Control System Design
The microcontroller or microprocessor forms the basis of most control systems. A dedicated microprocessor design allows the memory and interfacing to be designed separately, but, because of the range of microcontrollers now available and the additional system design work required, this is now much less likely to be a cost-effective solution. In either case, the peripheral interfacing must also be designed at component level.
By contrast, the PLC offers an off-the-shelf hardware package, requiring no external electronics to interface it. PLCs are generally designed around dedicated microcontrollers, with a built-in proprietary operating system. The program is traditionally written in ladder logic and is compiled automatically to machine code. Additional programming tools are also usually supplied by the individual manufacturer to meet more complex specifications and project management requirements. Overall, the PLC is robust, and easy to install and program, with a variety of communication interfaces to support system integration.
In industrial systems, the PC can function as a general purpose administrative computer, programming host, design workstation, SCADA display, network client or server. As a system controller, the PC is most often connected to client PLCs, robots and machine tools via
a network, with the PLCs controlling the target hardware, as in the FMS above. The PC can also act separately as a programming terminal for the different programmable devices, then SCADA host when the system is operational. It can also act as a computer-aided design or electronic computer-aided design (CAD/ECAD) workstation, component database server or just a plain old word processor!
Table 14.1 provides a comparison of the advantages and disadvantages of the different forms of system control outlined above. A reasonable working knowledge of all the options is required in order to select the most appropriate technology for any given application. The microcontroller is central to all these technologies.
Questions 14
1. Outline the differences between the Intel 8051 microcontroller and an equivalent PIC MCU in terms of their general internal architecture and consequent performance. (4)
2. State two advantages and two disadvantages of the conventional processor system over the microcontroller in designing a system to meet a particular specification. (4)
3. Explain briefly the advantages of using a PLC compared with a microprocessor system in
control applications. (4)
4. Draw a flowchart for Program 14.2 to show the control sequence clearly. (7)
5. List six possible functions of the PC in a production system. (6)
6. Match up the controller type with the most appropriate programming language or technique:
(a) Small microcontroller 1. ‘C’
(b) CISC Microprocessor 2. None
(c) Relay system 3. Mimic
(d) PLC 4. Assembler
(e) SCADA 5. Ladder Logic. (5)
Answers on pages 425e6. (Total 30 marks)
Activities 14
1. Log on to the Atmel website. Select a microcontroller from the list of available flash devices that is most similar to the 16F690 and compare its features and instruction set. Identify any advantages that the AVR microcontroller may have over the PIC.
2. Study the relay-based machine controller. Devise a circuit to switch a motor on and off using push buttons and a single relay. Why is this safer than using a simple mains switch?
3. Modify the PLC machine tool controller in Figure 14.8, and its program, to operate an alarm output if the machine overloads. The alarm is wired as another output.
4. Devise a block diagram of a domestic washing machine, controlled by a microcontroller.
Show interface blocks between the switched actuators and sensors and the microcontroller. Write a description of the operating sequence of the machine, and devise a flowchart for the control sequence, constructed so that it could be implemented in PIC assembly language.
5. By reference to the temperature controller design in Chapter 13, design the hardware interfacing for PIC implementation of the system shown in Figure 14.10. Select a suitable device according to the I/O and memory requirements, test Program 14.2 in the MPLAB simulator and implement the design using the most readily available construction techniques. Devise a target system to simulate the machine tool, and confirm correct operation in hardware.