Artículo Científico / Scientific Paper

MECHANICAL DESIGN OF AN ACTIVE ORTHOSIS: KNEE AND ANKLE Villa-Parra A.C1, Broche L2, Sagaró R3, Bastos T4, Zequera M5.

Resumen

Abstract

Las órtesis activas (OA) son dispositivos empleados para asistir el movimiento de las articulaciones en la marcha y para tareas de rehabilitación. Para el diseño mecánico de estos dispositivos es necesario conocer principios del movimiento humano y de ergonomía. Además se deben cumplir altas exigencias en cuanto al rendimiento de sus actuadores y a la resistencia de su estructura. En este contexto, el objetivo de este artículo es desarrollar el diseño mecánico de una órtesis activa para rodilla y tobillo. Se presenta una revisión de los requerimientos para la selección de actuadores en el campo de OA y un estudio biomecánico de la extremidad inferior. Con esta metodología se realizó el diseño mecánico de dos OA usando actuadores eléctricos para usuarios con deficiencia de flexión de rodilla y pie caído.

Active orthoses (AO) are robotic devices to assist the joint movement in walking and rehabilitation task. These devices need following principles of human motion and ergonomy in their mechanical design, in addition to have high requirements regarding their performance and structure. In this context, the main goal of this paper is to develop a mechanical design of AO for the knee and ankle. We present a review of the requirements for actuator selection in the AO field and a biomechanical study of the lower limb. This approach allows the development of a mechanical design of AO using electric actuators for users with knee flexion disability and foot drop.

Palabras Clave: órtesis activas, discapacidades, diseño mecánico.

ayudas

a Keywords: active orthoses, mechanical design, orthotics.

1

MSc. Electronic Engineering. Electronic Engineer. PhD. Student - Electrical Engineering Department, Universidade Federal do Espírito Santo, Brazil, and Universidad Politécnica Salesiana, Ecuador. 2 Mechanical Engineer. PhD Student - Mechanical and Design Engineering Department, Universidad de Oriente, Cuba. 3 Mechanical Engineering PhD. Professor - Mechanical and Design Engineering Department, Universidad de Oriente, Cuba. 4 Electric Engineering PhD. Professor - Electrical Engineering Department, Universidade Federal do Espírito Santo, Brazil. 5 Bioenginering PhD. Professor - Electronic Engineering Department, Pontificia Universidad Javeriana, Colombia. Corresponding author: e-mail: [email protected].

135

Artículo Científico / Scientific Paper Table 1: Design criteria for active orthosis

1. Introduction Active orthoses (AO) of lower limb are robotic devices that work in parallel with the human body [1] and are options for people with gait disorder who are unable to walk without assistance [2]. Also, these devices allow developing new clinical treatment strategies for rehabilitation and daily assistance [3]. AO have mechanical actuation to apply forces to the human limbs and compensate a deficient function in a joint [1]. These active devices have an onboard or tethered source of power, sensors and a computer to control the application of torque during gait [3]. The control system of AO has to be able to assist injured patients according to their physical condition [4] and can be induce an effective recovery [5]. For the knee joint, the active knee orthosis (AKO) have the functions of controling the knee flexion (bending) and knee extension (straightening) [2]. The device supplies assistive torque at the joint to alleviate the loading at the knee, and thus reduce the muscular effort required to perform activities of daily living [6]. Some studies report that reducing the pressure on the knee helps to relieve the strain and stress acting on the knee [5]. The active ankle foot orthosis (AAFO) provides support at the ankle and prevents foot drop during swing phase of the gait [7]. Foot drop is a common gait problem in the swing phase due to the failure to sufficiently activate the tibialis anterior prior to heel-strike [3], [8]. Some examples of research in this field are the self-adjusting orthosis for rehabilitation [2], the self-adjusting knee exoskeleton for robot-assisted treatment of knee injuries [9], an AAFO with ankle joint brake [10] and the AFO with a robotic tendon [11]. These devices have high requirements regarding their performance, weight and size of both actuator and structure, wearability of their design and safety of the user [12]. The AO design need following the basic principles of ergonomy. The design is based on the mechanism of the human body and its kinematics [13]. It also must be compact and light weight to minimize the energetic impact to the wearer [3].

CRITERIA

MECHANICAL STRUCTURE

ACTUATOR

*

Alignment with user’s joints Adaptability to different users Light weight and strong Stops to inhibit to go beyond the physiological ranges of motion Easy to wear and ergonomic Powerful (joint torques comparable to healthy individuals) Low mechanical impedance* Light weight and safety Highly compliant and zero backlash Compact design and efficient Positioning accuracy and repeatability

relationship between moment applied by the robotic orthosis and the joint angle.

Safety is a key concern for the robotic devices working in close proximity with human subjects [14]. To ensure that the AO moves together with the user, the mechanical structure is attached to the wearer’s joints. The structure can be equipped with encoders, sEMG electrodes, inertial and force sensors to capture the joint information of the user [5]. Table 1 summarizes the criteria for AO design indicated in [1], [3] and [15]. In the rehabilitation field, assisted gait training with AO has several advantages over manual physical therapy. It facilitates repetitive and prolonged training sessions, decreases the physical burden of therapists, while also significantly reduces the application related costs [1], [9]. The main goal of this paper is to present the mechanical design of AO for users with knee and ankle disabilities in the gait.

2. Methods 2.1 Biomechanical study Design requirements of AO can be determined by analyzing the human motion. According to the principles of anatomy, the lower limb can be divided into segments (thigh, calf, foot) and joints (hip, knee and ankle) [13]. The knee is the most heavily stressed joint because it has to support the 136

body’s weight and maintain the balance and stability. During normal gait, the ankle, shank and foot play important roles in all aspects of locomotion, including motion control, shock absorption, stance stability, energy conservation and propulsion [3]. The motion range of the knee and ankle is greater in the sagittal plane than in other planes during walking [5], [16]. For that reason, the knee movement of flexion-extension (F-E) and ankle dorsiflexion-plantar flexion (DF-PF) are where the greatest torques and angular displacements are applied during gait cycle (see figure 1 [17]).

(a) (b) Figure 1. Knee and ankle during walking [17]. (a) joint angle; (b) torque.

considering average heights (1.5 to 1.85 m) and weights (50 to 95 kg).

2.2 Selection of actuators Actuator selection is also a very difficult task since the space available for the actuators around the joints is quite limited [15]. Joint actuators are required to provide the adequate torque, angle values and power demand required during walking. For that, velocity–torque characteristics for targeted joint motions should be known in order to select the actuators [15]. Electric motors, are low-torque and high-velocity actuators, which require transmissions with the output at the joint [3]. The joint torque requirements for the actuators consider a mass that includes the structure and the actuator. This can be calculated by using (1) [15]. (1)

,

where TR is the required joint torque, TN is the normalized joint torque, and mT is the total mass carried by the AO, that can be calculated by (2). ,

(2)

where mexo is the mass of the exoskeleton, muser is the mass of the user, mact is the total mass of the actuators, and moe is the mass of auxiliary equipment including electronics, power source, etc. For this AO design, the approaches by Winter [16] are used, which are based on the anthropometric characteristics of the users

2.3 Active Knee Orthosis (AKO) Treatment of knee injuries requires reducing the pressure acting on this joint [5]. Thus, the AKO structure requires two braces (with soft straps) for thigh and shank segments. The software SolidWork FEM (Finite Elements Methods) Simulation 2013 was used for the structure design. The materials selected for the structure (thigh and leg) were stainless steel AISI 304. The knee joint orthosis is designed to satisfy the joint motion with angle range of 115º, because the rotation of the knee joint typically exceeds 90º during flexion and extension exercises [9] and the maximum angle for pick up an object from the floor is 117º [18]. Mechanical switch are placed as a measure of safety for the user. For the knee, the maximum available moment in human gait is 0.5 N·m /kg (Figure 1b). Based on the criteria of Table 1, the selected actuator for our design was the set RoboDrive Motor (servomotor) with Harmonic Drive gearbox [19], [20]. These actuators with compact design have a high torque and power density through maximum copper fill factor, with reduced power loss through optimized stator and rotor design [19]. The accurate and zero-backlash harmonic drive component sets can be integrated into the application to save space [7], [20]. The harmonic drive selected belongs to the CSD serie, model

137

(a)

(b)

(c)

Figure 2. (a) Structure of CAD Modeling; (b) Stress distribution in the structure; (c) Distribution of safety factor.

GR-2A, and type 32, with a 50 gear ratio and a weight of 0.51 kg. Applying the equations (1) and (2) with mexo = 3 kg, muser = 95 kg, mact = 0.51 kg, moe = 0.5 kg, the torque would be around 50 N·m. The system has a rotation speed of 60 rpm and a nominal torque of 71.5 N·m, being suitable for this application. This value is according to the literature, as [21] reports that the maximum torques range for a knee actuation system is 60 N·m. Figure 2a shows the CAD model of the AKO structure with actuators and the harmonic drive gear appearance. Applying FEM, the mechanical system was subjected to critical loads. Figures 2a-b show the maximum value of stresses of 15 MPa, giving a safety factor of 8. The AKO includes a potentiometer to measure the knee joint angle and two force sensor are attached to the segments in order to recognize the alignment of the structure and can be used to detect the gait phase. The AKO can include sEMG electrodes to obtain physiological information about movement intentionality and implement control tasks of fatigue adaptation and monitoring. The power supply will be located near of the waist.

needs to be 90º [3]. An AAFO with a control system can generate an assistive torque at initial swing phase for achieve to DF movement (see Figure 1a). For the ankle, the available moment in swing phase gait is close to zero (see Figure 1b). Following Mills [22], the moment in the swing phase may be considered as 0.04 N·m/kg. The AAFO needs to include heel strike and reduction of pressure at the metatarsal heads [23]. For that reason, an AAFO design needs to consider a specific customized structure. Figure 4a-d shows the sequence of the AAFO modeling. The design consists of a shank (adjusted to different diameters) and foot segments made of polypropylene, which is a material lightweight and strong. The AAFO is attached to the shank segment with a revolute ankle of aluminum that is powered by an electrical actuator (Rotary Voice Coil Actuators RA29) [24], used in such applications [25] and given the requirements and considerations of Table 1. For the AAFO, equation (2) consideres muser as the mass of the foot mfoot [26].

2.4 Active Ankle-Foot Orthosis (AAFO)

Applying (1), (2) and (3) with mexo = 0.25 kg (polypropylene structure), mact = 0.16 kg, muser = 95 kg, mfoot = 1.38 kg, moe = 0.60 kg, the torque would be around 0.1 N·m. The maximum resistive

To maintain foot clearance in the swing phase of the gait, the angle between the foot and shank

(3)

138

(a)

(b)

(c)

(d)

(e)

Figure 4. (a) Plaster cast of a right leg to develop the positive (solid) mold; (b) The AFO in polypropylene plastic; (c) 1. Power supply and electric circuit, 2. Shank segment, it can be adjusted to different diameters, 3. Powered joint, 4. Potentiometer, 5. Foot segment and sensor sole; (d) The modeling of the AAFO; (e) AAFO work positions.

torque of the motor is 0.22 N·m and can be used for this application. The AAFO includes a potentiometer to measure the ankle joint angle and two force sensors are attached to the sole in order to determine the beginning of the gait cycle and recognize the swing phase [10], [25]. The electric circuit with a dsPic microcontroller and 4 Ni-Cd batteries are attached at the back of the shank (see Figure 4d). The total weight of the AAFO does not exceeds 2 kg, which is the load that can produce an energetic impact on the wearer [3]. The mechanical switches and the total excursion angle of the motor guarantee the range maximum between 8º to -24º (see Figure 4e), according to figure 1a.

3. Results and Discussion As a result, it is verified that the selected actuators in AKO and AAFO are capable to be used by a user with disabilities of 95 kg. The weight of the orthosis is an important aspect of the design. In our case, the total weight of the Table 2: Active orthoses specifications PARAMETER Stand dimensions [mm] Lenght Width Height Weight [kg] Joint motion DC Power supply [V]

AKO

AAFO

540 10 30 4.10 115°/0° 6

294 121 348 1.01 8°/-24° 26

AKO is 4.1 kg which is acceptable if we compare it with the prototype WSE that has a weight of 15 kg excluding the actuators [15]. The FEM analysis demonstrates that the mechanical system can be employed to assist the (F-E) of the knee joint required during gait. For task of standing up motion, this orthosis cannot be consider because the torque value is around 100 N·m [12]. The total weight of the AAFO is 1.01 kg and is within the range of other prototypes (0.95 kg, [27] and 2.2 kg [10] ). The AAFO is only able to work and maintenance the ankle joint angle in 90º during the swing phase. For the entire gait cycle other actuators are necessary, because the stance phase requires a higher torque (Figure 1b). The KAO and AAFO actuators are provided high torques while operating in high speeds. In the case of AKO, the maximum angle of motion (115°) for human gait is not necessary, however this was considered in order to allow that the user can sit without removing the orthosis. The KAO anthropometric characteristics are 1.5-1.85 m and 50-95 kg. For AAFO the structure is customized for a user of 50-95 kg. Table 2 summarizes the AKO and AAFO specifications.

4. Conclusions In this paper the mechanical design of an AO for users with foot drop and knee flexion disabilities was presented. For the design and improvement of an AO in relation to the study of the biomechanical of the lower limb, it is important to 139

obtain a more comfortable and natural device to assist gait disorders. In FEM analysis, it was demonstrated that AKO is acceptable to use in all gait cycle. Mechanical switches are required as safety measure for the user because this guarantee physiological ranges of motion. A robust control algorithm is necessary to use the AO for human walking assistance or rehabilitation tasks, which is the next step of this work.

References [1] S. Hussain, S. Q. Xie, y G. Liu, «Robot assisted treadmill training: Mechanisms and training strategies», Medical Engineering & Physics, vol. 33, n.o 5, pp. 527-533, jun. 2011. [2] D. Cai, P. Bidaud, V. Hayward, F. Gosselin, y F. Fontenay Aux Roses, «Design of self-adjusting orthoses for rehabilitation», en Proceedings of the 14th IASTED International Conference Robotics and Applications (RA 2009), 2009, pp. 215–223. [3] K. A. Shorter, J. Xia, E. T. Hsiao-Wecksler, W. K. Durfee, y G. F. Kogler, «Technologies for Powered AnkleFoot Orthotic Systems: Possibilities and Challenges», IEEE/ASME Transactions on Mechatronics, vol. 18, n.o 1, pp. 337-347, feb. 2013. [4] K. Anam y A. A. Al-Jumaily, «Active Exoskeleton Control Systems: State of the Art», Procedia Engineering, vol. 41, pp. 988-994, ene. 2012. [5] K. H. Low y Y. Yin, «An integrated lower exoskeleton system towards design of a portable active orthotic device», International Journal of Robotics and Automation, vol. 22, n.o 1, pp. 32–43, 2007. [6] M. Chandrapal, «Intelligent Assistive Knee Orthotic Device Utilizing Pneumatic Artificial Muscles», University of Canterbury, Christchurch, New Zealand, 2012. [7] H. A. Quintero, R. J. Farris, y M. Goldfarb, «Control and implementation of a powered lower limb orthosis to aid walking in paraplegic individuals», en Rehabilitation Robotics (ICORR), 2011 IEEE International Conference on, 2011, pp. 1–6. [8] A. Gordon Robertson, Gary Kamen, Graham Caldwell, Joseph Hamill, y Saunders Whittlesey, Research Methods in Biomechanics, vol. 1. Human Kinetics, 2014. [9] M. A. Ergin y V. Patoglu, «A self-adjusting knee exoskeleton for robot-assisted treatment of knee injuries», en Intelligent Robots and Systems (IROS), 2011 IEEE/RSJ International Conference on, 2011, pp. 4917–4922. [10] N. Yoshizawa, «Active AFO with ankle joint brake friction control using force observer», en Engineering in Medicine and Biology Society (EMBC), 2012 Annual International Conference of the IEEE, 2012, pp. 1900–1903. [11] A. W. Boehler, K. W. Hollander, T. G. Sugar, y D. Shin, «Design, implementation and test results of a robust control method for a powered ankle foot orthosis (AFO)», en

Robotics and Automation, 2008. ICRA 2008. IEEE International Conference on, 2008, pp. 2025–2030. [12] N. C. Karavas, N. G. Tsagarakis, y D. G. Caldwell, «Design, modeling and control of a series elastic actuator for an assistive knee exoskeleton», en Biomedical Robotics and Biomechatronics (BioRob), 2012 4th IEEE RAS & EMBS International Conference on, 2012, pp. 1813–1819. [13] C. Chen, D. Zheng, A. Peng, C. Wang, y X. Wu, «Flexible design of a wearable lower limb exoskeleton robot», en Robotics and Biomimetics (ROBIO), 2013 IEEE International Conference on, 2013, pp. 209–214. [14] S. Hussain, S. Q. Xie, P. K. Jamwal, y J. Parsons, «An intrinsically compliant robotic orthosis for treadmill training», Medical Engineering & Physics, vol. 34, n.o 10, pp. 1448-1453, dic. 2012. [15] U. Onen, F. M. Botsali, M. Kalyoncu, M. Tinkir, N. Yilmaz, y Y. Sahin, «Design and Actuator Selection of a Lower Extremity Exoskeleton», IEEE/ASME Transactions on Mechatronics, pp. 1-10, 2013. [16] D. A. Winter, Biomechanics and Motor Control of Human Movement. John Wiley & Sons, 2009. [17] M. Whittle, Gait analysis: an introduction. Edinburgh; New York: Butterworth-Heinemann, 2007. [18] rat o n y oger . a e ncia nstituto de io ec nica de Valencia, 1998. [19] «Lightweight Torque Servo Motors: Flexible drive solutions through motors with RoboDrive technology - TQ Group GmbH». [Online]. Available in: http://www.tqgroup.com/en/products/product-details/prod/leichtbautorque-servomotoren/extb/Main/. [Access: 10-feb-2014]. [20] «Harmonic Drive AG». [Online]. Available in: http://www.harmonicdrive.de/english/products/harmonicdrive-gears/. [Access: 10-feb-2014]. [21] J. F. Veneman, «A Series Elastic- and Bowden-CableBased Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots», The International Journal of Robotics Research, vol. 25, n.o 3, pp. 261-281, mar. 2006. [22] P. M. Mills y R. S. Barrett, «Swing phase mechanics of healthy young and elderly men», Human movement science, vol. 20, n.o 4, pp. 427–446, 2001. [23] B. M. Kelly, M. C. Spires, y J. A. Restrepo, «Orthotic and prosthetic prescriptions for today and tomorrow», Phys Med Rehabil Clin N Am, vol. 18, n.o 4, pp. 785-858, 2007. [24] BEI KIMCO MAGNETICS, «Actuators Rotary Voice Coil Actuators», 2011. [25] I. Veneva, «Intelligent device for control of active ankle-foot orthosis», in 7th IASTED international conference on biomedical engineering; 17–19 feburary, 2010, pp. 100–105. [26] B. Chaffin, Gunnar B, J. Andersson, y Bernard J. Martin, Occupational Biomechanics, Wiley. 2006. [27] L. Eamer y R. Peruzzo, «Redesign of Ankle Foot Orthoses for Increased Stability and Mobility free ebook download», University of Toronto, 2008.

140