Piezoelectric Actuator Amplifier

    Everything you need to know about Piezoelectric Amplifiers

1.0 step1 What is a piezoelectric amplifier?

Piezoelectric amplifiers, also commonly referred to as amplified piezoelectric actuators, offer a huge advantage of large deformations with large strokes. The piezoelectric amplifiers can deform as much as 8%. These amplifiers can produce large strokes on both static and dynamic conditions because of the pre-stress applied to the ceramic plates combined with a mechanical amplifier. For these reasons, these amplifiers are ideal to be used for vibration generation, structure active damping, fluid control functions, energy harvesting, and micropositioning. With such important uses, the piezoelectric amplifiers are found in various domains, which includes optics, instrumentations, spacecraft and aircraft, production equipment, and machine tools.

The word piezo originates from the Greek word Piezein, which means to squeeze or press. Piezoelectricity refers to the electric polarity or the electricity generated in the dielectric crystals when it is subjected to mechanical stress. Likewise, theses dielectric poles can produce motion when subjected to electric charge.

Piezoelectricity was first discovered in 1880 by Curie brothers. They discovered that by applying electricity to quartz, the material could change its dimensions. Conversely, quartz was able to generate an electric charge when its dimensions were altered. The piezoelectric amplifier is a small precise amplifier that can produce small charges or precise movements as per user requirements.

Many piezoelectric amplifiers are also known to have an electrical effect due to the radiation or temperature effect around them. In this section, we will be looking into the electric effects limited to electric charge or mechanical stress.

2.0step2 What is the basics working principle of the Piezoelectric Amplifier?

Piezoelectric Amplifiers are ideal for driving high-frequency and high-capacitance piezoelectric devices. Piezoelectric amplifiers and transducers are usually capacitive and have the property of storing a strong charge. The high-capacitance results in a low impedance at higher frequencies. A high-current piezoelectric amplifier is needed to drive the electricity at ultrasonic frequencies. The amount of current required is calculated using Ohm’s Law, and it is directly proportional to the frequency, voltage, and the capacitance, as shown in the following equation.

Where;

I = current

V = Voltage

C = Piezo capacitance

For example, if the high-frequency piezoelectric actuator capacitance is 3.3µF, the maximum voltage required will be 20V, a frequency of 10kHz. The peak current will be 4.14A. For such conditions, a very high-current piezoelectric driver is needed to effectively drive the high amount of current. Frequencies any higher would need even higher current.

The way the piezoelectric amplifiers is defined into three main operation modes;

• Shear effect

The charge produced in this case is directly proportional to the applied force and is generated at an angle of 90°. The charge on the amplifier is directly dependent over the element shape and size.

• Longitudinal

The amount of the charge displaced is directly proportional to the force applied and is independent on the piezoelectric shape and size. The charge output of the piezoelectric amplifiers can be increased by properly putting several elements mechanically in series and by having the electricity in parallel.

• Transverse effect

The transverse effect on the piezoelectric amplifiers can be seen when force is applied along a neutral axis (y) and the charges are displaced in the (x) direction, perpendicular to the line of force. The amount of charge experienced by the amplifier is dependent on the geometrical dimension of the piezoelectric material used.

3.0 step3 What is the Theory and Modeling of the Piezoelectric Amplifier?

The basic theory behind piezoelectric amplifiers is based on electric dipoles. At a molecular level, the structure of the piezoelectric material is an ionic bonded crystal. At rest, the dipoles that are formed by the negative and positive ions cancel each other due to the symmetry of the crystal structure; thus, no electric charge is observed. When the material is stressed, and the symmetry is lost, a net dipole moment is created. This dipole moment creates an electric field across the piezoelectric crystals.

In this manner, the piezoelectric amplifier can generate an electric charge that is directly proportional to the pressure applied. These amplifiers can generate reciprocating motion if an AC voltage is applied across its terminals. Such amplifiers are ideal for dynamic or AC applications but are not well suited for static or DC applications. One of the main reasons for this is that such electrical charges produce decays over time due to the internal and input impedance of the sensors and then adversely affects their working capabilities.

3.1       Signal conditioning

The signal conditioning varies from one piezoelectric amplifier to another. They all are rated different and have different signal conditioning capabilities. Normal output voltages range from microvolts to hundreds of volts while the signal conditioning circuitry varies significantly. The key items to consider when designing the amplifier are;

• Input impedance
• Signal impedance
• Frequency of operation
• Mode of operation

This section assumes that the output of the sensor will have a small amount of amplification and that the output signal level by the piezoelectric amplifiers has the desired signal levels between 3 Volts to 5 volts for a full scale.

Normally for the high impedance of the piezoelectric amplifiers, there is a need for a high-input impedance. JFET or CMOS input op-amps such as the TLV2771 are preferred choices.

The following sections discuss two different circuits for signal conditioning. Figure A shows the voltage mode piezoelectric amplifier circuit, whereas, figure B shows the charge mode piezoelectric amplifier circuit. The voltage mode is usually used when the amplifiers in proximity to the sensor. On the other hand, the charge mode amplification is used when the piezoelectric amplifier is quite far away from the sensor.

3.2       Voltage mode amplifier

In a voltage-mode piezoelectric amplifier, the output is dependent over the amount of capacitance experienced by the sensor. The capacitance associated with the interface cable is known to directly affect the output voltage. If the cable is removed or moved from its original location, there can be some variations in C_c which can result in problems.

The Resistor R_b effectively provides a DC bias path for the amplifier input stage. On the other hand, the choice of R_f and C_f sets the upper cutoff frequency for the amplifiers.

Figure A

The lower cutoff frequency can be calculated via;

When designing a voltage mode piezoelectric amplifier circuit, it is important that the resistor R_b is to be selected with the highest resistance possible, and the interference cable is reduced to a minimum. In the circuit shown in figure A, a TLV2771 is used with an R_b of 10 MΩ. This configuration will result in a typical offset of 60µV over the commercial temperature range.

3.3       High-current piezoelectric amplifier

The piezoelectric transducer devices operating voltage can range anything from between 10V to 200V or in some cases even more. In addition to these, AC piezo devices that have any combination of high voltage, high capacitance or high frequency will need a high current driver. Signal and functions generators that are commonly found in the laboratories can have an output of less than 5V in a 50-ohm load. Their output voltages are significantly lower when the PZT transducer is lower than 50 ohms. Piezoelectric devices often require voltage in the range of at least 20V or more. Consequently, high voltage output and a high output current piezoelectric amplifier driver are required to properly drive such transducers.

3.4       Charge mode amplifier

In the charge mode piezoelectric amplifier, the charge mode will directly balance the charge injected into the negative input via the charging feedback capacitor (C_f). In the charge mode configuration, the resistor R_f bleeds off the charge from the capacitor C_f at a very low rate. The low rate of discharge prevents the amplifier from going into saturation. The resistor R_f provides the circuit with DC bias path for the negative input. The values of the C_f and the R_f are also set to a low cutoff frequency of the amplifier.

The action of the piezoelectric amplifier can maintain zero volt across the input terminals so that the stray capacitance presents in the circuit due to interference cabling do not present a problem. In the circuit shown in Figure B, the resistor Ri is providing ESD protection while the Resistor R_i and capacitor C_p and C_c combine to produce a roll-off at a significantly higher frequency.

Figure B

4.0 step4 Benefits

There are many different benefits of piezoelectric materials that make them effective for dynamic and quasi-static usage. The piezoelectric amplifiers have significant overload protection in which the amplifiers can directly react to the load experienced rather than straining under high loads. Most of the piezoelectric amplifiers have a pressure resistance of up to 3 x 10⁸ Pa. This is of a huge benefit for many users as this way their piezoelectric amplifiers can work effectively under massive overloading without any risk of pressure-induced destruction. Even if in some cases the amplifier is overloaded beyond its operational range, there will be no damage induced or no zero-point offsets, linearity changes or fatigue. This makes these amplifiers super durable and has good longevity.

The piezoelectric amplifiers also offer an added value regarding the stable sensitivity. The quartz amplifiers can feature a solid-state design and do not show signs of aging. They are not displaced under loading. Therefore, the changes to sensitivity are very minimal. No changes to sensitivity results in less frequent calibrations of the amplifiers. These benefits make the piezoelectric amplifiers both time and cost-effective in terms of maintenance.

Dimensions are often a very critical point when designing any cost-effective machine with small amplifiers. In this regard, piezoelectric amplifiers have a huge advantage. These amplifiers have minimal space. Therefore, the mass load they add to any product is negligible. Other benefits of piezoelectric amplifiers are that they have low acquisitions and lower life cycle costs.

Overall, piezoelectric amplifiers are one of the most cost-effective yet useful amplification technique that is deployed in all major industries and machinery.

5.0 step5 What are the Industrial Applications of Piezoelectric Amplifier?

With the benefits of the piezoelectric amplifiers discussed above, it can be seen that these amplifiers are quite versatile tools for the amplification or measurement of various processes. They are quite commonly used for process control, quality control, and research and development in many different industries. Though the piezoelectric effect was discovered in 1880 by Curie brothers, proper industrial use for the piezoelectric amplifiers was not seen until the 1950s. Since then, this technology has been increasingly used and has now become a mature technology.

The rise of piezoelectric technology is directly connected with a set of inherent advantages that it has. The high modulus of elasticity of many different piezoelectric materials is comparable to that of some metals and can go up to 10⁶ N/m².

Piezoelectric amplifiers have a wide array of industrial applications. The list of application for these amplifiers continues to grow and include;

• Aerospace – Piezoelectric amplifiers see its usage the most in the aerospace industry. These amplifiers are used in Wind tunnel and modal testing, landing gear hydraulics, ejection systems, aerospace structures. Since the next-gen aircrafts all make use of the fly-by-wire method of flying, the piezoelectric amplifiers can output precise movements at a press of a button or a swing of a joystick.
• Engine Testing – Gas exchange, combustion, dynamic stressing, indicator diagrams
• Ballistics – Piezoelectric actuators are also quite commonly used in the ballistics systems. These systems are usually small and require precision in movement and minute changes of direction. Hence, they are used in explosions, sound pressure distribution, combustion systems.
• Engineering – Building structures, Dynamic response testing, vibration isolation, ship structures, materials evaluations, structure testing
• Biomechanics – Sports, cardiology, neurology, ergonomics, components for force measurement all have piezoelectric amplifiers deployed in them.
• OEMs – molding, compressors, drilling of oil and gas, flexible structures, vibration testers, rockets, transportation systems, shock testers. For the OEMs, piezoelectric amplifiers are their backbone.
• Factory/industrial – Metal Cutting, machine health monitoring, machine systems, press and chimp force, automation processes all make use of piezoelectric amplifiers. One of the reasons for this is that these amplifiers are very small in size. In machines where space is premium, these actuators perform an excellent job by doing their task as well as using minimal space.

One of the disadvantages of piezoelectric amplifiers is that they cannot be used fully in static applications. When used with a static force, only a fixed amount of charge is obtained in the piezoelectric amplifiers. The piezoelectric amplifiers are also prone to high temperatures in some cases, and high temperatures can result in a drop of sensitivity and internal resistance.

It must also be noted that the piezoelectric amplifiers cannot be used for very fast processes or in some cases at ambient conditions too. Many of the piezoelectric amplifiers produce quasi-static measurements.

Despite the few operational limitations that piezoelectric amplifiers have, they are one of the most useful components in any type of machine. They tend to significantly lower the operation cost and improve the efficiency of the system. Alongside this, they are highly sensitive and can work with very small ranges of operations.

6.0 step6  What High-Frequency Piezoelectric Amplifier Impedance Matching?

6.1       Reactive Power vs. Real Power

The high-frequency piezoelectric amplifiers are highly capacitive. Its impedance is nearly all reactive without any real resistance experienced. These amplifiers do not dissipate real power; therefore, all the power is dissipated inside the amplifier.

6.2       Increase resistive impedance

One of the most effective technique to increase the piezoelectric amplifiers output current is to make the high-frequency PZT device resistivity higher. It can be done by adding more resistance in series to the amplifier. The resistor added should have a resistance between 0.5Z to Z. The value of Z should be equal to the piezoelectric amplifier. Since this procedure will result in high dissipating power, the resistor tends to become very hot. It must be ensured that the impedance matching resistor has the capacity to handle the power.

6.3       Low Impedance Piezoelectric amplifiers

The piezoelectric amplifiers with very small, built-in charge-to-voltage converters are also commonly referred to as low impedance units. These units can utilize some type of sensing element(s) as their high impedance counterparts.

7.0 step7  What is the Comparison between high and low impedance Piezoelectric  Amplifiers?

7.1       Similarities

Both these type of amplifiers utilizes the same category of piezoelectric materials and therefore, are AC coupled systems with a very limited low-frequency response. Both the type of amplifiers have quasistatic amplification capabilities.

7.2       High Impedance Piezoelectric amplifier

Usually, high impedance piezoelectric amplifiers are more versatile as compared to the low impedance amplifiers. The normalization, gain, reset, and time constant are all controlled via an external charge amplifier. Along with this, the time constants for the high impedance piezoelectric amplifiers are usually longer. This allows the amplifier to have easy short-term static calibration. Since these amplifiers do not have any built-in electronics, they have a significantly wider operating temperature range.

7.3       Low impedance piezoelectric amplifier

Generally, low impedance piezoelectric amplifiers are tailored to a custom or generic application. The low impedance piezoelectric amplifiers have an internal time constant and fixed range. This usually limits their intended use. For applications with very well-defined measuring frequency and temperature ranges, the low impedance piezoelectric amplifiers offer cheaper operating costs as opposed to high impedance amplifiers. In addition to this, these amplifiers can be used for general purpose cables in high humidity environment.

8.0 step8  What Types of Materials used Piezoelectric Amplifier?

There are many naturally occurring, and artificially made piezoelectric materials. The naturally available piezoelectric materials are topaz, Rochelle salt, Quartz, Tourmaline-group materials, and some organic substances such as enamel, rubber, dentin, wood, and silk. The artificially manufactured materials are PVDF, Polyvinylidene, difluoride, Lead Titanate, Barium Titanate, Potassium niobite, Lithium tantalite, Lithium niobite, and other lead-free piezoelectric ceramics.

It must be noted that though the materials mentioned above are all piezoelectric materials, not all materials can be used for the piezoelectric amplifiers. There are certain requirements that are supposed to be met by piezoelectric materials for them to be used as amplifiers. The materials used for the amplification purposes must have frequency stability, insensitivity to extreme temperatures and humidity, high output values and which can be available in various shapes or should be flexible enough to be manufactured into different shapes without adversely affecting their properties.

Unfortunately, there are no existing piezoelectric materials which have all these properties. Quartz is a highly stable crystal which is naturally available. However, Quartz has small output levels and amplifications. The highest output values can be obtained via Rochelle salt, but it is highly sensitive to environmental conditions, and it cannot be operated on temperatures exceeding 1150F.

9.0 step9 Piezoelectric amplifiers at PD (PiezoData Inc.) R&D Co., Ltd

We at PD (PiezoData Inc.) are manufacturers and suppliers of high-quality Piezoelectric amplifiers. We have decades of experience in piezoelectric research, development, and manufacturing. Therefore, from PZT power to amplifiers to actuators, we execute a full set of strict control over the quality of our components at each stage of manufacturing. We at Height Piezoelectric has excellent sourcing and supply chain at each production stage, which ensures cost-savings, excellence, and value-adding solution so that we can deliver what we promise to our customers.

We would love to work closely with you to offer the unique piezoelectric amplifier or any other piezo component to fir your requirements of piezoelectric amplifier solution. And deliver you with the best in class piezoelectric amplifiers.