How It Works: Linear Resonant Actuators

Diagram of a linear resonant actuator.
Diagram of the construction of a linear resonant actuator.

A linear resonant actuator is a vibration motor that produces an oscillating force across a single axis. Unlike a DC eccentric rotating mass (ERM) motor, a linear resonant actuator relies on an AC voltage to drive a voice coil pressed against a moving mass connected to a spring. When the voice coil is driven at the resonant frequency of the spring, the entire actuator vibrates with a perceptible force. Although the frequency and amplitude of a linear resonant actuator may be adjusted by changing the AC input, the actuator must be driven at its resonant frequency to generate a meaningful amount of force for a large current.

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Electrovibration and Touchscreens: Creating Virtual Textures

Graph of Perceived Friction by Voltage
A higher voltage results in a higher perceived friction.

In 1950, Edward Mallinckrodt, a researcher at Washington University in St. Louis, accidentally discovered the phenomenon of electrovibration (also known as electrostatic vibration). He noticed that a brass electric light socket had a different texture when a light was burning than it did when the light was turned off. Along with a team of researchers, he began exploring the phenomenon in more detail by running experiments using an aluminum plate with insulating varnish. They wrote:

If the dry skin of one’s finger is moved gently over a smooth metal surface covered with a thin insulating layer, and the metal is connected to the ungrounded side of an 110-v power line, the surface has a characteristic feeling that disappears when the alternating voltage is disconnected.

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Haptics and Emotion: How Touch Communicates Feelings

When we think about the content of a conversation, it’s easy to focus on just the verbal information exchanged through spoken words; however, there are many other factors that color our interpretation of conversations and, in turn, the information they communicate. One such consideration is the context provided by prosody—the intonation, stress, tempo, rhythm, and pauses in a person’s speech, all of which lend their voice a unique texture. The brain also employs detailed mappings that link different kinds of facial expressions and gestures with the emotions and nuances that they convey. In fact, up to 65% of the raw information in a conversation is exchanged nonverbally [1]. As we continue to investigate human communication, we uncover a highly complex, multi-modal system that comprises many of our senses—including our sense of touch.

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A Brief History of Haptic Feedback in Video Games

Picture of Fonz arcade cabinet
The original Fonz arcade cabinet

Currently, almost every modern video game console includes some form of vibrotactile feedback, but this was not always the case. As an increasing number of video games were made for computers and at-home entertainment systems, arcade game manufacturers sought ways to make their cabinet games more immersive. Though arcade controls were typically customized to each individual game, the increasing availability of video games outside of arcades placed pressure on companies to provide arcade visitors with experiences more uniquely tailored to branded game cabinets. In 1976, Sega’s game Moto-Cross (rebranded as Fonz) was the first to feature vibrotactile feedback, allowing each player to feel the rumble of their motorcycle as it crashed with another player’s bike on the screen. The control scheme was a success.

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How do devices provide haptic feedback?

Video game controllers, cell phones, wearables, and dozens of other consumer electronic devices make use of vibrotactile feedback to increase user engagement. There are three different types of hardware most frequently used to provide haptic feedback: eccentric rotating mass motors, linear resonant actuators, and piezoelectric actuators.

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Adding Senses to the Human Body

Expressive aphasia, also known as Broca’s aphasia, is a neurological condition characterized by an individual’s inability to produce grammatically correct speech, often due to a physical impact or alteration to the anterior regions of the brain, which impairs the proper function of neurons that would otherwise help construct vocalizations of grammatically correct sentences [1]. In the suburbs of Paris, France in 1861, Paul Broca identified the location of the region responsible for expressive aphasia after he conducted an autopsy of a patient incapable of uttering any word other than “tan” [2]. Though speculations of the structure and function of human consciousness had existed for several centuries, Broca’s discovery resulted in a new framework for understanding the brain’s role in producing conscious experience. The patient’s brain incurred a lesion from injury, and only a small subset of his cognitive function was impaired. Naturally, psychologists concluded that different parts of the brain mediate different cognitive processes. By 1874, Carl Wernicke discovered receptive aphasia (Wernicke’s aphasia), which results from damage to posterior regions of the brain [3], and increasing numbers of scientists began exploring which regions of the brain were responsible for different aspects of cognition. Brain science adopted a new goal: mapping the locations of the brain corresponding to each observable function in human consciousness.

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The First Haptic Wrist Band Design

In 1995, inventor Geir Jensen designed a piece to mount on a watch strap that could provide caller identification through haptic rhythms. His device was based on a frequency-controlled actuator that provides tactile feedback, but he never actually built the hardware. Instead, he submitted the idea to a technology competition and was rejected by the Norwegian Industrial and Regional Development Fund. Jensen was thinking two decades ahead of his time.

The Texas Instruments DRV2603: A Simpler Chip

In the past, we’ve discussed driving a linear resonant actuator using a DRV2605 haptic driver chip from Texas Instruments. Though the DRV2605 chip provides plenty of features (audio to haptics, licensed effects from Immersion, and flexible I2C or PWM input), it also requires a more complicated integration with an existing circuit. Though an extra capacitor or two doesn’t introduce too much complexity, the DRV2605 also isn’t suited for circuits attempting to drive multiple motors at the same time. Since the chip uses I2C, its address remains the same in every chip and cannot be modified. As a result, integrating multiple DRV2605 chips on a single I2C bus requires an I2C switch or multiplexor – multiple slaves cannot be controlled by the same master on the same bus.

There must be a simpler way! And Texas Instruments provides the DRV2603 to provide a simpler option. The DRV2603 haptic driver forgoes the licensed effects and audio input for a simple PWM-only input. Each motor driven with a DRV2603 chip only needs a single digital output pin from a signal source that is capable of producing a signal from 10kHz to 250kHz will be capable of driving multiple motors simultaneously.

For haptics projects that rely on multiple actuators to produce feedback, the DRV2603 provides a simpler way to get started using linear resonant actuators.

Audio to Haptics

Now, it’s common to use an audio signal to drive a haptic actuator and produce tactile effects that correspond to sounds. The Apple Watch and Taptic engine use this technique to render haptic feedback.

In general, the best approach for most consumer electronics devices involves the implementation of a haptic driver capable of consuming audio signals as input. The DRV2605 from Texas Instruments can provide this form of feedback with an eccentric rotating mass motor or linear resonant actuator.

Another option, which is less suited for most portable electronic devices, is to use a surface transducer. A surface transducer will frequently be used in speakers to produce vibrations in an enclosure that result in sound. Unfortunately, surface transducers consume a lot of power, but they are able to produce vibrations that propagate over a hard surface very quickly and consistently.