An Improved Variable M/T Method Based on Speed Estimation for Optical Incremental Encoders
Optical incremental encoders are widely used for the speed measurements in motor servo systems due to low cost and high performance. But it is difficult to use the encoders when wide speed range, high accuracy and extremely short responding time are requi
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An Improved Variable M/T Method Based on Speed Estimation for Optical Incremental Encoders Hui Wang and Jian-tao Pu
Abstract Optical incremental encoders are widely used for the speed measurements in motor servo systems due to low cost and high performance. But it is difficult to use the encoders when wide speed range, high accuracy and extremely short responding time are required at the same time in high performance servo systems. In this paper, an improved variable M/T method is introduced. In this method, both encoder pulse and high frequency clock pulse are counted in a variable interval which ensures the high measurement accuracy in both high speed and low speed. By speed estimation algorithm, the rapid response can be obtained even at very low speed. Keywords Optical incremental encoder • Motor servo system • Speed measurement • Variable M/T method
Introduction Optical incremental encoders are widely used as speed sensors on closed-loop speed control systems. With high noise immunity, low maintenance, and low cost, they are the preferred method for obtaining motor velocity information and are generally considered to be superior to direct current tachometers (Ekekwe et al. 2008). Optical incremental encoders produce two sequences of pulses with a 90 phase shift, which are called quadrature encoded pulses. As the motor rotating, the direction can be determined by detecting which of the two sequences is the leading sequence and the H. Wang Institute of Automation, Chinese Academy of Sciences, Beijing, China Beijing Information Science & Technology University, Beijing, China J.-t. Pu (*) College of Automation, Beijing Union University, Beijing, China e-mail: [email protected] R. Dou (ed.), Proceedings of 2012 3rd International Asia Conference on Industrial Engineering and Management Innovation(IEMI2012), DOI 10.1007/978-3-642-33012-4_2, # Springer-Verlag Berlin Heidelberg 2013
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speed can be determined by the pulse frequency. The speed accuracy is limited by the quantized speed measurement of the encoder, i.e. it is limited by the number of slits on the encoder disk (Merry et al. 2010). But higher resolution is limited by the manufacturing capacity and cost. Therefore a quadrupler is usually used to improve the encoder resolution which generates a decoded clock with four times frequency of each input sequence. Counting the decoded clock during a constant sampling period can get the pulse frequency. However this method is not applicable at low speed since the pulses are not frequently produced and no pulse will be detected in some sampling periods (Lilit Kovudhikulrungsri and Takafumi Koseki 2006; Takafumi Koseki et al. 2010). An alternative way is to measure the time interval between two consecutive pulses (Lilit Kovudhikulrungsri and Takafumi Koseki 2006). The approach has high resolution at low speed but low resolution at high speed. Combination of these methods is proposed by Ohmae et al. in (1982). This method combines the advantages of each method with high accuracy in w
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