Revolutionizing Haptic Feedback: The Advantages of Piezoelectric Transducers

Many touchscreens and handheld devices offer only basic or no haptic feedback. To meet growing user expectations, engineers are turning to piezoelectric transducers, which deliver superior physical and electrical performance compared to conventional vibration motors.

This article reviews piezo transducer principles, theory, and modeling, and discusses electronic circuits specifically designed to drive the unique characteristics of these transducers. It also explores haptic applications using piezoelectric technology and examines how amplifier input power relates to different piezo load configurations.

Note that haptic vibration from piezo actuators relies on the inverse piezo effect—converting electrical stimuli into mechanical motion. Any reference to the piezo effect in this context refers to this electrical‑to‑mechanical energy transfer.

Introduction to piezoelectric haptics

In most portable electronics, haptic feedback is generated by electromechanical (EM) transducers, such as eccentric rotating mass (ERM) motors and linear resonant actuators (LRAs). These devices are inexpensive, easy to use, and can operate from a battery‑level voltage.

However, EM transducers present several drawbacks:

• They resonate at a fixed frequency, and LRA units must be calibrated to a factory‑determined resonant frequency.
• EM devices are relatively large (3–5 mm tall), limiting integration into thin enclosures.
• They generate point‑source vibrations, making it difficult to create varied frequency patterns across a surface.
• They are energy‑inefficient, consuming significant power per haptic event.
• LRA motors can be fragile and susceptible to physical or electrical overstress.

Piezo transducers, on the other hand, generate mechanical vibrations through the inverse piezo effect by applying an AC voltage to a crystalline material. Their key advantages include:

• Ultra‑thin (<1 mm) and flexible, allowing mounting in diverse configurations.
• Surface‑wide vibration generation, enabling touch‑location sensitivity.
• High efficiency, especially when driven with an appropriate circuit.
• Wide frequency range and precise control over vibration patterns.
• Fast response due to negligible inertia.
• Zero EMI emissions.

Because piezo actuators require relatively high drive voltages—typically 60 V to 200 V peak‑to‑peak—and present a capacitive load, they benefit from specialized drive circuitry.

Piezo actuators come in various configurations. The most common form for haptics and audio is the bimorph bender, which is bonded to an internal surface—such as a device case or touchscreen—using adhesive. The following illustration shows a single‑layer surface‑mounted piezo actuator.

Figure 1: Bimorph piezo actuator construction

A bimorph actuator typically consists of one or more layers of polycrystalline ceramic deposited onto a conductive mechanical layer (e.g., brass or copper). After fabrication, a large DC polarizing voltage aligns the crystal domains, enhancing the inverse piezo effect and defining the direction of the generated force. Applying voltage in the same or opposite direction relative to the polarizing field yields different mechanical responses.

The illustration in Figure 1 shows a piezo actuator mounted orthogonally to the polarizing voltage. In this orientation, the applied electric field produces a force directed into the mounting base, resulting in minimal deflection. Conversely, if the base is mounted vertically and the free end is unconstrained, the actuator experiences greater deflection.

Mounting a piezo on a display panel, for instance, can transmit force to the surface, creating a haptic sensation for the user’s finger. Materials placed between the piezo and the mounting surface can absorb energy and attenuate vibration, especially if they are soft or pliable.

Piezo transducers also enable localized haptic feedback. By arranging multiple piezo elements beneath a touchscreen or keyboard, each element can be selectively energized when a touch is detected. This can be achieved with a high‑voltage multiplexer or separate amplifiers for each piezo.

Each ceramic layer generates a force proportional to the applied voltage; thus, an n‑layer actuator produces n times the force of a single layer.

>> Continue reading the complete article originally published on our sister site, Electronic Products.