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A brushless DC motor requires a rotor position sensor for speed and current control, but the sensor will increase the cost and size of motor and limit the industrial applications. In some specific applications, a brushless DC motor without a sensor is needed, which raises a problem of rotor position detection and speed control. In this paper, a new estimating method is proposed using an improved sliding mode observer, in which the discontinuous control is replaced by using the sigmoid function in order to reduce the chattering problem commonly found in the conventional sliding mode observer, then a phase lock loop based algorithm is adopted to calculate the rotor angular and velocity. The simulation results demonstrate that this method can achieve a good performance both in steady and transient operating.