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Most Important Things to do when Performing PLC Maintenance

Programmable Logic Controllers (PLCs) are built to withstand harsh and extreme environmental conditions, as they form an integral part of many industrial applications. They are durable and well-suited for factory floor use. Nevertheless, it is important you keep the PLCs up and running at peak efficiency all day since they are critical to the smooth operation of your systems and processes. The best way to keep up with a PLC system is with the right maintenance plan.

PLCs are generally designed to be easy to maintain, which ensures trouble-free operation. Still, like any other asset, certain maintenance measures should be performed periodically to keep the PLC system in good operating condition, once it is in place and operational. This article outlines some common tasks included in a checklist of well-planned preventive maintenance, aimed at minimizing the chances of PLC system malfunctioning.

PLC System Preventive Maintenance

The purpose of Preventive maintenance for a PLC system is to significantly reduce the failure rate of the controller’s components. A typical maintenance strategy consists of a few basic procedures. It is recommended that you perform preventative maintenance for the PLC system with regular equipment or machine maintenance. This will ensure that the controller and the machine are down for a minimum amount of time, as most PLC systems control both mission-critical and safety-critical processes which can only be stopped for short periods of time.

Scheduling for PLC preventative maintenance depends on the controller’s operating environment- for harsher operating environments, more frequent maintenance routines will be required. Also, the exact preventive maintenance plan you follow will be unique to your PLC system and your facility requirements. Here are a number of tasks commonly required in most PLC preventive maintenance strategies.

Checklist for PLC Preventive Maintenance

A) PLC Program and Data Backups 

Every so often, up-to-date and routine backup of the PLC’s programing is essential. This is done by saving a copy of your PLC program in a flash drive or a compact disk. After backing up the PLC programs, be sure to keep them safely away from high humidity, extreme temperatures, and exposure to RFI and EMI. This ensures that they will remain usable.

Hence, if something goes wrong making the PLC inoperable and you have to replace it, then you won’t risk losing production time and profits when the system is being reprogrammed; since you will be in a position to easily download the backup files onto a new PLC. Also, backing up the PLC program ensures continuity if the PLC system goes offline in the event of a power outage and the current programming is lost. The backup files can be also be used to verify the correctness of your current program whenever operational problems are encountered. Moreover, as a maintenance procedure you must keep current versions of the PLC operating programs and project backups in case anything goes wrong during the preventative maintenance procedures.

B) Check Program Functionality 

A single software bug in the PLC program could result in damage to hardware equipment and even loss of human life, as the control system is one of the most critical components of any machine or process in operation. When carrying out PLC preventive maintenance, check the functionality of the operating program to ensure the equipment or process being controlled is functioning as required. This is done by comprehensively testing the PLC program for correctness in functionality, safety, reliability, and predictability.

As most PLCs are fitted with software-checking programs, testing the operating program is relatively easy. Since you already have the backup files, reload the program into the PLC system and run a software test to check the programmed logic. The PLC should provide you with a printout of the output and input points that were used in the most recent operation, along with timer settings, counter presets, and other PLC configurations as well as the errors it has encountered. The printout details will allow you to counter-check the program functionality and see if the PLC is operating as intended. Also, testing the PLC program helps check for incorrect I/O devices addresses.

C) Check the Power LED Indicator 

The indicator lights on the PLC determine whether the PLC is functioning properly. When you turn on the PLC power supply, the power LED indicator should turn on. However, if it remains off or it keeps flickering this could be a preliminary sign of potential power supply issues. There are also fault indicators, which light up for about 1 to 3 seconds depending on the complexity and length of the PLC program. They include a ‘green’ RTCError indicator, a ‘red’ Pause indicator, and a ‘red’ RunError. If the MX-RTC is not installed, then the RTCError LED will remain on.

D) Inspect Battery Status 

The BATT LED indicator on the CPU module is designed as an easy way to check the status of the battery that supports your RAM memory module, and to enable you determine if the battery is due for replacement. The status of this indicator shows whether the available battery voltage is sufficient to provide backup power for the EPROM stored in the PLC, in the event of a power failure or when the machine is switched off for maintenance.

If the BATT LED on the CPU module is red/yellow or flickering, then it’s time to replace your battery as this indicates that the battery voltage is below low threshold and cannot back up the EPROM. Note, if you’re to replace the PLC battery it must be replaced with the exact type of your current battery module in terms of operating voltage and current.

E) Check the Operating Voltage and Current 

The BATT LED indicator on the CPU module is designed as an easy way to check the status of the battery that supports your RAM memory module, and to enable you determine if the battery is due for replacement. The status of this indicator shows whether the available battery voltage is sufficient to provide backup power for the EPROM stored in the PLC, in the event of a power failure or when the machine is swaAs part of regular maintenance practice, inspect your PLC power sources for any power spikes or shorts. Using a Voltmeter or a Multimeter verify that the AC input voltage to the PLC power supply matches the PLC voltage setting. PLC processors operate on DC power, so you should check the output voltage of the PLC power supply to ensure that it is within the appropriate range. Such power supplies are available in different power ratings and sizes, depending on the make of the PLC. The most common output voltage in majority of the PLC power supplies is 24 Volts DC. Also, check if the voltage setting of the PLC power supply conforms to the voltage ratings of the connected input and output devices.

In addition, check for AC ripples in the DC supplies using a digital meter set on a low AC voltage range. The measured AC voltage value should be fairly below the manufacturer’s specifications. As excess AC ripples have drastic effects on the operation of PLC processor components such as memory devices and microprocessors. The final voltage check is to measure the voltage of the PLC battery to make sure it is within the recommended values.

It is also important you measure the operating current of the PLC system, as the current rating directly affects how much work the PLC system can accomplish. Smaller PLCs have current ratings ranging between 2A(Amps) to 10A, while larger and more powerful PLC controllers can have current ratings of up to 50A. Using a digital Ammeter or Multimeter check the output and input current of the PLC power supply, to verify that it conforms with the reference current. itched off for maintenance. If the BATT LED on the CPU module is red/yellow or flickering, then it’s time to replace your battery as this indicates that the battery voltage is below low threshold and cannot back up the EPROM. Note, if you’re to replace the PLC battery it must be replaced with the exact type of your current battery module in terms of operating voltage and current.

F) Check Connections 

Periodically check all connections to the PLC system such as communication cables, terminal strips, sockets, plugs, field wiring, and I/O modules to ensure that they are connected properly. Loose connections could result in improper functioning of the PLC system and may as well lead to the damage of its components. For the I/O modules ensure that they are installed securely, and check for tightness in all the module connections. Also, inspect all connections to the field Input/Output devices and adjust them accordingly.

In addition, check that the PLC system itself is installed securely. Tighten any loose bolts, sockets, or screws to keep the system from rattling. This check should be performed more frequently if the PLC system is used in high-vibration applications. Constant vibrations are likely to periodically loosen terminal connections. It is therefore advisable that you interface a vibration detector with your PLC system as a preventive measure. This way the PLC will monitor high vibration levels in the location, which can cause loosening of terminal connections.

G) Inspect the Overall Condition of the PLC Components 

Visually inspect the CPU and the other PLC modules for wear, warping, distortion, or indications of overheated components like burnt odors or discoloration.

H) Check Operating Environmental Conditions 

Environmental factors like high humidity and extreme temperatures can negatively affect the PLC components. Be sure to check that these factors fall within safe ranges, for the PLC to operate properly. You could consider investing in multiple sensors which can monitor the environmental factors all day through and send you alerts if the safe ranges are exceeded. Also, ensure that heavy and noise/heat-generating equipment is not located too close to the PLC system.

I) Inspect the Cleanliness of your PLC Module

  • Dust Removal: First, ensure that no dirt or dust accumulates on the PLC components including its I/O modules, as the CPU unit and the I/O system are not designed to be dustproof. Building up dust on electronic circuitry and heat sinks could obstruct heat dissipation, causing the system to overheat. Also, if conductive dust gets to the PLC’s electronic boards, circuit malfunctions like short circuits are likely to occur. This may permanently damage the circuit board. The PLC system and its I/O modules should therefore be kept clean for optimal functionality. This can be achieved by cleaning off the dust on a regular basis. Also, it could be a good idea to install the PLCs in low-dust enclosures, but care should be taken to ensure that cooling vents are not blocked or clogged by dust. 
  • Clean and Change Filters: Regularly clean or replace any ventilation filters installed in the PLC enclosures. This helps in preventing dust buildup and ensures the presence of clear air circulation, making your system more reliable. Before committing to an exact frequency of replacing the ventilation filters, you should carry out routine checks to determine how quickly the filters clog up depending on the amount of dust generated in your facility.  
  • Maintain Ventilation: Along with regularly changing the ventilation filters, be sure to keep items such as installation manuals, tools, drawings, papers, or other materials away from the PLC system. Leaving these items on top of the unit, particularly the CPU rack or inside the enclosure can block airflow and create hot spots; ultimately causing the PLC system to malfunction. Hence, providing sufficient space for heat dissipation within the PLC enclosure is very essential. 
  • Insulation Against Airborne Debris: Even after keeping your factory as clean as possible, it might be difficult to keep out floating debris during regular machine operation. These debris are likely to systematically degrade the PLC components, especially if they are either conductive or corrosive contaminants. Try as much as possible to keep such potential airborne contaminants from your PLC by performing regular visual inspections for signs like black dust. Also, when necessary, you can use canned air to blow airborne debris from the vicinity of your PLC system, this will lower the likelihood of contamination. 

J) Audit the PLC Systems in Use 

It is important that you keep an up-to-date inventory of your PLCs. Often industries may be unaware of the number of PLCs they have in operation on their factory floor, hence, it is important you take time to check around and take the count of all of them. When doing so, be sure to record their details, such as their brand/make, type of programming, when last was their program backed up, and the machines they control. Such that in the following routine maintenances, you will only be conducting an audit to inspect each of the PLCs’ components including the I/O modules; to determine if they are in good operating conditions or they require replacement.

K) Keep Enough Spares On-Hand 

Any preventive maintenance plan including PLC maintenance requires that you stock a full set of replacement or spare parts. This practice will ensure that you have enough spares to handle your maintenance needs, ultimately minimizing downtime. Having the right spare part in stock means that you can replace a faulty component in a matter of a few minutes, instead of hours or days. It is recommended that you keep a spare for each of the components of the PLC’s main CPU board, regardless of how many CPU modules are in use. Also, each PLC power supply should have a backup. Record the location of each of the replacement parts for faster access when needed.

Generally, the amount of stocked spare parts should be 10% of the number of parts in use. But if a given PLC component is being used infrequently, you can then stock less than 10% spare parts for that particular component. However, there are some PLC applications that could require that you keep a complete CPU rack to act as a standby spare. This occurs in cases where a downed PLC system must be put into operation immediately, leaving no time to sort out the CPU boards.

L) Identify Updates Needed 

Check for any product notices, patches, recalls, or required upgrades for any of the PLC components and update accordingly.

M) Calibrate Analog Components

You should calibrate the circuit cards dealing with process control analogs every 6 months. While other devices like sensors should be serviced on a monthly basis. Additionally, PLC manufacturers provide a maintenance schedule for analog input devices. Be sure to follow this schedule for any of your PLC analog input devices. Note, analog inputs must be calibrated accurately and they should be cleaned regularly, as they are likely to encounter operational problems more often.

N) Check the Presence of Potential EMI or RFI Sources 

Radio-frequency interference (RFI) or electromagnetic interference (EMI) are major causes of erratic PLC processor issues, without a clear indication of the specific issue. Ideally, during preventive maintenance, you should perform an audit of the local wiring to identify potential sources of EMI that could disrupt the operation of your PLC system.

Also, ensure that the design of your PLC wiring takes into consideration the fact that high-current wires severely interfere with low-level components. In addition, handheld radios mainly used by maintenance personnel emit powerful Radiofrequency (RF) radiation which can disrupt the operation of the PLC if it’s unprotected. Long-term solutions to RFI and EMI problems normally involve improvements in power shielding, conditioning, and grounding.

How Frequently Should I Maintain My PLC System? 

The frequency of PLC preventive maintenance depends on the controller’s application and the nature of its operating environment. If the PLC system is used in extreme environments and high-vibration applications, then some maintenance tasks will need to be performed more frequently. Other tasks like dusting or cleaning ventilation filters are best performed daily to keep the PLC system in good working condition. Though some tasks like backing up the PLC’s programming are less independent of the working conditions and are often done on a semi-annual basis.

Lastly, before you schedule any PLC preventive maintenance, you should carry out some routine inspections to determine how frequently the terminal connections loosen or how often the parts wear out. Using this information, you can then set optimum intervals that are likely to keep your PLC system in good condition without having to perform preventive maintenance too often. Maintenance tasks can be tedious and time-consuming, even though they help prolong the service life of your PLC system and can save you lots of downtime costs and repair bills. For more information or to discuss which equipment might be best for your application, please visit our website here, or contact us at sales@pdfsupply.com or 1-919-535-3180. 

This entry was posted on October 4th, 2021 and is filed under PLC, Uncategorized. Both comments and pings are currently closed.

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