{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T01:12:09Z","timestamp":1760058729991,"version":"build-2065373602"},"reference-count":48,"publisher":"MDPI AG","issue":"9","license":[{"start":{"date-parts":[[2025,4,23]],"date-time":"2025-04-23T00:00:00Z","timestamp":1745366400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Vine&amp;Wine_PT","award":["67"],"award-info":[{"award-number":["67"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Agriculture needs to produce more with fewer resources to satisfy the world\u2019s demands. Labor shortages, especially during harvest seasons, emphasize the need for agricultural automation. However, the high cost of commercially available robotic manipulators, ranging from EUR 3000 to EUR 500,000, is a significant barrier. This research addresses the challenges posed by low-cost manipulators, such as inaccuracy, limited sensor feedback, and dynamic uncertainties. Three control strategies for a low-cost agricultural SCARA manipulator were developed and benchmarked: a Sliding Mode Controller (SMC), a Reinforcement Learning (RL) Controller, and a novel Proportional-Integral (PI) controller with a self-tuning feedforward element (PIFF). The results show the best response time was obtained using the SMC, but with joint movement jitter. The RL controller showed sudden breaks and overshot upon reaching the setpoint. Finally, the PIFF controller showed the smoothest reference tracking but was more susceptible to changes in system dynamics.<\/jats:p>","DOI":"10.3390\/s25092676","type":"journal-article","created":{"date-parts":[[2025,4,23]],"date-time":"2025-04-23T10:40:48Z","timestamp":1745404848000},"page":"2676","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Benchmarking Controllers for Low-Cost Agricultural SCARA Manipulators"],"prefix":"10.3390","volume":"25","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5307-0318","authenticated-orcid":false,"given":"V\u00edtor","family":"Tinoco","sequence":"first","affiliation":[{"name":"INESC TEC\u2014Institute for Systems and Computer Engineering, Technology and Science, 4200-465 Porto, Portugal"},{"name":"Department of Engineering, UTAD\u2014University of Tr\u00e1s-os-Montes and Alto Douro, 5000-801 Vila Real, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-0593-2865","authenticated-orcid":false,"given":"Manuel F.","family":"Silva","sequence":"additional","affiliation":[{"name":"INESC TEC\u2014Institute for Systems and Computer Engineering, Technology and Science, 4200-465 Porto, Portugal"},{"name":"ISEP\/IPP\u2014School of Engineering, Polytechnic Institute of Porto, 4200-072 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8486-6113","authenticated-orcid":false,"given":"Filipe Neves dos","family":"Santos","sequence":"additional","affiliation":[{"name":"INESC TEC\u2014Institute for Systems and Computer Engineering, Technology and Science, 4200-465 Porto, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2440-9153","authenticated-orcid":false,"given":"Raul","family":"Morais","sequence":"additional","affiliation":[{"name":"INESC TEC\u2014Institute for Systems and Computer Engineering, Technology and Science, 4200-465 Porto, Portugal"},{"name":"Department of Engineering, UTAD\u2014University of Tr\u00e1s-os-Montes and Alto Douro, 5000-801 Vila Real, Portugal"}]}],"member":"1968","published-online":{"date-parts":[[2025,4,23]]},"reference":[{"key":"ref_1","unstructured":"United Nations (2024, March 13). Global Issues: Population. Available online: https:\/\/www.un.org\/en\/global-issues\/population."},{"key":"ref_2","unstructured":"Jensen, K., Nielsen, S.H., Joergensen, R., Boegild, A., Jacobsen, N., Joergensen, O., and Jaeger-Hansen, C. (2012, January 15\u201318). A low cost, modular robotics tool carrier for precision agriculture research. Proceedings of the International Conference on Precision Agriculture, Indianapolis, IN, USA."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"135","DOI":"10.1016\/j.ifacol.2018.08.076","article-title":"Simulation as a Tool in Designing and Evaluating a Robotic Apple Harvesting System","volume":"51","author":"Wang","year":"2018","journal-title":"IFAC-PapersOnLine"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"106151","DOI":"10.1016\/j.compag.2021.106151","article-title":"Collision-free motion planning for the litchi-picking robot","volume":"185","author":"Ye","year":"2021","journal-title":"Comput. Electron. Agric."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"105837","DOI":"10.1016\/j.compag.2020.105837","article-title":"Development of an integrated 3R end-effector with a cartesian manipulator for pruning apple trees","volume":"179","author":"Zahid","year":"2020","journal-title":"Comput. Electron. Agric."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"160","DOI":"10.1016\/j.biosystemseng.2009.09.011","article-title":"Evaluation of a strawberry-harvesting robot in a field test","volume":"105","author":"Hayashi","year":"2010","journal-title":"Biosyst. Eng."},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Oliveira, L.F.P., Moreira, A.P., and Silva, M.F. (2021). Advances in Agriculture Robotics: A State-of-the-Art Review and Challenges Ahead. Robotics, 10.","DOI":"10.3390\/robotics10020052"},{"key":"ref_8","doi-asserted-by":"crossref","unstructured":"Cheng, C., Fu, J., Su, H., and Ren, L. (2023). Recent advancements in agriculture robots: Benefits and challenges. Machines, 11.","DOI":"10.3390\/machines11010048"},{"key":"ref_9","doi-asserted-by":"crossref","unstructured":"Tinoco, V., Silva, M.F., Santos, F.N., Rocha, L.F., Magalh\u00e3es, S., and Santos, L.C. (2021, January 28\u201329). A Review of Pruning and Harvesting Manipulators. Proceedings of the 2021 IEEE International Conference on Autonomous Robot Systems and Competitions (ICARSC), Santa Maria da Feira, Portugal.","DOI":"10.1109\/ICARSC52212.2021.9429806"},{"key":"ref_10","unstructured":"Hitbot Robotics (2023, April 11). How Much Does a Robotic Arm Cost?. Available online: https:\/\/www.hitbotrobot.com\/category\/product-center\/robotic-arm\/."},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Durrant-Whyte, H., Roy, N., and Abbeel, P. (2012). Learning to Control a Low-Cost Manipulator Using Data-Efficient Reinforcement Learning. Robotics: Science and Systems VII, MIT Press.","DOI":"10.7551\/mitpress\/9481.001.0001"},{"key":"ref_12","unstructured":"Nise, N.S. (2020). Control Systems Engineering, John Wiley & Sons."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Camacho, E.F., Bordons, C., Camacho, E.F., and Bordons, C. (2007). Constrained Model Predictive Control, Springer.","DOI":"10.1007\/978-0-85729-398-5"},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1689","DOI":"10.1109\/TMECH.2020.3028968","article-title":"An Observer-Based Neural Adaptive PID2 Controller for Robot Manipulators Including Motor Dynamics with a Prescribed Performance","volume":"26","author":"Shojaei","year":"2021","journal-title":"IEEE\/ASME Trans. Mechatron."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.oceaneng.2017.04.030","article-title":"Robust task-space control of an autonomous underwater vehicle-manipulator system by PID-like fuzzy control scheme with disturbance estimator","volume":"139","author":"Londhe","year":"2017","journal-title":"Ocean Eng."},{"key":"ref_16","doi-asserted-by":"crossref","unstructured":"Khamsehei Fadaei, M.H., Zalaghi, A., Rahmat Alhosseini Ghochan Atigh, S.G., and Torkani, Z. (March, January 28). Design of PID and Fuzzy-PID Controllers for Agile Eye Spherical Parallel Manipulator. Proceedings of the 2019 5th Conference on Knowledge Based Engineering and Innovation (KBEI), Tehran, Iran.","DOI":"10.1109\/KBEI.2019.8735095"},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Kumar, R., Srivastava, S., and Gupta, J.R. (2016, January 21\u201324). Modeling and control of one-link robotic manipulator using neural network based PID controller. Proceedings of the 2016 International Conference on Advances in Computing, Communications and Informatics (ICACCI), Jaipur, India.","DOI":"10.1109\/ICACCI.2016.7732054"},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Tang, Z., Yang, M., and Pei, Z. (2010, January 17\u201319). Self-Adaptive PID Control Strategy Based on RBF Neural Network for Robot Manipulator. Proceedings of the 2010 First International Conference on Pervasive Computing, Signal Processing and Applications, Harbin, China.","DOI":"10.1109\/PCSPA.2010.230"},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Siciliano, B. (2008). Springer Handbook of Robotics, Springer.","DOI":"10.1007\/978-3-540-30301-5"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Mart\u00ednez, C.M., and Cao, D. (2019). 2-Integrated energy management for electrified vehicles. Ihorizon-Enabled Energy Management for Electrified Vehicles, Butterworth-Heinemann.","DOI":"10.1016\/B978-0-12-815010-8.00002-8"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Pesce, V., Colagrossi, A., and Silvestrini, S. (2023). Chapter Ten-Control. Modern Spacecraft Guidance, Navigation, and Control, Elsevier.","DOI":"10.1016\/B978-0-323-90916-7.00009-3"},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Nadda, S., and Swarup, A. (2018, January 22\u201323). Integral Sliding Mode Control for Position Control of Robotic Manipulator. Proceedings of the 2018 5th International Conference on Signal Processing and Integrated Networks (SPIN), Noida, India.","DOI":"10.1109\/SPIN.2018.8474267"},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Bhave, M., Janardhanan, S., and Dewan, L. (2013, January 3\u20134). A robust third order sliding mode control of rigid underactuated robotic manipulator. Proceedings of the 2013 International Conference on Control, Computing, Communication and Materials (ICCCCM), Allahabad, India.","DOI":"10.1109\/ICCCCM.2013.6648907"},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Kameyama, T., Zhao, X., and Yang, Z.J. (2022, January 17\u201320). Event-triggered sliding mode control of a robot manipulator. Proceedings of the 2022 International Conference on Advanced Mechatronic Systems (ICAMechS), Toyama, Japan.","DOI":"10.1109\/ICAMechS57222.2022.10003318"},{"key":"ref_25","first-page":"1266","article-title":"Event-Triggered-Based Sliding Mode Asymptotic Tracking Control of Robotic Manipulators","volume":"71","author":"Li","year":"2024","journal-title":"IEEE Trans. Circuits Syst. II Express Briefs"},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Boukadida, W., Bkekri, R., Benamor, A., and Messaoud, H. (2017, January 19\u201321). Trajectory tracking of robotic manipulators using optimal sliding mode control. Proceedings of the 2017 International Conference on Control, Automation and Diagnosis (ICCAD), Hammamet, Tunisia.","DOI":"10.1109\/CADIAG.2017.8075717"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"Wanigasekara, C., Almakhles, D., Swain, A., Nguang, S.K., Subramaniyan, U., and Padmanaban, S. (2019, January 11\u201314). Performance of Neural Network Based Controllers and \u0394\u2211-Based PID Controllers for Networked Control Systems: A Comparative Investigation. Proceedings of the 2019 IEEE International Conference on Environment and Electrical Engineering and 2019 IEEE Industrial and Commercial Power Systems Europe (EEEIC\/I&CPS Europe), Genova, Italy.","DOI":"10.1109\/EEEIC.2019.8783404"},{"key":"ref_28","unstructured":"Omidvar, O., and van der Smagt, P. (1997). 6-The Neural Dynamics Approach to Sensory-Motor Control: Overview and Recent Applications in Mobile Robot Control and Speech Production. Neural Systems for Robotics, Academic Press."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"588","DOI":"10.22214\/ijraset.2020.6095","article-title":"Study of supervised learning and unsupervised learning","volume":"8","author":"Sharma","year":"2020","journal-title":"Int. J. Res. Appl. Sci. Eng. Technol."},{"key":"ref_30","unstructured":"Omidvar, O., and Elliott, D.L. (1997). Chapter 7-Neural Network Control of Robot Arms and Nonlinear Systems. Neural Systems for Control, Academic Press."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"3050","DOI":"10.1109\/LRA.2020.2975727","article-title":"Safe and Fast Tracking on a Robot Manipulator: Robust MPC and Neural Network Control","volume":"5","author":"Nubert","year":"2020","journal-title":"IEEE Robot. Autom. Lett."},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Zhou, J., Zheng, H., Zhao, D., and Chen, Y. (2021, January 16\u201319). Intelligent Control of Manipulator Based on Deep Reinforcement Learning. Proceedings of the 2021 12th International Conference on Mechanical and Aerospace Engineering (ICMAE), Athens, Greece.","DOI":"10.1109\/ICMAE52228.2021.9522377"},{"key":"ref_33","doi-asserted-by":"crossref","unstructured":"Li, Q., Nie, J., Wang, H., Lu, X., and Song, S. (2021, January 26\u201328). Manipulator Motion Planning based on Actor-Critic Reinforcement Learning. Proceedings of the 2021 40th Chinese Control Conference (CCC), Shanghai, China.","DOI":"10.23919\/CCC52363.2021.9550010"},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"1983","DOI":"10.1162\/neco_a_01079","article-title":"A Reinforcement Learning Neural Network for Robotic Manipulator Control","volume":"30","author":"Hu","year":"2018","journal-title":"Neural Comput."},{"key":"ref_35","first-page":"140","article-title":"Development of a tomato harvesting robot used in greenhouse","volume":"10","author":"Lili","year":"2017","journal-title":"Int. J. Agric. Biol. Eng."},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Mohamed, A., Shaw-Sutton, J., Green, B., Andrews, W., Rolley-Parnell, E., Zhou, Y., Zhou, P., Mao, X., Fuller, M., and Stoelen, M. (2019). Soft manipulator robot for selective tomato harvesting. Precision Agriculture\u201919, Wageningen Academic Publishers.","DOI":"10.3920\/978-90-8686-888-9_99"},{"key":"ref_37","doi-asserted-by":"crossref","unstructured":"Siciliano, B., and Khatib, O. (2016). Visual Servoing. Springer Handbook of Robotics, Springer International Publishing.","DOI":"10.1007\/978-3-319-32552-1"},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"012003","DOI":"10.1088\/1742-6596\/1500\/1\/012003","article-title":"Tomato Harvesting Arm Robot Manipulator; a Pilot Project","volume":"1500","author":"Oktarina","year":"2020","journal-title":"J. Phys. Conf. Ser."},{"key":"ref_39","doi-asserted-by":"crossref","unstructured":"Yaguchi, H., Nagahama, K., Hasegawa, T., and Inaba, M. (2016, January 9\u201314). Development of an autonomous tomato harvesting robot with rotational plucking gripper. Proceedings of the 2016 IEEE\/RSJ International Conference on Intelligent Robots and Systems (IROS), Daejeon, Republic of Korea.","DOI":"10.1109\/IROS.2016.7759122"},{"key":"ref_40","first-page":"20","article-title":"Development of an End-Effector for a Tomato Cluster Harvesting Robot","volume":"3","author":"Kondo","year":"2010","journal-title":"Eng. Agric. Environ. Food"},{"key":"ref_41","doi-asserted-by":"crossref","unstructured":"Hamill, P. (2013). A Student\u2019s Guide to Lagrangians and Hamiltonians\/Patrick Hamiltonians, Cambridge University Press.","DOI":"10.1017\/CBO9781107337572"},{"key":"ref_42","first-page":"265","article-title":"Design and Implementation of Sliding Mode Algorithm: Applied to Robot Manipulator-A Review","volume":"2","author":"Piltan","year":"2011","journal-title":"Int. J. Robot. Autom."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"5127","DOI":"10.1016\/j.ifacol.2017.08.781","article-title":"Sliding Mode Control for Nonlinear Manipulator Systems","volume":"50","author":"Zhang","year":"2017","journal-title":"IFAC-PapersOnLine"},{"key":"ref_44","doi-asserted-by":"crossref","unstructured":"Tinoco, V., Silva, M.F., Santos, F.N., Magalh\u00e3es, S., and Morais, R. (2023, January 26\u201327). Design and Control Architecture of a Triple 3 DoF SCARA Manipulator for Tomato Harvesting. Proceedings of the 2023 IEEE International Conference on Autonomous Robot Systems and Competitions (ICARSC), Tomar, Portugal.","DOI":"10.1109\/ICARSC58346.2023.10129554"},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"209320","DOI":"10.1109\/ACCESS.2020.3038605","article-title":"A Gentle Introduction to Reinforcement Learning and its Application in Different Fields","volume":"8","author":"Naeem","year":"2020","journal-title":"IEEE Access"},{"key":"ref_46","unstructured":"Hammoudeh, A. (2018). A Concise Introduction to Reinforcement Learning, Princess Suamaya University for Technology."},{"key":"ref_47","doi-asserted-by":"crossref","unstructured":"Oliveira, T.R., Fridman, L., and Hsu, L. (2023). Chattering in Mechanical Systems Under Sliding-Mode Control. Sliding-Mode Control and Variable-Structure Systems: The State of the Art, Springer International Publishing.","DOI":"10.1007\/978-3-031-37089-2"},{"key":"ref_48","doi-asserted-by":"crossref","first-page":"36","DOI":"10.1007\/s40435-024-01533-1","article-title":"A review of advanced controller methodologies for robotic manipulators","volume":"13","author":"Tinoco","year":"2025","journal-title":"Int. J. Dyn. Control"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/25\/9\/2676\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,9]],"date-time":"2025-10-09T17:20:20Z","timestamp":1760030420000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/25\/9\/2676"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2025,4,23]]},"references-count":48,"journal-issue":{"issue":"9","published-online":{"date-parts":[[2025,5]]}},"alternative-id":["s25092676"],"URL":"https:\/\/doi.org\/10.3390\/s25092676","relation":{},"ISSN":["1424-8220"],"issn-type":[{"type":"electronic","value":"1424-8220"}],"subject":[],"published":{"date-parts":[[2025,4,23]]}}}