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A comprehensive survey on humanoid robot development

Part of: Robot Athletes and Entertainers

Published online by Cambridge University Press:  03 December 2019

Saeed Saeedvand ,
Masoumeh Jafari ,
Hadi S. Aghdasi Open the ORCID record for Hadi S. Aghdasi [Opens in a new window]  and
Jacky Baltes
Saeed Saeedvand
Affiliation:
Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran e-mails: [email protected], [email protected], [email protected]
Masoumeh Jafari
Affiliation:
Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran e-mails: [email protected], [email protected], [email protected]
Hadi S. Aghdasi*
Affiliation:
Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz, Iran e-mails: [email protected], [email protected], [email protected]
Jacky Baltes
Affiliation:
Department of Electrical Engineering, National Taiwan Normal University, Taipei, Taiwan e-mail: [email protected]
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Abstract

The development of a versatile, fully-capable humanoid robot as envisioned in science fiction books is one of the most challenging but interesting issues in the robotic field. Currently, existing humanoid robots are designed with different purposes and applications in mind. In humanoid robot development process, each robot is designed with various characteristics, abilities, and equipment, which influence the general structure, cost, and difficulty of development. Even though humanoid robot development is very popular, a few review papers are focusing on the design and development process of humanoid robots. Motivated by this, we present this review paper to show variations in the requirements, design, and development process and also propose a taxonomy of existing humanoid robots. It aims at demonstrating a general perspective of existing humanoid robots’ characteristics and applications. This paper includes state-of-the-art and successfully reported existing humanoid robot designs along with different robots used in various robot competitions.

Type
Review
Information
The Knowledge Engineering Review , Volume 34 , 2019 , e20
Copyright
© Cambridge University Press, 2019 

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References

Abrate, F., Bona, B. & Indri, M. 2007. Monte Carlo localization of mini-rovers with low-cost IR sensors. In 2007 IEEE/ASME International Conference on Advanced Intelligent Mechatronics, 1–6. IEEE, December.CrossRefGoogle Scholar
Albers, A., Brudniok, S., Ottnad, J., Sauter, C. & Sedchaicharn, K. 2007. Design of modules and components for humanoid robots. In Humanoid Robots: New Developments, Armando Carlos de Pina Filho (ed). InTech, 1–16.Google Scholar
Al-Busaidi, A. M. 2012. Development of an educational environment for online control of a biped robot using MATLAB and Arduino. In 2012 9th France-Japan & 7th Europe-Asia Congress on Mechatronics (MECATRONICS)/13th Int’l Workshop on Research and Education in Mechatronics (REM), 337–344. IEEE.CrossRefGoogle Scholar
Albu-Schäffer, A., Haddadin, S., Ott, C., Stemmer, A., Wimböck, T. & Hirzinger, G. 2007. The DLR lightweight robot: design and control concepts for robots in human environments. Industrial Robot: An International Journal 34, 376–385.CrossRefGoogle Scholar
Allali, J., Deguillaume, L., Fabre, R., Gondry, L., Hofer, L., Ly, O., N’Guyen, S., Passault, G., Pirrone, A. & Rouxel, Q. 2016. Rhoban football club: Robocup humanoid kid-size 2016 champion team paper. In Robot World Cup, Springer, 491–502.Google Scholar
Allgeuer, P. & Behnke, S. 2018. Hierarchical and state-based architectures for robot behavior planning and control. arXiv preprint arXiv:1809.11067. Google Scholar
Allgeuer, P., Farazi, H., Schreiber, M. & Behnke, S. 2015. Child-sized 3D printed igus humanoid open platform. In 2015 IEEE-RAS 15th International Conference on Humanoid Robots (Humanoids), 33–40. IEEE.CrossRefGoogle Scholar
Almubarak, Y. & Tadesse, Y. 2017. Design and motion control of bioinspired humanoid robot head from servo motors toward artificial muscles. In Electroactive Polymer Actuators and Devices (EAPAD) 2017. International Society for Optics and Photonics, 101631U.Google Scholar
Al-Shuka, H. F. N., Allmendinger, F, Corves, B. & Zhu, W. H. 2014. Modeling, stability and walking pattern generators of biped robots: a review. Robotica 32, 907–934.CrossRefGoogle Scholar
Anderson, J., Baltes, J. & Cheng, C. T. 2011. Robotics competitions as benchmarks for AI research. The Knowledge Engineering Review 26, 11–17.CrossRefGoogle Scholar
Arvin, F., Samsudin, K. & Ramli, A. R. 2009. Development of a miniature robot for swarm robotic application. International Journal of Computer and Electrical Engineering 1, 436–442.CrossRefGoogle Scholar
Atkeson, C. G., Babu, B. P. W., Banerjee, N., Berenson, D., Bove, C. P., Cui, X., DeDonato, M., Du, R., Feng, S. & Franklin, P. 2015. No falls, no resets: reliable humanoid behavior in the DARPA robotics challenge. In 2015 IEEE-RAS 15th International Conference on Humanoid Robots (Humanoids), 623–630. IEEE.CrossRefGoogle Scholar
Bäck, I., Kallio, J. & Mäkelä, K. 2012. Enhanced map-based indoor navigation system of a humanoid robot using ultrasound measurements. Intelligent Control and Automation 3, 111.CrossRefGoogle Scholar
Baltes, J., Gerndt, R., McGill, S. & Sadeghnejad, S. 2016. RoboCup soccer humanoid league rules and setup. International RoboCup Federation, 1–35.Google Scholar
Baltes, J., Tu, K.-Y., Sadeghnejad, S. & Anderson, J. 2017. HuroCup: competition for multi-event humanoid robot athletes. The Knowledge Engineering Review 32, 1–14.CrossRefGoogle Scholar
Breazeal, C. 2003. Emotion and sociable humanoid robots. International Journal of Human-Computer Studies 59, 119–155.CrossRefGoogle Scholar
Cheng, H. H., Shaw, B. D., Palen, J., Larson, J. E., & Hu, X. 2001. A real-time laser-based detection system for measurement of delineations of moving vehicles. IEEE/ASME Transactions on Mechatronics 6, 170–187.CrossRefGoogle Scholar
Chestnutt, J., Lau, M., Cheung, G., Kuffner, J., Hodgins, J. & Kanade, T. 2005. Footstep planning for the honda asimo humanoid. In Proceedings of the 2005 IEEE international conference on robotics and automation, 629–634. IEEE.Google Scholar
Cho, B.-K., Kim, J.-H. & Oh, J.-H. 2011. Online balance controllers for a hopping and running humanoid robot. Advanced Robotics 25, 1209–1225.CrossRefGoogle Scholar
Dahiya, R. S., Metta, G., Valle, M. & Sandini, G. 2010. Tactile sensing-from humans to humanoids. IEEE Transactions on Robotics 26, 1–20.CrossRefGoogle Scholar
DeDonato, M., Dimitrov, V., Du, R., Giovacchini, R., Knoedler, K., Long, X., Polido, F., Gennert, M. A., Padır, T. & Feng, S. 2015. Human‐in‐the‐loop control of a humanoid robot for disaster response: a report from the DARPA Robotics Challenge Trials. Journal of Field Robotics 32, 275–292.CrossRefGoogle Scholar
Diftler, M. A., Culbert, C. J., Ambrose, R. O., Platt, R. & Bluethmann, W. J. 2003. Evolution of the NASA/DARPA robonaut control system. In 2003 IEEE International Conference on Robotics and Automation (Cat. No. 03CH37422), 2543–2548. IEEE.Google Scholar
Diftler, M. A., Mehling, J. S., Abdallah, M. E., Radford, N. A., Bridgwater, L. B., Sanders, A. M., Askew, R. S., Linn, D. M., Yamokoski, J. D. & Permenter, F. A. 2011. Robonaut 2-the first humanoid robot in space. In 2011 IEEE international conference on robotics and automation, 2178–2183. IEEE.CrossRefGoogle Scholar
Di Nuovo, A. G., Marocco, D., Di Nuovo, S. & Cangelosi, A. 2013. Autonomous learning in humanoid robotics through mental imagery. Neural Networks 41, 147–155.CrossRefGoogle ScholarPubMed
Duguleana, M. & Mogan, G. 2016. Neural networks based reinforcement learning for mobile robots obstacle avoidance. Expert Systems with Applications 62, 104–115.CrossRefGoogle Scholar
Duran, O., Althoefer, K. & Seneviratne, L. D. 2003. Pipe inspection using a laser-based transducer and automated analysis techniques. IEEE/ASME Transactions on Mechatronics 8, 401–409.CrossRefGoogle Scholar
Dutta, T. & Fernie, G. R. 2005. Utilization of ultrasound sensors for anti-collision systems of powered wheelchairs. IEEE Transactions on Neural Systems and Rehabilitation Engineering 13, 24–32.CrossRefGoogle ScholarPubMed
Englsberger, J., Werner, A., Ott, C., Henze, B., Roa, M. A., Garofalo, G., Burger, R., Beyer, A., Eiberger, O. & Schmid, K. 2014. Overview of the torque-controlled humanoid robot TORO. In 2014 IEEE-RAS International Conference on Humanoid Robots, 916–923. IEEE.CrossRefGoogle Scholar
Federation of International Sports Association (FIRA), “FIRA”, http://www.firaworldcup.org/.Google Scholar
International RoboCup Federation, “RoboCup Humanoid League”, https://www.robocuphumanoid.org/.Google Scholar
Ferro, F. & Marchionni, L. 2014. REEM: a humanoid service robot. In ROBOT2013: First Iberian Robotics Conference, 521–525. Springer.CrossRefGoogle Scholar
Ficht, G., Farazi, H., Brandenburger, A., Rodriguez, D., Pavlichenko, D., Allgeuer, P., Hosseini, M. & Behnke, S. 2018. NimbRo-OP2X: Adult-sized open-source 3D printed humanoid robot. In 2018 IEEE-RAS 18th International Conference on Humanoid Robots (Humanoids), 1–9. IEEE.CrossRefGoogle Scholar
Fujita, M., Kuroki, Y., Ishida, T. & Doi, T. T. 2003. A small humanoid robot sdr-4x for entertainment applications. In Proceedings 2003 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM 2003), 938–943. IEEE.Google Scholar
Goswami, A. 1999. Postural stability of biped robots and the foot-rotation indicator (FRI) point. The International Journal of Robotics Research 18, 523–533.CrossRefGoogle Scholar
Gouaillier, D., Hugel, V., Blazevic, P., Kilner, C., Monceaux, J., Lafourcade, P., Marnier, B., Serre, J. & Maisonnier, B. 2009. Mechatronic design of NAO humanoid. In 2009 IEEE International Conference on Robotics and Automation, 769–774. IEEE.CrossRefGoogle Scholar
Gutmann, J.-S., Fukuchi, M. and Fujita, M. 2004. Stair climbing for humanoid robots using stereo vision. In 2004 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)(IEEE Cat. No. 04CH37566), 1407–1413. IEEE.Google Scholar
Ha, I., Tamura, Y., Asama, H., Han, J. & Hong, D. W. 2011. Development of open humanoid platform DARwIn-OP. In SICE Annual Conference 2011, 2178–2181. IEEE.Google Scholar
Hochberg, U., Dietsche, A. & Dorer, K. 2013. Evaporative cooling of actuators for humanoid robots. In Proceedings of the 8th Workshop on Humanoid Soccer Robots, IEEE-RAS International Conference on Humanoid Robots, Atlanta.Google Scholar
Huan, Y., Dongdong, Y., WenXing, M. & Rong, X. 2016. ZJUDancer team description paper, International RoboCup Federation.Google Scholar
Huang, W., Chew, C.-M., Zheng, Y. & Hong, G.-S. 2008. Pattern generation for bipedal walking on slopes and stairs. In Humanoids 2008-8th IEEE-RAS International Conference on Humanoid Robots, 205–210. IEEE.Google Scholar
Hwang, K., Lee, S. W., Karng, S. W. & Kim, S. Y. 2008. Thermal performance of non-metallic two-phase cold plates for humanoid robot cooling. In 2008 11th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, 6–11. IEEE.Google Scholar
Jafari, M., Saeedvand, S. & Aghdasi, H. S. 2019. A hybrid Q-learning algorithm to score a moving ball for humanoid robots. In 5th Conference on Knowledge-Based Engineering and Innovation, Iran University of Science and Technology, Tehran, Iran, IEEE, 498–503.Google Scholar
Jeffers, J. R., Auyang, A. G. & Grabowski, A. M. 2015. The correlation between metabolic and individual leg mechanical power during walking at different slopes and velocities. Journal of Biomechanics 48, 2919–2924.Google Scholar
Jung, T., Lim, J., Bae, H., Lee, K. K., Joe, H. M. & Oh, J. H. 2018. Development of the humanoid disaster response platform DRC-HUBO+. IEEE Transactions on Robotics 34, 1–17.CrossRefGoogle Scholar
Kaneko, K., Kanehiro, F., Morisawa, M., Miura, K., Nakaoka, S. I. & Kajita, S. 2009. Cybernetic human HRP-4C. In 2009 9th IEEE-RAS International Conference on Humanoid Robots, 7–14. IEEE.CrossRefGoogle Scholar
Kawamura, K., Wilkes, D. M., Pack, T., Bishay, M. & Barile, J. 1996. Humanoids: future robots for home and factory. In Proceedings of the First International Symposium on Humanoid Robots, Waseda University, Tokyo, October 30–31, 53–62.Google Scholar
Khokar, K., Beeson, P. & Burridge, R. 2015. Implementation of KDL inverse kinematics routine on the Atlas humanoid robot. Procedia Computer Science 46, 1441–1448.CrossRefGoogle Scholar
Kim, J. Y., Park, I. W., Lee, J. & Oh, J. H. 2005. Experiments of vision guided walking of humanoid robot, KHR-2. In 5th IEEE-RAS International Conference on Humanoid Robots, 2005, 135–140. IEEE.Google Scholar
Koenig, N. & Howard, A. 2004. Design and use paradigms for gazebo, an open-source multi-robot simulator. In 2004 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) (IEEE Cat. No. 04CH37566), 2149–2154. IEEE.Google Scholar
Krotkov, E., Hackett, D., Jackel, L., Perschbacher, M., Pippine, J., Strauss, J., Pratt, G. & Orlowski, C. 2017. The DARPA robotics challenge finals: results and perspectives. Journal of Field Robotics 34, 229–240.CrossRefGoogle Scholar
Kuindersma, S., Deits, R., Fallon, M., Valenzuela, A., Dai, H., Permenter, F., Koolen, T., Marion, P. & Tedrake, R. 2016. Optimization-based locomotion planning, estimation, and control design for the atlas humanoid robot. Autonomous Robots 40, 429–455.CrossRefGoogle Scholar
Lapeyre, M., Rouanet, P., Grizou, J., Nguyen, S., Depraetre, F., Le Falher, A. & Oudeyer, P. Y. 2014. Poppy project: open-source fabrication of 3D printed humanoid robot for science, education and art. In Digital Intelligence 2014, 6.Google Scholar
Lee, D. & Nakamura, Y. 2007. Mimesis scheme using a monocular vision system on a humanoid robot. In Proceedings 2007 IEEE International Conference on Robotics and Automation, 2162–2168. IEEE.CrossRefGoogle Scholar
Lin, S. T., Hu, J., Shih, C. H., Huang, C. J. & Kuo, P. H. 2019. The development of supervised motion learning and vision system for humanoid robot. In Applied Mechanics and Materials, Trans Tech Publ., 188–193.Google Scholar
Lohmeier, S., Buschmann, T. & Ulbrich, H. 2009. Humanoid robot LOLA. In 2009 IEEE International Conference on Robotics and Automation, 775–780. IEEE.CrossRefGoogle Scholar
Mac, T. T., Copot, C., Tran, D. T. & De Keyser, R. 2016. Heuristic approaches in robot path planning: a survey. Robotics and Autonomous Systems 86, 13–28.CrossRefGoogle Scholar
Martin, T. B., Ambrose, R. O., Diftler, M. A., Platt, R. & Butzer, M. J. 2004. Tactile gloves for autonomous grasping with the NASA/DARPA Robonaut. In ICRA’04: Proceedings of the IEEE International Conference on Robotics and Automation, 2004, 1713–1718. IEEE.CrossRefGoogle Scholar
Mejías, A., Herrera, R., Márquez, M., Calderón, A., González, I. & Andújar, J. 2017. Easy handling of sensors and actuators over TCP/IP networks by Open Source Hardware/Software. Sensors 17, 94.CrossRefGoogle ScholarPubMed
Metta, G., Natale, L., Nori, F., Sandini, G., Vernon, D., Fadiga, L., Von Hofsten, C., Rosander, K., Lopes, M., Santos-Victor, J. & Bernardino, A. 2010. The iCub humanoid robot: an open-systems platform for research in cognitive development. Neural Networks 23, 1125–1134.CrossRefGoogle ScholarPubMed
Metta, G., Sandini, G., Vernon, D., Natale, L. & Nori, F. 2008. The iCub humanoid robot: an open platform for research in embodied cognition. In Proceedings of the 8th Workshop on Performance Metrics for Intelligent Systems, 50–56. ACM.CrossRefGoogle Scholar
Morita, T., Mase, K., Hirano, Y. & Kajita, S. 2007. Reciprocal attentive communication in remote meeting with a humanoid robot. In Proceedings of the 9th International Conference on Multimodal Interfaces, 228–235. ACM.CrossRefGoogle Scholar
Nikkhah, A., Yousefi-Koma, A., Mirjalili, R. & Farimani, H. M. 2017. Design and implementation of small-sized 3d printed surena-mini humanoid platform. In 2017 5th RSI International Conference on Robotics and Mechatronics (ICRoM), 132–137. IEEE.CrossRefGoogle Scholar
Nishiyama, T., Hoshino, H., Suzuki, K., Nakajima, R., Sawada, K. & Tachi, S. 1999. Development of surrounded audio-visual display system for humanoid robot control. In Proceedings of 9th International Conference of Artificial Reality and Tele-existence (ICAT’99), 60–67.Google Scholar
Oh, K. M., Kim, J. H. & Kim, M. S. 2005. Development of humanoid robot design process-focused on the concurrent engineering based humanoid robot design. In IDC International Design Congress (IASDR) 2005. International Design Congress.Google Scholar
Pashazadeh, S. & Saeedvand, S. 2014. Modelling of walking humanoid robot with capability of floor detection and dynamic balancing using colored petri net, International Journal in Foundations of Computer Science & Technology (IJFCST), 4, 1–10.Google Scholar
Pratt, J. E. & Tedrake, R. 2006. Velocity-based stability margins for fast bipedal walking. In Fast Motions in Biomechanics and Robotics, Springer, 299–324.CrossRefGoogle Scholar
Quigley, M., Conley, K., Gerkey, B., Faust, J., Foote, T., Leibs, J., Wheeler, R. & Ng, A. Y. 2009. ROS: an open-source Robot Operating System. In ICRA Workshop on Open Source Software, 5. Kobe, Japan.Google Scholar
Reil, T. & Husbands, P. 2002. Evolution of central pattern generators for bipedal walking in a real-time physics environment. IEEE Transactions on Evolutionary Computation 6, 159–168.CrossRefGoogle Scholar
ROBOTIS OP2-OP3. http://www.robotis.us/robotis-OP2-OP3/.Google Scholar
Romay, A., Kohlbrecher, S., Stumpf, A., von Stryk, O., Maniatopoulos, S., Kress‐Gazit, H., Schillinger, P. & Conner, D. C. 2017. Collaborative autonomy between high‐level behaviors and human operators for remote manipulation tasks using different humanoid robots. Journal of Field Robotics 34, 333–358.CrossRefGoogle Scholar
Sabe, K., Fukuchi, M., Gutmann, J.-S., Ohashi, T., Kawamoto, K. & Yoshigahara, T. 2004. Obstacle avoidance and path planning for humanoid robots using stereo vision. In IEEE International Conference on Robotics and Automation, 592–597. IEEE, 1999.Google Scholar
Saeedvand, S., Aghdasi, H. S. & Baltes, J. 2018. Novel lightweight odometric learning method for humanoid robot localization. Mechatronics 55, 38–53.CrossRefGoogle Scholar
Saeedvand, S., Aghdasi, H. S. & Baltes, J. 2019. Robust multi-objective multi-humanoid robots task allocation based on novel hybrid metaheuristic algorithm. Applied Intelligence 49, 1–31.CrossRefGoogle Scholar
Saeedvand, S., Jafari, M., Vahid, A., Arash, R. & Abbaszadeh, M. 2017. IRC Adult Size Humanoid Robot Soccer Team Description Paper 2017. RoboCup.Google Scholar
Sakagami, Y., Watanabe, R., Aoyama, C., Matsunaga, S., Higaki, N. and Fujimura, K. 2002. The intelligent ASIMO: system overview and integration. In IEEE/RSJ International Conference on Intelligent Robots and Systems, 2478–2483. IEEE.Google Scholar
Schmidt, P. A., Maël, E. & Würtz, R. P. 2006. A sensor for dynamic tactile information with applications in human–robot interaction and object exploration. Robotics and Autonomous Systems 54, 1005–1014.CrossRefGoogle Scholar
Schnekenburger, F., Scharffenberg, M., Wülker, M., Hochberg, U. & Dorer, K. 2017. Detection and localization of features on a soccer field with feedforward fully convolutional neural networks (fcnn) for the Adultsize humanoid robot Sweaty. In Proceedings of the 12th Workshop on Humanoid Soccer Robots, IEEE-RAS International Conference on Humanoid Robots, Birmingham.Google Scholar
Shukla, Y. M., Tamba, A., Pandey, S. & Sharma, P. 2014. A review and scope of humanoid robotics. In Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28–29 March 2014.Google Scholar
Silva, M. F. & Machado, J. A. T. 2012. A literature review on the optimization of legged robots. Journal of Vibration and Control 18, 1753–1767.CrossRefGoogle Scholar
Singler, B. 2019. Existential hope and existential despair in AI apocalypticism and transhumanism. Zygon® 54, 156–176.CrossRefGoogle Scholar
Stasse, O. & Flayols, T. 2019. An overview of humanoid robots technologies. In Biomechanics of Anthropomorphic Systems, Springer, 281–310.CrossRefGoogle Scholar
Taga, G., Yamaguchi, Y. & Shimizu, H. 1991. Self-organized control of bipedal locomotion by neural oscillators in unpredictable environment. Biological Cybernetics 65, 147–159.CrossRefGoogle ScholarPubMed
Tanie, K. 2003. Humanoid robot and its application possibility. In Proceedings of IEEE International Conference on Multisensor Fusion and Integration for Intelligent Systems, MFI2003, 213–214. IEEE.Google Scholar
Toyota Global Newsroom. 2017. Toyota Unveils Third Generation Humanoid Robot T-HR3. https://newsroom. toyota.co.jp/en/download/20110424.Google Scholar
Vukobratovic, M., Frank, A. A. & Juricic, D. 1970. On the stability of biped locomotion. IEEE Transactions on Biomedical Engineering BME-17, 25–36.CrossRefGoogle ScholarPubMed
Wang, J. M., Fleet, D. J. & Hertzmann, A. 2010. Optimizing walking controllers for uncertain inputs and environments. ACM Transactions on Graphics (TOG) 29, 73.CrossRefGoogle Scholar
Wang, S., Chaovalitwongse, W. & Babuska, R. 2012. Machine learning algorithms in bipedal robot control. IEEE Transactions on Systems, Man, and Cybernetics, Part C (Applications and Reviews) 42, 728–743.CrossRefGoogle Scholar
Weiss, K. & Woern, H. 2004. Tactile sensor system for an anthropomorphic robotic hand. In Proceedings of IEEE International Conference on Manipulation and Grasping (IMG 2004), Kobe, Japan, 895901.Google Scholar
Zhang, H. & So, E. 2002. Hybrid resistive tactile sensing. IEEE Transactions on Systems, Man, and Cybernetics, Part B (Cybernetics) 32, 57–65.CrossRefGoogle Scholar