Hydraulic control apparatus

DETAILED DESCRIPTION It is therefore an object of the present invention to provide a hydraulic control apparatus in which a command signal and an AC signal having a dither frequency are added to each other by an adder, a deviation signal corresponding to the difference between a signal outputted from the adder and a feedback signal representing pulse duration of exciting current of a solenoid is pulse width-modulated by a carrier wave having a constant frequency higher than the maximum frequency of the command signal, and the pulse width-modulated signal is supplied to the solenoid as exciting current, whereby the spool provided in a solenoid controlled valve to be controlled by the solenoid can be prevented from being hydraulically locked and a hydraulic system including the solenoid controlled valves can be controlled with a small amount of phase delay and a fast response and a high gain.
In order to achieve the object, a hydraulic control apparatus according to one preferred embodiment of the present invention has a switching element which is changed over by a control signal inputted to an input terminal thereof; and a solenoid controlled valve having a spool actuated by a DC solenoid to be driven by exciting current whose pulse duration has been controlled by the switching element, and is characterized by further comprising an adder for adding a command signal and an AC signal having a dither frequency to each other; an oscillator for generating a signal having a constant frequency higher than the maximum frequency of the command signal; a subtractor for subtracting a signal representing pulse duration of the exciting current and inputted through a low-pass filter from a signal outputted from the adder; and a comparator for comparing a signal outputted from the subtractor with a signal outputted from the oscillator, outputting a pulse width-modulated control signal to the input terminal of the switching element so as to control the ON and OFF of the switching element.
According to the hydraulic control apparatus, the adder adds the command signal and the AC signal having a dither frequency to each other, and the signal produced by the addition is inputted to the subtractor which determines the deviation between the signal generated by the addition and the feedback signal representing pulse duration of the exciting circuit of the solenoid and inputted through the low-pass filter