Producing over 31 000 biscuit products per year, Arnott’s Biscuits claims to be Australia’s largest biscuit manufacturer. The company wanted to implement an on-line batching system that would interface between its enterprise resource planning (ERP) system and plant floor systems. The goals of the project included implementation of a paperless system, elimination of manual data capture errors and streamlined operations.
Arnott's was looking for a single integrated control system which would interface its BPCS ERP software with its plant floor controllers, did not require programming skills and that would cater for an unlimited number of recipes. However, the company did not want to rely on PCs for process control.
As well as receiving shop orders from BPCS, Arnott's wanted the system to report back batch consumption and statistics from the plant floor to BPCS to integrate inventory control, accounting and quality control information.
Solution
Citect Professional Services worked alongside Arnott's engineering group to design, install and commission an entire control system from the sequence control and recipe handling to the ERP system interface. A manufacturing execution system (MES) was designed, which integrated the customer's BPCS, SQL Server database, CitectScada, Citect Plant2Business, Allen-Bradley PLC5s and ControlLogix PLCs, to provide a solution to the client's requirements. The system facilitated the flow of information from BPCS to the factory floor and back, without the need to re-enter data at any point. It provided for the fast and accurate flow of production planning information, the execution of production activities and the recording of materials used during manufacture.
The factory's Ethernet and AB Data Highway plus (DH+) networks link its scada stations with almost 100 individual Allen-Bradley PLC5 and SLC-500s, along with Arnott's BPCS AS400-based administration and warehousing computer system.
BPCS ERP and recipe database
The BPCS ERP system contains shop order requests, inventory control and accounting details. Shop order requests are transferred to the recipe database for production, while batch statistics are transferred from the recipe database to BPCS for inventory control and accounting purposes.
An SQL Server is used to store the recipes. Recipes include not only the batch parameters, but also the sequential control for the PLC program. The entire recipe is stored in tables which, once downloaded to the PLC, interface with a sequence engine to provide full sequential control of the process.
Recipes can be created and modified on-line via a 'Mixing Procedure Design Utility' which allows non-technical personnel to design, maintain and error check mixing recipes. As the recipes are created in a database and downloaded as required to the PLC, a potentially limitless number of recipes can exist for any mixer.
When shop order requests are received from BPCS, the recipe database associates the appropriate recipe with the shop order, and sends the request and recipe to the scada using Citect Plant2Business. On batch completion, statistics are uploaded from the scada back to BPCS.
CitectScada operator interface
The operator interface displays shop order requests and allows operators to select the most appropriate order for production. CitectScada then downloads the attached recipe to the PLC for production. The scada also provides the plant floor operator interface for normal HMI purposes. During production operators are prompted for, and are required to confirm manual operations at the scada terminal.
Improving plant performance
All plant floor control is performed by the PLCs. The recipe information downloaded to the PLCs includes the batch phase sequence and phases parameters. A table driven sequence engine was developed in the PLC ladder logic. Only the values in the tables that drive the PLC logic need to be changed to implement different mixing procedures for each product.
Multiple batches are possible in the sequence engine, enabling multiple runs of the same recipe sequence until the shop order is complete. To achieve this, the number of batches for a given recipe is calculated in the recipe database from the shop order request and downloaded with the recipe.
Batch statistics including detailed and accurate data such as temperature fluctuations are recorded through the sequence and reported to the scada system at the completion of a given batch for reporting to the BPCS. These statistics are compared to test results and product quality to improve product consistency and increase output.
Robustness
The design of the Arnott project caters for computer failure, since all sequence control associated with a current order is in the PLC. Should one of the subsystems, such as the recipe database or BPCS fail, the plant floor systems can continue operating until a change of recipes is required. Additionally, the Citect scada workstations offer multiple redundancy: operator intervention may occur at any station, removing any dependence on a single PC for system integrity.
Reliable transfer of information to and from the various devices throughout the plant is a vital part of the system. As such, application server redundancy and industry standard networking technologies were implemented.
The two application servers that run the scada application are each fully redundant (making four server computers in total) and are connected via the LAN. Switched Ethernet operating at 100 Mpbs at the switch connects all servers, hubs and routers. Hubs located in various areas throughout the plant are connected to the switch by fibre-optic cable. Industry standard protocols TCP/IP and IPX/SPX were used for the corporate system, recipe database, scada operator interface and PLC communications.
The commissioned system consisted of 25 CitectScada operator interfaces, four fully redundant CitectScada servers, one Windows 2000 file server, one Windows 2000 SQL server and 95 SLCs/PLCs.
For more information contact Niconette du Toit, Citect, +27 (0)11 699 6600, [email protected], www.citect.com
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