Theodorus J.A. de Vries

Brief Stability analysis of learning feed-forward control

In this paper, a learning control system is considered for motion systems that are subject to two types of disturbances; reproducible disturbances, that re-occur each run in the same way, and random disturbances. In motion systems, a large part of the disturbances appear to be reproducible. In the control system considered, the reproducible disturbances are compensated by a learning component consisting of a B-spline neural network that is operated in feed-forward. The paper presents an analysis of stability properties of the configuration in case of a linear process and second-order B-splines. The outcomes of the analysis are quantitative criteria for selection of the width of the B-splines, and of the learning rate, for which the system is guaranteed to be stable. These criteria facilitate the design of a learning feed-forward controller.

On admissible pairs and equivalent feedback-Youla parameterization in iterative learning control

This paper revisits a well-known synthesis problem in iterative learning control, where the objective is to optimize a performance criterion over a class of causal iterations. The approach taken here adopts an infinite-time setting and looks at limit behavior. The first part of the paper considers iterations without current-cycle-feedback (CCF) term. A notion of admissibility is introduced to distinguish between pairs of operators that define a robustly converging iteration and pairs that do not. The set of admissible pairs is partitioned into disjoint equivalence classes. Different members of an equivalence class are shown to correspond to different realizations of a (stabilizing) feedback controller. Conversely, every stabilizing controller is shown to allow for a (non-unique) factorization in terms of admissible pairs. Class representatives are introduced to remove redundancy. The smaller set of representative pairs is shown to have a trivial parameterization that coincides with the Youla parameterization of all stabilizing controllers (stable plant case). The second part of the paper considers the general family of CCF-iterations. Results derived in the non-CCF case carry over, with the exception that the set of equivalent controllers now forms but a subset of all stabilizing controllers. Necessary and sufficient conditions for full generalization are given.

Active damping within an advanced microlithography system using piezoelectric Smart Discs

Vibrations within high-precision machines are often badly damped, and may easily show up as a major factor in limiting the achievable accuracy. As it is hard to damp these vibrations by passive means, active vibration control may become inevitable within such machines. Robust active damping can be obtained by applying integral force feedback to so-called ‘Smart Discs’, i.e., active structural elements consisting of a piezoelectric position actuator and a collocated piezoelectric force sensor, which are inserted at appropriate locations within a machine frame. n nA wafer stepper, i.e., the advanced microlithography system that is at the heart of integrated circuit manufacturing, is an excellent example of a high-precision machine with a frame that has low structural damping. Badly damped vibrations of the lens of the wafer stepper may in future limit the attainable line width of the circuit patterns. For this reason an active lens support, based on Smart Discs, is developed. Experiments with the active lens support show that the relative damping of the dominant vibration modes of the lens is increased from 0.2% to 16%, which effectively results in 86% reduction of the vibration amplitude.

Inertia-driven controlled passive actuation

A serious problem with using electrical actuators in legged locomotion is the significant energy loss. For this reason, we propose and analyse an alternative means of actuation: Controlled Passive Actuation. Controlled Passive Actuation aims at reducing the energy flow through electric actuators by actuating with a combination of an energy storage element and a Continuously Variable Transmission. n nIn this work, we present a method where we apply a Continuously Variable Transmission with a storage element in the form of a mass to change the state of another mass (“the load”). An abstraction layer is created to abstract the inertia-driven Controlled Passive Actuation to a source of effort, a force actuator. On this abstracted system, feedback control can be applied to achieve control goals such as path tracking. n nWith simulations and experiments, we show that inertia-driven Controlled Passive Actuation can be used to control the state of an (inertial) load. The experimental results show that the performance of the system is affected by the internal dynamics and limited rate of change of the transmission ratio of the Continuously Variable Transmission.

Active damping of the 1D rocking mode

Active damping of a rotational vibration mode in the linear guidance of a precision machine in a one dimensional (1D) setting is considered in this paper. This so-called rocking mode presents itself in machines having linear actuation. The limitation this vibration mode imposes on the machine precision and closed-loop bandwidth can be overcome by means of active damping. Implementing active damping in a collocated fashion in combination with a passive control algorithm guarantees the robust stability of the system. Active damping control operates in parallel with the motion controller if one is present. These controllers operate completely independently of each other. The overall result is that a higher bandwidth can be achieved for the closed-motion-loop involving the actively damped plant. This improves the performance of the machine in terms of response time, settling time and steady-state and/or set-point error.

Active Structural Elements within a General Vibration Control Framework

High-precision machines typically suffer from small but annoying vibrations. As the most appropriate solution to a particular vibration problem is not always obvious, it may be convenient to cast the problem in a more general framework. This framework may then be used for frequency response analysis, which, together with close examination of the disturbance sources, leads to a solution in general structural terms, like ‘vibration isolation’, ‘stiffness enhancement’ or ‘damping augmentation’. In case it is not possible or unpractical to control the vibration passively, a solution based on active structural elements may be considered.

Design Procedure for a Learning Feed-Forward Controller

Abstract When a process is subject to reproducible disturbances, Learning Feed-Forward Control (LFFC) can be used to obtain accurate tracking. Until recently, LFFCs were designed by means of trial and error. This paper formulates a design procedure, according to which an LFFC can be designed in a structured way. The design is based on qualitative knowledge of the process and the disturbances. This design procedure results in a shorter design phase and better performing LFFC. As an example, the design procedure is used to construct an LFFC for a linear motor. Simulations show that the resulting LFFC is able to obtain accurate control.

Motor-gearbox selection for energy efficiency

Energy consumption of robotic systems is getting more and more attention. Previous work suggests that energy losses in the actuators of robots are a significant source of high energy consumption. A possible reason is that, so far, motor-gearbox selection methods have not focussed on energy efficiency. In this work, we present a model-based motor-gearbox selection procedure with the focus on key mechanisms of energy dissipation. Using an example, we evaluate the possible efficiency gains when selecting a motor-gearbox combination for energy efficiency. In this example, the losses are reduced by 56% to 40%. We conclude that selecting a motor-gearbox combination for energy efficiency can provide a significant improvement in the efficiency. However, the energy losses in the motor-gearbox combination remain significant.

A novel concept for a translational continuously variable transmission

Continuously Variable Transmissions have the potential to change robotics. When used in a drive train in combination with a spring, they could significantly reduce energy consumption. However, so far these advantages have been shown in theory only. To exploit the concept of a Continuously Variable transmission to its full potential it should have certain properties and be designed with some critical design points in mind. In this work, we propose three properties that a Continuously Variable Transmission needs to have and two critical design points that should be optimised in a design. We propose a novel concept of a lever with a movable pivot where the pivot always moves along the lever. A realisation of this concept is discussed and evaluated with respect to the required properties and critical design points. We conclude that the realisation has the required properties and that the performance on the critical design points can be improved by improving the mechanical design.

Applying energy autonomous robots for dike inspection

This article presents an exploratory study of an energy-autonomous robot that can be deployed on the Dutch dykes. Based on theory in energy harvesting from sun and wind and the energy-cost of locomotion an analytic expression to determine the feasible daily operational time of such a vehicle is composed. The parameters in this expression are identified using lab results and weather statistics. After an evaluation of the “Energy autonomous robot in the Netherlands” case, the results are generalised by looking at the effects of varying the assumptions. Based on this work, three conclusions can be drawn. Firstly, it is realistic to have an energy-autonomous walking dyke robot in the Netherlands. Secondly, the use of solar panels is probably not feasible if the amount of solar energy that is available is much less than assumed in the study. Finally, in this case study, the inclusion of a wind turbine typically offers a slight benefit. Furthermore, it gives a significant benefit in the months where the incident power of the sun is low, thus allowing a reasonable operational time during the winter.