PLC

PLCs were invented in the 60/70's for the automotive manufacturing industry. Since this time, they have developed into one of the most versatile tools used for industrial automation. A working knowledge of PLCs and other microprocessor based control systems are critical to technical personnel who are staying current with technology in industry.
Programmable Logic Controllers or PLC are the hub of many manufacturing processes. These microprocessor based units are used in processes as simple as boxing machines or bagging equipment to controlling and tracking sophisticated manufacturing processes. They are in virtually all new manufacturing, processing and packing equipment in one form or another. Because of their popularity in industry, it becomes increasingly more important to learn skills related to these devices. Click on the buttons to learn more about industrial automation and this invaluable tool.
The microprocessor or processor module is the brain of a PLC system. It consists of the microprocessor, memory
integrated circuits, and circuits necessary to store and retrieve information from memory. It also includes communications ports to other peripherals, other PLC's or programming terminals. Today's processors vary widely in their capabilities to control real world devices. Some control as few as 6 inputs and outputs (I/O) and others 40,000 or more. One processor can control more than one process or manufacturing line. Processors are often linked together in order to provided continuity throughout the process. The number of inputs and outputs PLCs can control are limited by the overall capacity of the PLC system
hardware and memory capabilities. The job of the processor is to monitor status or state of input devices, scan and solve the logic of a user program, and control on or off state of output devices.

RAM or Random Access Memory is a volatile memory that would lose it's information if power were removed..
This is why some processor units incorporate a battery back up. The type of RAM normally used is CMOS or
Complementary Metal Oxide Semiconductor. CMOS RAM is used for storage of the user's program (ladder logic diagrams) and storage memory.

ROM or Read Only Memory is a non-volatile type of memory. This means you don't need an external power source to keep information. In this type of memory, information can be read, but not changed. For this reason the manufacture sometimes calls this firmware. It is placed there for the internal use and operation of processor units.


EEPROM or Electrically Erasable Programmable Read Only Memory is usually an add-on memory module that is used to back up the main program in CMOS RAM of the processor. In many cases, the processor can be programmed to load the EEPOM's program to RAM if RAM is lost or corrupted.

Input Module
There are many types of input modules to choose from. The type of input module used is dependent upon what real world input to the PLC is desired. Some examples of inputs are limit switches, electric eyes, and pushbuttons. DC inputs, such as thumbwheel switches, can be used to enter integer values to be manipulated by the PLC. DC input cards are used for this application. Since most industrial power systems are inherently noisy, electrical isolation is provided between the input and the processor. Electromagnetic interference (EMI) and Radio Frequency Interference (RFI) can cause severe problems in most solid state control systems. The component used most often to provide electrical isolation within I/O cards is called an optical isolator or optocoupler. The wiring of an input is not complex. The object is to get a voltage at a particular point on the card. Typically there are 8 to 32 input points on any one input module. Each point will be assigned a unique address by the processor. Analog input modules are special input cards that use analog to digital conversion (A to D) to sense variables such as temperature, speed, pressure, and position. The external device normally is connected to a controller (transducer) producing an electrical signal the analog input card can interpret. This signal is usually 4 to 20 Ma or a 0 to 10 volt signal.

Output Module
Output modules can be for used for ac or dc devices such as solenoids, relays, contractors, pilot lamps, and LED readouts. Output cards usually have from 6 to 32 output points on a single module. The output device within the card provides the
connection from the user power supply to the load. Usually silicon controlled rectifiers (SCR), triac, or dry contact relays are use for this purpose. Individual outputs are rated most often at 2 to 3 amperes. Output cards, like input cards have electrical
isolation between the load being connected and the PLC. Analog output cards are a special type of output modules that use digital to analog conversion (D to A). The analog output module can take a value stored in a 12 bit file and convert it to an analog signal. Normally this signal is 0 -10 volts dc or 4 to 20 Ma. This analog signal is often used in equipment such as motor operated valves and pneumatic position control devices.

ANALOG

Analog inputs and/or outputs can be 0 to 10V; -10 to +10V; -5v to +5v; 0 to 20mA and 4 to 20mA.

The analog signals are sent to the PLC input where they are read in and converted into a numerical value.
Analog outputs are numerical values in the PLC which are sent to the outputs where they are converted into a variable signal (volts or mA).

The "raw" numerical value from an analog input signal can be processed further for use within the user program.

This raw value is usually scaled before such use to make it easier to process. i.e. a 20mA max signal maybe converted into 1024 units but in the real world 20mA may equate to, say, 600 degrees. It can be seen that if the actual input was 13mA to use the un-scaled value to set min/max alarms in the PLC would be difficult; therefore the user program firsts "scales" the raw values into something more sensible.

For example a sub-routine could scale the raw value to, 1024 units = 600 units, to set min/max alarms now would be easier. i.e.. 300 units = 300 degrees.

E.G.

Process = 0 to 600 degrees
Input = 0 to 20mA (0 degrees = 0 units; 600 degrees = 1024 units)
Raw value = 0 to 1024 units.
Scale = 0 to 600

The sub routine would have to carry some maths to perform the following.






MAX SCALED VALUE / MAX RAW VALUE * ACTUAL RAW VALUE

Max Scaled Value = 600
Max Raw Value = 1024
Actual Raw Value = 700 (variable)
Using the above equation the scaled value would be 410.16 (degrees)
If the actual raw value was 1000 the scaled value would be 585.94 (degrees)
If the actual raw value was 200 the scaled value would be 117.18 (degrees)