Linear wire encoder is one type of optical distance sensor. The linear wire encoder consists of an optical incremental encoder and a cable drum. When the optical incremental encoder shaft rotates, there is a corresponding pulse output. The discrimination of the direction of rotation and the increase or decrease the number of pulses are by virtue of the rear judgment circuit and counter. The starting point of the counting can be setting freely, and the infinite cumulative and measurement of multiple circles can be realized. When the number of pulses is fixed and the resolution needs to be increased, the original pulse number can be multiplied by using two signals of 90° phase difference A and B. The linear wire encoder is used in level control instrument, construction machinery and other related dimensional measurement and distance and position control, especially suitable for hydraulic cylinder control.