Leveraging disturbance observer based torque control for improved impedance rendering with series elastic actuators

Abstract

The fidelity with which series elastic actuators (SEAs) render desired impedances is important. Numerous approaches to SEA impedance control have been developed under the premise that high-precision actuator torque control is a prerequisite. Indeed, the design of an inner torque compensator has a significant impact on actuator impedance rendering. The disturbance observer (DOB) based torque control implemented in NASA's Valkyrie robot is considered here and a mathematical model of this torque control, cascaded with an outer impedance compensator, is constructed. While previous work has examined the impact a disturbance observer has on torque control performance, little has been done regarding DOBs and impedance rendering accuracy. Both simulation and a series of experiments are used to demonstrate the significant improvements possible in an SEA's ability to render desired dynamic behaviors when utilizing a DOB. Actuator transparency at low impedances is improved, closed loop hysteresis is reduced, and the actuator's dynamic response to both commands and interaction torques more faithfully matches that of the desired model. All of this is achieved by leveraging DOB based control rather than increasing compensator gains, thus making improved SEA impedance control easier to achieve in practice.

Compliance Shaping for Control of Strength Amplification Exoskeletons with Elastic Cuffs

“…It is also possible to directly reject disturbances in the open loop model [29], [30], which has shown to be an equivalent approach [31]. These force disturbance observers improve impedance rendering accuracy when used in a cascaded impedance controller [32].…”

Mechanical Design Procedures for Series Elastic Actuators

“…Ancak, yüksek tork çıkışı gerektiren görevlerde arzulanan sertlik değerinin yakalanması için yayların kalınlığı veya ağırlığı çok artacağından doğrusal yaylar tercih sebebi olmaktan çıkmaktadır. Düzlemsel burulma yayları, NASA (National Aeronautics and Space Administration Tr: Ulusal Havacılık ve Uzay Dairesi)'nın Robonaut projesinde [13] ve IIT (Instituto Italiano di Tecnologia Tr: İtalyan Teknoloji Enstitüsü )'nin yenilenen iCub projesinde başarıyla uygulanmış ve olumlu deneysel sonuçlara ulaşılmıştır [14]. Bu bağlamda düzlemsel burulma yayları bu çalışmada uygun bir aday olarak belirlenmiştir.…”

Impedance control of a cable-driven SEA with mixed H2/H∞ synthesis

“…For example, in Oblak et al (2010), Sergi and O'Malley (2015) and Vallery et al (2008), pure proportional-integraldifferential torque control was applied for SEA impedance control. In Kong et al (2009), Mehling et al (2015) and Yu et al (2015), disturbance observer (DOB)-based torque control methods were used to improve the impedance rendering accuracy and robustness. The aforementioned impedance control approaches were mostly implemented with the cascaded impedance-torque or impedance-torque-velocity control structure, wrapping impedance control around an inner torque or position control loop.…”