Preventive Maintenance for Control Systems

Overview

Most automation failures are preventable. The components that run your production — PLCs, drives, robots, and panels — all have known failure modes with known prevention steps. This checklist gives your maintenance team a structured approach to catching problems before they stop production.

Daily Checks (Operator Level)

• HMI alarms — Any active or unacknowledged alarms? Log them even if production is running.
• Panel door — Closed and latched? Open doors let in dust, moisture, and critters.
• Indicator lights — Any red or amber lights on PLC modules, drives, or network switches?
• Unusual noise — Fans running rough, drives buzzing, contactors chattering?
• Panel temperature — Touch test (exterior). Noticeably hot = investigate airflow.
• Robot cycle time — Running within normal range? Gradual slowdowns indicate mechanical wear.

Monthly Checks (Maintenance Level)

Electrical

• Panel air filters — Clean or replace. Clogged filters are the #1 cause of drive overtemp faults.
• Fan operation — All fans spinning? Replace any that are sluggish or noisy.
• Panel interior — Visual inspection for dust buildup, discoloration, burnt smell, condensation.
• Terminal blocks — Spot-check tightness on power terminals (120V+). Loose connections cause arcing.
• Wire routing — Nothing chafing, pinched, or contacting hot surfaces.

PLC & I/O

• PLC battery — Check battery status LED or diagnostics. Replace proactively if low.
• Module LEDs — All green? Note any amber/red indicators.
• I/O fault log — Review diagnostic buffer for intermittent communication errors.
• Memory usage — Check processor memory utilization. Consistently above 80% is a risk.

Drives

• Drive fan — Running and clean? Internal fans are often overlooked.
• Heat sink — Blow out dust with dry compressed air (low pressure).
• Output current — Compare to baseline. Rising current at same speed = mechanical issue.
• Fault history — Review last 30 days. Patterns indicate developing problems.

Network

• Switch LEDs — All ports showing expected link/activity?
• Cable connections — Check for loose, damaged, or unlabeled cables.
• Error counters — If using managed switches, check CRC errors and packet drops.

Quarterly Checks (Technician Level)

Thermal Inspection

• Infrared scan — Scan all power connections, breakers, contactors, and terminal blocks.
• Document hot spots — Anything 20°F+ above ambient needs investigation.
• Compare to baseline — Keep thermal images on file for trend analysis.

PLC & Program

• Backup — Create full PLC backup and store offsite.
• Compare program — Verify running program matches your master copy. Unauthorized changes happen.
• Check scan time — Trending upward? Investigate what's consuming cycle time.
• Battery replacement — If battery is over 2 years old, replace proactively.

Robots

• Full backup — Image backup to USB, copy to network storage.
• Grease joints — Follow manufacturer schedule (FANUC: typically every 3-5 years).
• Mastering check — Verify mastering marks align. Drift means positions are off.
• Cable inspection — Check dress pack, teach pendant cable, encoder cables for wear.
• Brake test — Run brake check on each axis.

Safety Systems

• E-stop test — Press every E-stop, verify machine stops and requires proper reset.
• Gate switch test — Open every safety gate, verify response.
• Light curtain test — Use test rod at required intervals per manufacturer spec.
• Safety relay indicator — Verify status LEDs match expected state.
• Document results — Safety tests should be logged with date, tester, and pass/fail.

Annual Checks (Planned Shutdown)

• Full retorque — Retorque all power connections to manufacturer specification.
• Contactor inspection — Check contact wear on all power contactors. Replace if pitted.
• Replace air filters — Annual replacement is cheap insurance even if they look clean.
• Calibrate analog instruments — Temperature sensors, pressure transducers, flow meters.
• UPS battery test — Load test the battery. Replace if over 3 years old.
• Full program archive — Backup PLC, HMI, drive parameters, robot programs. Store offsite.
• Update documentation — Mark up drawings with any changes made during the year.
• PLC firmware review — Check if updates are available. Don't update during production.
• Network infrastructure — Check switch firmware, replace aging cables, verify VLAN config.

Building Your Schedule

Making It Stick

The best checklist is worthless if nobody fills it out: