QuickPack - Technical Notes

Tech notes - 2

Using the Quickpack Actuator as a Bimorph

The QuickPack piezoelectric actuator can be operated in several different ways to create motion or force. One common mode of operation is to create a linear motion by operating the QuickPack actuator as a bimorph. Common applications for bimorph-style actuators include pneumatic control valves, fluid pilot valves, switches, relays, and pumps.

Versus traditional solenoid actuators, piezoelectric bimorphs offer several technical advantages. Piezoelectric actuators are smaller and lighter, and they require significantly less power than their solenoid counterparts. The QuickPack actuator operated in bimorph mode makes this unique piezoelectric technology suitable for high-volume products by packaging the raw piezoceramic elements in a protective skin with pre-attached electrical leads.

General

Bimorph actuators operate by having two independent, flat piezo elements stacked on top of each other. Driving one element to expand while contracting the other causes the actuator to bend, creating an out-of-plane motion. Although many mechanical arrangements are possible, typically a bimorph actuator design has rectangular piezoceramic elements clamped firmly at one end. (See sketch.) The result looks like a miniature diving board that bends up and down with applied voltage. Actuation occurs at the free end, away from the clamp. (NOTE: Bimorphs will also actuate with only one of the two layers activated.)

Design Considerations

The most important functional requirements that affect bimorph actuator design are force (or load, F) and stroke (or displacement, w). These functional requirements drive design parameters including length (L), width (b) and thickness (t) of the actuator, as shown by the equations below. For example, an increase in actuator length increases the stroke and decreases the force. An increase in width increases the force, but has little effect on stroke. An increase in thickness increases the force and decreases the stroke.

An additional design consideration is bandwidth (also operating frequency or response time). Force and stroke are typically specified under quasi-static conditions, meaning that the actuator is operating well below its first resonant mode. If the application requires the actuator to operate at a higher frequency, then the dynamics of the actuator may affect its force and stroke output. For high-bandwidth applications, the actuator is commonly designed so that the first resonant mode is 3x higher than the highest operating frequency for the application. Designing the actuator in this manner allows the force and stroke performance to be independent of frequency within the operating conditions of the application. The good news is that high bandwidths are easily achieved with QuickPack piezoelectric bimorph actuators by modifying basic design parameters. For example, by designing the actuator to be shorter or thicker, the bandwidth is increased.

Along with actuator dimensions, another design variable for the bimorph actuator is piezoceramic material selection. QuickPack actuators may be constructed with one of many blends of piezoceramic. For example, some piezoceramic blends provide higher force for a given geometry, but they have a higher capacitance and require slightly higher current for operation. Other piezoceramic blends are well suited for applications where the bimorph is to be energized statically for long periods of time. Still other piezoceramics are best for applications where operating frequencies are very high. When designing an actuator for a specific application, Midé engineers use their knowledge of the properties of various piezoceramic blends to choose the proper material.

Product Integration

The two most important aspects to product integration are the electrical connection to the actuator and the clamp. The QuickPack device solves many of the electrical connection issues by providing a standard connector that can mate with a cable or a receptacle on a printed circuit board. Soldered wires, which can be a source of reliability problems in “traditional” piezoceramic bimorphs, are not required with the QuickPack actuator. Several options for electrical connection exist for the QuickPack actuator, so consult Midé for recommendations for the best electrical connection method for any specific application.

The clamp or actuator mount is also a critical aspect of product integration. A firm clamp is required to achieve best actuator performance. Compliance in the clamp will degrade the actuator’s force performance. For a clamp to be “firm,” the rotational stiffness of the clamp area should be roughly three orders of magnitude (1000x) higher than the rotational stiffness of the bimorph that deflects outside the clamp. “Rotational stiffness” in this case can be thought of as a measure of how difficult it is to bend the bimorph around the clamp edge.

To achieve a firm clamp for a typical bimorph actuator, Midé recommends using a clamp constructed from material that has high mechanical stiffness (steel or ceramic), and applying 3000 PSI clamping pressure over at least 0.25 inches of the actuator length. This will provide proper performance of the actuator without risking damage to the actuator from too much clamp pressure. (NOTE: The QuickPack device’s polyimide coating mechanically protects the piezoceramic elements and electrodes against cracking in the clamping region, allowing Midé to recommend an ideal clamping pressure of 3000 PSI for QuickPack bimorphs. This ideal clamping pressure could fracture raw piezoceramic materials.)

Prototype Planning

To develop a custom prototype actuator, Midé recommends that its customers prepare answers to the following questions in order to help define the application requirements:

• What are the force and stroke required for the application?
• How will the actuator operate dynamically (frequency, duty cycle, response time, etc.)?
• What (if any) are the electrical constraints for the application (power, voltage, capacitance, etc.)?
• What (if any) are the geometrical constraints for the application (length, width, height, etc.)?
• What are the expected operating conditions (temperature, chemical environment, expected life)?

For Further Assistance

If you have additional questions about using the QuickPack actuator as a bimorph, or about the general use and operation of QuickPack products, please contact Midé . You will speak to a knowledgeable representative that will help you with any specific question you may have. And if you are designing a product with high-volume production potential, please contact Midé early in your design process. Midé can provide you with recommendations for reducing your production system costs and maximizing the performance of the QuickPack product for your specific application.

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For more information:

Please email Conor Clery (Products), or call: 781-306-0609 x292