Most touchscreen panels have a limited type of haptic feedback or none at all. This is also true for many types of handheld or wearable devices like watches, touchpads, keyboards, a mouse, etc. The desire for improved haptic feedback is leading some to take a closer look at piezo transducers to generate haptic signals, which provide a number of physical and electrical improvements over traditional vibration generators.
This article reviews piezo transducer principles, theory, and modelling. It includes a discussion of electronic circuits that are specifically designed to drive the unique characteristics of piezo transducers and shares examples of haptic applications using piezoelectric transducers. The article also examines the relationship of amplifier input power with respect to piezo load configurations.
Note that haptic vibration from piezo actuators uses the inverse piezo effect (i.e., vibration from electrical stimulus). Any mention of piezo effects refers to this electrical-to-mechanical energy transfer.
Introduction to piezoelectric haptics
Today, in most handheld or portable electronic devices, a haptic vibration is created by an electromechanical (EM) transducer that converts electrical signals into mechanical vibrations. These include eccentric rotating mass (ERM) actuators and linear resonant actuators (LRAs). These types of EM transducers are low-cost, are fairly easy to use, and can be powered from a battery-level voltage.
There are, however, a number of disadvantages to EM transducers:
- They are resonating devices that create a specific vibration frequency and, in the case of an LRA, it must be calibrated to a resonant frequency that is stochastic from the factory.
- EM devices are physically large and tall (3 to 5 mm high), reducing the ability to mount them into thin enclosures.
- They produce point-source vibrations and cannot create various frequency patterns onto a surface.
- They are inefficient, requiring significant energy per haptic event.
- LRA devices are somewhat fragile and can be destroyed by either physical or electrical overstress (e.g., a drop).
In comparison, piezo transducers are not based on EM energy conversion and excel as a haptic vibration generator. They generate mechanical vibrations though the inverse piezo effect by creating crystalline vibrations from applied electromotive force (i.e., EMF), typically from an AC voltage source.
Piezo transducers can be advantageous due to several important properties:
- They are thin (<1 mm), are flexible, and can be mounted in a variety of options and shaped to nearly any desired pattern.
- They produce vibrations over a surface area and can be touch-location–sensitive.
- They are highly efficient, depending on the method used to drive the piezo.
- They can reproduce any vibration frequency over a wide frequency range.
- They can generate a pattern of haptic signals that can be amplitude- or frequency-modulated.
- They have very little inertia and, therefore, have a very fast response time.
- They produce no EMI emissions.
Note that piezo actuators require a relatively high voltage-drive signal to create significant mechanical vibration, typically 60 V to 200 V peak to peak. Also, piezo actuators are primarily a capacitive load to the drive circuit and, therefore, benefit from specialized electronic drive circuitry. More on this subject will be discussed later.
A detailed discussion of piezo actuator construction and physics is beyond the scope of this paper; however, a brief description follows. Piezo transducers are manufactured in a variety of different physical configurations, depending on the application. A piezo actuator that is most generally used for haptic and audio reproduction takes the form of a bimorph bender that would be mounted (i.e., glued) to an internal surface that is part of a handheld or wearable case or a touchscreen, for instance. An example of a single-layer piezo actuator that is surface-mounted is shown in Fig. 1.
Figure 1: Bimorph piezo actuator construction
As shown in Fig. 1, a bimorph bender is generally composed of one or more layers of polycrystalline ceramic material screened onto a conductive, mechanical layer (e.g., brass or copper). After the layers have been created, a large DC polarizing voltage is applied across the piezo structure to align the crystal domain boundaries to strengthen the inverse piezo-effect force that will be generated (i.e., increasing force generated per voltage EMF). The polarizing voltage then defines the direction of the mechanical force generated with applied voltage. Increasing applied voltage in the direction of the polarizing voltage increases the mechanical force or bending displacement. Polarization to the piezo layers can be applied in the same direction or in opposite directions. Each method has its advantages and can be used to create piezo effects as desired.
The illustration in Fig. 1 shows a piezo actuator mounted to a surface that is orthogonal to the polarizing voltage. This configuration (with the applied EMF as shown) generates a force into the mounting base, and consequently, there is little deflection of the piezo. If the base was mounted vertically to the piezo actuator (shown in dotted lines) and the opposite end of the actuator was unconstrained, this would cause a larger deflection of the piezo.
An example of the mounting shown in Fig. 1 would be to a display screen generating a force that is conducted to a surface. This creates maximum conductive force and minimum deflection. This method could be used, for example, in generating a haptic vibration to fingers on a touch-activated display screen. It should be noted that any material that is present between the piezo and the mounting surface absorbs mechanical energy and tends to attenuate the conducted vibration, especially if the material is soft or pliable.
A piezo transducer can also be used to provide localized haptic feedback. This can be accomplished by arranging a number of piezo elements under a touchscreen or keyboard display, for example, so that each piezo element provides a haptic feel localized to its placement. When a touch is sensed, not only does the display produce the X-Y location of the touch, but a piezo driver is enabled that energizes that particular piezo actuator. This can be accomplished by using a high-voltage MUX or from separate piezo amplifiers.
Each layer of polycrystalline ceramic produces a force proportional to the applied voltage, and n-layers produce n times the force generated.
>> Continue reading the complete article originally published on our sister site, Electronic Products.
|Tim Dhuyvetter is senior principal member of the technical staff, Mobile Solutions, Maxim Integrated.|