The Science of Touch in Electronics Haptics, it Used to be All about Resonant Frequency

Button shell

The first rigid support developed by KEMET was a round button, named Button shell. The construction and components disposition are reported in Fig. 4 while materials and layers thickness are listed in the Table 2.

Foam
As discussed previously, in order to provide a light support to the FFAA when working, the active part of the FFAA can be supported by a thin foam layer placed below the actuator itself (Fig. 5). It can be integrated to the rigid part of the shell in several ways: the seat of the shell is designed with a rigid bottom or an external rigid support is placed in the cavity.

Independently by the support chosen, one important consideration that needs to be done in the design phase of the seat is that it needs to be big enough to allow the wires layout without constrains. In Table 3 are reported the main properties of the foam used in Button shell.

Soldering and wiring
Another important aspect of the rigid shell design is the shape of the seat frame. The frame can be composed of a single tooth (Fig. 6a circled area) usable as a reference to facilitate the FFAA positioning near to the soldering area. A second design approach of the frame shape can be to add recesses on the edges (Fig. 6b) in order to hide the conductive paste drops at the four corners of the actuator assembly; this will be helpful with the target of obtaining a flat surface of the FFAA with the surface covering layer. Until now KEMET experience in wiring and final assembling of the FFAA into the fully integrated device, is fully manual. Therefore, it was observed that also the soldering of cables to the actuator plays an important role regarding mechanical stresses and potential failures. Examples of wires and soldering paste used in Button shape are reported in Table 4.

During assembly and later during handling of the FFAA, wires work as leverages towards the actuator. It could happen that during final assembly, just because of rough handling, the wiring can be partially disconnected from FFAA jeopardizing the overall manufacturing process. Moreover, stronger mechanical stresses can potentially affect the solder paste at the four corners of the actuator and therefore the film layers constituting the actuator. To avoid any mechanical stress on the FFAA due to wires, cable soldering can be performed both following the same plane of the Flex Assembled Actuator, or normal to it (Fig. 7). All the assembly and testing steps need to build the final integration shape shall be treated carefully, guaranteeing anyhow the easiest path to the driver connectors.

Double sided adhesive tape
Either heat activated or pressure sensitive adhesives can be used to attach 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. Mechanical adhesion is easier compared to heat activated adhesion: it is suggested to avoid air bubbles within the adhesion area.

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

• to finish the user facing surface after the actuator positioning in the designated area
• to keep the FFAA in the designated position providing the mechanical constraints without affecting the vibration performances
• to guarantee electric shock and humidity protection.

Assembly procedure suggestions
The following suggestions for assembly are referred to the Button shell shape described above but are appropriate for the application of any general surface cover positioned on a rigid structure with a seat:

• Concerning the double-sided adhesive, both paper layers protecting the adhesive should pre-cut for an easy removal in the next steps. Moreover, to ease the surface cover positioning, the adhesive could be pre-assembled on the inner side of the cover (a paper layer will remain on the cover side intended for the adhesion on shell side). The cover should be placed with the exposed adhesive area on the edge of the shell. Then the paper layer should be removed while making the adhesive layer adhere to the shell (flex PCB side of the assembled actuator). The double side adhesive shall also be mechanically activated by gently pressing the cover to make it adhere, while also removing small air bubbles.
• During the assembly process the rigid shell could be hold steady (for instance in a vice).
• FFAA should be kept in the correct position into the shell adding a layer of adhesive Kapton between FFAA and the shell, paying attention to make it tight without air bubbles.
• It is good practice, at the end of the assembly, to test the assembled actuator for actuator activation, Capacitance and Insulation Resistance at HV.

Other integration ideas
Below are reported other three integration examples of KEMET FFAA in well know shape, specifically a joystick, a curved button and a mouthpiece. The structure of the different devices is depicted in Fig. 8.

Flexible shape integration
Since the main characteristics of the FFAA are its flexibility, conformability and localized haptic sensation, there might be soft structures, where it can be effectively used. Gloves and shirts are potential applications as soft and flexible shapes. Effective integration in soft shapes may be achieved following most of the suggestions and instructions given for the hard shapes. The easiest and smaller shape possible, requires just to cover the FFAA with proper surface cover and adhesive (both materials reported in this paper are suitable for the purpose) without any supporting mean and in this configuration, no foam layer is required. Wiring should be secured within the two layers of covers and the double sided adhesive and can be positioned directly on the soldering areas between FFAA and cables.

It is possible also to create a large shape or surface equipped with more than one FFAA, once the cable path and general architecture of the FFAA layout is defined, the main topic to be addressed is the mechanical fixing of FFAA to the designated area(s) and how to hide the wires. For both purposes, in KEMET integrated devices, the approach used to resolve the issue was to apply a small layer of foam between the two cover layers. Materials and layers thickness, used for the construction of flexible pads, are listed in the Table 5. Moreover, to hide and host the wires, tracks were carved on the foam with a laser engraver. The FFAA should be positioned on the top side of the foam with the actuator facing the foam (flex PCB substrate of the actuator visible). The wires should be placed on the rear side of the foam; therefore, the foam should be pre-cut to allow the wires to pass through.

In case a multipolar cable is needed to connect several actuators to the power supply, a mechanical constraint, such as some Kapton strips, is needed to lock the cable to the foam and/or final cover. In Fig. 9 is reported the structure of a flexible and soft pad able to integrate one to four (and potentially more) actuators.

Effect design
Haptic effects on the market are subject to the related technology. Each technology has its own frequency and amplitude range where the haptic effect can perform at its best. As indicated in the datasheet available in the KEMET Website, the Film Flex Assembled Actuator module can be driven by voltage waveforms with the characteristics reported in the following table.

Regardless the complex phenomenon which happens in the raw material when voltage is applied, the final effect of the Voltage waveform application is the vibration of the actuators, the haptic effect. The final outcome to the user depends not only on the waveforms but also on the individual perception. As previously reported, the most sensitive areas of human skin are located at the pads of our fingertips with a high sensitivity towards transient and oscillating external stimuli. The high sensitivity can detect a transient skin displacement on nanometer-scale, covering the frequency range from 20 Hz up to over 800 Hz, with a peak corresponding to 250 Hz.

KEMET developed some waveforms to drive its FFAA and to show its capabilities through a Demo kit which provides an extensive multisensory user experience including touch, sound and sight stimuli. Voltage peak to peak values (around 200V) and frequency ranges (less than 200Hz) of the waveforms, which drive the KEMET FFAA, seem to be well aligned to the values identified in literature as suitable to be recognised as the stimuli for the human fingertips. Regardless the individual perception and sensitivity, peaks of the waveform are connected to the perception on the skin of the vibration, while frequency is associated to overall content to be transmitted.

Either waveforms with regular period or without a specific period have a meaning in the haptic simulation. Clock, heartbeat, and other periodic waveforms have been implemented as driving haptic effects for the FFAA and can be easily identified by the human fingertips. As well, some waveforms without a specific period, which has been developed to reproduce a kind of electric discharges or water movements (random waveforms), are still well identified by the human fingers; examples of both random and periodic waveforms are reported in Fig. 10. Some waveforms have been obtained adapting their equivalent sound waveforms. Sound effect might be reproduced by the FFAA; it can be reduced or avoided filtering frequencies above 200 Hz and tuning the overall waveform.

Regardless some hints on how to mimic the natural touch sensations through the waveforms building criteria, there will be as many waveforms as the vary many surfaces. Therefore, for any application some time should be spent in defining the best waveform (or set of waveforms) to get to the most natural fingertips response.