PLC Maintenance Management

Today PLC (Programmable Logic Controller) is a common equipment in most factories. This tiny computer is very popular because of its flexibility and reliability. The PLC replaces thousands of relays that were in older electrical panels, and allows the maintenance technician to change the way a machine works without having to do any wiring. The program is typically in ladder logic, which is similar to the wiring schematics maintenance electricians are already accustomed to working with. Inputs to a PLC can be sensors, bar codes, switches, machine operator data, etc. Outputs from the PLC can be motors, air solenoids, indicator lights, etc. Therefore, most machines today use PLC to control and to troubleshoot the machine.

Since PLC plays vital roles in most machines in your factory today, if anything went wrong with the PLC , the whole plant or some production lines will stop and you lost a lot of money during this downtimes.

To prevent this ,the PLC itself also need maintenance. The first step to take is to perform a PLC audit. You have to open every electrical panel, write down the PLC brand, model, and other information. Then analyze the audit information and risk, and act on that analysis. Once you have collected the basic information in your Plant wide audit, you need to analyze the information to develop an action plan based on risk analysis. In the risk analysis, bottlenecks and other factors will help you assess priorities. Starting with the highest priority PLC, you will need to ask more important questions. For example:

- Do we have the most common spares for the PLC?
- Is the vendor available 24/7? Or even in business any more?
- Do we have a back up copy of the PLC program?
- Does our program copy have descriptions so we can work with it reliably and efficiently?
- Do we have the software needed to view the PLC program?
- Are our maintenance personnel trained on that PLC brand?
- Do we have at least one trained person per shift to maintain and troubleshoot our PLCs?
- How to train our maintenance people to use that PLC?
- Is there any self-study PLC training kit from the vendor?

Don't for get to write down the corporate policy and procedures for the PLC. if policy and procedures are not written and enforced, we eventually stray back to the old unreliable ways.

Following are some examples:
1. All personnel working with PLCs will be trained on that PLC equipment.
2. Backup copies of the PLC programs will be made every 3 months regardless of change status.
3. If a PLC program has been changed or updated
- It will be documented in the software electronics copy ,printed copy and in the Maintenance program (CMMS).
- All programs must be stored in high reliable data storage media.
- Multiple copies will be stored on maintenance room, maintenance manager's office and HQ.
- All EEPROM must be updated to current version.
- etc.
4. Future PLC purchases
- If possible , try to setup common brand in all equipment (Standardization of PLC types) Omron , Servo , Mitsubishi, SIEMENS
- Looking for the model which has backup EEPROM option for zero downtime in some failure modes and have line filter on all 110v control voltage cable.
- All manual must be customer's native language.

5. Don't try to force inputs and outputs on or off. This type of action must be treated as a Safety issue. If not avoidable, they shall be removed with in 24 or 48 hours with more permanent solution found. All forces should be documented in software and a written log before being enabled.

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