Touch with Input: Sensing using linear resonant actuators

As wearables and handheld devices shrink in size, haptics are becoming an increasingly important channel for feedback, whether through silent alarms or the subtle “click” sensation of pressing a button on a touchscreen. Haptic feedback, which is common in almost all wearables and mobile phones, is typically achieved by linear resonance actuators (LRA), a small linear motor that uses resonance to provide a powerful haptic signal in a small package. However, the touch and pressure sensing needed to activate haptic feedback often rely on additional independent hardware, which can increase the price, size and complexity of the system.

ACM UIST 2020 “Touch with Input: In “back emf in linear Resonant Actuators for Touch, pressure, and Environmental Perception,” we demonstrate that the widely available LRA can sense a wide range of external information, such as touch, percussion, and pressure in addition to being able to convey information about contact with skin, objects, and surfaces. We achieved this using an off-the-shelf LRA by multiplexing the drive with short pulses of custom waveforms designed to be induced using a backemf voltage. We demonstrated the potential of this approach in enabling expressive discrete buttons and vibrating haptic interfaces, and showed how it could lead to rich sensing opportunities for integrated haptic modules in mobile devices, increasing sensing capabilities with fewer components. Our technology may be compatible with many existing LRA drivers because they already employ backemf induction to automatically adjust the vibration frequency.

Principle of counter electromotive force in LRA

Inside the LRA housing is a magnet attached to a small mass, and both move freely on the spring. The magnet moves in response to the excitation voltage introduced by the voice coil. The motion of the oscillating mass produces a counter electromotive force, or counter electromotive force, which is a voltage proportional to the rate of change of magnetic flux. A larger oscillation velocity produces a larger counter emf voltage, while a rest mass produces a zero counter EMF voltage.

Active counter electromotive force for induction

Touching or contacting the LRA during vibration changes the velocity of the internal mass as the energy dissipates into the contact object. This applies to soft materials that deform under pressure, such as human bodies. For example, a finger in contact with an LRA will absorb different amounts of energy depending on the contact force. By driving the LRA with a small amount of energy, we can measure this phenomenon using the back emf voltage. Because induction using backemf behavior is an active process, the key insight to enable this work is to design custom non-resonant driver waveforms that allow continuous induction while minimizing vibration, sound, and power consumption.

We measure the back EMF from the floating voltage between the two LRA leads, which requires a temporary disconnection of the motor driver to avoid interference. When the driver is disconnected, the mass still oscillates inside the LRA, generating an oscillating counter EMF voltage. Since commercial backemf sensing LRA drivers do not provide raw data, we designed a custom circuit capable of picking up and deflating backemf voltages. We also generated custom drive pulses to minimize vibration and energy consumption.

application

LRA used in mobile phones behave the same whether they are on a table, on a soft surface or held in a hand. This can cause problems, as vibrating phones can slide off glass tables or make loud and unnecessary vibrating sounds. Ideally, the LRA on the phone automatically adjusts to its environment. We demonstrated our approach to sensing using THE LRA counter-emf technology by connecting directly to the Pixel 4’s LRA and then classifying whether the phone was held, placed on a soft surface (foam), or placed on a table.

We also presented a prototype that demonstrated how the LRA could be used as a combined input/output device in portable electronics. We connected two LRAS, one on the left side of the phone and one on the right. These buttons provide tap, touch and pressure sensing. They are also programmed to provide tactile feedback when a touch is detected.

There are many wearable tactile AIDS, such as sleeves, vests and bracelets. In order to transmit tactile feedback with a dedicated skin, the tactor must apply the correct pressure; It must not be too loose or too tight. Currently, the typical way to do this is through manual adjustments, which can be inconsistent and lack measurable feedback. We show how LRA counter emf technology can be used to continuously monitor fitted bracelet devices and alert users if it’s too tight, too loose, or just right.

Evaluate the LRA as a sensor

The LRA works well as a pressure sensor because it has a secondary response to the magnitude of the force during touch. Our approach applies to all five off-the-shelf LRA types we evaluated. With typical power consumption of just 4.27 mA, all-day sensing will only shorten the Pixel 4 phone’s battery life from 25 hours to 24 hours. Power consumption can be greatly reduced by using low-power amplifiers and using active sensing only when needed, such as when the phone is active and interacting with the user.

The challenge with active sensing is to minimize vibration so that they are not detectable when touched and do not produce audible sounds. We optimized the active sensing to produce only 2 dB of sound and a peto-peak acceleration of 0.45 m/s 2, almost imperceptible to the fingers and quiet, compared to the conventional 8.49 m/s 2.

In the future, we plan to explore other sensing technologies, perhaps measuring electric currents could be an alternative. In addition, the use of machine learning may improve sensing and provide more accurate classification of complex counter-emf patterns. Our method can be further developed to achieve closed-loop feedback with the actuator and sensor, which will allow the actuator to provide the same force regardless of external conditions.

We believe this work opens up new opportunities to provide rich interactive and closed-loop feedback haptic actuators using existing ubiquitous hardware.

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