Pneumatic fluid-powered actuation
Pneumatic fluid-powered actuators and robots have the advantages of flexibility, lightweight nature, safe interaction, ease of installation, and are comfortable to wear compared with their rigid counterparts. Pneumatic systems are usually made from soft materials such as polymers and fabrics, which can provide sufficient compliance, high force/volume ratio and a high degree of biocompatibility. A well-known example is the McKibben artificial muscle, which was invented by Richard H Gaylord [22] in 1958 and popularised by Joseph L McKibben at the beginning of 1960s. It has been used as a pneumatic actuator for orthotic systems [23-25] since the 1960s. However, the limited force (100 N – 300 N) generated by the McKibben artificial muscle and the requirement of a compressed air tank limits its application to finger prehension orthosis. However, different designs in terms of muscle length, diameter, weaving angle and weaving fabric can significantly affect the static force of the McKibben muscle. With the development of advanced technologies in materials and actuation, the McKibben muscle was further developed and introduced into robotic devices to mimic the compliance of natural skeletal muscle by Bridgestone and other industrial companies in 1980s and 1990s [26]. Recently, with the growing need for safe interaction and flexibility, pneumatic fluid-powered artificial muscles and robots have been widely used for rehabilitation and assistive devices [13][16]. In this section, rehabilitation and assistive devices enabled by pneumatic fluid-powered actuators and robots that have been developed in the last ten years for use on both the upper and lower body are reviewed and discussed.
Soft assistive devices, such as ankle-foot orthosis, have been developed for gait assistance and rehabilitation. Shorter et al. [27] developed a novel untethered ankle-foot orthosis (PPAFO) based on a pneumatic fluid-powered rotary actuator to provide untethered gait assistance for ankle-foot rehabilitation. A portable pneumatic power source, a rotary actuator, embedded electronics and sensors were integrated to construct an untethered device, as shown in Figure 2a. The total weight of the PPAFO was 3.1 kg, which was distributed on a belt. The orthosis was able to provide plantar flexor and dorsiflexor torque assistance using a bidirectional pneumatic rotary actuator. The system positional response speed of the orthosis was 600°/s for a 0.98 kg load and 900°/s with no load, using a driven pressure of 0.62 MPa. The untethered ankle-foot orthosis demonstrates that the use of pneumatic fluid-powered actuators provides the capability to actuate a joint, with the advantages of a high force/weight and force/volume ratio. The results from non-disabled walking trials showed that a peak assistive torque of 9.2 Nm can be achieved at an operating pressure of 0.62 MPa, which validates that the PPAFO can provide sufficient plantar flexor assistance for gait modification.
Park et al. developed an active soft ankle-foot orthotics that was powered by pneumatic artificial muscles for gait assistance [28]. The device was fabricated with flexible and soft materials that mimic the muscle-tendon-ligament structure. The prototype weighed 950 g and was able to provide 12° dorsiflexion from a resting position of an ankle joint and a 20° dorsiflexion from plantarflexion. The active orthotics include three anterior muscles for assisting dorsiflexion in alleviating the drop-foot condition and can be expanded to include posterior muscles for actively assisting plantarflexion in a complete gait cycle. Thalman et al. [29] designed a soft one-size-fits-all ankle-foot orthosis (SR-AFO) using pneumatic soft fabric actuators to assist in inversion-eversion (IE) ankle support and gait rehabilitation, as shown in Figure 2b. The Exosuit was constructed from a custom neoprene fabric sleeve, which could be worn as a boot and adapted to the foot size of most adults. The soft fabric actuators were designed to operate vertically on both sides of the ankle, starting at the base of the talus and across the medial and lateral malleolus of the ankle. These actuators were deliberately used to mimic the placement of conventional ankle braces to prevent medial and lateral instability and provide IE ankle support. More importantly, the fabric actuators can be pneumatically controlled to increase IE ankle stiffness while not affecting ankle dorsiflexion-plantarflexion (DP) stiffness. The ability to actively vary the device stiffness provides an effective approach to prevent ankle buckling in the IE direction, while providing a comfortable, dynamic solution and behaving as a garment when inactive. Testing on six healthy subjects showed that the SR-AFO device could change eversion stiffness from approximately 20 to 70 Nm/rad at 30 kPa, while minimising the change in dorsiflexion stiffness. The variable stiffness of the soft pneumatic actuators makes the ankle-foot orthosis highly versatile and functional in multiple directions, as compared to conventional rigid motors. In addition, with an in-depth understanding of the kinematic and dynamic responses of the lower limb during walking, the controllability and portability of the SR-AFO can be further enhanced.
The development of soft wearable Exosuits is another area that is attracting increasing interest. A soft, ultra-lightweight lower-extremity robotic Exosuit for augmenting normal muscle function by 50% was developed by Wehner et al. [30]. The Exosuit was produced using a virtual anchor technique that connects McKibben pneumatic actuators through a soft network based on an inextensible webbing with triangulated attachment points. As shown in Figure 2c, pneumatic actuators are used to generate joint torques in the sagittal plane at the ankle, knee, and hip. This was achieved by connecting pneumatic muscles to the virtual anchor points (red points in Figure 2c) that were constrained using the soft inextensible webbing. The reaction force from the pneumatic muscles can be redirected to the connecting anchors. The pneumatic muscles used for the Exosuit were able to achieve a 90% of max force (235 N) in 0.316 s during inflation and a decrease from a maximum force to 10% of its maximum force in 0.098 s during deflation. The results demonstrated that the Exosuit had little effect on walking kinematics and the metabolic power (386.7 W), which is used to estimate the energy demands of acceleration and deceleration events of activities, which was almost identical to the average power (381.8 W) without the suit. The soft Exosuit is ultra-lightweight with a total weight of only 9121 g, which has significantly reduced mechanical impedance and inertia compared with traditional rigid exoskeletons. Another lightweight wearable augmented walking suit (AWS) was developed by Thakur et al. [31], using pneumatic gel muscles (PGMs) that were able to generate assistive forces of 30 N at 60 kPa and 44 N at 100 kPa at an operating pressure range from 50 to 300 kPa. The walking suit provides both waist and knee support by attaching the PGM along the rectus femoris muscle. Force sensitive resistor sensors were arranged in the shoe to detect forces during the walking and thereby provide feedback to the assistive suit. A proportional feedback controller was used to switch on the PGMs to provide an assistive force during the swing phase of the human gait cycle, which was identified by the force-sensitive resistor (FSR) sensors placed in the shoes. This form of an augmented walking suit is untethered, portable, and lightweight with a total weight of 1.2 kg. Experiments on subjects demonstrated that the AWS could achieve a significant reduction in the activity of most muscles, with a maximum of 44% reduction in the maximum voluntary contraction (MVC) of rectus femoris muscle at 60 kPa, and a maximum of 27.6% reduction in MVC of biceps femoris muscle at 100 kPa while walking. The AWS is lightweight, portable and easy to use. To improve the controllability of the suit, inertial measurement units (IMUs) can be used to achieve improved motion detection for feedback control during a walking stance. It was noted that the AWS could be more user-friendly and comfortable by integrating the controller, battery, and air tank into the waist support belt.
Soft exosuits are also being developed for patient rehabilitation and disease prevention. For example, Deep Vein Thrombosis (DVT) is a severe medical condition that can affect patients who are bed-ridden for a long period after a stroke. Blood clots in the deep veins of the lower extremity can significantly affect and deteriorate normal blood flow. The current state-of-the-art methodologies to prevent DVT can be categorised as pharmacological prophylaxis and mechanical prophylaxis. While the pharmacological prophylaxis uses anticoagulant drugs to prevent blood clotting, mechanical prophylaxis systems are normally developed to promote venous blood flow and resolve the venous stasis. However, commercial mechanical prophylaxis devices have led to side effects such as skin necrosis, blisters, and injury due to rigid patient-machine interactions. A soft wearable suit has therefore been considered as a new approach to improve the patient experience and tackle challenges associated with DVT. Low et al. [32] developed an Exosock for DVT, primarily as an alternative to mechanical prophylaxis. The Exosock was driven by soft double-extension pneumatic actuators which can be attached to the lower extremity of the body, as shown in Figure 2d. The soft Exosock consists of five different modules including a sock, knee sleeve, soft double-extension actuators placed within a fabric, joint angle sensor and a programmable pump-valve controller. When the actuator is inflated, it extends and guides the foot in a plantarflexion motion; when the actuator is deflated, it retracts to its original length in which the resultant tension assists in dorsiflexion motion of the ankle. An inertial measurement unit (IMU) was attached to the metatarsal region of the foot to measure the ankle joint angle in real-time and provide feedback to the controller. The soft actuator was able to generate a peak force of 33.2 ± 0.3N at 100% strain. Pilot results on a healthy subject showed that the Exosock was able to achieve an average of 16.4 ± 1.3° of passive ankle dorsiflexion consistently with low deviation, which provides a promising solution for DVT prevention. The real-time feedback and the wireless control capabilities of the Exosock can allow therapists or doctors to monitor ankle exercises and control the Exosock remotely; however the overall efficacy of the device for DVT prevention needs to be quantified with the use of clinical trials.
Total knee arthroplasty (TKA) is a surgical process to restore the knee joint function by replacing a damaged, worn, or diseased knee with an artificial joint. After a TKA procedure, patients often have a reduced range of motion and low quadricep strength; there is therefore a need for an assistive device. However, conventional metal-based exoskeletons have size and weight limitations. As a solution, a lightweight pneumatic exoskeleton for TKA patients was developed by Ezzibdeh et al. [33] that used a fabric pneumatic actuator as the bending joint. The exoskeleton consisted of a plastic structure body with on-board electronics, a power pack in the waist belt, a knee joint enabled by the fabric pneumatic actuator, and four Velcro straps to secure the device in place, as seen in Figure 2e. The device had a built-in force-feedback controller and a non-linear spring to assist in detecting the intent of the patient to move and provide continuous assistance. The positive results showed that the pneumatic exoskeletons are promising in assisting patients after TKA procedures.
Baiden and Ivlev [34] proposed a concept of an active human-robot-interaction (HRI) control for orthoses using soft pneumatic actuators. A 2-DoFs exoskeleton robot was applied to implement the concept and analyse HRI control strategies. The robot was comprised of two lower extremities orthoses equipped with soft actuators that were directly attached to human knee and ankle joints, as shown in Figure 2f. To realise rotary motion and allow the device to mimic human kinematics, a soft pneumatic actuator with pleated rotary elastic chambers (REC) was developed and used to create the orthoses [35]. A knee joint that combined a series of two REC-actuators were able to achieve a full range of motion from 0° to 90°, while at the ankle joint a single REC-actuator was able to rotate from 0° to 45°. A master-slave with a compliant position control strategy was developed for HRI, in which a reference movement was achieved from the healthy leg as the master, and the desired control signals were generated and transferred to the impaired leg to achieve precise movement. This approach can be highly suitable for hemiplegia patients who are paralysed on one side of their body but are able to readily move their limbs on the unaffected body part and balance using a support frame. The HRI concept was proven using human sit-to-stand and stand-to-sit examples. The controllable stiffness of the REC actuators was also exploited to achieve a spring-like behaviour, with an independent stiffness-position or stiffness-torque control to further enhance the HRI concept [36]. A linear relationship between the torque and driven pressure was assumed for the REC systems, independent stiffness-position and stiffness-torque control can be realised using a position and torque look-up table determined from experimental calibration, which effectively includes any non-linearities of the system. The desired torque and stiffness need to be accurately predicted to maintain an appropriate pressure within the supply pressure range and the torque-angle hysteresis will need to be compensated.
Fang et al. designed a foldable pneumatic bending actuator (FPBA), which were fabricated using thermoplastic polyurethane fabric materials and developed for an FPBA-based knee Exosuit [37]. FPBAs were formed in a bellow structure and could be folded into fan shapes to achieve different folding angles from 0 to 360°. With an increase in pressure from 0 to 40 kPa, the folding angle was varied from 30° to 180°, and the output torque was up to 25.74 Nm. The Exosuit consisted of an onboard electronic and sensing system, an off-board pneumatic and control system, and a knee sleeve that attached the FPBA behind the user’s knee. Five healthy subjects were recruited to test the knee Exosuit. The results showed that FPBAs were able to generate bending motions with a large range of motion that could achieve human-scale torque levels at a low input pressure (up to 40 kPa), which can effectively assist in knee movement. The FPBA can bend at any angle without airflow restrictions with the advantages of low cost, ease of manufacturing, mechanical flexibility, and comfortability. However, the FPBA is currently tethered and not portable since it employs a miniature pump; however, the design of a compact power supply could enable an untethered FPBA to be produced. Furthermore, human intention recognition, such as biomechanical information recognition and surface electromyography recognition, can be used to realise human-robot-interaction control, which can improve the controllability of the FPBA.