HVAC Control in the New Millennium.pdf - HVAC.Amickracing

HVAC Control in the New Millenniumcommon open protocol for industrial automation.Fieldbus devices and host control systems provide an alternative toanalog control. Fieldbus is an all-digital, serial, two-way communicationssystem to interconnect sensors, actuators, and controllers. It servesas a local-area network for instruments used in control and automationapplications. Fieldbus provides reduced infrastructure requirements especiallyin the area of field cabling. Each fieldbus segment is capable ofsupporting multiple devices. The expensive cable run between the fielddevicejunction box and the control system will be much smaller.Since Fieldbus can drop spurs along its length, this reduces thenumber of home run cables required. A digital signal can also be superimposedover an analog signal which carries the control variable measurement.Savings can also be achieved with fieldbus multivariable transmitters.These allow a single device to transmit more than one variable. Thesignal to be multiplexed can be temperature or pressure measurements.Fieldbus and digital signals also offer increased reliability. This increasedreliability comes from the use of the diagnostic capabilities of thesystem. The network transmits this information to the network, where itcan be used to monitor the health of the system.According to some studies, when a problem occurs, 2/3 of the repairtime is spent in diagnosis and only 1/3 goes to the actual repair orreplacement. Reduction of the time needed to diagnose the problem canhave a significant impact on maintenance costs.Foundation Fieldbus technology comprises the physical layer, thecommunication stack, and the user application. These components fit theOSI communication model. The physical layer allows low-speed devicesto run on twisted-pair cable.The link active scheduler (LAS) is in charge of access to the bus. Alink master is a device that can be an LAS. The LAS capability can betransferred to secondary link masters, providing redundancy for the system.Fieldbus defines an eighth layer, the user layer. The user layer definesblocks that represent the functions and data in a device.You can implement function blocks on the fieldbus for PID and PDfunctions which could each reside in separate devices, such as a transmitter,a loop controller, and a valve.In these systems, Fieldbus provides a type of distributed control, inwhich the control function exists in the field rather than in larger central©2001 by The Fairmont Press, Inc. All rights reserved.