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High-resolution haptic experience: a different feeling!

While most smartphone and tablet users have already experienced haptic technology, the term haptics itself is poorly understood by consumers. In its basic definition, “haptics” refers to the science of haptic feedback. The most basic form of application of haptic technology is that the phone vibrates to alert the user of an incoming call or a new text message in the inbox. In these cases, the phone utilizes haptic technology to draw the user’s attention.

About one-third of smartphones use haptic feedback technology that does more than just vibrate alerts. A common example is the subtle vibration feedback that users experience when using their phone to compose an email or send a text message. When a key is pressed successfully, the phone will vibrate once to confirm. With haptic feedback, users rarely make input errors, and the operating experience is more satisfying.

Enhance user experience with haptic technology

More and more mobile devices, such as mobile phones and tablets, are using touch functions. The touch interface is so simple and intuitive that a three-year-old would unlock the smartphone and tap the YouTube icon to view a video playlist. However, touchscreens have a serious limitation: because there is no physical or mechanical feedback, user interaction or reminders cannot be achieved. Designing excellent haptic feedback capabilities can greatly enhance the overall user experience of touch-enabled mobile devices.

The application of haptic technology goes far beyond user reminders or keystroke confirmations. Standard gesture operations, such as swiping to unlock, fingers to zoom pictures, and drag up and down to turn pages, can all have their own tactile/perceptual features. The haptic feedback intensity also increases when the user views the image at its maximum size. Haptic feedback is also faster when scrolling quickly. When this context-sensitive feedback is combined with audible feedback, the resulting user experience is highly satisfying and intuitive.

Plus, haptic feedback brings another level of fun. Many people use mobile devices to play games. Haptic feedback technology can greatly improve the gaming experience. For example, in a first-person shooter game, the protagonist of the game can really feel the shock of weapon shooting. Users can feel the bumps and bumps of the car in a racing game, feel the spring of a slingshot rope in the popular Angry Birds game, feel the real guitar strings or piano keys in a playing game, and so on. The imagination of game developers will create unlimited experience possibilities!

Inertial Haptic Actuators (ERM/LRA)

Standard haptic feedback in mobile phones is accomplished through a small motor called an eccentric rotating mass actuator (ERM). The motor is driven by a voltage and starts to rotate, and the user can feel the vibration. The haptic feedback driver chip drives the motor differentially, so the motor spins when a positive voltage is applied and stops when a negative voltage is applied in reverse polarity. This method works well when used for vibrating reminders. However, when using ERM for other haptic feedback applications, such as gaming, the device battery power drops rapidly.

ERMs have inertia and require overdrive to spin faster. Start-up time is the time it takes for the motor to reach 90% of its rated acceleration and typically ranges from 50 to 100 ms. The time frame for motor braking or stopping is similar. Triggering a very simple haptic feedback event (eg: click) takes about 100 to 200 ms. If the application requires repeated haptic feedback events, the delay of motorized haptic feedback is less than ideal.

Another aspect of ERM is the hum generated by the rotating motor, a noise audible to the human ear. This problem can be partially alleviated if haptic feedback is used in conjunction with audible feedback. However, in a quiet conference room, if someone is texting, everyone can hear the noise. In addition, the haptic feedback effect of ERM may become inconspicuous due to user actions. Vibration frequency and amplitude, related to single control voltage.

Another inertial actuator, the Linear Resonant Actuator (LRA), is used in some smartphones for haptic feedback and vibration alerts. Compared with ERM, the mechanical structure of LRA is different. It consists of a mass mounted on a spring that vibrates in a linear motion. LRAs must be driven at narrow resonant frequencies. Also, its startup time is slightly better than ERM.

Depending on the manufacturer, the start-up time ranges from 40 to 60 ms (Figure 1), which is better than the ERM’s start-up time (50 to 100 ms). By modulating the resonant carrier amplitude, various haptic feedback effects can be produced.

High-resolution haptic experience: a different feeling!

picture1 LRA The typical start-up time is40 arrive60ms

High-resolution haptic feedback

High-definition (HD) TVs have a higher resolution than standard-definition TVs, so they can provide a sharper and clearer picture. Likewise, high-resolution haptic feedback allows users to experience a more pronounced vibration effect than the humming vibration of an inertial drive. Piezo or ceramic haptic feedback actuators are used to achieve HD haptic feedback, providing a beautiful experience that is different from ERM/LRA.

Piezo Actuator

When a differential voltage is applied across the piezoelectric actuator, it bends and deforms, creating vibrations. Piezo actuators require high pressure to deform. Depending on the manufacturer, the voltage can be 50 to 150VPP. Higher voltages require fewer piezoelectric layers; therefore, the voltage is 150VPPthe piezoelectric actuator has about 4 layers, and the voltage is 50VPP, there may be 16 to 24 layers. At higher voltages, piezoelectric actuators have less capacitance due to the reduced number of layers. In other words, less current is required to drive a low-capacitance haptic feedback actuator.

Piezo actuators take the form of discs or rectangular strips, also known as benders. The piezoelectric disc deforms vertically and can be used for Z-axis vibration. Piezo benders can be mounted directly on a “floating” touchscreen to vibrate the screen (Figure 2a). Piezobenders can also be mounted into a small module, which in turn can be mounted to the device’s housing or PCB, allowing the entire device to vibrate (Figure 2b). Piezo modules are popular because of their simple mechanical integration.

High-resolution haptic experience: a different feeling!

picture2 Piezo Actuator Form Factor

What makes piezo actuators high resolution?

Four factors differentiate piezoelectric actuators from inertial actuators:

1. Faster startup: Due to its inherent mechanical properties, piezoelectric actuators have very short startup times—generally less than 15ms, which is 3 to 4 times faster than ERM. Overall haptic feedback event duration is 70ms shorter than ERM. Figure 3 describes this in detail.

2. Higher bandwidth: The higher bandwidth of the piezoelectric actuator (see Figure 4) can provide a more delicate combination of tactile feedback and achieve more effects.

3. Lower noise: Unlike ERM, piezoelectric actuators have no mechanical noise generated by rotating masses.

4. Stronger vibration: Piezoelectric modules can generate higher vibration intensity. Figure 5 shows the acceleration characteristics of a commercially available piezoelectric module, while Figure 6 shows the acceleration characteristics of a commercially available LRA. We can see that compared to LRA below 1.5 GPPof peak-to-peak acceleration, the piezo actuator produces 3 GPPof peak-to-peak acceleration. This higher vibration intensity means that piezoelectric modules are ideal for large-screen smartphones and tablets.

High-resolution haptic experience: a different feeling!

picture3 Typical start-up times for piezo modules are~14ms

High-resolution haptic experience: a different feeling!

picture4 Piezo actuators (ideal modules) have higher bandwidth

High-resolution haptic experience: a different feeling!

picture5 Acceleration characteristics of piezoelectric modules

High-resolution haptic experience: a different feeling!

picture6 LRAs acceleration characteristics

Current consumption of piezoelectric actuators

Although piezoelectric actuators require higher voltages than standard inertial actuators, the actual current consumption is lower than ERM and about the same size as LRA (see Table 1).

surface1 Power consumption of haptic feedback actuators

in conclusion

Compared with inertial actuators, piezoelectric actuators have large performance and cost advantages. It has a shorter startup time and helps keyboard applications achieve strong and brisk click confirmations. Its high-bandwidth advantage helps achieve haptic effects that are more perceptible to users, which is critical for gaming applications. Piezoelectric actuators have stronger vibrations and can bring haptic feedback to some large consumer devices such as tablets and e-readers. In conclusion, piezoelectric haptic feedback technology has many compelling properties that can enhance the haptic feedback experience and help improve the overall user experience of mobile devices.

TI supplies both analog input (DRV8662) and digital input (DRV2665) piezoelectric haptic feedback drivers that can be used with a variety of piezoelectric actuators on the market. In addition, TI also provides a number of “refreshing” demos for designers.

Related websites and reference documents

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