Electronics Overview

Piezos, with their electromechanical coupling, can be challenging to electrically interface with.  This article provides some information to help you interface with them and develop your electronics.

Manuals

The following two links are to manuals for the EPA-008 and EPA-104 which are high voltage amplifiers for driving piezos.  You will need a function generator as well to drive your piezo.

• EPA-008
• EPA-104

The following two older kit manuals have good background on interfacing with piezos.  We are updating these documents and will be publishing them shortly.

• GS (Generator / Sensor) Kit
• MA (Motor / Actuator) Kit

Driving a Piezo

Quasi-Static Operation

A piezoelectric actuator operating below its fundamental resonance can be treated simply as a capacitive load. The circuit must supply charge to cause a motion, and must withdraw charge to cause a retraction ( i.e. charge applied to the device does not bleed off internally ). When held motionless in any position, piezoelectric actuators draw negligible current, typically much less than a microamp.

Near Resonance Operation

A piezoelectric actuator operating near resonance can be modeled as a capacitor (having a value equal to the transducer capacitance) with a resistor in parallel ( typically 10 to 100 ohms ). The power dissipated by this resistance represents the work which the actuator does on its environment. The drive circuit must have sufficient current capacity to maintain the desired voltage on the resistor.

Charge / Discharge Protection

Instantaneous charging or discharging of piezoelectric actuators causes acoustic shockwaves within the piezoceramic which can lead to localized stress concentrations and fracture. Therefore, the peak current to any actuator must be limited. One simple method places a protection resistor in series with the actuator, the value of which can be estimated using the following relation: 

For a series operated cantilevered bending element, substituting for C and Fr:

This essentially limits operation to a frequency region below the fundamental resonance.

Output Stage Protection

Piezoelectric bending elements can generate high voltages ( >100 volts ) under external vibration, shock, or temperature shifts. lf these conditions are expected, the drive circuitry of the output stage must be protected against transient voltages of all polarities

Electrical Isolation

The outer electrode surfaces of certain motor elements are electrically "live" in many configurations. For product or experimental safety, consideration should be given to insulating or shielding the electrodes, mount, and power take-off sections of the motor element.

Electrical Breakdown

The highest value of applied electric field is determined by electrical breakdown occurring either through the body of the piezoceramic sheet or over the its edges. Pieces of dust and debris adhering to edges can initialize edge discharge at fields as low as 400-800 volts/mm. However, the discharge arc vaporizes the debris, thereby cleaning itself. A number of these edge debris arcs may occur during the initial energization of the bending motor, but they will not occur again. Continuous breakdown occurs around 3,000-4,000 volts/mm, usually at impurity or defect regions within the bulk of the material. This can lead to a short circuit across the sheet due to vapor deposition of electrode or shim material near the site of arcing. A current limiting resistor or in-line fuse is recommended when excessive electric fields are used. 

Electrical Losses

The bulk resistivity of piezoceramic is - 10 ¹² Q-cm. Therefore, electrical losses are minimal under static or low frequency operation. However, dielectric losses are significant under cycled operation and can lead to heating under high frequency /high power operation. The loss tangent, the ratio of series resistance to series reactance, for PZT-5A is -0.015. 

Piezo Electrical Output

Piezoelectric generators are usually specified in terms of their short-circuit charge and open-circuit voltage. Short-circuit charge, Qs, refers to the total charge developed, at the maximum recommended stress level, when the charge is completely free to travel from one electrode to the other, and is not asked to build up any voltage. Open-circuit voltage, Vo, refers to the voltage developed, at the maximum recommended stress level, when charge is prohibited from traveling from one electrode to the other. Charge is at a maximum when the voltage is zero, and voltage is at a maximum when the charge transfer is zero. All other values of simultaneous charge and voltage levels are determined by a line drawn between these points on a voltage versus charge line, as shown below. Generally, a piezo generator must move a specified amount of charge and supply a specified voltage, which determines its operating point on the voltage vs. charge line. Work is maximized when the charge moved permits one half the open circuit voltage to be developed. This occurs when the charge equals one half the short-circuit charge.

Circuit Considerations

Piezo Charge and Voltage Generator

The following shows the equivalent circuit for a piezo charge generator where Cp is the piezo capacitance and Rp is the internal (or bulk) resistance of the piezo device. Since the resistivity of piezoceramic is of the order of ~1012 ohm-cm, Rc is generally neglected. 

Since voltage can be calculated from the charge delivered by a piezo device according to the relationship that V = Q / Cp, The following shows a piezo device modeled as a voltage generator. Voltage output, proportional to the stress or strain of the piezo device, provides a sensing function.

Electrical Time Constant

The charge on a piezo sensor decays with time at a rate determined by its RC time constant. An RC circuit exhibits the exponential decay. Typically, charge is generated quickly. If the discharge time constant is large, it will not decay faster than the charging rate. Clearly, a long time constant is necessary if low frequencies need to be monitored. 

Input Signal Circuits

The signals from piezo sensors can be fed to meters, oscilloscopes, and circuits. The circuits are numerous, ranging from simple to complex, whose purpose is to filter or cancel unwanted portions of the signal, to amplify or switch the signal (transistors and operational amplifiers), or to make decisions based on information from signals (digital logic). 

Input Stage Protection

Piezoelectric elements can generate high voltages ( >>100 volts ) under external vibration, shock, or temperature shifts. If these conditions are expected, the circuitry of the input stage must be protected against transient voltages of all polarities. This is commonly accomplished with a high resistance bleed resistor placed in parallel with the piezo element. 

Cables

An op amp should be kept as close to the piezo sensor as possible. Shielded coax cable should be used when possible, although this can add leakage and capacitance to the circuit. Motion of the cable is another source of noise. The cable should be held down firmly to eliminate any movement or vibration. Polar plastic materials, used for cable insulation, can generate charge. Teflon is a good choice of material to minimize this problem.

Commercially Available Solutions

Driving a Piezo

We offer several amplifers and inverter options online. There are other commercially available options too. Our blog on how to drive a piezoelectric actuator goes into more depth the different options available.   Below is a summary table from that blog.

Energy Harvesting

The piezo’s output will be a relatively high voltage, low current AC signal that needs to be conditioned for use with most other electronics.  We have a commercially available circuit online. We have also partnered with Linear Technology who offers a number of commercially available chips and demonstration boards for energy harvesting applications.  The following solutions are recommended:

• LTC3588-1 – Nanopower Energy Harvesting Power Supply
• Demo board from Linear Technology
• Breakout board from Spark Fun
• DC2042A – Energy Harvesting Multi-Source Demo board
• LTC3588-2 – Nanopower Energy Harvesting Power Supply (higher voltage)
• LTC3330 –Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Life Extender
• Demo board from Linear Technology
• LTC3331 – Nanopower Buck-Boost DC/DC with Energy Harvesting Battery Charger
• Demo board from Linear Technology

Linear Technology offers a number of piezoelectric energy harvesting electronic solutions. They have also partnered with Wurth Elektronik to offer an energy harvesting kit that incorporates a microprocessor, data acquisition and wireless communication; this solution provides everything you need to develop a full up system powered off piezoelectric and three other types of energy harvesting inputs.