To construct a control system without position sensors for PMSM with low cost and high reliability,the pole position detection techniques of PMSM based on SMO were studied.First,a SMO based on sliding mode control and vector control was designed to estimate the back EMF of motor,and then a position detected system based on PLL was constructed.Results of experiment and simulation show that the designed SMO can accurately estimate the back EMF,but chattering at high frequency will occur.However,the PLL based system can effectively reduce the effect of chattering and improve the precision of position detection.