Compliant robots

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Contents

Overview

Compliant Actuators

To define what a compliant actuator is, a definition of a non-compliant actuator —better known as a stiff actuator—is useful. A stiff actuator is a device, which is able to move to a specific position or to track a predefined trajectory. Once a position is reached, it will remain at that position, whatever the external forces exerted on the actuator (within the force limits of the device).

A compliant actuator on the other hand will allow deviations from its own equilibrium position, depending on the applied external force. The equilibrium position of a compliant actuator is defined as the position of the actuator where the actuator generates zero force or zero torque. This concept is specifically introduced for compliant actuators, since it does not exist for stiff actuators.

Some remarks must be made concerning terminology. Since compliance is the opposite of stiffness, both terms are used to describe the compliant or non-stiff behavior of an actuator. To describe an actuator with a variable stiffness, the term adjustable compliance can be used, but also variable compliance, adjustable stiffness, and controllable stiffness are used. These examples are given to show that there is not a standard terminology (yet) to describe these types of actuators. Also the term Variable Impedance Actuators is sometimes found. Here impedance can be both compliance as also the damping factor.

Examples of compliant actuators

Passive compliant actuators contain an elastic element, e.g. a spring which can store energy, which is not the case for actuators with active compliance, where the controller of a stiff actuator mimics the behavior of a spring [1]. The latter has the disadvantage that no energy can be stored in the actuation system and due to the limited bandwidth of the controller, no shocks can be absorbed. An advantage of active compliance is that the controller can make the compliance online adaptable. Online adaptability means that the compliance can be adapted during normal operation. A famous robot that uses active compliance for safety is the Kuka/dlr leightweight arm [7].

The most well know example of a passive compliant actuator is the original Series Elastic Actuator SEA [17]), which is a spring in series with a stiff actuator. The compliance of this actuator is fixed and is determined by the selection of the spring; thus, the physical compliance cannot be changed during operation. To obtain variable stiffness, the virtual stiffness of the actuator is adjusted by dynamically adjusting the equilibrium position of the spring.

To combine energy storage and adaptable compliance, an elastic element to store energy is needed, together with a way to adapt the compliance. A substantial number of designs have been developed. An overview is presented in 'Van Ham et al. [21], some classes are:

  • Antagonistic-controlled stiffness: two actuators with nonadaptable compliance and nonlinear force-displacement characteristics are coupled antagonistically, working against each other. By controlling both actuators and using nonlinear springs, the compliance and equilibrium position of this antagonistic setup can be set. To be able to vary the compliance, it is required that the spring characteristic of the two actuators is non-linear, while the resulting spring characteristic is linear. Examples in this section includes the use of pneumatic muscles [4], the VSA joint [18], AMASC [11],. . .
  • Structure Controlled Stiffness: Unlike the previous concept, structure control modulates the effective physical structure of a spring to achieve variations in stiffness. When using a beam as elastic element, the stiffness depends on material modulus, the moment of inertia, and the effective beam length (Mechanical Impedance Adjuster [14]). By adjusting one of these parameters during operation, the stiffness can be controlled. In the Jack Spring [9], the number of active coils in a helical spring is adjusted to vary the stiffness.
  • Mechanically Controlled Stiffness: Similar to structure control, mechanical control also adjusts the effective physical stiffness of the system. However, in this case, the full length of the spring is always in use. The variation is done by changing the pre-tension or the preload of the spring, as is the case in the MACCEPA [22] and the VS-Joint [27]. These actuators require only one compliant element. The complete actuator behaves as a torsion spring where the spring characteristics and the equilibrium position can be controlled independently during operation.
  • Variable Transmission Ratio: Also by changing the transmission ratio differ- ent stiffnesses can be obtained. In the versatile energy efficient (V2E2) actuator [19] an energy efficient actuator is achieved by a combination of an Infinite Vari- able Transmission (IVT) and an elastic element. Another approach to change the transmission ratio is by using a lever mech- anism. A lever has three prin- cipal points; the pivot: the point around which the lever can rotates, the spring point: the point at which springs are located and the force point: the point at which the force is applying to the lever. By changing the position of one of the three one obtains a variable stiffness which is independent controlled from the equilibrium position. Examples here are the AWAS-I [13], AWAS-II [12] and vsaUT [25].

Robotic applications of compliant actuators

Industrial robotics focuses on the interface between motor and loads, which is “as stiff as possible”. This rule of thumb arose because stiffness improves the precision, stability and bandwidth of position control. A welding robot for ex- ample needs stiff actuators. But a precise tracking of a trajectory is not always that stringent anymore. New requirements arise to let a robot walk or run and those can be found in compliant actuation: good shock tolerance, lower reflected inertia, more accurate and stable force control, less damage during inadvertent contact, and the potential for energy storage. For robots that need to interact with humans safety is of primary concern. The biggest danger presents when working in close proximity with robots is the potential for large impact loads resulting from the large effective inertia (or more generally effective impedance) of many robotic manipulators.

So compliant actuators can be found in walking and running robots for energy efficiency since the can exploit the natural dy- namics like is found in passive walkers like Denise [26], Nagoya walkers , Flame [8], Lucy [23]. . . or because they can store and release energy like Mowgli [15], BiMasc [10],. . . Also for prosthesis like ankle-foot prosthesis like from H. Herr [2], AMPfoot,... uses compliant elements to walk energy-efficient.

However, passive compliance can greatly improve robot safety in cases of im- pact between a human and a robot. It also has the ability to store energy (very similar to loading a spring). However, when this energy is suddenly released, it can result in high speed motions of the robot, and correspondingly in a high risk for humans in case of collision [20]. So the control must be adapted to this as is done e.g. in a proxy based sliding mode controller. Examples of robots including compliance for safety are DLR hand/arm system [6], Meka Robot, Probo [5], Ecce Robot [16],. . . . Also for rehabilitation purposes as exoskeletons compliant actuators are used for safety and also adaptability to be able to give assistance as needed as is for example the case in Lopes [24], Knexo [3],. . .

Researchers

Bram Vanderborght (Vrije Universiteit Brussel) [1]

Publications

[1] A. Albu-Schaeffer, O Eiberger, M. Grebenstein, S. Haddadin, Ch. Ott, T. Wimboeck, S. Wolf, and G Hirzinger. Soft robotics: From torque feed- back controlled lightweight robots to intrinsically compliant systems. IEEE Robotics and Automation Magazine, 15(3):20–30, 2008.

[2] Samuel Au and Hugh Herr. On the design of a powered ankle-foot prosthe- sis: The importance of series and parallel motor elasticity. IEEE Robotics & Automation Magazine.

[3] P. Beyl, M. Van Damme, R. Van Ham, B. Vanderborght, and D. Lefeber. Design and control of a lower limb exoskeleton for robot-assisted gait train- ing. Applied Bionics and Biomechanics, 6(2):229–243, 2009.

[4] S. Davis and Darwin G. Caldwell. Pneumatic muscle actuators for hu- manoid applications - sensor and valve integration. In IEEE-RAS Interna- tional Conference on Humanoid Robots, pages 456–461, December 2006.

[5] Kristof Goris, Jelle Saldien, Bram Vanderborght, and Dirk Lefeber. Me- chanical design of the huggable robot probo. International Journal of Hu- manoid Robotics, 8(3):481–511, 2011.

[6] M. Grebenstein, A. Albu-Schäffer, T. Bahls, M. Chalon, O. Eiberger, W. Friedl, R. Gruber, U. Hagn, R. Haslinger, H. Höppner, et al. The dlr hand arm system. volume 11, 2010.

[7] S. Haddadin, A. Albu-Sch "affer, and G. Hirzinger. Requirements for safe robots: Measurements, analysis and new insights. International Journal of Robotics Research, 28(11-12):1507–1527, 2009.

[8] D. Hobbelen, T. de Boer, and M. Wisse. System overview of bipedal robots flame and tulip: Tailor-made for limit cycle walking. In IEEE/RSJ Inter- national Conference on Intelligent Robots and Systems (IROS2008), pages 2486–2491, 2008.

[9] K.W. Hollander, T.G. Sugar, and D.E. Herring. Adjustable robotic tendon using a "jack spring"T M . In 9th International Conference on Rehabilitation Robotics (ICORR 2005), pages 113–118, June-July 2005.

[10] Jonathan W Hurst, Joel Chestnutt, and Alfred Rizzi. Design and phi- losophy of the BiMASC, a highly dynamic biped. In IEEE International Conference on Robotics and Automation (ICRA 2007), pages 1863–1868, April 2007.

[11] Jonathan W. Hurst and Alfred A. Rizzi. Series compliance for robot actu- ation: Application on the electric cable differential leg. IEEE Robotics & Automation Magazine, 15(3):2008, 2008.

[12] A. Jafari, N.G. Tsagarakis, and D.G. Caldwell. Awas-ii: A new actuator with adjustable stiffness based on the novel principle of adaptable pivot point and variable lever ratio. In Robotics and Automation (ICRA), 2011 IEEE International Conference on, pages 4638–4643. IEEE, 2011.

[13] A. Jafari, N.G. Tsagarakis, B. Vanderborght, and D.G. Caldwell. A novel actuator with adjustable stiffness (AwAS). In IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS 2010), pages 4201– 4206. IEEE, 2010.

[14] T. Morita and S. Sugano. Design and development of a new robot joint using a mechanical impedance adjuster. In Robotics and Automation, 1995. Proceedings., 1995 IEEE International Conference on, volume 3, pages 2469–2475. IEEE, 1995.

[15] R. Niiyama, A. Nagakubo, and Y. Kuniyoshi. Mowgli: A bipedal jumping and landing robot with an artificial musculoskeletal system. In Robotics and Automation, 2007 IEEE International Conference on, pages 2546–2551. IEEE, 2007.

[16] V. Potkonjak, B. Svetozarevic, K. Jovanovic, and O. Holland. Anthro- pomimetic robot with passive compliance-contact dynamics and control. In Control & Automation (MED), 2011 19th Mediterranean Conference on, pages 1059–1064. IEEE, 2011.

[17] G.A. Pratt and M.M. Williamson. Series elastic actuators. In IEEE Inter- national Workshop on Intelligent Robots and Systems (IROS 1990), pages 399–406, Pittsburgh, USA, 1990.

[18] R. Schiavi, G. Grioli, S. Sen, and A. Bicchi. Vsa-ii: A novel prototype of variable stiffness actuator for safe and performing robots interacting with humans. In Robotics and Automation, 2008. ICRA 2008. IEEE Interna- tional Conference on, pages 2171–2176. IEEE, 2008.

[19] S. Stramigioli, G. van Oort, and E. Dertien. A concept for a new energy efficient actuator. In Advanced Intelligent Mechatronics, 2008. AIM 2008. IEEE/ASME International Conference on, pages 671–675. IEEE, 2008.

[20] M. Van Damme, B. Vanderborght, B. Verrelst, R. Van Ham, F. Daerden, and D. Lefeber. Proxy-based sliding mode control of a planar pneumatic manipulator. The International Journal of Robotics Research, 28(2):266, 2009.

[21] Ronald Van Ham, Sugar Thomas, Bram Vanderborght, Kevin Hollan- der, and Dirk Lefeber. Compliant actuator designs: Review of actuators with passive adjustable compliance/controllable stiffness for robotic appli- cations. IEEE Robotics and Automation Magazine, 16(3):81 – 94, Septem- ber 2009.

[22] Ronald Van Ham, Bram Vanderborght, Michael Van Damme, Bjorn Ver- relst, and Dirk Lefeber. MACCEPA, the mechanically adjustable compli- ance and controllable equilibrium position actuator: Design and implemen- tation in a biped robot. Robotics and Autonomous Systems, 55(10):761–768, October 2007.

[23] Bram Vanderborght, Ronald Van Ham, Björn Verrelst, Michaël Van Damme, and Dirk Lefeber. Overview of the lucy project: Dynamic sta- bilization of a biped powered by pneumatic artificial muscles. Advanced Robotics, 22(25):1027–1051, 2008.

[24] J.F. Veneman, R. Kruidhof, E.E.G. Hekman, R. Ekkelenkamp, E.H.F. Van Asseldonk, and H. van der Kooij. Design and evaluation of the lopes exoskeleton robot for interactive gait rehabilitation. Neural Systems and Rehabilitation Engineering, IEEE Transactions on, 15(3):379–386, 2007.

[25] L.C. Visser, R. Carloni, F. Klijnstra, and S. Stramigioli. A prototype of a novel energy efficient variable stiffness actuator. In Engineering in Medicine and Biology Society (EMBC), 2010 Annual International Conference of the IEEE, pages 3703–3706. IEEE, 2010.

[26] M. Wisse. Essentials of Dynamic Walking : Analysis and Design of Two- Legged Robots. PhD thesis, Technische Universiteit Delft, 2004.

[27] Sebastian Wolf and Gerd Hirzinger. A new variable stiffness design: Match- ing requirements of the next robot generation. In IEEE International Con- ference on Robotics and Automation (ICRA 2008), pages 1741–1746, May 2008.

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