Publications of Pouya Mohammadi

Niels Dehio, Joshua Smith, Dennis Leroy Wigand, Pouya Mohammadi, Michael Mistry and Jochen J. Steil, "Enabling Impedance-based Physical Human-Multi-Robot Collaboration", International Journal of Robotics Research (TO BE PUBLISHED) 2020,

Pouya Mohammadi, Enrico Hoffman, Niels Dehio, Milad Malekzadeh, Martin Giese, Nikos Tsagarakis and Jochen Steil, "Compliant Humanoids Moving Toward Rehabilitation Applications: Transparent Integration of Real-Time Control, Whole-Body Motion Generation, and Virtual Reality", IEEE Robotics & Automation Magazine (RAM), pp. 83-93, Dec 2019, DOI: 10.1109/MRA.2019.2940970
This paper was selected for presentation in ICRA-2020 as well.

Humanoids fascinate us through their anthropomorphic appearance, high dexterity, and potential to work in human-tailored environments and interact with humans in novel applications. In our research, we promote two real-world applications in physiotherapeutic juggling and assisted walking using two compliant humanoids (COMANs), COMAN and COMAN+. We focus on rehabilitation, which, as a result of changing demographics, is becoming an increasingly crucial application field. However, as with most humanoid experiments, the realization of these scenarios is challenging because the hardware is brittle, the software is complex, and control remains highly demanding. In this article, we describe an integrative and transparent control architecture that alleviates this complexity by strictly adhering in design and implementation to a componentbased approach. It promotes flexibility and reusability and allows transparent switching among different humanoid robots, between simulation and the real world, and among control paradigms. It also orchestrates the integration of real-time (RT) and non-RT (NRT) components, including a virtual reality (VR) framework, toward rich user interaction.

Pouya Mohammadi, Daniel Kubus and Jochen Steil, "Exploiting Environment Contacts of Serial Manipulators", 2019 International Conference on Robotics and Automation (ICRA), pp. 197-203, May 2019, DOI: 10.1109/ICRA.2019.8794027

We explore the characteristics of secondary contacts when applying forces with the end-effector of a robot and address the question when these secondary contacts can increase maximum applicable end-effector forces or reduce required actuator efforts. To this end, we formalize the effect of such secondary contacts in terms of required actuator efforts and derive efficiency bounds depending on the contact characteristics and robot configuration. Our findings are confirmed by experiments with a redundant serial manipulator.

Pouya Mohammadi, Enrico Hoffman, Luca Muratore, Nikos Tsagarakis and Jochen Steil, "Reactive Walking Based on Upper-Body Manipulability: An application to Intention Detection and Reaction", 2019 International Conference on Robotics and Automation (ICRA), pp. 4991-4997, May 2019, DOI: 10.1109/ICRA.2019.8794309

In this paper, we look at the challenge of human robot interaction in locomotion. We consider a hand-in-hand interaction scenario where a human compliantly interacts with the upper-body of an impedance controlled humanoid. By exploring the velocity transmission of the robot arms, and the interaction in terms of robot arms manipulation quality evaluated through the monitoring of their manipulability the proposed method derives suitable reactive steps in appropriate directions to ensure that the robot manipulation ability is maintained with the robot arms providing high capacity of motion along the different directions. The proposed approach can be combined with different walking pattern generators and is not tailored to a specific one used in this work. The results of the proposed method are experimentally validated on the COMAN + humanoid robot showing the efficacy of the method to generate reactive stepping driven by the interaction and manipulation motion of the human operator. Besides, the work also provides a real-time software architecture to control humanoid COMAN+, but it is also flexible to be used for the control of other robot platforms.

Sugeeth Gopinathan, Pouya Mohammadi and Jochen Steil, "A Comprehensive Evaluation of Manipulability based Control Schemes for pHRI (to be submitted)", Transactions on Human-Robot Interaction, July 2020,

Recent research in the field of physical Human-Robot Interaction (pHRI) suggests that incorporating human factors into the interaction strategies could improve the quality of pHRI. In this article, we introduce the concept of manipulability based stiffness adaptation for improved pHRI. We show that by using manipulability, it is possible to quantify and combine the task specificity and the user’s physical parameters. We consider and compare different strategies based on manipulability to adapt the stiffness of a KUKA LWR online during the execution of a collaborative task. This article reports the results of a user study conducted with 40 participants for evaluating different manipulability based approaches. The results show that considering task parameters and human-specific factors, improves both the interaction quality and task performance.

Pouya Mohammadi, Milad Malekzadeh, Jindrich Kodl, Albert Mukovskiy, Dennis Wigand, Martin Giese and Jochen Steil, "Real-time Control of Whole-body Robot Motion and Trajectory Generation for Physiotherapeutic Juggling in VR", 2018 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), pp. 270-277, Oct 2018, DOI: 10.1109/IROS.2018.8593632

Motor rehabilitation is in increasingly high demand to deal with minor functional motor impairments resulting from stroke, cerebellar ataxia, or Parkinson's disease. Juggling physiotherapy has shown to induce brain plasticity and to improve coordination and balance in this context. The physiotherapy, however, relies on large number of repetitions to be effective which prompts to deploy robots to release the burden on therapists both in terms of time as well as physical strain. This paper provides a framework to enable juggling games for patients in interacting with robots through Virtual Reality (VR). A set of throwing motions is recorded from the therapist and is retargeted to the humanoid robot COMAN's wrist. The respective whole-body motion is then solved in a stack of Quadratic Programs (QP) in a real-time architecture that integrates OROCOS and Gazebo. The resulting motion is finally streamed to VR for animation of the robot and the thrown ball, which the user can catch in VR using a controller device. We regard the VR setting as an essential step towards physiotherapeutic robotic juggling, because it ensures safety of the patients and effective testing of the methods and already has potential for actual therapeutic intervention. The control framework, however, is already validated in this paper for switching to full real-time operation on the physical robot.

Dennis Wigand, Pouya Mohammadi, Enrico Hoffman, Nikos Tsagarakis, Jochen Steil and Sebastian Wrede, "An open-source architecture for simulation, execution and analysis of real-time robotics systems", 2018 IEEE International Conference on Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR), pp. 93-100, May 2018, DOI: 10.1109/simpar.2018.8376277

The specification and analysis of the timing are an integral part of a robotics system that requires to be highly reliable. Especially since the demand for robots, which are applied in collaborative environments, is increasing drastically, robots need to be even more reliable and safe. In this paper, we propose a workflow for timing specification and analysis of real-time sensitive component-based robotics systems. Further, we introduce CoSiMA, a C++ based architecture that combines technologies, which are well-known in the domain of robotics. CoSiMA offers the ability to model, simulate, deploy, and analyze a robotics system on different robotic platforms. In addition to that, it offers a real-time safe mechanism to collect execution time data of a system, run in simulation or on the real hardware, to investigate and ensure the desired behavior of the robot. In order to depict the proposed workflow, we implemented an experimental system using CoSiMA, which lets the humanoid robot COMAN perform a Zero Moment Point-based walk on a straight line.

Zeeshan Shareef, Pouya Mohammadi and Jochen Steil, "Improving the Inverse Dynamics Model of the KUKA LWR IV+ using Independent Joint Learning", IFAC-PapersOnLine, pp. 507–512, 2016, DOI: 10.1016/j.ifacol.2016.10.653

Abstract: In this paper, we discuss the improvement of the inverse dynamics models of the KUKA LWR IV+ by a recently proposed approach called Independent Joint Learning (IJL). In IJL, the error between the torques from the real robot and the torques from inaccurate dynamics model is estimated using only joint-local information. Due to the reduced model complexity IJL can be used for task-to-task transfer learning and to a task different from the trained tasks. In this paper, we implemented IJL to improve the accuracy of the already existing KUKA LWR IV+ inverse dynamics model and our results show a significant improvement. We also discuss IJL for different types of input datasets and compared them in terms of performance.

Marco Cognetti, Pouya Mohammadi, Giuseppe Oriolo and Marilena Vendittelli, "Task-oriented whole-body planning for humanoids based on hybrid motion generation", 2014 IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 4071-4076, Sep 2014, DOI: 10.1109/IROS.2014.6943135

This paper considers the problem of planning the motion of a humanoid robot that must execute a manipulation task, possibly requiring stepping, in environments cluttered by obstacles. The proposed method explores the submanifold of the configuration space that is admissible with respect to the assigned task and at the same time satisfies other constraints, including humanoid equilibrium. The exploration tree is expanded using a hybrid scheme that simultaneously generates footsteps and whole-body motions. The algorithm has been implemented for the humanoid robot NAO and validated through planning experiments and dynamic playback in V-REP.

Marco Cognetti, Pouya Mohammadi and Giuseppe Oriolo, "Whole-body motion planning for humanoids based on CoM movement primitives", 2015 IEEE-RAS 15th International Conference on Humanoid Robots (Humanoids), pp. 1090-1095, Nov 2015, DOI: 10.1109/HUMANOIDS.2015.7363504

This work addresses the problem of whole-body motion planning for a humanoid robot that must execute a certain task in an environment containing obstacles. A randomized planner is proposed that builds a solution by concatenating whole-body motions. Each whole-body motion is generated so as to realize a center of mass (CoM) movement selected from a set of primitives and simultaneously accomplish a portion of the task. The CoM primitives are representative of typical humanoid actions such as walking gaits (static and dynamic), and can in principle include more sophisticated movements (e.g., jumping, crouching, etc). Implementation on the NAO humanoid proves that the proposed method generates sensible plans for a variety of composite tasks requiring a combination of navigation and manipulation.

Jindrich Kodl, Albert Mukovskiy, Pouya Mohammadi, Milad Malekzadeh, Nick Taubert, A Christensen, Tjeerd Dijkstra, Jochen Steil and Martin Giese, "{O}nline planning and control of ball throwing by the humanoid robot {COMAN} and validation exploiting {VR} in rehabilitation scenarios with ataxia patients", In Proc. of CYBATHLON Symposium on Assistive and Wearable Robotics (AsWeR), May 2019,

Sugeeth Gopinathan, Pouya Mohammadi and Jochen Steil, "Improved Human-Robot Interaction: A manipulability based approach", pp. , 06 2018,

Industrial robots, thanks to advancements in hardware and control design, become more suitable for physical human-robot interaction and collaboration. In this contribution we focus on taking into account human morphology when interacting with robot because in a realistic human-robot interaction, the human physical attributes contribute to quality of task execution, as well as to human comfort. To this aim, we introduce a novel manipulability based stiffness adaptation for a 7 DOF compliant robot and discuss the preliminary results which shows the importance of taking into consideration the human physical charactersitcs for designing controllers for physical Human-Robot Interaction.