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

Control Technology, Microelectronics and Nanotechnologydesigned around a magnetic drum with 8064 words of 20-bit length. Theexternal input/output supported 540 1-bit digital inputs. The computerwas designed to interface directly with existing analog control systems.The system acted as an enhancement to the existing control instrumentationand it functioned as a supervisory control device.The software program consisted of models of the process. Thesewere usually simple linear programming algorithms. The models providednew sets of operating conditions (setpoints) which could improveperformance. The process was moved from one set of steady-state conditionsto another in response to changes in uncontrolled disturbancessuch as ambient conditions.Within a few years, other companies introduced their versions ofcontrol computers. These companies included IBM, General Electric,Control Data Corporation and Foxboro. The initial improvements involvedincreased memory size. The use of magnetic core memory increasedthe computational speed from milliseconds to microseconds. Amajor step came with the replacement of solid-state circuitry (diodes,resistors, and transistors) with integrated circuits. This technology providedincreased speed and capacity, lower power requirements, smallersize and improved reliability.An evolution in control computers came with the development ofdirect digital control (DDC). Here, the function of the analog instrumentationwas incorporated in the computer and the analog controllers wereeliminated. The goal was to reduce the control system cost and increasedesign flexibility for new applications.Initially, these computers and especially the software, which hadbecome complicated, were not sufficiently reliable. In many cases, whenthe computer went down, control was lost completely.When minicomputers replaced these older computers, it was feasibleto provide redundancy in the form of dual computers. This solvedthe reliability problem.DCSIn 1976 when Honeywell announced the first distributed digitalcontrol system (DCS), the system was based on redundancy using microprocessor-basedcontrollers, redundancy in communications and redundancyin operator interfaces.Most large-scale control applications now use microcomputer-©2001 by The Fairmont Press, Inc. All rights reserved.