College of Engineering Student : Yang Zhou, Yousef Mohammed Alshahrani

Computer-Controlled Lower Limb Exoskeleton Ambulation System for Paraplegia

Advisor : Chaoyang Chen

INTRODUCTION

In USA, there are approximately 20 million patients suffered from paralysis caused by spinal cord injury (SCI) with extreme limitations in overground ambulation. Powered exoskeletal-assisted walking device makes it possible for paraplegias to re-walk and return to normal daily life. Several companies including Rewalk Robotics, Indego, and Ekso Bionics have developed powered exoskeleton devices that enable paraplegias to restore basic upright activities. However, the prices of their devices may be unaffordable (over $70,000 USD) for the patients. Long term goal of our research is to develop cost-effective products. In this study, we developed a lower cost, high efficient exoskeleton device that can restore the natural gait patterns with standing and sitting abilities.

METHODS

Light weight bipedal lower limb exoskeleton was designed using NX software. The steel joints were designed based on human knee joint and hip joint anatomical structures and manufactured by professional service. Steel joints and other parts were assembled in our labs. On each leg, two stepper motors with 3 Nm torque, ACME threaded rods and nuts were used to control the movement of exoskeleton joints. The stepper motors were driven by microstep drivers (MB450A) using two 12V 7Ah batteries in series connection (total 24 V). And necessary calculations were performed to estimate the torque generated by the stepper motor and threaded rod structures. Arduino microcontroller platforms (4 UNO r3 boards, 1 Mega2560 r3 boards) were used as computer control system. Arduino Mega2560 r3 was the central control board which received user’s manual input signals and sent output signals to individual UNO boards. Each UNO board controlled one stepper motor with a different controlling algorithms. Four motors work together to achieve sitting, standing or walking motions. Arduino IDE (Integrated Development Environment) was used to write the controlling algorithms using C++ language. Four momentary switches were used to control standing, walking, stopping, and sitting operations separately (Fig. 1).

RESULTS

The exoskeleton performed a normal human gait patterns. The maximum load of this stepper motor driven actuation produced up to 176 lbs load. The maximum walking speed was 108 m/6 mins (Table 1). in walking function compared with the gait of the Rewalk and Indego exoskeletons. Sitting and standing-up speed was designed to be 3 seconds for better motion control. Using the 24V 7Ah battery system, the operation sustaining time was 3 hours.

Figure 2. A shows code for central controlling computer microchip. B shows lateral view of exoskeleton. C shows front view of exoskeleton.

DISCUSSION

To date, there are several exoskeletons that were either on the market or under development for paraplegias. In USA, there are 3 commercialized exoskeletons, including Rewalk, Bionics, and Indego. However they are cost-expensive with a sale price more than $70,000. Paraplegia may not be able to offer it. One of our research purposes is to develop a cost-effective lower limb exoskeleton walking aiding device. To archive this goal, we have used stepper motor instead of servo motors as used in other exoskeleton system. Servo motor system required sensors and decoders to provide position signals feedback to computer control system. Because stepper motor can be precisely controlled by pre-programed computer algorithms without needing feedback signals, hence stepper motor actuation system may not need sensor system. This makes it possible to develop less complexity of algorithms and required less firmware, thus reducing the costs. In terms of kinematics, our exoskeleton provided a physiologic walking speed and load capacity, which is similar to other commercialized exoskeletons (Table 1). Future studies include clinical trails before final delivery to clinical applications. We will also develop muscle bio-electrical signals (EMG) or brain wave signals (EEG) decoding and encoding systems for intuitive exoskeleton motion controls.

CONCLUSIONS

Our computer-controlled lower limb ambulation system can provide a cost-effective walking aide device for paraplegias. The stepper motor control mechanism also provides a platform as an alternative approach for intuitive exoskeleton controls.

REFERENCES

A B C

Figure 1. Control Structure of the exoskeleton device (microstep drivers are not shown). 64 I/O Mega microchip was used to control 4 individual Uno microchips and associated stopper motors as joint actuators.

Table  1  Load  capacity  and  opera5on  speed  of  exoskeletons    

1. Ajax Yang et al. Assessment of In-Hospital Walking Velocity and Level of Assistance in a Powered Exoskeleton in Persons with Spinal Cord Injury. Top Spinal Cord Inj Rehabil 2015; 21(2); 100-109 2. Clare Hartigan et al. Mobility Outcomes Following Five Training Sessions with a Powered Exoskeleton. Top Spinal Cord Inj Rehabil 2015; 21(2); 93-99

Exoskeleton Maximum  Load  (lbs) Walking  Speed  (m/6min)

Our  Design 176 108 Rewalk 220[1] 144[1]

Indego 224[2] 86.3[2]