Page 312 - Handbook of Biomechatronics

304 Andres F. Ruiz-Olaya et al.

The input is the EMG signals and the output is force estimation. The muscle
model can be obtained by using the parametric and nonparametric muscle
model. The parametric muscle model is commonly implemented using
the Hill-based muscle model (Anam and Al-Jumaily, 2012). This model
can be regarded as the biological mechanics of the musculoskeletal limb
model and it is composed of three elements: a contractile element (CE),
a series element (SE), and a parallel element (PE). The Hill-based model
generates the output as the function of EMG activity and the muscle length.
The nonparametric muscle model does not need information of muscle and
joint dynamics (Anam and Al-Jumaily, 2012).
Based on the physical parameters, the exoskeleton control system can be
classified into position, torque-force, and force interaction controllers
(Anam and Al-Jumaily, 2012). The position control scheme is commonly
utilized to make sure the exoskeleton joints turn in a desired angle. The con-
trol system based on torque-force controller is generally applied in the
low-level controller; meanwhile, the high-level controller is the impedance
controller which controls the interaction force between human and the
exoskeleton (Anam and Al-Jumaily, 2012). The main goal of torque/force
controller is to provide proper help for the users in performing a task so that
the force of human-exoskeleton interaction goes to zero. The impedance
controller is an extension of position control and it does not only control
the position and the force but also control a relation and an interaction
between the exoskeleton and the human body, the output of the impedance
model is the force that becomes the reference force for the force-torque con-
troller. This interaction is applied as the high-level controller; its main goal is
to provide proper help for the users in performing a task so that the force of
human-exoskeleton interaction goes to zero. The interaction force can be
controlled by either the impedance controller or the admittance controller
(Anam and Al-Jumaily, 2012). The basic characteristic of the impedance
controller is that it accepts position and produces force. While, the admit-
tance controller is the opposite of the impedance controller; it accepts the
force and yields the position (Anam and Al-Jumaily, 2012).
From the hierarchy point of view, the exoskeleton control system can
be grouped into three levels, which they are task-level, high-level, and
low-level controllers (Anam and Al-Jumaily, 2012). The task-level control-
ler is the highest level controller whose function is based on the task
designed. The high-level controller is responsible for controlling the force
of human-exoskeleton interaction based on the information from the task-
level controller. In the low-level controller, which is the lowest level, the