Kraft Foods (Beaver Dam, WI) wanted to integrate HART capability into its analog acquisition system to measure volume before and after a solids separation system—similar to a large centrifuge—in its dairy processing facility. Kraft didn't want a separate HART data collection system running in tandem with the primary analog acquisition system. Setting up both adds cost with additional hardware and cabling requirements.

Engineers avoided duality using Spectrum Controls' HART-enabled I/O modules for the Allen-Bradley ControlLogix PLC that incorporate analog input and output capability as well as the ability to read and write HART data to field instruments over the existing wiring system. This eliminates the need for external hardware and provides full access to HART data via the control system network.

Kraft has used HART primarily for maintenance and commissioning of its systems for six years, according to Steven Stippich, plant engineer. Normally, Kraft only had access to this data if it was deemed critical. Kraft would then implement a multiplexer system and interface it to the PLC using a communications adapter. "HART I/O provides a seamless interface to our HART-enabled instruments when compared to other PLC interface solutions," Stippich says.

Stippich claims cost savings associated with HART I/O are dramatic. A typical multiplexer installation requires additional wiring and panel space to accommodate connections to the multiplexer and the bridge wiring between the multiplexer and analog I/O. A communications bridge module is necessary to tie the multiplexer data into the PLC backplane. "[Using HART I/O], installation is as simple as installing a standard analog module," Stippich says. "It does not require extra panel space or wiring."

Savings per HART I/O point is more than 600% and is similar to standard analog acquisition costs; $131 compared to $818 per point for one multiplexer configuration.

I/O devices measure the traditional analog current loop value and polls the flowmeter (Kraft uses an Endress+Hauser Promass 63 mass flowmeter), using HART communications, and provides variables including flow rate, density, temperature, and totalized volume. These measurements can then be used to calculate the efficiency of the separator.

An added benefit to the system is that the HART data are now available to the operator from a workstation via the PLC's Ethernet network. Plant asset management software monitors device health and troubleshooting may be performed real-time without leaving the operator workstation. HART-enabled I/O has allowed Kraft a simple, cost-effective, and flexible solution to control and monitor its processes.

Integrated HART I/O for programmable controllers is a logical evolution of the technology. HART protocol originated in the late 1980s and remains a key tool for improving plant operations. Traditional HART applications of the protocol have included plant asset management, commissioning, periodic maintenance activities, and large-scale distributed control systems. Learn more about HART at www.hartcomm.org.

For more information, enter 500 at www.controleng.com/freeinfo or visit www.spectrumcontrols.com