Patent ID: 6296081
Filing Date: 2001-10-02
Classification: B66F,G01S

Abstract:
A lift cylinder comprising:a cylinder body, the cylinder body being occupied by oil;a piston that is moved axially in the cylinder body by oil pressure within a limited range of movement;a transducer device located near one end of the cylinder body out of the range of movement of the piston, wherein, in response to an electric signal, the transducer device transmits an ultrasonic signal toward a reflective surface that moves integrally with the piston, and the transducer device receives an ultrasonic signal reflected from the reflective surface and produces an electric signal corresponding to the received ultrasonic signal;a temperature sensor located in the cylinder body to detect the temperature of the oil; anda controller for judging whether the piston is fully retracted, wherein the controller derives a current threshold value from the currently detected oil temperature based on a predetermined relationship between oil temperatures and threshold values, and wherein the controller produces a measured value that represents the time period from when an ultrasonic signal is sent to when a reflection of the ultrasonic signal is received by the transducer device and wherein the controller compares the current threshold value to the measured value to determine whether the piston is fully retracted.