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:  Friction damper 
1. Abstract of Cited Paper (BIB001):  This paper introduces the development of a semi-active friction based variable physical damping actuator (VPDA) unit. The realization of this unit aims to facilitate the control of compliant robotic joints by providing physical variable damping on demand assisting on the regulation of the oscillations induced by the introduction of compliance. The mechatronics details and the dynamic model of the damper are introduced. The proposed variable damper mechanism is evaluated on a simple 1-DOF compliant joint linked to the ground through a torsion spring. This flexible connection emulates a compliant joint, generating oscillations when the link is perturbed. Preliminary results are presented to show that the unit and the proposed control scheme are capable of replicating simulated relative damping values with good fidelity. 
2. Abstract of Cited Paper (BIB002):  The new areas of technical exploitation of robotics systems has recently set new trends for the robotic actuation by demanding more versatile systems which can cope with unpredictable interactions within not well defined environments and work in close vicinity with the human. Following these trends, this work presents the development of a new actuation system with embodied characteristics such as passive compliance and variable physical damping. Compared to the other existing compliant linear or rotary actuators the proposed CompAct unit has the ability to regulate the oscillations induced by the introduction of the compliance by means of a variable physical damping actuator (VPDA) unit. Apart from facilitating the control the VPDA unit can assist in managing the energy transfer from/to the compliant module. The mechatronics, model and control scheme of the CompAct are analysed. The overall system is evaluated with experimental trials performed using a prototype unit. Preliminary results are presented to show that the unit and the proposed control scheme are capable of regulating the impedance components (stiffness and damping) within a wide range and with good fidelity. 
3. Abstract of Cited Paper (BIB003):  This paper presents the design of an electrically actuated, proportional brake that provides a significantly greater torque-to-weight ratio than a magnetic particle brake (MPB) (considered a benchmark of the state of the art) without sacrificing other characteristics, such as dynamic range, bandwidth, or electrical power consumption. The multidisc brake provides resistive torque through a stack of friction discs, which are compressed by a dc-motor-driven ball screw. Unlike nearly all other proportional brakes, which operate in a normally unlocked mode, the brake presented here is designed such that it may be configured in either a normally unlocked or normally locked mode. The latter enables lower electrical energy consumption and added safety in the event of electrical power failure in certain applications. Following the device description, experimental data are presented to characterize the performance of the brake. The performance characteristics are subsequently compared to those of a commercially available MPB of comparable size. 
4. Abstract of Cited Paper (BIB004):  Research on legged locomotion has encountered its limits in human-made actuation technology. Building a structure and an actuation system capable of performing like a biological muscle for the dynamic locomotion of a mid- to large-sized machine is prohibitive. Novel technologies are being explored with the aim of improving actuator performance. This paper describes the development of a robotic leg for agile locomotion. The main aim is to imitate the multidisciplinary performance of the natural muscle by means of hybridizing conventional and new technologies for the development of large power, large force, lightweight, force-controlled, compliant robotic legs. A preliminary leg prototype has been developed. The actuation system is based on the hybrid use of series elasticity and magneto-rheological dampers which provides variable compliance for natural-looking motion and improved interaction with the ground. Experiments with the real leg prototype provide a positive assessment of the target leg features. 
5. Abstract of Cited Paper (BIB005):  Many forms of actuators have been developed with the capability of braking. Most of these braking mechanisms involve numerous mechanical components, that wear with time and lose precision, furthermore the mechanism are difficult to scale down in size while maintaining relatively large holding torques. In this paper, we propose the use of an off-the-shelf economic material, Hot-Melt-Adhesive (HMA), as a brake mechanism. HMA exhibits visco-elastic characteristics and has interesting properties as it can change phases from solid to plastic to liquid and vice versa. Its advantage is that it is reusable and durable. Experiments were performed to display the holding strength as well as the HMAs visco-elasticity in its solid state as a brake mechanism. The HMA requires no constant application of power when solid, and acts as a brake and visco-elastic damper depending on temperature. Results show that HMA can add compliance and high torque braking of joints.

Literature Review:
A Friction Damper (FD) is composed of an actuator that applies a normal force, F n , on the output shaft (see Fig. 11a ). A frictional damping force F d is produced as a consequence of relative motion. Because of the relative motion, a frictional damping force F d is produced as a consequence of the relative motion, and described by: where f is the friction coefficient andq r the relative speed between the actuator and the output shaft. A more complex and realistic mathematical model can be found in BIB001 . Pure dry or lubricated friction is of no practical use because constant levels of friction can be optimized for a specific task however if the system has nonlinear dynamics or task variability (this is quite common in robotics systems) the global system performance might be deteriorated. If, however, the friction is modulated the performance can be improved. The challenge is therefore the control of the friction to obtain the desired damping characteristics. If the friction damper is properly controlled it can emulate viscous characteristics or any other type of nonlinear damping. It is also interesting to notice that this type of dampers can produce high damping forces at low velocities. Drawbacks of FD, pointed out in BIB001 , are hysteresis and the presence of a static friction band  that can cause irregular behavior. This principle has been used in BIB002 and BIB003 where three piezo stacks connected in parallel apply a braking force between two contact surfaces. This configuration demonstrates low energy consumption while being small, lightweight and clean. CompAct © BIB002 combines the piezoelectric-based friction damper with a series elastic actuator of  marginally increasing the volume, without affecting the total length. Multidisc brakes actuated by electric motors have been tested in BIB004 and  uses magnetic particle brakes.In BIB005 a powder brake was used for haptic rendering at high force bandwidth and magnitude. A Hot-Melt-Adhesive (HMA) as implemented in BIB006 exhibits visco-elastic characteristics that can reversibly change phase from solid to plastic to liquid.