INTRODUCTION TO PROGRAMMABLE LOGIC CONTOLLERS (PLCs) - PART 2

 Programming

Early PLCs, up to the mid-1990s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs.[2] Some proprietary programming terminals displayed the elements of PLC programs as graphic symbols, but plain ASCII character representations of contacts, coils, and wires were common. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were minimal due to lack of memory capacity. The oldest PLCs used non-volatile magnetic core memory. More recently, PLCs are programmed using application software on  personal computers, which now represent the logic in graphic form  instead of character symbols. The computer is connected to the PLC  through USB, Ethernet, RS-232, RS-485, or RS-422  cabling. The programming software allows entry and editing of the  ladder-style logic. In some software packages, it is also possible to  view and edit the program in function block diagrams, sequence flow  charts and structured text. Generally the software provides functions  for debugging and troubleshooting the PLC software, for example, by  highlighting portions of the logic to show current status during  operation or via simulation. The software will upload and download the  PLC program, for backup and restoration purposes. In some models of  programmable controller, the program is transferred from a personal  computer to the PLC through a programming board which writes the program  into a removable chip such as an EPROM. 

 PLC Hardware Components A central processing unit (CPU) serves as the brain of the  PLC. It is a -16 or -32 bit microprocessor consisting of a memory chip  and integrated circuits for control logic, monitoring, and  communicating. The CPU directs the PLC to execute control instructions,  communicate with other devices, carry out logic and arithmetic  operations, and perform internal diagnostics. The CPU runs memory  routines, constantly checking the PLC (PLC controller is redundant) to  avoid programming errors and ensure the memory is undamaged. 

Functionality

The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems, and networking.  The data handling, storage, processing power, and communication  capabilities of some modern PLCs are approximately equivalent to desktop computers.  PLC-like programming combined with remote I/O hardware, allow a  general-purpose desktop computer to overlap some PLCs in certain  applications. Desktop computer controllers have not been generally  accepted in heavy industry because the desktop computers run on less  stable operating systems than do PLCs, and because the desktop computer  hardware is typically not designed to the same levels of tolerance to  temperature, humidity, vibration, and longevity as the processors used  in PLCs. Operating systems such as Windows do not lend themselves to  deterministic logic execution, with the result that the controller may  not always respond to changes of input status with the consistency in  timing expected from PLCs. Desktop logic applications find use in less  critical situations, such as laboratory automation and use in small  facilities where the application is less demanding and critical, because  they are generally much less expensive than PLCs. 

 

Basic and complex functions

The  most basic function of a Programmable logic controller (PLC) is to  receive inputs from status components, which can be from sensors or  switches. Some of the basic components of a PLC are input modules, a  central processing unit, output modules, and a programming device. When  an input is activated, some output will also be activated by whatever  the machine is told to do. Some examples of this are setting a timer to  10ms, activating the timer and once 10ms have passed a siren goes off.  Some advantages to using a PLC over other programming devices are the  user doesn't have to rewire anything, the PLC has very little downtime  in between running different programs, the user can program off-line,  and PLC's aren't time constrained. If the user tells the PLC to perform  an output in 10ms, it will perform the output in 10ms unlike other  programs like LabView which can have a delay in activation.  

 Timers and counters

The  main function of a timer is to keep an output on for a specific length  of time. A good example of this is a garage light, where you want power  to be cut off after 2 minutes so as to give someone time to go into the  house. The three different types of timers that are commonly used are a  Delay-OFF, a Delay-ON, and a Delay-ON-Retentive. A Delay-OFF timer  activates immediately when turned on, counts down from a programmed time  before cutting off, and is cleared when the enabling input is off. A  Delay-ON timer is activated by input and starts accumulating time,  counts up to a programmed time before cutting off, and is cleared when  the enabling input is turned off. A Delay-ON-Retentive timer is  activated by input and starts accumulating time, retains the accumulated  value even if the (ladder-logic) rung goes false, and can be reset only  by a RESET contact. Counters are primarily used for counting items such as cans going  into a box on an assembly line. This is important because once something  is filled to its max the item needs to be moved on so something else  can be filled. Many companies use counters in PLC's to count boxes,  count how many feet of something is covered, or to count how many  pallets are on a truck. There are three types of counters, Up counters,  Down counters, and Up/Down counters. Up counters count up to the preset  value, turn on the CTU (Count Up output) when the preset value is  reached, and are cleared upon receiving a reset. Down counters count  down from a preset value, turns on the CTD (Count Down output) when 0 is  reached, and are cleared upon reset. Up/Down counters count up on CU,  count down on CD, turn on CTUD (Count Up/Down output) when the preset  value is reached, and cleared on reset. 

 Programmable logic relay (PLR)

In  more recent years, small products called PLRs (programmable logic  relays), and also by similar names, have become more common and  accepted. These are much like PLCs, and are used in light industry where  only a few points of I/O  (i.e. a few signals coming in from the real world and a few going out)  are needed, and low cost is desired. These small devices are typically  made in a common physical size and shape by several manufacturers, and  branded by the makers of larger PLCs to fill out their low end product  range. Popular names include PICO Controller, NANO PLC, and other names  implying very small controllers. Most of these have 8 to 12 discrete  inputs, 4 to 8 discrete outputs, and up to 2 analog inputs. Size is  usually about 4" wide, 3" high, and 3" deep. Most such devices include a  tiny postage-stamp-sized LCD screen for viewing simplified ladder logic  (only a very small portion of the program being visible at a given  time) and status of I/O points, and typically these screens are  accompanied by a 4-way rocker push-button plus four more separate  push-buttons, similar to the key buttons on a VCR remote control, and  used to navigate and edit the logic. Most have a small plug for  connecting via RS-232 or RS-485 to a personal computer so that  programmers can use simple Windows applications for programming instead  of being forced to use the tiny LCD and push-button set for this  purpose. Unlike regular PLCs that are usually modular and greatly  expandable, the PLRs are usually not modular or expandable, but their  price can be two orders of magnitude less than a PLC, and they still offer robust design and deterministic execution of the logics. 

 PLC compared with other control systems

  Allen-Bradley PLC installed in a control panel       Control center with an Allen-Bradley PLC for a RTO.   PLCs are well adapted to a range of automation  tasks. These are typically industrial processes in manufacturing where  the cost of developing and maintaining the automation system is high  relative to the total cost of the automation, and where changes to the  system would be expected during its operational life. PLCs contain input  and output devices compatible with industrial pilot devices and  controls; little electrical design is required, and the design problem  centers on expressing the desired sequence of operations. PLC  applications are typically highly customized systems, so the cost of a  packaged PLC is low compared to the cost of a specific custom-built  controller design. On the other hand, in the case of mass-produced  goods, customized control systems are economical. This is due to the  lower cost of the components, which can be optimally chosen instead of a  "generic" solution, and where the non-recurring engineering charges are  spread over thousands or millions of units. For high volume or very simple fixed automation tasks, different techniques are used. For example, a cheap consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities. A microcontroller-based  design would be appropriate where hundreds or thousands of units will  be produced and so the development cost (design of power supplies,  input/output hardware, and necessary testing and certification) can be  spread over many sales, and where the end-user would not need to alter  the control. Automotive applications are an example; millions of units  are built each year, and very few end-users alter the programming of  these controllers. However, some specialty vehicles such as transit  buses economically use PLCs instead of custom-designed controls, because  the volumes are low and the development cost would be uneconomical.[14] Very complex process control, such as used in the chemical industry,  may require algorithms and performance beyond the capability of even  high-performance PLCs. Very high-speed or precision controls may also  require customized solutions; for example, aircraft flight controls. Single-board computers  using semi-customized or fully proprietary hardware may be chosen for  very demanding control applications where the high development and  maintenance cost can be supported. "Soft PLCs" running on desktop-type  computers can interface with industrial I/O hardware while executing  programs within a version of commercial operating systems adapted for  process control needs.[14] Programmable controllers are widely used in motion, positioning, or  torque control. Some manufacturers produce motion control units to be  integrated with PLC so that G-code (involving a CNC machine) can be used to instruct machine movements.[15][citation needed] PLCs may include logic for single-variable feedback analog control loop, a proportional, integral, derivative (PID) controller.  A PID loop could be used to control the temperature of a manufacturing  process, for example. Historically PLCs were usually configured with  only a few analog control loops; where processes required hundreds or  thousands of loops, a distributed control system (DCS) would instead be used. As PLCs have become more powerful, the boundary between DCS and PLC applications has been blurred. PLCs have similar functionality as remote terminal units.  An RTU, however, usually does not support control algorithms or control  loops. As hardware rapidly becomes more powerful and cheaper, RTUs,  PLCs, and DCSs are increasingly beginning to overlap in  responsibilities, and many vendors sell RTUs with PLC-like features, and  vice versa. The industry has standardized on the IEC 61131-3  functional block language for creating programs to run on RTUs and  PLCs, although nearly all vendors also offer proprietary alternatives  and associated development environments. In recent years "safety" PLCs have started to become popular, either  as standalone models or as functionality and safety-rated hardware added  to existing controller architectures (Allen-Bradley Guardlogix, Siemens  F-series etc.). These differ from conventional PLC types as being  suitable for use in safety-critical applications for which PLCs have  traditionally been supplemented with hard-wired safety relays. For example, a safety PLC might be used to control access to a robot cell with trapped-key access,  or perhaps to manage the shutdown response to an emergency stop on a  conveyor production line. Such PLCs typically have a restricted regular  instruction set augmented with safety-specific instructions designed to  interface with emergency stops, light screens, and so forth. The  flexibility that such systems offer has resulted in rapid growth of  demand for these controllers.