Piezo Stack
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The SA stack actuators are high-performance piezoelectric stack actuators with an epoxy coating for improved mechanical and humidity protection. The SA actuators are matched to the range of PiezoDrive amplifiers and driver modules. Applications include: Nanopositioning, Microscopy, Precision Machining, Vibration Control, Hydraulic Pumps, Valves, and Optics.
Stack actuators should not be exposed to significant tensile loads, unequally distributed loads, off-axis loads, bending moments, or torque. To reduce mounting errors, a ceramic or stainless steel ball end can be used to interface the stack actuator to the load. Flexural mechanisms are also recommended.
PiezoDrive stack actuators can be supplied with ceramic ball ends to improve the load distribution, simplify the mounting, and reduce bending moments during service. One or two ball ends can be specified by adding one of the following suffixes to the order code: -TBE (Top Ball End), -BBE (Bottom Ball End), or -2BE (Both Ball Ends). E.g. SA070718-2BE.
The range is specified for an applied voltage of -30V to +150V. If the input voltage is unipolar (0V to +150V) the specified range is reduced by a factor of 0.75. When a stack actuator is driving a stiff spring, the range is reduced by the factor$$\\frac{k_p}{k_p+k_L}$$ where \\(k_p\\) is the actuator stiffness and \\(k_L\\) is the load stiffness.
The total stack dimensions are based on the width and length of the ceramic listed in the specifications. The length specification includes the piezoelectric stack and two 1-mm thick ceramic end plates. The length tolerance is +/- 0.5 mm.
Thorlabs' Discrete Piezoelectric Stacks consist of multiple piezoelectric chips stacked face-to-face and bonded via epoxy and glass beads. By combining many chips, a stack is able to achieve a free stroke displacement that is significantly larger than that of its single chip counterpart while maintaining sub-millisecond response times and a low drive voltage range.
Ceramic plates cap the two mounting surfaces of the piezoelectric stack, which are located at opposite ends of the stack. One plate is flat, and the other can be chosen to be flat or hemispherical. The hemispherical plate is attached so that the curved surface is exposed to the interface with the load.
The ceramic plates assist in both distributing the applied force of the load over the mounting surface of the stack and in directing the force along the actuator's axis of translation. The flat and hemispherical ceramic plates offer different advantages. When the load can be directed along the translating axis of the actuator, the flat plate with its large contact area provides a better option for achieving maximum force transfer. The hemispherical ceramic plate makes it possible to interface the actuator with an off-axis load, as the curved surface routes the applied force along the actuator's translating axis.
The PKJCUP, PKFCUP, and PKGCUP conical end cups, which are compatible with ball contacts with diameters between 1.5 to 7.0 mm, can also be used to safely interface the load and the actuator. Please see the Operation tab for more information on interfacing piezoelectric actuators with loads, special operational considerations, and data that will allow the lifetimes of these actuators to be estimated when their operational conditions are known.
A ceramic layer covering the other four sides of the stack acts as a barrier against moisture. The ceramic layer offers better protection against moisture than an epoxy coating. For harsh environments, we also offer discrete stacks with hermetically sealed housings. For convenience, the stacks have pre-attached 75 mm long wires and are wrapped in polyimide tape. Piezo chips with custom dimensions, voltage ranges, and coatings are available. Please contact Tech Support for more information.
Our piezoelectric chips are fabricated in our production facility in China, giving us full control over each step of the manufacturing process. This allows us to economically produce high-quality products, including custom and OEM devices. A glimpse into the fabrication of our piezoelectric chips follows. For more information about our manufacturing process and capabilities, please see our Piezoelectric Capabilities page.
Interfacing a Piezoelectric Stack with a LoadPiezoceramics are brittle and have low tensile strength. Avoid loading conditions that subject the actuator to lateral, transverse, or bending forces. When applied incorrectly, an external load that may appear to be compressive can, through bending moments, cause high tensile stresses within the piezoelectric device. Improperly mounting a load to the piezoelectric actuator can easily result in internal stresses that will damage the actuator. To avoid this, the piezoelectric actuator should be interfaced with an external load such that the induced force is directed along the actuator's axis of displacement. The load should be centered on and applied uniformly over as much of the actuator's mounting surface as possible. When interfacing the flat surface of a load with an actuator capped with a flat mounting surface, ensure the two surfaces are highly flat and smooth and that there is good parallelism between the two when they are mated. If the external load is directed at an angle to the actuator's axis of displacement, use an actuator fitted with a hemispherical end plate or a flexure joint to achieve safe loading of the piezoelectric stack.
To accommodate a variety of loading conditions, these discrete piezoelectric stacks may be purchased with either two flat ceramic end plates or one flat and one hemispherical end plate. In addition, Thorlabs offers Conical End Cups which feature concave surfaces that can interface with Ø1.5 mm to Ø7.0 mm hemispherical or curved contacts. To attach a load to the piezoelectric stack, we recommend using an epoxy that cures at a temperature lower than 80 C (176 F), such as our 353NDPK or TS10 epoxies or Loctite Hysol 9340. Loads should be mounted only to the faces of the piezoelectric stack that are fitted with the flat or hemispherical ceramic end plates; the other sides of the actuator do not translate, and mounting a load to a non-translating face may lead to the mechanical failure of the actuator. Some correct and incorrect approaches to interfacing loads with piezoelectric stacks capped with both kinds of end plates are discussed in the following.
The image at left presents incorrect (A, far-left) and correct (B, near-left) methods for using a piezoelectric stack to actuate a lever arm. The correct method uses a hemispherical end plate so that, regardless of the angle of the lever arm, the force exerted is always directed along the translational axis of the actuator. The incorrect interfacing of the stack and the lever arm, shown at far-left, endangers the stack by applying the full force of the load to one edge of the stack. This uneven loading causes dangerous stresses in the actuator, including a bending moment around the base.
The image at right shows one incorrect (near-right, A) and three correct approaches for interfacing a flat-bottomed, off-axis load with a piezoelectric stack. Approaches A and B are similar to the incorrect and correct approaches, respectively, shown in the image at left. Correct approach C shows a conical end cup, such as the PKFCUP, acting as an interface. The flat surface is affixed to the mating surface of the load, and the concave surface fits over the hemispherical dome of the end plate. In the case of correct approach D, a flexure mount acts as an interface between the off-axis flat mounting surface of the load and the flat mounting plate of the actuator. The flexure mount ensures that the load is both uniformly distributed over the surface plate of the actuator and that the loading force is directed along the translational axis of the actuator.
Operating Under High-Frequency Dynamic ConditionsIt may be necessary to implement an external temperature-control system to cool the device when it is operated at high frequencies. The maximum operating temperature of these devices is 130 C (266 F), and high-frequency operation causes the internal temperature of the piezoelectric device to rise. The dependence of the device temperature on the drive voltage frequency for each product can be accessed by clicking the Info icons, , below. The temperature of the device should not be allowed to exceed its specified maximum operating temperature.
Estimating the Resonant Frequency for a Given Applied Load A parameter of significance to many applications is the rate at which the piezoelectric actuator changes its length. This dimensional rate of change depends on a number of factors, including the actuator's resonant frequency, the absolute maximum bandwidth of the driver, the maximum current the piezoelectric device can produce, the capacitance of the piezoelectric stack, and the amplitude of the driving signal. The length of the voltage-induced extension is a function of the amplitude of the applied voltage driving the actuator and the length of the piezoelectric stack. The higher the capacitance, the slower the dimensional change of the actuator.
Quick changes in the applied voltage result in fast dimensional changes to the piezoelectric stack. The magnitude of the applied voltage determines the nominal extension of the stack. Assuming the driving voltage signal resembles a step function, the minimum time, Tmin, required for the length of the actuator to transition between its initial and final values is approximately 1/3 the period of resonant frequency. If there is no load applied to the piezoelectric stack, its resonant frequency is ƒo and its minimum response time is:
Applying a load to the actuator will reduce the resonant frequency of the piezoelectric stack. Given the unloaded resonant frequency of the actuator, the mass of the stack, m, and the mass of the load, M, the loaded resonant frequency (ƒo') may be estimated: 59ce067264
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