Abstract: An operating frequency setting unit outputs an ultrasonic emitting command to a variable oscillator circuit in an operating frequency setting mode to cause a detector to emit an ultrasonic wave a plurality of times while varying the frequency thereof. At this time, emission or reflected waveform data are inputted via a band pass filter to detect a resonance frequency of a system including the piezoelectric element of the detector and an LPG tank on the basis of the difference between attenuation characteristics of waveforms. The detected resonance frequency is registered in a memory part as an operating frequency. Thus, it is possible to provide a general purpose ultrasonic sensor usable for any one of a plurality of vessels having any wall thickness and material when an object is detected while the detector is mounted on the outside face of a vessel.

Abstract: A push button switch provided with a multistage actuation interlock function is composed of a casing; a pair of push buttons arranged side by side passing through an upper lid of the casing; a plurality of movable bodies arranged movably in a direction perpendicular to a depression direction of the push buttons into the casing; a plurality of movable contacts attached to the movable bodies, respectively; and a plurality of fixed contacts arranged on a base portion of the casing in confronting positional relationship with respect to the movable contacts. When each of the push buttons is depressed at each of the multistage stroke, the movable body is shifted to bring the movable contact into contact with the fixed contact, respectively in sequence. Further, an interlock plate is interposed between and associated with the two push buttons in the casing.

Abstract: A system is provided for controlling a PWM inverter having three main circuit arms in which free-wheel diodes are connected in parallel to positive and negative side switching elements. The inverter takes three phase alternating current from an intermediate portion of the arms on the basis of a voltage supply of a direct current and an ON and OFF operation of the switching elements. The control system includes a circuit for detecting a zero-crossing point of the three phase alternating current. A wave generating circuit generates a PWM wave at every main circuit arm, which changes the level thereof corresponding to ON and OFF operation of the switching elements and is inverted to each other in the positive and negative sides. An on-delay circuit delays a level change timing from ON to OFF of the PWM wave for a short circuit preventing duration.