Hiding robot inertia using resonance

Abstract

To enable compliant training modes with a rehabilitation robot, an important prerequisite is that any undesired human-robot interaction forces caused by robot dynamics must be avoided, either by an appropriate mechanical design or by compensating control strategies. Our recently proposed control scheme of "Generalized Elasticities" employs potential fields to compensate for robot dynamics, including inertia, beyond what can be done using closed-loop force control. In this paper, we give a simple mechanical equivalent using the example of the gait rehabilitation robot Lokomat. The robot consists of an exoskeleton that is attached to a frame around the patient's pelvis. This frame is suspended by a springloaded parallelogram structure. The mechanism allows vertical displacement while providing almost constant robot gravity compensation. However, inertia of the device when the patient's pelvis moves up and down remains a source of large interaction forces, which are reflected in increased ground reaction forces. Here, we investigate an alternative suspension: To hide not only gravity, but also robot inertia during vertical pelvis motion, we suspend the robot frame by a stiff linear spring that allows the robot to oscillate vertically at an eigenfrequency close to the natural gait frequency. This mechanism reduces human-robot interaction forces, which is demonstrated in pilot experimental results. Abstract-To enable compliant training modes with a rehabilitation robot, an important prerequisite is that any undesired human-robot interaction forces caused by robot dynamics must be avoided, either by an appropriate mechanical design or by compensating control strategies. Our recently proposed control scheme of "Generalized Elasticities" employs potential fields to compensate for robot dynamics, including inertia, beyond what can be done using closed-loop force control. In this paper, we give a simple mechanical equivalent using the example of the gait rehabilitation robot Lokomat. The robot consists of an exoskeleton that is attached to a frame around the patient's pelvis. This frame is suspended by a springloaded parallelogram structure. The mechanism allows vertical displacement while providing almost constant robot gravity compensation. However, inertia of the device when the patient's pelvis moves up and down remains a source of large interaction forces, which are reflected in increased ground reaction forces. Here, we investigate an alternative suspension: To hide not only gravity, but also robot inertia during vertical pelvis motion, we suspend the robot frame by a stiff linear spring that allows the robot to oscillate vertically at an eigenfrequency close to the natural gait frequency. This mechanism reduces human-robot interaction forces, which is demonstrated in pilot experimental results.

A MUltidimensional Compliant Decoupled Actuator (MUCDA) for Pelvic Support During Gait

“…Nevertheless, it has been demonstrated that on average, individuals post-stroke have a walking cadence of as much as 80% that of able-bodied individuals [41]. Furthermore, the use of a spring to compensate for inertial effects of an exoskeleton is effective across a range of frequencies [30]: a spring structure demonstrably reduces vertical interaction forces even if the stiffness is not optimal. Therefore, despite being based on walking and step cadence data for able-bodied subjects, the proposed device should also be suitable for gait training for many individuals post-stroke.…”

“…As no active elements are present to compensate for robot dynamics, the passive structure itself must minimize the forces. To achieve this goal, the eigenfrequency of the oscillating mass-spring assembly must be in the same range as an average human gait frequency [30]. The medium walking cadence is expected to be f 0 = 0.75 Hz and the estimated mass of the oscillating parts of the device m = 33.6 kg, which is mainly caused by the mass of an additional exoskeletal structure for the legs, giving a resulting stiffness k z equal to mω 2 0 .…”

“…Furthermore, passive compliance can allow some freedom around an equilibrium configuration without enabling excessive deviation from this point, for example, to provide guidance. With appropriate tuning, passive compliance can also compensate for periodic inertial forces [30].…”

The Resonating Arm Exerciser: design and pilot testing of a mechanically passive rehabilitation device that mimics robotic active assistance

“…In addition, for both RAE and assist-as-needed robotic therapy devices, the amount of assistance is tunable by altering system parameters (stiffness and damping and therefore the movement amplification gain). Programmable resonance has been proposed previously as a method to hide the inertia of a gait robot [28], and a robotic-assistance algorithm that assists rhythmic movement by adapting to the frequency of a user’s movements has been proposed as a method of assisted rehabilitation [18]. …”

Cooperative Control Design for Robot-Assisted Balance During Gait

“…The rotation axes for hip flexion/extension and ab/adduction intersect approximately in the center of the human hip joint. In this experiment, dynamics of the robot that could have influenced aband abduction movements were passively compensated by springs, following the method in [28]. Lateral translation was actuated, so that the device was able to apply balance-assisting forces to the pelvis in lateral direction.…”