Modular hydraulic servos can be described as those that include an individual hydraulic power source for each individual actuator. These are packaged in such a manner that the servo can be installed or removed readily by detaching a few bolts and an electrical connector. The resultant high density package retains all the advantages of the superior performance of a hydraulic servo and acquiries the simplicity, ease of maintenance and reliability of an all-electric servo. Modular hydraulic servos have been developed primarily for short duration missile control but have merit for continuous duty aircraft flight control.
Since each modular servo has its own hydraulic supply, the designer is free of multi-channel cross-talk that has previously necessitated the use of a constant-pressure hydraulic system. Multi-pressure, multi-flow and other variations are now feasible. In addition to linear operation, bi-stable and tri-stable operation can be used to advantage.
The high density packaging requirements have resulted in various design techniques. Submerging all operating components within a reservoir is a particularly attractive approach. Access to remove components in such a system is thus discouraged and contamination control is greatly enhanced. The modular servo is maintained by replacement with a spare rather than serviced in the field. Modular servos generally incorporate load mounting shafts, bearings, and supports as part of the package so that the servo designer maintains full responsibility for the various load parameters, e.g., backlash, friction, elasticity, and others, as they intimately enter into servo performance. This responsibility normally is shared by various mechanical electrical, and structural engineers with the result that many desirable servo techniques are often rejected prematurely.