How ultrasonic motor moves camera lens
How ultrasonic motor moves camera lens | ||
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Any automatic focusing camera must have a motor to move the lens elements to bring the subject into focus. That's not a simple task when you consider how much speed and precision the camera requires and how little space the lens provides to hide a motor. As with the autofocus mechanisms devised to measure the distance from camera to subject, camera makers have come up with several ingenious ways to slip motors into minuscule spaces. We'll look here at a motor designed by Canon that many other camera manufacturers have adapted: the ultrasonic motor (USM). The ultrasonic motor is built on a phenomenon called the piezoelectric effect. The effect turns up in certain substances, such as the ceramic lead zirconium titanate (PZT). When an electrical voltage is applied to a strip of PZT, the ceramic expands in one direction and compresses in the other. If the voltage's polarity—the plus or minus charge—is reversed, the ceramic now compresses in the first direction and expands in the second. To create a piezo bender, or bimorph, PZT is bonded to both sides of a thin strip of steel spring metal. A positive charge is applied to one side and a negative charge is applied to the other side. Now the only way the ceramic can expand or contract is to bend the metal strip. The negatively charged side bends out, and the other, positively charged side bends inward. If the charges are reversed, the bimorph bends the opposite way. The next step in creating a piezo motor, or actuator, is to send opposite charges to alternating sections of the bender. The charge on one section makes the bender bow out at the same time the opposite charges going to the sections on either side make them curve inward. By using an alternating current that switches its polarity several times a second, the bender seems to ripple as the adjacent sections bend first one way and then the other, looking like waves that have up-and-down motion but no lateral movement. The amplitude of the combined waves is only about 0.001mm, but it's enough movement to power adjustments on even a weighty telephoto lens. The final step in creating an ultrasonic motor is to mold the bender into a circle. An elastic material studded with flexible nubs is bonded to the circle's rim, creating a stator, which is the stationary part of a machine that moves a rotor. It looks like an endless caterpillar, a resemblance that's more than superficial. As the piezo strip makes waves, the feet press against the rotor, turning the lens elements. Each of the two layers of PZT has its own AC voltage that is slightly out of sync with the other. This allows the autofocus control to determine which way the rotor turns. When both springs are turned off, the friction between the stator and rotor holds the focus steady. | ||