Improvement of the modeling of a hydraulic drive for exoskeleton application based on measurements

Abstract

A hydraulic drive for the application to an upper limb exoskeleton is evaluated. The actuator supports the flexion of the elbow joint. The aim of the work is to analyse and characterise the electro-hydraulic drive for the application to the exoskeleton. Therefore, an evaluation of the relevant parameters of the drive is performed. These parameters are essential for the selected control strategy of the application.First, the electro-hydraulic drive is measured using a hydraulic circuit including a variable speed electric motor and a constant displacement axial piston pump producing the volumetric flow rate. The output flow characteristic of the drive is performed. A pump characterisation is derived for prescribing the motor current and knowing the corresponding volumetric pump output. A initial simulation model of the drive is compared to measurement results of the hydraulic circuit. Additional parameters are implemented into the simulation model of the pump and evaluated due to the improved simulation model, a model prediction control is possible. The measured input of the muscle tension of the wearer is taken as signal and depending on a given supporting factor the controller sets the motor current. The characterisation of the pump allows the controller to calculate the current signal of the motor in order to predict the resulting volumetric flow of the system.Second, the actuator is place onto an upper limb exoskeleton. The exoskeleton supports the elbow joint flexion during the load lifting process. The hydraulic cylinder is linked by a kinematic mechanism to the exoskeleton joint. Due to the known volumetric flow rate, depending on the motor current, the angular motion of the exoskeleton joint with the linked mechanism can be predicted. Using the signal of the muscle tension allows the setting of the exoskeleton force with the actuator input.