Open-loop Control Method Research Articles

Scanning optimization of an electrothermally-actuated MEMS mirror for applications in optical coherence tomography endoscopy

Probe is an important component of the optical coherence tomography (OCT) system. Electrothermally-actuated micro-electromechanical (MEMS) mirrors are especially suitable for OCT endoscopic imaging due to their large scan angle, low driving voltage and high fill factor. However, endoscopic OCT imaging quality would be seriously affected by the nonlinear scanning characteristics of electrothermal MEMS mirrors as well as the distortion due to the need of placing the MEMS mirror on a 45° slope. In this work, an open-loop control method based on the transfer function of the MEMS mirror is proposed to optimize the overall optical trajectory scanned on the sample under a MEMS OCT probe. The transfer function of the electrothermal MEMS mirror is established and the relation between the MEMS mirror’s tilt angle and the final scanning trajectory on the sample is calculated. A PID control model based on the transfer function is developed and employed to generate the driving voltage signals that can be applied to the MEMS mirror for obtaining undistorted scanning trajectories on the sample. Experiments show that the proposed method can correct the scanning nonlinearity and the distortion without adding extra hardware. For instance, the linear scanning ratio is improved from 72% up to 98%; the fan-shaped distortion is reduced by a factor 3–5. This method is also applied to a swept-source OCT system and the image distortions are compensated effectively.

Design and control of 2-DoF joystick using MR-fluid rotary actuator

The purpose of this paper is to design a control algorithm for a 2-DoF rotary joystick model. Firstly, the structure of the joystick, which composes of two magneto-rheological fluid actuators (shorten MRFA) with optimal configuration coupled perpendicularly by the gimbal mechanism to generate the friction torque for each independent rotary movement, is introduced. The control strategy of the designed joystick is then suggested. Really, because of two independent rotary movements, it is necessary to design two corresponding controllers. Due to hysteresis and nonlinear dynamic characteristics of the MRFA, controllers based an accurate dynamic model are difficult to realize. Hence, to release this issue, the proposed controller (named self-turning fuzzy controllers-STFC) will be built through the fuzzy logic algorithm in which the parameters of controllers are learned and trained online by Levenberg-Marquardt training algorithm. Finally, an experimental apparatus will be constructed to assess the effectiveness of the force feedback controls. Herein, three experimental cases are performed to compare the control performance of open-loop and close-loop control method, where the former is done through relationship between the force at the knob and the current supplied to coil while the latter is realized based on the proposed controller and PID controller. The experimental results provide strongly the ability of the proposed controller, meaning that the STFC is robust and tracks well the desirable force with high accuracy compared with both the PID controller and the open-loop control method.

Realization of desired damping characteristics based on an open-loop-controlled magnetorheological damper

The realization of the desired damping characteristics based on magnetorheological (MR) dampers is important for semi-active control and useful for the matching process of suspension damper. To reduce the cost of the control system and improve the output accuracy of the desired damping force, this study proposes an open-loop control method featuring an accurate inverse model of the MR damper and a tripolar current driver. The reversible sigmoid model is used to accurately and quickly calculate the desired current. Furthermore, the change characteristic of the desired current is analyzed qualitatively and quantitatively, which shows that the desired current needs to change suddenly to make the actual damping force velocity curve quickly approach the desired one. To meet the demand of the desired current, a tripolar current driver controlled by an improved PI control algorithm is proposed, which is with fast response and low noise. Finally, the bench test verifies that the control system can achieve different desired damping characteristics well, and the inherent error in this process is explained through the gap between the available damping force area and the desired damping characteristic curve and the crossover phenomenon of the dynamic characteristic curves of the MR damper.

Sensorless Combined Control of In-wheel BLAC Motors for Mobility Scooter

This paper presents a speed control for in-wheel brushless AC (BLAC) motors using a simple hall sensor to improve steering and driving performance of mobility scooters. To achieve smooth steering and motion controls, the sensor-sensorless combined estimation method is proposed. Furthermore, a modified current model reference adaptive system (MRAS) is also proposed to increase the dynamic response and estimation accuracy. The dead-time effect is directly compensated to improve the sensorless performance in the lower speed region. From standstill to the controllable minimum speed region, fast open-loop control method is applied to the traction motors. The vehicle velocity is then controlled using the speed information obtained from the proposed sensorless-sensor combined estimation scheme. To verify the effectiveness, a prototype mobility scooter was manufactured and the proposed control method was tested. Experiment results shows that the steering and motion of the vehicle are improved and the overall performance is increased compared to that of the conventional driving method.

A novel soft-start method for SVG based on D-axis orientation

In this paper, a novel method for shock-free soft-start is proposed. The novel method can realize the aim of no-overshoot and inrush of current during the entire soft-start process, thus the start process is safe, stable, and reliable. When the uncontrolled rectification stage ends, a sinusoidal signal synchronized with the grid voltage is generated through dq coordinate transformation and D-axis orientation, then the signal is transmitted to the three-phase inverter bridge. The AC voltage output of the inverter bridge is superimposed on both ends of the soft-start resistor along with the grid. The open-loop control method is adopted to gradually reduce the modulation ratio of the sinusoidal PWM wave until the capacitor voltage on the DC side reaches the rated voltage value, then the soft-start resistor is removed, which means the shock-free soft-start process is completed. Experimental results prove the effectiveness and feasibility of the control strategy proposed in this paper.

Analysis and control optimization of positive pressure fluctuation in electromechanical oxygen regulator

The Electromechanical Oxygen Regulator (EMOR) is a new type of aviator oxygen equipment. Positive pressure refers to the pressure difference between the breath pressure and the ambient pressure during pressurized oxygen supply. The phenomenon of positive pressure fluctuation was believed to reduce the system performance. The current open-loop control method cannot solve this problem. In this paper, the mathematical model was established and main factors were analyzed. By combining experimental research and simulation calculation, it was determined that pressure fluctuation was caused by inlet pressure and diaphragm deformation together. With the increase of pulmonary ventilation volume, the influence of inlet pressure on fluctuation decreases gradually, while the proportion of diaphragm deformation increases rapidly. A closed-loop control strategy of Proportional Resonant with Feedforward Compensation (PRFC) was proposed to solve the problem and control parameters were obtained through co-simulation. The effectiveness of the control strategy was verified by experiments. The results show that the control strategy can enhance the anti-disturbance ability of the system and significantly reduce the pressure fluctuation range, which is beneficial to improving the overall system performance.

Capacitor voltage ripple reduction in MMC-HVDC system using flat bottom current method

Modular-multilevel converters (MMCs) are considered a useful technology in high-voltage direct current (hvdc) transmission systems. Since it offers many advantages like modular configuration with scalable output voltage. Voltage ripples on sub-module (SM) capacitors in MMC and circulating currents are two major challenges, which can affect the converter stability and performance. Conventionally, direct modulation method is used to insert or bypass SMs in MMC producing circulating current and SMs capacitor voltage oscillations. The closed loop and open loop control methods are used to overcome drawbacks in direct modulation method. However, these methods rely on the accurate measurements and estimation of SM capacitor voltages. A flat bottom current method is proposed in this paper to minimize the amplitude of ripple voltages on SM capacitors in MMC-based HVDC systems. Limited capacitor energy changes leading to reduce capacitor voltage ripples are observed by the proposed methodology using direct-modulation method without using circulating current suppression controller. The proposed method is tested on a two-terminal MMC-based HVDC system implemented by vector control approach in PSCAD/EMTDC software. Analytical results are verified by simulation results in reducing capacitor voltage ripples based on the direct modulation approach.

High-bandwidth open-loop motion control of a piezo-positioning mechanism based on a composite electromechanical dynamic model

The piezo-positioning system with high dynamic performance is urgently needed in the field of precision manufacturing and precision instrumentation. The observed rate-dependent hysteresis presents more challenges and difficulties for accurately modeling the response behavior of the piezoelectric stack actuators over the last several decades. Our previous research shows that the rate-dependent characteristics may be caused by the dynamic characteristics of the power amplifier. In order to achieve the high-bandwidth and high-speed motion control accuracy, we propose an open-loop compensation control method of a piezo-positioning mechanism based on a composite electromechanical dynamic model. The dynamic model can describe the dynamic characteristics of the power amplifier, the inverse piezoelectric effect and the dynamics of the mechanism. Experimental results show that the proposed control method can achieve precise motion control of the piezo-positioning mechanism within 1–800 Hz bandwidth.

Two-degree-of-freedom flow-induced vibration suppression of a circular cylinder via externally forced rotational oscillations: comparison of active open-loop and closed-loop control systems

In the current work, both active open-loop and closed-loop control approaches are considered to reduce two-degree-of-freedom vortex-induced vibration (VIV) of a circular-section cylinder, by means of externally imposed rotational oscillations. The open-loop control method utilizes pre-specified rotary oscillations, while the active closed-loop controller computes the proper angular velocity depending on the feedback signal of lift coefficient. The interactions between fluid and structure are realized by utilizing of the moving mesh methodology by means of user-defined function, which incorporated into the central code of computational fluid dynamic solver. According to the numerous set of numerical simulations, three optimum open-loop control systems for appropriate suppression of cylinder vibration are selected. In particular, it is observed that the VIV suppression capability of the closed-loop control strategy in comparison with optimum open-loop systems is more desirable despite the lower level of consuming control effort. Moreover, it is detected the excellent performance of selected closed-loop controller with constant proportional control gain in the entire range of reduced velocities, while the optimum open-loop systems are executable mainly near their corresponding reduced velocities. In particular, the selected closed-loop VIV control system (Kp = 10) is found to suppress the maximum transverse response amplitudes by 99.6%, 99.5%, and 99.3% for reduced velocities Vr = 5, 6, and 7, respectively, while the associated in-line displacement amplitudes decrease by 99.2%, 98.3%, and 96.2%. Also, the closed-loop controller does not require to find the “lock-on” frequency to execute appropriate de-synchronization type action and can effectively shift the C(2S)-mode vortices to the typical von Karman 2S-mode vortex shedding.

Open-loop control of directed self-assembly of colloidal particles in a microfluidic device

Directed self-assembly provides a practical route for the manufacture of structured materials. Future processes based on self-assembly will need to be equipped with suitable automated controllers for improved performance. Open-loop control is needed when real-time observation of a self-assembly system is difficult. An automated open-loop control method for directed self-assembly of microspheres in a microfluidic device with multiple electrodes is developed and characterised experimentally. The control objective is to create an elementary particle density distribution between two regions in the device. An empirical model is developed to describe the motion of the particles such that open-loop control becomes feasible. Experimental characterization shows the effectiveness of the approach. Finally, it is demonstrated how a first-principle model describing electroosmotic flow can also be used to predict the particle motion, which provides opportunities to develop open-loop control schemes for directed self-assembly systems for which no real-time observation is available in future work.

Open-loop optimal control of a flapping wing using an adjoint Lattice Boltzmann method

We present the usage of an adjoint Lattice Boltzmann Method (LBM) for open-loop control of two-dimensional flapping wing motion. Movement of the wing is parametrised with periodic B-Splines, while the wing interacts with the surrounding flow via an Immersed Boundary (IB) method. Multi-objective optimisation is performed using a gradient optimisation algorithm, for which sensitivities are calculated with an adjoint method. The objectives selected were the mean lift force and mechanical power. To achieve performance suitable for optimisation, we also present an efficient GPU implementation of the LBM and adjoint LBM. The Immersed Boundary approach employed for the LBM is verified against results from the literature, while for the flapping case it is compared with two different Finite Volume Method (FVM) approaches. The obtained Pareto front of optimal designs shows a clear discrepancy between the power consumption and the mean lift force. A significant improvement of the basic wing design is demonstrated, and highlights the applicability of adjoint LBM simulations in complex open-loop control problems.

Flexible Bio-tensegrity Manipulator with Multi-degree of Freedom and Variable Structure

Conventional manipulators with rigid structures and stiffness actuators have poor flexibility, limited obstacle avoidance capability, and constrained workspace. Some developed flexible or soft manipulators in recent years have the characteristics of infinite degrees of freedom, high flexibility, environmental adaptability, and extended manipulation capability. However, these existing manipulators still cannot achieve the shrinking motion and independent control of specified segments like the animals, which hinders their applications. In this paper, a flexible bio-tensegrity manipulator, inspired by the longitudinal and transversal muscles of octopus tentacles, was proposed to mimic the shrinking behavior and achieve the variable motion patterns of each segment. Such proposed manipulator uses the elastic spring as the backbone, which is driven by four cables and has one variable structure mechanism in each segment to achieve the independent control of each segment. The variable structure mechanism innovatively contains seven lock-release states to independently control the bending and shrinking motion of each segment. After the kinematic modeling and analysis, one prototype of such bionic flexible manipulator was built and the open-loop control method was proposed. Some proof-of-concept experiments, including the shrinking motion, bending motion, and variable structure motion, were carried out by controlling the length of four cables and changing the lock-release states of the variable structure mechanism, which validate the feasibility and validity of our proposed prototype. Meanwhile, the experimental results show the flexible manipulator can accomplish the bending and shrinking motion with the relative error less than 6.8% through the simple independent control of each segment using the variable structure mechanism. This proposed manipulator has the features of controllable degree-of-freedom in each segment, which extend their environmental adaptability, and manipulation capability.

Mechanism analysis and control design of selective excitation of adjacent energy levels

Femtosecond coherent anti-Stokes Raman scattering (CARS) generated by femtosecond laser with a large spectral range will inevitably face the problem that several adjacent vibration levels are excited simultaneously. In this paper, the internal control mechanism of open-loop coherent control methods is analyzed and summarized, and a design method forthe optimal control parameters to realize the selective excitation of adjacent energy levels is proposed. Finally, numerical simulations for selective excitation of adjacent energy levels are done to verify correctness and effectiveness of the method proposed, showing that the method proposed in this paper can optimize the ratio between the resonant peaks of two resonant energy levels to about 150–250.

Stability analysis and motion control of spinning electrodynamic tether system during transition into spin

Spinning electrodynamic tether systems are considered one of the most promising potential applications of tethered satellite systems, which require little fuel and avoid the equilibrium limit of conventional vertical electrodynamic tether systems. Therefore, spinning electrodynamic tether systems have good prospects in debris removal, orbit reboost, payload transportation, and so on. It has been found that, if not carefully controlled, tethers easily become slack or sagging during the transition process from equilibrium state into spin. In this regard, this paper mainly focuses on the transition process of spinning electrodynamic tether systems from the initial equilibrium state into the final spinning state with expected angular velocities. Conditions of dynamic equilibrium position and minimum current for acceleration (critical current) are firstly derived in this paper, which provide references for future space tether experiments. The motion of spinning electrodynamic tether systems is described by the Lagrangian model, which takes orbital motion into consideration. By considering power limits of electrodynamic tether systems, this paper proposes two open-loop control methods for the safe transition into spinning state as nominal control algorithms with different mission objectives. The first method (direct transition) provides a near time-minimum solution, and the second method (swinging transition) provides a minimum current-energy solution for acceleration into spin. Considering perturbations in space, including inhomogeneous distribution of magnetic induction, varying mass distribution of the system and so on, an adaptive sliding mode controller is proposed to regulate the system and to track nominal trajectories of acceleration. The effectiveness of the proposed control methods is validated by numerical results.

Kinematics-based end-effector path control of a mobile manipulator system on an uneven terrain using a two-stage Support Vector Machine

SUMMARYA mobile manipulator system (MMS) consists of a robotic arm mounted on a mobile platform that is used in rescue and relief, space exploration, warehouse automation, etc. As the total system has 14 Degrees of Freedom (DOF), it does not have a closed-form inverse kinematics (IK) solution. A learning-based method is proposed, which uses the forward kinematics data to learn the IK relation for motion of an MMS on a rough terrain, using a one-class support vector machine (SVM) framework. Once trained, the model estimates the joint probability distribution of the MMS configuration and end-effector position. This distribution is used to find the MMS configuration for a given desired end-effector path. Past research using a Kohonen Self organizing map (KSOM) neural network-based open-loop control method has shown that the MMS deviates from its desired path while moving on an uneven terrain due to unknown disturbances such as wheel slip, slide, and terrain deformation. Therefore, a new sequential two-stage SVM-based end-effector path-tracking control scheme is proposed to control the end-effector path. In this scheme, the error in the end-effector path is continuously tracked with the help of a Microsoft Kinect 2.0 (Microsoft Regional Sales, Singapore 119968) and is sent as a feedback to the controller. Once the error reaches a threshold value, the error correction step of the controller gets activated to correct the error until the desired accuracy is reached. The effectiveness of the proposed approach is proved through extensive simulations and experiments conducted on 3D terrain in which it is shown that the end effector can follow the desired path with an average experimental error of around 2 cm between the desired and final corrected path.

Payload twisting dynamics and oscillation suppression of tower cranes during slewing motions

Challenging and dangerous material-handling applications in construction have motivated the study of the dynamics and control of tower cranes. Manipulating tower cranes is difficult while moving large-size payloads because of the inevitable payload swing and twisting about the cables. Although significant work has been directed at reducing the swing of point-mass loads, much less effort has been directed at limiting the payload twisting. A nonlinear dynamic model of tower cranes carrying distributed-mass beams is described. Furthermore, an open-loop control method is proposed to reduce both the swing and twisting of the payloads during slewing motions. Simulations and experiments demonstrate the theoretical dynamic behavior of the model and validate the effectiveness of the proposed control method.

Multiflexoelectric Actuation and Control of Beams

Flexoelectric materials subject to inhomogeneous electric field can actuate and control flexible structures based on the converse flexoelectric effect. An atomic force microscope (AFM) probe placed on top of a flexoelectric patch can generate a nonuniform electric field with an excitation control voltage, and consequently, an internal actuation stress is generated in the flexoelectric material. The induced flexoelectric membrane force and bending moment, in turn, influence the vibration of the structure. Because of high stress concentration of the flexoelectric actuation, multiple AFM probes/patches are necessary to alleviate potential fracture. In this study, the vibration control effectiveness of an elastic cantilever beam with multiple AFM probes/patches placed at optimal locations is evaluated. The beam is excited initially with an external mechanical force. In case studies, both open-loop and closed-loop control methods are imposed, and optimal combinations of three actuator positions are chosen to minimize the tip displacement. For the open-loop responses, amplified tip displacements are also observed due to overactuation. Displacement feedback control is proposed. Free vibration, and open-loop and closed loop vibration control are compared for the first three beam modes. The results prove that multiple flexoelectric actuators can effectively accelerate the beam damp process with closed-loop control.

Fractional-order modeling and control of ionic polymer-metal composite actuator

Fractional-order (FO) calculus-based modeling and control of ionic polymer-metal composite (IPMC) actuators is studied in this paper. Since IPMC actuators exhibit FO dynamics, a suitable compensator must be designed for control thereof. In this work, we employ FO proportional-integral-derivative (FOPID) controllers, as well as FO inverse model (FOINVM) based control. Only open-loop control methods are considered. Since process model-based control is used, the IPMC actuator is modeled using a FO transfer function, whereby a process identification is performed. Additionally, the designed controller is implemented in hardware that allows the whole control system to be easily miniaturized and considered in embedded applications. All of the experiments presented in this paper are carried out with the IPMC actuator placed in a controlled inert environment. The results of the real-time control experiments are presented and discussed and indicate the validity of the considered modeling and control approach.

Optimization-based real-time open-loop control of an optofluidic refractive phase modulator.

We present a novel open-loop control method for an electrostatically actuated optofluidic refractive phase modulator, and demonstrate its performance for high-order aberration correction. Contrary to conventional electrostatic deformable mirrors, an optofluidic modulator is capable of bidirectional (push-pull) actuation through hydro-mechanical coupling. Control methods based on matrix pseudo-inversion, the common approach used for deformable mirrors, thus perform sub-optimally for such a device. Instead, we formulate the task of finding driving voltages for a given desired wavefront shape as an optimization problem with inequality constraints that can be solved using an interior-point method in real time. We show that this optimization problem is a convex one and that its solution represents a global minimum in residual wavefront error. We use the new method to control both the refractive phase modulator and a conventional electrostatic deformable mirror, and experimentally demonstrate improved correction fidelity for both.

Towards Additive Manufacturing: Pumping Flow Rate with Time-Dependent Material Rheology in 3D Cementitious Material Printing

3D printing has been getting increasing attention from both industry and academy in recent years. Unlike traditional construction processes, 3D cementitious material printing requires more accurate control on the pumping flow rate, which is known to be affected by the material rheology. In this paper, the effect of fresh rheology on the flow rate of cementitious material was studied experimentally in 3D cementitious material printing process. The material viscosity was measured via a large gap vane viscometer at different time points after mixing, and printing tests were conducted at the same time period to measure the flow rate based on volume conservation principle. Experiments showed that the flow rate was significantly affected by rheology change with respect to time. An open loop control method was then implemented to harmonize the flow volume per unit length during printing processes to improve printing quality.