Feedforward Compensation Control Research Articles

Optical crystal temperature tuner based on feedforward-feedback compound control

To tune the output wavelength of the laser, the operating temperature of the optical crystal needs to be controlled precisely in a wide range. In this paper, a temperature tuning control method for optical crystals with high temperature control precision and large temperature control range based on semiconductor refrigerator (TEC) was studied. The operating characteristics of TEC,the thermal effects and heat transfer mechanisms of optical crystals were analyzed. A temperature-current double closed loop temperature control model was established. This temperature-current double closed loop control model takes control of both the electrical characteristic's variations of working TEC and the temperature variations of the optical crystal. To overcome the inherent hysteresis characteristic of feedback control, the feedforward compensation control based on the double closed loop control was introduced. The TEC driver module based on the ARM+FPGA architecture was designed to achieve the high-frequency and high-precision PWM drive control. In conclusion, an optical crystal temperature tuning system was completed. Experimental results show that the overall temperature control accuracy of this system is better than 0.002ºC and the system can quickly stabilize at any set point temperature in a wide temperature range of -30-130ºC, and it has good robustness to abnormal environment, which meets the requirements of tunable laser for temperature range and accuracy of optical crystal.

High-Accuracy Force Control With Nonlinear Feedforward Compensation for a Hydraulic Drive Unit

The hydraulic drive unit (HDU) applied in a hydraulic drive legged robot joints adopts an outer loop impedance control method based on the hydraulic control's inner loop during motion so that the entire system obtains certain compliance characteristics. There are two types of inner loops, namely, the position closed loop control and the force closed loop control; this paper studies a system in which the inner loop is a force closed loop control system and improves its accuracy in order to provide references for the outer loop impedance control. Therefore, designing a compensation method that improves the inner loop force control's accuracy has important research significance. In view of the above research significance, this paper first deduces a mathematical model for the force closed loop control and simplifies the sixth-order mathematical model to find the transfer function of each part. Second, combined with influence factors, such as pressure-flow nonlinearity, friction nonlinearity, and complex and variable loads on the system, the feedforward compensation controller of the force control input is derived. Considering the practical application in engineering, the controller order is reduced, and a partial compensation is achieved. Finally, on the HDU performance test platform, the force control performance is quantitatively analyzed by inputting typical signals and random signals. The experimental results show that the feedforward compensation controller can greatly improve the system force control performance with different input signals. The above research results can be combined with a corresponding disturbance rejection strategy for the force control system to provide an important reference and experimental basis for the hydraulic inner loop control strategy of force-based impedance.

A Non-Smooth Composite Control Approach for Direct Torque Control of Permanent Magnet Synchronous Machines

Considering the effects of disturbances in permanent magnet synchronous motor (PMSM), in this paper, a non-smooth composite control approach, which includes finite time disturbance observer (FTDO) for feedforward compensation and finite time control (FTC) for feedback control, is proposed to improve the anti-disturbance performance of PMSM. First, the FTDO is used to estimate the lumped disturbances, such as friction, parameter perturbation, and load variation. Then, the observed value is added to the speed controller as a feedforward compensation to eliminate the effect of disturbance. Second, FTC is introduced into the feedback control design part. In the end, by utilizing Lyapunov theory, the stability analysis of the overall closed-loop system is demonstrated. In contrast to the conventional asymptotically stable control strategy, the proposed composite scheme can provide not only a faster dynamic response but also a stronger capacity of disturbance rejection. The simulation and experimental tests are presented to demonstrate the superior properties of the proposed control scheme.

A Novel Speed Control Method Based on Port-Controlled Hamiltonian and Disturbance Observer for PMSM Drives

This paper is mainly focusing on the problem of speed tracking control for permanent magnet synchronous motor (PMSM) drive system subject to matched and mismatched disturbance. This is achieved by a novel Port-Controlled Hamiltonian control method with nonlinear disturbance observer. Different from the traditional speed-current cascade control for PMSM, the speed-current single loop controller is designed. Firstly, the Port-Controlled Hamiltonian model of PMSM is established, then the Hamiltonian speed controller is designed by using interconnection assignment and damping assignment method. However, the standard Hamiltonian controller cannot achieve the satisfying behavior in the presence of the parameter variations and external disturbance, thus a nonlinear disturbance observer is designed to estimate the lump disturbance of the system, which is used to the feed-forward compensation control. Finally, simulation and experiment on the speed control system of PMSM are implemented. The results show that the proposed method has fast speed control performance and strong robustness for the different disturbance.

An improved scheme for eliminating the coupled motion of surgical instruments used in laparoscopic surgical robots

Considering the nonlinear characteristics such as backlash hysteresis and coupled motion commonly exist in cable-driven mechanism of laparoscopic surgical robot end-effector, it is a great challenge to control the motion of robotic end-effector precisely during the surgical procedure. Due to the effects of coupled motion, the surgical end-effector will not move accurately as surgeons expected. Previous studies mostly focused on the design of special compensation mechanisms and software compensation algorithms to solve coupled motion problem. However, these approaches are limited because the backlash hysteresis is ignored and the mechanism of end-effector is restricted. This paper shows an improved scheme to eliminate the coupled motion of end-effector and reduce the position tracking error. The proposed decoupling scheme is conducted in three stages. Firstly, the time and frequency domain information of the driving motor current and the motion information of surgical instrument are extracted in real-time. Thereafter, a feedforward neural network is designed to identify the movement stage of end-effector. Finally, a prediction model is designed to predict the coupling error, after that the coupling error can be eliminated by using feedforward compensation control. An experimental platform was set up to verify the effectiveness of the proposed control scheme, and the results of corresponding comparative experiments revealed that the proposed strategy can substantially improve the tracking accuracy.

Accurate flight path tracking control for powered parafoil aerial vehicle using ADRC-based wind feedforward compensation

Winds are the major contributors to the tracking error in the control process of the powered parafoil, making it difficult to achieve better control performance. To address this problem, an accurate flight path tracking control approach for the powered parafoil combining active disturbance rejection control (ADRC) and wind feedforward compensation is proposed. The wind impacts on the lateral heading and longitudinal altitude dynamics of the powered parafoil are analyzed firstly. Moreover, the wind feedforward compensation control, the lateral heading and longitudinal altitude tracking controller are designed to directly compensate the wind disturbance in the control process, such that the tracking precision and disturbance-rejection capacity can be enhanced simultaneously. Eventually, the mathematical simulation, robustness test, and experimental results demonstrate that the proposed control approach achieve better tracking performance and robustness against the variable wind disturbance than the conventional ADRC and PID.

An improved force-based impedance control method for the HDU of legged robots

Hydraulic drive mode enables legged robots to have excellent characteristics, such as greater power-to-weight ratios, higher load capacities, and faster response speeds than other robots. Nowadays, highly integrated valve-controlled cylinder, called hydraulic drive unit (HDU), is employed to drive the joints of these robots. However, various robot control issues exist. For example, during the walking process of legged robots, different obstacles are encountered, making it difficult to control such robots because the load characteristics of the ends of their feet change with the environment. Furthermore, although the adoption of HDU has resulted in high-performance robot control, the hydraulic systems of these robots still have problems, such as strong nonlinearity, and time-varying parameters. Consequently, robot control is very difficult and complex. This paper proposes an improved second-order dynamic compliance control system, impedance control, for HDU. The control system is designed to rectify the issues affecting the impedance control accuracy of the dynamic compliance serial–parallel composition between the HDU force control inner loop and the impedance control outer loop. Specifically, it consists of a compliance-enhanced controller and a feedforward compensation controller for the force control inner loop. Furthermore, the dynamic compliance composition of the inner and outer HDU control loops is rearranged. The results of experiments conducted indicate that the proposed method significantly improves the control accuracy compared to that of traditional force-based impedance control.

Evolutionary Algorithm-Based Friction Feedforward Compensation for a Pneumatic Rotary Actuator Servo System

The friction interference in the pneumatic rotary actuator is the primary factor affecting the position accuracy of a pneumatic rotary actuator servo system. The paper proposes an evolutionary algorithm-based friction-forward compensation control architecture for improving position accuracy. Firstly, the basic equations of the valve-controlled actuator are derived and linearized in the middle position, and the transfer function of the system is further obtained. Then, the evolutionary algorithm-based friction feedforward compensation control architecture is structured, including that the evolutionary algorithm is used to optimize the controller coefficients and identify the friction parameters. Finally, the contrast experiments of four control strategies (the traditional PD control, the PD control with friction feedforward compensation without evolutionary algorithm tuning, the PD control with friction feedforward compensation based on the differential evolution algorithm, and the PD control with friction feedforward compensation based on the genetic algorithm) are carried out on the experimental platform. The experimental results reveal that the evolutionary algorithm-based friction feedforward compensation greatly improves the position tracking accuracy and positioning accuracy, and that the differential evolution-based case achieves better accuracy. Also, the system with the friction feedforward compensation still maintains high accuracy and strong stability in the case of load.

Dynamic feedforward control of spatial cable-driven hyper-redundant manipulators for on-orbit servicing

SUMMARYThe hyper-redundant manipulators are suitable for working in the constrained on-orbit servicing environment due to the extreme flexibility. However, its modelling and control are very challenging due to the characteristics of non-linearity and strong coupling. In this paper, considering the multi-level mapping among the motors, cables, joints, and end-effector, a proportional derivative (PD) with dynamic feedforward compensation control system is designed. The corresponding control system is divided into five parts: controller, planner, actuator, manipulator, and sensor. The actual control torque consisting of the desired feedforward torque and the feedback torque is generated by the controller. In order to improve the tracking accuracy and maintain rapid response, the torque, which is calculated by the dynamics model of the traditional joint-driven manipulator, is regarded as the desired feedforward torque. The parameters of interest are the angle and velocity of the universal joint and motors. The planner plans and converts the desired parameters of the universal joint to corresponding motors. Combining with the feedback angles and velocities signals of the corresponding motors, the feedback torque can be calculated by the PD control module. Finally, typical cases of six universal joints (12DOFs) manipulators are simulated and experimented. The results demonstrate that the method is very efficient for controlling spatial cable-driven hyper-redundant manipulators.

Combined Speed and Current Terminal Sliding Mode Control With Nonlinear Disturbance Observer for PMSM Drive

A terminal sliding mode control (SMC) method based on nonlinear disturbance observer is investigated to realize the speed and the current tracking control for the permanent magnet synchronous motor (PMSM) drive system in this paper. The proposed method adopts the speed-current single-loop control structure instead of the traditional cascade control in the vector control of the PMSM. First, considering the nonlinear and the coupling characteristic, a single-loop terminal sliding mode controller is designed for PMSM drive system through feedback linearization technology. This method can make the motor speed and current reach the reference value in finite time, which can realize the fast transient response. Although the SMC is less sensitive to parameter uncertainties and external disturbance, it may produce a large switching gain, which may cause the undesired chattering. Meanwhile, the SMC cannot keep the property of invariance in the presence of unmatched uncertainties. Then, a nonlinear disturbance observer is proposed to the estimate the lump disturbance, which is used in the feed-forward compensation control. Thus, a composite control scheme is developed for the PMSM drive system. The results show that the motor control system based on the proposed method has good speed and current tracking performance and strong robustness.

高加速度运动系统的非线性摩擦前馈补偿控制

高加速度运动系统的非线性摩擦前馈补偿控制

A New Feed-forward Compensation Inverter Control Strategy

With the development of new energy research, photovoltaic inverter is widely used in solar photovoltaic power generation system. In this paper, a new static feedforward compensation control strategy for current loop is proposed for the shortcomings of the single-phase PV inverter in the use of the typical Ⅰ-type system in the double closed-loop control system. Through the improved Park transformation, the equivalent model of the single-phase inverter is obtained. The control equation of the current is obtained by the method of feed-forward decoupling control in the equivalent inverter model. For the interference signal in the current control equation, The static feed-forward compensation control method eliminates the interference signal present in the inner loop of the current. Through this control method, the current loop in ensuring the performance of good follow at the same time effectively improve the current loop anti-jamming performance. At the same time, current loop control and voltage outer loop control using PI control, the formation of double closed-loop control system. The simulation results show that the anti - jamming performance of the double closed-loop inverter control system with static feed-forward compensation is better, the output waveform distortion rate of the inverter is low, and the PV inverter is stable in the presence of disturbance.

Hysteresis linearization control of a novel hybrid vibration isolator

The undesired hysteresis exists widely in smart-material actuators, which significantly reduces the accuracy and response speed of the actuators. In this paper, the static experiment of our previously developed hybrid vibration isolator (HVI) employing piezoelectric actuator is implemented to choose appropriate preload of the HVI. The preload-dependent hysteresis of the HVI is also proved in the static experiment. To achieve the hysteresis linearization control of the HVI, two linearization methods named the feedforward linearization and feedforward compensation and PI feedback hybrid linearization control are presented, respectively, which are based on the Bouc-Wen model and corresponding parameter identification model. To evaluate the effectiveness of the proposed linearization controllers for the HVI, the experiments are implemented. The experiment results demonstrate that both linearization controllers for the HVI can linearize the hysteresis characteristics and improve the HVI control accuracy. In addition, the feedforward compensation and PI feedback hybrid linear controller can achieve higher linearity than feedforward compensation controller.

Double Position Servo Synchronous Drive System Based on Cross-Coupling Integrated Feedforward Control for Broacher

Synchronization errors directly deteriorate the machining accuracy of metal parts and the existed method cannot keep high synchronization precision because of external disturbances. A new double position servo synchronous driving scheme based on semi-closed-loop cross-coupling integrated feedforward control is proposed. The scheme comprises a position error cross-coupling feedforward control and a load torque identification with feedforward control. A digital integrated simulation system for the dual servo synchronous drive system is established. Using a 20 t servo broacher, performance analysis of the scheme is conducted based on this simulation system and the simulation results show that systems with traditional parallel or single control have problems when the worktable works with an unbalanced load. However, the system with proposed scheme shows good synchronous performance and positional accuracy. Broaching tests are performed and the experimental results show that the maximum dual axis synchronization error of the system is only 8 μm during acceleration and deceleration processes and the error between the actual running position and the given position is almost zero. A double position servo synchronous driving scheme is presented based on cross-coupled integrated feedforward compensation control, which can improve the synchronization precision.

An electrical-coupling-suppressing MEMS gyroscope with feed-forward coupling compensation and scalable fuzzy control

This paper proposes a novel electrical coupling suppressing and drive closed loop control method for a MEMS gyroscope with feed-forward coupling compensation (FCC) and scalable fuzzy control. Theoretical analysis of the novel method is described in detail, and it is very simple to realize. Experimental results demonstrate that the electrical anti-resonant peaks located at the amplitude-frequency and phase-frequency responses are both eliminated by FCC control, and the height of the amplitude resonant peak increases more than 24 dB over 1800 Hz span. In addition, the overshoot of the transient response with scalable fuzzy control is smaller than 5\%, and the settling time is less than 15 ms. The stabilities of the resonant amplitude and phase of the drive-mode velocity with scalable fuzzy control are about 15 ppm and 11 ppm, respectively. The scale factor of the gyroscope is measured to be 33.98 mV/deg/s with nonlinearity about 0.08\%. Furthermore, the bias instability of the gyroscope with wavelet analysis is improved to be about 6.3 deg/h from 25.2 deg/h of the gyroscope without wavelet analysis.

Feedback Control Strategy to Eliminate the Input Current Harmonics of Matrix Converter Under Unbalanced Input Voltages

It is generally considered that feedback control of input currents in the matrix converter (MC) is hard to be realized due to the coupling of input control and output control, which reduces the degree of freedom and robustness. Moreover, under unbalanced input voltages, the coupling also results in severe distortion of input currents when the commonly used feedforward compensation control method with fixed input power factor is adopted. To address these two issues, this paper proposes a feedback control strategy on the input side of MC. This strategy is based on a control method which can modify input reference currents. The input control strategy is embedded into the output control strategy and thus is the inner loop of the system control. The input-side controllers can be designed to achieve expected input control objectives and maintain the output performance at the same time. On this basis, resonant controllers are applied to regulate input currents and instantaneous active power, so as to directly eliminate the input current harmonics and meanwhile ensure the load absorbing constant active power under unbalanced input voltages. The validity and feasibility of the proposed strategy are verified by the simulation and experimental results.

Adaptive positioning control of an ultrasonic linear motor system

A 3PRR parallel precision positioning system, driven by three ultrasonic linear motors, was designed for use as the object stage of a scanning electron microscope (SEM). To improve the tracking accuracy of the parallel platform, the positioning control algorithms for the drive joints needed to be studied. The dead-zone phenomenon caused by static friction reduces the trajectory tracking accuracy significantly. Linear control algorithms such as PID (Proportion Integration Differentiation) are unable to compensate effectively for the dead-zone nonlinearity. To address this problem, two types of feedforward compensation control algorithms have been investigated. One is constant feedforward with the integral separation PID; the other is adaptive feedback and feedforward based on the model reference adaptive control (MRAC). Simulations and experiments were conducted using these two control algorithms. The results demonstrated that the constant feedforward with integral separation PID algorithm can compensate for the time-invariant system after identifying the dead-zone depth, while the adaptive feedback and feedforward algorithm is more suitable for the time-varying system. The experimental results show good agreement with the simulation results for these two control algorithms. For the dead-zone nonlinearity caused by the static friction, the adaptive feedback and feedforward algorithm can effectively improve the trajectory tracking accuracy. A positioning system has been built for the driven joints of the 3-PRR in SEM.The dead-zone of the system has been identified and applied to compensation.The proposed control algorithm improves the tracking precision significantly.

Improved V/f control strategy for microgrids based on master–slave control mode

The V/f control adopted by the master power supply has problems of slow dynamic response, poor anti-interference ability in response to micro-source output power fluctuations and loads abrupt change. Aiming at problems of the output voltage mentioned above, an improved V/f control strategy based on compound control is proposed in this study. The improved V/f control strategy is composed of two parts, feedforward compensation and robust feedback control. The design of the feedforward compensator is realised by the method of approximate full compensation. The design of the robust feedback controller is transformed into solving a convex optimisation problem with linear matrix inequality constraints and linear objective functions. The designed V/f control strategy is verified by a variety of comparative tests. Test comparisons show that the controller can get better voltage output characteristics, dynamic characteristics and robustness in response to micro-source output power fluctuations, loads abrupt change or non-linear loads and unbalanced loads.

Research on nonlinear friction compensation of harmonic drive in gimbal servo-system of DGCMG

The purpose of this paper is to present our results in overcoming the influence of the nonlinear friction afforded by harmonic drive to the gimbal servo-system of double-gimbal control momentum gyro (DGCMG). The existence of compliance and oscillation inherent in harmonic drive systems, and the lack of any technical information on the internal dynamics of the transmission, make the development of friction compensation in harmonic drive system extremely challenging. In this paper, the modeling of nonlinear friction in harmonic drive gear transmission in gimbal servo-system of the DGCMG is proposed. The relationship among the nonlinear friction, the angular velocity and the angular position with an improved Coulomb-Viscous model is derived, and the experiments to identify the various parameters of the improved model are given. At last a feed-forward compensation controller based on the improved model is designed to carry out the friction compensation study.

Motion compensated controller for a tendon-sheath-driven flexible endoscopic robot

A tendon-sheath system (TSS) has the advantages of being relatively compact in size, flexible and low cost, and therefore is favoured in building flexible endoscopic robots to pass through long and tortuous human lumen. TSS, however, is prone to nonlinear behaviors such as backlash, hysteresis and direction dependent properties. A compensation technique is required to improve its positioning performance. Tension and elongation models of TSS are analyzed. A feedforward motion compensation controller is designed to compensate the asymmetric backlash behavior of the TSS in real time. Motion tracking experiments were conducted on a TSS driven two DOFs continuum manipulator. The results showed that using the proposed compensation methods, tracking error can be reduced by 74%. The proposed compensation method is useful for controlling flexible continuum robots, which are anticipated to have emerging roles in assisting surgeons to perform the increasingly technically challenging endoscopic procedures. Copyright © 2016 John Wiley & Sons, Ltd.