Haptic Feedback Technology: An Innovator of Realistic Vibration Experience
Why do people give such a weird name to such a “cool” functional technology? Literally, it confuses me. The word “Haptics” comes from the Greek word “ἅπτω,” which means “I stare, I touch.” Basically, haptic systems use haptic vibrations to provide operational feedback. The Greeks didn’t use the word much since it was invented, and it didn’t give it a new meaning until modern haptic technology was widely used in all walks of life. First used in aviation, this technology allows pilots to “feel” the simulated vibration of the joystick when the plane’s engines are turned off. On older planes, this vibration is real, but with improved control systems, the plane will detect it and force it back into the system.
In recent years, haptic systems have expanded into simulation and electronics. Devices that allow users to feel and feel things in remote (or virtual) environments are already widely used in mining, architectural design, education, and even telemedicine. On a more personal level, haptic feedback technology can allow you to watch a movie in silence while being reminded that you have a meeting to attend, or being reminded of a lottery winning text message without your neighbors ignoring it. In the gaming world, when your car is about to pull off the road, or you get injured in a Halo grudge match (Xbox game), because your controller has embedded actuators integrated and programmed into the game , haptic feedback technology will alert you.
How important this technology is to you, let’s talk about how it works! Basically, there are 2 types of tactile sensing technologies on the market today: the old school and the new school. However, both factions are inherently motor-based. Each topology has its own advantages and disadvantages and unique features. We now delve into each topology.
Deflection Mass (ERM) – Old School
Deflection mass is the oldest and most mature haptic feedback technology on the market. Think back to all the vibration-capable devices from your childhood, most of which were made by ERM. As shown in Figure 1, the ERM consists of an eccentric rotating mass that generates an omnidirectional vibration when it rotates, and the vibration spreads throughout the device. For example, the vibration can be used to alert you when your phone is in silent or vibration mode.
Figure 1. Structure diagram of deflection mass (ERM) haptic actuator
Unfortunately, the ability to form complex waveforms is limited due to the structural issues of the ERM. The frequency and amplitude of each wave are coupled together to the input control voltage, allowing you to use only one variable to create different vibration effects. In general, you only get different combinations of pulses or speeds, which are similar to Morse code. Compared to newer technology, this method of waking up the motor to let it work and then stop it has certain limitations. ERM is a relatively slow option when speed and response time are required. However, the advantage of this technique is that, since it has been around for quite some time, it is one of several cost-effective options currently available.
Linear Resonance Actuator (LRA) – Emerging
The next generation of haptic feedback technology is the linear resonant actuator, which has been widely adopted by many new handheld device manufacturers. An LRA is basically a spring-connected magnet, surrounded by a coil, and housed in a box-shaped enclosure, as shown in Figure 2. The magnets are controlled to move in a linear fashion, eventually reaching the resonant frequency. This operation at the resonant frequency allows the driver to operate at lower power consumption, on average 30% lower than the ERM; however, it is limited by this frequency.
Efficiency and performance are greatly reduced when the LRA drive frequency is shifted outside this resonant frequency band. This becomes a design issue that needs to be addressed, as the spring constant can change due to losses, temperature fluctuations, or other environmental changes, such as whether the LRA device is stuck, etc. (If not, there is no performance concern.)
Figure 2 Linear Resonant Actuator (LRA) Haptic Actuator
Although there is no flexibility in frequency, the amplitude of the input signal can still be adjusted. The effect of sending this signal is to add extra degrees of freedom and unique waveforms that cannot be achieved with ERM. In terms of response time, LRAs also beat ERMs, as they can provide keystroke confirmation feedback for typing multiple letters in a second, making them ideal for text messaging or any input application.
We’ve covered both the old and new schools of haptic actuators, but there’s still another actuator we haven’t covered. This actuator is not a motor type, it has amazing response time, high energy efficiency, and has a much smaller size than ERM and LRA. This ideal new device is called a piezoelectric actuator.
To be precise, piezoelectric technology is not cutting-edge technology, because it has been around for decades and basically consists of a film (vibration-voltage converter). Previously, this technology has been used in many energy harvesting applications and driving speakers, but now it will enable the most complex and detailed haptic feedback experience. Brand new applications take this mature technology into a new field. Standard piezoelectric actuator technology uses a thin strip or disk, bends them and bounces back, creating vibrations by applying a voltage across them (Figure 3). One way to use a thin strip is to mount the piezoelectric strip ends to the touchscreen and then attach the center of the strip to the device housing. Afterwards, the touchscreen is mounted into a housing so that the strips can “float” so that the piezoelectric vibrations of the screen can be visibly felt. This experience is called “local touch.” You can still feel some vibration from the device itself, but most of it comes from the screen. If the screen is not required to vibrate locally, another topology known as a drop-in module can be used. It is similar to a piezo actuator, but less functional: the level of vibration precision is not as high as that of a local piezo haptic, but it can greatly reduce the complexity of the design.
Figure 3 Piezoelectric haptic actuators typically use a thin strip or flat disc that vibrates when a voltage is applied
Piezoelectric haptics do not have any frequency or amplitude limitations, and designers can achieve waveforms beyond what is possible with LRAs and ERMs. While you won’t be able to feel the precise tactile feedback you get when you press a mechanical button, leveraging piezo-type haptics will make the two feel very close in the future. Embedding multiple piezo modules in a design creates a high-precision haptic feedback experience that vibrates only part of, but not all, areas of the touchscreen. In capacitive touch-driven applications, each touch point (finger) can feel its own unique wave response, rather than the entire screen vibrating.
One disadvantage of piezoelectric actuators is that most systems require about 100-200 volts peak-to-peak (Vpp) voltage to drive the entire device. Multilayer piezoelectric actuators can reduce this system voltage to 50 Vpp, but this multilayer piezoelectric actuator is expensive. Figure 4 describes the characteristics of this actuator in terms of speed and response time. ERMs and LRAs have response times in the 30 to 60 ms range, while piezo actuators are typically less than 2 ms! This property makes them much more effective than ERM and LRA. With piezoelectric technology, you can achieve higher speeds, get the ideal vibration waveform faster, return to rest faster, and consume less energy.
Figure 4 Piezo haptics have extremely short startup times compared to ERM and LRA technologies
As “cool” as these actuators are, there is only one component in the wide variety that fits into the actuator. What makes this drive “great” is the support of many other products. The one component that supports the most drives is the physical drive. There are many such physical drives on the market, but only a few are specifically designed for piezoelectric actuator drives.
TI’s DRV8662 is a 200-Vp-p piezoelectric haptic driver with an integrated boost converter. With a fast 1.5ms start-up time, this piezo driver is versatile enough to be used in any high-end piezo haptic system design. The input voltage can be single-ended or differential, and can use a 3.0-5.5V power supply. Thanks to the integrated power switch and diode, the transformer is no longer required. Therefore, when using small packages, the above specifications mean that you can use less board space and a lower total system cost. Piezo haptic feedback technology is a game-changer for today’s haptic solutions, helping customers achieve the most realistic and unexpected user experiences.
To learn more about TI’s haptic feedback solutions, visit the following links:
For more details on the DRV8662 Piezo Haptic Driver, please visit: www.ti.com.cn/product/cn/drv8662
Check out a video of TI’s haptic feedback technologists explaining the technology and answering questions that may arise in your application: http://www.ti.com/drv8662video-ca
For design technical support and questions from TI experts, please visit: www.deyisupport.com
The Links: 2MBI200U4H-120 PM400DSA060