A milling machine has a frame, ground engaging tracks that support the frame, a first actuator connecting the frame to a first track of the ground engaging tracks and a second actuator connecting the frame to a second track from the ground engaging tracks. The milling machine has an orientation sensor that determines an orientation of the frame. The milling machine has a controller that operates the first and second actuators to raise or lower the frame. The controller determines the frame orientation using the orientation sensor. The controller also determines a velocity error between actuator velocities of the first and second actuators based on the frame orientation and a target orientation of the frame. The controller determines a control parameter for the second actuator based on the velocity error and operates the second actuator using the determined control parameter.

A valve device having a valve housing (10, 54, 94, 166) and fluid ports (16, 18) disposed therein, which are connected to or separated from one another in a fluid-conveying manner by means of a valve piston (22) displaceable along its longitudinal axis (24) in its various travel positions, and having at least one latching device (26, 28) for latching the valve piston (22) in at least one of its travel positions, wherein individual latching means (30, 38, 170) of said latching device, when it is actuated by an external force, take latching positions spatially different from one another, which is characterized in that, with respect to the longitudinal axis (24) of the valve piston (22), the individual latching means (30, 38, 170) of the latching device (26, 28) take positions differing from one another both in the axial and in the radial direction in the individual latching positions.

An electro-hydraulic servo valve, which has an adjustable null position and can be directly driven by the motor, is provided and includes four parts: a slide valve assembly, a two-dimensional motor, a magnet resetting-to-null mechanism, and a displacement sensor, and the four parts are set coaxially arranged. The slide valve assembly is a conventional two-dimensional servo valve structure. The two-dimensional motor may directly drive the valve core to rotate to further control a size of an opening opened by a valve. The magnet resetting-to-null mechanism is configured to reset the valve to the null position. The displacement sensor may monitor a position of the valve core in real-time and feedback signals to achieve closed-loop controlling.

A hydraulic system of a construction machine includes: control valves interposed between a variable displacement main pump and hydraulic actuators; and first solenoid proportional valves connected to pilot ports of the control valves. The hydraulic system further includes: a regulator that changes a displacement of the main pump; and a second solenoid proportional valve connected to an auxiliary pump by a primary pressure line, the second solenoid proportional valve outputting a secondary pressure to the regulator through a secondary pressure line. A switching valve is interposed between the auxiliary pump and the first solenoid proportional valves, and includes a pilot port that is connected to the secondary pressure line by a pilot line.

A hydraulic system of a construction machine includes: control valves interposed between a variable displacement main pump and hydraulic actuators; and first solenoid proportional valves connected to pilot ports of the control valves. The hydraulic system further includes: a regulator that changes a displacement of the main pump; and a second solenoid proportional valve connected to an auxiliary pump by a primary pressure line, the second solenoid proportional valve outputting a secondary pressure to the regulator through a secondary pressure line. A switching valve is interposed between the auxiliary pump and the first solenoid proportional valves, and includes a pilot port that is connected to the secondary pressure line by a pilot line.

A directional control valve includes: a housing having a cylindrical spool hole communicating with an inlet port letting a fluid from an external supply source at a predetermined pressure and an outlet port flowing the fluid to a working cylinder; a spool movable in the spool hole axially and changing an amount of flow of the fluid; a first solenoid driving section having a first needle coupled to or integral with a first end portion of the spool and driving the spool; and a second solenoid driving section having a second needle coupled to or integral with a second end portion of the spool and driving the spool, the directional control valve including a pressure equalization circuit equally applying a pressure to the end portions.

A stepper motor driven actuator system is provided. The system includes a stepper motor, a cam, and a gearbox system. The gearbox system operatively connects the stepper motor to the cam. The cam rotates in response to stepping of the stepper motor. The system also includes a valve having a control piston located therein. The control piston is configured to translate in response to rotation of the cam. The system further includes a rotary actuator. The rotary actuator is fluidly connected to the valve, and the rotary actuator is configured to rotate the cam in response to translation of the control piston.

A multi-control valve unit includes a housing with built-in spools that are parallel to each other. The housing includes pilot chambers formed therein, the pilot chambers corresponding to both ends of the spools. Solenoid proportional valves are mounted to the housing, such that the solenoid proportional valves are connected to the respective pilot chambers. A hydraulic pressure generator is mounted to the housing, such that the hydraulic pressure generator is connected to the solenoid proportional valves. The hydraulic pressure generator includes an electric motor and a pump.

A multi-control valve unit includes a housing with built-in spools that are parallel to each other. The housing includes pilot chambers formed therein, the pilot chambers corresponding to both ends of the spools. Solenoid proportional valves are mounted to the housing, such that the solenoid proportional valves are connected to the respective pilot chambers. A hydraulic pressure generator is mounted to the housing, such that the hydraulic pressure generator is connected to the solenoid proportional valves. The hydraulic pressure generator includes an electric motor and a pump.

A servo valve comprising: a fluid transfer valve assembly comprising a supply port and a control port (P.sub.A, P.sub.B); a moveable valve spool arranged to regulate flow of fluid from the supply port to the control port in response to a control signal; and a drive means configured to axially move the valve spool relative to the fluid transfer assembly in response to the control signal to regulate the fluid flow; wherein the drive means comprises a piezo actuator element mounted within and axially movable with the valve spool, the piezo actuator configured to extend axially in response to the control signal to cause corresponding axially movement of the spool.