Active paraffin wax phase change actuators for intelligent process control

Abstract

Process fluctuations due to changing environmental conditions, thermal loads and wear are known problems in industrial production. When accumulated, they can lead to instabilities in process chains. Paraffin wax-based phase change actuators (PCA) are an available, robust solution for stabilization and error compensation. Consisting of two deep-drawn steel cups, joined by laser welding, they enclose a paraffin wax core. Phase transformation of the paraffin wax leads to a significant volume expansion, generating the actuating force. The PCAs are suitable even for highly rigid systems with achievable actuating forces of above 85 N/mm² of effective actuator area. Additionally, the particularly compact housing offers high integration capability. Passive PCAs, activated by environmental heat, are already integrated in industrial applications. For active application, an energy supply via additional external heating elements, e.g. heating tapes, is required. This contradicts the concept of a simple and compact overall system and thus the integration capability. The overriding challenge results in implementing the PCA as an actively controllable, integrated overall system. This paper presents an approach based on printed electronics, consisting of a conductive carbon ink, applied to a corresponding substrate by means of screen-printing. Electrical connections are subsequently attached and the heating unit is integrated into the PCA. This work shows challenges within design and implementation of these high-performance heating electronics. It discusses different inks, substrates and their necessary properties for PCA usage. Concepts for electrical conduction through the sealed actuator housing are presented. Finally, the performance of these PCAs is discussed and compared with passive, externally activated PCAs.