From torque-controlled to intrinsically compliant: humanoid robots.
There is one further strong reason why researchers build humanoid robots: understanding and technically reproducing such seemingly simple human tasks like dexterous grasping and manipulation, balancing, walking and running, perceiving the surrounding environment for planning and executing daily tasks are still largely unsolved questions, at least when compared to the human performance. Thus humanoid robot research helps to answer fascinating questions about human capabilities on the one hand, provides clues to build more dexterous, efficient and general purpose machines on the other hand.
In this paper we give an overview of the advancements in humanoid robotics at the German Aerospace Center (DLR) over the last decade. The development started with focus on dexterous, bimanual manipulation with the wheel-based humanoid Rollin' Justin and continued with legged locomotion on TORO. Both robots are characterized by torque-controlled actuators, capable of emulating the adaptable human muscle compliance by feedback control. A new generation of actuators is developed for the humanoid upper body HASY (Hand Arm System), in which "muscle compliance" is realized mechanically, by variable compliance actuators. This step promises increased impact robustness and energy efficiency by elastic energy storage, but raises at the same time substantial additional challenges regarding mechatronic integration and control.
The DLR Light-Weight-Robot-III  represents the third
generation of torque-controlled robot arms developed at DLR. One of the main features of this robot is a tight mechatronic integration of strain-gauge-based torque sensors at the power-output side of the drive units. Such torque sensors allow for effective vibration damping in highly dynamic operations. Moreover, low-level torque feedback loops produce a highly sensitive back-drivable closed-loop behavior despite the highly geared drive units required by the lightweight construction of the joints. As a result, model-based nonlinear control approaches, such as impedance control, can be implemented successfully based on this technology. These drive units build a mature technological basis for the complex humanoid robots Rollin' Justin and TORO (Figure 1).
Autonomous Compliant Manipulation with Rollin' Justin
The mobile humanoid robot Rollin' Justin is utilized as a research platform for autonomous planning and control of manipulation tasks in human environments. The system consists of an omnidirectional platform, an articulated torso, two seven degrees-of-freedom arms, two four-fingered dexterous hands, and a multi-sensory head (see Table 1 & Table 2). The hands are equipped with position and torque sensors and can thus be used for complex manipulation tasks: for example for handling tools or unscrewing the lid of a container or bottle. Rollin' Justin can be operated without wires for about one hour. The size and geometry of the footprint of the mobile base can be adapted to the task by coordinating the movements of the four steerable springborne wheels. Overall, the robot can reach objects up to a height of 2.7m while still fitting through standard doorways. The vision system consisting of RGB-D cameras mounted in the head and the platform and a stereo camera pair allows for the 3D reconstruction of the environment.
With Rollin' Justin we aim to create a cognitive robotic system that is able to reason about compliant manipulation tasks, based on intelligent decisions according to the actual state of the environment. In order to cope with a wide variety of tasks, we utilize a knowledge-based hybrid reasoning system to plan the task execution autonomously on the symbolic level (i.e. which actions have to be scheduled to satisfy the commanded goal state) and on the geometric level (i.e. what are appropriate task parameters to manipulate the objects involved in the actions) . Moreover, during the reasoning procedure, the robot parametrizes the control level for each task execution individually. A hierarchical whole-body impedance control framework  builds the behavioral basis for the higher-level reasoning system (Figure 2). For each task, the robot selects and parameterizes the required control strategies (e.g. Cartesian impedance control, singularity avoidance, and self-collision avoidance) and the controller parameters (i.e. Cartesian trajectory, Cartesian stiffness, and maximum allowed Cartesian forces) based on the requirements of the objects involved in the task execution and the environment.
Exemplary tasks in domestic environments involve wiping of windows, cleaning the dishes and collecting dust or shards with a broom, as demonstrated in the video . These tasks share the need for coordinated whole-body motions, while a tool is guided along a surface in contact. The tasks can be executed with the same overall control strategy, only requiring a different parameterization.
Balancing and Walking with TORO
While Rollin' Justin's main focus is on safe human-robot interaction, complex whole-body motions, bimanual manipulation and other high-level tasks, the bipedal humanoid TORO was built with the aim of evaluating similar torque-based control concepts also for a legged robot. The relevant tasks for TORO include bipedal walking and multi-contact balancing, i.e. compliant stabilization against external disturbances while sustaining two or more end-effectors in contact. In contrast to the dexterous torque-controlled hands of Rollin' Justin, the hands of TORO are human hand prostheses (iLimb ultra) allowing for a robust grip in multi-contact operations but without sensor feedback. Six-axis force-torque sensors in the feet allow measurement of the Zero-Moment-Point (ZMP), i.e. the torque-free point of action of the gross contact force, an inertia measurement unit (IMU) in the trunk is used for real-time control. In accordance with the aim of studying dynamic walking approaches, the feet were designed relatively small, having a size of 19 x 9,5cm. The multi-sensory head consists of a stereo camera, a RGB-D sensor and an additional IMU, which are fused by an onboard computer to provide an ego-motion estimation (based on an extended Kalman filter) and a mapping of the environment. The onboard batteries in the backpack allow for an autonomous operation of up to 1h.
Our approach for the generation and stabilization of walking motions is based on the concept of Capture Point, which is defined as the point on the floor where the robot has to place the ZMP in order to stop within one step. It can be shown that the use of the Capture Point as a state variable separates the overall dynamics into the stable and unstable part. For gait stabilization we utilize an underlying position-based ZMP controller and treat the ZMP as the control input. Moreover, from the Capture Point dynamics one can also see that a sequence of constant ZMP locations (associated with the footsteps) leads to a Capture Point trajectory, which geometrically is simply a connection of lines (zig-zag-curve, Figure 3). As a consequence, the trajectory generation can be performed in a highly efficient way as part of the real-time controller .
Motivated by the successful implementation of torque-based impedance controllers for manipulation with Rollin' Justin, we developed a balancing controller for TORO which builds up on the torque-controlled operation mode of the joint drive units. The controller aims at generating a desired wrench (6-dimensional force-torque vector) at the CoM of the robot . This desired wrench contains a compensation of the robot's total gravity force and a proportional and derivative control action responding to deviations of the CoM and hip orientation from a desired equilibrium configuration. Then a set of contact forces for the end-effectors in contact is computed by an optimization formulation considering unilaterality and friction cone constraints. Finally, the contact forces for all end-effectors are realized by mapping the forces into desired joint torques, which are transferred as set-points to the underlying joint torque controllers. The algorithm has been evaluated in a series of balancing experiments with two (only feet) to four (feet and hands) end-effectors in contact (see Figure 3), including balancing on movable inclined planes, rocks, and even on compliant surfaces (sports mattresses). Current extensions of this controller focus on the realization of dynamic changes in the number of contacts as well as on combinations with the Capture-Point-based algorithm for gait stabilization.
THE HAND ARM SYSTEM
What does it aim at?
The Hand Arm System is a DLR development towards the next generation of humanoid robots in terms of mechatronic design. The aim is to reach the performance of human beings in terms of speed, force and accuracy . Its design philosophy is to understand the biological system and implement the technology to provide a functional equivalent but avoid making a blind copy of the biology.
How does it work?
In humans, the elasticity provided by the muscles, tendons and ligaments decouples the link position from the drive position. Generally speaking, the energy introduced into the system, no matter whether caused by a collision, external forces or acceleration of the link inertia, is converted to elastic energy. This power source can be used to regain kinetic energy and therefore enhances the dynamics of the system. This motivated the introduction of mechanical springs, placed between the output of the gear box and the link to provide a similar behavior. Moreover, by using several nonlinear mechanisms actuated by two motors per joint, it is possible to adjust the stiffness of the joints and adapt to the task requirements.
The Hand Arm System is an upper body humanoid robot with two arms and hands. All of its 48 joints are actuated with nonlinear, adjustable stiffness mechanisms. It is equipped with more than 300 sensors and too motors that are controlled at a frequency of 3kHz. We experimented with different concepts of implementing variable intrinsic compliance .
The platform is used to investigate and experiment modeling and control but also on new planning and grasping strategies (Figure 4).
A typical application demonstrating the potential of compliant actuators is illustrated in Figure 1 (bottom). The arm is driven in mechanical resonance to achieve link velocities above the motor velocity and allow impact torques which are above the maximum motor and gearbox torques. The impact force peak is absorbed and smoothened by the spring. Despite the large actuator compliance, positioning precision is achieved by iterative learning control.
A disadvantage of the very compliant actuation is the low damping of the system when performing fast positioning motions based only on motor position information. However, measuring the joint torque and its derivative based on the spring deflection  allows applying nonlinear control techniques to effectively damp out these oscillations (Figure 5).
CONCLUSIONS AND OUTLOOK
Our overall goal is to develop safe and robust humanoid robots that are capable of performing a multitude of complex tasks and hereby contributing to human welfare. While a decade ago, humanoids seemed far too complex for realistic scenarios, the current results encourage us to imagine first applications within the next decade. Possible fields of use include service robotics, industrial coworkers, search and rescue, space applications and medical robotics, to name but a few. Teleoperated scenarios are feasible in short term, developing in long term towards shared or even full autonomy. Still, advancements have to be made in almost all areas, starting from mechatronic robustness, reliability and energy efficiency, over multimodal perception and control up to autonomous planning and Al-based reasoning. Development of interaction interfaces and communication modalities to humans will play an increasingly important role in the future.
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Christian Ott (1) Alexander Dietrich Daniel Leidner Alexander Werner Johannes Englsberger Bernd Henze Sebastian Wdlf Maxime Chaldn Werner Friedl Alexander Beyer Oliver Eiberger Alin Albu-Schaffer
German Aerospace Center (Ola) WeBling, Germany
(1) Corresponding author
TABLE 1 Overview of the main characteristics of the humanoid systems described in this article. Attribute TORO Rollin'Justin Weight 76 kg about 200 kg Load capacity 10 kg 20 kg Battery time 1 h 1 h (during operation) (during operation) Height 1.74 m 1.91 m Locomotion velocity up to 0.5 m/s up to 2 m/s Vision system Head: 1 RGB-D Head: 1 RGB-D camera & set of camera & set of stereo cameras stereo cameras Mobile platform: 3 RGB-D cameras Additional 1 IMU in the head 1 IMU in the head sensors 1 IMU in trunk 1 IMU in mobile 1 FTS in each foot platform Attribute Hand/Arm System Weight 35 kg Load capacity 15 kg Battery time n.a. Height 1.8 m (including stand) Locomotion velocity -- Vision system Head: 1 RGB-D camera Additional -- sensors TABLE 2 Degrees of freedom of the three humanoid systems described in this article. Subsystem TORO Rollin'Justin Hand Arm System Arms 2x6 2x7 2 x (7+7) Hands 2x6 2x12 2 x (18+18) Torso 1 3 -- Neck/Head 2 2 (4 under development) Locomotion 2x6 (legs for 8 (for free -- System walking) planar motion) Sum 39 51 104
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|Title Annotation:||Focus on Dynamic Systems & Control|
|Comment:||From torque-controlled to intrinsically compliant: humanoid robots.(Focus on Dynamic Systems & Control)|
|Author:||Ott, Christian; Dietrich, Alexander; Leidner, Daniel; Werner, Alexander; Englsberger, Johannes; Henz|
|Date:||Jun 1, 2015|
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