MES Systems: The Missing Link Between ERP and the Factory Floor

What Is a MES System?

A Manufacturing Execution System (MES) is a software layer that bridges the gap between production planning at the ERP level and actual control on the factory floor. MES tracks every production operation from the moment a work order begins until the finished product is delivered.

Why Do You Need MES?

Without MES, factories face a critical information gap:

The production manager cannot see order status in real time
Quality data is recorded on paper and gets lost or delayed
There is no clear visibility into causes of downtime and waste
ERP planning relies on estimates instead of actual data

MES closes this gap by providing a live and accurate view of everything happening in the factory.

The Three Layers: ERP, MES, and Control

The information architecture of a modern factory consists of three integrated layers:

Top Layer: ERP (e.g., SAP, Oracle)

Resource and budget planning
Inventory and procurement management
Long-term scheduling (weeks and months)

Middle Layer: MES

Execution of production orders
Real-time material and quality tracking
Short-term scheduling (hours and days)

Bottom Layer: Control (PLC, SCADA)

Direct control of machines and motors
Data collection from sensors
Immediate execution (milliseconds)

Integration between these layers ensures that planning decisions translate into actions on the factory floor, and that actual data flows back to improve future planning.

Core MES Functions

A comprehensive MES covers 11 core functions according to the ISA-95 standard:

Production Order Management: Receiving orders from ERP and distributing them to production lines
Operations Scheduling: Determining the sequence and timing of each operation on each machine
Resource Management: Tracking availability of machines, workers, and tools
Product Tracking: Knowing the location of every part at every production stage
Quality Management: Recording inspection results and linking them to production batches
Maintenance Management: Scheduling preventive maintenance and logging breakdowns
Data Collection: Automatic recording of machine and process data
Performance Analysis: Calculating indicators such as OEE and productivity
Document Management: Providing work instructions and drawings electronically
Labor Management: Tracking worker skills and task assignments
Track and Trace: Linking every product to the raw materials and processes it went through

ISA-95 Standard: Structuring Integration

ISA-95 is the international standard that defines how data is exchanged between business systems (ERP) and control systems (PLC/SCADA) through MES.

The Five Levels of ISA-95

Level 0: The physical process (motors, valves, sensors)
Level 1: Direct control (PLCs, controllers)
Level 2: Supervision and monitoring (SCADA, HMI)
Level 3: Manufacturing operations management (MES)
Level 4: Business planning (ERP)

Data Models

ISA-95 defines standardized data models for information exchange:

Product Model: What we are making and how
Resource Model: What is available (machines, workers, materials)
Schedule Model: When to execute each operation
Performance Model: What actually happened compared to the plan

OEE: Overall Equipment Effectiveness

OEE is the gold standard metric for measuring production line productivity. It combines three factors into a single number:

The Formula

OEE = Availability x Performance x Quality

Availability

The ratio of actual running time to planned production time:

Availability = (Planned Time - Downtime) / Planned Time

Performance

The ratio of actual speed to maximum speed:

Performance = (Parts Produced x Ideal Cycle Time) / Running Time

Quality

The ratio of good parts to total parts produced:

Quality = Good Parts / Total Parts

OEE Benchmarks

Below 65%: Needs fundamental improvement
65-75%: Average
75-85%: Good
Above 85%: World-class performance

Practical Example: Tracking a Production Order From Start to Finish

Let us follow the journey of a production order for 500 parts through a smart factory:

Phase 1: Order Receipt

ERP sends a production order to MES containing: the product, quantity, deadline, and required materials.

Phase 2: Scheduling

MES calculates the optimal distribution of the order across available machines while considering other active orders.

Phase 3: Execution

The work order appears on the operator's screen with detailed instructions
The system automatically records the start and end of each operation
Quality data is collected at every inspection point

Phase 4: Live Monitoring

A dashboard displays: parts produced, completion percentage, current OEE
Instant alerts when downtime occurs or quality deviates

Phase 5: Closure and Reporting

MES sends the actual production report back to ERP
A complete record is saved for every part (materials, machines, workers, inspection results)

Summary

MES is the beating heart of the digital factory, connecting ERP planning with factory floor execution. It provides a live view of every production operation, from receiving the work order to delivering the product. The ISA-95 standard organizes integration between different layers, and the OEE metric offers a clear picture of equipment efficiency. Start by tracking OEE manually for a single line, then gradually transition to a comprehensive MES system.