What is the 'tactile feedback' used in all the trendy HMIs? How to use it to your advantage?

When we "deal" with electronic devices, we need a human-machine interface (HMI) - through HMI, people can send out instructions, and the machine will respond according to the instructions and let the user perceive it. In most cases, the main way for people to perceive machine feedback is through vision and hearing, that is, obtaining image and sound information from the machine.

But sometimes machines can also "communicate" with people in other ways, such as touch. We are familiar with this kind of feedback from machines. For example, when a phone is on silent mode and there is an incoming call, the phone will vibrate to let the user know; or when we click a virtual button on the touch screen, we will feel like we are pressing a physical mechanical button...Haptics technology is what provides us with this tactile experience.

Simply put, tactile feedback technology is to generate a controlled vibration signal through electronic components to give users a tactile experience. Figure 1 shows a typical tactile feedback system: by sending a signal to the electronic device to trigger an event - this signal may be user input or from a sensor - the main control chip sends instructions to the tactile feedback driver IC based on the event, which drives the actuator (transmitter) to generate a corresponding vibration signal in the electronic device, which is felt by the user.

Figure 1. Typical haptic feedback system (Image source: Taxes Instruments)

It can be seen that the tactile feedback system is not complicated, but as users' demands become more and more sophisticated, for example, they will require a more timely response to trigger events, and different trigger events will produce different waveforms and bring different tactile sensations... The ever-improving user experience has put higher and higher requirements on the design of tactile feedback solutions, especially the selection and application of the two core components, the actuator and the driver IC, which need to be carefully considered according to the final application.

The "tactile sensation" is generated by the actuator. Although it is just a vibration signal, due to the different ways of generating vibration, the tactile feedback has great differences in intensity, waveform, response time, system power consumption, noise, etc.

There are currently three types of actuators commonly used in tactile feedback design. Let’s introduce them one by one.

The first is the deflection mass motor (ERM), which generates vibration by driving an eccentrically rotating motor. Its technology is mature and has obvious cost advantages, so it is widely used. However, since ERM is an inertial element, there is a significant time delay in starting (usually more than 100ms), so the response speed is slow, the power consumption is high, the noise is loud when working, and it can only generate simple waveforms, which are its inherent shortcomings.

The linear resonant actuator (LRA) is the second type. The structure of LRA is a magnet mounted on a spring. By applying a driving signal to the coil surrounding it, the magnet is controlled to move linearly to reach the resonant frequency. Developers can control the change of the waveform by modulating the resonance amplitude. Although both LRA and ERM are inertial tactile actuators, LRA has a faster response speed, can achieve more diverse waveforms, and has lower power consumption, so it is used in portable devices such as smartphones. The main weaknesses of LRA are its narrow operating bandwidth, weak environmental anti-winding resistance, and changes in consistency over long-term use.

The third type of piezoelectric actuator (Piezo) is a new technology developed in recent years. As the name implies, it generates vibrations through the piezoelectric effect. When voltage is applied to both ends of the piezoelectric actuator, it will bend and deform, thereby generating vibrations. It is precisely because of this non-inertial actuation mechanism that it has many obvious advantages over ERM and LRA:

• Fast startup: usually can reach 10mS or even lower.

• Rich waveforms: With higher bandwidth, more tactile feedback effects can be achieved.

• Strong vibration: can produce higher peak-to-peak acceleration and provide stronger tactile sensation.

• Low noise: No humming noise from ERM motors.

• Small size: The appearance can be made very thin and light, which is convenient for integration into space-constrained applications.
  

However, the piezoelectric actuator has a higher driving peak voltage V _pp (usually 50-200V), requires a special driving circuit, and consumes more power than LRA during operation, which is its weakness. However, because it can provide users with a more timely and delicate "HD" tactile experience, it has received more and more attention, and the product is constantly being optimized and upgraded.

Table 1. Comparison of haptic feedback actuator performance

For example, TDK 's PiezoHapt series The ultra-thin actuator ( PHUA8060-35A-33-000 ) adopts a single-layer piezoelectric design with a thickness of only 0.35mm, an extremely short response time of 4ms, and supports low-voltage drive of ≤24V. At a frequency of 200Hz (sine wave) and a peak voltage of 24V, the typical vibration displacement is 55 µm , and the amplitude of the vibration can be controlled by changing the driving voltage, thereby generating a more variable waveform. It is an ideal choice for touch feedback on smartphones and tablets. TDK also provides a PiezoHapt Kit evaluation development tool to facilitate developers to quickly get started with design-in.

Figure 2. TDK’s PHUA8060-35A-33-000 PiezoHapt ultra-thin actuator

Figure 3. TDK's PiezoHapt series of ultra-thin actuators (right) can generate richer vibration waveforms compared to ERM (left). (Image source: TDK)

After choosing the right actuator for haptic feedback, the next step is to match it with a suitable driver IC. The role of the haptic feedback driver is to bridge the main controller and the actuator, provide the necessary analog functions and digital interfaces, and convert the main controller's instructions into the actuator's drive signals.

When selecting a haptic feedback driver IC, the following factors need to be considered:

• Driving capability: It needs to meet the driving voltage and current required by the actuator. For example, the driving voltage of ERM and LRA is usually only 2V-5.5V, while the piezoelectric actuator requires tens of volts or even hundreds of volts.
• Control precision: Whether the product provides unique functions to improve control precision and provide a more delicate touch.
• Digital interface: If communication with a host controller is required, the driver usually needs to have a data interface such as ^I2C .
• Power consumption: Since the actuator is in standby mode most of the time, the standby power consumption cannot be ignored.
• Design resources: Can you provide software algorithms, waveform libraries and other resources that match the hardware, as well as a development environment?
  
  

Figure 4. DA7280 haptic control driver IC system block diagram (Image source: Dialog)

In addition to DVR2667, TI also provides other piezoelectric haptic driver product options: DRV2665 has the same basic analog and digital features as DVR2667, but does not have 2kB RAM for waveform storage; DRV8662 also integrates high-voltage boost and piezoelectric driver, but does not have a digital interface. The rich product portfolio undoubtedly provides developers with more options.

If needed, TI also has a more comprehensive range of tactile drivers covering three actuator types for you to choose from. It is particularly worth mentioning that TI also pre-loads royalty-free tactile waveforms designed and licensed by Immersion in the DVR2605 driver, which can be regarded as a great "benefit" for developers.

After selecting core components such as actuators and haptic driver ICs according to the needs of the final application, we can start designing the haptic feedback solution. Compared with other electronic design solutions, the haptic feedback system is not complicated, but both hardware and software preparations must be made at the beginning of the design. Let's take TI's products as an example for a brief explanation.

First, developers need to prepare an evaluation/development board based on the selected tactile driver IC. Taking TI's piezoelectric tactile driver DVR2667 as an example, its corresponding evaluation board is DRV2667EVM-CT , whose onboard resources include a microcontroller, piezoelectric actuator, waveform samples, and capacitive touch buttons, which can fully demonstrate and evaluate the functional characteristics of DVR2667. (For a detailed introduction to the development board, you can Click here Get it)

Figure 6. DRV2667EVM-CT, an evaluation board for the DVR2667 piezo haptic driver

Chip manufacturers also provide corresponding software development environments to support the development of tactile feedback solutions in conjunction with hardware evaluation/development tools. For example, the Haptic Control Console (HCC) provided by TI is a software development environment with a visual graphical interface. Through this software tool, developers can evaluate, debug and develop applications for TI's series of touch feedback products.

Figure 7. Haptic Control Console graphical user interface (Image source: Taxes Instruments)

Taking DVR2667 as an example, after the developer downloads HCC from TI's website and installs it on the computer, it connects to the DRV2667EVM-CT evaluation board through an adapter USB2ANY . Then, click the [Connect] button in the HCC interface on the computer to connect to the evaluation board and develop through the HCC graphical interface.

The HCC window interface is divided into two parts: [Setting] and [Work Mode]. In the [Setting] interface, developers can change the output voltage of the drive actuator by adjusting the gain. In the [WorkMode] interface, there are three modes to choose from:

• Internal Trigger: Developers upload waveform files to the driver's built-in RAM through the RAM Manager, and also support user-defined waveforms.

• External Analog : This mode supports receiving signals via analog or PWM input.

• FIFO Playback : Users can control the actuator in real time, output a 200Hz sine wave or a custom waveform to the actuator, with a duration of 5-20mS.

Such rich and flexible functional options allow developers to fully tap the potential of tactile feedback products and explore more application possibilities. Visit TI's website to download and learn about HCC's detailed usage guide and its support for different tactile feedback solutions.

In addition to chip manufacturers, we can also find many third-party tactile feedback development kits, such as Adafruit 2305 and Seeed 102990020. These increasingly sophisticated software and hardware development tools make today's tactile feedback application development easier.

With the advancement of technology, rich design resources from components to development tools have made innovative applications of tactile feedback possible, which has also taken the HMI experience in various fields that we are familiar with to a new level.

• In the field of mobile communications, tactile feedback not only helps to achieve a larger screen-to-body ratio for mobile phones, but also can bring personalized functional experience to APPs.

• In the industrial field, tactile feedback is combined with touch screens to replace physical buttons, enabling HMI graphical upgrades.

• In the field of autonomous driving, tactile feedback can become a new way for people and vehicles to "communicate" with each other to ensure driving safety. For example, when the vehicle deviates from the lane, the tactile feedback system will vibrate to remind people through the steering wheel or seat.

• In the emerging VR/AR field, tactile feedback can give users a more realistic sense of reality in the "virtual" world, and we can already see many games starting to test the waters.