Proposal of a Mechanism for an Electrical Elbow Prosthesis

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1 Proposal of a Mechanism for an Electrical Elbow Prosthesis D. A. Contreras, A. Ramírez-García, F. J. Gallegos, I. Bazán Instituto Politécnico Nacional, EIME Zacatenco México D.F. dianalecontreras@gmail.com, alramirezga@ipn.mx, fgallegosf@ipn.mx, ibazan@ipn.mx Abstract This work focuses on the description of the system of mechanism for a electrical prosthesis. Considering system performance and system of transmission, both will allow to make the movements of the electrical prosthesis. The electrical prosthesis was designed to replace the function of the elbow joint of an upper limb. In the system of transmission is based on rotary motion since it is similar to the movements of the elbow joint which is flexion and extension, to carry out these movements were determined using the gears; of which were calculated to produce them, it is described in this work. On the other hand, the system performance was chosen by using the electrical actuators for high efficiency, high availability and variety of sizes. To be able to define our system of mechanism of a electrical prosthesis, it should be considered several criteria listed in this work, and finally the torque output was calculated to know how much weight can load the prosthesis designed. Keywords gear; prosthesis; servomotor; system of activation; system of transmission; torque I. INTRODUCTION Today, electrical prostheses have evolved with the development of technology. In general, an electrical prosthesis is constituted by an electronic system, a control system and a mechanical system. These prostheses are controlled by biosignals to generate an electrical signal due to the chemical interaction of body. This signal is detected by sensors, after is processed by an electronic circuit, and finally is used in the control of actuators that are coupled to a mechanism to generate movements. This project proposes a kind of mechanism for a electrical prosthesis that simulates the movements of a joint of the elbow that are extension-flexion. To determine the type of mechanism should know the specifications of an electrical prosthesis, as well as the system performance and the system of transmission. The first system is the type of source that a mechanism requires to generate movements in the prosthesis; the second system is responsible for producing movements through of mechanical elements. After the choice of the type of system performance and transmission, it is to perform the calculations for the elaboration of the mechanism. Finally a prototype of prosthesis should be characterized to determine if it meets the specifications. II. DEVELOPMENT A. pecifications In the design of mechanism of a prosthesis the specifications should be established so it is non-invasive and is easy to use for users, these specifications are shown in Table I, which consists of a research of commercial prostheses [1, 2, 3, 4]. TABLE I. Range of motion (flexio-extension): PECIFICATION OF COMMERCIAL PROTHEE Commercial electrical prostheses Utah Arm NY Electric Elbow Boston Elbow Otto Bock Load capacity: 1 Kg Kg 4.5 Kg Time without load in flexion: Time without load in extension: 1.2s 1.1s 0.655s Take into account these specifications, in the design of mechanism is essential to investigate the system of activation and the system of transmission to determine the type of mechanism that we have to use. The following describes each type of system. B. ystem of activation This system has elements that are responsible for producing mechanical performance, these elements are called actuators. An actuator is an inherently mechanical device whose function is to provide force to move a mechanism. There are three types of actuators as pneumatic, hydraulic and electrical ; in the first, its energy source is the pressurised air and its application in the field of prosthesis is when only required two states as open-close of a gripper (hand); in the second, its source is the fluid (mineral oil is the most common) and applies to largest robots so that requires greatest speed and highest mechanical resistance; finally in the third its energy source is electricity and are used in robots because of requires accuracy and repeatability. Electrical actuators are more 1s 128

2 applicable than other in the design of prostheses because these have a high efficiency, high availability and compact sizes. There are several types of these actuators as step, DC, servo, etc. The step have a high torque at low speeds, high precision when calculating the position, it is inexpensive and requires little maintenance. The DC have a fast response, good performance, low cost, also if the voltage applied in its terminals is reverse so its direction of rotation also changes. These are used in companies such as Otto Bock and Centri [5]. The servo are very common for applications in prosthesis, because these are efficients, rotate approximately 0-180, are easy to control and have a high torque. C. ystem of transmission An objective in the design of any prostheses is to simulate as close body movements. In this work, the simulation of the movements are extension and flexion, these movements have to be soft but electrical actuators work at excessive speed, that's why system of transmission will require reducing speed. There are three most common types of system in prostheses. The first is the gears, these are toothed wheels that are compacted, an example of its use is in the Utah Arm prosthesis [6]. The second is the bands that have great flexibility, are not noisy and don t require lubrication; and the third is the straps, these are flexibles and no slips. D. Materials Moreove of the design is the support that serves to contain and to hold the elements of the systems. This support should be light and tough, which the following materials are proposed: polypropylene is a type of widespread plastic and is usually used for the sockets of the prostheses, aluminum is a nonferrous metal and is light due its low density and titanium is a corrosion-resistant, non-toxic material and is usually used for internal prostheses. The Table II, describes more features of these materials. Material TABLE II. Polypropylene Aluminum Titanium TYPE OF MATERIAL IN PROTHEE Characteristics Density [g/cm 3 ] Elastic limit [MPa] 11x x10 2 Tensile force [MPa] Density [g/cm 3 ] 2.7 Elastic limit [Mpa] 13x x10 4 Resistencia a la tracción [MPa] Density [g/cm 3 ] 4.51 Elastic limit [Mpa] Tensile force [MPa] As shown, polypropylene is the lightest and titanium is the most resistant. E. Matrices of decision This paper has been proposed several elements for each system, you must decide which electrical actuator and mechanism, you will use. These elements will be evaluated with matrices of decision. Its procedure is to evaluate each element (types of electrical actuators and mechanisms), taking into account the specifications of a prosthesis. Numeric values shown in Table III are used for its evaluation. TABLE III. VALUE TO EVALUATE Values 9 Good 6 Regular 3 Bad The procedure is the following, both system of activation and system of transmission are set specifications, by each specification is assigned a percentage from 0 to 100% according to its compliance with the design of a prosthesis, then the assigned value is multiplied by the percentage for the weighted average (p.p.), finally adds the p.p. of all specifications and gets a total average. The Table IV and Table V are matrices of decision of each one of the systems. The servomotor had higher percentage similarly the gears for the system of transmission. F. Calculations Then, the servomotor that decided to use was the HD- 1501mg, its properties are: operating voltage of 4.8V-6.0V (DC), with speed of 0.16sec/60º-0.14sec/60º (without load), torque of 15.5 kg cm-17kg.cm with angle of 180 ±10. According to the Table V, it was decided to make a pair of gears by its simple design, in addition to reducing speed and increasing torque. A pair of gears is composed of two gears one small and one large. To perform its calculations are required to know the transmitted power (P) and the angular velocity (ω) of the servomotor, which the calculations are described below: tep 1: To specify the number of teeth of a small gear (N ) known as sprocket. In [7] is suggested to use under 35 teeth because there is a transmission rotational movement. In our case was elected N = 32 teeth. tep 2: To calculate the ratio of speeds (VR) that is expressed in many forms as shown in (1), it is used most often for the analysis of the functioning of the gears. Knowing speed of sprocket (n =71rpm) which is the speed of the servomotor previously mentioned, and the speed of large gear that it is proposed by the designer, in this case we decided n L =47rpm; therefore VR=1.51. n DL NL VR n D N L tep 3: To calculate the number of teeth of a large gear (N L ) is using (1), it requires knowing VR and N. Later, the size of teeth and the distance between teeth were made using of the measures standards [8] knowing N and N L as shown in Fig. 1. L 129

TABLE IV. MATRICE OF DECIION ON YTEM OF ACTIVATION ystem of activation Torque 20% value Control 20% Acquisition 20% Consumed energy 10% Weight 10% ize 10% Cost 10% Total average 100% Range value p.

3 TABLE IV. MATRICE OF DECIION ON YTEM OF ACTIVATION ystem of activation Torque 20% value Control 20% Acquisition 20% Consumed energy 10% Weight 10% ize 10% Cost 10% Total average 100% Range value p.p. value p.p. value p.p. Value p.p. Value p.p. value p.p. value p.p. tep ervo DC TABLE V. MATRICE OF DECIION ON YTEM OF TRANMIION ystem of transmission Efficiency of transmission 25% ize 20% Weight 20% Cost 15% Noise 10% Maintenance 10% Total average 100% Range value p.p. value p.p. value p.p. Value p.p. Value p.p. value p.p. Gears Bands traps For the measurement of the length of the forearm were taken into account the anthropometric measures [9] of Mexicans between 18 and 65 years to see Fig. 2. Fig. 2. Measures taken from Mexicans. Fig. 1. Measures standards of the gears. 130

Look at the Table VI with respect to Fig. 2, the difference between the height of the elbow and the height of the wrist are used to calculate the measure of the forearm. TABLE VI.

4 Look at the Table VI with respect to Fig. 2, the difference between the height of the elbow and the height of the wrist are used to calculate the measure of the forearm. TABLE VI. MEAURE ANTHROPOMETRIC OF MEXICAN Measures 6. the height of the shoulder m 7. the height of the elbow m 8. the height of the flexed elbow m 9. the height of the wrist m 10. the height of the knuckle m The gears were manufactured with Nylamid L (black) by its properties of molybdenum disulfide that can reduce its friction, has high thermal resistance and high wear resistance; in addition this material is very common for the manufacture of gears. o the structure of the prosthesis was aluminum for high resistance and light weight. ubsequently, calculating the output torque of mechanism is as follows: tep 1: According to the definition of torque (2), where F is the force and r is the distance; we can calculate the force of the sprocket (F ) with the torque of the sprocket (T =15.5 Kg cm), which is the same torque of the servomotor because it is attached to the sprocket. And the radius of the sprocket (r =2.6 cm), which is obtained from the previous calculation of gear. F Tr tep 2: To calculate torque of the large gear (T L ) that is the total torque of pair of gears, is used (2) where F is the same force of the large gear then F L = N and the radius of the large gear is r L =3.6 cm. This can be seen in Fig. 3. Fig. 4. Pair of gear, where it has N =32 teeth with a diameter of 5.4 cm and N L=48 teeth with a diameter of 7.5 cm. Measurement of the forearm on the Table VI was 24.3 cm, the calculated measures are send to manufacture the prosthesis, shown in Fig. 5. Fig. 5. The prosthesis of elbow joint. In a) is a side view showing positioned sprocket, b) is another side view, there was placed the servomotor HD mg and in c) is front view on the left side was placed the pair of gear. The T L was Kg cm, which is higher than T =15.5 Kg cm, it is consistent with the literature about the increase of torque. On the other hand, the outputs of T P are shown in the Table VII with different masses. Fig. 3. Drawing shafts of torque, radio and force in the pair of gears, both torques are perpendicular to the radius and the force. Finally, the torque of prosthesis (T P ) was calculated by applying (3), where it was considered θ=90 because it is showing greater force, r P is the measurement of the forearm and F g is the force due to acceleration which is measured by multiplying the mass (m=1 Kg, 1.5 Kg, 2 Kg) by gravity (g=9.81 m/s²). T r sen( ) F P P Likewise, the force required to load a certain weight is known. III. REULT Measures of pair of gears are shown in Fig. 4, these were obtained through the calculations previously mentioned. g TABLE VII. THE OUTPUT OF TORQUE OF PROTHEI WITH EVERAL MAE Mass (Kg) Force (N) Torque (Nm) Later, the tests were performed as shown in the Fig. 6 which could load Kg, but we must take into account the weight of the tube simulating the forearm, this is Kg, as well as the support of the gear that it is Kg. Therefore, the total weight that can be loaded is kg, i.e. the T P is around 2.4 Nm. 131

Fig. 6. Measuring torque of the lever arm of the prosthesis, where r P is the distance from forearm, F g is the force due to acceleration with a weight of 0.500 Kg and θ is the angle of 90. IV.

5 Fig. 6. Measuring torque of the lever arm of the prosthesis, where r P is the distance from forearm, F g is the force due to acceleration with a weight of Kg and θ is the angle of 90. IV. DICUION In other prostheses use DC as hand prosthesis [10, 11] but in our case a high torque is desired to support the load that's why using a servomotor. A disadvantage would be the price because the greater is the torque, the higher is the cost. On the other hand, the system of transmission is widely used the train of gears which consists of more two wheels toothed; but we used a servomotor as it contains a system of gears in its box so it is not required of a very complex system of transmission that's why we chose the pair of gears. [1] Motion control, Inc. [online]. (2013, February 4). Available from: [2] Homer prosthetics and orthotics [online]. (2013, March 11). Available from: [3] Upper limb prosthetics technology [online]. (2013, March 28). Available from: [4] Ottobock [online]. (2013, April 1). Available from: [5] Centri AB [online]. (2013, April 12). Available from: [6] E. K. Iversen, J. R. Linder and H. H. ears, Prosthetic arm powered by an ultrasonic motor, U.. Patent B1, July 23, [7]. Joseph and U. John, Teoría de máquinas y mecanismos, Michigan: MCGraw Hill, 2001, pp [8] L. M. Robert, Diseño de elementos de máquinas. 4 th ed., México: Pearson Educación, 2006, pp [9] A. Rosalío, P. Lilia and G. Elvia, Dimensiones antropométricas de población latinoamericana, Mésico: Universidad de Guadalajara, Centro de Investigaciones en Ergonomía.México, [10] D.W. Zhao, L. Jiang, H. Huang, M.H. Jin, H.G. Cai and H. Liu, Development of a multi-dof anthropomorphic prosthetic hand, Proceedingsof the IEEE International Conference on Roboticsand Biomimetics, [11] C. Kyu-Jin, R. Josiah and A. Harry, BC hand: a lightweight robotic handwith an MA actuator array implementing C-segmentation, IEEE International Conference onrobotics and Automation, pp , V. CONCLUION After an extensive review of fundamentals for a design in prosthesis, we can say that according to the specifications of an electrical prosthesis, you can decide what kind of system of activation and transmission will be used in the prosthesis. In the system of activation, the actuator more used is type electrical so we chose the servomotor because of high torque since one of our purposes is to load more than 1 Kg according to the characteristics of commercial prostheses. In the system of transmission, we used of a reduction system which was implemented in a pair of gears to have a greater torque. The type of material selected for their resistance, their light weight and their low cost were the Nylamid and the aluminum. Finally it was verified that the torque increases with the pair of gear also it can support 1 Kg. As future work, we will characterize the prosthesis according to specifications, these are the total weight, the range of motion and time in which occurs the flexion-extension movement. ACKNOWLEDGMENT Thanks to Instituto Politécnico Nacional, Escuela uperior de Ingeniería Mecánica y Eléctrica (EIME), Unidad Zacatenco and to Consejo Nacional de Ciencia y Tecnología (CONACYT) with No. of registration for their support during the study Mastery. REFERENCE 132

Design and Control of an Anthropomorphic Robotic Arm

Journal Of Industrial Engineering Research ISSN- 2077-4559 Journal home page: http://www.iwnest.com/ijer/ 2016. 2(1): 1-8 RSEARCH ARTICLE Design and Control of an Anthropomorphic Robotic Arm Simon A/L