Automation of a capacitance dilatometer using distributed control
A comprehensive and novel system of automatic control has been designed and constructed using distributed intelligence techniques to control a complex item of equipment for the measurement of linear thermal expansion over the temperature range 1.5K - 300K. The system is designed to perform its own self-calibration automatically before the experiment commences. The low temperature dllatometer developed for this research project incorporates the most sensitive length change sensor available; a three-terminal capacitance transducer. This transducer technique has been refined to resolve length changes of 10-5A and more importantly, measure length against temperature profiles to better than 10-2A. The period of time required to collect a full set of data measurements on any particular specimen was in excess of 100 hours. Forming the heart of the automated control system is an assembly of Intel MCS-Sl single chip microcontrollers connected together on a serial link consisting of a simple pair of wires. The approach has been to divide the system automation into a number of specific control tasks and to allocate a different task to each controller. The Instrument has been used as a diagnostic tool to investigate the properties of lead glasses, and in particular to study the possibility of negative thermal expansion existing at low temperatures. Thermal expansion measurements were also performed to observe the phase change within single crystals of polydiacetylene and to research into low temperature phenomena occurlng within the crystal.