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Tactile response of the piezoelectric motor based solutions

In Electronic Infomation Category: T | on April 22,2011

With the touch screen handheld consumer devices gradually replace mechanical buttons, the lack of tactile response, consumers are starting to put forward the demand for real-time response. Users are accustomed to operating a successful button press that key input of the mechanical touch of the keyboard shown in Figure 1. Recently, the lack of good tactile feedback design, which led to the demand for e-touch response system.

Figure 1. Based on the software press the activation button

Achieved using piezoelectric tactile feedback is a more promising approach, this approach has been the application of a small number of consumer devices for many years. Piezoelectric tactile feedback has many advantages, including: fast response, ultra-thin appearance, low power consumption and TPS3305-18DGN datasheet and a large can take advantage of the piezoelectric materials and TPS3305-18DGN price and assembly processes.

Piezoelectric properties and TPS3305-18DGN suppliers and compare There are various piezoelectric materials

shape, size, thickness, voltage, force and rated capacitance, can be processed into a specific shape to meet the needs of special applications and packaging, and can provide single and multi-layer structure . Can be achieved over a strong piezoelectric tactile feedback and a variety of touch.

Work in and around the resonance piezoelectric applications include:

vibration excitation and eliminate

mini pump

Micro engraving systems

ultrasonic drilling / welding / sculpture / anatomy / measurement Working at the resonance point

applications include the following:

tactile response

Image Stabilization

AF system

fiber optic calibration

structural deformation

wear compensation

Piezoelectric works

Below the resonant frequency, the piezoelectric body can be easily simulated with a capacitor. When DC voltage is applied to both ends of the piezoelectric body, structure and physical experience of different shapes have different piezoelectric strain (Figure 2).

Figure 2. Simplified model of piezoelectrics

Coulomb law states that the Q = CV, but in the piezoelectric body capacitance is not constant, because the distance between capacitor plates will change with the voltage change.

When voltage is applied to the piezoelectric body, due to the distance between the plates change (Fig. 3A), the capacitance will change accordingly. Piezoelectric displacement is proportional to the electric field strength, while the electric field between the plates is a function of voltage and distance. Applied voltage and the force generated by piezoelectric actuators to maintain a reasonable proportional relationship (Figure 3C).

In most piezoelectric actuator range of movement, the piezoelectric equivalent capacitance of the charge and maintains approximately proportional to the displacement relationship. If the equivalent capacitance is no leakage current between the plates, even if the plate and the voltage source off, still able to maintain displacement.

Figure 3. Displacement and the force and the applied voltage

Force is proportional to the applied voltage piezoelectric vitro (Figure 3). Force (as opposed to time) are the main factors affect the tactile response, which determines the users feeling can be improved using the multilayer piezoelectric displacement.

Piezoelectric model

Operation of piezoelectric motor system in the main media can be used in parallel capacitor CP series formed by the LRC network simulation (Figure 4). Prior to the resonant frequency, the impedance will be the same as the capacitance decreases with frequency increase. So, when working in far below the piezoelectric resonance frequency, can only simulate a capacitor CP.

Figure 4. Piezoelectric crystal impedance and frequency

Piezoelectric resonance frequency can work to meet the self-oscillation at a fixed frequency requirements, such as ultrasonic oscillator. However, tactile feedback for the piezoelectric actuator is usually far below the resonant frequency of the work in the position.

For audio applications, efficiency is most concerned about, the contrast tactile feedback, haptic feedback, the key issue is not efficiency, but the persons touch. More than a few megabytes Hz vibrations do not give good tactile feedback, but unnecessary power consumption. Cycle more than a few milliseconds of vibration can produce a strong touch, but also have do not want to hear the clicking.

Figure 5 shows a typical touch waveform, the waveform simulation of a mechanical key press and release the feelings. The rising edge of the waveform, P0 to P1, reflecting the compression of the tactile response; falling, P2 to P3, reflecting the release of the tactile response. From P1 to P2 is the user holding down the time duration of mechanical buttons, determined by the user.

Figure 5. Waveforms of a typical example of tactile feedback

When building a piezoelectric-based tactile feedback system, you first need to decide is to use single or multi-layer piezoelectric actuator (Fig. 6). Table 1 summarizes the comparison of two piezoelectric types.

Table 1. Single and multi-layer piezoelectric actuator of the advantages of comparison

Figure 6. Left 100VP-P layer of piezoelectric film (SLD); upper right picture shows the 120VP-P multilayer piezoelectric strip (MLS); the lower right picture shows the 30VP-P multilayer piezoelectric Article (MLS)

Scheme selection Single-layer or multilayer structure


Table 1 provides information on recommended single-layer piezoelectric actuator. Single-chip supplier and has large volume production, multi-layer piezoelectric production is relatively small. In addition, many low-cost single-layer piezoelectric, which uses multiple piezoelectric very important program. For example, many mobile phones on the market have been installed behind the screen more than single-layer piezoelectric film, in which case the cost of using the multi-layer piezoelectric would be much higher.

Discrete or single-chip solution?

Piezoelectric tactile feedback on the shortcomings of the program is one of relatively high complexity, a typical piezoelectric solution uses discrete components to achieve the haptic feedback system, additional discrete components including a microcontroller, flyback boost or integrated charge pump, flyback transformers or inductors, and various resistors, capacitors, diodes and transistors. The DC motor of the haptic feedback based on program needs little or no external components. Single-chip solution

tactile feedback, such as compared with traditional discrete designs MAX11835 has many advantages: smaller printed circuit board size, lower power consumption, streamlined bill of materials (BOM) and a simple software support. Taking into account the very small size of the piezoelectric body, MAX11835 for handheld devices is a very attractive solution.

Figure 7 shows the single-chip high voltage drive controller block diagram of the tactile feedback:

Figure 7. Piezoelectric actuator using the tactile feedback scheme diagram

MAX11835 single-chip optimization program has the following features:

support for single and multi-layer piezoelectric actuator

user-definable waveforms on-chip storage (via serial port)

chip waveform generator

embedded DC-DC Boost Controller

operating voltage range to meet the needs of a typical cell phone battery

small package size

Low Power

The importance of power management

Piezoelectric motor drive relative to the DC power consumption is very low, though, there are still some other power factors to consider:

Every time touch the power from the main power consumption

Each touch of the waveform type

number of times per second touch

The power consumption of high-voltage boost circuit

MAX11835 touch drive controller for various piezoelectric actuators and high voltage power capacitors were measured. MAX11835 can boost converter feedback loop playback software control of storage in the waveform, the test waveforms including sine wave and 20Hz 100Hz ramp.

Figure 8,9 A and 9B show the MAX11835 driver 175V 100Hz sine wave output, but also the main draw of the transformer winding current.

Figure 8. MAX11835 boost output voltage and current waveform of the power supply

Figure 9A. 100Hz Continuous sine wave, the power curve of the load

Figure 9B. Peak boost supply current changes with the load curve,

Test conditions: frequency = 100Hz sine wave; boost power supply voltage = 4.2V; boost power supply decoupling capacitors = 10uF; use 6:1 transformers.

Press the button is the most common operation, the waveform shown in Figure 10 requires charging 40ms, 10ms discharge. Slow charging process of the touch screen is not easy to be detected, but the feeling is as rapid discharge of a mechanical key release.

Figure 10. Press the button analog waveform

Figure 11. Power and piezoelectric voltage curve, with the single and multi-layer piezoelectric analog button press. When the voltage exceeds 180V, MAX11835 the primary clamp open, power consumption will rise sharply.

Continuous waveform shown in Figure 11. With the reduced duty cycle while the power consumption decreases linearly. In mechanical load (half blocking force) and load the piezoelectric actuator data were not significantly different. Figure 12 shows the MAX11835

boost the efficiency of the process, with the load power consumption energy consumption divided by the boost of energy (VBST) were measured.

Figure 12. Energy conversion efficiency: energy consumption and VBST load energy consumption. When the voltage exceeds 180V, MAX11835 the primary clamp open, rapid increase efficiency.

Figure 12, the efficiency increases with the increase in load capacitance, because the only boost circuit consumes static power.

MAX11835 power consumption compared with the motor drive

MAX11835s power relative to the motor drive is very low, the motor drive, including polarization rotation (ERM) model, linear oscillating drive (LRA) type and voice coil type.

Based motor drive typically require low-voltage (1.8V to 3V), the current is quite large. In addition, General Motors, off characteristics, in particular the ERM model, the simulation does not have the ideal tactile feedback necessary.

Table 2 and Figure 13 shows the measurement results of a large number of drivers tested in two modes, continuous and pulse work. The actual situation is usually not continuously work, because a lot of touch operation is very short, even if the simulation of textured surfaces simulation.

Table 2. Motor drive power consumption

Figure 13. Table 2 Comparison of the drive, the data shown in Table 2

Figure 14 shows the continuous power consumption. The figure by the amplitude of piezoelectric 180V, frequency of 100Hz Continuous sine wave drive. Other drive or from the 3VDC 2VRMS (LRA and voice coil) drivers.

Figure 14. All under the continuous drive power consumption

Figure 15 shows the pulse operation mode of power, the figure 50ms drive pulse from the drive, push button operation of this simulation. Piezoelectric actuator drive amplitude of 180V, the drive voltage for the other 3VDC or 2VRMS (LRA and voice coil).

Figure 15. All kinds of drivers working in the pulse mode power consumption


From the above discussion, many conclusions can be drawn. Obviously, based on a variety of reasons, single (non Multi) piezoelectric actuators are more attractive to the current design:

Lowest cost

Number of supply channels

Mass production

Provides custom design

Can be installed in the LCD on the back or side

Data show that tactile feedback circuit should be a detailed calculation of the power consumption of power, wave amplitude, the type and duration of power will affect the size and tactile response.

Will also affect the number of times per second to touch power, need to consider the operation of rolling or sliding, or tap or slow type, etc. These factors will affect the power consumption. Finally, the measurements were normalized to a touch operation per second, for comparison purposes.

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