Implementing tactile (haptic) feedback in consumer-electronic devices enhances the user’s experience. It provides a sense of touch in a user-interface design and is the newest major interface on smartphones and other portable consumer-electronic devices. Several haptic technologies are available now, including but not limited to vibration motor actuation, piezoelectric actuation, and electro-active polymer actuation. This article explains the basics of piezoelectric-based actuation and how it offers a fast response time, thin profile, and low power, all of which are important in handheld applications.
With the advent of touch-screens replacing mechanical buttons on portable handheld consumer devices, the loss of tactile feedback has created a need for real-time feedback. Users are familiar with the “push to activate” feel of mechanical feedback to indicate a successful entry, for instance, on a keypad (Figure 1). Recently, the lack of good tactile feedback has fueled the demand for adding electrically-based tactile feedback systems.
Figure 1. Push-to-activate software-based buttons.
One of the more promising approaches for real-time tactile feedback uses piezoelectric actuation, which has been available in a small number of consumer devices for several years. Piezo-based haptics offer several advantages including a fast response time, thin profile, low power, and a wide range of available piezo characteristics and mounting techniques.
Piezo characteristics and comparison
Piezos are available in many different shapes, sizes, thicknesses, voltage ranges, force and capacitance ratings. They can be made into custom shapes for specific applications or packaging constraints, and are offered as single-layer or multilayer structures. Multiple piezos can be used to provide a stronger haptic response and more localized haptic feedback.
Piezo actuator applications for operation at or near resonance include:
- Vibration stimulation and cancellation
- Microengraving systems
- Ultrasonic drilling/welding/knives/scalpels/scalars
Applications for operation well below resonance include:
- Tactile feedback
- Image stabilization
- Autofocus systems
- Fiber-optic alignment
- Structural deformation
- Wear compensation
The basics of piezo operation
Well below resonance, the piezo can be modeled as a simple capacitor. Based on its configuration and physical shape, the piezo will change shape as a (DC) voltage is applied across its terminals (Figure 2).
Figure 2. Simplified piezo model.
Coulomb’s law states that Q = CV. But in the piezo, C is not constant since the spacing between electrodes changes with applied voltage.
When voltage is applied to the piezo, the capacitance also changes because the distance between the electrodes is changing. Displacement of the piezo is proportional to the electric field, and the electric field is a function of the electrode voltages and the distance between the electrodes. Applied voltage does maintain a reasonably proportional relationship with the force generated by the piezo actuator.
Charge on the piezo capacitance holds a good proportional relationship with the displacement over most of the piezo actuator’s range of motion. The displacement remains even if the electrodes are disconnected from the voltage source, except for the small leakage current found with any normal capacitor.
Figure 3: Displacement and force vs. applied voltage.
Force is proportional to applied voltage across the piezo (Figure 3). Force (versus time) is the dominant factor in tactile feedback; it determines a “good” user feel. Improving displacement can be accomplished using multilayer (stacks) of piezos.
When voltage is applied to the piezo, the capacitance changes because the distance (Figure 3A) between the electrodes changes. Displacement of the piezo is proportional to the electric field, and the electric field is a function of the electrode voltages and the distance between the electrodes. Applied voltage does maintain a reasonably proportional relationship with the force generated by the piezo actuator (Figure 3C).
A piezo model
The electromechanical system of motion in a piezo is modeled by series LRC in parallel with the primary dielectric capacitor, CP (Figure 4). Impedance rolls off like a capacitance until resonance is reached. Operating well below the resonant frequency, the piezo can be modeled as a simple capacitor, Cp.
Figure 4. Piezo impedance vs. frequency
Piezos can be used at the resonance frequency for free-running fixed-frequency applications such as ultrasonic vibrators. However, piezo actuators used for tactile feedback are typically operated well below natural resonance.
For tactile feedback, the central issue is not the efficiency of driving the piezo, a concept often associated with audio applications. The main issue is about “feel,” a human touch. Frequencies above a few hundred Hertz do not provide good tactile response and consume unnecessary power. Slew times faster than a few milliseconds provide a strong tactile feel but produce an undesirable audible click.
Figure 5 shows a typical waveform for a good tactile feel. This waveform mimics the feel of a typical press and release on a mechanical button. The rising edge of the waveform, P0 to P1, provides the pressing-down tactile response, while the falling edge of the waveform, P2 to P3, provides the releasing tactile response. The time from P1 to P2 is the time that the user holds the “mechanical” button down; that interval will be determined by the user.
Figure 5. Example waveform for a “good” tactile feedback response.
When implementing a piezo-based tactile feedback system, one of the first decisions is whether to use single-layer or multilayer piezo actuators (Figure 6). A summary comparison of the two piezo types is shown in Table 1.
Figure 6. A 100VP-P single-layer piezo disc (SLD) at left; a 120VP-P (top right) and 30VP-P (lower right) multilayer piezo strip (MLS).
Table 1. Benefit comparison between single-layer and multilayer piezo actuators
Part 2 of this article looks at design decisions including single-layer or multilayer configuration, discrete components vs. a single-chip solution, The importance of power management, and motor actuator power.
- Information Display, October 2009, pages 18-21
- Maxim Integrated Products, Datasheet for MAX11835
About the author
Tim Blankenship joined Maxim in 2005 and is currently a Product Definer for Touch Interface Products. Prior to joining Maxim he spent 13 years working in the LCD area for several companies in Austin, Texas and, before moving to Texas, spent 12 years working in various design roles, including memory, telecom, and CMOS mixed-signal systems at Harris Semiconductor in Palm Bay, Florida. He earned an MSEM degree from the Florida Institute of Technology in 1990 and a BSEE degree from the University of Florida in 1980, and can be reached at email@example.com .
The author wishes to thank Mohammad Umair, Paul Nickson, Antony Routledge, and Stephen Bradshaw at Maxim for supporting this project and collecting data on the various haptic actuators.