Practical Design Considerations for Performance and Robustness in the Face of Uncertain Flexible Dynamics in Space Manipulators

Low frequency dynamics introduced by structural flexibility can result in considerable performance degradation and even instability in on-orbit, robotic manipulators. Although there is a wealth of literature that addresses this problem, the author has found that many advanced solutions are often precluded by practical considerations. On the other hand, classical, robust control methods are tractable for these systems if the design problem is properly constrained. This paper investigates a pragmatic engineering approach that evaluates the system’s stability margins in the face of uncertain, flexible perturbation dynamics with frequencies that lie close to or within the bandwidth of the nominal closed-loop system. The robustness of classical control strategies is studied in the context of both collocated (joint rate) and non-collocated (force/torque and vision-based) feedback. It is shown that robust stability and performance depend on the open-loop control bandwidth of the nominal control law (as designed for a simplified, rigid plant). Namely, the designed bandwidth must be constrained to be lower than the minimum flexible mode frequency of the unmodeled dynamics by a given factor. This strategy gives credence to popular heuristic methods commonly used to reduce the effect of unmodeled dynamics in complex manipulator systems.

Improved Adaptive Augmentation Control for a Flexible Launch Vehicle with Elastic Vibration

The continuous development of spacecraft with large flexible structures has resulted in an increase in the mass and aspect ratio of launch vehicles, while the wide application of lightweight materials in the aerospace field has increased the flexible modes of launch vehicles. In order to solve the problem of deviation from the nominal control or even destabilization of the system caused by uncertainties such as unknown or unmodelled dynamics, frequency perturbation of the flexible mode, changes in its own parameters, and external environmental disturbances during the flight of such large-scale flexible launch vehicles with simultaneous structural deformation, rigid-elastic coupling and multimodal vibrations, an improved adaptive augmentation control method based on model reference adaption, and spectral damping is proposed in this paper, including a basic PD controller, a reference model, and an adaptive gain adjustment based on spectral damping. The baseline PD controller was used for flight attitude control in the nominal state. In the non-nominal state, the spectral dampers in the adaptive gain adjustment law extracted and processed the high-frequency signal from the tracking error and control-command error between the reference model and the actual system to generate the adaptive gain. The adjustment gain was multiplied by the baseline controller gain to increase/decrease the overall gain of the system to improve the system’s performance and robust stability, so that the system had the ability to return to the nominal state when it was affected by various uncertainties and deviated from the nominal state, or even destabilized.

TIME DOMAIN STUDY ON THE CONSTRUCTION MECHANISM OF MILLING STABILITY LOBE DIAGRAMS WITH MULTIPLE MODES

The stability lobe diagram (SLD) is an important expression way of milling stability prediction result. The SLD obtained by only selecting the most flexible mode fails to predict the chatter if the milling process is dominated by multiple modes. To reveal the relationship between the SLD with multiple modes and the SLDs corresponding to each single mode, this paper studies the construction mechanism of the SLD with multiple modes by using a time domain method. First, the milling dynamic model of the tool with multiple modes is established. Then, the numerical method based on the Newton-Cotes rules is used to solve the milling dynamic model with multiple modes whose solution is in the form of the SLD. It shows that the SLD with multiple modes can be approximated by using the lowest envelope of the SLDs corresponding to each single mode. Finally, two study cases are adopted to verify the construction mechanism of the SLD with multiple modes. To verify the correctness of the SLD with multiple modes, a series of milling tests are carried out. The experimental results agree with the simulation results, which means the proposed time domain method can reveal the construction mechanism of the SLD with multiple modes.

Time Domain Study on the Construction Mechanism of Milling Stability Lobe Diagrams With Multiple Modes

The stability lobe diagram (SLD) is an important expression way of milling stability prediction result. The SLD obtained by only selecting the most flexible mode fails to predict the chatter if the milling process is dominated by multiple modes. To reveal the relationship between the SLD with multiple modes and the SLDs corresponding to each single mode, this paper studies the construction mechanism of the SLD with multiple modes by using the time domain method. First, the milling dynamic model of the tool with multiple modes is established. Then, the numerical method based on the Newton-Cotes rules is used to solve the milling dynamic model with multiple modes whose solution is in the form of the SLD. It shows that the SLD with multiple modes can be approximated by using the lowest envelope of the SLDs corresponding to each single mode. Finally, two study cases are adopted to verify the construction mechanism of the SLD with multiple modes. To verify the correctness of the SLD with multiple modes, a series of milling tests are carried out. The experimental results agree with the simulation results, which means the proposed time domain method can reveal the construction mechanism of the SLD with multiple modes.

A Novel Design of Water-Activated Variable Stiffness Endoscopic Manipulator with Safe Thermal Insulation

In natural orifice transluminal endoscopic surgery (NOTES), an ideal endoscope platform should be flexible and dexterous enough to go through the natural orifices to access the lesion site inside the human body, and meanwhile provide sufficient rigidity to serve as a base for the end-effectors to operate during the surgical tasks. However, the conventional endoscope has limited ability for maintaining high rigidity over the length of the body. This paper presents a novel design of a variable stiffness endoscopic manipulator. By using a new bioplastic named FORMcard, whose stiffness can be thermally adjusted, water at different temperatures is employed to switch the manipulator between rigid mode and flexible mode. A biocompatible microencapsulated phase change material (MEPCM) with latent heat storage properties is adopted as the thermal insulation for better safety. Experiments are conducted to test the concept design, and the validated advantages of our proposed variable stiffness endoscopic manipulator include: shorter mode activation time (25 s), significantly improved stiffness in rigid mode (547.9–926.3 N·cm2) and larger stiffness-adjusting ratio (23.9–25.1 times).

Vibration Control of Flexible Joint Robots Using a Discrete-Time Two-Stage Controller Based On Time-varying Input Shaping and Delay Compensation

Most industrial serial robots use decentralized joint controllers assuming rigid body dynamics. To prevent exciting the flexible mode, gains are kept low, resulting in poor control bandwidth and disturbance rejection. In this paper, a two-stage flexible joint discrete controller is presented, in which the decentralized approach is extended with a stiffness to take into account the dominant coupling mode. In the first-stage, an input shaping feedforward shapes the rigid closed-loop dynamics into desired dynamics that does not produce link vibrations. Robotic dynamic computation based on a recursive Newton-Euler Algorithm is conducted to update the feedforward link inertia parameter during robot motion. A second-stage is added to increase disturbance rejection. A generalized Smith predictor is developed to compensate for delay and feedback sensor filtering. An effective methodology is presented to optimize the control loop gains. Numerical simulations and experiments on a six-joint robot manipulator confirm that the proposed controller improves control performances in terms of bandwidth, vibration attenuation, and disturbance rejection.

Adaptive Polynomials for the Vibration Analysis of an L-Type Beam Structure with a Free End

Vibration analysis using the component mode method has been less popular than before, since computers are powerful enough to solve complicated structures by a single large finite model. However, many structural engineers designing local structures on a ship still need simple tools to check anticipated vibration problems during their design work. Since most of local structures on a ship are simple enough to consist of several substructures, the component mode method could be of use as long as good, natural mode functions can be provided so that reasonable natural frequencies can be yielded. In this study, since mode polynomials based on static deflection of cantilever beams fail to work to cover the various configurations of L-type beams with a free end, two alternatives are suggested. One is based on more flexible mode functions—we call them adaptive polynomials. The other is a purely mathematical approach, which makes realistic mode functions unnecessary. Suggested alternatives yield very good numerical results.