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Unit 7 Discrete Controllers Assigned Core Text Reading for This Unit Unit 7 Discrete Controllers Assigned Core Text Reading for this Unit: Groover, M. P. (2008), Automation, Production Systems, and Computer- Integrated Manufacturing, 3rd ed., Chapter 9. 7.1 Unit Introduction 7.2 Unit Learning Objectives 7.3 Discrete Process Control 7.4 Ladder Logic Diagrams 7.5 Programmable Logic Controllers 7.6 Personal Computers Using Soft Logic 7.7 Unit Review 7.8 Self-Assessment Questions 7.9 Self-Assessment Answers Section 7.1 Unit Introduction Automation is possible because of sensors, actuators, machine tools such as robots or machines and controllers that execute a program of instructions. Many controllers in manufacturing are specific to the machine tool that in turn is specific to a particular machine tool supplier (e.g. Bridgeport, Kuka Robotics, Siemens and so on). In this unit we examine general controllers that are typically not linked to machine tools or robots and are used to control a variety of independently configured sensors and actuators using discrete control. Discrete control is implemented via two principle industrial controllers: BULLETLIST Programmable Logic Controllers (PLCs) Personal Computers (PCs). ENDLIST KEYPOINT Discrete control is implemented via two principle industrial controllers: the programmable logic controller, and the personal computer. END KEYPOINT LEARNING ACTIVITY 7.1 Learn more about PLCs and PCs at the following web-sites: http://en.wikipedia.org/wiki/Programmable_logic_controller http://en.wikipedia.org/wiki/Personal_computer END LEARNING ACTIVITY 7.1 The unit begins with an examination of discrete process control, its principles of logic control and sequencing. This is followed by an analysis of ladder logic diagrams, whereby logical relationships between various elements may be mapped. Programmable logic controllers are then discussed: their elemental components are specified, their operative cycle is defined, and their additional capabilities are outlined. Subsequently an analysis of personal computers using soft logic is provided; PCs have begun to take-over some of the application work formerly performed by PLCs. Section 7.2 Unit Learning Objectives After completing this unit, and the assigned reading and exercises supplied, you should be able to: BULLET LIST Define discrete process control Explain the two types of discrete process control systems: logic control, and sequencing Explain the concept of a ladder logic diagram Define a programmable logic controller Outline the advantages of programmable logic controllers over older systems of control Specify the steps in the operating cycle of a programmable logic controller Outline the position of personal computers in relation to programmable logic controllers, in terms of their relative markets Specify the two approaches used in personal computer control systems ENDLIST Section 7.3 Discrete Process Control Parameters and variables that change value at discrete moments in time fall under the scope of discrete process control systems. These parameters and variables are typically binary (i.e. on or off; true or false; present or not present; high or low; etc.), and are represented by two values, 1 or 0. Typically, binary values are input and output signals to and from the controller. The input signals may be generated by process limit switches or photo-sensors; while output signals turn on and off switches, motors, valves and other binary actuators related to the process. KEYPOINT Discrete process control systems utilise parameters and variables that change value at discrete moments in time, in binary fashion. END KEYPOINT There are two classes of discrete process control—logic control and sequencing—outlined in general terms in Table 7.1. Table 7.1: Process Control Types Control Type Description Logic Control Also known as combinational logic control, this control type uses a switching system infrastructure so that outputs from the system, at any given moment, are dependent upon the system inputs. The system does not utilise memory, or previous input signals to determine the output signal; nor does it have any operating characteristics that perform directly as a function of time. Sequencing This uses internal timing devices to determine when to initiate changes in output variables. Many sequencing applications require the controller to provide a pre-scheduled set of on/off values for the output variables, which operates in an open-loop control system. KEYPOINT There are two classes of discrete process control: logic control, and sequencing. END KEYPOINT Logic control may be illustrated by means of a robotic example, whereby a robot is programmed to perform a machine-loading application. The robot is required to pick up a raw part from a conveyor, and to place it correctly in a production machine. First, the raw part must be in the correct location on the conveyor for the robot to handle it: this is achieved by means of a limit switch that senses a part’s presence and proceeds to stop the conveyor to enable part-pickup via an ON signal to the robot controller. Next, the production machine must have completed production on the previous raw part: this is ensured by an ON signal sent from the machine itself to the robot controller. Finally, the previous part must have been removed from the production machine: this is determined by a photo- detector that determines the presence or absence of the part, and—on sensing no part—sends an ON signal to the robot controller. These three separate ON signals, once received by the robot controller, allows the robot to perform the action for which it was programmed. No previous conditions or past history has any bearing on the robot’s actions. It is just the presence or absence of these three ON signals that determines whether the robot will be actuated or not. Figure 7.1 illustrates a typical discrete process controller. Figure 7.1: Discrete Process Controller The three basic elements of logic control are the logic gates AND, OR, and NOT. The value of the output from each of these logic gates is determined by the received inputs, such that the input and outputs are in binary format. In industrial control 0 is defined as OFF, and 1 is defined as ON. KEYPOINT The three basic elements of logic control are the logic gates AND, OR, and NOT. END KEYPOINT The three logic gates are outlined in detail in Table 7.2. Table 7.2: Logic gates in industrial control Logic gate Description AND Outputs a value of 1 if all inputs are 1; otherwise it outputs 0. May be used to indicate that two or more actions have been successfully completed, therefore signalling that the next step in the process may be initiated. OR Outputs a value of 1 if either of the inputs are 1; otherwise it outputs 0. Often used in manufacturing for safety monitoring initiatives. NOT Outputs a value the reverse of its single input. If 1 is input, then 0 is output; if 0 is input, then 1 is output. Often used to open a circuit upon receipt of a control signal. KEYPOINT The AND logic gate outputs a value of 1 if all inputs are 1; otherwise it outputs 0. The OR logic gate outputs a value of 1 if either of the inputs are 1; otherwise it outputs 0. The NOT logic gate outputs a value the reverse of its single input. END KEYPOINT These basic elements form the basis of Boolean algebra and truth tables. Boolean algebra dates back to the 1840s when George Boole invented it to test logical statements for their accuracy; it has found a renewed use in digital logic systems. Truth tables are an extension of this Boolean algebra, and represent all possible logical combinations in a particular instance. In Boolean algebra the AND logic gate is expressed as: Y = X1 . X2 that is, Y is true if both X1 and X2 are true; otherwise it is false. The corresponding truth table, showing all logical conditions for Y, is as follows: Inputs Output X1 X2 Y = X1 . X2 0 0 0 0 1 0 1 0 0 1 1 1 In a similar fashion we can outline the OR function in Boolean logic (Y = X1 + X2) and its truth table as: Inputs Output X1 X2 Y = X1 + X2 0 0 0 0 1 1 1 0 1 1 1 1 Finally, the NOT function in Boolean logic (Y = X1), and truth table: Input Output X1 Y = Xˉ1 0 1 1 0 In addition there is the NAND and NOR gates: the NAND gate is achieved by combining an AND gate and a NOT gate in sequence; while the NOR gate is obtained by combining an OR gate followed by a NOT gate. These logic gates may be represented in various ways, but typically either the US or the ISO symbol conventions are chosen; these are outlined in Figure 7.1. Figure 7.1: Symbols used for the various logic gates: US and ISO KEYPOINT The logic gates of logic control form the basis for Boolean algebra and truth tables. END KEYPOINT LEARNING ACTIVITY 7.2 Visit the following web-sites to learn more about Boolean logic, including an account of its laws and theorems: http://en.wikipedia.org/wiki/Introduction_to_boolean_algebra http://en.wikipedia.org/wiki/Boolean_algebra_(logic) http://en.wikipedia.org/wiki/Boolean_logic http://www.play-hookey.com/digital/boolean_algebra.html END LEARNING ACTIVITY 7.2 EXAMPLE 7.1 On a particular piece of operator-controlled production equipment, the production process may only be performed by the operator activating two safety switches, located at some distance from each other. This is to prevent the equipment from accidentally starting whilst the operator is loading or unloading the machine. The switches have to be depressed together by the operator using both hands. (a) What is the truth table for this operation? (b) What is the Boolean logic expression for this operation? (c) What is the logic network diagram for the operation (in ISO symbol terminology)? (d) What is the ladder logic diagram for the system? Solution (a) Truth table for the operation is as follows: Where X1 is first switch, and X2 is second switch, and Y is the output of switch activation.
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