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Some crystal structures give an electrical charge when stressed or twisted, which is known as the piezoelectric effect. This is used in many electronic circuits. A piezo linear actuator uses this effect in reverse, by having an electrical current cause movement in the crystal. These actuators are often found in microelectronic, or very small, motors and where small movements are needed in a straight line, such as micro switches.
There are two main types of piezo actuators, stepping and continuous action, which differ by the action that results from the electrical input. A stepping piezo crystal moves a measurable amount with each electrical activation. This is the type of movement normally associated with a piezo linear actuator, which is moving back and forth in a linear or straight-line motion.
Many piezo crystals also have resonant frequencies, where an electrical input of a specific voltage will cause the crystal to resonate, or vibrate at a specific rate. Resonant frequency effects can be used for continuous actuators, where each vibration causes a small movement. By taking advantage of the resonance, the small movements can be combined into a motion that appears continuous.
A piezo linear actuator uses a friction pad at one surface of the crystal that can be placed adjacent to a rod, wheel or other device that needs to be moved. When the crystal moves, the friction pad transfers that movement to the device. After the electrical input is stopped, the crystal returns to its original shape and the friction pad moves away from the device until the next activation.
Layering or stacking individual crystals together can result in larger movements. Electrical connections are made to each crystal in the stack, and when activated the combined movement is approximately the sum of each crystal in the stack. Placing crystals opposite each other along the length of the actuator can create longer linear movements. The electrical input alternates to the crystals on each side of the piezo linear actuator rod, which causes it to move farther than a single input.
Movement of the crystal structure is not only dependent on the electrical charge, but the polarity or direction of electron flow. Reversing the electrical polarity can cause the crystals to move or deform in the opposite direction. This effect is used to move a piezo linear actuator in the opposite direction, or back and forth by repeatedly changing the polarity.