Apr. 12, 2019

Basic Types and Important Data





Programmable logic controllers I/O channel specifi cations include total number of points, number of inputs and outputs, ability to expand, and maximum number of channels. Number of points is the sum of the inputsand the outputs. PLC may be specifi ed by any possible combination of these values. Expandable units may be stacked or linked together to increase total control capacity. Maximum number of channels refer to the maximum total number of input and output channels in an expanded system. PLC system specifi cations to be considered include scan time, number of instructions, data memory, and program memory. Scan time is the time required by the PLC to check the states of its inputs and outputs. Instructions are standard operations (such as mathematical functions) available to PLC software. Data memory is the capacity for data storage.





Program memory is the capacity for control software storage. Available inputs for programmable logic controllers include DC, AC, analog, thermocouple, RTD, frequency or pulse, transistor, and interrupt inputs. Outputs for PLC include DC, AC, relay, analog, frequency or pulse, transistor, and triac. Programming options for PLC include front panel, hand held, and computer.Programmable logic controllers can also be specifi ed with a number of computer interface options, network specifi cations, and features. PLC power options, mounting options, and environmental operating conditions are all also important to be considered.





PLCs are usually available in these three general types:





(1) Embedded.

The embedded controllers expand their field bus terminals and transform them into a modular PLC. All embedded controllers support the same communication standards such as Ethernet TCP/IP. The industrial PC and compact operating units belonging to PLC product spectrum are also identical for all controllers.





(2) PC-based.

This type of PLCs is of slide-in card for the PC that extends every PC or IPC and transforms it into a fully fledged PLC. In the PC, the slide-in card needs only one PCI bus slot and runs fully independently of the operating system. PC system crashes leave the machine control completely cold.

















(3) Compact.

The compact PLC controller unites the functions of an operating unit and a PLC. To some extent, the compact controller already features integrated digital and analog inputs and outputs. Further fi eld bus terminals in the compact PLCs can be connected via an electrically isolated interface such as CANopen.





Installation and Maintenance





(1) Control design considerations





(a) Systematic design for process control.





First, you need to select an instrument or a system that you wish to control. An automated system can be a machine or a process and can also be called a process control system. The function of a process control system is constantly watched by input devices (sensors) that give signals to a PLC controller. In response to this, the PLC controller sends a signal to external output devices (operative instruments) that actually control how the system functions in an assigned manner (for simplification it is recommended that you draw a block diagram of opera tions’ flow).





Second, you need to specify all input and output instruments that will be connected to a PLC controller. Following identification of all input and output instruments, corre sponding designations are assigned to input and output lines of a PLC controller. Allotment of these designations is, in fact, an allocation of inputs and outputs on a PLC controller that corresponds to inputs and outputs of a system being designed.





Third, make a ladder diagram for a program by following the sequence of operations that was determined in the first step, and then programming the completed ladder logic diagrams. Finally, the program is entered into the PLC con troller memory. When programming is finished, checkups are done for any existing errors in a program code (using functions for diagnostics) and, if possible, an entire operation is simulated. Before this system is started, you need to check once again whether all input and output instruments are connected to correct inputs or outputs. By bringing supply in, the system starts working.





(b)Memory considerations





The two main factors to consider when choosing memory are the type and the amount. An application may require two types of memory: nonvolatile memory and volatile memory with a battery backup. A non volatile memory, such as EPROM, can provide a reliable, permanent storage medium once the program has been created and debugged. If the application will require on-line changes, then it should probably be stored in read/write memory supported by a battery. Some controllers offer both of these options, which can be used individually or in con junction with each other. The amount of memory required for a given application is a function of the total number of inputs and outputs to be controlled and the complexity of the control program. The complexity refers to the amount and type of arithmetic and data manipulation functions that the PLC will perform. For each of their products, manufacturers have a rule-of-thumb formula that helps to approximate the memory requirement. This formula involves multiplying the total number of I/O by a constant (usually a number between 3 and 8). If the program involves arithmetic or data manipulation, this memory approximation should be increased by 25–50%.





(c) Software considerations.





During system implementation, the user must program the PLC. Because the programming is so important, the user should be aware of the software capabilities of the product they choose. Generally, the software capability of a system is tailored to handle the control hardware that is available with the controller. However, some applications require special software functions that are beyond the control of the hardware components. For instance, an application may involve special control or data acquisition functions that require complex numerical calculations and data-handling manipulations. The instruction set selected will determine the ease with which these software tasks can be implemented. It will also directly affect the time required to implement and execute the control program.





(d) Peripherals.





The programming device is the key peripheral in a PLC system. It is of primary importance because it must provide all of the capabilities necessary to accurately and easily enter the control program into the system. The two most common types of programming devices are handheld units and personal computers. Handheld units, which are small and of low cost, are typically used to program relatively small control programs in small PLCs. The amount of

information that can be displayed on a handheld unit is normally a single program element or, in some cases, a single program rung. Personal computers provide a better way to program a system if the control program is large. Many PLC manufacturers provide software that allows their PLCs to be

programmed using a standard PC. However, expansion boards or special interfacing cables may be required to link the personal computer with the programmable controller.





In addition to the programming device, a system may require other types of peripherals such as line printers or color displayers at certain control stations to provide an interface between the controller and the operator. If a PC is used as a graphic interface to a PLC system, both systems must have compatible DDE (dynamic data exchange) drivers to properly interface with peripherals. Peripheral requirements should be evaluated along with the CPU, since the CPU will determine the type and number of peripherals that can be interfaced to the system. The CPU also influences the method of interfacing, as well as the distance that peripherals can be placed from the PLC.





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