期刊文献+
共找到66篇文章
< 1 2 4 >
每页显示 20 50 100
Adaptive Gain Tuning Rule for Nonlinear Sliding-mode Speed Control of Encoderless Three-phase Permanent Magnet Assisted Synchronous Motor 被引量:1
1
作者 Ghada A.Abdel Aziz Rehan Ali Khan 《CES Transactions on Electrical Machines and Systems》 CSCD 2023年第3期301-310,共10页
In this paper, an adaptive gain tuning rule is designed for the nonlinear sliding mode speed control(NSMSC) in order to enhance the dynamic performance and the robustness of the permanent magnet assisted synchronous r... In this paper, an adaptive gain tuning rule is designed for the nonlinear sliding mode speed control(NSMSC) in order to enhance the dynamic performance and the robustness of the permanent magnet assisted synchronous reluctance motor(PMa-Syn RM) with considering the parameter uncertainties. A nonlinear sliding surface whose parameters are altering with time is designed at first. The proposed NSMSC can minimize the settling time without any overshoot via utilizing a low damping ratio at starting along with a high damping ratio as the output approaches the target set-point. In addition, it eliminates the problem of the singularity with the upper bound of an uncertain term that is hard to be measured practically as well as ensures a rapid convergence in finite time, through employing a simple adaptation law. Moreover, for enhancing the system efficiency throughout the constant torque region, the control system utilizes the maximum torque per ampere technique. The nonlinear sliding surface stability is assured via employing Lyapunov stability theory. Furthermore, a simple sliding mode estimator is employed for estimating the system uncertainties. The stability analysis and the experimental results indicate the effectiveness along with feasibility of the proposed speed estimation and the NSMSC approach for a 1.1-k W PMa-Syn RM under different speed references, electrical and mechanical parameters disparities, and load disturbance conditions. 展开更多
关键词 Permanent magnet assisted synchronous reluctance motor Nonlinear sliding mode speed control speed estimation Parameter uncertainties Sliding mode estimator
下载PDF
A Sensor-less Surface Mounted PMSM for Electronic Speed Control in Multilevel Inverter
2
作者 S.Dinesh Kumar A.Jagadeeshwaran 《Intelligent Automation & Soft Computing》 SCIE 2023年第5期2201-2215,共15页
Recent advancements in power electronics technology evolves inverter fed electric motors.Speed signals and rotor position are essential for controlling an electric motor accurately.In this paper,the sensorless speed c... Recent advancements in power electronics technology evolves inverter fed electric motors.Speed signals and rotor position are essential for controlling an electric motor accurately.In this paper,the sensorless speed control of surface-mounted permanent magnet synchronous motor(SPMSM)has been attempted.SPMSM wants a digital inverter for its precise working.Hence,this study incor-poratesfifteen level inverter to the SPMSM.A sliding mode observer(SMO)based sensorless speed control scheme is projected to determine rotor spot and speed of the multilevel inverter(MLI)fed SPMSM.MLI has been operated using a multi carrier pulse width modulation(MCPWM)strategy for generation offif-teen level voltages.The simulation works are executed with MATLAB/SIMU-LINK software.The steadiness and the heftiness of the projected model have been investigated under no loaded and loaded situations of SPMSM.Furthermore,the projected method can be adapted for electric vehicles. 展开更多
关键词 Surface-mounted permanent magnet synchronous motor sensorless speed control multilevel inverter torque estimation multi carrier pulse width modulation
下载PDF
INDUCTION MOTOR SPEED CONTROL SYSTEM BASED ON FUZZY NEURAL NETWORK 被引量:1
3
作者 徐小增 李叶松 秦忆 《Transactions of Nanjing University of Aeronautics and Astronautics》 EI 2004年第3期195-199,共5页
A fuzzy neural network controller with the teaching controller guidance and parameter regulations for vector-controlled induction motor is proposed. The design procedures of the fuzzy neural controller and the teachin... A fuzzy neural network controller with the teaching controller guidance and parameter regulations for vector-controlled induction motor is proposed. The design procedures of the fuzzy neural controller and the teaching controller are described. The parameters of the membership function are regulated by an on-line learning algorithm. The speed responses of the system under the condition, where the target functions are chosen as I qs and ω, are analyzed. The system responses with the variant of parameter moment of inertial J, viscous coefficients B and torque constant K tare also analyzed. Simulation results show that the control scheme and the controller have the advantages of rapid speed response and good robustness. 展开更多
关键词 induction motor fuzzy neural network vector control speed control system
下载PDF
Design of hydraulic motor speed control system based on co-simulation of AMESim and Matlab_Simulink 被引量:1
4
作者 孟凡虎 赵素素 +2 位作者 雷晓顺 王娜 高峰 《Journal of Measurement Science and Instrumentation》 CAS CSCD 2016年第3期279-285,共7页
In order to design an effective hydraulic motor speed control system, Matlab_Simiulink and AMESim co-simulation technology is adopted to establish more accurate model and reflect the actual system. The neural... In order to design an effective hydraulic motor speed control system, Matlab_Simiulink and AMESim co-simulation technology is adopted to establish more accurate model and reflect the actual system. The neural network proportion-integration-differentiation (PID) control parameters on-line adjustment is utilized to improve system accuracy, celerity and stability. Simulation results indicate that with the control system proposed in this paper, the system deviation is reduced, therefore accuracy is improved; response speed for step signal and sinusoidal signal gets faster, thus acceleration is rapidly improved; and the system can be restored to the control value in case of interfering, so stability is improved. 展开更多
关键词 speed control system CO-SIMULATION neural network proportion-integration-differentiation (PID) control
下载PDF
Speed Sensorless Vector Control of Induction Motor Based on Reduced Order Extended Kalman Filter 被引量:1
5
作者 杨文强 贾正春 许强 《Journal of Southeast University(English Edition)》 EI CAS 2001年第1期41-45,共5页
A speed sensorless vector control system of induction motor with estimated rotor speed and rotor flux using a new reduced order extended Kalman filter is proposed. With this method, two rotor flux components are sele... A speed sensorless vector control system of induction motor with estimated rotor speed and rotor flux using a new reduced order extended Kalman filter is proposed. With this method, two rotor flux components are selected as the state variables, and the rotor speed as an estimated parameter is regarded as an augmented state variable. The algorithm with reduced order decreases the computational complexity and makes the proposed estimator feasible to be implemented in real time. The simulation results show high accuracy of the estimation algorithm and good performance of speed control, and verify the usefulness of the proposed algorithm. 展开更多
关键词 extended Kalman filter flux estimation speed estimation speed sensorless vector control induction motor
下载PDF
Speed Control of Travelling Wave Type Ultrasonic Motors Using Artificial Neural Network
6
作者 杨忠 金龙 《Journal of Southeast University(English Edition)》 EI CAS 1999年第2期63-68,共6页
Ultrasonic motor (USM) is a newly developed motor, and it has some excellent performances and useful features, therefore, it has been expected to be of practical use. However, the driving principle of USM is different... Ultrasonic motor (USM) is a newly developed motor, and it has some excellent performances and useful features, therefore, it has been expected to be of practical use. However, the driving principle of USM is different from that of other electromagnetic type motors, and the mathematical model is complex to apply to motor control. Furthermore, the speed characteristics of the motor have heavy nonlinearity and vary with driving conditions. Hence, the precise speed control of USM is generally difficult. This paper proposes a new speed control scheme for USM using an artificial neural network. An accurate tracking response can be obtained by random initialization of the weights of the network owing to the powerful on line learning capability. Two prototype ultrasonic motors of travelling wave type were fabricated, both having 100 mm outer diameters of stator and piezoelectric ceramic. The usefulness and validity of the proposed control scheme are examined in experiments. 展开更多
关键词 artificial neural networks ultrasonic motors travelling wave type speed control
下载PDF
Predictive Control Model Based on the MPC Algorithm on Three Different Road Sections
7
作者 Xiaoxi Liu Xixi Zheng +1 位作者 Shuming Zhang Zhidong Wang 《Journal of Electronic Research and Application》 2024年第2期116-123,共8页
Predictive control is an advanced control algorithm,which is widely used in industrial process control.Among them,model predictive control(MPC)is an important branch of predictive control.Its basic principle is to use... Predictive control is an advanced control algorithm,which is widely used in industrial process control.Among them,model predictive control(MPC)is an important branch of predictive control.Its basic principle is to use the system model to predict future behavior and determine the current control action by optimizing the objective function.This paper discusses the application of MPC in the prediction and control of the speed of vehicles to optimize traffic flow.It is a valuable reference for alleviating traffic congestion and improving travel efficiency and smoothness and provides scientific basis and technical support for future highway traffic management. 展开更多
关键词 MPC algorithm Variable speed limit control HIGHWAY
下载PDF
Design of speed controller for electronic fuel injection gasoline generator based on feed-forward PID control
8
作者 赵自庆 刘昌文 张平 《Journal of Measurement Science and Instrumentation》 CAS CSCD 2015年第4期354-363,共10页
As for the application of electronic fuel injection (EFI) system to small gasoline generator set, mechanical speed controller cannot be coupled with EFI system and has the shortcomings of lagged regulation and poor ... As for the application of electronic fuel injection (EFI) system to small gasoline generator set, mechanical speed controller cannot be coupled with EFI system and has the shortcomings of lagged regulation and poor accuracy, a feed-forward control strategy based on load combined with proportional-integral-differential (PID) control strategy was proposed, and a digital speed controller applied to the electrical control system was designed. The detailed control strategy of the controller was intro- duced. The hardware design for the controller and the key circuits of motor driving, current sampling and angular signal captu- ring were given, and software architecture was discussed. Combined with a gasoline generator set mounted with EFI system, the controller parameters were tuned and optimized empirically by hardware in loop and bench test methods. Test results show that the speed deviation of generator set is low and the control system is stable in steady state; In transient state the control system responses quickly, has high stability under mutation loads especially when suddenly apply and remove 100% load, the speed deviation is within 8% of reference speed and the transient time is less than 5 s, satisfying the ISO standard. 展开更多
关键词 gasoline generator digital speed controller electronic fuel injection (EFI) feed forward proportional-integral-differential (PID) control
下载PDF
ADAPTIVE VARIABLE STRUCTURE CONTROLLERS AND APPLICATION TO ENGINE IDLE SPEED CONTROL SIMULATION 被引量:3
9
作者 Hui Qing Quyang MinggaoState Key Laboratory of AutomotiveSafety and Energy,Tsinghua University,Beijing 100084, China 《Chinese Journal of Mechanical Engineering》 SCIE EI CAS CSCD 2003年第2期171-174,共4页
A neural-network-based adaptive variable structure control methodology isproposed for the tracking problem of nonlinear discrete-time input-output systems. The unknowndynamics of the system are approximated via radial... A neural-network-based adaptive variable structure control methodology isproposed for the tracking problem of nonlinear discrete-time input-output systems. The unknowndynamics of the system are approximated via radial basis function neural networks. The control lawis based on sliding modes and simple to implement. The discrete-time adaptive law for tuning theweight of neural networks is presented using the adaptive filtering algorithm with residueupper-bound compensation. The application of the proposed controller to engine idle speed controldesign is discussed. The results indicate the validation and effectiveness of this approach. 展开更多
关键词 variable structure control adaptive control neural networks nonlineardiscrete-time systems idle speed control
下载PDF
Structure modification and constant remelting speed control of a 120-t three-phase electroslag furnace 被引量:3
10
作者 Wang Changzhou Song Jinchun 《China Foundry》 SCIE CAS 2012年第4期370-375,共6页
The traditional large electroslag remelting furnaces have many shortages,such as high short-network impedance and inductance,long maintenance time for electrode replacement,low stiffness of driveline,and low control a... The traditional large electroslag remelting furnaces have many shortages,such as high short-network impedance and inductance,long maintenance time for electrode replacement,low stiffness of driveline,and low control accuracy of remelting speed.The present research was aimed to solve these problems through structure modification and constant remelting speed control for a 120-t electroslag remelting(ESR) furnace.Based on the technique of three-phase double electrodes in series,the short-network system and the structure of the 120-t ESR furnace were improved;and a continuous feeding system for the self-consumption electrode was proposed.A selfdesigned fully hydraulic driveline system with three degrees of freedom was successfully applied to the 120-t ESR furnace.An electrode auto-replacement system and the S-style speed-control curve of electrode-feeding system were designed on the basis of the soft measurement/sensing model on the remaining electrode length so as to obtain a high accuracy control system for constant remelting speed.The experiment products showed good surface quality and cross-sectional results,indicating good system control,and verifying the effectiveness of the structure modification of the furnace. 展开更多
关键词 electroslag remelting furnace structure improvement design constant remelting speed control
下载PDF
Constant Speed Control of Four-stroke Micro Internal Combustion Swing Engine 被引量:2
11
作者 GAO Dedong LEI Yong +1 位作者 ZHU Honghai NI Jun 《Chinese Journal of Mechanical Engineering》 SCIE EI CAS CSCD 2015年第5期971-982,共12页
The increasing demands on safety, emission and fuel consumption require more accurate control models of micro internal combustion swing engine (MICSE). The objective of this paper is to investigate the constant spee... The increasing demands on safety, emission and fuel consumption require more accurate control models of micro internal combustion swing engine (MICSE). The objective of this paper is to investigate the constant speed control models of four-stroke MICSE The operation principle of the four-stroke MICSE is presented based on the description of MICSE prototype. A two-level Petri net based hybrid mode/ is proposed to mode/ the four-stroke MICSE engine cycle. The Petri net subsystem at the upper level controls and synchronizes the four Petri net subsystems at the lower level. The continuous sub-models, including breathing dynamics of intake manifold, thermodynamics of the chamber and dynamics of the torque generation, are investigated and integrated with the discrete model in MATLAB Simulink. Through the comparison of experimental data and simulated DC voltage output, it is demonstrated that the hybrid model is valid for the four-stroke MICSE system. A nonlinear model is obtained from the cycle average data via the regression method, and it is linearized around a given nominal equilibrium point for the controller design. The feedback controller of the spark timing and valve duration timing is designed with a sequential loop closing design approach. The simulation of the sequential loop closure control design applied to the hybrid model is implemented in MATLAB. The simulation results show that the system is able to reach its desired operating point within 0.2 s, and the designed controller shows good MICSE engine performance with a constant speed. This paper presents the constant speed control models of four-stroke MICSE and carries out the simulation tests, the models and the simulation results can be used for further study on the precision control of four-stroke MICSE. 展开更多
关键词 MICSE constant speed control hybrid model spark timing valve duration
下载PDF
Application of Super-Synchronization Speed Control Technology in Two 80 MVA Motor-Generator Units of HL-2A 被引量:2
12
作者 李华俊 杜沧 +8 位作者 宣伟民 彭建飞 胡浩天 刘伶 康利 徐丽荣 黄昭荣 王芬 王小平 《Plasma Science and Technology》 SCIE EI CAS CSCD 2007年第2期231-234,共4页
Two sets of super-synchronization speed control assemblies for two 80 MVA motor-generator units have been developed successfully in order to satisfy the demand of the toroidal field system in the HL-2A tokamak. Based ... Two sets of super-synchronization speed control assemblies for two 80 MVA motor-generator units have been developed successfully in order to satisfy the demand of the toroidal field system in the HL-2A tokamak. Based on the three-phase logical no-circumfluence a.c./a.c. cycloconverter, the speeds of two 2500 kW double fed drive motors have been regulated by means of the vector control technology. The maximum operating speed of each motor- generator unit has been raised from 1488 rpm (revolutions per minute) to 1650 rpm and the released energy of each unit during a pulsed discharge can reach 500 MJ. As a result, the toroidal field system has the capacity to provide 2.8 tesla (T) in HL-2A experiments. 展开更多
关键词 double fed motor a.c./a.c CYCLOCONVERTER vector control super-synchro nization speed control assembly
下载PDF
EXPERIMENT STUDY ON SPEED RATIO CONTROL OF METAL V-BELT TYPE CVT 被引量:2
13
作者 WangHongyan QinDatong +1 位作者 ZhangBoying QiuXiding 《Chinese Journal of Mechanical Engineering》 SCIE EI CAS CSCD 2004年第1期11-15,共5页
Based on the control scheme of force and position, the controlling andtesting system of metal V-belt type CVT is developed. Having taken account of the complex nonlinearcharacteristics of the CVT shift dynamics and th... Based on the control scheme of force and position, the controlling andtesting system of metal V-belt type CVT is developed. Having taken account of the complex nonlinearcharacteristics of the CVT shift dynamics and the saturation and nonlinear characteristics of thespeed ratio control valve, the speed ratio fuzzy controller based on the speed ratio feedback isdesigned. The experiment results show that the developed speed ratio control system is practical andfeasible. 展开更多
关键词 CVT speed ratio control Fuzzy control EXPERIMENT
下载PDF
Model predictive control of rigid spacecraft with two variable speed control moment gyroscopes 被引量:3
14
作者 Pengcheng WU Hao WEN +1 位作者 Ti CHEN Dongping JIN 《Applied Mathematics and Mechanics(English Edition)》 SCIE EI CSCD 2017年第11期1551-1564,共14页
In this paper, an attitude maneuver control problem is investigated for a rigid spacecraft using an array of two variable speed control moment gyroscopes (VSCMGs) with gimbal axes skewed to each other. A mathematica... In this paper, an attitude maneuver control problem is investigated for a rigid spacecraft using an array of two variable speed control moment gyroscopes (VSCMGs) with gimbal axes skewed to each other. A mathematical model is constructed by taking the spacecraft and the gyroscopes together as an integrated system, with the coupling interaction between them considered. To overcome the singular issues of the VSCMGs due to the conventional torque-based method, the first-order derivative of gimbal rates and the second-order derivative of the rotor spinning velocity, instead of the gyroscope torques, are taken as input variables. Moreover, taking external disturbances into account, a feedback control law is designed for the system based on a method of nonlinear model predictive control (NMPC). The attitude maneuver can be realized fast and smoothly by using the proposed controller in this paper. 展开更多
关键词 integrated system variable speed control moment gyroscope (VSCMG) nonlinear model predictive control (NMPC)
下载PDF
Turbine speed control system based on a fuzzy-PID 被引量:2
15
作者 孙建华 汪伟 余海燕 《Journal of Marine Science and Application》 2008年第4期268-272,共5页
The flexibility demand of marine nuclear power plant is very high,the multiple parameters of the marine nuclear power plant with the once-through steam generator are strongly coupled,and the normal PID control of the ... The flexibility demand of marine nuclear power plant is very high,the multiple parameters of the marine nuclear power plant with the once-through steam generator are strongly coupled,and the normal PID control of the turbine speed can't meet the control demand.This paper introduces a turbine speed Fuzzy-PID controller to coordinately control the steam pressure and thus realize the demand for quick tracking and steady state control over the turbine speed by using the Fuzzy control's quick dynamic response and PID control's steady state performance.The simulation shows the improvement of the response time and steady state performance of the control system. 展开更多
关键词 turbine speed control system fuzzy control PID control
下载PDF
Vector control of induction motor based on fractional-order intelligent-integral speed controller 被引量:2
16
作者 MIAO Zhong-cui HAN Tian-liang +1 位作者 DANG Jian-wu JU Mei 《Journal of Measurement Science and Instrumentation》 CAS CSCD 2018年第2期125-133,共9页
To improve dynamic and static performances and robustness of the induction motor speed control system based on vector control,an improved fractional-order intelligent proportional integral(IPIλ)controller was applied... To improve dynamic and static performances and robustness of the induction motor speed control system based on vector control,an improved fractional-order intelligent proportional integral(IPIλ)controller was applied to the speed controller of the vector control system,which combined the intelligent fractional integral with the proportion according to the variation of deviation.Compared with proportional integral(PI)and fractional-order proportional integral(FOPI)controllers,the IPIλcontroller achieved better control performance.The stimulation results indicate that the IPIλcontroller can not only track the given speed quickly and accurately,but also have better anti-interference and robustness for load and parameters variations. 展开更多
关键词 fractional-order intelligent-integral induction motor speed controller
下载PDF
Asymptotic Tracking Control for Wind Turbines in Variable Speed Mode 被引量:1
17
作者 Wei Lin Zongtao Lu Wei Wei 《IEEE/CAA Journal of Automatica Sinica》 SCIE EI CSCD 2017年第3期569-576,共8页
This paper presents a variable speed control strategy for wind turbines in order to capture maximum wind power.Wind turbines are modeled as a two-mass drive-train system with generator torque control.Based on the obta... This paper presents a variable speed control strategy for wind turbines in order to capture maximum wind power.Wind turbines are modeled as a two-mass drive-train system with generator torque control.Based on the obtained wind turbine model,variable speed control schemes are developed.Nonlinear tracking controllers are designed to achieve asymptotic tracking for a prescribed rotor speed reference signal so as to yield maximum wind power capture.Due to the difficulty of torsional angle measurement,an observer-based control scheme that uses only rotor speed information is further developed for global asymptotic output tracking.The effectiveness of the proposed control methods is illustrated by simulation results. 展开更多
关键词 Asymptotic tracking partial-state and output feedback two-mass drive-train model variable speed control wind turbines
下载PDF
Frequency-Speed Control Model Identification of Ultrasonic Motor Using Step Response 被引量:1
18
作者 史敬灼 张彩霞 《Transactions of Nanjing University of Aeronautics and Astronautics》 EI CSCD 2015年第2期192-198,共7页
Control model of ultrasonic motor is the foundation for high control performance.The frequency of driving voltage is commonly used as control variable in the speed control system of ultrasonic motor.Speed control mode... Control model of ultrasonic motor is the foundation for high control performance.The frequency of driving voltage is commonly used as control variable in the speed control system of ultrasonic motor.Speed control model with the input frequency can significantly improve speed control performance.Step response of rotating speed is tested.Then,the transfer function model is identified through characteristic point method.Considering time-varying characteristics of the model parameters,the variables are fitted with frequency and speed as the independent variables,and the variable model of ultrasonic motor system is obtained,with consideration of the nonlinearity of ultrasonic motor system.The proposed model can be used in the design and analysis of the speed control system in ultrasonic motor. 展开更多
关键词 ultrasonic motor(USM) speed control characteristic point method
下载PDF
MOTION VELOCITY SMOOTH LINK IN HIGH SPEED MACHINING 被引量:9
19
作者 REN Kun FU Jianzhong CHEN Zichen 《Chinese Journal of Mechanical Engineering》 SCIE EI CAS CSCD 2007年第2期17-20,共4页
To deal with over-shooting and gouging in high speed machining, a novel approach for velocity smooth link is proposed. Considering discrete tool path, cubic spline curve fitting is used to find dangerous points, and a... To deal with over-shooting and gouging in high speed machining, a novel approach for velocity smooth link is proposed. Considering discrete tool path, cubic spline curve fitting is used to find dangerous points, and according to spatial geometric properties of tool path and the kinematics theory, maximum optimal velocities at dangerous points are obtained. Based on method of velocity control characteristics stored in control system, a fast algorithm for velocity smooth link is analyzed and formulated. On-line implementation results show that the proposed approach makes velocity changing more smoothly compared with traditional velocity control methods and improves productivity greatly. 展开更多
关键词 High speed machining Motion velocity link S-type control equation
下载PDF
FPGA Based Speed Control of SRM with Optimized Switching Angles by Self Tuning 被引量:1
20
作者 P. Saravanan R. Arumugam M. Senthil Kumaran 《Circuits and Systems》 2016年第8期1530-1545,共16页
The electromagnetic torque and speed in Switched Reluctance Motor (SRM) greatly depend on the excitation parametersi.e. turn-on angle, turn-off angle, dwell angle and magnitude of the phase currents of its phases... The electromagnetic torque and speed in Switched Reluctance Motor (SRM) greatly depend on the excitation parametersi.e. turn-on angle, turn-off angle, dwell angle and magnitude of the phase currents of its phases. At lower speeds, a change in the current contributes the torque requirement which can be achieved either by voltage control (pulse width modulation) or instantaneous current control techniques. At high speeds, due to high back EMF, the regulation of current is crucial and achieved with the control of switching angles of phases. This type of control is referred as average torque control, where the torque is averaged over one stroke (2π/N<sub>r</sub>). With constant dwell angle, advancing the phase angle influences the current into the phase winding at minimum inductance position. It has more time to get the current out of the phase winding before the rotor reaches the negative inductance slope. To maintain the speed of the motor at different load conditions, the turn-on and turn-off angles are adaptively varied. The change in dwell angle may be required where the turn-on and turn-off angle may not be sufficient to reach the required speed. In this paper, a new algorithm is proposed for self tuning of switching parameters of SRM. The proposed algorithm is simulated in MATLAB-Simulink and experimentally validated with Field Programmable Gated Array (FPGA) using MATLAB- system generator environment. 展开更多
关键词 Switched Reluctance Motor speed Control Optimized Switching Angles FPGA Nexys-4 MATLAB-System Generator
下载PDF
上一页 1 2 4 下一页 到第
使用帮助 返回顶部