  This paper proposes a simultaneous control method for the angle and stiffness
of the joint in an antagonistic pneumatic artificial muscle (PAM) actuator
system using only pressure measurements, and clarifies the allowable references
for the PAM actuator system. To achieve a sensor-less control, the proposed
method estimates the joint angle and contraction forces using an unscented
Kalman filter that employs a detailed model of the actuator system. Unlike
previous control methods, the proposed method does not require any encoder and
force sensor to achieve angle and stiffness control of the PAM actuator system.
Experimental validations using three control scenarios confirm that the
proposed method can control the joint angle and stiffness simultaneously and
independently. Moreover, it is shown that a reference admissible set can be
used as an indicator to establish reference values by demonstrating that the
reference set covers the experimentally obtained trajectories of the angle and
stiffness.
