Motorized drive gives linear motion with resolution better than 0.1 mm

July 1, 2000
A new piezoelectric device that turns a screw promises linear positioning with unheard-of precision and range. Now in screw travel you can get what you call for ... no more, no less.

Inventors at New Focus Inc., Sunnyvale, Calif., call it a Picomotor, and they designed it originally as a positioner for lasers and other optic systems. However, its uses could run well beyond such devices. If you are hunting for a motorized linear actuator that offers resolution better than 0.1 mm, minimum rotation of 2 mrad, thrust exceeding 2 lb, rotary speed of 2 to 3 rpm, and stroke limited only by the length of screw you want to use, this could be your prize.

What’s piezoelectric action?

Maybe you remember the piezoelectric phenomenon from Physics class: For some materials mechanical strain in a crystal causes electrical charges on the crystal’s surface. The converse effect is that, if an electrical charge is applied to two surfaces, the crystal mechanically deforms. In general, there is a linear relationship between sizes of the electrical and mechanical factors. On a macro scale, application of a voltage to a shaped piezoelectric material causes the material to deform. Such deformation can be controlled in size and direction. Many actuators are based on piezoelectric action.

According to Frank Luecke, New Focus vice president, piezo-driven actuators historically have relied on contraction and expansion of the piezo device to produce positioning. The piezo devices themselves are the positioning elements. In one way or another, however, such actuators have suffered backlash, hysteresis, or creep, and they require constant voltage to hold position. For example, the hysteresis effect means that a piezoelectric device has a slightly different displacement for the same voltage, depending on whether the voltage is increasing or decreasing.

The new device uses the piezoelectric element only to operate a sort of ratchet, which either moves the screw a small amount or doesn’t move it at all, depending on the speed of application of a voltage pulse. If the voltage pulse changes too rapidly, no motion is produced. For lesser rates of change, the applied voltage produces screw motion. Figure 1 shows the principle of operation. New Focus engineers liken it to the “table-cloth trick”: If you pull the table cloth slowly toward you, everything on it will come along. If you yank the cloth, you remove the cloth while everything that had been on it now rests on the bare table. You may wish to try this trick at home. We don’t.

Nevertheless, the simile is appropriate, for it relies on friction and inertia. At low speed, the coefficient of static friction is not overcome, and all parts move together — the screw turns. (The table setting moves with the cloth.) At high speed, the inertia of the mass to be moved prevents movement; the screw cannot turn. (The table setting stays while the cloth leaves.) To reverse rotation of the screw, just reverse fast and slow directions of the electronic pulse train. Friction force at the threads holds the screw when no voltage is applied at the piezoelectric device. A spring, rather like the springs on some wooden clothes pins, applies that holding force in a clamping action. You can overcome that force externally if you wish; that is, you can turn the screw by hand in order to make a coarse adjustment.

The manufacturer provides the actuators in several versions, some intended as micrometer-replacement actuators (MRAs) as in Figure 2; others as motorized mounts and positioners for mirrors and similar systems.

The drivers

A driver for a Picomotor generates 120-V waveforms required to drive the piezoelectric device. They alter screw rotation direction by changing the rise and fall times of the pulse. As described above, due to inertia the screw doesn’t turn during fast rise or fall. During slow rise or fall, the screw turns. Therefore, a pulse with a fast rise time and slow fall time results in rotation that is, say, counterclockwise; conversely, a pulse with slow rise time and fast fall time results in clockwise rotation.

The drive circuit consists of a voltagecontrolled oscillator, some logic elements, and a power field-effect-transistor (FET) circuit which produces the 120-V pulses. Drivers are available both for single- axis and multi-axis operation. Figure 3 shows a typical schematic of the singleaxis driver system. You can operate the drivers with a control pad for manual input, or with a D connector for electronic input. With the D connector, you can use either low-voltage analog (62.4 V) or digital signals.

The control pad is an option that provides manual control of up to three axes of operation. The pad has three slide potentiometers to operate three axes, such as those in a typical mirror mount. Sliding a potentiometer slightly forward produces slow screw advance; pushing it farther advances the screw faster. Logarithmic slide response lets the highest speed be 1,000 times that of the lowest. Slide movement in the opposite direction reverses actuator movement.

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