Control Systems Terminology PDF

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CONTROL SYSTEMS TERMINOLOGY 2.4.1 Block diagrams: Fundamentals A block diagram is a shorthand, pictorial representation of the cause-and-effect relationship between the input (desired value) and output (actual value) of a physical system. It provides a convenient and useful method for characterising the functional relationships among the various components, is made possible by using a block diagram. As illustrated in Fig. 2.3, a block diagram consists of a specific configuration of four types of elements: blocks summing points pickoff points whether the signal is entering or leaving.

arrows indicating

QVSTF,M

INPUT OUTPUT

Figure 2.3: General block diagram Control systems can either be open-loop (without feedback) or closed-loop (with feedback). In paragraph 2.6, the term "feedback" is explained.

2.4.2 Block diagrams of continuous feedback control systems The blocks representing the various components of a control system are connected in a fashion which characterises their functional relationships within the system. The basic configuration closed-loop (feedback) control system with a single input and a single output (abbreviated SISO) is illustrated in Fig. 2.4 for a system with continuos signals only.

Control

Disturbance

Reference

Signal

FEEDBACK PATH

Figure 2.4 2.4.3 Terminology of the closed-loop block diagram

It is important that the terms used in the closed-loop block diagram be clearly understood.

Lowercase letters are used to represent the input and output variables of each element as well as the symbols for the blocks gl, g2, and h. These quantities represent functions of time, unless otherwise specified.

For example

In subsequent chapters, we use capital letters to denote Laplace transformed or z-transformed quantities, as functions of the complex variable s, or z, respectively, or Fourier transformed quantities (frequency functions), as functions of the pure imaginary variable jo. Functions of s or z are often abbreviated to the capital letter appearing alone. Frequency functions are never abbreviated.

R(s) may be abbreviated as R, or F(z) as F. R(jo) is never abbreviated.

The letters r, c, e etc., were chosen to preserve the generic nature of the block diagram. This convention is now classical.

The plant (or process, or controlled system) g2 is the system, subsystem, process, or object controlled by the feedback control system.

The controlled output c is the output variable of the plant, under the control of the feedback control system.

The forward path is the transmission path from the summing point to the controlled output c.

The feedforward (control) elements gl are the components of the forward path that generate the control signal u or m applied to the plant.

Note: Feedforward elements typically include controller(s), compensator(s) (or equalisation elements), and/or amplifiers.

The control signal u (or manipulated variable m) is the output signal of the feedforward elements gl applied as input to the plant g2.

The feedback path is the transmission path from the controlled output c back to the summing point.

The feedback elements h establish the functional relationship between the controlled output c and the primary feedback signal c.

Note: Feedback elements typically include sensors of the controlled output c, compensators, and/or controller elements.

The reference input r is an external signal applied to the feedback control system, usually at the first summing point, in order to command a specified action of the plant. It usually represents ideal (or desired) plant output behaviour.

The primary feedback signal b is a function of the controlled output c, algebraically summed with the reference input r to obtain the actuating (error) signal e, that is, r ± b = e.

Note: An open-loop system has no primary feedback signal.

The actuating (or error) signal is the reference input signal r plus or minus the primary feedback signal b. The control action is generated by the actuating (error) signal in a feedback control system.

Note: In an open-loop system, which has no feedback, the actuating signal is equal to r.

Negative feedback means the summing point is a subtractor, that is, e = r - b. Positive feedback means the summing point is an adder, that is, e = r + b. The disturbance signal d is the unwanted signal that tends to affect the controlled variable. The disturbance may be introduced into the system at many places.

2.5 OPEN-LOOP AND CLOSED-LOOP CONTROL SYSTEMS Control systems are classified into two general categories: open-loop and closed-loop (a system with feedback) systems. The distinction is determined by the control action, that quantity responsible for activating the system to produce the output.

The term control action is classical in the control systems literature, but the word action in this expression does not always directly imply change, motion, or activity. For example, the control action in a system designed to have an object hit a target is usually the distance between the object and the target. Distance, as such, is not an action, but action (motion) is implied here, because the goal of such a control system is to reduce this distance to zero.

An open-loop control system is one in which the control action is independent of the output, (Fig. 2.5).

Open-loop control systems are the simplest form of controlling devices. Unfortunately, it has the disadvantage of inaccurate control action since it has no way of automatically correcting its output, for feedback comparison is absent. See paragraph 2.6 for an explanation of the term "feedback". Name of element or Mathematical operation

Figure

INPUTOUTPUT

2.5: Block diagram of an open loop system

Two outstanding features of open-loop control systems are: 1.

Their ability to perform accurately is determined by their calibration. To calibrate means to establish or re-establish the input-output relation to obtain a desired system accuracy.

2.

They are not usually troubled with problems of instability, a concept to be subsequently discussed in detail.

A closed-loop control system is one in which the control action is somehow dependent on the output.

Closed-loop control systems are more commonly called feedback control systems, and are considered in more detail beginning in the next section.

Figure 2.6 is an example of a closed-loop system with unity feedback. This is the simplest form of a closed-loop control system. INPUT

SUMMING POINT

OUTPUT

Figure 2.6: Block diagram of a closed loop control system with unityfeedback To classify a control system as open-loop or closed-loop, we must distinguish clearly the components of the system from components that interact with but are not part of the system.

For example, the driver in Example 2.5 was defined as part of that control system, but a human operator may or may not be a component of a system.

Example 2.6 Most automatic toasters, Fig. 2.7, are open-loop systems because they are controlled by a timer. The time required to make "good toast" must be estimated by the user, who is not part of the system.

Control over the quality of toast (the output) is removed once the time, which is both the input and the control action, has been set. The time is typically set by means of a calibrated dial or switch. This dial or switch is the reference selector

Example 2.7 The DC shunt motor of Fig. 2.8 is another example of an open-loop system.

For a given value of current, a required value of voltage is applied to the armature to produce the desired value of the motor speed. In this case the motor is the dynamic part of the system, the applied armature voltage is the input quantity, and the speed of the shaft is the output quantity. A variation of the speed from the desired value, due to a change of mechanical load on the shaft, can in no way cause a change in the value of the applied armature voltage to maintain the desired speed. Therefore, the output quantity has no influence on the input quantity.

Voltage selector

Voltage source for field Voltage source for armature

Figure 2.8 Example 2.8 Another example of an open-loop system, is the water tank level control system shown in Fig. 2.9. The water level, h, must be kept within reasonable, acceptable limits even though the outlet flow rate is varied. By irregular manual adjustment of the inlet water flow, h can be kept constant within limits.

INLET FLOW

STEM

OUTLET FLOW

Figure 2.9 Neither the inlet flow rate, nor the outlet flow rate, of the water level can be measured accurately, for the human factor is involved.

The block diagram of the water tank level control system, is shown in Fig. 2.10. Control system Operator and TAPI INPUT (Desired tank level) OUTPUT

Figure 2.10 (measured level)

Example 2.9 An autopilot mechanism and the aeroplane it controls is a closed-loop (feedback) control system. Its purpose is to maintain a specified aeroplane heading, despite atmospheric changes it performs this task by continuously measuring the actual aeroplane heading, and automatically adjusting the aeroplane control surfaces (rubber, ailerons, etc.) so as to bring the actual aeroplane heading into correspondence with the specified heading. The human pilot or operator who presets the autopilot is not part of the control system.

Example 2.10 Another example of a closed-loop system, is the water tank level control system shown in Fig. 2.14. The desired water level, h, can be maintained within quite accurate tolerances, even though the outlet flow rate is varied. The error signal is then amplified to drive a motor in order to adjust the valve which regulates the inlet flow rate.

SYSTEM INLET FLOW

OUTLET FLOW

Figure 2.11 -59_

Fig. 2.12 shows the block diagram of the water tank level control system with feedback....