Engineering Smart Technologies

Tech notes - 1

Attaching the Quickpack/PowerAct Transducer to a structure with epoxy

Many applications of the QuickPack piezoelectric transducer require the device to be attached to the surface of a structure. For these applications, proper attachment of the QuickPack transducer is critical to its performance, because the QuickPack device must transmit mechanical strain energy to and from the structure.

The QuickPack transducer can be attached to a structure by any one of several means. There are two common forms of attachment: embedding the device into a composite structure, and bonding the device to the structure with epoxy. When the QuickPack transducer is bonded to a structure with epoxy, the choice of epoxy should consider many factors, especially performance and cost. For performance considerations, the epoxy should produce a uniform, thin, stiff bond layer. The thinner and stiffer the bond layer, the better the results. For this reason, a high-performance structural epoxy is often the best choice.

Midé ships, free of charge, sample quantities of a two-part, 24-hour cure, high-performance structural epoxy with QuickPack product orders. Midé recommends that this epoxy be used for bonding the device to a structure, because it provides excellent adhesion to the QuickPack actuator’s protective polyimide coating. (NOTE: The sample epoxy shipped with the QuickPack products should be used before the expiration date marked on the package.) Another example of a suitable high-performance structural epoxy is listed below.

ECCOBOND© 45 CLEAR
Richey Electronics 508-664-9355
87 Concord Street 508-664-9356 fax
No. Reading, MA 01864
http://www.richeyelec.com

In cases where assembly costs are a greater consideration, an epoxy that requires a 24-hour cure cycle may not be the best choice. Elevated temperatures can accelerate the cure cycle of most two-part epoxies,including the epoxy Midé provides. Some applications that require high-volume manufacturing processes may call for a different type of adhesive. If you are considering using a QuickPack product for a high-volume application, consult Midé for product integration assistance.

The numbered steps in this QuickNOTE document provide detailed instructions recommended for bonding QuickPack transducers to structures. Italicized steps are required only if a vacuum bonding technique is used. The vacuum bonding technique applies uniform pressure to the QuickPack transducer during the cure cycle and helps to remove trapped air bubbles in the epoxy. An alternative bonding method is simply to apply weights to the QuickPack product to provide roughly 14.7 PSI during the cure cycle. In both cases, the goal is to achieve a uniform, thin bond layer.

Before you start

Once the QuickPack transducer is properly attached to the structure, it cannot be removed without destroying it. Therefore, it is important to determine exactly where you want to place the device before you mix the epoxy. If the desired location is flat, any of the standard QuickPack models may be used. If the desired location is not flat, please note that standard flat QuickPack products may conform to structures with slight curvatures (radius > 0.5m). Tighter curvatures may require a custom QuickPack device.

Surface Preparation

1. To improve adhesion, lightly sand the bonding location, and clean residue with isopropyl alcohol. The structure and the QuickPack transducer should be free of dirt, grease, and oils. Isopropyl alcohol is the best cleaning agent. (Do not use acetone or other organic solvents.) Lightly sanding the surface of the structure should improve adhesion, but do not sand the QuickPack product, as that would damage the protective polyimide coating.
   
2. Place Teflon® tape on the area where the QuickPack transducer will be attached. For standard QuickPack transducers, Midé recommends 0.010 inch thick Teflon tape.
3. While holding the QuickPack transducer down at its desired location on the Teflon tape, trace, cut and remove the tape to create a pocket for the QuickPack transducer.
4. Place Teflon tape on the QuickPack transducer, covering the top surface of the transducer and encircling (i.e., the top and bottom) the flex tab and electrical connector. For standard QuickPack transducers, Midé recommends 0.005 inch thick Teflon tape.
   Teflon tape on the structure and on the QuickPack device helps to position the transducers precisely and to make clean-up much easier.
5. Place bonding tape on the structure to surround bonding area.
   Bonding tape placed around the bonding area will create a seal when a vacuum is applied. If the structure is relatively small, the bonding tape can be placed on a larger bonding surface prepared with a release film to protect it. In this case, the small structure (with QuickPack transducer) would be placed on the larger bonding area when a vacuum is applied.
6. Cut air weave and bagging material to the size of the bonding area.
   The air weave material is a fabric that distributes the vacuum pressure and prevents the bagging material from blocking the vacuum line. The bagging material is a flexible plastic sheet that provides an air-tight seal over the bonding area. The air weave sheet should be large enough to cover the QuickPack transducer and the vacuum hose, but small enough to fit within the bonding tape perimeter. The plastic bagging sheet should be larger than the bonding tape perimeter.
7. Mix the epoxy in appropriate proportions.
8. Spread the epoxy in a thin, even layer on the structure and on the QuickPack transducer.
   For convenience, the epoxy that Midé provides comes in a pre-proportioned mixing dispenser.
9. Place the QuickPack transducer on the structure.
   After the QuickPack transducer is in place, excess epoxy can be wiped off.
10. Lay down the air weave material over the bonding area.
11. Slide the vacuum hose under the air weave, and seal the hose with bonding tape.
12. Lay down the bagging material over the bonding area.
    The vacuum hose should be connected to a standard vacuum pump with the line closed during set-up.
13. Open the vacuum line slowly (approximately five seconds).
14. Close vacuum line to check for leaks.
15. Fix leaks as necessary and open vacuum line again.
    It is recommended that the integrity of the vacuum seal be checked to confirm that proper pressure is applied to the QuickPack transducer during the cure cycle.
16. Remove pressure after 8 to 12 hours to clean up excess epoxy flow.
17. Re-establish pressure for the remainder of the 24-hour epoxy cure cycle.
    Excess epoxy is most easily removed halfway through the cure cycle, after it has solidified but before it has fully set. This is done by temporarily removing any vacuum bag or weight, scraping or peeling away the excess epoxy, and replacing the vacuum bag or weight to re-establish pressure for the remainder of the cure cycle.
18. After the epoxy has cured, remove pressure, bagging material, air weave and bonding tape, and peel off Teflon tape.

For further assistance

If you have additional questions about attaching the QuickPack transducer to the surface of a structure, 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 can provide you with recommendations for reducing your production system costs and maximizing the performance of the QuickPack product for your specific application.

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.

Tech notes - 3

High volume low cost of QuickPack/PowerAct transducers for acoustical/strain actuation applications

Integration Requirements:

Mide has evaluated various methods for integrating QuickPack piezoelectric transducers into OEM products for acoustical and other strain actuation applications. This document describes an example of a high volume/low cost process that can provide OEMs with a reliable method of bonding. This high volume/low cost integration process was originally developed for a QuickPack QP15N to be bonded to a plastic panel with a minimum fixture time of 30 seconds.

Multiple adhesives were evaluated from different suppliers to determine which adhesive would best meet the needs of this application. These adhesives were checked for cure time, bond strength, thermal shock resistance, and hardness. After rigorous testing Mide determined that Loctite®498, an ethyl modified cyanoacrylate, was the best adhesive1.

High Volume Bonding Procedure:

The described integration process is capable of producing 2,500 assemblies in an 8 hour shift utilizing one operator. The only operator intervention will be to load and unload individual plastic panels and to stock bulk QuickPack transducer components into a stacking elevator. The system can be fully automated to eliminate the need for an operator.

Figure 1. Bonding Process: An 8-position rotary indexed table transports parts through station 1 through 8.

Station 1:

Operator loads plastic panels into a fixture nest of rotary center precision indexer. Indexer includes:

• eight position, 32” positive-locking rotary indexable table
• panel holding fixtures/nests at each of 8 positions
• two point doweling for mounting of fixtures
• welded base with heavy-duty locking casters and tote bins
• pneumatic based control system

Station 2:

Dispense adhesive onto panel. Adjustable dispensing valves are set to dispense precise and repeatable beads of adhesive to specified location on panel. Adhesive dispensing station includes:

• fixturing for dual adhesive dispenser
• X,Z dispensing motion controller with pneumatic control system/interface to indexing dial
• pressure sensor
• external controls to adjust dispensing valve pressure and time

Station 3:

Pick and place QuickPack transducer onto adhesive QuickPack transducers are available from a part feeder and stack elevator. A vacuum based pick and place mechanism will provide accurate placement (+/- 0.015”) of the transducer onto the panel.

An automatic mechanical clamp will apply force onto the transducer and maintain adequate adhesive fixturing pressure as the assembly rotates through stations 4-7

Stations 4-7:

Adhesive cure. Assembly rotates through 4 stations for curing. No processes are performed on the assembly.

Station 8:

Unclamp and prepare for unload. An automatic unclamping device will free the transducer/panel assembly and prepare for unloading at Station 1

Station 1:

Unload bonded transducer/panel assembly. Operator unloads assembly and prepares to load next panel.

Mide can work with customers to modify this process for unique product and integration requirements.

Tech notes - 4

Integrating the Quickpack OmniCom Transducer Into a Handheld Product

The Midé OmniCom transducer is a member of the QuickPack actuator family of products. It is a packaged, piezoceramic device that is operated as a bimorph actuator to provide tone alert, vibration alert and speaker functions. By combining 3 features into a single compact device, manufacturers of handheld devices can add functionality, and reduce the size and complexity of their products.

Operating the OmniCom transducer requires clamping the ”tail end” of the transducer and attaching this clamp to the housing of your product. By applying a voltage to the clamped transducer, you will actuate the tip of the device which will produce an audible or vibratory output. (See Figure 2.) Supplying varying frequency and voltage inputs changes the mode of operation: tone, speaker, vibration.

There are a variety of methods for integrating the OmniCom transducer into an OEM product. This note outlines a reliable integration method for the OmniCom transducer. It is the method used to fabricate the OmniCom demonstration unit. Midé can work with OEM manufacturers to design and implement alternative methods.

1. Select Location

The OmniCom transducer requires a 0.67” × 1.83” area. During operation the maximum displacement at the tip is 0.17” (since there is no deflection at the clamp end, other handheld product components may be located nearer to the transducer). The OmniCom transducer resonates the OEM product housing itself, so it should be placed in a location which provides a suitable case panel radiating surface of at least 1.40” × 2.25”. While this panel area may house other device components, the panel itself should be free of stiffening ribs. The wall thickness of this panel area should be 0.060”. (For demonstration and evaluation purposes, an existing housing may be modified by milling to the specified thickness and removing any ribs etc.). Ideally, the actuator should be clamped near one edge of the panel, on axis with the center line of the panel surface. This allows the transducer to resonate a larger panel area without being constrained by the stiffness of the product walls or other stiffeners. (See Figure 3.)

The OmniCom device operates without any special enclosure or speaker grille and does not require the OEM product housing to be sealed.

2. Clamp Device

A firm clamp is required to achieve the optimal performance. The OmniCom demonstration units use a 6061-T6 aluminum or 303 stainless steel clamp. Other materials like ceramic or plastics may be used. When evaluating alternative clamp materials ensure that the clamp does not creep nor deform during operation.

The clamp should be positioned over a 0.6" x 0.2" area of the piezoceramic portion of the transducer. The suggested clamp size is 0.7" x 0.5".(see figure 4) Two #2-56 UNC screws are used to fasten the clamp to the product housing. Tightening screws to apply a 2.5 inlb. torque (280 lbs. total) force provides ideal performance. While tests have shown that varying the clamping pressure may produce satisfactory performance; at the extremes, under-clamping will lead to lower outputs and over-clamping may damage the device. (NOTE: The QuickPack device's protective coating mechanically protects the piezoceramic elements and electrodes against cracking in the clamping region allowing ACX to recommend approximately 2300-3000 PSI for QuickPack bimorphs. This ideal clamping pressure could fracture unprotected, "raw" piezoceramic materials).

Screws / actuator holes provide ideal registration actuator holes provide ideal registration features for high volume manufacturing. Mide can work with OEMs to evaluate alternative clamping options such as insert molding and bonding.

3. Electrical Connection

The OmniCom transducer provides two exposed and knurled brass contact pads. Electrical contact can be made to these pads by leaf springs, pogo pins or conductive elastomers, or wires can be soldered directly to the pads.

4. Driving Conditions

The QuickPack actuators are capacitive devices; the OmniCom transducer has a capacitance of 40nF. Like most piezoceramics, QuickPack OmniCom actuators require high voltage and low current. The electronics subsystem that drives the OmniCom transducer should be capable of delivering 0.5W power, a bandwidth of at least 6.5KHz and should be able to reach peak voltages of ±75V. The Mide 1224/5 QuickPack Power Amplifier can easily meet these needs for lab evaluation. Discrete drive electronics or an ASIC, such as SIPEX model SP4515 , are appropriate for production solutions. Figure 5 illustrates the functionality of the complete system.

Omnicom system architecture.

Tech notes - 5

Integrating the Quickpack qp16fs into a flat panel moniter

The Mide QuickPack QP16FS transducer is a member of the QuickPack actuator family of products. It is a packaged, piezoceramic device that may bonded to a surface and operated as an alert device or a full range speaker. With a thinness of only 0.013", its an ideal solution for thin products and tight spaces. And since the device does not require any grilles or speaker holes, it provides an extremely robust solution for products that will be used in harsh environments.

Operating the QP16FS transducer requires bonding the device to the OEM product housing. Supplying the transducer with an audio signal drives the transducer which induces curvature in housing --providing speaker capability from an extremely thin device.

There are a variety of methods for integrating the QP16FS transducer into an OEM product. This note outlines a reliable integration method for the QuickPack transducer. It is the method used to fabricate the Mide Invisible Speaker demonstration unit. Mide can work with OEM manufacturers to design and implement alternative methods.

1. Select Location

Since the QuickPack transducer resonates the OEM product housing itself, it should be placed in a location that provides a suitable case panel radiating surface. For optimum performance the QP16FS transducer should be bonded to a flat panel with twice the actuator's area (~ 1.41" x 2.82"). This area should be free of stiffening ribs. A typical panel made of ABS/Polycarbonate or similar plastic should be between 0.050" and 0.070" thick. (contact ACX for recommendations on alternate materials.) If the actuator is bonded to a larger structure which is nominally thicker, the surface should be thinned locally to the recommended thickness in a ~1.41" x 2.82"area. This thickness provides a suitable radiating surface while preventing most visible shadows on the outer plastic surface. A thicker, more stiff panel will reduce the sound output of the speaker.

2. Prepare Surface

Prior to bonding, the surface of the plastic and the surface of the actuator should be cleaned with a mild solvent such as isopropanol (acetone is not recommended) to remove any oils or release agents left during handling of the parts. Although excess epoxy on the panel or around the edges of the QuickPack actuator will not affect performance, masking may be desired for aesthetic purposes. Teflon tape can be used to mask around the bond location, preventing excess epoxy from adhering to the surrounding panel. When the Teflon tape is removed from around the actuator, the exposed panel and actuator surfaces are left clean.

3. Bond

Mide recommends Loctite 498 or similar quickcuring cyanolacrylate adhesive. This adhesive provides suitable strain transfer and meets typical OEM environmental requirements including temperature and high thermal cycling. Spread the adhesive uniformly on the QP16FS surface. Be careful not to cover the exposed solder pads with adhesive. Quickly place the actuator on the panel in center or slightly off center of the 1.41" x 2.82" area. (Center actuation increases sensitivity of the transducer, but off center actuation improves flatness by reducing excitation of standing wave modes.) Hold the actuator in place with a teflon coated flat block of sufficient size to cover the actuator, but small enough not to touch any nearby structural features which are thicker than the QP16FS. The thickness of the bond layer should be minimized for optimum performance. Hold the block in place with a weight of 20-30 lbs for 3-5 minutes. Remove the weight and block (use slight twisting motion if the teflon coated block sticks to the excess adhesive).

4. Electrically Connect

Solder electrical leads to the exposed solder pads and drive transducer with a 50V power amplifier or with a 10V audio amp and ACX audio transformer.

5. Prevent buzzing

Since the product housing is vibrating, the OEM must take care to ensure that nearby components, such as cables or other parts of the housing do not rattle or buzz. ACX recommends that components be spaced at least 0.06" from the vibrating housing part or be rigidly mounted to the vibrating structure. Cables and wires may be easily rigidly mounted using an adhesive or tape.

Any components closer than the minimum distance but not rigidly attached to the vibrating structure must be protected from impact with the structure by closed cell foam (0.03" minimum thickness recommended) or other soft elastic material such as neoprene. A foam gasket would be appropriate, for example, to prevent a monitor bezel from buzzing against the LCD.