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Introduction

This application note describes the items to consider when integration electro-active polymer actuator onto flexible and rigid bodies. This application note considers the following items:

  • Mechanical integration
  • Effect design

For product specifications, product part numbers and safety information please refer to the datasheet “KEM_P0106_FFAA”.

Mechanical Integration

General Guidelines

It is recommended to:

  • Attach the Haptic Modules assembly to the user-facing surface on the substrate side. Attaching the assembly to the surface on the actuator side will counter-act the vibration of the system.
  • Minimize the resistance to the vibration added by the integration by choosing the right material and geometry for the user facing surface (also referred as “cover” in this document) and the target area. There are a variety of textile and polymer blend sheets suitable to use on the user facing surface of the integration. Examples are PU-coated textiles.

Material selection

The following parameters/specifications must be considered when selecting material for the user facing surface:

Resistance to bending

  • It depends both on the stiffness and the thickness of the material used. The stiffer and thicker the sheet is, the lesser of the actuator’s energy is transformed into a perceivable haptic sensation. If the sheet is too thin or flexible, the haptic module starts to show on the surface, more so if the integration is not flat. If the sheet is too flexible, a stiffener layer could be added on the actuator side of the sheet.

Wear rate

  • Scratch and wear resistance are important design factors for the user facing surfaces. If the surface is passivated with a polymer-based material, it is important to make sure that the protection does not wear out throughout the lifetime of the product

Water Resistance

  • The whole integration of the haptic module should be waterproof. Failure to do so may result in electrical shortage of the actuator, harming the actuator, the driving board, and the user.

Conformability

  • The user facing surface can also wrap around the whole integrated product part for seamless construction.

Integration on to Rigid Bodies

The above assembly integrates three different shapes, which can easily be found in several human-machine interfaces: buttons, joysticks, touch pad. It is built with the integration of four actuators in three hard button shapes.

Each button has the same integration structure as per the following:

Shell

In order to obtain the best performances, the actuator supporting structure should be designed to:

  • Guarantee the mechanical support of the Flex Assembled Actuator in the perimeter around the active part of the actuator (free margin of the flex pcb of the Flex Assembled Actuator).
  • Avoid interference with the actuator, to avoid damages or stress
  • Allow the wiring to easily pass thorough without applying any mechanical stress to the flex PCB and the overall Flex Assembled Actuator.
  • Guarantee a soft mechanical support of the actuator (active part of the Flex Assembled Actuators) when working
  • Guarantee that the outline of the Flex Assembled Actuators is not visible on the user side

The assembly above shows an example of opening. Its dimensions are shown in the table below.

The frame where to put the actuator has a tooth (circled area) to facilitate the actuator position, where the soldering points are.

Foam

The active part of the assembled actuator can be supported by a thin foam layer placed below the actuator itself. The foam support can be placed into a cavity in the mechanical body, drive wires can be fed through holes in the cavity if space allows. Below are some characteristics of foam options of various durometers.

In the current example, the foam used has the following characteristics:

Actuators

Electrical connections to the actuators should be done in such a way as to:

  • Allow proper electrical connections
  • Avoid mechanical stress to the Flex Assembled Actuator, specifically to the flex pcb
  • Provide the easiest path to the driver connectors

Soldering can be done following the same plane as the actuator or normal to it (see picture above). It is important to provide adequate strain relief.

Note: Polarity is not relevant; actuators are AC voltage powered.

Important wiring parameters

Double-Sided Adhesive Tape

Both heat activated or pressure sensitive adhesives can be used to fix the assembled actuators to the user facing surface. The choice depends on the shell material and on the user facing surface material. Heat activated adhesive should be avoided on the area where the actuator is since the bonding temperature must not exceed 100°C on the area where the actuator is. Mechanical adhesion is easier compared to heat activated adhesion: it is suggested to avoid air bubbles within the adhesion area.

Cover

Cover should be chosen to avoid actuator performances reduction due to stiffness or thickness. Moreover, it has the following functions:

  • To finish the user facing surface after having placed the actuator into the designated area
  • To keep the Flex Assembled Actuator in the designated position mechanical constraints without affecting the vibration performances
  • To guarantee electric shock and humidity protection.

Other points to facilitate assembly include:

  • Double-sided adhesive should be pre-assembled to the internal side of the cover.
  • To keep the Flex Assembled Actuators in the correct position into the shell, a layer of adhesive Kapton can be added on the shell.
  • Keep the shell compressed (for instance in a vise)
  • Place the cover with the exposed adhesive area on the edge of the shell and remove the paper layer while making the adhesive layer adhere to shell (flex pcb side of the assembled actuator).
  • Gently press the cover to make it adhere and remove the completed shell from the vice.

In addition to functionality, the actuator should be tested for capacitance and insulation resistance after assembly.

Other examples

Integration on to Flexible Bodies

The actuators may also be integrated onto flexible, non-rigid bodies.  This allows to for the reliable transfer of energy between the actuator and the subject when embedded into a flexible substrate. The table below shows materials involved in the process of integrating onto flexible bodies.

Integrating onto a flexible body follows the same process as with a rigid body. Integrating smaller actuators do not require the use of a foam core. Wiring should be secured with the two layers of covers and the double sided adhesive and can go directly to the connector for the power supply.

To have a bigger shape and to place more than one assembled actuator, a foam substrate can be used. In the following, few suggestions can be given for the application of the actuators when using a foam substrate.

  • In order hide the wiring, it is possible to create channels in the foam with a laser to hide the wires. Kapton strips can be used to keep the wires in the desired tracks.
  • The assembled actuator should be positioned on the observe side of the foam with the actuator facing the foam (flex substrate of the actuator visible). The wires should be housed on the reserve side of the foam. Therefore, the foam should be pre-cut in a way to allow the wires to pass through.
  • In case a heavy cable is needed to connect several actuators to their power supply, a mechanical strain relief is needed to lock the cable to the foam and/or final cover.

Haptic Effect Design

As indicated in the datasheet, the actuator module can be driven by voltage waveforms with the following characteristics:

Some haptics effects have been obtained adapting their equivalent sound waveform. Sound effect might be reproduced by the haptic actuator; it can be reduced or avoided keeping frequency below 200 Hz and adapting the overall waveform.

Note: For the waveform design, in addition to the characteristics of the Film Flex Assembled Actuator, the parameters of the driving electronics must be considered, e.g.:

  • Peak to peak Voltage
  • Current
  • Slew rate
  • Frequency
  • Temperature

References

Haptics In Neuroscience

https://www.computer.org/csdl/journal/th/2016/04/07782886/13rRUyY294K

The Sensory Neurons of Touch

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3811145/

Somatosensation

https://courses.lumenlearning.com/boundless-biology/chapter/somatosensation/#:~:text=Meissner’s%20corpuscles%2C%20also%20known%20as,low%2Dfrequency%20vibration%20or%20flutter

Tactile Sensibility in The Human Hand: Relative and Absolute Densities of Four Types of Mechanoreceptive Units in Glabrous Skin

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1979.sp012619