In two-hand safety operations in dangerous industrial operations, such as a pneumatic press, the hands of an operator must be engaged for safety reasons. This condition is usually achieved by arranging two pushbuttons (Say, PB1 and PB2) at a distance.
That means the operator cannot press the pushbuttons with one hand. At times, operators tend to latch one of the pushbuttons by other means, operate the machine with one hand, and adjust the workpiece or tools with the other hand.
The pushbuttons must be pressed within a short time, say one second, to get the anti-tie-down feature. A ladder program to implement this control requirement by taking a lamp (Q100.1) as the representative output device.
Ladder Program
Programmable Logic Controllers and Programming Concepts
With Electrical, Pneumatic, and Hydraulic Applications
A PLC system is usually designed with LEDs on the CPU, power supply, and I/O modules for fault diagnosis. It is usually provided with built-in fault analysis measures to carry out self-testing and display fault codes, which can then be interpreted by looking up the code in a list provided by the manufacturer to give the source of the fault and possible method of rectification.
Moreover, the PLC may be provided with a backup power source to keep the system running in the event of a power failure.
A PLC system may fail due to the following reasons: hardware problems, alteration of user programs, improper wiring, power failures, ground integrity, electromagnetic interference, loss of communication, and excess heat.
Programmable Logic Controllers and Programming Concepts
With Electrical, Pneumatic, and Hydraulic Applications
Joji Parambath
Hardware Faults
The hardware problems in PLCs are caused by CPU, memory, I/O modules, and field devices.
CPU
The troubleshooting engineer must identify the source of the problem using LED indicators, and the troubleshooting table indicating the problem description, probable causes and recommended action against each status indication.
Memory
The PLC memory can become corrupted, or the program may get altered by external factors such as electromagnetic interference, radio frequency interference, improper wiring and grounding, heat, and disruptions of power.
I/O Modules
The I/O modules in PLCs are provided with status indication LEDs. I/O failures are caused by errors in the PLC configuration, loose I/O blocks, broken wires, and incompatible modules.
Field devices
The field devices such as sensors and solenoids may malfunction due to loose connections, damage to the circuitry, and device failure.
Power Failure
A PLC power supply may fail due to blackout, loose connections, or broken wires. A sudden failure of the power supply can cause the scrambling of its memory and loss of process data.
Faulty Grounding
A good grounding of the PLC system is essential for the safety of the system and personnel. Ground wires must be fully intact.
Electrical Noise
A common cause of electrical noise in a PLC system is electromagnetic interference, which typically occurs, for example, when a large electric motor is started nearby.
Network and Communication Failures
The PLC communication network can fail due to hardware faults, power supply failures, wrong configuration and setting of network, and incompatible components.
Excess Heat and Vibration
A PLC system must be safeguarded from the excess heat and vibration and anything that generates excess heat should be kept at a safe distance from the PLC.
A PLC system with communication interfacing capabilities is configured using several networked components, such as PLCs, a computer, communication modules, and HMI. These components can exchange data between two or more communication partners through the network using specific protocols for handling the data traffic.
A network is characterized by the type of hardware involved, the communication procedure, and the data transfer rate. Accordingly, PLC communication networks are multi-point interface (MPI), Profibus, Industrial Ethernet, and point-to-point (PTP) connections.
Multi-Point Interface (MPI)
Multi-point interface (MPI), used in SIEMENS PLC systems, is designed as a programming device interface and is intended for networking a small number of CPUs to exchange small volumes of data.
PROFIBUS
Profibus complies with the standard EN 50170 and is used for networking field devices for communication. This type of connection is a local area network that provides a high-speed data communication bus to which devices within the local area can be connected.
Industrial Ethernet
Industrial Ethernet is a network for connecting PLCs and computers. It is suitable for the high-speed exchange of large quantities of data and facilitates communication between the communication partners.
Point-to-point (PTP) Connection
In a point-to-point connection, a communications processor forms the interface between a PLC and a communications partner. Serial communication occurs when bits belonging to each byte of information are transmitted one after the other in a serial order.
Programmable Logic Controllers and Programming Concepts
With Electrical, Pneumatic, and Hydraulic Applications
Joji Parambath
Cables
Cables are used as the transmission medium for communication among devices connected in a PLC system. There are two types of cables available. They are: (1) Copper cables and (2) Fibre-optic cables.
Copper cable can be sub-divided into: (1) 2-core cable, (2) coaxial cable, and (3) shielded twisted-pair cable
Fibre-optic cable can be sub-divided into: (1) glass fibre-optic cables and (2) plastic fibre-optic cables
RS-232C
RS-232C interface is a kind of serial physical interface standard. This method is based on 1:1 communication, which is generally applied to the point-to-point communication within a short distance of 15 meters.
RS-485
RS-485 is a network interface that uses a master-slave communication method. That is one master communicating with multiple slaves (1: N). This network can be used as a two-wire or four-wire network. RS-485 network supports long-distance links, and the cable length can be up to 1200 m.
RS-422A
RS-422A is a network interface that also uses a master-slave communication method. This network can be used as a two-wire or four-wire network. RS-422A network supports long-distance links, and the cable length can be up to 1200 m.
Authored by
Joji Parambath
Author
Fluid Power Educational Series
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