Machine Design

Tiny And Precise

Carl A. Nardell
Michigan Aerospace Corp.
Ann Arbor, Mich.

Force Motor PZT measures 44 mm long and 15 mm in diameter. A piece of Invar in the load path elastically stretches during motor operation. The thin-walled section is 1.15-mm thick and acts as a stiff spring. The PZT pushes against the body and is constrained by the 8-mm nut, stretching the thinwalled member. The Force Motor can travel up to 4 mm and has a tuning accuracy of 0.04 nm.

A PZT motor is driven by an analog servo using proportional and integral feedback. The microcontroller module accepts data from the telemetry module and can accept data from another computer. The microcontroller controls both the command module and stepper motor module.

The red and blue graphs illustrate the performance of a PZT (feedback capacitance and displacement, respectively) without the controller in the loop. The green graph illustrates the performance of the ForceMotor with the capacitive feedback controller implemented.

Miniaturized electronics, optics, and laboratory work demands motors that can precisely move submicron distances. Piezoelectric transducer (PZT) motors provide that sort of fine control, with resolutions as small as an Angstrom (10 –10 m).

They are used in semiconductor manufacturing applications for positioning wafers and etching equipment, and in optics applications for precise positioning of elements in interferometry and fiberoptic assemblies. Other applications include spaceflight where optical components must be positioned to 1/1,000 of a wavelength of visible light.

Accurate control of displacement is the main advantage of PZTs. There are a few other options for this range of motion, notably ferroelectric motors and some differential screw designs. But PZTs avoid some of the problems that plague larger motors; specifically, backlash (lost motion at the beginning of a reversed motion) and slipstick (the sudden surge of motion realized at the moment static frictional forces are overcome).

On the other hand, PZT motors do have some limitations. Piezo devices expand and contract based on the voltage potential applied across the material. This expansion moves the traveling end of the motor over some working distance. For a given voltage, the distance changes depending on a host of factors including temperature, humidity, and the direction of the motor. Some applications can tolerate the variations, but others add feedback loops to control the hysteresis.

PZTs are also susceptible to cracking if subjected to shear or bending moments. So when mounting the motor, installers take care to either eliminate shock loads or position the motor to minimize the bending moments it sees. Another way to reduce the impact of bending moments and increase the range of motion is to drive the motor using an analog servo with proportional and integral feedback, which also improves the accuracy of the motor.

Finally, the price paid for precise motion is small travel distance. The PZT range of motion is typically limited to less than 1 micron (10– 6 m) for a single element. An individual element is a piece of ceramic that comes in a variety of shapes and sizes. Generally, the larger the size, the higher the voltage required to get a certain amount of travel. Designers stack smaller elements to get more travel distance without having to go with a single, larger, monolithic element which would require much higher voltage. The smaller elements allow the use of lower voltages and increase the range of travel for the motor to about 3 the standard for PZTs.

A capacitive feedback system eliminates hysteresis. Two variably-gapped capacitance plates each mount to both ends of the PZT cylinder. The PZT cylinder lengthens or shortens when a voltage is applied. This action changes the distance between the surface of the plates. The change in capacitance lets the microcontroller calculate the motor expansion or contraction, allowing for repeatable positioning.

Telemetry also provides temperature and voltage information to calculate the position of the traveling end of the motor. Temperature affects performance because the materials expand or contract with changing temperatures. A typical PZT may have a thermal expansion coefficient on the order of 100 mm/m/°C. This may not seem like much, but becomes important when working with billionths of a meter. Part of the reason for using a PZT is to actively eliminate thermal expansion. It is therefore critical that the components in the feedback loop have temperature coefficients that approach zero. Use of Invar, a metal with a verylow thermal-expansion coefficient, reduces the motor's metering system thermal coefficient to 10–2mm/m/°C.

Strength and range issues are addressed by adding an Invar strength member to the motor that eases the mounting requirements for the PZT. Unlike other commercially available systems, these motors can withstand a modest bending moment because the Invar member is stretched by the piezoelectric element when in operation.

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