Introduction to Automatic Control

MAK331E System Dynamics & Control Introduction to Automatic Control

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Introduction to Automatic Control

References: Modern Control Engineering, K. Ogata, 4th ed., pp. 1-8

Modern Control Systems, R.C. Dorf, R.H. Bishop, 12th ed., pp. 1-48

Contents

Definitions........................................................................................... 2

Examples of Control Systems. 4 Automobile steering control system... 4 Manually controlled closed-loop system for regulating the level of fluid...... 4 Speed control system..... 5 Temperature control system (Electric furnace)... 6 Temperature control of the passenger compartment of a car... 6 A three-axis control system......7

Control System Design....................................................................... 8 Design example: Turntable speed control 9 Design example: Insulin delivery control system.............10


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Definitions System. A system is a combination of components that act together and perform a certain

objective. A system is not limited to physical ones. The concept of the system can be applied to abstract, dynamic phenomena such as those encountered in economics. The word system should, therefore, be interpreted to imply physical, biological, economic, and the like, systems.

Plant. A plant may be a piece of equipment, perhaps just a set of machine parts functioning

together, the purpose of which is to perform a particular operation. Any physical object to be controlled (such as a mechanical device, a heating furnace, a chemical reactor, or a spacecraft) is a plant.

Process. A process is a progressively operation that consists of a series of controlled

actions or movements systematically directed toward a particular result or end. Any operation to be controlled is a process. Examples are chemical, economic, and biological processes.

Controlled Variable and Manipulated Variable. The controlled variable is the quantity or

condition that is measured and controlled. The manipulated variable is the quantity or condition that is varied by the controller so as to affect the value of the controlled variable. Normally, the controlled variable is the output of the system.

Control. Control means measuring the value of the controlled variable of the system and

applying the manipulated variable to the system to correct or limit deviation of the measured value from a desired value. In studying control engineering, we need to define additional terms that are necessary to describe control systems.

Disturbance. A disturbance is a signal that tends to adversely affect the value of the output

of a system. If a disturbance is generated within the system, it is called internal, while an external disturbance is generated outside the system and is an input.

Feedback Control. Feedback control refers to an operation that, in the presence of

disturbances, tends to reduce the difference between the output of a system and some reference input and does so on the basis of this difference. Here only unpredictable disturbances are so specified, since predictable or known disturbances can always be compensated for within the system.

Open-loop control. Open-loop control systems use an actuating device to control the

process directly without using feedback. Those systems in which the output has no effect on the control action are called open-loop control system. In other words, the output is neither measured nor fed back for comparison with the input.

One practical example is a washing machine. Soaking, washing, and rinsing in the washer operate on a time basis. The machine does not measure the output signal, that is, the cleanliness of the clothes. In the open control system, the accuracy of the system depends on calibration. In the presence of disturbances, an open-loop control system will not perform the desired task. For instance, traffic control by means of signals operated on a time basis is another example of open-loop control (also an electrical toaster).

Actuating device process

Desired output response output


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Closed-loop control. Closed-loop control systems use a measurement of the output and

feedback of this signal to compare it with the desired output (reference or command).

A feedback control system often uses a function of a prescribed relationship between the output and reference input to control the process. On the other hand, stability is a major problem in the closed-loop control system, which may tend to overcorrect errors and thereby can cause oscillations of constant or changing amplitude. Closed-loop control systems have advantages only when unpredictable disturbances and/or unpredictable variations in system components are present. But, the closed-loop control system is generally higher in cost and power. Feedback control systems are not limited to engineering. The human body, for instance, is a highly advanced feedback control system. Both body temperature and blood pressure are kept constant by means of physiological feedback. It makes the human body relatively insensitive to external disturbances, thus enabling it to function properly in a changing environment. Furthermore, as the systems become more complex, the interrelationship of many controlled variables must be considered in the control scheme.

Output variables

controllercomparison

measurement

Desired output responses

process

Desired output response output

controllercomparison

measurement

+

error


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Examples of Control Systems Automobile steering control system

The desired direction is compared with a measurement of actual direction in order to generate a measure of the error. This measurement is obtained by visual and tactile (body movement) feedback. There is an additional feedback from the feel of the steering wheel by the hand (sensor).

A negative feedback system block diagram depicting a basic closed-loop control system. Manually controlled closed-loop system for regulating the level of fluid in a

tank

The input is a reference level of fluid that the operator is instructed to maintain. (This reference is memorized by the operator.) The power amplifier is the operator, and the sensor is visual. The operator compares the actual level with the desired level and opens or closes the valve (actuator), adjusting the fluid flow out, to maintain the desired level.

Process

(desiredoutput)

Actual output

ActuatorControl device

Sensor

+ Error Input

FeedbackMeasured Output

Automobile

Actual direction of travel Steering

mechanism Driver

Measurement (visual and tactile)

+ Error Desired direction of travel


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Speed control system

The amount of fuel admitted to engine is adjusted according to the difference between the desired and the actual engine speeds.

The sequence of actions may be stated as follows: The speed governor is adjusted such that, at the desired speed, no pressured oil will flow into either side of the power cylinder. If the actual speed drops below the desired value due to disturbance, then the decrease in the centrifugal force of the speed governor causes the control valve to move downward, supplying more fuel, and the speed of the engine increases until the desired value is reached. On the other hand, if the speed of the engine increases above the desired value, then the increase in the centrifugal force of the governor causes the control valve to move upward. This decreases the supply of fuel, and the speed of the engine decreases until the desired value is reached. In this speed control system, the plant (controlled system) is the engine. The difference between the desired speed and the actual speed is the error signal. The control signal (the amount of fuel) to be applied to the plant (engine) is the actuating signal. The controlled variable is the speed of the engine. The external input to disturb the controlled variable is the disturbance. An unexpected change in the load is a disturbance.

Process (engine)

Actuator (valve)

Controller (the speed governor)

Measurement (the piston-cylinder

mechanism)

+ Error Referenceengine speed (input)

Actual engine speed (output)

Process (tank)

Actuator (valve)

Controller (operator)

Measurement (visual)

+ Error Referencelevel of fluid (input)

Actual level of fluid (output)


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Temperature control system (Electric Furnace)

The temperature in the electric furnace is measured by a thermometer, which is analog device. The analog temperature is converted to a digital temperature by an A/D converter. The digital temperature is fed to a controller through an interface. This digital temperature is compared with the programmed input temperature, and if there is any discrepancy (error), the controller sends out a signal to the heater, through an interface, amplifier, and relay, to bring the furnace temperature to a desired value.

Temperature control of the passenger compartment of a car

Passenger compartment

Desired temperature

(output)

Heater of air conditioner

Controller

Sensor

(input)

Radiation heat sensor

Sensor

Ambient temperature

Sun

Passenger compartment temperature

Process (electric furnace)

Actuator (relay)

Controller (the computer)

Measurement (the thermometer)

+ Error Reference temperature (input)

Actual temperature (output)

Interface A/D Converter

Relay

Heater

Program input

Electric Furnace

Computer

Interface Amplifier


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The desired temperature (converted to a voltage) is input to the controller. The actual temperature of the passenger compartment must be converted to a voltage through a sensor and fed back to controller for comparison with input. Note that the ambient temperature and radiation heat transfer from the sun, which are not constant while the car is driven, act as disturbances. The controller receives the input signal, output signal, and signals from sensors from disturbance sources. The controller sends out an optimal control signal to the air conditioner or heater to control the amount of cooling air or warm air so that the passenger compartment temperature is about the desired temperature.

A three-axis control system for inspecting individual semiconductor wafers with a highly sensitive camera

This system uses a specific motor to drive each axis to the desired position in the x-y-z-axis, respectively. The goal is to achieve smooth, accurate movement in each axis. This control system is an important one for the semiconductor manufacturing industry.

Process (electric furnace)

Actuator (relay)


Measurement (the thermometer)


Actual temperature (output)

Process (passenger

compartment)

Actuator (air-conditioner

or heater)


Measurement (the sensors)



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Control System Design

The design of control systems is a specific example of engineering design. The first step in the design process consists of establishing the system goals. For example, our goal is to control the velocity of a motor accurately. The second step is to identify the variables that we desire to control. For example, the velocity of the motor. The third step is to write the specifications in terms of the accuracy we must attain. This required accuracy of control will then lead to the identification of a sensor to measure the controlled variable. The next step consists of identifying an actuator. This will depend on the process. For example, if we wish to control the speed of a rotating flywheel, we will select a motor as the actuator. The sensor, in this case, will need to be capable of accurately measuring the speed. The next step is the selection of a controller, which often consists of a summing amplifier that will compare the desired response and the actual response. The final step in the design process is the adjustment of the parameters of the system in order to achieve the desired performance. If we can achieve the desired performance by adjusting the parameters, we will finalize the design and proceed to document the results. If not, we will need to establish an improved system configuration and perhaps select an enhanced actuator and sensor. Then we will repeat the design steps until we are able to meet the specifications. The performance specifications will describe how the closed-loop system should perform and will include (1) good regulation against disturbances, (2) desirable responses to commands, (3) realistic actuator signals, (4) low sensitivities, and (5) robustness.

If the performance does not meet the specifications, then iterate the configuration

1. Establish control goals

2. Identify the variables to control

3. Write the specifications for the variables

4. Establish the system configuration

5. Obtain a model of the process, the actuator, and the sensor

6. Describe a controller and select key parameters to be adjusted

7. Optimize the parameters and analyze the performance

If the performance meets the specifications, then finalize the design.


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Design example: Turntable Speed Control Many modern devices use a turntable to rotate a disk at a constant speed. For example, a CD player, a computer disk drive, and a phonograph record player all require a constant speed of rotation in spite of motor wear and variation and other component changes. Our goal is to design a system for turntable speed control that will ensure that the actual speed of rotation is within a specified percentage of the desired speed. We will consider a system without feedback and a system with feedback. To obtain disk rotation, we will select a DC motor as the actuator because it provides a speed proportional to the applied motor voltage. For the input voltage to the motor, we will select an amplifier that can provide the required power. The open-loop system (without feedback) is shown in Figure (a). This system uses a battery source to provide a voltage that is proportional to the desired speed. This voltage is amplified and applied to the motor. The block diagram of the open-loop system identifying control device, actuator, and process is shown in Figure (b).

To obtain a feedback system, we need to select a sensor. One useful sensor is a tachometer that provides an output voltage proportional to the speed of its shaft. Thus the closed-loop feedback system takes the form shown in Fig. (a). The block diagram model of the feedback system is shown in Fig. (b). The error voltage is generated by the difference between the input voltage and the tachometer voltage.

Process Turntable

Actuator DC motor

Control device Amplifier

Sensor Tachometer

+ Error Desired speed (voltage)

Actual speed

(b)

Measured speed (voltage)

Process Turntable

Actuator DC motor

Control device Amplifier

Desired speed (voltage)

Actual speed

(b)


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The feedback system is superior to the open-loop system, because the feedback system will respond to error and work to reduce them. Design example: Insulin Delivery Control System

Our goal is (step 1) to design a system to regulate the blood sugar concentration of a diabetic. The blood glucose and insulin concentrations for healthy person are shown in Figure. The system must provide the insulin from a reservoir implanted within the diabetic person.

Thus the variable we wish to control (step 2) is the blood glucose concentration. The specification for the control system (step 3) is to provide a blood glucose level for the diabetic that closely approximates (tracks) the glucose level of a healthy person. In step 4, we propose a preliminary system configuration. An open-loop system would use a preprogrammed signal generator and miniature motor pump to regulate the insulin delivery rate as shown in Figure (a). The feedback control system would use a sensor to measure the actual glucose level and compare that level with the desired level, thus turning the motor pump on when it is required, as shown in Figure (b).

Human body, blood, and pancreas

Motor, pump,

and valve

Amplifier

Sensor

+ v(t) Desired glucose level

(b)

Measured glucose level

Insulin delivery rate

Human body, blood, and pancreas

Motor, pump,

and valve

Programmed signal generator

(a)

v(t) Motor voltage

I(t)

Documents

Introduction to Automatic Control