The available tools and methods for vibration and motion control serve well for the majority of existing needs. In many applications however, purely mechanical systems may be performance-limited, regardless of the sophistication of design. Methods that can change or adapt mechanical properties may increase performance and enable new applications. A combined capability in both vibration and motion control could allow the combination of two devices or subsystems into one. Such a device might be useful in a suspension or leveling system, or a repositioning system incorporated into a recoil damper. This research attempted to address requirements for combined motion and vibration control. To that end, we successfully designed and tested a concept device that served both functions utilizing smart materials.
Magnetorheological (MR) fluids provide a means for actively controlling the properties of dampers. Such fluids are typically comprised of medium viscosity oils with small ferrous particles mixed throughout. These particles align with external magnetic fields creating chains that block normal flow. In the presence of such a field the material behaves as a Bingham Plastic. No flow will occur below a threshold shear until an applied force of sufficient magnitude breaks the particle chains. Once these chains are broken, the fluid behaves in a typical Newtonian manner with a viscosity $\mu$.
While MR fluid alone is not sufficient for actuation, it can be used in combination with other components to create an actuator that retains an adjustable damping capability. The architecture of the new device was similar to typical MR dampers: a piston immersed in an MR fluid chamber. A current loop produced the necessary magnetic field across the fluid gap with the actuator housing providing the flux return path. The key to the new design lay in the addition of a second piston and intervening fluid chamber separated by a piezoelectric stack actuator. Provided the field was of sufficient magnitude, the particle chains would check flow through the gap and effectively lock the piston in place. By properly timing the actuation of the two coils and the piezoelectric stack, we could achieve clamp-extend-clamp-contract inchworm movement. Output force was delivered via expansion or contraction piezoelectric. This technique permitted both positive and negative output forces making it a truly push and pull actuator. The device as built required three separate power sources for the two piston coils and the piezoelectric stack with a microcontroller commanding the necessary stepping signals.
The damping characteristics of the device were typical of other MR dampers. Bingham plastic behavior was measured as a function of coil current. Likewise under no-load conditions, inchworm movement proceeded in the step like manner seen in similar device types. Blocked force output however, was lower than predicted by a factor of 10. Shear deformation of the “locked” MR volume was responsible for this deficiency. While the particle chains do check the fluid flow, the MR volume influenced by the magnetic field behaves like a viscoelastic under axial loads; they stretch a significant amount before they are broken. As a result, available output force is divided between the external load and the annular shaped “MR spring”.
Shrinking the gap would increase the effective stiffness of the volume as well as the total achievable locking force. However, larger viscous forces during piezo movement would prevent reasonable actuation velocities. Thus, the design shows exceptional promise, but more work will be required to increase the output power to more useful levels.