  This paper expounds the design and control of a new Variable Stiffness Series
Elastic Actuator (VSSEA). It is established by employing a modular mechanical
design approach that allows us to effectively optimise the stiffness modulation
characteristics and power density of the actuator. The proposed VSSEA possesses
the following features: i) no limitation in the work-range of output link, ii)
a wide range of stiffness modulation (~20Nm/rad to ~1KNm/rad), iii)
low-energy-cost stiffness modulation at equilibrium and non-equilibrium
positions, iv) compact design and high torque density (~36Nm/kg), and v)
high-speed stiffness modulation (~3000Nm/rad/s). Such features can help boost
the safety and performance of many advanced robotic systems, e.g., a cobot that
physically interacts with unstructured environments and an exoskeleton that
provides physical assistance to human users. These features can also enable us
to utilise variable stiffness property to attain various regulation and
trajectory tracking control tasks only by employing conventional controllers,
eliminating the need for synthesising complex motion control systems in
compliant actuation. To this end, it is experimentally demonstrated that the
proposed VSSEA is capable of precisely tracking desired position and force
control references through the use of conventional
Proportional-Integral-Derivative (PID) controllers.
