Passive Bilateral Teleoperation
Project: Reconfigurable Control of Robotic Systems Over Networks
Márton Lırinc
Dept. Of Electrical Engineering
Sapientia University
Overview
• What is bilateral teleoperation?
• Notion of passive systems
• Methods for ``passivation‘‘ of teleoperation systems
- Scattering matrix approach
- Wave variables approach
- Passivity observer/controller approach
- The problem of packet loss
• Some closing slides...
Bilateral teleoperation
• Basic elements of the system:
Master robot - Slave robot
• Unilateral teleoperation: the
operator moves the slave
(generally low power robot),
the master follows the motion
of the slave
• Bilateral teleoperation: the
master (when it is in contact
with the environment) reflects
back the contact force to the
slave, which is reflected to the
operator
Early teleoperation system
(no network)
The elements of
a networked teleoperation system
Operator Haptic Device
(Slave) and
Display
Network - Internet
Master robot Camera
Environment
The elements of
a networked teleoperation system II.
Master (m) and slave (s) robot model:
x – position
f - force
Networked teleoperation
• The control objectives in the teleoperation systems are:
- the stability of the system (especially when contact with
environment is expected)
- telepresence, transparency of the teleoperation: position
tracking and force reflection
• The Internet in the control system is modeled as a variable
(bidirectional) time delay + package losses
• The network induced delay further decrease the stability
properties
• According to previous experiments the typical operator
behavior in teleoperation systems: move and wait
Passive electrical
and mechanical networks
• One-port network, two port network, multiport networks
•
•
•
•
f – voltage
v – current
f·v – power
Energy - integral
of power
• The network is passive if the sum of energy flows always
positive (always more energy flows in than out)
Passive electrical
and mechanical networks II.
Examples: Mechanical – electrical equivalency
f – force - voltage (input)
v – speed - current (output)
Stability:
• The passive systems dissipates energy through resistors,
friction ..
• Passive systems are BIBO (Bounded Input Bounded Output)
Stable.
• Power continuous interconnection of passive systems
yields passive systems
Passivation of non-passive networks
Series passive element
• If RN < 0 (negative
resistor) the system is
not passive
• Passivation: R>RN
Parallel passive element
• If Vs> U>0 not passive
• Passivation: RL>RsU/(Vs-U)
R>0
• … or with passive feedback controllers
Back to teleoperation systems …
• The Human is assumed a passive controller.
• The Environment is assumed a passive spring + damper,
with high spring constant.
• We have to make the system highlighted below passive by
controlling correspondingly the robot actuators. Based on
the available measurements (delayed, sometimes lost..)
Back to teleoperation systems … II.
• The objectives can be formulated as:
Passive teleoperation - Scattering approach
• Applicable for linear systems, ideas taken from the
transmission line theory:
• The hybrid matrix:
Definition
Interpretation
Ideal
Passive teleoperation - Scattering approach II.
• Passivity theorem:
• Sketch of the proof:
Passive teleoperation - Scattering approach III.
Passivation with constant time delays:
• Master (m) and slave (s) robot model:
• Controller: (slave: position error + velocity error)
• Passivation of the communication channel:
• Resulting scattering matrix:
Passive teleoperation – Wave variables
• Instead of exchanging as reference signals the power
variables force and velocity the wave variables are
transmitted
• For constant delay we have same passivity results as
in the case of the scattering variables
Passive teleoperation – Wave variables II.
• Time varying delays: the wave variables are amplified
with a gain that depends on the rate of the delay
• With this modification the communication channel is
passive (proof is omitted).
Passive teleoperation – Wave variables III.
• Position drift: General wave or scattering variables encode
information about velocities and forces on both sides of the
teleoperation channel; however, no explicit information about
position is available which may result in position mismatch
between the master and slave systems due to initial transient
response or numerical roundoff errors.
• Modification (for constant delay): the position is sent across the
communication channel both from master to slave and viceversa,
where it is used in a proportional type control.
• With this modifications the passivity is preserved and the position
coordination can be guaranteed.
• Similar results are available for time varying delays
Passive teleoperation – Wave variables IV.
• Impedance matching: it is well known that if the load that
terminates the line has a different impedance than the
characteristic impedance of the transmission line then
wave reflections occur. In the case of bilateral
teleoperation, such reflections degrade the performance
of the system.
• This led to the introduction of impedance-matching
elements b at each end of the communication channel
Passive Teleoperation:
Passive Observer/Controller
• ‚No delay‘ case: Passivity Observer
• Passivity Controller: (α1, α2 is not zero only if the ‚passivity
decreases‘)
Passive Teleoperation:
Passive Observer/Controller II.
• Define the directional energy flows:
• Use two observers:
• The controllers have similar forms:
Packet loss and passivity
• Packet loss is an inherent problem with packet-switched
networks due to several factors such as transmission
time-outs, transmission errors and limited buffer size.
• Last packet strategy:
• Null packet strategy: the
the passivity is compromised:
passivity preserves, but
coordination is compromised:
Packet loss and passivity II.
• Summer, Packet Formatter
• Packet Reader, Subtractor
Packet loss and passivity III.
• Interpolator – in the case of n packet loss we apply the
mean value of the n loss packets n times
• It can be shown that the energy of the system not
increases in the case of packet loss (E0 ≥ Er)
Alternative representations of
teleoperation systems
• Electrical network representation:
• Network as a flexible beam:
Predictive displays
Augmented Reality view of the environment.
• A virtual slave robot is moved real time
on the top of the video feedback.
• After a confirmation from the operator, the real slave robot
follows the motion of the virtual slave robot.
•
Some Youtube videos
with passive teleoperation experiments
http://www.youtube.com/watch?v=GW7XaB4ffsE
http://www.youtube.com/watch?v=GW7XaB4ffsE
http://www.youtube.com/watch?v=Gcf6MHi24Rk
http://www.youtube.com/watch?v=iUgGpArMi9Y
What we want to do?
• The human operator is not necessarily passive in Internet based
teleoperation. The deviation between the seen motion (visual
feedback) and felt motion (force feedback) may de-passify the
operator.
• Human in the Loop problem in a teleoperation approach.
• Stochastic modeling approach to describe the common human +
network + teleoperator behavior.
• Quantifying real-time the quality of the video and signal transmission
– controller, path planner re-tuning in the function of the network
• Video transmission for reliable teleoperation (real time zoom, real
time image quality change in function of the motion)
• Image processing methods, use of extra sensors to help the operator