An optical red green blue sensor for monitoring the degradation of magnetorheological fluids in flow-recirculating high-torque clutch actuators

Abstract

The aerospace industry is gradually moving toward “more electric aircraft” to reduce its environmental footprint. Developing all-electric actuators brings a reliability challenge because conventional geared-motor actuators are susceptible to jamming failures from their metal-to-metal gear contacts. Magnetorheological clutch actuators solve this challenge using a layer of fluid to transmit torque and are not exposed to potential jam failures, making them particularly attractive for high reliability applications such as primary flight controls. A key challenge that must be addressed for a widespread deployment of the magnetorheological fluid technology in aerospace is the ability to monitor magnetorheological fluid condition while it degrades in operation. To date, no such efforts have been reported in the literature. Recent studies on magnetorheological fluid durability have shown that mixing the magnetorheological fluid using a magnetic screw pump principle can significantly increase the life of the fluid by up to 50%. This article presents the design, prototyping, and testing of a proof-of-concept magnetorheological fluid condition monitoring sensor, capitalizing on the flow-recirculating properties of magnetic screw magnetorheological clutches to continuously provide a well-mixed and homogeneous fluid sample to the sensor. The proposed sensing principle uses optical red green blue sensor placed in the fluid circulation path to measure the fluid color during degradation. The sensor has been tested up to fluid failure on a high-torque (60 N m) magnetorheological clutch mounted on a fully instrumented durability test bench. Tests have been performed with two types of fluid: a commercially available fluid, the Lord 140CG, and a homemade fluid based on perfluoropolyether oil, the GPL-103. Results with the Lord fluid demonstrate a strong correlation between the decrease in torque-to-current performance with fluid brightness. The average of three aging tests on Lord 140CG fluid show a 12% ± 1% decrease in brightness at end-of-life regardless of operating conditions such as torque, shear rate, and dissipated power. These results suggest that, for the Lord 140CG fluid, brightness is directly linked to the fluid degradation state and independent of operating conditions, which makes it a more accurate metric to quantify durability than life dissipated energy since the latter can vary significantly depending on operating conditions. Tests made with the GPL-103 based fluid did not show such a strong correlation, which means that optical sensing of magnetorheological fluid condition must be carefully calibrated for each individual fluid and clutch design. Results from this study suggest that optical sensing is a relevant method to measure the magnetorheological fluid condition in flow-recirculating clutches.