Robotic lower limb exoskeletons using proportional myoelectric control

Abstract

Robotic lower limb exoskeletons have been built for augmenting human performance, assisting with disabilities, studying human physiology, and re-training motor deficiencies. At the University of Michigan Human Neuromechanics Laboratory, we have built pneumatically-powered lower limb exoskeletons for the last two purposes. Most of our prior research has focused on ankle joint exoskeletons because of the large contribution from plantar flexors to the mechanical work performed during gait. One way we control the exoskeletons is with proportional myoelectric control, effectively increasing the strength of the wearer with a physiological mode of control. Healthy human subjects quickly adapt to walking with the robotic ankle exoskeletons, reducing their overall energy expenditure. Individuals with incomplete spinal cord injury have demonstrated rapid modification of muscle recruitment patterns with practice walking with the ankle exoskeletons. Evidence suggests that proportional myoelectric control may have distinct advantages over other types of control for robotic exoskeletons in basic science and rehabilitation.

Figure 1

University of Michigan Human Neuromechanics Laboratory exoskeletons. Above left: ankle exoskeleton with artificial pneumatic muscle providing plantar flexor torque. Above middle: knee and ankle exoskeleton with artificial pneumatic muscles providing extensor and flexor torques. Above right: hip exoskeleton with a pneumatic cyclinder providing extensor and flexor torques.

Figure 2

Schematic of the proportional myoelectric controller. Surface electromyography electrodes recorded muscle activation signals that were processed with high-pass filtering, rectification, and then low-pass filtering to yield a control signal for the air pressure regulator.

Figure 3

Mean soleus EMG patterns for a subject with incomplete spinal cord injury. The ASIA-D subject completed 24 minutes of walking with the robotic ankle exoskeleton providing plantar flexor assistance under proportional myoelectric control of the soleus. Prior to training with the exoskeleton powered (Pre-Unpowered, dark blue), the soleus EMG profile is abnormal in that it does not have an increase at the end of stance (40–65% of the gait cycle). After 24 minutes of walking with the powered exoskeleton (Powered-24 minutes, pink), the subject recruited soleus primarily in late stance. When the exoskeleton was turned off (Post-Unpowered, light blue), the soleus recruitment pattern remained.