Tuesday, March 11, 2008

Matters that must be known before installing the PLC

Safety Procedure
Safety Procedure is a prevention action so that is not happened the accident at the time of work excecution that may be happened to the worker or other people, machine, equipment and environmental in any time and any where.

The safety equipment for example:
  • Wear
  • Safety shoes
  • Helmet
  • Gloves
  • Safety glasses
  • Masker, etc
Prevention of the happening of accident in the work place must pay attention some factors for example :
  • Ascertaining equipments in good condition
  • Ascertaining wear in good condition
  • Must discipline in using the equipment
  • Must attentive and concentration at the work
  • Ascertaining has comprehended the way of operation of the machine or equipment
  • Ascertaining body condition before working in good health

Equipments and materials of installation of the PLC are attached as according to specification of devices, standard and conditions
The PLC (Programmable Logic Controller) is an automatic controller based on microcontroller and could be programmed. The PLC is used to replace the conventional controller or the controller based on relay. Each brand of the PLC have the different specification but principally, its installation procedure much the same because hardwares of the PLC are only consist of several components; such as Power Supply Unit, CPU Unit, Memory Unit and I/O Unit. Installation of the PLC must as according to installation guide because wrong installation can result damage at the PLC.

Hardwares of the PLC
Hardwares of the PLC are only consist of several components; such as Power Supply Unit, CPU Unit, Memory Unit and I/O Unit. At the small PLC; Power Supply, CPU, Memory and I/O stay in the same Unit but at the big PLC; Power Supply, CPU, Memory and I/O in the separate Unit.
See the picture below;
Block diagram of PLC

The small Omron PLC

The big Omron PLC

The small LG PLC

The big LG PLC

Power Supply
The Power Supply is used to give power to all the parts. The most PLC works with power 24 VDC or 220 VAC. The big PLC has the separate power supply and the small PLC has power supply which is one. The Power Supply is also provided by the battery back up, when happened the failure of power, automatically battery will replace the power supply to supply the CPU, so that the program of memory user do not lose.

CPU Unit (Central Processing Unit)
Without seeing to type of PLC ( small, medium, big) prosesor and memory always in the same unit. This unit is referred as the CPU. The CPU is the brain of a PLC. The CPU is a 8 bit, 16 bit or 32 bit microcontroller. The CPU has function to handle communications with the eksternal apparatus, interconnectivity between the parts of PLC, executes the program, managements the memory, observing or perceiving the input signal and gives the output signal ( as according to the program or process run). The CPU is also provided by the indicator lamp as indicator of the happening of mistake and damage.
See the picture below;
The CPU of Omron PLC

CPU Indicators

I/O Unit
The I/O Unit is the interface unit has function to convert the input signal and output signal so that the CPU can communicate with the external apparatus like the switch, stepping motor, solenoid, sensor, etc. The I/O Unit has two type; Discrete I/O and Analog I/O.
See the picture below;
The I/O Unit of Omron PLC

Memory Unit
The Memory Unit has function to save the data and program will be used by PLC. This Memory is divisible into two type that is ROM and RAM. ROM contains the data like facility of the logic program, facility to edit the program, facility to monitor the program, facility for the communications, etc. The Data will be save permanently and will not lose though the power supply is off. While RAM contains the data of user program, like ladder diagram, memory data, I/O status, etc. The Data can be writed and read. RAM has the character is not permanent, if the power suplply is off the data will lose. To avoid the mentioned, so the power supply is provided by the battery back up to supply if the especial power supply is off.
Input Devices
PLCs can receive input from either automated or manual devices. The PLC could receive data from the user via a pushbutton switch, keyboard, or similar device. Automated input could come from a variety of devices: microswitches, timers, encoders, photosensors, and so on. Some devices, like the Limit Switch shown below, turn ON or OFF when the equipment actually makes contact with it. Other devices, like the Photoelectric Switch and Proximity Switch shown below, use other means, such as light or inductance, in order to get information about the equipment being monitored. See the picture below;

Output Devices
A PLC can output to a myriad of devices for use in automated control. Almost anything that you can think of could be controlled (perhaps indirectly) by a PLC. Some of the most common devices are motors, Solenoids, Servomotors, Stepping Motors, valves, switches, indicator lights, buzzers, and alarms. Some of these output devices; such as the motors, Solenoids, Servomotors, Stepping Motors, and valves; affect the controlled system directly. Others; such as the indicator lights, buzzers, and alarms; provide output to notify personnel.

The necessary environmental conditions for installing the PLC
Do not install the PLC in any of the following locations. Doing so will affect PLC life and may affect operating performance.
  • Locations subject to ambient temperatures lower than 0°C or higher than 55°C
  • Locations subject to drastic temperature changes or condensation
  • Locations subject to ambient humidity lower than 10% or higher than 90%
  • Locations subject to corrosive or flammable gases
  • Locations subject to excessive dust (especially iron dust) or chloride
  • Locations that would subject the CPU to direct shock or vibration
  • Locations that would subject the PC to water, oil, or chemical reagents
  • Locations exposed to direct sunlight

Take appropriate and sufficient countermeasures when installing systems in the following locations:
  • Locations subject to static electricity or other forms of noise
  • Locations subject to strong electromagnetic fields
  • Locations subject to possible exposure to radioactivity
  • Locations close to power supplies

The operating environment of the PLC System can have a large effect on the longevity and reliability of the system. Improper operating environments can lead to malfunction, failure, and other unforeseeable problems with the PLC System. Be sure that the operating environment is within the specified conditions at installation and remains within the specified conditions during the life of the system.