Michel Franken

Bilateral Telemanipulation With Time Delays: A Two-Layer Approach Combining Passivity and Transparency

In this paper, a two-layer approach is presented to guarantee the stable behavior of bilateral telemanipulation systems in the presence of time-varying destabilizing factors such as hard contacts, relaxed user grasps, stiff control settings, and/or communication delays. The approach splits the control architecture into two separate layers. The hierarchical top layer is used to implement a strategy that addresses the desired transparency, and the lower layer ensures that no “virtual” energy is generated. This means that any bilateral controller can be implemented in a passive manner. Separate communication channels connect the layers at the slave and master sides so that information related to exchanged energy is completely separated from information about the desired behavior. Furthermore, the proposed implementation does not depend on any type of assumption about the time delay in the communication channel. By complete separation of the properties of passivity and transparency, each layer can accommodate any number of different implementations that allow for almost independent optimization. Experimental results are presented, which highlight the benefit of the proposed framework.

Endoscopic camera control by head movements for thoracic surgery

In current video-assisted thoracic surgery, the endoscopic camera is operated by an assistant of the surgeon, which has several disadvantages. This paper describes a system which enables the surgeon to control the endoscopic camera without the help of an assistant. The system is controlled using head movements, so the surgeon can use his/her hands to operate the instruments. The system is based on a flexible endoscope, which leaves more space for the surgeon to operate his/her instruments compared to a rigid endoscope. The endoscopic image is shown either on a monitor or by means of a head-mounted display. Several trial sessions were performed with an anatomical model. Results indicate that the developed concept may provide a solution to some of the problems currently encountered in video-assisted thoracic surgery. The use of a head-mounted display turned out to be a valuable addition since it ensures the image is always in front of the surgeons eyes.

Friction compensation in energy-based bilateral telemanipulation

In bilateral telemanipulation algorithms based on time-domain passivity, internal friction in the devices poses an additional energy drain. Based on a model of the friction, the dissipated energy can be estimated and reclaimed inside the energy balance of the control algorithm. As long as the estimate is conservative, passivity of the entire system is maintained. In this paper we consider two types of friction and discuss the influence of two types of measurement noise. Without noise compensation the dissipated energy is largely overestimated. A compensation method based on the probability density of the noise is proposed. This leads to an energy estimate which is always conservative even in the presence of measurement noise and does not require additional filtering. Simulation results are provided that show the increase in obtained transparency when this energy compensation technique is applied.

Bilateral telemanipulation: Improving the complementarity of the frequency- and time-domain passivity approaches

Passivity of bilateral telemanipulation systems ensures stability of the interaction with such systems. In the frequency domain, passivity of a linear time invariant approximation of the system can be designed for a considered set of operating conditions. Non-linear control structures have been proposed that enforce passivity of the system in the time domain. In this paper, extensions are proposed that increase the complimentarity of the frequency- and time-domain approaches. The combination of both approaches allows a guaranteed measure of transparency to be designed in the frequency domain for a desired set of operating conditions. For operating conditions outside the desired set, stable interaction is guaranteed by the non-linear passivity enforcing control structure. Simulation results of the combined approach are presented that show that the stability properties of the bilateral controller designed in the frequency domain are improved and the transparency properties are improved with respect to those of the standard passivity-enforcing algorithm in the time-domain.

Analysis and simulation of fully ankle actuated planar bipedal robots

This paper deals with the analysis of planar bipedal robots, based on passive dynamic walkers, which are actuated only by actuation of the ankle joints. An overview of the major design characteristics of such robots and their influence on the feasibility of a stable limit cycle is presented. It is shown that robots which are fully powered by ankle actuation require a mass ratio of at least 10:1 between the upper and lower limb to obtain sufficient ground clearance during the swing phase at a wide range of walking speeds. The effect and necessity of the offset in the footshapes of many passive dynamic walkers is shown and the influence of the moment of push off on the required energy injection is treated. The results of the analysis are supported by simulations with a dynamic model of such a robot. The simulated model exhibits a very natural looking gait and walks with a wide range of velocities at low mechanical cost of transport. Simulation results are provided which confirm that pushing off before the swing leg collides with the floor is energetically more efficient than pushing off after the impact as also known from previous literature.

Stability of position-based bilateral telemanipulation systems by damping injection

In this paper two different approaches to guarantee stability of bilateral telemanipulation systems are discussed. Both approaches inject damping into the system to guarantee passivity of the interaction with the device in the presence of time delays in the communication channel. The first approach derives tuning rules for a fixed viscous damper, whereas the second approach employs modulated dampers based upon the measured energy exchange with the device and enforces passivity in the time domain. Furthermore, a theoretical minimum damping injection scheme is sketched that shows that the fixed damping approach is inherently conservative with respect to guaranteeing stability. Experimental results show that both the theoretical minimum damping scheme and a time domain passivity algorithm are successful in stabilizing the telemanipulation system for large time delays with lower gains of the damping elements than derived by the fixed damping injection approach. However, as damping is inherently present in the system, the fixed damping tuning rules can be used to identify if a time domain passivity algorithm is needed given boundary conditions on the actual time delays.

On the use of shunt impedances versus bounded environment passivity for teleoperation systems

This paper analyses and compares two passivity-based approaches that allow to include a-priori knowledge on the dynamic range of the human operator and/or the environment. This can lead to less conservative teleoperation systems compared to systems designed to be purely passive or absolutely stable. The first approach under investigation is a method where the absolute stability is analysed of a teleoperation system augmented with shunt impedances in series and/or parallel with the teleoperation system. It is shown that the traditional interpretation of the use of shunt impedances is not valid and a more accurate description of how to use this method is presented. The second approach under investigation is the bounded environment (operator) method. It is shown that the original idea to restrict the analysis to the so-called worst-case scenarios of a pure mass and a pure stiffness as environment can be too simplistic. Illustrative examples with mass-spring-damper systems fixed to the ground and floating objects as environments are made to demonstrate this in detail. In conclusion, this paper shows that embedding environment knowledge into the controller analysis/design is not straightforward and further research should be dedicated to determine which bounds should be used to obtain practically stable systems for different applications.

Multi-dimensional passive sampled Port-Hamiltonian systems

Passivity of virtual environments running in discrete time is a sufficient condition for stability of the system. The framework for passive sampled Port-Hamiltonian systems allows multi-dimensional virtual environments exhibiting internal dynamic behavior to be computed on a discrete medium in a passive manner. It is shown that a causality analysis is required in the framework to detect if any of the model elements have, a time dependent change of energy function in the energy balance of the system. The Standard Linear Solid model, which is often used to simulate the visco-elastic interaction with soft biological tissue is used as an example. Simulated and experimental results are provided to demonstrate the benefit of the described framework. It is shown that using this approach a multidimensional model which is passive in the continuous domain remains passive in the discrete domain, whereas a standard discretization approach can become non-passive.