Position Control Loop Research Articles

On-line contour error estimation and control for corner smoothed five-axis tool paths

The dimension accuracy of the part is directly affected by the contour error. The improvement of contour accuracy has been one of the research hotspots in five-axis Computer Numerical Control (CNC) machining. Because of the inexplicit mathematical expressions of the point-to-curve distance function for free-form parameterized splines, most existing contour error estimation algorithms of the multi-axis CNC machining are based on linear or circular approximation, while the geometric characteristics of the tool path are not taken into account. However, the corner smoothed five-axis tool paths are composed of straight lines and micro splines with few control points, which have clear and simple mathematical expressions and make it possible to directly calculate the contour error from the actual tool pose to the reference tool path. Considering the geometric characteristics of the corner smoothed five-axis tool paths, this paper proposes an on-line and high-accuracy contour error estimation algorithm for five-axis CNC machining. The estimated contour error components of each axis are compensated to the position-loop controller of each axis to realize contour error control. Simulation and experimental results show that the proposed method has higher precision than the state of art methods, and contour errors are effectively suppressed by on-line compensation.

Disturbance Observer-Based Modeling and Parameter Identification for Synchronous Dual-Drive Ball Screw Gantry Stage

A novel algorithm is proposed to identify the parameters of a synchronous dual-drive ball screw gantry system in this paper. The modeling of the gantry system is described in this paper, and dynamic equations are used to derive the relationship from the motor to the coupling mechanism. The modeling results are used to design the algorithm used to identify the model parameters. The proposed algorithm uses two disturbance observers (DOB) to identify the parameters for the coupling mechanism and sliding stages. The sinusoidal position and velocity commands for the proposed identification algorithm are relatively smooth to avoid damaging the mechanism. The proposed method is easy to implement in the position control loop and velocity control loop. Finally, simulation and experimental results show that the proposed DOBs can accurately estimate the parameters for a gantry system, and the estimated results can be further used to design a corresponding controller for the gantry stage.

Attitude Control in Ascent Phase of Missile Considering Actuator Non-Linearity and Wind Disturbance

During the ascent phase of a missile, a challenging problem occurs that blocks the construction of a high-precision attitude control scheme, which directly affects accurate modeling including disturbances: non-linearities of an actuator, rapidly time-varying parameters, un-modeled dynamics, etc. In order to improve the control performance, an active disturbance rejection control (ADRC) scheme, considering non-linear dynamics of the actuator and wind disturbance during the ascent phase, is proposed in this paper. An expand state observer (ESO) is planned to estimate and compensate the actuator’s non-linear dynamics, flight model uncertainties, and wind disturbance. Therefore, the complex non-linear time-varying control problem is simplified into a linear time-invariant control problem. The pitch attitude control system is controlled by the cascade method and ADRC controllers are designed for actuator close loop and attitude control loop, respectively. The simulation results show that ADRC has strong robustness under different dead-zones and external disturbances of the actuator. On the other hand, ADRC can effectively suppress the external atmospheric disturbance. Compared with the traditional gain-scheduling control scheme, the ADRC scheme can significantly reduce the overloading of the system and shows remarkable performance for tracking as well as wind resistance.

Standoff Tracking of a Moving Target for Quadrotor Using Lyapunov Potential Function

This paper presents a control scheme for standoff tracking of a ground moving target by a quadrotor unmanned aerial vehicle (UAV). The control system is decoupled into outer loop for position control and inner loop for attitude regulation. In the outer loop design, the standoff motion of the vehicle is described in a cylindrical coordinate system attached to the target. After that, the standoff tracking guidance law is designed based on a Lyapunov potential function which can guarantee the stability of the movement. The acceleration signals are produced from the proposed guidance law, and converted to Euler angle commands for the inner control system. A integral backstepping controller is developed to stabilize the attitude of the quadrotor. In particular, the disturbance observer technique is used to deal with the correction terms that account for a non-uniform moving target and constant wind. Numerical simulations are performed to verify the feasibility and performance of the proposed control scheme.

NEMS Electrostatic RF Wakeup Switch with Pt FIB Contact

Recent pushes to reduce wireless sensor node power consumption have targeted sensors detecting the signal of interest, e.g. acoustic, acceleration, rotation, and use the energy in the signal to trigger switches. Regardless of the sensing modality however, all wireless sensors include some form of data communication radio, which in many cases is constantly consuming power. We take inspiration from a very low sub-threshold current and low switching-energy NEMS switch to implement a near zero-power laterally actuated NEMS electrostatic RF wake-up switch to reduce passive power draw by the RF electronics [1]. The switch is operated by holding a mechanical contact gap just outside of thermal fluctuations and an RF signal acts to physically close the switch. Focused ion beam is utilized to achieve sub-100 nm platinum contact gaps, allowing for below 3V operation and improved reliability. Preliminary results are presented from a custom vacuum probe station utilizing a switch position control loop for the contact-based switch operating off resonance.

Trajectory Tracking of Flexible-Joint Robots Actuated by PMSM via a Novel Smooth Switching Control Strategy

This paper presents the trajectory tracking control of a flexible-joint manipulator driven by permanent magnet synchronous motor (PMSM). Combining the PMSM electrical equation and mechanical equation of robotic manipulators, a novel smooth switching control scheme is proposed. Firstly, the position loop controller of the system is designed, with an improved hierarchical sliding mode control (IHSMC) algorithm proposed to further the response speed of the system, additionally, a robust interconnection and damping assignment passivity-based controller (IDA-PBC) is designed to improve the steady state performance of the system. Then, the IDA-PBC control strategy is leveraged to design the current loop controller of the system, on which basis a hybrid controller with smooth switching is designed. Furthermore, Gaussian function is applied as the smooth switching function of the hybrid controller to promote the switching performance. As a result, the hybrid controller has both good dynamic and steady performance. The simulation results verify the effectiveness of the algorithms.

Math simulation of attitude stabilization accuracy and performance for an earth remote sensing spacecraft

The paper discusses a math simulation of an Earth remote sensing spacecraft attitude control loop dynamics during imaging. The spacecraft is intended for imaging a swath along the flight path, stereo imaging, area and corridor imaging at an angle to the flight path. To provide the spacecraft with high maneuverability during imaging it is assumed that the control loop uses as its effectors one-axis powered gyros. Studies were conducted of the feasibility of using this spacecraft for performing a series of area and corridor surveys. Selected for the attitude control loop were the control parameters which allow meeting the specified performance. Simulation results have confirmed that the spacecraft is capable of performing these kinds of surveys and meeting the specified accuracy levels. Key words: Earth remote sensing spacecraft, control moment gyros, single-axis powered gyros, kinetic momentum, stabilization, angular rate, precession.

On Stability of Virtual Torsion Sensor for Control of Flexible Robotic Joints with Hysteresis

SUMMARYThe aim of the virtual torsion sensor (VTS) is to observe the nonlinear deflection in the flexible joints of robotic manipulators and, by its use, improve positioning control of the joint load. This model-based approach utilizes the motor-side sensing only and, therefore, replaces the load-side encoders at nearly zero hardware costs. For being applied in the closed control loop, the stability and robustness of VTS are most crucial. This work extends the previous analysis by a general case of nonlinear joint stiffness with hysteresis and provides straightforward conditions with respect to the system dynamics. The dissipativity and passivity of the torsion-torque hysteresis map are analyzed and discussed in detail. The absolute stability of VTS inclusion into position control loop is shown based on the equivalent loop transformations and Popov criteria, including the sector conditions. Illustrative numerical examples of the control error dynamics and its convergence are provided.

Optimal Design and Force Control of a Nine-Cable-Driven Parallel Mechanism for Lunar Takeoff Simulation

Traditional simulation methods are unable to meet the requirements of lunar takeoff simulations, such as high force output precision, low cost, and repeated use. Considering that cable-driven parallel mechanisms have the advantages of high payload to weight ratio, potentially large workspace, and high-speed motion, these mechanisms have the potential to be used for lunar takeoff simulations. Thus, this paper presents a parallel mechanism driven by nine cables. The purpose of this study is to optimize the dimensions of the cable-driven parallel mechanism to meet dynamic workspace requirements under cable tension constraints. The dynamic workspace requirements are derived from the kinematical function requests of the lunar takeoff simulation equipment. Experimental design and response surface methods are adopted for building the surrogate mathematical model linking the optimal variables and the optimization indices. A set of dimensional parameters are determined by analyzing the surrogate mathematical model. The volume of the dynamic workspace increased by 46% after optimization. Besides, a force control method is proposed for calculating output vector and sinusoidal forces. A force control loop is introduced into the traditional position control loop to adjust the cable force precisely, while controlling the cable length. The effectiveness of the proposed control method is verified through experiments. A 5% vector output accuracy and 12 Hz undulation force output can be realized. This paper proposes a cable-driven parallel mechanism which can be used for lunar takeoff simulation.

A new hybrid control methodology for a morphing aircraft wing-tip actuation mechanism

ABSTRACTThe focus of this paper is on the modelling of miniature electromechanical actuators used in a morphing wing application, on the development of a control concept for these actuators, and on the experimental validation of the designed control system integrated in the morphing wing-tip model for a real aircraft. The assembled actuator includes as its main component a brushless direct current motor coupled to a trapezoidal screw by using a gearing system. A Linear Variable Differential Transformer (LVDT) is attached on each actuator giving back the actuator position in millimetres for the control system, while an encoder placed inside the motor provides the position of the motor shaft. Two actuation lines, each with two actuators, are integrated inside the wing model to change its shape. For the experimental model, a full-scaled portion of an aircraft wing tip is used with the chord length of 1.5 meters and equipped on the upper surface with a flexible skin made of composite fibre materials. A controllable voltage provided by a power amplifier is used to drive the actuator system. In this way, three control loops are designed and implemented, one to control the torque and the other two to control the position in a parallel architecture. The parallel position control loops use feedback signals from different sources. For the first position control loop, the feedback signal is provided by the integrated encoder, while for the second one, the feedback signal comes from the LVDT. For the experimental model, the parameters for the torque control, but also for the position control-based encoder signal, are implemented in the power amplifier energising the electrical motor. On the other hand, a National Instruments real-time system is used to implement and test the position control-based LVDT signal. The experimental validation of the developed control system is realised in two independent steps: bench testing with no airflow and wind-tunnel testing. The pressure data provided by a number of Kulite sensors equipping the flexible skin upper surface and the infrared thermography camera visualisations are used to estimate the laminar-to-turbulent transition point position.

Neural Network-based Robust Adaptive Certainty Equivalent Controller for Quadrotor UAV with Unknown Disturbances

In this paper, a robust adaptive neural network certainty equivalent controller for a quadrotor unmanned aerial vehicle is proposed, which is applied in the outer loop for position control to directly generate the desired roll and pitch angles commands and then to the inner loop for attitude control. The newly proposed controller takes into account the vehicle’s kinematic and modelling error uncertainties which are associated with external disturbances, inertia, mass, and nonlinear aerodynamic forces and moments. The control method integrates an adaptive radial basis function neural networks to approximate the unknown nonlinear dynamics with certainty equivalent control technique, in this way leading to the fact that precise dynamic model and prior information of disturbances are not needed. The adaptation law was derived by using a Lyapunov theory to verify the stability and superiority of the new algorithms. The performance and effectiveness are also verified by carrying out several simulations. It was shown from the analysis that the altitude, position, and attitude tracking errors are converged to zero and the closed loop stability is guaranteed under extreme conditions.

Position Based Impedance Control Strategy for a Lower Limb Rehabilitation Robot.

Active rehabilitation training can improve patients' neural engagement and the rehabilitation effects. A position based impedance control strategy is proposed for implement of active training in this paper. Firstly, it is necessary to accurately estimate patients' active torques during the training process, which can be calculated by using the human-robot dynamic model. A novel friction model is designed, where the influence of the joint coupling factor and viscosity is considered to improve the model accuracy. In order to prevent sudden changes of the interaction force, a virtual tunnel around the reference trajectory is established to limit the motion range to ensure patient safety in case of emergency, such as muscle spasm. Then, a position based impedance control strategy designed in the joint space is used to convert human torques into the required movement. The strategy is implemented by a double closed loop control structure, the outer loop of which converts the interaction force into angular, angular velocity, and angular acceleration deviations by the inverse impedance equation. The regulated trajectory will be used as reference for the inner position control loop. The experimental results show that the proposed system dynamic model can be used to accurately estimate the patients' active torques, and the proposed control strategy can be used to design a compliant human-robot interaction interface, so that patients can complete the training task comfortably and naturally, and also safety of the training can be ensured.

Improved study on position measurement system for linear motor applied in electromagnetic launch system

The linear motor applied in electromagnetic emission system uses a closed loop position control strategy, which needs a set of position measurement system with high reliability, high resolution and integration to achieve real-time acquisition and analysis of position signals. The existing position controller is based on the simple logic chip design without memory function, and does not have the storage analysis and preprocessing function to position signals. Therefore, the system has insufficient scalability, low integration and reliability. Aiming at the improvement of the existing position measurement system, an intelligent position measurement system integrating the functions of position signals acquisition, processing and uploading, data storage and analysis is proposed in this paper, and its working principle and system composition are discussed in detail. The position, speed and acceleration obtained on the electromagnetic emission platform are in good agreement with the expected value of the system. As results, the feasibility and accuracy of the improved integrated intelligent position measurement system are verified, and the control performance of the system is also satisfied well, which can be good guidance and reference for subsequent engineering practice.

Perancangan dan Simulasi Sistem Kendali Kolektor Surya pada Sistem Pembangkit Daya Tenaga Matahari

This paper discussed about simulation of closed loop position control system of DC servo motor in solar collector thermal power plant application. When the input position that simulated with step signal given to motor, the positional information from an output potentiometer which is mechanically coupled to a motor is feedback to control amplifier. The references position input from input step is combined with the feedback signal at the input of the amplifier which drivers the motor in proportion to the difference between two signal. In this case, the position of DC servo motor controlled with PID controller. Simulation was created by Simulink toolbox of Matlab 6.50. we found constant PID control are Kp = 1, Ki = 0,4, and Kd = 0,5.

Robust Attitude Control for Quadrotors Based on Parameter Optimization of a Nonlinear Disturbance Observer

This paper presents a nonlinear disturbance observer (NDOB) for active disturbance rejection in the attitude control loop for quadrotors. An optimization framework is developed for tuning the parameter in the NDOB structure, which includes the infinity-norm minimization of the weighted sum of noise-to-output transfer function and load disturbance sensitivity function. Subsequently, the minimization generates an optimal value of the parameter based on the tradeoff between disturbance rejection and noise propagation in the system. The proposed structure is implemented on PIXHAWK, a real-time embedded flight control unit. Simulation tests are carried out on a custom built, high-fidelity simulator providing physically accurate simulations. Furthermore, experimental flight tests are conducted to demonstrate the performance of the proposed approach. The system is injected with step, sinusoidal, and square wave disturbances, and the corresponding system tracking performance is recorded. Experimental results show that the proposed algorithm attenuates the disturbances better compared to just a baseline controller implementation. The proposed algorithm is computationally cheap, an active disturbance rejection technique and robust to exogenous disturbances.

Position‐heading quadrotor control using LPV techniques

This work presents a solution to the position-heading control problem of a small quadrotor. The main idea of this study is to exploit the advantages of the linear parameter varying (LPV) control approach together with a feedback linearisation approach for designing a cascade control structure without the need to perform simplifications with respect to the reference dynamic model used in the literature. The use of a LPV representation of the attitude subsystem allows to consider it as independent of the position one, allowing the use of a cascade control structure. For the inner attitude control loop, the proposed quasi-LPV-linear-quadratic regulator (LQR) controller and proportional-integral observer (PIO) are solved in an optimal manner by solving a set of linear matrix inequalities (LMIs). While for the outer position control loop, a feedback linearisation approach is developed before designing the corresponding controller and PIO also minimising the LQR problem by means of LMIs. Furthermore, another outer loop, interchangeable with the position one, such that controls the velocities in the body frame is proposed. This velocities controller is also solved in an optimal manner using a combination of feedback linearisation and LPV techniques. The proposed solutions are applied to an AscTec Hummingbird unmanned aerial vehicle in simulation.

Adaptive fault tolerant control for a small coaxial rotor unmanned aerial vehicles with partial loss of actuator effectiveness

This paper investigates the trajectory tracking problem for a small coaxial-rotor unmanned aerial vehicle (CRUAV) with partial loss of actuator effectiveness. The CRUAV model is decomposed into a dual loop structure based on back-stepping design idea. First, certain performance function specified a priori by the designer is introduced into the position loop such that the original position tracking error is transformed into an equivalent constrained variable providing for the performance judgment of the position loop. Then, a fault-tolerant control scheme is proposed based on adaptive strategies for compensating the effect caused by various adverse factors. Subsequently, an attitude control loop is derived by incorporating adaptive compensation method. It is proved that the proposed dual-loop structure is able to guarantee the satisfaction of the pre-specified constraint on the transformed errors. Finally, the effectiveness and benefits of the design dual-loop control system are validated via computer simulation.

Implementation of Predictive Controllers for Matrix-Converter-Based Interior Permanent Magnet Synchronous Motor Position Control Systems

This paper proposes the implementation of predictive controllers for matrix-converter-based interior permanent magnet synchronous motor (IPMSM) position control systems. A model-based predictive position controller and a model-free predictive current controller (MFPCC) for IPMSMs are investigated here. The proposed predictive controllers can improve the dynamic responses, including transient, load-disturbance, and tracking responses. In addition, the proposed MFPCC does not require any parameters such as inductance, resistance, or back-electromotive force of the IPMSM, and its performance does not deteriorate because of variations in the virtual dc-link voltage. Only the stator current difference is used to predict the future sampling current. A detailed stability analysis of the current-loop control and position-loop control of the IPMSM position control system is discussed. Several experimental results are included to validate the theoretical analysis. The experimental results show that the proposed predictive controllers have better performance than PI controllers. A 32-bit digital signal processor, a TMS-320LF-2407A, is used to execute the predictive controllers. The proposed control system is easily applied in industry due to its systematic design procedure.

Saturated control design of a quadrotor with heterogeneous comprehensive learning particle swarm optimization

Abstract This paper presents a novel saturated control method for a quadrotor to realize three–dimensional spatial trajectory tracking with heterogeneous comprehensive learning particle swarm optimization (HCLPSO). First, the model of the quadrotor is decomposed into cascaded control structure (CCS) with inner attitude control loop and outer position control loop. Second, the saturated control is deployed to confine the thrust force of the quadrotor in the outer position control loop. Then, the inner attitude reference signals can be generated with the outer position control loop through the cascaded control mode. Third, to alleviate the difficulty of the parameter adjustment, HCLPSO algorithm is applied in the control parameter optimization for the quadrotor. The optimization results of the HCLPSO are compared with that of particle swarm optimization (PSO), comprehensive learning particle swarm optimization (CLPSO), genetic algorithm (GA), and differential evolution (DE). At last, simulation results certify that the spatial trajectory tracking control of the quadrotor can be successfully achieved with the proposed method.

INCREASING PRODUCTIVITY OF CUTTING PROCESSES BY REAL-TIME COMPENSATION OF TOOL DEFLECTION DUE TO PROCESS FORCES

The Internet of Production (IoP) describes a vision in which a broad range of different production data is available in real-time. Based on this data, for example, new control types can be implemented, which improve individual manufacturing processes directly at the machine. A possible application scenario is a tool deflection compensation. Although the problem of tool deflection is well known in the industrial field, a process-parallel compensation is not common in industrial applications. State-of-the-art solutions require time and cost consuming tests to determine necessary cutting parameters. An NC-integrated compensation that adapts the tool path in real-time will make these tests obsolete and furthermore enables higher chip removal rates. In this paper, a control-internal real-time compensation of tool deflection is described, which is based on a process-parallel measurement of process forces. The compensation software is designed as an extension to the NC kernel and thereby integrated into the position control loop of an in-series NC. The compensation movements are generated by manipulating the reference values of the feed axes. The approach is investigated by experiments with linear axis movements. During these tests, a significantly reduction of geometrical machining errors is possible.