Soft Coupling - MES in Vehicle Manufacturing

Working as execution system, MES needs to integrate with lineside devices and control systems intensely, which is quite different than other IT systems.

For example, ERP releases orders with unit of day, manages warehouse with unit of day.

But for MES, in situations of working with lineside devices, normally it’s required to work with unit of second.

So MES requires very high availability.

And on the other hand, compared to PLC, MES is not so stable. So MES users will suspect MES availability. So users require contingency plan of MES downtime.

The so-called soft coupling design is used to suit such cases.

Soft coupling is the opposite of hard coupling.

Hard coupling means both sides of an integration should always be active at same time, 1 side failure will cause the other side failure as well. Examples are Cummins NGMES, Ford NGAVS, Volvo Andon.

As for soft coupling, while one side fails, won’t affect work of the other side.

Here’s some examples.

 

Case 1: cache data with barcode labels.

Challenge: one vehicle final assembly shop, manufactures passenger car, SUV, MPV, with 51 jobs per hour, how to ensure devices error proofing?

Solution: Print out error proofing barcode labels in assembly sheet.

When vehicle leaves PBS Out, MES prints out assembly sheet of it, with error proofing barcode labels of all devices. Each label includes all necessary information such as model type, process parameters for one device at one station. MES prints out these labels based on process configuration, this step doesn’t need to interact with PLC.

When vehicle comes to error proofing station, operator scans error proofing barcode label, then device PLC reads label and recognize character string from it, and then splits string and decodes error proofing command. This step is done by PLC based on configuration, doesn’t need to interact with MES.

 

Case 2: cache data with RFID Tag.

Challenge: one engine assembly shop, manufactures many different types at same day, and processes are modified quite often, so MES is required to suit the process changes, and should have minimum impact to lineside production.

Solution: store process data in big storage RFID Tag.

Firstly, design a process management module in MES, so users can define and modify process of each model type.

Secondly, when engine lifts on, MES queries process configuration of the engine based on its model type, and get process parameters of each station, and transfers to PLC, and then PLC writes data into 64KB RFID Tag.

Thirdly, when engine comes to assembly station, PLC reads station’s process parameters via RFID Tag, and based on that, handles logic of assembly/torque/test accordingly. When assembly is completed, PLC writes traceability data into RFID Tag.

At last, when engine lifts off, PLC sends all additional data back to MES.

We can see that, PLC only interacts with MES in real time at Lift On and Lift Off stations; for most assembly stations, the operations are handled by PLC itself.

Besides that, because the processes are configured in MES, the configuration data are stored in MES, so we can make changes easily with help of MES User Interface.

Diagram 3.7-1 shows related process:

Diagram 3.7-1: cache data with RFID Tag

 

Case 3: cache data in IT PLC.

Challenge: one vehicle scheduling system, can download work orders into conveyor line PLCs, and collects vehicle travel records from line PLCs. Now it’s required to have data buffer of 2 hours, so if MES or IT network is not available in short time, it wouldn’t impact lineside operations.

Solution: cache data in IT PLC.

As lineside key device, IT PLC can communicate and exchange data with conveyor line PLCs in real time. In the same time, IT PLC can store data including the buffer data of work orders for next 2 hours and offline vehicle travel records of last 2 hours.

Before work starts, planners will freeze orders in MES, which triggers system to download orders (together with model type and other key vehicle data) to IT PLC, and then IT PLC buffers the data inside specific Data Blocks.

When work starts, conveyor line PLCs request to download orders from IT PLC, then IT PLC will search from local buffer and transfer to line PLCs; when IT PLC finds its buffer is lower than safety number, then it will request MES to download new orders. When MES is not functioning for short time, if IT PLC and line PLCs are still working normally, then IT PLC can transfer its buffer orders, so the lineside operation can still continue without any impact.

At vehicle tracking stations, line PLCs collect vehicle information from RFID Tag, and upload to IT PLC. IT PLC checks if MES is online, if so then upload data to MES, otherwise buffer data in local Data Blocks. When MES is back to online, IT PLC will upload local buffered vehicle travel record back to MES.