Tri State Hydraulics

3 Closed-Loop Servo...

...Valve Technology Credited to Moog

The Moog Serve Valve was an innovation introduced by William C. Moog with a patent in 1951.  This servo valve is sometimes called a Mechanical Feedback (MFB) Servo Valve to differentiate if from some valves that were developed later with on-board electronic feedback mechanisms.  Prior to 1951, hydraulic valves were primarily open loop systems where the positioning of the spool was at best determined through empirical results in the laboratory under ideal conditions.  However, in practice the controlled variable-pressure hydraulic fluid will have some variation over time leading to imprecise positioning of the spool in the chamber.  The servo valve as an innovation that revolutionized the accuracy of industrial machines relied upon the introduction of the feedback mechanism used in the early hydraulic servo valve.  The role of the feedback mechanism is to precisely determine the position of the spool in the valve and signal the stopping of the spool at a position that is proportional to the electric input of the valve.

The Nozzle Flapper of the Moog Servo Valve features a continuous flow of a small amount of fluid through two small nozzles facing each other.  Between these jets is the flapper wire that is connected to the torque motor.  The control sends an electrical control signal to the torque motor, which in turn controls the opening of the nozzle-flapper interface to regulate the flow of the controlled variable-pressure hydraulic fluid from the two nozzles.  The pressure from the hydraulic fluid will be directed to one side of a sliding valve referred to as the spool.


Figure 3 - Moog G761 Series Servo Valve Cutaway showing Ball-in-Hole Technology (See Section 6)

Closing the loop between the spool and the control algorithm incorporates the servo control that effectively revolutionized the industry.

The spool is precisely machined to fit in a cylindrical chamber where it is displaced by the controlled variable-pressure hydraulic fluid.  The are orifices in the cylindrical chamber that allow high-pressure hydraulic fluid to flow directly into the hydraulic cylinder.  Accurately controlling the position of the spool in the chamber is critical to the precise delivery of high-pressure fluid to the cylinder.

The feedback mechanism is a cantilever spring with a spherical ball bonded to the end.  The ball is in direct contact with the spool in the chamber and is deflected as the spool moves transversally in the cylinder.  The position displacement of the spool provides the position feedback for the servo valve control algorithm.  The control algorithm closes the position loop between the spool and valve flapper providing hydraulic equipment builders with precision control.

Servo valve vendors publish system specifications from the command input and output response.  Each servo valve model is designed to achieve a specific performance criteria in terms of flow/pressure gain, frequency response, and precision while being subjected to wide spectrum of environmental and temperature ranges.  This is aboutas far as most design engineers ordinarily go to make a product selection.  For the engineer to asses that these performance specifications can be sustained throughout the life expectancy of the servo valve it is valuable to understand which components in the servo valve control loop are critical to early fatigue.  Early fatigue in the internal servo valve components generally do not exhibit any external signs of degradation such as leakage. However, early fatigue definitively affects the dynamic performance of the servo valve.  Once the servo valve is mounted in the equipment, most equipment operators will not be able to visually see any performance changes unless the system fails catastrophically.


PUMPS, VALVES & MOTORS MOOG Valves Design of Long Life... 3 Closed-Loop Servo...