Closed Loop Digital and Computer Control of an Ultragravimetric Microbalance

摘要

This report summarizes the development of a control system for anultragravimetric microbalance. Included are the different control methodologies investigated and the characteristics of each. These methodologies are based on two types of control systems: one is a digital system implemented totally in hardware and the other a computer controlled system. The intent of the project was to replace the existing analog controller with a system that was more stable, had better response to transient conditions, was less temperature sensitive and could be operated unattended for long periods of time. Much of the work to date, on the computer controlled system, has concentrated on bringing the balance to equilibrium quickly and reliably, maintaining equilibrium once achieved and reliably tracking mass change. Data analysis and reduction techniques are still being developed and evaluated. As yet, there is no indication of the ultimate sensitivity obtainable with a computer controlled system, but a sensitivity of 0.2 microgram, with a sample mass of 5 grams, should be rountinely obtainable. In addition to the electrical/electronic characteristics, there is some discussion of the mechanical characteristics that affect the performance of the balance. But because of time constraints, the mechanical aspects are not discussed in detail. An adsorption/desorption experiment, on a devitrifying, microcircuit package, sealing glass (type 75-83), is presented to demonstrate the use of the system and illustrate the mass tracking behavior. (KR)