Abstract
BackgroundLoad cells are often used in rehabilitation robotics to monitor human–robot interaction. While load cells are accurate and suitable for the stationary end-point robots used in rehabilitation hospitals, their cost and inability to conform to the shape of the body hinder their application in developing affordable and wearable robotic orthoses for assisting individuals in the activities of daily living. This exploratory work investigates the possibility of using an alternative technology, namely compliant polymeric air cushions, to measure interaction forces between the user and a wearable rigid structure.MethodsA polymeric air cushion was designed, analyzed using a finite element model (FEM), and tested using a bench-top characterization system. The cushions underwent repeatability testing, and signal delay testing from a step response while increasing the length of the cushion’s tubes. Subsequently, a 3D printed wrist brace prototype was integrated with six polymeric air cushions and tested in static conditions where a volunteer exerted isometric pronation/supination torque and forces in vertical and horizontal directions. The load measured by integrating data recorded by the six sensors was compared with force data measured by a high quality load cell and torque sensor.ResultsThe FEM and experimental data comparison was within the error bounds of the external differential pressure sensor used to monitor the pressure inside the cushion. The ratio obtained experimentally between the pressure inside the pressure cushion and the 8 N applied load deviated by only 1.28% from the FEM. A drift smaller than 1% was observed over 10 cycles. The rise times of the cushion under an 8 N step response for a 0.46, 1.03, and 2.02 m length tube was 0.45, 0.39, and 0.37 s. Tests with the wrist brace showed a moderate root mean square error (RMSE) between the force estimated by the pressure cushions and the external load cells. Specifically, the RMSE was 13 mNm, 500 mN, and 1.24 N for forearm pronation/supination torque, vertical force, and horizontal force, respectively.ConclusionsThe use of compliant pressure cushions was shown to be promising for monitoring interaction forces between the forearm and a rigid brace. This work lays the foundation for the future design of an array of pressure cushions for robotic orthoses. Future research should also investigate the compatibility of these polymeric cushions for data acquisition during functional magnetic resonance imaging in shielded rooms.
Highlights
Load cells are often used in rehabilitation robotics to monitor human– robot interaction
Many of these robotic devices include wearable orthoses as parts of exoskeleton systems [3] or as hand-held devices, such as those used for grasping, that are end effector-based [4]. These systems have been developed world-wide for rehabilitation protocols that monitor the user’s improvement using feedback sensors that measure various metrics from the user via signal processing and data acquisition software [5]. Sensors such as force sensing resistors (FSRs) [6], capacitive force sensors [7], load cells [8], and torque sensors [9] are integrated into these systems to measure the applied forces that are exerted on the mechanical system
If the force sensors detect that the difference of applied forces between the two arms is greater than 1 Nm, the bimanual wearable robotic device (BWRD) applies resistance to the motion of both arms through its motor and brake system, prompting the user to correct the imbalance of the forces that they are applying on the exoskeleton arms
Summary
Load cells are often used in rehabilitation robotics to monitor human– robot interaction. While load cells are accurate and suitable for the stationary end-point robots used in rehabilitation hospitals, their cost and inability to conform to the shape of the body hinder their application in developing affordable and wearable robotic orthoses for assisting individuals in the activities of daily living This exploratory work investigates the possibility of using an alternative technology, namely compliant polymeric air cushions, to measure interaction forces between the user and a wearable rigid structure. Many of these robotic devices include wearable orthoses as parts of exoskeleton systems [3] or as hand-held devices, such as those used for grasping, that are end effector-based [4] These systems have been developed world-wide for rehabilitation protocols that monitor the user’s improvement using feedback sensors that measure various metrics from the user via signal processing and data acquisition software [5]. Alternative methods for measuring arm forces through muscle activity, such as electromyography (EMG) [10], can sometimes be too sensitive to environmental conditions such as electric and magnetic noise, and have small signal to noise ratios [11]
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