A hybrid hydraulic actuation system is proposed as an active pitch link for

rotorcraft applications. Such an active pitch link can be used to implement Individual

Blade Control (IBC) techniques for vibration and noise reduction, in addition to

providing primary control for the helicopter. Conventional technologies like electric

motors and hydraulic actuators have major disadvantages when it come to applications

on a rotating environment. Centralized hydraulic system require the use of

mechanically complex hydraulic slip rings and electric motors have high precision

mechanical moving parts that make them unattractive in application with high centrifugal

load. The high energy density of smart materials can be used to design

hydraulic actuators in a compact package. MagnetoRheological (MR) fluids can be

used as the working fluid in such a hybrid hydraulic actuation system to implement

a valving system with no moving parts. Thus, such an actuation system can be

theoretically well-suited for application in a rotating environment.

To develop an actuation system based on an active material stack and MR

fluidic valves, a fundamental understanding of the hydraulic circuit is essential. In order to address this issue, a theoretical model was developed to understand the

effect of pumping chamber geometry on the pressure losses in the pumping chamber.

Three dimensional analytical models were developed for steady and unsteady

flow and the results were correlated to results obtained from Computation Fluid

Dynamic simulation of fluid flow inside the pumping chamber. Fundamental understanding

regarding the pressure losses in a pumping chamber are obtained from

the modeling process. Vortices that form in the pumping chamber (during intake)

and the discharge tube (during discharge) are identified as a major cause of pressure

loss in the chamber. The role of vortices during dynamic operation is also captured

through a frequency domain model.

Extensive experimental studies were conducted on a hybrid hydraulic system

driven by a pump (actuated by a 2" long and 1/4" diameter Terfenol-D rod) and a

Wheatstone bridge network of MR fluidic valves. The Wheatstone bridge network

is used to provide bi-directionality to the load. Through a variety of experimental

studies, the main performance metrics of the actuation system, like output power,

blocked force, maximum no-load velocity and efficiency, are obtained. The actuation

system exhibits a blocked force of 30 N and a maximum no-load velocity of 50

mm/s. Extensive bi-directional tests were also done for cases of no-load, inertial

load and spring load to establish the frequency bandwidth of the actuator. The

actuation system can output a stroke of 9 mm at an output actuator frequency of

4 Hz. An analytical model was developed to predict the performance of the hybrid

hydraulic actuation system. A state space representation of the system was derived

using equations derived from the control volume considerations. The results of the analytical model show that the model predicts the frequency peak of the system to

within 20 Hz of the actual resonance frequency.

In the third part of this dissertation, the effectiveness of the hybrid hydraulic

actuation system is evaluated in a rotating environment. A piezoelectric stack that

is driven by three PI-804.10 stacks was attached at the end of a spin bar. After

balancing the spin bar using a counterweight, the spin bar is spun to an RPM of

300. This simulates a centrifugal loading of 400 g, which is slightly higher than the

full-scale centrifugal loads experienced by a pitch link on a UH-60. The performance

of the actuator was measured in terms of velocity of an output cylinder shaft. Since

some deterioration of performance was expected at 300 RPM, the output cylinder

was redesigned to include roller bearings to support the excess force. Through no

load and load tests, the effectiveness of the current hybrid actuation system design

was shown as the performance of the system did not deteriorate in performance with

greater centrifugal acceleration.