  Although parallel manipulators (PMs) started with the introduction of
architectures with 6 Degrees of Freedom (DoF), a vast number of applications
require less than 6 DoF. Consequently, scholars have proposed architectures
with 3 DoF and 4 DoF, but relatively few 4 DoF PMs have become prototypes,
especially of the two rotation (2R) and two translation (2T) motion types. In
this paper, we explain the mechatronics design, prototype and control
architecture design of a 4 DoF PM with 2R2T motions. We chose to design a 4 DoF
manipulator based on the motion needed to complete the tasks of lower limb
rehabilitation.
  To the author's best knowledge, PMs between 3 and 6 DoF for rehabilitation of
lower limbs have not been proposed to date. The developed architecture enhances
the three minimum DoF required by adding a 4 DoF which allows combinations of
normal or tangential efforts in the joints, or torque acting on the knee. We
put forward the inverse and forward displacement equations, describe the
prototype, perform the experimental setup, and develop the hardware and control
architecture. The tracking accuracy experiments from the proposed controller
show that the manipulator can accomplish the required application.
