Best Practices: Object-Oriented Technology Processes Potatoes

Few sexy buzzwords deliver what they promise, but a recent food processing application that went from drawing board to full production in eight months is helping the term "object-oriented technology" fulfill its early expectations.

In November 1995, Cavendish Farms selected Trihedral Engineering Ltd., located here, to design and install the control systems for a new potato products plant in Summerside, Prince Edward Island, Canada.

The system, for plant process and building services, involved 7,500 I/O points, 4,500 tags in 30 vendor-supplied control systems, 900 motors, and a daily throughput volume of 1 million pounds of products. The plant's equipment --package boilers, refrigeration compressors, steam packagers -- had standalone control systems from multiple vendors and a variety of communication protocols.

The task of integrating these diverse parts into a homogeneous plant system fell to three engineering groups; results had to be supported by one maintenance department, and had to be completed before deadline.

: An operator in the control room at Cavendish Farms' potato processing plant monitors some of the 1 million pounds of products handled each day.

Objectified solution

Components common to all systems were first "objectified" using Trihedral's WEB software. Human-machine interface and programmable logic controller components were represented by standard building blocks (objects) common to all systems. The components were then developed and tested independently, ranging from simple digital inputs and outputs to complex PID (proportional integral derivative) controllers and motors.

Having common building blocks allowed the plant's systems to share one foundation, and to be built efficiently in the same manner, independent of operations. The motor object (made up of 19 I/O points, representing such functions as speed setpoint, feedback, start/stop permissives and protection) was common to most systems, yet it obviously functioned quite differently in each area.

Within WEB, the Cavendish Farms "plantHELP" configuration resides in a SQL-compliant database that can be modified on-line using WEB tools, or off-line using an Excel spreadsheet. Properties entered into the spreadsheet would address all concerns in one place, such as I/O address, logging, alarming, trending, maintenance, and control.

For integration, engineers entered information as single line items in the spreadsheet tables as it pertained to a particular motor in the system. They were not bogged down by excess details about how to link the I/O address to trending and alarming or how to construct maintenance log information.

These functions occurred automatically because they were encapsulated within the motor object. Since the motor object was designed to be universal (polymorphic), it was the same object and had the same code, with only the properties being different. This greatly simplified maintenance, speeded development, and lead to a homogeneous plant system.

: A screen display for Cavendish Farms' dryer system is one of 30 processes at the facility using Trihedral's object-oriented WEB monitoring and control software.

Up and running

Now installed and running, Cavendish Farms' operators control the plant's system from a single interface in a central control room connected to 24 workstations with Microsoft Windows NT throughout the facility. Stations include 11 NEMA 4X-rated stations on the plant floor and four workstations in supervisor/manager offices. The supervisory operator in the control room, the operator on the plant floor, or the plant manager in the office all see the status of any system in the plant presented in the same graphical format.

The operator is able to click on the object, start or stop a motor, for example, and view any status or alarm information about that motor. Adding a new screen or a custom dialog box for a new piece of equipment can be done easily by the plant maintenance staff. The resulting system builds-in flexibility and maintainability from the start, without sacrificing speed, according to Trihedral officials

Copyright by CAHNERS PUBLISHING COMPANY.

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