Introduction of Kepware OPC (MES in vehicle manufacturing)

This article will introduce most popular OPC product: Kepware OPC Server and its plug-ins.

 

1.    What’s OPC

OPC is short of OLE for Process Control.

While OLE is short of Object Linking and Embedding.

An example of OLE is to insert an Excel table to Word.

 

Diagram 3.5-1: Function of OPC

 

From diagram 3.5-1, we can see that the key function of OPC is: to convert address of PLC Data Block into Memory Tag, which can be read/write by MES via OPC client.

In best practice, if PLC needs to talk to upper systems, normally it will store data into specific interface Data Block, which can be used by upper system easily:

 

Tag	Data Type	Address
HB_FLAG	Bool	DB1010.DBX0.0
DATA_READY	Bool	DB1010.DBX0.1
STATON_NUMBER	Int	DB1010.DBD2
SN	String[40]	DB1010.STRING4

 

But these address can not be accessed by MES directly.

After OPC mapping, these PLC Tags are similar to any other memory variables, and OPC server can read/write them directly.

And we can install OPC client in MES server, so that we can read/write these Tags easily.

Till now there’re 2 typical standard OPC protocols: OPC DA and OPC UA.

OPC DA is short of OPC Data Access, it’s based on Microsoft’s DCOM technology, it has been used for decades, it’s very popular, but it can only be installed in Windows platform. Due to limitation of DCOM, OPC DA has big security concerns, and it’s very complicated to configure settings.

OPC UA is short of OPC Unified Architecture, it can be used in Windows platform, and can also support Linux + Java environment, but it’s not used widely.

 

2.    DataLogger

DataLogger is a plug-in of Kepware, it can log OPC Tag’s values into Database automatically.

For the request of PLC data collection, this plug-in can match such request, it only needs some simple settings, doesn’t require any coding.

After data is logged into Database, we can write logic in Database Procedures or Application Layer, to handle the data accordingly.

For example, we need to run PMC in a Welding Shop, to collect equipment status data, then we can use OPC DA + DataLogger to collect data, then we write a Trigger and a Stored Procedure to handle the data.

 

3.    ODBC Driver

DataLogger can only write Database, cannot read data from it, so data can only transfer from PLC to MES.

But in some cases, PLC needs to read data from Database.

For example in Welding Shop, we need to download Orders and sync time to PLC, then we can use ODBC Driver to accomplish it.

ODBC Driver configures Database as a special device, uses ODBC engine to connect to Database and read/write data.

We can create 2 interface Table, one for storing Orders, one for time synchronization.

OPC server can read these 2 Tables, and then write data to PLC via OPC DA.

 

4.    Advanced Tags

Advanced Tags is a group of plug-ins, here we introduce the most popular one – Link Tag.

Link Tag can link 2 tags, so if 1^st tag changes its value, then 2^nd tag will change automatically.

Here we introduce 2 use cases.

Use case 1: PLC communication. We do assembly in area A, and do checking in area B. If we failed checking in area B, then Andon system needs to notify area A PLC. Considering that the distance between 2 areas is quite long, and there’s not interaction between 2 processes. So we don’t need to add any additional hardware such as Coupler, instead we can use OPC DA + Link Tag to communicate.

Use case 2: PLC updates data to MES. DataLogger can write Database with limitation: it can only insert data. But ODBC Driver can update Table. So we can use OPC DA to monitor PLC Tags, and with help of Link Tag and ODBC Driver, we can map them into data which is stored in Database.

 

5.    IoT Gateway

IoT Gateway is short of Internet of Things Gateway.

Kepware provides 4 types of IoT Gateway:

-          MQTT client

-          REST client

-          REST server

-          ThingWorx Gateway

The first 3 types are quite simple to configure and use.

MQTT is short of Message Queuing Telemetry Transport, is a specific message queuing service used in long-distance and narrow-band network. For example in Machining Shop, network signal is not quite stable, in that case we can use MQTT to collect PLC data without upgrading network.

Rest is short of Representational State Transfer, is a widely used protocol which transfer data in real time and stable.

REST client can monitor PLC tag, once it changes, then it will call MES REST server, to push data to MES. For example as for vehicle travel record of AVI system, we can use REST client to upload data in real time.

REST Server provides a listener, which expose PLC tags, and MES can call commands via REST client. For example if MES needs to lock an Order, then it can write data to PLC via REST client.

As for ThingWorx Gateway, it should be configured with Thingworx platform together.