The D633 and D634 Series are Direct Drive Valves (DDV) with electric closed loop spool position control.

These valves are throttle valves for 3-, 4-, and 2x2-way applications. They are suitable for electrohydraulic position, velocity, pressure, or force control systems including those with high dynamic response requirements.

The spool drive device is a permanent magnet linear force motor which can actively stroke the spool from its spring centered position in both directions. This is an advantage compared with proportional solenoids with one force direction only. The closed loop spool position electronics and pulse width modulated (PWM) drive electronics are integrated into the valve.

The integrated electronics of the valves is a new development featuring SMD technology with pulse width modulated (PWM) current output stage and requires a 24 VDC power supply.

• Directly driven by a permanent magnet linear force motor with high force level
• No pilot oil flow required
• Pressure independent dynamic performance
• Low hysteresis and low threshold
• Low current consumption at and near hydraulic null
• Standardized spool position monitoring signal with low residual ripple
• Electric null adjust
• With loss of supply voltage, or broken cable, or emergency stop the spool returns to its spring centered position without passing a load move position.

The position control loop for the spool with position transducer and linear force motor is closed by the integrated electronics. An electric signal corresponding to the desired spool position is applied to the integrated electronics and produces a pulse width modulated (PWM) current to drive the linear force motor. An oscillator excites the spool position transducer (LVDT) producing an electric signal proportional to spool position.

The demodulated spool position signal is compared with the command signal and the resulting spool position error causes current in the force motor coil until the spool has moved to its commanded position, and the spool position error is reduced to zero. The resulting spool position is thus proportional to the command signal.

The linear force motor is a permanent magnet differential motor. The permanent magnets provide part of the required magnetic force. For the linear force motor the current needed is considerably lower than would be required for a comparable proportional solenoid. The linear force motor has a neutral mid-position from which it generates force and stroke in both directions. Force and stroke are proportional to current.

High spring stiffness and resulting centering force plus external forces (i.e. flow forces, friction forces due to contamination) must be overcome during out-stroking. During backstroking to center position the spring force adds to the motor force and provides additional spool driving force which makes the valve very less contamination sensitive. The linear force motor needs very low current in the spring centered position.

Proportional solenoid systems require for the same function two solenoids with more cabling. Another solution uses a single solenoid, working against a spring. In case of current loss in the solenoid, the spring drives the spool to the end position by passing through a fully open position. This can lead to uncontrolled load movements.