Fine digital temperature control for metrological applications

摘要

Fine regulation and control of temperature is of fundamental importance in all fields of metrology and in particular in dimensional metrology. Examples are the regulation of the frequency of laser sources or the dimensional stabilization of reference cavities. This paper is dedicated to a small but significant experiment in digital control suggested by the Length Division of the "G. Colonnetti" Metrology Institute. In order to better tune the frequency regulation field of a laser source with the absorption lines of the reference molecular transition [1] it was necessary to regulate and stabilize the reference temperature of the laser source independently of the temperature of the measurement bench. In particular, the reference temperature of the source, understood as the temperature of the metal base of its case, must be regulated to a precise value (approx=30°C) and kept there despite variations in the power from the source itself and from its electronics. Particular tolerances are not required (<0.1K), but the start-up transient must be rapid (<15 min) and with no appreciable overshoot (<0.5K). To make the digital control loop, commercially-available transducers and actuators were used: thermistors and Peltier cells (TEC, Thermo-Electric Cooler). The Digital Control Unit (DCU) was designed and built by two students as part of their degree thesis, with the help of modern design techniques [2] learned during the course of Process Control held by one of the authors at Politecnico di Torino, Faculty of Engineering, Vercelli, Italy. The method entails an in-depth study of the dynamics of the command-measurement chain and of the disturbances affecting that chain. Fine dynamics is used in a numerical simulator that accompanies the entire project as test and inspection bench; the project dynamics is a simplified discrete-time state equation dynamics around which the digital control algorithms are synthesized. The result is a system of thermal control with performance that is markedly better than the commercially-available control used previously, as may be seen in Figure 3. The paper briefly illustrates the salient points of the control strategy and the results of experimental tests.