Guide to Haptic actuators in product design – Haptic actuators & types

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What are Haptic actuators?

Haptic actuators are used in mobile phones, touch screens, gaming controllers and gaming chairs to enhance the user experience by using haptic feedback to communicate with their sense of touch.

Haptic feedbacks such as vibrotactile, force, electro tactile, ultrasound tactile and thermal feedback explore users sense of touch via haptic actuators to notify, alert and simulate their senses for virtual experiences.

The most popular type of haptic feedback is phone vibration, which employs a tiny spinning motor as the haptic actuator. However, these are not the only actuators available on the market.

Types of Haptic actuators?

There are several more types of actuators used in diverse applications that can be grouped under the six basic types of haptic actuators listed below:

        • Eccentric Rotating Mass (ERM)
        • Linear Resonant Actuators (LRA)
        • Piezo Haptic actuator
        • Thermoelectric device
        • Solenoid actuator
        • Ultrasonic transducer or sensor


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Eccentric Rotating Mass

Eccentric rotating mass actuators are the oldest and one of the most common types of haptic actuators used in the market today. As the name implies, the ERM actuators are small magnetic DC motor that spins an eccentric unbalanced weight to create the desired vibrations.

Coreless ERM internal design
Coreless ERM internal design (Source – precisionmicrodrives.com)

As one of the most matured technologies in this haptic feedback, they are available in various power and performance specifications. Due to its mass and out of balance inertias, an ERM’s starting and shutdown times are comparably slower than LRA or Piezoelectric actuators and in the region of 50 to 100ms. The frequency of the motor influences the amplitude or the vibration strength of the output, therefore producing complex and precise waveforms challenging. You can read about how ERM works in a detailed note here.

Eccentric rotating mass actuators
Eccentric rotating mass haptic actuator types (image source-ifixit,precisionmicrodrives,google)

They are available in either Coin or Cylinder form with a range of motor technologies, including iron core, coreless and brushless.

Eccentric rotating mass vibration motors are cheap to fabricate. While they are not staggeringly cutting edge, they are utilized in the automobile sector to give haptic feedback in touch screen displays and dashboards.

Advantages & disadvantages of ERM

Advantages of ERM – Low cost, widely available, mature technology, Wide range of specifications & simple design and use

Disadvantages of ERM – Vibration signal amplitude relies on driven frequency, High power requirement, slow response (starting and stopping)

Linear Resonant Actuators (LRA)

Linear resonant actuators (LRA) function like a speaker, which vibrates to send haptic feedback. In its simplest form, it is a spring-mass system where the mass moves linearly back and forth with the help of a voice coil resulting in the desired haptic feedback.

Linear resonant actuators haptic (source:precisionmicrodrives.com)

As seen in the exploded view of an LRA, a mass and magnet combination is suspended above the voice coil by a wave spring within the magnetic field of the permanent magnet. The spring constant of the spring defines the resonant frequency of the spring-mass system. The AC drives the voice coil, which will exert a mechanical force on the mass proportional to the current. It will result in the magnet moving linearly on the spring, with the displacement will increase and being maximum if the drive frequency matches the resonance frequency of the moving mass.

LRAs are increasingly common in smartphones, small touch screen tablets, portable navigation devices, portable gaming consoles and automobiles for basic vibration notifications.

Over the last several years, most phone manufacturers have converted from Eccentric Rotating Mass (ERM) vibration motors to the more efficient, responsive Linear Resonant Actuator (LRA) motors.

Apple's taptic engine
Apple’s Taptic engine (Source:patentlyapple.com)

Apple introduced its version of LRA on their devices a few years ago called tactile haptics (shown above).

Advantages & disadvantages of LRA

Advantages of LRA – More acceleration for the same size compared to ERM, Easy amplitude modulation, LRA consumes less power compared to ERM, quicker response, Smaller in size, high power efficiency due to high Q, Reliable

Disadvantages of LRA – Only vibrates in one axis, Resonant frequency varies between each LRAs, due to manufacturing variations. Lower vibration amplitude outside resonant frequency due to narrow operating bandwidth

Piezo Haptic actuator

Piezo haptic actuator uses the piezo effect to generate vibration using piezoelectric material mounted in a cantilever beam configuration. Piezoelectric materials such as ceramics, crystals, bone, DNA and proteins, generate an electric charge in response to applied mechanical stress and vice versa.

Piezo haptic actuator
Piezo haptic actuator (source:piezo.com & TDK)

The piezoelectric effect, which causes the material to compress or stretch when an electric signal is applied, is used in piezoelectric actuators to cause vibration. The most common piezoelectric material for piezoelectric actuators is a kind of ceramic known as PZT. The bender and stack are the two most prevalent piezoelectric structures.

Because the actuator’s deflection is directly proportional to the control signal, it can be either made to oscillate or hold its position.

The picture below is a Stack version of a piezoelectric actuator made by sandwiching piezoelectric material between two metal plates.

piezoelectric-effect-stacker-bender (Source:piezo.com.TDK)

When an AC voltage is applied, piezoelectric material expands and contracts, which cause the element stack to warp up and down, as shown above, resulting in vibration.

In the case of the bender configuration, two pieces of piezoelectric materials are attached to form a cantilever beam.  A voltage signal across the piezoelectric materials will bend and create significant displacement near the tip of the beam. Due to dual piezoelectric elements, the deflection is more compared to stack embodiment.

These actuators are more precise than ERM or LRA because they can vibrate at a wide range of frequencies and in more than one direction, unlike other haptic actuators.

Unlike ERM and LRA, the amplitude and frequency of the deflection can be controlled individually, allowing for considerably more sophisticated and detailed signal waveforms.

Some of the applications of Piezoelectric haptic actuators:

      • Audio systems such as sounders, fire alarms and receivers
      • Data storage includes MR head testing, spin stands, disk testing
      • Precision mechanics such as linear drives, shock wave generation micro-pumps and active vibration cancellation
      • Life science and medical technology share nano-litre pumps, scanning microscopy, micro dispenser, and cell penetration
      • Optics, photonics, for example, autofocus system, fibre optic alignment, laser tuning, image stabilization and many more

Advantages & disadvantages of Piezo Haptic actuator

Advantages of Piezo Haptic actuator – Non-magnetic, holding power not required, Quick rise and fall time (1ms), higher resonance frequency, multiple vibration patterns, Low power consumption, smaller size and high precision

Disadvantages of Piezo Haptic actuator – Higher voltage, Nonlinear, sensitive to electrical overdrive

Thermal haptic actuators

Most thermal feedback devices or actuators employ thermoelectric devices (TE) as their thermal source. Sometimes referred to as TED, it is a heat transfer solid-state module that transforms electrical current into heat flux using the Peltier effect and vice versa using the Seebeck effect.

thermal-haptic-actuator (source:azom.com & TEGway)

Peltier based haptic actuators are difficult to use due to their rigid substrate structure. Thermoelectric devices designed for an electric power generator based on the Seebeck effect have more flexibility and outstanding thermal output performance. Hence it was more suitable for a haptic thermal device such as a glove or flexible wearable.

Thermal haptic actuators are used in a wide range of applications such as VR headsets, gamepads, joysticks, and others to enhance the thermal haptic experience.

Advantages & disadvantages of thermal haptic actuators

Advantages of thermal haptic actuators – Spot cooling or heating, high precision temperature control, compact, lightweight, vibration-free, noiseless, Solid-state

Disadvantages of thermal haptic actuators – Latency, Hot and cold sides are very close to each other, Technologies not yet fully matured

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Solenoid haptic actuators

Solenoid actuators, also known as impact actuators, forced impact actuators, or accelerated Ram actuators, are built similarly to Solenoids or bigger versions of linear resonant actuators. When compared to other actuation systems, Solenoid haptic actuators offer the broadest frequency response range.

A solenoid generates an electromagnetic field around a moveable magnetic material core known as an armature. When the coil is energized it creates an electromagnetic field, which pulls and pushes the armature up and down the coil cavity, converting electrical energy into mechanical cyclic motion.

Sometimes the springs at each end of the mass prevent it from striking the outer casing or end stop. As a result, its building layout is comparable to that of a large LRA.  In another form, the solenoid actuator is coupled to a touch surface through a lever and suspended via haptic suspension mounts. When the user touches the surface, it jogs it to mimic a button press.

In some designs, the inner mass strikes an end stop incorporated into the motor housing, causing an impact to alert the user.

They are generally employed in loud locations when the use of ERM or LRA is challenging due to ambient vibration.

      • VR
      • Game controller
      • Virtual button
      • Mobile phone
      • Automotive dashboards and displays

Advantages & disadvantages of Solenoid haptic actuators

Advantages of Solenoid haptic actuators – Wide range of frequency response, higher forces and vibration amplitudes, impact sense

Disadvantages of Solenoid haptic actuators – Expensive as it required custom drivers, comparably large, higher power consumption

Ultrasonic transducer or sensor

Not every haptic technology necessitates users coming into contact with a surface. Contactless haptic technology eliminates the need for wearables or controllers. It generates tactile sensations in mid-air using technologies such as ultrasound or lasers.

Ultrasonic actuators use speakers or integrated 3D ultrasound sensors to transfer tactile effects onto a user’s hands.

Advantages & disadvantages of Ultrasonic actuators

Advantages of Ultrasonic actuators – Contactless experience for the user,

Disadvantages of Ultrasonic actuators – Expensive, only suitable for specific applications

 

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