C Chenyang Ding

Modeling and control of a 6-DOF contactless electromagnetic suspension system with passive gravity compensation

A contactless Electro-Magnetic Isolator (EMI) is designed for gravity compensation of a heavy payload by passive Permanent Magnetic (PM) force and control by active Lorentz force. The theoretically calculated PM force and torque produced by this EMI are presented. To characterize the EMI and to evaluate the vibration isolation performance, the Single EMI System (SEMIS) is designed by adding three horizontal and three vertical Lorentz actuators for control. It is a six Degrees-of-Freedom (DOF) contactless electromagnetic suspension system possessing the properties of inherent instability, nonlinearity, no mechanical contact, passive gravity compensation. The physical SEMIS model is developed based on reasonable assumptions and approximations. It is subsequently linearized for control design. The vibration isolation performance of the two control strategies, decoupled control and decentralized control, are simulated and compared. The results show that decentralized control has more significant coupling than the decoupled control for only two DOF-pairs.

Stabilization and vibration isolation of a contactless Electromagnetic Isolator: A Frequency-Shaped Sliding Surface Control approach

A Frequency-Shaped Sliding Surface Control (FSSSC) approach is applied to an unstable model of a candidate Electro-Magnetic Isolator (EMI) design which has three Degrees Of Freedom (DOF). The EMI is designed to achieve contactless passive gravity compensation for heavy load by permanent magnets. The 3-DOF model can be regarded as three exactly the same double-integrators disturbed by the nonlinear and coupled passive force which results in its inherent instability. The sliding surface is designed based on relative displacement and payload acceleration feedback to achieve low-frequency vibration isolation. To avoid the algebraic control loop, a linear converging controller is designed instead of the conventional switching control. Regardless of the plant uncertainties, the closed-loop transmissibility converges to the designed transmissibility with increasing open-loop gain. A sufficient condition for the closed-loop stability is developed. Both time domain and frequency domain performance of the designed controller is evaluated by simulation. It shows that robust vibration isolation performance is achieved despite of the nonlinear and coupled passive force.

Robust vibration isolation by Frequency-Shaped Sliding Surface Control with geophone dynamics

The Frequency-Shaped Sliding Surface Control (FSSSC) has been recently applied to the Active Vibration Isolation System (AVIS) and the robust skyhook performance is experimentally validated. However, the performance of this approach is theoretically limited by the sensor dynamics. This paper generalizes the FSSSC approach as a two-step AVIS control design method. The first step is to design the sliding surface which determines the designed performances. The second step is to design the regulator which guarantees the convergence of the system dynamics. As long as this convergence is guaranteed, the designed performances would be realized. The vibration isolation of the original plant is therefore transformed to the regulation of a new system which is composed of the original plant and the sliding surface. As the regulator design has been well studied in the literature, this paper focuses on the sliding surface design. An example sliding surface design to achieve low-frequency vibration isolation is provided. The FSSSC of an example 1-DOF plant using both original and the improved sliding surface are compared. Theoretical calculations show that the improved sliding surface has no theoretical performance limit and achieves robust vibration isolation at much lower frequencies than the original design.

Modeling and realization of a 6-DoF contactless electromagnetic anti-vibration system and verification of its static behavior

This paper concerns measurements on an electromagnetic six Degrees-of-Freedom (DoF) anti-vibration system that has been realized recently. The heart of this system is a fully passive permanent-magnet device which acts as a contactless magnetic spring. As such, the gravity force of a floating rigid metrology frame (730 kg) is compensated by the passive interaction between the permanent magnets in this device. The low position dependency of this force, or stiffness, is an important system property for floor vibration isolation. This 6-DoF system is stabilized by closed-loop controlled Lorentz actuators based on position feedback. The static force and torque of this system have been obtained experimentally to validate the modeling and design of the device. The results indicate a temperature sensitivity of 1.70/00/K which corresponds to -12.1 N/K compared to the vertical force of 7.2 kN. The passive force and torque produced by the gravity compensator have linear relation with translational and rotational displacements. The predicted low stiffness property of this system is validated by the stiffness matrix derived from static measurements. The total power consumption is position dependent and remains within a range of 0.3~6 W.

Robust Vibration Isolation of a 6-DOF system using modal decomposition and sliding surface optimization

For a high-performance 6-DOF Active Vibration Isolation System (AVIS), the vibration isolation performance (transmissibility) is the most important criterion and the disturbance rejection performance (compliance) has lower priority. The strategy of combining modal decomposition and frequency-shaped sliding surface control is applied based on the measurement scheme of relative displacement and payload absolute acceleration. Modal decomposition decouples the six modes and calculates the equivalent sensor noises for each mode. The designed performances, transmissibility and sensitivities to the two sensor noises, depend solely on the sliding surface design. The sliding surface is optimized for each mode with predefined constraints which are derived from common industrial requirements. The regulator is designed to realize the designed transmissibility for each mode and to achieve low compliance. The numerical example of the sliding surface optimization gives better result than the manual design. This strategy designs the four performances step by step and iterative design is not necessary.

Optimal static decoupling for the decentralized control : an experimental study

Abstract A three Degrees-Of-Freedom (DOF) vibration isolation system with a certain degree of symmetry shows significant cross coupling. The decentralized control is intended and it demands excellent decoupling performance. The Vaes optimization algorithm is applied to find the optimal static decoupling matrices. However, experiments show that the measured μ interaction measure is not consistent with calculation results and thus not optimized. An iterative procedure based on the Vaes algorithm is proposed to obtain the optimal static decoupling matrices. The Vaes algorithm is also modified to improve the numerical stability and to speed up the optimization process. The proposed iterative procedure and the modified Vaes optimization algorithm are validated on the 3-DOF vibration isolation system.

Vibration isolation of a feedback linearized model for a contactless Electromagnetic Isolator by Virtually Varying Mass control

Two active vibration isolation methods, Virtually Increasing Mass (VIM) control and On-Off Mass (OOM) control, based on absolute acceleration feedback are proposed for a contactless Electro-Magnetic Isolator (EMI) being designed for heavy payload. They are applied to a stabilized feedback-linearized model for the vertical DOF of a candidate EMI design. The control objectives of this application are seeking, vibration isolation and force disturbance rejection. The vibration isolation performance (transmissibility) up to 1000 [Hz] is evaluated by simulation with both white noise vibrations and sweep sine vibrations. Simulation shows that the transmissibility magnitude peak is shifted to lower frequency by the proposed vibration isolation methods. The high-frequency nonlinear behaviors are analyzed. Besides, the step response to a constant force disturbance is not compromised and the seeking performance is acceptable. Above all the simulation results, the proposed control methods are feasible for the presented application.

Robust vibration isolation by frequency-shaped sliding surface control with floor velocity/acceleration measurement

This paper studies control of 1-DOF Active Vibration Isolation System (AVIS) using Frequency-Shaped Sliding Surface Control (FSSSC) approach based on the measurement schemes of the relative displacement and the floor absolute velocity/acceleration. Seismic velocity/acceleration sensors usually have large dimensions and mass which sometimes cause difficulties to fix them to the payload. The FSSSC approach has been recently applied to AVIS control and generalized as a two-step AVIS control design method. It is simple and has validated robust performance. Assuming linear regulation, both sliding surface design and the robustness of the realized performance are studied. The realized transmissibility is limited by the displacement sensor noise at high frequencies. The FSSSC designs of an example 1-DOF plant with the measurement schemes of the relative displacement and the floor absolute velocity/acceleration are provided. Theoretical calculations show that the robust low-frequency vibration isolation can be achieved but the performance robustness is limited at high frequencies due to the displacement sensor performance.

Closed-loop identification of an unstable electromagnetic isolator under floor vibration

This paper studies closed-loop identification of the Permanent Magnetic (PM) force produced by an unstable contactless Electro-Magnetic Isolator (EMI) designed for passive gravity compensation of heavy payload. According to the criterions of high force density and minimized stiffness, the EMI magnetic topology parameters can be optimized based on theoretically calculated PM force. To validate this design method, the PM force has to be measured accurately. One solution is closed-loop identification of the unstable suspension system formed by a rigid payload suspended by a single EMI. The two objects that can be directly identified are the passive force (the sum of the PM force and gravity force) and the payload mass. The PM force can be calculated by the passive force minus the gravity force. To guarantee the closed-loop stability and floor vibration suppression with unknown plant parameters, the generalized frequency-shaped sliding surface control approach can be applied. An iterative method is proposed to directly estimate the payload mass. To evaluate the proposed methods, the identification of the passive force and the payload mass is simulated using a 1-DOF model taken from a candidate design of the EMI. The results show that the estimation errors for both objects are reasonably small. The mass identification has improved performance by iterative estimations.