A low cost wireless multi-channel measurement system for strain gauges

This paper describes the development of a wireless measurement system for multiple strain measurements on rotational structures. Impulse to this development has been given by an industrial need to measure strains on complex loaded rotating cylinders. After an introduction into the basics of strain measurements and signal conditioning, the principles of the low cost, wireless microphone system are explained, followed by a discussion on the data acquisition process. Finally, preliminary validation results are discussed. The major challenge for this development being wireless multi-channel data transmission, other design criteria like portability, lightweight and ease of setup were to be fulfilled at low cost, without compromising on reliability, accuracy and bandwidth. The developed system is multipurpose as it is not limited to strain gauge signals only, but can be used for other sensors too. 1. Problem statement Strain gauges have a long tradition in measuring strains on structural components. Usually, a wire network connects these sensors with their signal conditioning and data acquisition system. However, in case of rotating structures, sensor data must be transmitted in a wireless way. Commercially available wireless systems like inductive (contactless) or slip rings (contact) systems have to be tailored to the application and hence are very expensive. For one-off measurements or multipurpose measurement needs, a low cost, flexible and adaptive wireless measurement system is needed. Such need was expressed within an industrial project concerning the replacement of existing but degraded cylinders used in film production by new lightweight versions. The current cylinders, made of glass reinforced polyester, are 195 cm long, have a diameter of 30 cm, contain 2 stainless steel inserts at both ends and weigh about 30 kg in total. During film production, storage and shipment, film is wound onto these cylinders at high speed (300m/min) to a total weight of 2 ton, aged at elevated temperature, stored and unwound again, resulting in many cycles of severe alternating as well as long term loads throughout the service life of the cylinder. The majority of these cylinders has been in service for many years and needs to be replaced in the near future. However, their laminate lay-up, fibre reinforcement and shell thickness have been evolving over many years based on best practices, without detailed structural analyses. Hence, in order to design and develop a new generation of cylinders, an updated set of design criteria had to be specified. However, although loading conditions during winding on full or half width and during storage can be assessed qualitatively, no quantitative design data corresponding to real service loads were available. During film winding e.g. static axial compression,