Non-collocated Sensors Research Articles

CONTROL OF A FLEXIBLE ROBOT WITH FUZZY LOGIC AND A NONCOLLOCATED SENSOR LOCATION

Nonadaptive and fuzzy logis system adaptive control (FLSAC) strategies are used to track the end effector of a flexible space robot with sensors collocated at the joints, noncollocated at the end effector and at 0.5m from the elbow joint. Collocated joint sensors satisfy hyperstability conditions but fail to capture nonminimum phase (NMP) response that causes control action delay. Noncollocated sensors capture NMP response. Results are poor for nonadaptive control with collocated and noncollocated sensors but precise control is achieved for FLSAC regardless of sensor location.

Velocity Feedback with a Non-Collocated Pair of Sensor and Actuator for Tip Vibration Suppression of a Beam

This paper presents a theoretical and experimental study of a non-collocated pair of piezopolymer PVDF sensor and piezoceramic PZT actuator, which are bonded on a cantilever beam, in order to suppress unwanted vibration at the tip of the beam. The PZT actuator patch was bonded near the clamped part and the PVDF sensor, which was triangularly shaped, was bonded on the other part of the beam. This is because the triangular PVDF sensor is known that it can detect the tip velocity of a cantilever beam. Because the arrangement of the sensor and actuator pair is not collocated and overlapped each other, the pair can avoid so called "in-plane coupling", which can be found at a matched piezoelectric sensor and actuator pair and restricts the stability and performance of direct velocity feedback control. The test beam is made of aluminum with the dimension of 200 × 20 × 2 mm. Before control, the sensor-actuator frequency response function is confirmed to have a nice phase response without accumulation in a reasonable frequency range of up to 5000 Hz. The feedback control attenuates the magnitude of the first two resonances in the error spectrum of about 6 -7 dB.

Shape optimization of piezoelectric sensors or actuators for the control of plates

This paper deals with a new method to control flexible structures by designing non-collocated sensors and actuators satisfying a pseudo-collocation criterion in the low-frequency domain. This technique is applied to a simply supported plate with a point force actuator and a piezoelectric sensor, for which we give some theoretical and numerical results. We also compute low-order controllers which stabilize pseudo-collocated systems and the closed-loop behavior show that this approach is very promising.

궤환 모델 개선법을 위한 모드 분리 제어기

A novel concept of feedback loop design for modal test and model updating is proposed. This method uses the closed-loop natural frequency information for parameter modification to overcome the problems associated with the conventional method employing the modal sensitivity matrix. To obtain new modal information from closed-loop system, controllers should be effective in changing modal data while guaranteeing the stability of closed-loop system. But it is very hard to guarantee the stability of the closed-loop system with non-collocated sensor and actuator set. In this research, we proposed a controller called mode-decoupling controller that can change a target mode as much as the designer wants guaranteeing the stability of closed-loop system. This controller can be computed Just using measured open-loop modeshape matrix. A simulation based on time domain input/output data is performed to check the feasibility of proposed control method.

A time delay method to solve non-collocated input estimation problems

Fundamental properties of using time delay to solve ill-posed input estimation problems are studied. It is shown that for systems with non-collocated sensors and inputs these problems are ill-posed, and how a sensor-specific time delay approach can be used to formulate a modified, well-posed problem. Invertibility of the associated block Toeplitz matrix and its relation to truncation errors are discussed. Moreover, it is shown that the time delay approach, in order to produce an acceptable solution, requires that certain conditions regarding the positioning of sensors are fulfilled. Some numerical examples to illustrate various aspects of the time delay approach are also given. The issues are treated in the setting of mechanical systems.

Active vibration isolation of a flexible structure mounted on a vibrating elastic base

The problem of isolating the vibration at any location on a flexible structure mounted on a vibrating flexible base is considered using a Kalman-based active feedforward–feedback controller (KAFB) with non-collocated sensors and actuators. The control strategy developed in this study focuses on lowering the force transmitted to the structure through its vibrating elastic foundation in the presence of process and measurements noise. A state-space model of the structure is constructed from the natural frequencies and mode shapes generated via finite element modal analysis of the structure. The important aspect of the proposed control strategy is that, while it's design is based on a full order model of the physical structure (plant), its implementation is reduced to the realization of a second order estimator regardless of the order of the plant model, and with negligible effect on its accuracy and performance. Therefore, the proposed control strategy requires low computational effort, which makes it well suited for real time control applications. Another unique aspect of this control strategy is its agility and speed in compensating for any phase or magnitude mismatch between transmitted force and control force. Moreover, the stability of the control system is implicitly attained by the controllability condition posed by the Kalman filter on the model. Thus, proper choice of Kalman gains will drive the states of the structure, at the sensor location, ideally to zero. In addition to that, digital implementation of the proposed controller can be easily done considering the fact that the discrete Kalman filter is exact. Numerical simulation of the controller performance is carried out and the results are presented.

Hysteresis, Creep, and Vibration Compensation for Piezoactuators: Feedback and Feedforward Control

We present an integrated two-step approach, which combines feedback and feedforward control, to compensate for the effects of hysteresis, creep, and vibration in piezoactuators. In this approach, the control of hysteresis and creep is decoupled from the control of vibrational dynamics. First, high-gain feedback control is used to minimize positioning error caused by hysteresis and creep. Second, an inversion-based feedforward approach, which can achieve exact tracking for general output trajectories, is used to compensate for error due to vibration at high scan rates. The feedforward approach is applicable to minimum (collocated sensor and actuator) and nonminimum phase (noncollocated sensor and actuator) positioning systems. The decoupling of hysteresis and creep control from vibration control simplifies the inversion-based approach, and the use of feedback provides robustness. We show significant improvement in positioning precision and scanning rate, and illustrate our results with an experimental piezoactuator scanner that is used in Atomic Force Microscope (AFM) applications.

ACHIEVING MINIMUM PHASE TRANSFER FUNCTION FOR A NONCOLLOCATED SINGLE‐LINK FLEXIBLE MANIPULATOR

ABSTRACTA noncollocated system has the potential of providing more precise tracking, improved disturbance rejection and increased bandwidth at the sensor location, but is considerably more difficult to stabilize than a collocated system due to its nonminimum phase nature. For a flexible manipulator, the problem becomes even more complicated because the system is inherently infinite dimensional. In this paper, a single‐link flexible manipulator having a tip payload and a noncollocated actuator and sensor is investigated using a linear distributed parameter model. With the joint torque as the input and the joint angle plus a weighted value of tip deflection as the measured output, an exact transfer function involving transcendental functions is derived. Using the methods of infinite product expansion, the root locus, and the asymptotic property of the roots of the transcendental equation, a necessary and sufficient condition in terms of the weighting factor of tip deflection is obtained such that the transfer function does not have any open right‐half plane zeros. This condition depends neither on the physical properties of the link nor on the mass properties of the tip payload. In order to correct the misinterpretation which has occurred in some closely‐related works concerning the stability of the infinite‐dimensional zero‐dynamics, the equivalence of the zeros of the transfer function and the eigenvalues of zero‐dynamics is also verified. Numerical results for the closed‐loop performance of a single‐link flexible arm using collocated and noncollocated PD controllers are given to show the efficacy of the proposed minimum phase transfer function approach.

Adaptive neural control for space structure vibration suppression

Despite recent advances in efficiency, current methodologies for space structure control design still engage significant human resources for engineering development and routine maintenance. The adaptive neural control (ANC) program is part of an effort to develop neural network based controllers capable of self-optimization, on-line adaptation and autonomous fault detection and control recovery. This development in addition supports the long-term space exploration objectives for which autonomous spacecraft involving self-reliant control systems are a necessity. The ANC program comprises two phases. The first, basic phase focused on the development of efficient and completely autonomous neural network feedforward control for the case of broadband disturbances. Algorithms were developed that work with no prior modeling information about the system to be controlled and adapt to changing conditions, while minimizing or eliminating the introduction of extraneous training signals. The algorithms were demonstrated experimentally on an optical structure testbed at Harris. The second phase of the program demonstrated a more complex neural controller on the advanced space structures technology research experiments (ASTREX) test facility at the Air Force Research Laboratory capable of the fault-tolerant adaptive control of multiple sensors and actuators. This system used six actuation channels of the existing ACESA struts on the ASTREX structure to simultaneously cancel three independent tonal disturbances in the 10-15 Hz band, measured at non-collocated sensors on the secondary tower of the structure. The system demonstrated impressive fault-recovery performance, maintaining good cancellation performance with successive actuators disabled. Cancellation of individual tones was between 25 and 55 dB, with over 27 dB attenuation realized root mean square. The algorithm required very low computational throughput, operating at a sample rate of 1/20 Hz. The results of the ANC program show that adaptive cancellation systems can reduce vibrations in precision structures without prior modeling information and can adapt successfully to certain failures in actuators or sensors, optimally reconfiguring themselves without human intervention. These capabilities should significantly reduce the expense of designing and maintaining vibration control systems for spacecraft.

Robust strategies for active vibration control of plate-like structures: Theory and experiment

The theoretical modelling of the vibration of a plate-like active structure incorporating piezoelectric actuators is presented using the Rayleigh-Ritz method. The model was used to design multi-variable controllers (linear quadratic Gaussian and H∞) for a cantilever plate with two piezoelectric actuators providing the controlled strain input to the plate with two non-collocated sensors providing the feedback signals. A technique for the optimal placement of piezoelectric actuators is developed using modal controllability and a genetic algorithm as the search engine. The proposed controllers were then implemented experimentally and the measured results compared favourably with those predicted from simulation studies. The results show that considerable disturbance attenuation can be achieved in lightly damped modes of the structure using optimal placement of piezoelectric actuators and multi-variable controllers.

Modeling and Control of Noncollocated Flexible Structures

A new and comprehensive method for modeling and control of noncollocated flexible structures is proposed. The first step of the design is obtaining reduced order model which includes a delay between input excitation by the actuator and output measurement by the noncollocated sensor. A fixed order observer-predictor control scheme for compensating the response delay is used. In addition, a new algorithm for calculating the state feedback and observer gain matrices is introduced. It is intended to overcome the spillover effects by suppressing the residual subsystem response.

Frequency Domain Parametric Identification of a Flexible Manipulator Model for Structural Control Applications

The problem of modeling a single-link flexible manipulator having a revolute joint, is rather complex. It involves complex interaction and coupling between the desired rigid body motion and the undesired flexible motion. Analysis is performed on a representative case study of a Pinned-Free Euler-Bernoulli beam with noncollocated sensors. Numerical method for accurate computation of the transfer function poles and zeros is proposed. Experimental investigations that have been carried out on a real laboratory model of a single link flexible manipulator is reported. This model consists of a prismatic beam free at one end and attached to a DC motor at the other end. Using a dual channel Dynamic Signal Analyzer DSA, the manipulator has been excited by a harmonic torque and the structural displacement at several locations have been measured using vibration accelerometer. The DSA curve fitting capability are used to determine the best fit estimate of the manipulator's experimental transfer function poles and zeros.

Stable Adaptive Control of Non-Collocated Systems Using the Progressive Learning Method

A novel approach to adaptive control of non-collocated robotic systems is presented, and its feasibility and effectiveness are verified through experiments using an ultra-high speed direct-drive robot having structural flexibility. This approach, called "progressive learning," allows the system to learn parameters recursively and progressively, starting with the ones associated with low frequencies and moving up to the ones with a full spectrum. Even though the system has non-collocated sensors and actuators and the relative order is three or higher, the learning process is guaranteed to converge by exciting the system with a particular series of reference inputs having an appropriate frequency spectrum. Experimental results are provided to demonstrate the effectiveness of this method. Implementation issues and practical considerations are addressed as well.

Passivity motivated controller design for flexible structures

When actuators and sensors on a controlled mechanical structure form naturally passive pairs, which usually requires physical collocation, very robust stabilizing controllers can be constructed. If the number of such naturally passive pairs is small, the closed loop performance may not be satisfactory. This motivates the incorporation of additional sensors in the control design. However, the passivity property no longer holds for these sensors. Some thoughts are presented on extending the passivity based design to incorporate these non-collocated sensors by using some nominal model information to synthesize a fictitious strictly passive output. Simulation and experimental studies are presented to show the merits of this approach. >

Active Sound Attenuation in Finite-Length Ducts Using Close-Form Transfer Function Models

In this paper, the problem of active sound cancellation in finite-length ducts is investigated. The closed-form solution of a one-dimensional wave equation is obtained as the plant model. The controllability, observability, and transmission zeros are discussed based on the transfer function model. For ducts with totally reflective boundaries, stabilization can be achieved by using a speaker (actuator) and a microphone (sensor). Cases of collocated and noncollocated sensors and actuators are presented. A repetitive control algorithm was developed to drive the actuator so that harmonic noises in a duct are attenuated. For a duct with partially reflective boundaries, the application of repetitive control prevents sound from leaking out of the duct at a chosen end. A simulation study demonstrating the effects of this feedback control scheme is also presented.

Contact transition control with semiactive soft fingertips

We address the problem of controlling contact forces generated when the fingers of a robot close on an object or make contact with surfaces in the robot's environment. The use of controllable fingertip materials is proposed to enhance system performance and avoid contact instability problems that can arise with noncollocated sensors and actuators. We describe a novel fingertip employing an electrorheological fluid and present the results of experiments to evaluate its capabilities when used with a robot contacting a stationary object at various speeds. Having characterized the fingertip behavior as a function of applied voltage, we turn to the question of optimal control, Using a simplified dynamic model of a robot equipped with a controllable electrorheological fingertip, we obtain a piece-wise optimal solution for the fingertip damping as a function of time, to minimize settling time while satisfying constraints on the contact forces.

Dynamic Analysis of Noncollocated Flexible Arms and Design of Torque Transmission Mechanisms

A new actuation method for one-link flexible arms is presented. The endpoint control of a flexible arm has been known as a nonminimum phase system due to the noncollocated sensor and actuator. By relocating the actuator near the endpoint, the system can be modified to approximate a minimum phase system. In order to implement this, transmission mechanisms are developed which transform the actuator torque to a combination of force and torque and transmit them to an appropriate point on the arm link. Exact pole-zero configurations are analyzed with regard to the location of the actuation point and the type of actuator used. Guidelines for design of the transmission mechanisms and the actuation points are developed with respect to the operation bandwidth, stability and controllability. A prototype flexible arm is designed based on the design guidelines and open-loop and closed-loop tests are performed to verify the effectiveness.

Active beam vibration control using PZT actuators

Abstract Smart materials of lead-zirconate-titanate (PZT) piezoelectric ceramics have attracted attention in recent years for active vibration control, acoustic noise suppression, health monitoring and damage assessment. We at McDonnell Douglas Aerospace have been studying these ceramics for use as sensors and actuators in various space and aircraft structures. In this paper, we will report results of a recent study on active vibration control using monomorph PZT actuators. The experiments were performed on thin aluminum cantilever beams. Collocated and non-collocated sensors and actuators were employed. Two control techniques: the classical velocity feedback and adaptive feedback controls, were investigated. We have obtained significant damping and broadband vibration attenuation of greater than 30 dB using the classical control with the single-input single-output feedback approach. A 24 dB reduction has also been achieved using the adaptive control with the multiple-input single-output approach. Detaile...

Control of lightly damped, flexible modes in the controller crossover region

Many proposed future space structures have closely spaced, lightly damped flexible modes that must actively be controlled to achieve desired performance objectives. These densely packed structural modes require a controller to roll off in a region of modes that have some amount of modeling error or uncertainty in their damping levels, natural frequencies, and/or mode shapes. Therefore, a delicate balance must be found between the level of closed-loop performance of the control system and its robustness to various types of uncertainty. This paper focuses on using the structured singular value (u) control design technique to achieve performance and robustness objectives in the presences of uncertainty flexible modes in the controller crossover region. Controllers are synthesized for a lightly damped, flexible structure experiment with noncollocated sensors and actuators to show the role played by uncertainty modeling in achieving desired performance objectives.

位置決め機構の物理パラメータの一同定方法

The positioning mechanism can be regarded as a simple vibration system with one degree of freedom. Generally speaking, it is difficult to identify the physical parameters of this mechanism without decomposing. In this paper, an identification method of physical parameters for positioning mechanism is described. Firstly it is shown that the viscous damping of slide mechanism can be derived by measuring the frequency response from the collocated sensor to the non-collocated sensor. Moreover, the spring constant between the mechanisms also can be derived more accurately in comparison with the value based on the impulse response. Secondary the equivalent viscous damping is calculated by means of the constant velocity test or the frequency response. Then the viscous damping of rotational mechanism may easily be derived by subtracting the converted viscous damping, which is the value of slide mechanism at the position of motor axis, from the equivalent viscous damping. Finally, the proposed identification method is applied to an actual positioning mechanism.