Abstract
HERA is a space robot with elastic joints and flexible limbs. This paper describes the modelling and control of the HERA joints. Based on analytical studies and measurements, a dynamic model for the joints is presented, which can be used, together with the HERA equations of motions, to simulate HERA operations. The torque characteristics of the Brushiess DC motor are studied and based on measurements a Harmonic drive model, valid during forward and backward drive, is presented. This gear model includes the Harmonic drive flexibility, load-dependent friction and hyster- A derived control design model has been used to investigate the influence of the joint flexibility and limb flexibility on HERA performance. The performance limitations of decentralized joint control under motor velocity and/or joint position feedback are studied as function of the lowest structural resonance frequency and the lowest anti-resonance frequency. It has been pointed out how the performance limitations and the selection of the controller gains can be based on these frequencies.
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