Variable impedance actuators: A review <s> Inherent damping <s> A novel force/torque control actuator called series damper actuator (SDA) is proposed, modeled and analyzed. Compared to conventional force/torque control schemes and series elastic actuator (SEA), SDA has good output force/torque fidelity, low output impedance and large force/torque range. Furthermore, varying damping coefficient endows the SDA with more advantages and makes the system more versatile. An experimental SDA system is developed, in which a magnetorheological (MR) fluid damper is employed as the series damper. The experimental results show that SDA system is an effective force/torque control actuator. <s> BIB001 </s> Variable impedance actuators: A review <s> Inherent damping <s> In this paper, we propose a sophisticated design of human symbiotic robots that provide physical supports to the elderly such as attendant care with high-power and kitchen supports with dexterity while securing contact safety even if physical contact occurs with them. First of all, we made clear functional requirements for such a new generation robot, amounting to fifteen items to consolidate five significant functions such as “safety”, “friendliness”, “dexterity”, “high-power” and “mobility”. In addition, we set task scenes in daily life where support by robot is useful for old women living alone, in order to deduce specifications for the robot. Based on them, we successfully developed a new generation of human symbiotic robot, TWENDY-ONE that has a head, trunk, dual arms with a compact passive mechanism, anthropomorphic dual hands with mechanical softness in joints and skins and an omni-wheeled vehicle. Evaluation experiments focusing on attendant care and kitchen supports using TWENDY-ONE indicate that this new robot will be extremely useful to enhance quality of life for the elderly in the near future where human and robot co-exist. <s> BIB002 </s> Variable impedance actuators: A review <s> Inherent damping <s> The development of actuation systems with intrinsic compliance has recently gained increased attention due to the benefits that compliance brings with respect to interaction, safety, and energy efficiency. However, the incorporation of compliance into the actuation system results in systems with more complex dynamics that are not trivial to control due to the induced oscillations and the decreased mechanical bandwidth. To assist the control of robots powered by compliant actuators, this study proposes the use of physical damping in the structure of the compliant actuator. A motivation study is presented to demonstrate the improvements in compliant joint performance gained by introducing physical damping. The variable physical damping actuator (VPDA) aims to improve the control of compliant actuators with the provision of a desired physical damping level on demand. A single degree-of-freedom compliant joint is used to evaluate the ability of the VPDA in replicating different values of physical damping when the link of the compliant joint is perturbed. The performance of the unit is experimentally verified, showing that the VPDA system (mechanism and its control) is capable of generating desired viscous damping levels with good fidelity assisting to regulate the generated oscillations. <s> BIB003 </s> Variable impedance actuators: A review <s> Inherent damping <s> Design issues, dynamic modeling, trajectory planning, and feedback control problems are presented for robot manipulators having components with mechanical flexibility, either concentrated at the joints or distributed along the links. The chapter is divided accordingly into two main parts. Similarities or differences between the two types of flexibility are pointed out wherever appropriate. <s> BIB004
Inherently compliant actuators do have drawbacks: the mechanical resonance is decreased, compromising achievable bandwidth BIB003 [79] [80]  . The introduction of two complex conjugate poles also creates a sharp increase in phase lag, decreasing the stability margin when controlling the joint for link quantities and making the control difficult on the motor side due to the introduction of an anti-resonance at the same frequency. This creates problems particularly in the position/velocity control of such actuators at frequencies well below the mechanical resonance imposed by the compliance BIB004 . Furthermore, the induced underdamped oscillatory modes reduce the achievable performance of the position/velocity control. These oscillations may be suppressed by active damping control if the actuator dynamics are sufficiently high, however the resulting system would require a substantial amount of energy to accomplish such a highly dynamic task. Inherent damping devices (fixed damping actuators and variable damping actuators) are designed to control robotic joints by providing physical damping. The Series Damper Actuator (SDA) BIB001 , is a fixed damping actuator which uses a viscous damper between the actuator and the load. Twentyone has a torsion bar as an elastic element and a rotary damper around the bar as a pseudo damper BIB002 . Dampers have been thoroughly studied and have many different operating principles (Fig. 11) . The most relevant are discussed here and displayed in Fig. 4. 