Vibrotactile Feedback Design Issues In Mobile Devices

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and improve the user experience. ERM vibration motors and electromagnetic actuators are generally presented as linear resonance actuators (LRA), and specific modifications of the voice coils and solenoid type actuators. So far PrecizeMicrodrives^TM supplies wide range of ERMs which are able to generate magnitudes of vibration signals from 0.5G to about 150G. However, the maximum vector of torque can vary from a normal to tangential direction and cannot exactly be specified. Among Voice Coil Actuators (VCAs), HiWave/Tectonic Ltd, and Redux Labs produce a variety of highly efficient one-directional electromagnetic transducers that support generating oscillations in the entire audio frequency range, while the vibrations below 500Hz can also be used for tactile stimulation. The inertia of the magnet mass enables the moving coil to impart bending waves into the interaction environment. Other technologies, like piezoelectric actuators, allow developers to create bending actuators, enhanced with inertial mass, which can be used for tactile stimulation. At a minimum deflection of 100 μm, these types of actuators are able to exert forces of about ~0.3N and higher that can be properly assembled into the specific design-configuration to generate bending waves in the screen overlay.

AAC Technologies Inc. has developed piezoelectric vibrotactile actuators for haptic applications. These are comparatively powerful and efficient in converting inertial mass moments into vibrotactile feedback for mobile devices. However, their force and displacement are function of the electric field and voltages applied to the piezoelectric elements to generate well-perceivable haptic signals (generally, the applied voltage varies from 30V to 180V and higher). Furthermore, amplified piezoelectric actuators can mechanically be magnified. The mechanical amplification is obtained due to an external elliptical spring frame made of stainless steel that magnifies vibrations along the short axis when piezoelectric actuator extends along the long axis. The pre-stressed and magnified flextensional frames have better performances than traditional mechanical amplifiers based on lever arms and flexure pivots, but require higher voltages and greater power, preventing their use for mobile application. Furthermore, recent LRAs developed by AAC, utilize composite materials instead of piezoelectric ceramics, and perform better at generating low frequency haptic signals.

However, force-moment response delay can be an issue in such actuators, as they need to balance generated forces with acceleration. Thus, these types of actuators may not be ideal for providing short precise information signals.

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Essentially, there are two key issues to consider while designing vibrotactile feedback for mobile and handheld devices. Firstly, it is important to define and target a specific stimulation, and outline the

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physical signal parameters that can provide this perceptual stimulation for most users. Once this process is completed it is crucial to select the correct actuator or actuation technology which can, in most environments, reliably and repeatedly generate the intended signal through the device.

And secondly, designers must account for and mitigate the environmental factors which may hinder or alter the physical signal reaching the skin contact (at the desired area of interaction on the mobile device). This includes, but is not limited to signal mediation, ensuring signal integrity, and reducing signal attenuation or signal to noise ratio. In this section, we discuss these issues and other limitations which need to be considered while designing vibrotactile feedback in mobile devices (adapted from Farooq et al., (2016a) and Farooq et al., (2016b)).

Limitation in Current Actuators

If we consider actuators with eccentric or inertial mass, they generally do not have a direct link for the transition of generated force-moments and torques via the movable mass or the shaft from which the force is exerted to the surface of interaction. Therefore, only a minor portion of the force’s component produced can be delivered to the specific location of the human finger. Due to this key drawback, eccentric or inertial mass rotating motors can only be utilized for raw vibration feedback associated with the shaking of the entire device and not delivering localized haptic content or events at the point of interaction. Such actuators lack the ability to generate complex vibrotactile signals and patterns such as normal (vertical) or tangential (horizontal, lateral and longitudinal) screen micro-displacements localized to a desired location on the screen at the point of contact.

Similarly, the energy transformation of piezoelectric actuators into mechanical movements is often subject to friction between movable components of actuators and inertia of their moving mass. Therefore, part of the energy that was not transformed into movement will be converted into heat which has to be dissipated with a heat sink that should also be considered in designing of haptic transducers. Self-locking linear actuators like PIShift Piezo Motors (inertia based drives), which are powerful linear actuators (holding force of 10N), resolve the heat generation issue but cannot compare well, as they are slow in providing maximum velocity (5mm/s at 15watts consumption), needed for desired long-stoke actuation.

In essence, each type of actuator utilized for haptic signal generation has its unique advantages and disadvantages which need to be considered when designing perceivable haptic parameters and the entire system.

Because most systems do not employ these actuators directly to the actuation surface it is also important to mitigate side effects which may hamper or interfere in signal propagation. During this research, we have

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been able to closely work with manufacturers of actuators (FUKOKU Co., Ltd, and AAC Technologies Holdings Inc.) to test and evaluate the latest prototypes of piezoelectric exciters. Voice coil transducers are also able to generate powerful vibration signals and micro-displacements (HiWave technologies PLC) in a wide range of frequencies and magnitudes (see Table 1). As speculated by Fröjd and Ulriksen (2015), voice coil transducers can be a more efficient alternative to piezoelectric transducers in generating mechanical waves within a low frequency range.

Need for the Mediation of Vibrotactile Signals

Looking at the tactile signal being developed for touchscreen interaction, it is obvious that the signal being generated is greatly attenuated or altered before it reaches the touchscreen itself. Considering the tactile signals as haptic energy being transferred to the skin contact via the touchscreen, neglecting the decay in mechanical energy propagation from the actuator to specific receptors in the skin, means we are ignoring the mutation of the actuation signal (see section 3.8). This transmission mutation largely attenuates the magnitude of the tactile signal, interferes with noise, renders it weak and it becomes less informative (Mortimer et al., 2007) as it reaches the main interaction area (touchscreen). There are two key factors adversely affecting signal integrity, as discussed in Chapter 3.

Firstly, mechanical impedance of each component involved in the mechano-transduction process (by transferring the tactile information signals from the actuator to the point of contact) is largely inconsistent.

With the concept of direct touch (Poupyrev et al., 2002), as different materials having different physical properties (density and mechanical impedance), modulate energy of stimuli differently, efficiency of the transmitted signal becomes very poor.

And secondly, investigations into the mechanical impedance of the skin at the fingertip have shown a nonlinear increase in stiffness when pressure is produced against the contact surface until a maximum skin indentation of approximately 3 mm (Gerling, 2010; Gennisson et al., 2004), which mean that the skin receptors (in the fingertip) are greatly dampened with interacting with the hard glass surface. This happens because the skin is depressed against the hard glass surface, dampening not only the mechanoreceptors in the skin but also changing the mechanical impedance of the touchscreen surface (i.e. point of contact) (Evreinov et al, 2016c).

Putting the two issues together would suggest that primarily the applied signal is greatly contaminated during transmission from the source to the area of the touch contact, and once the information signal does reach the point of contact (on the touchscreen surface), fingertip receptors are unable to detect and recognize the entire signal as they are considerably deadened in interaction with a ridged glass surface. Essentially this means that not even a fraction of the original generated signal reaches the user or receiver, hampering any possibility of effective information feedback.

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