【正文】 and the innerloop is the traditional PI current controller. It has the integrative merits of the linear and nonlinear control methods. Moreover, a load torque observer is advanced to enhance the antidisturb ability of the servo system when the brushless DC motor running with motor parameters change and/or external load variation. The proposed control method can obtain the excellent position tracking performance as well as the good speed response ability. Simulation and experiment results have verified the validity and effectiveness of this control scheme.Keywords Brushless DC motor。 observerI. INTRODUCTIONSince the traditional brush be taken place by the electrical brush and the permanent rotor substituted for excitation winding, the Brushless DC motor (BLDCM) has less volume, light weight, less inertia and high precision position control and easy to speed regulation. To be as a kind of servo mechanical organism, the BLDCM can improve the system reliability and reduce the electrical sparkle. The ability of the brushless DC motor applying to servo environment has been analyzed and its advantage and disadvantage was pared to PMSM from the view of power density, torque ripple andparameters sensitivity in [1]. The servo control algorithm of BLDCM can be divided into linear PID control and nonlinear control. The traditional PID control algorithm is easy to be implemented, but it cannot be apply into high dynamic performance area [26]. The nonovershoot BLDCM digital position control system was achieved in [7] based on the fuzzy method. The variable structure control has been implemented in the position servo of BLDCM control system, and it has been verified good performance in the application of directdrive robotic arm in [8]. Generally, the nonlinear control method, such as fuzzy and variable structure, can make the system obtain high quality dynamic and stable performance and has good robustness. Of course, it’s more plex and more difficult to be carried out than the linear control one. A new servo control method based on second discrete smooth trajectory filter (STF) is proposed in this paper, it is a production of the bination of linear and nonlinear control method. Using the nonlinear filter to replace of the position loop and speed loop in the BLDCM control system and design the load observer simultaneously, the good position servo trajectory performance is obtained. The proposed control law is easy to be performed in DSP and the simulation results has verified its validity and effectiveness.II. SECOND DISCRETE STFThe design of the STF is mainly based on variable structure (VS) control techniques proposed by Utkin [9]. With a proper choice of a sliding surface and a boundary layer, it is possible to guarantee good performances both in transient and in the final steadystate conditions. The kernel of this controller is built around a nonlinear statevariable filter, which ensures that bounded first and second output derivatives are available to generate feedforward control actions. This control scheme is especially suitable for the two or higher order motion control systems, it can provide a reference input which satisfies the dynamic and stable performance of the whole system. In [10], the modified continuous second order smooth trajectory was presented and the short time trajectory stable problem was tackled. Based on the continuous time results, Zanasi has proposed a discrete time nonlinear control law that guarantees the minimum time global stabilization of a chain of two discrete integrators with bounded input [11]. The smooth trajectory filter was applied into PMSM servo motion system and accurate simulation and experiment results were obtained in [12]. The designed modified second smooth filter structure in this paper is shown as Fig. 1. The two discrete integrators in Fig. 1 has different structure, this choice has been made in order to guarantee the same dynamic characteristic as the continuous STF. This controller structure is similar with which proposed in [11], the only difference between them is that they have diverse nonlinear feedback controller.Assume T is the sample period of controller, and let, then can be calculated as (1)So the dynamic state model of second order smooth trajectory filter C3 in Fig. 1 can be depicted as follow: (2)To do the vector transform for, and let (3) (4)Then (5)Where .Thus, equation (3), (4) and the follow equation constitute the controller C3 in Fig. 1. (6)Whereis the tracking error, is the velocity error, ,is the discretetime derivative of signal rn and σn =0 is the sliding mode surface.Fig. 1 Second discrete nonlinear smooth tracking filterAs is seen from the initial controller in [11] and the improved controller in this paper, both them have the same bound limitation of the derivatives of input , . However, the effect of the second derivative is not considered in the former under the precondition of the first derivative being piecewise constant, which limits the types of signal as those meeting the demand, such as square wave, slope wave, the sawtoothed wave and so on. On the contrast, the latter design proposed by this paper can be available to the broader range of signal than the former, only with the bound limitation of the derivatives of input as above, such as sine wave which can not fit for the former.III. MODEL OF BLDCMIn the ideal condition, the three phase voltage equations in a matrix form for the BLDCM are represented as (7)Where, ua, ub and uc are stator phase voltage。 L is the selfinductance。 r39。 ke is the back electromotive force constant。 L39。 and D is the differential operator.Further, the above equation can be simplified as (8)Where u is the terminal voltage。 ia, ib and ic are the line currents。 PI controller。她淵博的學(xué)識(shí)、精湛的學(xué)術(shù)、嚴(yán)謹(jǐn)?shù)淖黠L(fēng)、創(chuàng)新的思想熏陶著我，深刻的影響著我的學(xué)習(xí)和生活，必將使我終生受益！衷心的感謝劉金鳳老師對(duì)我的諄諄教誨和悉心關(guān)懷！四年來(lái)，我收獲良多，因?yàn)橛泻芏嗳私o予我無(wú)私的幫