Nominated Lecture: The Mechanics of Instrumentation

Abstract

In this lecture instrumentation will be interpreted as the ways and means to transport and modify signals so as to make them suitable for the input channels of human beings and automatic controllers. A difference may be made between measuring and control systems; the first present the value of the measured quantity in an appropriate way, the second also formulate and execute command signals. Modern control theory and techniques have an increasing influence on the arts of instrumentation. An important part of all instrumentation systems is the transducers, which convert the value of the signal to be measured to another physical dimension, more suited for transport or evaluation. Typical are the conversions to electric voltages, pneumatic or hydraulic pressures. The main requirements for the design of a transducer are a low energy consumption from the measuring source, fast response and low susceptibility to disturbing signals and noise. At the input there exist besides the value of the signal to be measured xm an associate signal ym, such that xmym has the dimension of energy W (watt). The generalized impedance Z is defined as Z = xm/ym, so that W = xm2/ Z. In general W should be small, requiring Z being large, sometimes approaching infinity. On the other hand, examples will be given where Z must have a certain specified value. A low value of W can be obtained either by using a very sensitive primary sensor and applying sufficient signal amplification afterwards or by using feedback techniques. Most of the primary sensors transfer the measured quantity into a displacement or elongation of the measuring element. A fast response requires small moving parts and a high stiffness. This lowers the sensitivity and efficiency of the signal conversion. Feedback techniques make it possible to increase the high-frequency response without undue miniaturization of the mechanical parts. The shape and the way of loading of the measuring elements have great influence on the efficiency of signal conversion and a figure of merit will be defined and applied to a number of common elements. An instrumentation system should be insensitive to disturbing signals and noise. This requires no, or a non-changing, interaction between adjacent components. Further the design should be such that such disturbances as temperature and barometric changes, accelerations, vibrations and gravity have no influence on the output. The first step is to make the design as insensitive as possible for these errors and thereafter by means of series—or parallel-compensation techniques—the remaining effects can be reduced to the required values.