A PLC is an industrial computer that is capable to do discrete or sequential logic in a factory environment. The PLC was developed to replace the mechanical relays, timers, and counters. The PLC is considered as the heart of the control system in an automated system. The PLC can monitor the control system state by the input device signal and based on the program logic it would determine the course of action to be carried out through the output device. The PLC can be considered as the combination of the solid-state logic device.
- How to do the PLC maintenance and how to select a PLC?
- What are the components of a PLC?
- PLC communications
- What are the types of programming languages used in PLC?
- What are the different operating modes in PLC?
We must take proper care while installing a PLC so that we can reduce the damages to the automated control systems. We must avoid the installation of PLC in certain places in a factory.
- It must not be installed in places where it would get direct exposure to the sunlight
- The temperature of the location where the PLC is installed must not exceed more than 55 degrees Celsius.
- The relative humidity range 10%-90% RH
- It must not be installed in places where flammable or poisonous gases are present
- It should not be installed in places where there is a lot of vibrations
- It shouldn’t be installed in places where oil or chemical dust is present
- We must allow enough space for air circulation
- The power lines and high voltage equipment can cause electrical noise in a PLC
- We must not install the PLC in a panel that has high voltage equipment
- There should be at least 200 mm distance between the PLC and the nearby power lines
- The PLC must be installed in a way that it can be accessed for operation and maintenance
- It must not be installed near a high voltage equipment
Installing the CPU unit and I/O unit
The small PLC must be installed in a way that it should have adequate cooling, the small PLC must not be installed in the incorrect position. It can be installed on a horizontal surface. The I/O units can be installed in a way that inserting the proper modules in their correct locations.
We must connect the I/O units as shown in the wiring diagram, most of the switches are attached to the panel, sensors, and solenoid which is located at the machine which is to be controlled.
Special I/O connections
We need to give special care while connecting certain field devices, this type of connection is leaky inputs and inductive loads. Certain field devices would have leakage current even they are not working and this leaky input can trigger an input circuit and thus misoperation. So this can be prevented by using a bleeding resistor, place this resistor across the input. This resistor would provide resistance to the circuit and because of this, there will be a voltage drop between the leaky field device and the input circuit. The inductive loads can interrupt the current and this would create voltage spikes and to avoid this we can use a snubber circuit.
Installing the PLC in a panel cabinet
- While installing sufficient air circulation is required
- Cooling fan
- The device which generates heat must be located under the PLC
- There shouldn’t be any high voltage equipment
What are the factors that must be considered while wiring a PLC?
- We must use short cables as possible
- We must use a single line between the equipment and do not connect the cable to make it long
- We shouldn’t make sharp curves in the cable
- We need to keep the area of the system and the control wiring away from the high voltage wiring
- We need to separate the input wiring, output wiring, and other wiring types
- The AC and DC wiring must be separate
- All the components must be grounded
- We must make sure that the wire that is used is of the correct gauge and it must be able to handle the maximum possible current
- We can label the field wire and termination point using a reliable labeling method
- The wire bundling techniques can be used to simplify the connections, the bundles which carry the same type of signals must be kept in separate ducts so that we can avoid interference.
- In case if we need to place the I/O wiring and the power cables in the same duct, then they should be shielded from each other by using metal plates.
How to avoid the voltage spike problems in a PLC?
The PLC is composed of many active components like the transistor, IC, etc. All these components would burn if there is lightning and causes surge voltage. So we need to prevent this problem, we must make sure the protective earth terminal and PLC on the functional earth ground with a minimum of 100ohm. The cable which is used for the grounding must be 2mm square. Coating and corrosion of paint must be removed at the grounding connection, the site grounding, and the green cable must be terminated at the cable lug. This cable must tighten the lug and solder. While grounding the PLC if the independent grounding is not possible then do the joint grounding. The grounding wire must be thicker and the grounding point must be close to the PLC so that the distance to the grounding cable can be decreased.