Multisensor thiết bị đo đạc thiết kế 6o (P7)

MEASUREMENT AND CONTROL INSTRUMENTATION ERROR ANALYSIS Systems engineering considerations increasingly require that real-time I/O systems fully achieve necessary data accuracy without overdesign and its associated costs. In pursuit of those goals, this chapter assembles the error models derived in previous chapters for computer interfacing system functions into a unified instrumentation analysis suite, including the capability for evaluating alternate designs in overall system optimization. This is especially of value in high-performance applications for appraising alternative I/O products. ...
Nội dung trích xuất từ tài liệu:
Multisensor thiết bị đo đạc thiết kế 6o (P7) Multisensor Instrumentation 6 Design. By Patrick H. Garrett Copyright © 2002 by John Wiley & Sons, Inc. ISBNs: 0-471-20506-0 (Print); 0-471-22155-4 (Electronic)7MEASUREMENT ANDCONTROL INSTRUMENTATIONERROR ANALYSIS7-0 INTRODUCTIONSystems engineering considerations increasingly require that real-time I/O systemsfully achieve necessary data accuracy without overdesign and its associated costs.In pursuit of those goals, this chapter assembles the error models derived in previ-ous chapters for computer interfacing system functions into a unified instrumenta-tion analysis suite, including the capability for evaluating alternate designs in over-all system optimization. This is especially of value in high-performanceapplications for appraising alternative I/O products. The following sections describe a low data rate system for a digital controllerwhose evaluation includes the influence of closed-loop bandwidth on intersampleerror and on total instrumentation error. Video acquisition is then presented for ahigh data rate system example showing the relationship between data bandwidth,conversion rate, and display time constant on system performance. Finally, a high-end I/O system example combines premium performance signal conditioning withwide-range data converter devices to demonstrate the end-to-end optimization goalfor any system element of not exceeding 0.1%FS error contribution to the total in-strumentation error budget.7-1 LOW-DATA-RATE DIGITAL CONTROL INSTRUMENTATIONInternational competitiveness has prompted a renewed emphasis on the develop-ment of advanced manufacturing processes and associated control systems whosecomplexity challenge human abilities in their design. It is of interest that conven-tional PID controllers are beneficially employed in a majority of these systems at 147148 MEASUREMENT AND CONTROL INSTRUMENTATION ERROR ANALYSISthe process interface level to obtain industry standard functions useful for integrat-ing process operations, such as control tuning regimes and distributed communica-tions. In fact, for many applications, these controllers are deployed to acquireprocess measurements, absent control actuation, owing to the utility of their sensorsignal conditioning electronics. More significant is an illustration of how controlperformance is influenced by the controller instrumentation. Figure 7-1 illustrates a common digital controller instrumentation design. Forcontinuity, the thermocouple signal conditioning example of Figure 4-5 is em-ployed for the controller feedback electronics front end that acquires the sensedprocess temperature variable T, including determination of its error. Further, thetransfer function parameters described by equation (7-1) are for a generic dominantpole thermal process, also shown in Figure 7-1, that can be adapted to otherprocesses as required. When the process time constant 0 is known, equation (7-2)can be employed to evaluate the analytically significant closed-loop bandwidthBWCL –3 dB frequency response. Alternately, closed-loop bandwidth may be evalu-ated experimentally from equation (7-3) by plotting the controlled variable C risetime tr resulting from setpoint step excitation changes at R. 1 s KPKC 1 + + C 2 Is 2 D 0s = · (7-1) R 1 s 1 s 1 + KPKC 1 + + 1 + KPKC 1 + + 2 Is 2 D 2 Is 2 D 1 s 1 + KPKC 1 + + 2 Is 2 DBWCL = Hz dominant-pole closed-loop bandwidth 2 0 (7-2) 2.2 BWCL = Hz universal closed-loop bandwidth (7-3) 2 tr For simplicity of analysis, the product of combined controller, actuator, andprocess gains K is assumed to approximate unity, common for a conventionallytuned control loop, and an example one-second process time constant enables thechoice of an unconditionally stable controller sampling period T of 0.1 sec (fs = 10Hz) by the development of Figure 7-2. The denominator of the z-transformed trans-fer function defines the joint influence of K and T on its root solutions, and hencestability within the z-plane unit circle stability boundary. Inv ...