Control device for variable hydraulic machines

A control device for variable displacement hydraulic machines equipped with a servo system (4) connected to a swash plate so that the displacement volume of the hydraulic machine is infinitely variable, comprising:

• a sensor connected with an electronic control unit (12) for controlling the swash angle position of the swash plate, and
• an electrohydraulic convertor (25;
• 17;
• 18, 19;
• 10, 18;
• 17) for pressure controlling the servo system (4) connected to the electronic control unit (12) and the servo system (4) so as to form a closed loop control system, the electrohydraulic convertor having a movable spool therein and receiving an electrical input current from the electronic control unit (12) and having an output pressure to the servo system (4) which is proportional to the input current,
• a pilot pressure passage fluidly connecting the output pressure of the electrohydraulic converter with the spool so as to provide a hydraulic pilot pressure signal based upon the output pressure of the electrohydraulic converter,
• the electrohydraulic convertor receiving at an end of the spool therein the hydraulic pilot pressure signal based upon the output pressure of the electrohydraulic convertor whereby the pilot pressure signal urges the spool into a position which allows the output pressure to drain from the servo system back through the electrohydraulic convertor during an electrical failure affecting the electrohydraulic convertor and thereby allows the servo system to seek to a neutral position.

A method for the infinite control of the volume flow of a hydraulic machine whereby a servo system (4) is connected to a swash plate of the hydraulic machine;

• comprising:
• pressure controlling of the servo system (4) by means of an electrohydraulic convertor (25;17),
• conducting a feedback signal indicative of a swash angle position of the swash plate of the corresponding hydraulic machine from a sensor to an electronic control unit (12), controlling a swash angle position of the swash plate of the corresponding hydraulic machine by means of an electrical input current generated by the electronic control unit (12) and received by the electrohydraulic convertor (25;17) such that the electrohydraulic convertor (25;
• 17) has an output pressure proportional to the input current,
• conducting a hydraulic pilot pressure signal from the output pressure of the electrohydraulic convertor to an end of a spool in the electrohydraulic convertor so as to urge the spool into a position which allows the output pressure to drain from the servo system back through the electrohydraulic convertor during an electrical failure affecting the electrohydraulic convertor,
• whereby the servo system seeks a neutral position during an electrical failure.