PLC

Checklist to Carry out PLC Preventive Maintenance Activity

  • A preventive maintenance checklist is a group of tasks that the technician must complete to close a preventive maintenance work order.
  • The purpose of a preventive maintenance checklist is to confirm for maintenance tasks are correctly done.
  • The maintenance schedule of Programmable Logic Controllers depends upon the environment of the controllers.
  • Frequent maintenance of the controller is most important for the controller used in a harsher environment.
  • Scheduling a periodic maintenance routine will increase the durability of Programmable Logic Controllers and reduces the probability of system malfunction.

Below mentioned practices must be followed to maintain the controller in a good operating environment.

Checklist to Carry out PLC Preventive Maintenance Activity

Backup the PLC program logic:

  • The copy of updated PLC program must be copied as a backup by uploading the program from controller to the maintenance computer during the maintenance routine.
  • If the PLC controller gets failed to operate and needs to be replaced, this backup file saved in the maintenance computer can be easily downloaded to the newly installed PLC.

Check LED Indicators:

  • All LED indicator lamps provided must be checked continuously.
  • If the power LED indicator gets turned off/blinking or if the battery LED indicator is off/blinking, it represents a low battery or potential power supply issue.

Replace the Battery:

  • If the Battery OK LED is blinking or off, then the battery must be replaced to avoid catastrophic or destructive problems.

Check Operating Environment:

  • Check the temperature, humidity, and other environmental factors to ensure that our controller is functioning in proper conditions. Ensure good ventilation in the cabinet by cleaning the filters

Check Operating Voltage:

  • Check the input voltage required to power up the controller and ensure that the supply voltage is within range and free from power spikes or burnout conditions.

Check Program Functionality:

  • During scheduled maintenance, check the controller functionality to ensure the system is operating as intended.

What are the actions taken for PLC system maintenance?

Three easy actions are taken for PLC system maintenance

1. Preventive Maintenance:

  • Preventive maintenance of programmable logic controllers reduces the chances of damage to system components.
  • Maintenance of PLC must be scheduled with regular machines or equipment so that both equipment and controller can be down for a low time period.

Guidelines for preventive measures are shown below:

A.  Cleaning or Replacing a filter must be done as per the schedule that has been installed in the panel at a frequency of dust in that location this will ensure clean air circulation inside the panel having a controller.

B.  Do not allow any dust particles to get accumulated because the controller CPU and I/O’s are not designed to be dirt-proof. If nonconductive dust particles get accumulated on heat sinks and electronic circuitry it may clog heat dissipation that causes a malfunction in the circuit. If conductive dust particles get accumulated on electronic circuitry it causes a short circuit that result in permanent damage to the circuit board.

C.  Periodic Inspection of I/O module connections to ensure that all plugs, sockets, terminal strips, and modules have good connections and are securely installed this type of inspection is usually done for the controller installed in a vibrating area which could loosen terminal connections.

D.  Note that the programmable logic controller must be located far away from heavy or noise-generating equipment.

E.  Unnecessary items such as documents or drawing sheets, and installation manuals must be kept away from the equipment enclosure, on the top of the CPU rack, or enclosures that may block ventilation and create hot spots in the system. 

F.  If the controller is located in an environment that exhibits vibration, install a vibration detector/sensor that can interface with PLC as a preventive measure for a controller enclosure and can monitor higher levels of vibration, which causes the loosening of connections

2.  Spare parts:

  • Stocking the required spare parts on hand is a good idea to reduce the downtime or shut down of the process for minutes, instead of hours or days resulting from component failure or damage.
  • The main CPU component must be maintained with one spare each, despite of what number of CPUs used.
  • A power supply unit either main or auxiliary must have a backup. Certain applications require a complete CPU rack as a standby spare. Using of redundant controller protects the process automatically in case of hardware failure.
  • According to the Thumb rule “Stocking of at least 10 to 15% of spare units of total components used in the process must be in hand”

3.  Replacements of I/O modules:

  • If an I/O module needs to be replaced, the user must replace the correct module.
  • Most of the systems allow the replacement of the modules when powered up, but some systems require a power supply to be disconnected.
  • An operator must check for inductive loads if the failure reoccurs in a relatively short period even after the replacement of I/O modules.
  • The inductive loads may produce voltage and current spikes.
  • If the fuse blows again after the replacement of the module, the problem may be that the output current limit of the module is exceeded or the output device may be shorted.

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