The actuators most used today in haptic systems are
direct-current electrical motors. They are easy to install, quiet,
and relatively simple to control. However, their reduction
mechanisms can introduce backlash and high inertia, which are
undesirable in haptic applications. In this brief, we investigate
the use of electropneumatic actuators in a tele operation system.
Compared to the electrical actuators, pneumatic actuators
have higher force-to-mass ratio and can generate larger force
without any reduction mechanism. Moreover, they are inert to
magnetic fields, which is crucial in certain applications such
as robot-assisted surgery under magnetic resonance imaging
guidance [3], [4].
Due to the above advantages, pneumatic actuators have
been used in new applications, such as telerobotics, in recent
years. For instance, Tadano and Kawashima [5] propose a
forceps manipulator for a surgical master–slave system, which
estimates external forces without using any force sensor.
Pneumatic muscle actuators have been also recently used in
the tele operation system [6]. They are compact and have high
power/weight density, but they are difficult to control and
require an accurate experimental characterization.
All of the prior works used proportional servo valves
in pneumatically actuated systems because they allow the
achievement of high performances in various position or force
control tasks. However, they are typically expensive because
they require high-precision manufacturing. Therefore, in this
brief, fast-switching ON/OFF valves are used due to their
advantages in terms of low cost and small size. Thanks to
the breakthroughs in valve technology such as leak reduction,
miniaturized of mechanical elements, and fast electronic components;
the solenoid valves now are faster and more accurate
than the former valves. One of the objectives of this brief is to
show that good transparency in tele operation can be obtained
with these inexpensive components.
The traditional method for controlling a system with a
solenoid valve is pulse-width modulation (PWM) [7]–[9].
A main problem with PWM control is chattering, which is
caused by the high-frequency switching of the valve [10].
Chattering can drastically reduce the lifetime of the
valve and generate noises possibly disturbing for certain
applications.
To overcome the drawbacks of PWM-based control of
solenoid valves, this brief presents a new control method
inspired by the hybrid theory recently developed for asynchronous
and synchronous electrical motor control [11]–[13].
This approach is used in a switching-based hybrid system,
which includes continuous actuators and a discrete controller
with a finite number of states. In the case of alternating
current motor drives, contrary to conventional vector control
such as proportional-integral control in which the inverter model is not taken into account by the controller, hybrid
control considers the state of the inverter a control variable.Therefore, it allows obtaining faster torque dynamics than the
vector-control algorithm. Our contribution in this brief consists
of applying the previously developed hybrid algorithm to
the pneumatic master–slave system actuated by fast-switching
ON/OFF valves. The four-channel (4CH) bilateral tele operation
architecture is used in order to test the efficiency of the
proposed control laws. The reason for this is that the 4CH
method is the most general tele operation control architecture
compared to position error based, direct force reflection, and
shared compliance control methods and it can achieve superior
transparency [14], [15]. Finally, an analysis of the controller
parameters is carried out to achieve satisfactory performance
in terms of tele operation transparency.
For the sake of simplicity, the master and slave actuators
are considered to be identical in this brief. In the experiments,
the master and the slave are one-degree-of-freedom
(1-DOF) pneumatic manipulators. It should be noted that this
brief does not deal with the presence of time delay in the
communication channel of the tele operation system—for time
delay compensation in haptic tele operation, the readers may
refer to [16] and [17]. Such a delay is commonly presented in
remote tele operation systems, but is generally not significant
in local tele operation systems.
The structure of this brief is as follows. First, the modeling
of the pneumatic manipulator composed of a cylinder and four
solenoid valves is presented in Section II. Section III describes
the hybrid principle, which is implemented in a 4CH bilateral
control system. Section IV presents experimental results that
validate the proposed theories. Finally, concluding remarks
appear in Section V