Some principles of measurement and control
John Flavell Coales · Journal of Scientific Instruments · 1956
Measurements can be divided into two broad classes: those which only have to be observed and recorded and those which are used to control sources of power in order to change some quantity such as position, speed, temperature or pressure. In both cases the principles are the same and there are certain limitations from which all measuring instruments suffer, e.g. they all disturb the physical system on which the measurements are being made and they all have inertia and damping to some degree. Also, no measuring instrument is completely stable and all exhibit unwanted fluctuations if the sensitivity is increased to the limit of usefulness. In the paper these limitations are discussed together with the principles underlying the design of measuring systems to reduce their effect to a minimum. The simple theory of linear control systems is developed and it is shown that the only real difference between measuring systems and control systems lies in the magnitude of the power required at the output. Negative feedback in a closed-loop system is shown to be the equivalent of the restoring force in an open-loop system.