How to create a touch screen you can “feel”

Thanks to smartphones, touch screens have been part of modern life for more than a decade. It is no exaggeration to say that the touch screen marks a revolution in Human-Machine Interfaces (HMI). Since the advent of touch screens, mechanical HMI devices such as keyboards, trackballs and styluses have gradually been replaced by fingers, which are all you need to operate them.

As screen sizes grew larger, the few remaining buttons on phones became increasingly redundant and were “removed” entirely from some models a few years ago. Hence the screens of the latest smartphones are all touch-sensitive. However, this is not without its problems for users. For example, it is difficult to distinguish one phone from another when a few of these dark-screened phones are placed alongside each other.

Furthermore, while the tactile experience of using mechanical HMI devices with buttons provides clear feedback that allows users to “feel” and validate their actions, an ordinary touch screen does not. For example, when you click on a virtual button on a touch screen, it doesn’t necessarily ensure that the "touch" has been registered and its corresponding function activated.

To solve this problem, other complementary technologies were developed. For example, haptic feedback technology is able to compensate for the shortcomings of touch screens. Basically, haptic feedback is the generation of special vibrations by a special actuator to mimic the feeling of pressing a physical, mechanical button. The same actuator can also generate different vibration signals that correspond to different actions in order to achieve more realistic and personalized effects.

Thanks to this ability, haptics is an excellent “companion” for touch screens in that it gives users a “sense” of “touch”. Although the main sources of external information are visual and auditory, many users agree that the tactile stimulus enabled by haptics actually enriches the user experience. Thus, haptics has become an increasingly important technical option for HMI designs.

Haptic feedback actuator

As mentioned, one of the most critical components for achieving haptic feedback is the actuator. Choosing the right haptic feedback actuator is the critical first step in the design.

Actuators used for haptic feedback generally fall into three categories:

• Eccentric rotating mass (ERM): It generates vibration through an eccentrically rotating motor. It’s one of the most mature and widely used technologies, and has obvious advantages in terms of cost. However, due to the inertia of the ERM, the response speed (start time) is slow, the noise during operation is relatively loud, and the resulting waveform is relatively simple.
   
• Linear resonant actuator (LRA): Though LRA and ERM are both inertial haptic actuators, their mechanical structures are different. The LRA is essentially a magnet surrounded by a coil and connected to a spring. The electrical signal in the coil controls the magnet's linear movement to reach the resonant frequency and achieves different haptic effects through the modulation of different resonance amplitudes. Compared to ERM, LRA gives a faster response, consumes less power, and can produce more vibration waveforms. However, ERM also has its weaknesses – its operating bandwidth is narrow, it is sensitive to the environment, and its long-term performance can suffer from inconsistency.
   
• Piezoelectric actuator: Piezoelectric actuators are a rising star in the field of haptic feedback. They generate vibrations through the piezoelectric effect – the device produces a bend when the voltage is applied, which consequently generates a vibration. Compared with inertial actuators such as ERM and LRA, the piezoelectric actuator is not limited by frequency and amplitude, and can generate richer and more delicate vibration waveforms. Its response time can be mere milliseconds. It makes little noise, has larger amplitudes of vibrations, and can be made into a more compact size. However, the peak voltage (100-200 volts) required to drive piezoelectric actuators is higher, requiring a dedicated driver that consumes more power than the LRA.

At the onset of the design process, the developer can accurately measure the cost, response time, power consumption, vibration waveform and other features of various haptic feedback actuators, and then select the one best suited to the application at hand.

HD driver design

Obviously, designing a complete haptic feedback system, especially an “HD” haptic feedback system that provides complex waveforms and a better user experience, requires components in addition to an actuator. These include:

• Haptic driver: The haptic driver is interposed between the main controller and the actuator and contains the necessary analog function and digital interface to provide drive signals to the actuator in accordance with the instructions of the main controller.
• Software: A waveform used to generate haptic feedback that can be run in an application processor, microcontroller or integrated driver, depending on the system’s needs.

Inevitably, the rise of haptic feedback has also helped related IC companies identify business opportunities and launch products and solutions around their ecology.

For example, Dialog Semiconductor released three haptic driver IC products, including the DA7280, DA7281 and DA7282, this year. The DA7280, for example, is a low-power LRA and ERM haptic driver IC with a drive capability of up to 1 kHz, which provides a more detailed and complex “HD” vibration waveform with a drive current of up to 500mA. At the same time, 360nA's no-load power consumption (76% lower than rival products) and supporting power management modes make it outstanding in terms of power consumption efficiency.

Figure 2. DA7280 haptic driver IC system block diagram (Source: Dialog)

Dialog's other two haptic driver IC products also have their own strengths. The DA7281 is designed for multi-drive systems. Its two I2C address pins can simultaneously integrate up to four DA7281s in the same system. The DA7282's ultra-low standby power consumption is even more outstanding. It provides a full-sleep mode with a standby current of only 5nA. Together, these products form a complete portfolio of haptic driver ICs to address different design needs.

In short, the combination of haptic feedback technology and touch screens continues to excite the imaginations and fire up the senses of users. Ultimately, the brand that is able to give users the most complete “sensory” experience will emerge the winner.