High-resolution tactile experience: a different feeling!

While most smartphone and tablet users have experienced haptic technology, the term haptics itself is less well understood by consumers. At its most basic definition, haptics refers to the science of tactile feedback. In its most basic form, haptics is used when a phone vibrates to alert the user to an incoming call or a new text message in the inbox. In these cases, the phone uses haptics to get the user's attention.

About one-third of smartphones use tactile feedback technology, and its functions are no longer just vibration reminders. A common example is that when using a mobile phone to write an email or send a text message, the user will feel a subtle vibration feedback. After successfully pressing a key, the phone will vibrate once to confirm. When using tactile feedback, users rarely make input errors and the operation experience is more satisfactory.

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 can unlock a smartphone and click on the YouTube icon to view a video playlist. However, touch screens have a serious limitation: there is no physical or mechanical feedback, so user interaction or notifications cannot be achieved. Designing a good tactile feedback function can greatly enhance the overall user experience of touch-based mobile devices.

The applications of haptics extend far beyond user alerts or key confirmations. Standard gestures such as sliding to unlock, pinching to zoom, and dragging to turn pages can all have their own haptic/perceptual characteristics. The haptic feedback increases in intensity when the user maximizes the image size. The haptic feedback also comes faster when scrolling quickly. If this context-sensitive feedback is combined with audio feedback, the resulting user experience can be highly satisfying and more intuitive.

Haptic feedback also brings another kind 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 game protagonist can actually feel the shock of a weapon shooting. Users can feel the collision and bumps of the car in a racing game, feel the elastic force of releasing the slingshot rope in the popular "Angry Birds" game, and actually feel the guitar strings or piano keys in a performance game. The imagination of game developers will create endless experience possibilities!

Inertial tactile actuator (ERM/LRA)

Standard haptic feedback in mobile phones is achieved through a small motor called an eccentric rotating mass actuator (ERM). The motor is driven by a voltage to start rotating, and the user can feel the vibration. The haptic feedback driver chip drives the motor differentially, so the motor rotates when a positive voltage is applied and stops when the opposite polarity, or negative voltage, is applied. This method is very effective when used for vibration reminders. However, when ERM is used for other haptic feedback applications, such as gaming, the device battery power drops rapidly.

ERMs have inertia and need to be overdriven to spin faster. Startup time is the time it takes for the motor to reach 90% of its rated acceleration and is typically in the range of 50 to 100 ms. The time it takes to brake or stop the motor is in a similar range. Triggering a very simple haptic feedback event (e.g., a click) takes about 100 to 200 ms. If the application requires repeated haptic feedback events, the latency of motor-based haptic feedback is less than ideal.

Another aspect of ERM is the hum of the rotating motor, which is audible to the human ear. If tactile feedback is combined with acoustic feedback, this problem can be partially alleviated. However, in a quiet conference room, if someone is texting, everyone can hear this noise. In addition, the tactile feedback effect of ERM can be muted by user operation. Vibration frequency and amplitude, related to a single control voltage.

Another type of inertial actuator, the linear resonant actuator (LRA), is used in some smartphones to provide tactile feedback and vibration alerts. The LRA has a different mechanical structure than the ERM. It consists of a mass mounted on a spring that vibrates in a linear motion. The LRA must be driven at a narrow resonant frequency. In addition, its startup time is slightly better than that of the ERM.

Depending on the manufacturer, the attack time ranges from 40 to 60 ms (Figure 1), which is better than the ERM attack time (50 to 100 ms). By modulating the amplitude of the resonant carrier, a variety of tactile feedback effects can be produced.

Figure 1 The typical startup time of LRA is 40 to 60ms

High-resolution tactile feedback

High-definition (HD) TVs have higher resolution than standard-definition TVs, so they provide sharper and clearer images. Similarly, high-resolution haptic feedback can also make users feel more obvious vibrations than the buzzing vibrations of inertial actuators. Piezoelectric (piezo) or ceramic haptic feedback actuators are used to achieve HD haptic feedback, providing a different and better experience than ERM/LRA.

Piezoelectric Actuator

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

Piezoelectric actuators come in the form of disks or rectangular strips, also called benders. The piezoelectric disk deforms vertically and can be used for Z-axis vibration. Piezoelectric bender can be mounted directly on a "floating" touch screen to achieve screen vibration (Figure 2a). Piezoelectric bender can also be mounted in a small module, which can be mounted on the device's housing or PCB to achieve overall device vibration (Figure 2b). Piezoelectric modules are popular because they are easy to integrate mechanically.

Figure 2: Shape factor of piezoelectric actuator

What gives piezoelectric actuators high resolution?

Four factors distinguish piezoelectric actuators from inertial actuators:

1. Faster actuation: Due to its inherent mechanical properties, piezoelectric actuators have very fast actuation times—typically less than 15ms, 3 to 4 times faster than ERMs. The overall haptic feedback event duration is 70ms shorter than that of ERMs. Figure 3 illustrates this in detail.

2. Higher bandwidth: The higher bandwidth of piezoelectric actuators (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 mass.

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 the piezoelectric actuator generates a peak-to-peak acceleration of 3 GPP compared to the peak-to-peak acceleration of the LRA of less than 1.5 GPP. This higher vibration intensity means that piezoelectric modules are ideal for large-screen smartphones and tablets.

Figure 3 Typical startup time for a piezoelectric module is ~14 ms

Figure 4 Piezoelectric actuators (ideal modules) have higher bandwidth

Figure 5 Acceleration characteristics of piezoelectric module

Figure 6 LRA acceleration characteristics

Current consumption of piezoelectric actuators

Although piezoelectric actuators require higher voltage than standard inertial actuators, the actual current consumption is lower than that of ERMs and is about the same as that of LRAs (see Table 1).

Table 1 Power consumption of tactile feedback actuator

in conclusion

Compared with inertial actuators, piezoelectric actuators have great performance and cost advantages. It has a shorter startup time, which helps keyboard applications achieve strong and light click confirmation. Its high bandwidth advantage can help achieve tactile effects that are easier for users to perceive, which is crucial for gaming applications. Piezoelectric actuators have stronger vibrations and can bring tactile feedback to some large-size consumer devices (such as tablets and e-readers). In short, piezoelectric tactile feedback technology has many attractive features that can enhance the tactile feedback experience and help improve the overall user experience of mobile devices.