How to Drive Piezoelectric Actuators

Posted by Jeff Court on February 29, 2016

Piezoelectric Actuator Amplifier image

In my work I often have the need to power piezoelectric transducers for many different applications so I thought I would put this post together to help people choose the best method for driving piezoelectric actuators for their specific application.

Operating a piezoelectric transducer as an actuator requires that the correct electrical input be used.  The two main components of the electrical input are the drive frequency and voltage amplitude. The input frequency determines how fast the piezo will vibrate or change state.  A piezo controlling a valve will typically have a state of “open” or “closed” indicating two discrete positions and change position as the valve needs to be opened or closed, a piezoelectric fan may operate at a constant peak voltage and constant frequency for its entire life, and a piezoelectric speaker may continuously change the operational frequency to emit the desired tones.  These applications highlight the different driving needs which will be covered in this post. 


Considerations When Choosing an Amplifier

As amplifiers are evaluated for use with a system, it is important to understand the key parameters, and how they will affect the operation of the piezo.  Knowing the following basic requirements will help facilitate the best amplifier selection.  You also need to understand the key specifications of your piezoelectric device, specifically you need to know the capacitance and drive frequency/voltage as these impact the current and power you need from the amplifier.  Our piezo products data sheet has all of the relevant information you would need for the piezo actuator.

Output Voltage

The voltage range in which the amplifier will be operating.  Piezo amplifiers with an output voltage on the order of 100 V to 200 V are typical.  If the actuator is to be driven to vibrate back and forth, it is common to require the unit to be driven to a negative voltage as well.  Amplifiers will typically state an operational voltage range such as 0-200V or +/- 100V, for example.  Most of the actuator products we provide are rated for +/- 200 V.

Output Current

The current output of the amplifier.  High voltage amplifiers will typically have a low output current (<1 A), but piezo actuators require very little current.  For example, Mide's piezo fan draws about 5mA.  A single amplifier will typically have enough power to drive multiple actuators simultaneously.  PiezoDrive offers a nice calculator to determine the current draw of your actuator to help with choosing the proper amplifier. 

Bridging

This feature allows one unit to be bridged with a second unit to increase the output current or maximum operational voltage.

Bias  

A DC voltage that exists on the amplifier when no signal is applied can be considered the bias.  Instead of generating a positive and negative voltage, some amplifiers may choose to generate just a positive voltage.  This takes the 'neutral' point of the actuator from ground to the bias voltage.  A bit more care must be taken when operating a unit with a DC bias.

Frequency

Simply stated this is the range of drive frequencies in which the amplifier can operate.   However, this is often a common source of user confusion.  In most cases the usable frequency is less than the stated absolute frequency and is limited by output power vs. capacitance of the load being driven.

Voltage Gain

Amplifiers operate based on the premise of scaling an input voltage to a higher voltage.  This voltage gain (measured in volts per volt) is typically linear and fixed.  When operating at higher voltages and near the limits of the equipment, the gain may not be linear and can indeed fluctuate with drive frequency, temperature, and other parameters.

Noise

The amount of electrical noise that may be experienced.  Lower noise will typically cost more money and result in a more accurate output waveform.  However, for many applications, small amounts of output noise may not be detrimental to the system.

Maximum Power

Even if an amplifier meets all of the above requirements for your application, it is important to keep in mind that it may not be able to meet all of the requirements at the same time!  Amplifiers will spec a maximum power (watts) that can be supplied.  This power will relate to the drive frequency, voltage, and piezo capacitance.  In order for the amplifier to provide enough power to the piezo, it must be able to meet all of the requirements simultaneously.

This PiezoDrive calculator can help you determine if an amplifier will meet the power requirements of your piezo application.


Different Options to Drive a Piezoelectric Actuator

Piezoelectric devices are generally operated at high voltage so a means to generate that high voltage is needed.  There are several methods to generate this voltage, and selecting the correct one will be important for your application.  The methods and products discussed here cover a wide range of applications, but if you are still unsure what is appropriate for your system, feel free to reach out to us for help.

There are several different types and styles of amplifiers available on the market.  Choosing the correct one will depend on where you are in your development effort and what final requirements have been established.  The following table provides a high level overview of the amplifiers  to be discussed.

 

Amplifier or Driver Type

Relative Size

Power Output

Example Application

COTS Integrated Circuits

small

low

Body Worn Haptics

Amplifier Modules

medium

moderate

Automobile / Aircraft

Benchtop Amplifiers

large

high

Research & Development

DC Power Supplys

varies

varies

Valve

Mains (Wall) Power

small

high

Piezo Fan

Custom

varies

varies

OEM solutions

Benchtop Amplifiers

4_PiezoDrive_Bench.jpg

Benchtop amplifiers are often chosen at the beginning of a development effort due to their high performance.  They function well over a wide range of frequencies (including DC) and voltages while producing a clean signal with minimal noise and distortion.  This allows for the R&D activities to focus on the piezos performance and how it interacts with the rest of the system without worrying about the power supply and amplifier.  The consistent and high performance does come at a cost.  Benchtop amplifiers are often large, heavy, and power hungry.  They may sacrifice electrical efficiency and generate significant amounts of waste heat to provide the user with the desired signal.  When the performance of the system achieves desired levels, the amplifier can then be swapped for a more appropriate solution.

The PiezoDrive PD200 amplifier is a good example of a bench top amplifier.  The unit offers a significant power output of 60W.  The linear amplifier is great for research and development work due to its reliable response and low output noise.  BNC connections and mains power allow you to get up and running quickly.


COTS Integrated Circuits

1_DRVChip.jpg

A quickly growing market is haptics, which involves the application of force and vibration to communicate via touch to a user.  Perhaps the most commonly used haptic solution is the vibrate function on a cell phone.  Haptics has been traditionally dominated by off balance motors and linear resonant actuators, both based on electromagnetic technology.  More recently however, the market has been shifting towards piezoelectric actuators due to the higher energy efficiency, higher bandwidth, solid state technology, and control authority.  Applications such as body worn solutions and hand held electronics require an IC / chip style amplifier due to size, weight, and heat rejection requirements.  These ICs are directly integrated into the circuit boards of the device and normally run on an input of 3.3 VDC or 5 VDC.  A significant limitation of chip based solutions is the maximum power output of the devices.  Since the ICs are small, they don’t have the physical hardware required to generate very large voltages.  Their size also limits the device’s ability to reject heat, requiring the output power of the device to be limited to prevent damage to the driver.

Texas Instruments offers a line of chip based amplifiers marketed as piezo haptic drivers.  Example part numbers are DRV2667 and DRV8662.  These ICs are approximately 4 mm x 4 mm in size and operate on 3-5 VDC.  This is one of the chips my company uses in the haptics applications due to its size, low power consumption, and ease of implementation.  The unit is available in several form factors including the raw chip ($4), an evaluation board ($100), and even a custom breakout board ($28).  Since TI is interested in selling chips and not full piezo driver systems, they offer a lot of reference designs that can be used to create custom electronics boards.  User yurikleb on Instructables offers a tutorial to get the chip running with an Arduino.

OPA2544.jpg

A higher power chip based offering is the Texas Insturments OPA2544.  This dual opamp takes a wide range of input voltages (+/- 10 VDC to +/- 35VDC) is able to provide a maximum output of up to 4 amps and has an output voltage up to +/- 70 V.

Since this unit packs so much power in a small package, it will be importnat to keep it cool during operation.  A hot opamp can limit the output current or even fail.  This unit costs approximatly $24 in low quantities.  At these power levels, the unit can operate Mide's piezos around 20 kHz.  For users seeking a much higher frequency, the LT1210 offers a reasonable power output (albeit at a lower voltage) in the megahertz frequency range.


Amplifier Modules

2_PDm200B.png

When a small size is required, but a significant amount of power is needed, an amplifier module is typically the best option.  I do a lot of work with synthetic jet actuators, which are often located on cars, trucks, or aircraft where space and weight are at a premium.  Modules will often require a DC power supply such as 12, 24, or 48 VDC, making them ideal for automotive and aerospace applications where DC voltage is readily available.  Module style amplifiers are can be rough in appearance consisting of exposed circuit boards, wires, and heat sinks.  However, when the amplifiers are integrated into the final system, they are typically hidden (under the hood of a car, for example) like other electrical hardware.

PiezoDrive offers a wide selection of module based amplifiers including the PDm200B.  This module based amplifier operates on an input DC voltage of +/- 12 VDC to 34 VDC.  It offers a high output voltage of +/- 200 V and a peak output current of 300 mA.  For even greater output, the unit can be bridged with a second for an output of +/- 400 V peak.  This unit is a nice balance of size and power.  It offers a clean interface with clearly labeled inputs and outputs. 

3_piezoMaster.jpg

Viking Industrial Products offers many piezo amplifier modules.  The VP7206-48H805 model for example is capable of an output voltage range of 800 Vpp at an output current of 200 mA.  With an average output power of 15W and a peak power output of 150 W, these units offer a lot of power in a small form factor.  A drawback of the amplifiers is that they operate on a bias voltage.  Instead of being centered on 0 V, the driving waveform has a bias of half the output voltage range.  Care must be taken when using the units.  Midé has found that if you try to connect a piezo while the amplifier is running, it could damage the piezo due to the bias voltage.  Additionally, connecting to the units is a bit clumsy when compared to the PiezoDrive modules.  When all hooked up, there are many wires coming from the amplifier that the user must solder to or connect with clips.


DC Power Supply

In many piezo driving applications such as speakers, haptics, and fans, the unit is driven at a constant or varying frequency so that it vibrates back and forth to create force, motion, or sound.  However, there are also many applications where the piezo is controlled to a specific position, such as in a pneumatic valve application.  In a valve the only positions needed may be ‘open’ and ‘closed’.  Such bi-state operations can be achieved with a standard DC power supply.  When not powered, the piezo can relax to a known state.  Applying a DC voltage across the piezo will cause it to strain and change shape to the second state.  This DC voltage can be a battery, voltage rail available in the system, or created by means of a voltage booster (operational amplifier, DC to DC converter, or transformer).


Mains (Wall) Power

An often overlooked way to drive a piezo is with mains (wall) power.  In the United States, a typical wall outlet will provide 120 VAC RMS (170 VAC peak) at 60 Hz.  In many other parts of the world the voltage is on the order of 220 VAC RMS at 50Hz.  Mains power can be useful in driving piezos in situations where the drive frequency and amplitude are not critical.  Alternatively, the piezoelectric actuator can be designed to operate directly on mains power.  This is how we chose to design our piezo fan called PiezoFlo which you can see a video of here.  Since the piezo fan will be integrated into existing systems that are probably mains powered (such as a TV, computer, or LED lamp), powering the piezo fan with mains power reduces the electrical hardware requirements.   When operating on mains power, it is not a bad idea to include hardware for current limiting or fusing since mains electricity can have surges and supply a significant amount of power.

Learn More on PiezoFlo Webpage


Custom Amplifier Solution

The final method to drive a piezo is with a custom amplifier solution.  These solutions typically will require significantly more electrical engineering knowledge than a commercial off the shelf (COTS) solution.  Custom solutions can be optimized for operating parameters, size, weight, power, and integration.  The end result is often very similar to a module solution in appearance and components used, but optimized for the final system.


Overview of Piezo Amplifiers

 

Component

DRV2667

OPA2544

PDm200B

VP7206-48H805

PD200

Manufacturer

Texas Instruments

Texas Instruments

PiezoDrive

Piezo Master

PiezoDrive

Type

Chip

Chip

Module

Module

Bench Top

Output Voltage

200 Vpp

140 Vpp

400 Vpp

0-800 V

200 Vpp

Output Power

1 W*

10 W*

10 W*

15 W

60 W

Dimensions

4 x 4 x 1 mm

20 x 20 x 5 mm

71 x 38 x 40 mm

64 x 100 x 32 mm

275 x 141 x 64 mm

Approximate Cost

$4

$24

$270

$380

$1,890

Datasheet

Link

Link

Link

Link

Link

* Parameters are estimated

 

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Jeff Court
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Jeff Court on February 29, 2016
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Topics: Piezoelectrics


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