Lower Limb Robotic Exoskeleton

An exoskeleton is a structure used in the human body, which serves as a support and is used to assist movements and/or increase the capabilities of the human being. These may be passive or active, that is to say, whether or not they contain actuators for movement and whether or not they need a control system associated with the actuators' movement [1].

There are three types of exoskeletons: a) rehabilitation, used to recover the physical abilities of patients after suffering accidents, injuries, neurological damage, heart attacks, etc.; b) locomotion assistance, which provide external support to the human skeleton, as well as substitution of motor function; c) improvement of physical abilities, whose aim is to provide assistance during movement to patients, are especially used in the elderly, workers, military, etc. [2].

Most exoskeletons are designed for lower extremities, as they are the most vulnerable to injury and concentrate most of the human weight. The lower limbs allow us to move, so they are the basis of a person’s independence [1]. The development of these devices is based on the anthropometry and biomechanics of the human being, since these must act in synchrony with the human body. Anthropometric measurements should be taken into account, so that the dimensions of the body fit. On the other hand, the biomechanical dimension is responsible for analyzing each joint and reproducing its movements within the device [2]. 

In the biomechanical design of the exoskeleton of lower limbs, it is important to define parameters such as degrees of freedom of movement, joints, etc., of the human body. In order to maintain the centre of gravity when performing movement, sufficient force must be required on the flexor tendons and extensors of the hip, knee and ankle [2].

In this project an exoskeleton of lower limbs was developed, whose main objective was to recreate the gait cycle, using servomotors, ARDUINO and Medium Density Fiberboard (MDF) 3 mm.

Keywords: exoskeleton, lower limb, biomechanics, arduino, servomotor, gait cycle, leg, knee, ankle, hip, movement, joints.

References

[1] M. Alejandro, C. Cardona, F. R. Spitia, and A. Baradica López, “Exoesqueletos para potenciar las capacidades humanas y apoyar la rehabilitación,” Org.co. [Online]. Available: http://www.scielo.org.co/pdf/rinbi/v4n7/v4n7a08.pdf. [Accessed: 27-Feb-2023].

[2] L. Dunai Dunai, I. Lengua Lengua, I. Lengua Lengua, and B. Defez Garcia, “DISEÑO DE UN EXOESQUELETO DE EXTREMIDADES INFERIORES,” Dyna, vol. 94, no. 1, pp. 297–303, 2019.

• Medium Density Fiberboard (MDF) 3mm
• Servomotors MG995
• Stoppers
• Wooden sticks (Φ6 x15 mm)
• M3x10 mm screws
• M3 hex nuts
• M4x10 mm screws
• M4 hex nuts
• Laser cutting machine
• Creality Ender-3 S1 Pro 3D printer
• Arduino UNO
• Protoboard
• Jumper wires
• Voltage source at 5 V
• Banana plugs

First, an exoskeleton was designed by hand to visualize what it would look like on lower limbs considering the biomechanics and joint physiology of the legs. 

Each piece corresponding to the hip, thigh, tibia and foot were then designed in SolidWorks. Each one contains boreholes that serve to add the servomotors and join the pieces with each other. These pieces were cut in MDF, while the stoppers were designed to be printed on a 3D printer using PLA filaments.

When the pieces were already cut and the stoppers had been printed, the exoskeleton was assembled. As seen in the images, each leg has 2 sides: one contains the servomotors and the other does not.

The servomotors were attached to their corresponding parts using screws and hex nuts. On the other hand, the side that do not contain servomotors were joined using wooden sticks and 3D printed stoppers on each side of the sticks to prevent them from falling out of place.

The servomotors were programmed on Arduino so they would recreate the gait cycle. Another code was developed to make sure that each one of the servomotors worked correctly, making them move from 0° to 90°.

Additionally, 2 more codes were developed. The first one makes the exoskeleton walk on a surface, while the second one makes it walk faster.

After assembling and programming the exoeskeleton, it was able to recreate the gait cycle successfully.