Precision moves with pint-sized motors

Innovative actuators and stages based on piezoelectric materials give superfine motion in a compact package. A typical use for PZT actuators is in the precision positioning of microscope objectives. The MIPOS 500 stage (top) from Piezosystem Jena Inc. is one example of a device designed for this application.-It travels up to 500 m and has a stiffness of 0.27 N/ m. General-purpose PZT actuators generally come in a stainless-steel case, but Nano-P-Series devices from Mad City Labs Inc. (bottom) are Invar, a low thermal-expansion alloy as strong as stainless steel. The coefficient of thermal expansion (CTE) of Invar is 23 times smaller than that of aluminum. This gives the Nano-P devices thermal stability and minimizes positioning errors from thermal drift. Range of motion can span tenths of nanometers to tens of micrometers depending on the model. The typical PZT stack contains thin metal sheets between discs of PZT ceramic. The sheets serve as electrical contacts. PZT doesn't handle pulling forces well, so actuators typically incorporate a preload. Most manufacturers encapsulate the stack in a polymer to guard against contamination, though actuator maker PI has devised a ceramic coating it says better withstands humidity. PZT stack actuators can only tolerate axial stress, so they may use ball or flexible tips to help avoid tilting and shearing forces. PZT motors from Nanomotion Inc. wiggle like an inch worm as they move loads. The stack has a hole through the center and rides on a shaft. The stack expands, a clamp on the top locks, the stack contracts, and the clamp on the bottom locks while the top clamp lets go. PZT is good for more than just micrometerscale movements when it is built into linear motors. Motors from EDO Corp. use PZT stacks to push objects at top speeds of 300 mm/sec through a sequence of events depicted here. The action end of the motor consists of a shoe supported on two ends by PZT stacks. The device drives objects by driving one of the stacks on and off, forcing it to lengthen and contract, thus pushing the shoe against the object surface on each cycle. To push the object in the opposite direction, the motor applies a drive signal to the PZT