You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Variable impedance actuators: A review 
Chapter Title:  Active impedance by control 
1. Abstract of Cited Paper (BIB001):  Purpose – The paper seeks to present a new generation of torque‐controlled light‐weight robots (LWR) developed at the Institute of Robotics and Mechatronics of the German Aerospace Center.Design/methodology/approach – An integrated mechatronic design approach for LWR is presented. Owing to the partially unknown properties of the environment, robustness of planning and control with respect to environmental variations is crucial. Robustness is achieved in this context through sensor redundancy and passivity‐based control. In the DLR root concept, joint torque sensing plays a central role.Findings – In order to act in unstructured environments and interact with humans, the robots have design features and control/software functionalities which distinguish them from classical robots, such as: load‐to‐weight ratio of 1:1, torque sensing in the joints, active vibration damping, sensitive collision detection, compliant control on joint and Cartesian level.Practical implications – The DLR robots are excellent rese... 
2. Abstract of Cited Paper (BIB002):  This paper proposes an effective framework of human-humanoid robot physical interaction. Its key component is a new control technique for full-body balancing in the presence of external forces, which is presented and then validated empirically. We have adopted an integrated system approach to develop humanoid robots. Herein, we describe the importance of replicating human-like capabilities and responses during human-robot interaction in this context. Our balancing controller provides gravity compensation, making the robot passive and thereby facilitating safe physical interactions. The method operates by setting an appropriate ground reaction force and transforming these forces into full-body joint torques. It handles an arbitrary number of force interaction points on the robot. It does not require force measurement at interested contact points. It requires neither inverse kinematics nor inverse dynamics. It can adapt to uneven ground surfaces. It operates as a force control process, and can therefore, accommodate simultaneous control processes using force-, velocity-, or position-based control. Forces are distributed over supporting contact points in an optimal manner. Joint redundancy is resolved by damping injection in the context of passivity. We present various force interaction experiments using our full-sized bipedal humanoid platform, including compliant balance, even when affected by unknown external forces, which demonstrates the effectiveness of the method. 
3. Abstract of Cited Paper (BIB003):  Transforming research results into marketable products requires considerable endurance and a strong sense of entrepreneurship. The KUKA Lightweight Robot (LWR) is the latest outcome of a bilateral research collaboration between KUKA Roboter, Augsburg, and the Institute of Robotics and Mechatronics at the German Aerospace Center (DLR), Wessling. The LWR has unique characteristics including a low mass-payload ratio and a programmable, active compliance which enables researchers and engineers to develop new industrial and service robotics applications with unprecedented performance, making it a unique reference platform for robotics research and future manufacturing. The stages of product genesis, the most innovative features and first application examples are presented. 
4. Abstract of Cited Paper (BIB004):  Legged robots have the potential to serve as versatile and useful autonomous robotic platforms for use in unstructured environments such as disaster sites. They need to be both capable of fast dynamic locomotion and precise movements. However, there is a lack of platforms with suitable mechanical properties and adequate controllers to advance the research in this direction. In this paper we are presenting results on the novel research platform HyQ, a torque controlled hydraulic quadruped robot. We identify the requirements for versatile robotic legged locomotion and show that HyQ is fulfilling most of these specifications. We show that HyQ is able to do both static and dynamic movements and is able to cope with the mechanical requirements of dynamic movements and locomotion, such as jumping and trotting. The required control, both on hydraulic level (force/torque control) and whole body level (rigid model based control) is discussed.

Literature Review:
Active impedance by control is when an actuator mimics the impedance behavior using software control BIB002 . Based on the measured output state, a correction is calculated by the controller and set by the (stiff) actuator. This type of VIA has an actuator, sensor and controller that are fast enough for the application, but no energy can be stored and due to the limited bandwidth of the controller no shock can be absorbed (e.g. hitting with a bat will not be handled by the system with the desired impedance setting). Similarly, exploiting energy efficient natural dynamics (cf passive walkers BIB001 ), is not possible since energy is required to move. Also the impedance controller is quite complex and requires accurate system dynamics models. An advantage of controlled impedance is that it can adapt both the damping and stiffness (contributing to the impedance of the system) online and this in a theoretical infinite range and with infinite speed. This technique pioneered by DLR and commercialized by Kuka is now considered mature BIB004 . Active impedance control has been shown in hydraulic actuators on systems such as the quadruped HyQ BIB005 and Sarcos BIB003 . Fasse et al.  present an active controlled impedance where 3 currents are controlled on the 2 or more available independent windings in the stator or rotor of an electromagnetic motor. This differs from the state feedback strategies described above as the control of torque, stiffness and damping is independent of the feedback from the output, ensuring passivity.