Flexible joint control : robustness analysis of the collocated and non-collocated feedbacks

In this paper, we propose a discussion on the robustness and performance properties of a proportional-derivative controller applied to a very flexible joint. Because of the flexible mode due to in-joint compliance, the classical collocated control does not allow to obtain good rigid mode dynamics with a correct phase margin in low and high frequency, and the non-collocated control does not allow to damp correctly the rotor mode. The simultaneous analysis of discrete root loci and Nichols plots leads to a phase control law with a derivative term built from both input and output velocities. Simulations taking into account various real non-linearities and measurement imperfections are proposed to validate this improved control design

Active damping based on decoupled collocated control

High-precision machines typically suffer from small but persistent vibrations. As it is difficult to damp these vibrations by passive means, research at the Drebbel Institute at the University of Twente is aimed at the development of an active structural element that can be used for vibration control. The active structural element, popularly referred to as ‘Smart Disc’, is based on a piezoelectric position actuator and a piezoelectric force sensor

ASCIE: An integrated experiment to study CSI in large segmented optical systems

A description of the advanced structures/control integrated experiment (ASCIE) experimental setup, a generic test bed for several essential technologies was presented. In particular its multi-input, multi-output, non-collocated control system and its complex structural dynamics, characteristic of large segmented systems make it an ideal test bed for Control-Structure Interaction (CSI) experiments. The high accuracy of its measurement system will make it possible to investigate the dynamics of microvibrations and its implication for the CSI phenomenon

Hybrid iterative learning control of a flexible manipulator

This paper presents an investigation into the development of a hybrid control scheme with iterative learning for input tracking and end-point vibration suppression of a flexible manipulator system. The dynamic model of the system is derived using the finite element method. Initially, a collocated proportional-derivative (PD) controller using hub angle and hub velocity feedback is developed for control of rigid-body motion of the system. This is then extended to incorporate a non-collocated proportional-integral-derivative (PID) controller with iterative learning for control of vibration of the system. Simulation results of the response of the manipulator with the controllers are presented in the time and frequency domains. The performance of the hybrid iterative learning control scheme is assessed in terms of input tracking and level of vibration reduction in comparison to a conventionally designed PD-PID control scheme. The effectiveness of the control scheme in handling various payloads is also studied

Prediction and improvement of the maximum achievable damping with collocated control

Active damping can be realised robustly through the use of a position actuator, a collocated force sensor, and control based on ‘Integral Force Feedback’ (IFF). Instead of a pure integrator, it is also possible to use a first-order lowpass-filter in the feedback loop (‘leaking IFF’). For both cases, the maximum achievable relative damping for a certain vibration mode can easily be predicted. If the achievable damping is too low, it is possible to improve this by means of ‘crosstalk-compensation’. A close look at these strategies reveals that there is a one-to-one relation between ‘leaking IFF’ and ‘crosstalk-compensation’. The presented theory is verified by means of active damping experiments within the lens support of a wafer stepper

Robust saturated control of human-induced floor vibrations via a proof-mass actuator

This paper is concerned with the design of a robust active vibration control system that makes use of a proof-mass actuator for the mitigation of human-induced vibrations in floor structures. Ideally, velocity feedback control (VFC) is unconditionally stable and robust to spillover effects, interlacing of poles and zeros of collocated control is then accomplished. However, the use of a proof-mass actuator influences the system dynamics and the alternating pole-zero pattern of the system formed by the actuator and structure is no longer fulfilled. However, a controlled migration of the two zeros of the root locus plot at the origin, resulting from the acceleration output, can be achieved by adding a feed-through term (FTT) to the structure acceleration output. That is, the FTT enables us to control the position of a pair of complex conjugate zeros (an anti-resonance in the frequency domain). This paper proposes the introduction of an FTT designed in such a way that the anti-resonance at the origin is located between the actuator resonance and the structure fundamental resonance. Hence, an integral controller leads to infinite gain margin and significant phase margin. Simulation and experimental results on a concrete slab strip have validated the proposed control strategy. Significant improvements in the stability properties compared with VFC are reported

Remarks on input-to-state stability of collocated systems with saturated feedback

We investigate input-to-state stability (ISS) of infinite-dimensional collocated control systems subject to saturated feedback. Here, the unsaturated closed loop is dissipative and uniformly globally asymptotically stable. Under an additional assumption on the linear system, we show ISS for the saturated one. We discuss the sharpness of the conditions in light of existing results in the literature.Comment: 12 page

Collocated versus Non-collocated Multivariable Control for Flexible Structure

Future space structures have many closely spaced, lightly damped natural frequencies throughout the frequency domain. To achieve desired performance objectives, a number of these modes must actively be controlled. For control, a combination of collocated and noncollocated sensors and actuators will be employed. The control designs will be formulated based on models which have inaccuracies due to unmodeled dynamics, and variations in damping levels, natural frequencies and mode shapes. Therefore, along with achieving the performance objectives, the control design must be robust to a variety of uncertainty. This paper focuses on the benefits and limitations associated with multivariable control design using noncollocated versus collocated sensors and actuators. We address the question of whether performance is restricted due to the noncollocation of the sensors and actuators or the uncertainty associated with modeling of the flexible structures. Control laws are formulated based on models of the system and evaluated analytically and experimentally. Results of implementation of these control laws on the Caltech flexible structure are presented