Hydraulic control system

In a hydraulic system including a hydraulic drive unit for moving a load, said drive unit being connected to a hydraulic circuit through which hydraulic fluid flows from and back to a fluid reservoir via a pressure fluid supply means under the control of flow-control valve means having a main control plunger, an electrically operated means to position said main control plunger, a flow meter having a displaceable measuring element to sense the flow of said hydraulic fluid, an electric displacement measuring means connected to the flow meter so as to detect the displacement amplitude of said measuring element and to convert said amplitude of said measuring element into an electric actual-value signal proportional to the existing fluid flow, means for sending an adjustable electric desired-value signal, electric comparing means to compare an input of said actual-value signal with an input of said desired-value signal in order to provide an output difference signal and means connecting the output of said electric comparing means to said electrically operated positioning means in order to position the main control plunger of said flow-control valve means in response to the difference signal,

• the improvement for regulating and maintaining a substantially constant velocity of movement of said hydraulic drive unit, irrespective of disturbances including fluctuations in the pressure of the fluid supply, which comprises:
• a flow meter arranged in said hydraulic circuit, said flow meter having a housing of constant inner diameter which encloses at least one pressure chamber around a longitudinal flow axis with radially disposed and axially displaced inlet ports and an outlet port to conduct the hydraulic fluid therethrough;
• and
• a measuring element of said flow meter having sensor means which are axially movable in said housing between said biased spring means such that said sensor means, in the rest position, cover a radially disposed outlet port while the inlet ports are disposed outside the range of movement of said sensor means on either side of said outlet port.