Overview of exoskeleton

1.2.1 Exoskeleton of rigid frame

Robotic exoskeletons have been developed for more than two decades for many applications. In fact, the General Electric Company manufactured the first set of exoskeleton in the world, called ‘Hardiman’, in 1966 [1]. Exoskeleton is a multifunctional assistive device, having different benefits for different applications.

Generally, the main purpose of an exoskeleton can be divided into enhancement and assistance [2]. It can also be divided into four purposes including assistance, enhancement, protection, and detection [3]. An exoskeleton for assistance helps physically impaired individuals to regain ability that healthy individuals normally have, like standing, walking, or grabbing. Exoskeletons for enhancement improve wearer’s ability such as carrying heavier load, running faster or endurance augmentation. The functions of protection can be very diverse due to the versatile targets of protection and the mechanisms or materials used in design. For example, space suit is designed to protect astronaut from harsh environment of pressure and temperature extremes in the

outer space. As for the detection scenario, exoskeleton obtains information from human body with sensors. The information obtained can be used for rehabilitation, treatment, or as feedback signal for controlling the exoskeleton. In recent year, researches of exoskeleton are always combined with the applications of the four purpose mentioned above. Exoskeletons can be stationary devices, mounted on treadmill and used for gait rehabilitation [4, 5], or retraining for individuals suffering disabilities or injury [6].

Figure 1-1 shows the gait training system using a robotic orthosis, Lokomat [4].

Jezernik et al. developed a control algorithm that made Lokomat automatically adapt the gait trajectory, which was recorded by tracking the hip and knee joint-angles, to meet that of “free motion” of the healthy leg of the patient suffering hemiplegia, with force-feedback proportional-integrative (PI) control. Then, the gait trajectory with inverse-phase adaptation is tracked by a position controller to move the impaired leg for gait raining.

Figure 1-1 Gait training system with robotic orthosis Lokomat [4]

doi: 10.6342/NTU201701368

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Mobile exoskeleton can be divided into two categories based on different scenarios. For paralyzed individuals, it provides full support and control of patient’s lower limbs to help to regain walking ability [7]. The other type of mobile exoskeleton can be applied on most users for specific purposes such as to restore grabbing and gesturing ability [8], to assist heavy lifting [9, 10], or to prevent fatigue [11].

Particularly, several mobile exoskeletons have been designed to enhance locomotion and shown to improve load carrying capacity significantly. For instance, the quasi-passive leg exoskeleton test demonstrated by Walsh and his team at MIT, applying springs at hip extension, hip flexion, ankle plantar flexion, ankle dorsiflexion, and a variable damper at knee, can bypass 80% of 36 kg load to ground [12].

Most exoskeletons use rigid frame as main structure [4, 5, 7, 13, 12]. With additional support and highly geared motor or hydraulic system, most of them are excellent in bypassing load and providing full support for limbs [13, 12]. For example, the exoskeleton BLEEX, developed by Kazerooni et al. in 2004, is built by using numerous sensors and hydraulic actuators, as shown in Figure 1-2 [14]. The BLEEX was composed of two powered anthropomorphic legs, a power supply, and a backpack-like frame for load carrying. With the close-loop control including an inclinometer input, BLEEX achieves self-balancing and thus frees wearer from any load at walking speed below 3.2 km/hr.

Figure 1-2 Berkeley Lower Extremity Exoskeleton (BLEEX) [13]

As the improved versions of BLEEX, ExoHiker^TM [13] and ExoClimber^TM [15]

were developed in 2005. ExoHiker^TM [13], as shown in Figure 1-3(a), weights 14kg including power unit, batteries and on-board computer. It is able to give its wearer a load-free walk with 70kg mounted. With a solar panel and an 80 W-hour battery, ExoHiker^TM is capable of carrying 70 kg for about 21 hours while the wearer feels no load on his/her shoulder. In the same year, a 22 kg-weight version, ExoClimber^TM [15]

as shown in Figure 1-3(b), was built. Despite its weight, ExoClimber^TM [15]retains the same long-term load carrying capability as ExoHiker^TM, and is able to do stair ascent rapidly with 70 kg load while remaining load-free user experience for the wearer.

Figure 1-3(c) shows UCLC^TM(Human Universal Load Carrier), which combines ExoHiker^TM and ExoClimber^TM. UCLC^TM has a better load capacity of 90 kg, and decreases oxygen consumption of users walking at a speed of 3.2 km per hour with 36 kg load by about 15%. The function of load easing for user is a great development, especially for those who carrying load for long-term task like soldiers [16] and mountain rescue team. [17]

Figure 1-3 (a)ExoHiker^TM [15]; (b)ExoClimber^TM [16]; (c) UCLC^TM [17].

Though exoskeletons with rigid frame excel in load carrying and are able to provide reliable support for limbs, their high inertial due to the materials of the structure needs to be driven by high power actuators, causing high power consumption and the compromise between actuator’s power and user’s weight. Also, the misalignment between biological joint and the fixed rotation center of robotic joint greatly affects nature gait, deviating the muscle-skeletal of human from efficient walk. Furthermore, the tuning of the complicated control as well as multiple sensors and actuators for a powered exoskeleton is time-consuming and costly. For instance, hybrid assistive limb-5 (HAL-limb-5), shown in Figure1-4(a), developed by Tsukuba University in Japan and the robotic company Cyberdyne, has myoelectricity sensors, angle sensors, floor reaction force sensors (COP/COG sensors), and etc. The positions of sensors are shown in Figure1-4(b). In the patent of HAL-5 lower limbs, the myoelectricity sensors are marked as 38a, 40a, 42a, and 44a on the thigh. The force sensors are marked as 50a, 50b, 52a, and 52b in the picture, and the angle sensors are combined with each power unit. Though the HAL-5 can multiply wear’s strength by two to ten times depending on its type and last for almost five hours after recharging, it costs $14000~19000 as a price for commercial use [18], and takes two months to calibrate to achieve optimal function for each individual [19]. [20, 21]

Figure 1-4 (a) HAL: the Hybrid Assistive Limb [20] (b) Patent of HAL [21]

To overcome these disadvantages, some researches of exoskeleton start to focus on developing lighter, simpler, and more flexible exoskeletons by using different kinds of concepts. Without a doubt, the use of lighter materials in structure is one of the solutions. Taking the HAL-5 mentioned as an example, the latest version of HAL-5 consists of a frame made of nickel molybdenum and extra-super-duralumin, and is further strengthened with plastic casing. With these light materials, the HAL-5 weighs 22 kg as a full body exoskeleton, and achieves weight of 15 kg for its lower-limb-only version [19]. On the other hand, since full-body or full-limb exoskeletons inevitably need large structures and complex control to maintain its function and reliability, exoskeletons for certain joint assistance is another solution. Taking the Honda Walking Assist Device with Stride Management System (WADSMS) [22], as shown in Figure 1-5, for example, the device mainly actuates hip joint in flexion and extension with motors approximately collocated with hip joint of the wearer in the sagittal plane. The WADSMS consists of a waist frame in which controller and battery are installed, and two thigh frames on where motors and angle sensors are mounted. The waist frame and the thigh frame are connected with passive joint, which allows a limited amount of hip abduction and adduction. This device functions by applying small forces to maintain wearer’s cadence under an efficient range. By limiting its function as an exoskeleton, the WADSMS achieves a lightweight for about 2.8kg including battery.

Figure 1-5

The Walking Assist Device with Stride Management System (WADSMS) [22]

1.2.2 Soft exoskeleton and exosuit

In addition to these two methods mentioned above, the change in actuators or mechanisms of transmission is another way, in which pneumatic muscles and tethered actuation are used mostly. Pneumatic artificial muscle is a kind of actuator made of flexible materials and will change its original length and spring constant when charged with pressurized fluid. Because pneumatic artificial muscle actuates like human muscle does, it is often installed near a certain muscle to provide assistive contraction force.

Figure 1-6 show an active soft orthotic device for ankle, designed by Yong-Lae Park and his team. This device mimics the biological muscle-tendon architecture [23]. By using artificial muscles, wearer is freer to move without kinematic limit of exoskeleton;

the total weight of the device is much lighter since the heavy rigid frame used to transmit force is not needed anymore. However, the use of artificial muscle faces two major difficulties: (a) the lack of well-known properties of artificial muscles, which require lots of tuning and tests before the device design stage; (b) the lack of adequate air source, which greatly limits the mobility of exoskeleton. A portable system of this kind requires an air compressor, a high-pressure vessel, and a power source for the compressor. These components of personal gear size, are not yet commercially available like batteries and motor drivers.

In addition to rigid structures, cables are also good medium for force transmission.

Figure 1-6 Main design of the active soft orthotic device, highlighting key components [23]

Despite the fact that cables mainly transmit tensile force, cables can be quite weight efficient for exoskeleton if we make good use of our bones. Accordingly, a new type of exoskeletons that treat Bowden cables as the main force transmitter, named as

‘Exosuit’ by Walsh and his research team in Wyss Institute at Harvard University has been proposed. This type of exoskeleton consists of mostly soft textures and Bowden cables. By attaching the ends of cables appropriately, scrolling and tensioning the cables in parallel with muscle by motor, torque is generated directly on biological joints without greatly constraint wearer’s limbs. Based on this concept, Figure1-7 shows series of designs published by the team at Harvard University to assist the three most energy-consuming motions during walking like hip flexion [24, 25, 26, 27], hip extension [24, 26, 27, 28],and ankle plantar flexion [24, 25, 26, 27],. The exosuits do face several difficulties like other exoskeletons do. Since exosuits only actuate to generate assistive force when certain muscle contracts, the timing to actuate become a major factor that affect the effectiveness of exosuit. In the lack of rigid frame that can track position of limbs with force or angle sensors, the task of accurate timing is even more difficult. To track or estimate position of limbs more accurately, a soft strain sensor as shown in Figure 1-8, capable of measuring 250% strain was developed [29], and other methods like foot switch [24, 25], IMU [30], and load cell [31] were used to track the phase of gait to compute average cadence or to give the right timing for actuation. Another major difficulty is that the assistive force is directly exerted on human body and utilizes bone structure to withstand compression stress. Therefore, the assistive force cannot be either too weak or too strong. Otherwise, it will be either useless or dangerous. To deal with it, gait analysis is conducted to evaluate actuation speed and force, and flexible materials are used to provide sufficient compliance. As a result, the exosuits achieve an averagely 15% reduction of metabolic rate in treadmill walking at constant speed of 1.5 m/s, while carrying load equivalent to 30% of body mass [32]. [33]

Figure 1-7(a) Exosuits for multi-joint assistance [33] (b) Exosuits for hip extension assistance [28] (c) Force diagram of Exosuits for hip extension assistance [26]

Figure 1-8 Soft strain sensor that is capable of measuring 250% strain [29].