signal
4. Sending a correcting order (the error signal) that will properly change the existing conditions tothe desired result.
Although not technically accurate by definition, open-loop control systems are also often referred to in the Navy and many publications as servo systems even though they lack one of the basic requirements.
A closed-loop control system is one in which an input forcing function is determined in part by the system response. The measured response of a physical system is compared with a desired response. The difference between these two responses initiates actions that will result in the actual response of the system to approach the desired response. This in turn drives the difference signal toward zero. Typically the difference signal is processed by another physical system, which is called a compensator, a controller, or a filter for real-time control system applications.
- Closed-Loop system utilizes feedback to measure the actual system operating parameter being controlled such as
temperature, pressure, flow, level, or speed. This feedback signal is sent back to the controller where it is compared
with the desired system set point. The controller develops an error signal that initiates corrective action and drives the
final output device to the desired value.
All automatic control systems use negative feedback for controlling a physical parameter like position, velocity, torque etc. The parameter which has to be controlled is sensed by a suitable transducers and fed back to the input, for comparison with the reference value (see Figure 20.1. This subtraction of the sampled output signal with that of reference input is called as negative feedback. The difference signal, called the ``error" is then amplified to drive the system (referred to as actuation ) in such a manner that the output approaches the set reference value. In other words the system is designed to minimize the error signal.
All practical loads have inertia and spring constants due to which there is a delay in actuation. Hence, even though a system may be designed for negative feedback, due to inherent time lags, the feedback may turn into positive feedback at certain frequencies. If the loop gain is more than unity at some frequency at which the feedback is positive, the system will oscillate. Hence, in designing control systems great care has to be taken to avoid such situations.
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