Discrete stacking offers the widest range of ceramic materials, flexibility in the shape of the ceramic elements, and better control of heat generated during high frequency operation, but voltages of 500 V – 1000 V usually are needed to operate these actuators.
Co-fired actuators are driven by much lower voltages, 100 V – 150 V, but most co-fired stacks are prepared from low Curie point ceramics, and are incompatible with high electrical input.
In either construction, the stack of ceramic elements can be simply coated with an insulating material — a bare stack — or it can be encased in a stainless steel or other casing, to protect it from mechanical damage and environmental effects, or to incorporate mechanical pre-stress. The casing and pre-stress also direct shear forces to the pre-stress mechanism.
The mechanical displacement (stroke) for a stack actuator is proportional to the height of the stack of elements; the blocking force is proportional to the cross-sectional area of the stack:
| Stack Height | Stack Cross-Section | Stroke | Blocking Force |
| h | A | S | F |
| 2h | A | 2S | F |
| h | 2A | S | 2F |
| 2h | 2A | S | 2F |
Depending on the ceramic material used, the maximum strain (stroke/stack height) can equal up to 0.15-0.20% of the height of the stack. Strokes are additive for multiple actuators in series. Operating an actuator of stack height (h) at half its potential strain, compared to operating a stack of height (h/2) at full strain, reduces power consumption by 50%. For a particular ceramic, strain and other properties are consistent, whether the ceramic is used in a co-fired or discretely stacked actuator.
Strain should not be the only consideration when selecting a stack actuator. A suitable ceramic for a particular application offers the best balance of electromechanical characteristics, including low power consumption, minimal heat generation, and compatibility with the intended operating environment (temperature, humidity, etc.). Because piezoelectric charge constants typically increase with increasing temperature or electric field strength, the stroke or blocking force for an actuator can exceed the anticipated value. The manner in which the actuator is mounted (and pre-stressed) can further complicate the situation. Consequently, the most reliable indicators of actuator performance are measurements made under anticipated operating conditions. It is important to deal with a knowledgeable supplier, such as APC International, who can assist in selecting or designing an actuator with the best characteristics for a specific task.
To prevent damage to the ceramic elements or electrodes, mount a bare stack actuator only at its endfaces. An encased stack can be mounted at the endfaces or clamped at its circumference. In coupling an actuator to a mechanical system, exclude bending, tilting, or shear forces that could inhibit the stroke or damage the actuator. A planar face and the center of a spherical tip provide suitable contact. Planar face – planar face contact is acceptable if one of the faces is free moving. If the stroke for an actuator proves insufficient for a particular task, an alternative to using a larger actuator, or multiple actuators in series, is to amplify the stroke by modifying the mounting configuration.
In micro-positioning applications, a stack actuator should be used in combination with a sensor capable of monitoring the absolute position of the actuator and, if necessary, adjusting the voltage.
For more information on our stack and co-fired stack actuators, contact our team or request a quote today.