收费全文 | 78320篇 |
免费 | 7540篇 |
国内免费 | 4175篇 |
电工技术 | 4573篇 |
技术理论 | 3篇 |
综合类 | 5585篇 |
化学工业 | 12812篇 |
金属工艺 | 4617篇 |
机械仪表 | 5143篇 |
建筑科学 | 5770篇 |
矿业工程 | 2264篇 |
能源动力 | 1938篇 |
轻工业 | 7020篇 |
水利工程 | 1645篇 |
石油天然气 | 3977篇 |
武器工业 | 674篇 |
无线电 | 8837篇 |
一般工业技术 | 9179篇 |
冶金工业 | 3406篇 |
原子能技术 | 935篇 |
自动化技术 | 11657篇 |
2024年 | 315篇 |
2023年 | 1257篇 |
2022年 | 2391篇 |
2021年 | 3216篇 |
2020年 | 2492篇 |
2019年 | 2062篇 |
2018年 | 2437篇 |
2017年 | 2496篇 |
2016年 | 2396篇 |
2015年 | 3231篇 |
2014年 | 4067篇 |
2013年 | 5010篇 |
2012年 | 5619篇 |
2011年 | 6071篇 |
2010年 | 5495篇 |
2009年 | 5401篇 |
2008年 | 5130篇 |
2007年 | 4878篇 |
2006年 | 4702篇 |
2005年 | 3892篇 |
2004年 | 2701篇 |
2003年 | 2229篇 |
2002年 | 2066篇 |
2001年 | 1732篇 |
2000年 | 1505篇 |
1999年 | 1438篇 |
1998年 | 1073篇 |
1997年 | 939篇 |
1996年 | 822篇 |
1995年 | 666篇 |
1994年 | 527篇 |
1993年 | 415篇 |
1992年 | 304篇 |
1991年 | 242篇 |
1990年 | 177篇 |
1989年 | 149篇 |
1988年 | 114篇 |
1987年 | 82篇 |
1986年 | 78篇 |
1985年 | 36篇 |
1984年 | 29篇 |
1983年 | 23篇 |
1982年 | 32篇 |
1981年 | 14篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 13篇 |
1975年 | 9篇 |
This paper presents a position control strategy for a planar active-passive-active (APA) underactuated manipulator with second-order nonholonomic characteristics. According to the structural characteristics of the planar APA system, we divide the system into two parts: a planar virtual Pendubot (PVP) and a planar virtual Acrobot (PVA). For the PVP, we mainly fulfill the target angle of the first link, which is calculated through the geometry method, and make the system stable. In this stage, via keeping the states of the third link being zero, the system is reduced to the PVP. Meanwhile, we design an open-loop control law based on the nilpotent approximation (NA) model of the PVP to make the second link stable and the first link stabilize at its target angle. Then, the planar APA system is reduced to a PVA with all links’ angular velocities being zero. For the PVA, we mainly realize the other two links’ target angles obtained via the particle swarm optimization (PSO) algorithm. Thus, the control objective of the planar APA system is achieved. Finally, above control strategy is verified by simulation results.
相似文献Unlike a fully-actuated manipulator, the position-posture control of a planar underactuated manipulator (PUM) is more difficult, but the research on it is significant due to the wide practical applications. The existing control methods consider no external disturbance and are involved in the staged control idea, bringing the problems of nonsmooth control torque and time-consuming. A novel one-stage control approach is proposed in this paper for the position-posture control of a three-link PUM with the first free joint under the external disturbance. By analyzing the coupling relationship between its active joints and free joint, the position-posture control is transformed into the trajectory tracking control. Unlike the general trajectory planning, the trajectories of the active joints are planned to include several parameters. Meanwhile, the parameters are solved using a chaos particle swarm optimization algorithm to guarantee that all joint angles can reach to their desired angles. Then, to obtain the high trajectory tracking accuracy at every moment under the external disturbance, the nonlinear disturbance observer is constructed and a nonlinear fast terminal sliding mode tracking controller is designed. Finally, the feasibility and superiority of this strategy are verified via two simulations.
相似文献Three phase brushless DC motor has the characteristics of high power density, simple structure and excellent speed regulation performance, which is widely used in the field of electric drive. Open-end winding brushless DC motor has no neutral point, and each phase winding is electrically isolated, which can realize the independent control of each phase winding. Compared with Y-connected brushless DC motor, the open-end winding brushless DC motor has a wider range of speed regulation under the same voltage, and has certain fault-tolerant performance, which is suitable for the application of low voltage and large current. Compared with the multiphase fault-tolerant motor, open-end winding brushless DC motor has simpler structure and higher winding utilization rate. However, the open-end winding brushless DC motor still has torque ripple, among which the cogging torque is related to the motor body structure, while commutation torque ripple is affected by the mutual inductance of the winding and the current freewheeling loop. The commutation process of the open-winding brushless DC motor is analyzed, and it is concluded that the current change rates of the turn-off phase and turn-on phase are equal under the ideal back electromotive force(back-EMF), which can ensure that the non-commutation phase current and the electromagnetic torque kept constant. The overlapping commutation with phase current closed-loop method is proposed to suppress the commutation torque ripple of open-end winding BLDC motor, and the applied voltage on turn-off phase and delayed time of turn-off phase in overlapping commutation are derived, the sliding mode observer is used to calculate the electromagnetic torque. MATLAB simulation and experimental results verify the effectiveness of this method.
相似文献