A comparison of stack load-line (including blocked force and free displacement) as well as dynamic response of two
single crystal PMN-32%PT stacks is provided in this study. The first stack is a 7mm diameter by 0.5mm thickness 60
layer single crystal stack while the second stack is a 6mm diameter by 0.3mm thickness 100 layer single crystal stack.
Blocked force and free displacement measurements were both performed under DC driving conditions. Free
displacement measurements showed that under 500V driving conditions displacements approaching 87&mgr;m (~2500ppm)
and 48&mgr;m (~1450ppm) were obtained for the 6mm and 7mm diameter stacks, respectively. Experimental blocked force
measurements correlated well with theoretical predictions with experimental values approaching 709N and 685N for the
6mm and 7mm diameter stacks, respectively. The error between the theoretical predictions and experimental values was
attributed to the linear load line assumption in the theoretical model whereas the stack stiffness is dependent upon the
applied force. Dynamic measurements performed under a pre-stress of 4MPa indicated an increase in the strain at
frequencies above 500Hz for driving frequencies up to 1000Hz. This was unexpected as the PMN stack resonance was
calculated to be on the order of several kHz.
Smart materials' ability to deliver large block forces in a small package while operating at high frequencies makes them extremely attractive for converting electrical to mechanical power. This has led to the development of hybrid actuators consisting of co-located
smart material actuated pumps and hydraulic cylinders that are connected by a set of fast-acting valves. The overall success of the hybrid concept hinges on the effectiveness of the coupling between the smart material and the fluid. This, in turn, is strongly dependent on the resistance to fluid flow in the device. This paper presents results from three-dimensional unsteady simulations of fluid flow in the pumping chamber of a prototype hybrid actuator powered by a piezo-electric stack. The results show that the forces
associated with moving the fluid into and out of the pumping chamber
already exceed 10% of the piezo stack blocked force at relatively low frequencies ~120 Hz and approach 40% of the blocked force at 800 Hz. This reduces the amplitude of the piston motion in such a way that the volume flow rate remains approximately constant above operating frequencies of 500 Hz while the efficiency of the pump decreases rapidly.
Smart materials' ability to deliver large block forces in a small package while operating at high frequencies makes them extremely
attractive for converting electrical to mechanical power. This has led to the development of hybrid actuators consisting of co-located
smart material actuated pumps and hydraulic cylinders that are connected by a set of fast-acting valves. The overall success of the hybrid concept hinges on the effectiveness of the coupling between the smart material and the fluid. This, in turn, is strongly dependent on the resistance to fluid flow in the device. This paper presents the results of two and three dimensional simulations of fluid flow in a prototpype hybrid actuator being developed for aerospace applications. The steady simulations show that losses in the device result primarily from three dimensional effects and that two dimensional approaches can underestimate losses by approximately a factor of 40. The effects of varying design parameters like the pumping chamber height, discharge port location, and discharge port chamfer are also explored and are found to have significant impacts on performance. Three dimensional, unsteady simulations demonstrate how resistance to fluid flow in the pump reduces the amplitude of the piezo displacement and thus limits the flow rate of the device.
Magnetorheological (MR) fluids can be used in a variety of smart semi-active systems. MR dampers have especially great potential to mitigate environmentally induced vibration and shocks. MR fluid can be used effectively in valve networks to control the flow from a hydraulic source to enable a fully active actuator. These devices are simple, have few moving parts and can be easily miniaturized to provide a compact, high energy density pressure source. The present study describes a prototype MR-piezo hybrid actuator that combines the piezo-pump and MR valve actuator concepts, resulting in a self-contained hydraulic actuation device without active electro-mechanical valves. Durability and miniaturization of the hybrid device are major advantages due to its low part count and few moving parts. An additional advantage is the ability to use the MR valve network in the actuator to achieve controllable damping. The design, construction and testing of a prototype MR-piezo hybrid actuator is described. The performance and efficiency of the device is derived using ideal, biviscous and Bingham-plastic representations of MR fluid behavior. Unidirectional performance, or constant velocity actuator shaft motion, is assessed analytically and compared to experimental data. A experimental assessment of the magnetorheological birectional flow control capability is also provided.
Dampers based on electrorheological (ER) and magnetorheolgical (MR) fluids can be analyzed under assumptions of quasi-steady, fully developed flow behavior. Models that have been used to characterize ER and MR dampers include the Bingham-plastic, the Herschel-Bulkley and biviscous models. In the Bingham-plastic and the Herschel-Bulkley models, the fluid exhibits rigid behavior in the preyield flow region. The difference between these two models lie in the modeling of the postyield behavior. In the case of the Bingham-plastic model, the postyield behavior is such that the shear stress is proportional to the shear rate. In contrast, the Herschel-Bulkley model assumes that the shear stress is proportional to a power law of the shearrate. In the biciscous model, the relationship between the shear stres and shear rate is linear in both the preyield and postyield regions with constant values of viscosities for the two regions. However, the preyield flow behavior exhibits a much high viscosity than that in the postyield. In the propose model, the assumption of preyield rigid behavior within the Herschel-Bulkley model has been relaxed while the postyield relationship based on the power law has been retained. Here the fluid undergoes Newtonian preyield viscous flow and has a non-Newtonian postyield behavior. Based on this model, we have analyzed the performance of a rectangular duct ER or MR valve. Typical results include shear stress and velocity profiles across the valve gap, equivalent damping and damping coefficients.
Magnetorheological(MR) fluids are suspensions of magnetic particles in a carrier fluid. The rheological properties of MR fluids undergo changes on application of magnetic field. MR fluids made using nanometer sized soft magnetic iron particles were studied for their benefits vis-a-vis flow characteristics and settling properties. Three kinds of MR fluids were synthesized using the microwave process (1) 30 micrometers sized iron particles (2) 26.5 nm sized iron particle (3) a mixture of micron and nanometer sized iron particles . The fluids were suspensions of micron, nanometer as well as a mixture of nanometer and micron sized powders. Standard static and dynamic yield stress measurements were performed using a parallel disc oscillatory rheometer. The flow properties of these fluids were then characterized using the Bingham-Plastic, Eyring-Plastic and the Herschel-Buckley models. This paper investigates the rheological properties of these fluids and assesses their advantages and disadvantages. The pure micron powder yielded an MR fluid with highest yield stress, but had the most rapid settling. The nano sized powders overcame the problem of settling because of the predominance of thermodynamic forces at that scale, but they yielded a considerably lower yield stresses. A hybrid combination of micron and nano sized powders was an effective compromise to exploit the high yield stress provided by the micron powders and the self dispersing properties of the nano sized powders. We characterized these mixtures using existing rheological models. A key conclusion is that it may be possible to synthesize MR fluids with significant yield stress, while mitigating settling through the use of nanoscale powders.