Factory Integration and Control Systems

INDUSTRY TRENDS

Factory Integration Technologies - (Part 1)

Numerous systems and techniques have gained popularity over time to assist operators, supervisors and managers control and optimise the operations of their factories. Some of the strategies that have evolved include CIM (Computer Integrated Manufacturing); JIT (Just In Time); FMS (Flexible Manufacturing Systems); HMI (Human Machine Interfaces); MES (Manufacturing Execution System); MRP (Material Requirements Planning); MRP II (Manufacturing Resources Planning); ERP (Enterprise Resources Planning); and SCM (Supply Chain Management).

Most of these strategies exist as comprehensive software applications running on networks of computers - some with an array of supporting procedures. While they address different, but sometimes overlapping requirements, an attribute they share is that they integrate two or more traditionally independent functions of an organisation to improve efficiency, increase quality and reduce costs. For example, CIM integrates the product design and production planning functions with factory floor set-up, production control and monitoring. MRPII integrates order entry, purchasing, scheduling, warehouse and production systems.

The components of factory management systems generally fall into one of the following levels:

Control System
Supervisory Control
MIS (Management Information System)

The objective of this article is to introduce some of the technologies that are used to provide factory integration and show how they are being used to improve production efficiency. In Part 1 of this article, we'll introduce some of the integration technologies being used at the Control System level to achieve integration of control system devices with each other and with higher level systems. Part 2, in the next issue of CSE Connection, will continue with the integration technologies being used at the Supervisory Control and MIS levels of an organisation.

Control System Integration

Integration at the Control System level involves peer-to-peer transfer of information between equipment such as PLCs (Programmable Logic Controllers), CNC machine tools, robots and other controllers to allow efficient and reliable operation for process coordination, material handling and activity synchronisation. It also provides interfaces to allow integration of the Control System with the Supervisory Control and MIS levels.

MAP

One of the early developments to address integration at the Control System level was called MAP (Manufacturing Automation Protocol). It is an open communication standard that was initiated in 1980 by General Motors in the USA. At that stage, GM had over 40,000 intelligent devices on their factory floors with over 85% of them in isolated islands of automation. GM estimated that communication and integration costs accounted for up to 50% of Control System budgets.

MAP's strategy was to define one communication standard that all control systems vendors would comply with, addressing all aspects of communication including physical/electrical interfaces, network protocols, data format and syntax. Products began to appear in 1984 and a number of successful systems were installed - mainly in the automotive industry.

Despite its aim of providing a single, standard communication interface, MAP has failed to achieve wide spread popularity. It is quite complex and expensive and has suffered from a lack of support from vendors. One of the problems for an organisation wanting to adopt MAP is that there is no migration path for existing non-MAP equipment.

Fieldbus Networks

More recently, a number of new Fieldbus standards have been developed. These are high-speed networks specifically designed for control system applications and peer-to-peer communication at the Control System level. Some of these networks address specific control system requirements such as intrinsically safe systems for hazardous environments, bus-powered devices, fast machine control, and safety systems that require triple redundancy.

There are still integration problems if different standards are used by equipment within the same site, but many control system vendors support a number of these standards so there is a better chance of integrating equipment. An example of some of these standards are summarised below:

Ethernet

Faced with numerous proprietary and standard Fieldbus networks, many organisations are selecting Ethernet for a wide range of applications on the factory floor. There are a number of benefits in using Ethernet in areas beyond traditional business information networks. These include its low cost, high reliability, ease of configuration and management and easy connectivity to company intranets or the Internet.

This trend is also encouraged by some users' concerns that existing Fieldbus networks may not provide enough bandwidth for current and future requirements. For example, the data transfer rate of the existing H2 Foundation Fieldbus specification is 2.5 megabits per second (Mbps). ControlNet is 5 Mbps and Profibus has a maximum transfer rate of 12 Mbps. As a minimum, Ethernet has a transfer rate of 10 Mbps, with 100 Mbps Fast Ethernet rapidly becoming a de facto standard and the Gigabit Ethernet specification to be released in 1998.

Unlike the Fieldbus networks, there is currently no standard "user layer" and Ethernet itself doesn't define the format or interpretation of data transmitted. Therefore, it is generally not possible to achieve peer-to-peer communication between different vendors' equipment by simply connecting them to an Ethernet network. However, Ethernet's ability to easily communicate with multiple devices and manage the traffic between control systems and computers make it attractive to integrate the Control System with the Supervisory and MIS levels of an organisation.

There are widespread concerns about Ethernet's lack of determinism (its ability to deliver critical data within a given time) and limited redundancy that may prevent its widespread use for peer-to-peer communication at the Control System level. These issues are now being addressed by the IEEE 802.3 Standards Committees with new specifications for Redundant Links and Message Prioritisation being released in 1997 and 1998 respectively.

OPC

To address the integration of control system devices with high level systems, a standard called OPC (OLE - Object Linking and Embedding - for Process Control), has been under development since 1995. Now under the control of the OPC Foundation, the objective of OPC is to define an open, flexible, plug-and-play standard for the communication interface to control system devices. Based on Microsoft's OLE and COM (Component Object Model) technologies, OPC consists of a standard set of interfaces, properties and methods for use in process control and manufacturing automation applications.

The benefit of OPC is that the 'client' system, whether it is an HMI system in the control room or a reporting application on a manager's PC, only needs to understand one interface to get data from any device. Previously, proprietary interfaces and applications were required for each vendor's equipment. OPC exposes the data from any control system in the same way. So any OPC client application can be connected to any vendor's OPC-compliant server in the same way and expect the same behaviour from the server. Where Ethernet, for example, standardises the communication channel between devices, OPC standardises the interface presented by each device.

At this stage, OPC servers take the form of software components running on Microsoft platforms that present the standard OPC interface to applications and communicate with attached control system devices in their own proprietary communication standard. With the reliance on Microsoft's OLE and COM technologies, an obvious limitation of OPC is multi-vendor support in non-Microsoft computers at the management level. In these cases, an option is to use a Microsoft Windows PC to provide a gateway to the factory floor for non-Microsoft environments.

Factory Window

Factory Window is an integration technology for flexible manufacturing systems developed by CSE Technology. It is a software based communication tool which allows a wide range of devices used in factories to communicate with each other and with application software running in host computers. Typically, this equipment consists of CNC machines, robots, AGVs, automated warehouses, PLCs and other intelligent controllers.

Factory Window uses software called Drivers to communicate with each Control System in its own native protocol. The Driver translates the messages from the Control System's protocol to a common protocol based on the MMS standard, as used by MAP. This means that the Drivers make the interface to all Control Systems appear the same - as if all Control Systems were using MMS based standards for communication. This allows peer-to-peer communication support in Factory Window, as well as vertical communication to application software running in the same computer, or on other computers connected to a TCP/IP network.

Factory Window runs on PCs, workstations and other host computers, allowing normally incompatible systems to be seamlessly integrated.

Part 2 of this article introduces some of the integration technologies used at the Supervisory Control level of an organisation. Part 3 concludes with the integration technologies used at the MIS level.