Correspondence Lesson 9: Programmable Logic Controllers

Computer-based controllers (most commonly called programmable controllers or programmable logic controllers, PLCs) are the first controlling system able to harmoniously tie other control systems together. Essentially, PLCs are specially designed computers equipped with input and output (I/O) devices to interface with industrial controls and machines. They are housed in high-strength cases, for enduring

Computer-based controllers (most commonly called programmable controllers or programmable logic controllers, PLCs) are the first controlling system able to harmoniously tie other control systems together. Essentially, PLCs are specially designed computers equipped with input and output (I/O) devices to interface with industrial controls and machines. They are housed in high-strength cases, for enduring rough industrial environments.

At the core of the PLC is a central processing unit (CPU), which performs internal calculations. PLC processors are computer microchips, designed to support a fixed number of discrete, analog, or specialty I/O ports. They run programs, perform mathematical functions, and communicate with other processors (if programmed). PLC processors also monitor the status (ON/OFF) of the input devices.

Memory. Memory chips in PLCs serve two main purposes: • Retain information that the processor needs for logical decisions. This part of the memory is sometimes referred to as "storage" or "data table," where the status (ON/OFF) of all the discrete inputs and outputs is stored. Numeric values of timers and counters may also be stored in this memory. • Retain programming instructions. This is often referred to as the "user program memory" and contains ladder-diagram instructions. This section of memory is generally many times larger than the storage or data table. Instructions are in the memory via a programming device, magnetic tape, or systems computer.

This memory may vary from a few thousand bytes to several million bytes. This measurement is expressed in "K" (Kilo) for thousands of bytes or "M" (Mega) for millions of bytes. A byte is a grouping of eight bits of binary information, used in a computer's internal operations.

There are two general classes of memory: "volatile" and "nonvolatile." Volatile memory will lose its contents if power is lost. Therefore, it's necessary to have battery backup power available at all times.

Volatile memory is Random Access Memory (RAM). There are two common types of RAM memory chips: MOS (metal-oxide semiconductor) and CMOS (complimentary metal-oxide semiconductor).

Nonvolatile memory is Read Only Memory (ROM). It retains its information with power loss; thus, it doesn't require battery backup power.

There are several types of ROM:

Programmable Read Only Memory (PROM). The manufacturer generally programs PROM through use of special programming equipment. Once programmed, you cannot erase it or alter it.

Erasable Programmable Read Only Memory (EPROM). You can erase an EPROM chip by the using an ultraviolet light source. After you completely erase the program chip, you can make changes to the program.

Electrically Alterable Read Only Memory (EAROM). An erasing voltage applied to the proper pin of an EAROM chip will completely erase the program.

Electrically Erasable Programmable Read Only Memory (EEPROM). EEPROM, also known as "E-squared prom", is a nonvolatile memory that offers the same programming flexibility as RAM. The program can easily be changed with a manual programming unit.

PLCs come in many different memory sizes. The amount required depends on the user's application. Sizes are generally expressed in kilobytes: 2K, 4K, 16K, and so on. (Note: Due to the quirks of the computer business, 1 kilobyte is actually 1024 bytes, not the 1000 bytes you would expect.)

Many PLCs also come with a special feature called Error Detection and Correction (EDC). EDC ensures that the thousands of bit transfers taking place in the memory are processed accurately, by finding and correcting errors without interrupting the program operations.

While the CPU and memory are enough to handle most calculations and operations, they're not suited to connect to machines or sensors. To allow the processor to work along with the varied machines and mechanical devices, you must use I/O devices.

I/O devices. I/O devices connect many types of machines and sensors. Some I/O devices send or receive digital signals, others send or receive analog signals.

The rating of an I/O device must coordinate with the rating of the equipment or sensors it connects. Working with the PLC manufacturer can help you to properly coordinate pertinent characteristics. It is usually necessary to incorporate special relay devices or transformers to coordinate different voltage or current values. To keep track of details of the manufacturing process, many types of sensors are used with PLC-controlled systems. These devices are the "I" or inputs to the I/O devices. The most common types are proximity sensors and photoelectric sensors (which sense the location of parts during the manufacturing process).

Networking. Networking systems allow many systems and controllers to communicate with each other. The most common systems are Bus, Ring, Star and Mesh networks. These systems are the same ones used for computer networking. A network of coaxial, twisted pair, or optical fiber cables run between the units. This wiring must be used with networking software, which oversees the communications between the various machines. Special integrated circuit cards must also be installed in various machines to accommodate such systems. More industrial automation systems are going to a network setup, due to data-processing advantages. For network wiring, installers must follow the manufacturer's instructions.

Programming. PLC manufacturers developed a language of commands, instructions, and operations, presented in a form similar to the relay ladder-type diagram. However, in drawing the circuit for a programmable logic controller, programmers do not have symbols like those used in electromechanical circuit diagrams (i.e. push buttons, limit switches, and pressure switches). Programming devices use relay contact symbols for component switches. Each action is represented by a specific instruction. The instruction tells the processor to do something with the information stored in the data or user table (see Fig. 2, on the original article's page 52).

Programming language adopted by PLC manufacturers uses basic ladder logic symbols. For more complex symbols, such as timers, a generic rectangle is used to store the parameters of the device. Whether the input switches are normally open or normally closed will affect the state of the logic to be programmed in the PLC. For example, a normally closed stop switch in a motor control circuit is necessary to interrupt the flow of current to the motor controller. However, in a PLC circuit the switch could be either programmed open or programmed closed. The important condition is the state of the logic in the PLC ladder diagram.

In industrial applications where the PLC will replace the relay logic circuit, generally the operator controls and process switches will remain as they are wired. Therefore, the PLC logic will need to reflect the normal condition of the operator or process switches.

Control signals. In electromechanical control, a contact is closed to energize a circuit or open to de-energize a circuit. In solid-state control, these closed and open contact concepts are referred to simply as 1 and 0, respectively. In PLCs, the inputs and outputs are always such as to allow a signal to pass or to prevent it from passing.

Control depends on initial and continuing information from the machine and/or the process. This information is gathered from inputs such as pushbutton switches, limit switches, transducers, and other control devices. An input module then communicates the status of the discrete inputs to the processor. The signal from the input must be prepared for access to the processor. Field-supplied voltages from the input may cover a range of 24V to 240V AC/DC. The input signal must be converted to a low-level DC logic voltage. This voltage varies with different manufacturers. If the input is alternating current, a rectifier converts it to direct current, and a resistor reduces the voltage. If the input signal is analog (as it would be with many temperature, speed, and pressure sensors), the signal is converted to a binary through an analog to digital converter.

Signal isolation. Because of power quality concerns, incoming signals are isolated before they enter the processor. Optoisolators, reed relays, or transformers can perform this task. Optoisolators, the most common of the three, operate by changing the electrical signal into a light signal, sending the light signal to a second circuit where it is changes back into an electronic signal. The optoisolator transmits the signal while eliminating any electrical current that could pass stray voltages.

Troubleshooting. It's difficult to offer a complete and comprehensive approach to troubleshooting the PLC, since each manufacturer takes a slightly different approach to the design of the unit. However, one area generally applies to all units: the peripheral devices. This hardware at least is the same in most cases. It consists of components supplying input information for motion, pressure, and temperature, and components controlling all outputs, such as solenoids, relays, motor starters, visual indicators, and alarm systems. The area between the inputs and outputs is the responsibility of the central processing unit.

In most cases the PLC manufacturer will supply visual indication of all incoming and outgoing signals. However, there are several areas in which the use of a meter to check volts as well as an ohmmeter to check continuity is useful.

One manufacturer of PLCs has provided diagnostic monitoring I/O modules. They provide the troubleshooter with information to quickly identify the possible source of a problem. In practice, the module actually "learns" the operation of a process. When the operator is satisfied the process is correct, the monitoring module observes the PC and the system hardware.

In this article, we've covered only the basics of programmable controllers. If you are new to the use of these controllers, you will have to familiarize yourself with specific types of controllers, the control devices that will be used with them, and have a clear idea of your control objectives.

Sidebar: Electronic Troubleshooting

The electrician doesn't need to be an electronics technician to locate and repair problems associated with solid-state equipment. The following hints will help you locate common problems associated with solid-state equipment.

• Actual test voltages and other measurements should be recorded when the equipment is in good working order. They can then be compared to measurements during troubleshooting. This will speed up the process.

• Look for obvious problems by visual inspection (physical damage, overheated terminations, broken wires, etc.).

• Measure supply voltage and current. Is it within the limits of the equipment?

• Check all circuit breakers and fuses. If circuit breakers or fuses are open, close them only if the fault is cleared.

• High temperature is the leading cause of failures of electronic equipment. Equipment should be placed in a well ventilated area so heat can escape.

• Transient AC voltage peaks or voltage surges in control circuits may last only milliseconds, but can cause problems in the control circuit. A digital multimeter with peak-hold function can be used to determine whether a transient AC peak problem exists.

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