Piezo Sensors Finding New Uses in Pressure Sensing Applications and Energy Harvesting Sources

Piezo Sensors
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Piezoelectric materials are among the oldest material-based components in the engineer’s toolkit. The piezoelectric effect was discovered in 1880 by French physicist Pierre Curie and his brother Jacques, who found that when stress (pressure) is applied to a crystal or ceramic material, or even bone, the material produces a distinct and repeatable voltage in proportion to that stress. Soon after, they demonstrated the reverse effect—when a voltage is applied to the crystal, it undergoes a tiny change or deformation in size or strain.

These two principles have been adapted to many applications; the best-known one is use of quartz as a resonant element in an oscillator circuit. Piezo materials are also used as actuators that can be directed to move in precise nanometer-size steps, such as for micromotors in applications ranging from high-end scientific instruments down to autofocusing in mass-market consumer cameras.  

1215 Piezo Sensors Thrive Under Stress Take Strain Off Users Minds In Article 1
Figure 1: The piezoelectric effect has a dual nature: a) applying stress on a crystal causes it to generate a voltage; while b) applying a voltage to the crystal causes it to deform, both in predictable and repeatable relationships. (Source: Seiko Epson Corp.) 

Even a gas-fired grill can have a piezoelectric element, using a built-in crystal that generates a spark of several hundred volts when the user turns a knob and so squeezes the crystal. This spark ignites the gas and initiates a flame (but the current level is so low that there is no danger of shock).

Since they have a crystalline structure, many piezoelectric materials are quite hard and can withstand a great deal of stress without changing their basic characteristics or failing. For this reason, they are often used as transducers for sensing pressure-related physical parameters, which includes pressure itself, as well as its related manifestations: acceleration, impact, altitude (air pressure), and vibration. Depending on material, these sensors can withstand and work at accelerations of several thousand g.

Using Piezo Devices as Strain Sensors

As a basic transducer, the piezo-based unit is very simple and rugged, but it has a unique electrical aspect. Due to its inherently high electrical impedance, it can only deliver a sufficiently strong signal to the electronic circuitry if the interfacing signal-conditioning circuit also has very high impedance. This impedance matching is a consequence of the power-transfer law, where the impedance of a source and its load must be complex conjugates to achieve maximum power transfer. [Note that this same consideration occurs in other contexts as well, including an RF power amplifier driving an antenna, or an antenna connected a low-noise amplifier (LNA) is a wireless system.]

One way to satisfy these interface requirements is to connect the piezo sensor to a junction field-effect transistor (JFET). This discrete device typically has an input impedance at least 1 GΩ (109 Ω) along with bandwidth into the high kHz and even higher, which is needed for high-frequency vibration measurements. While using a JFET is a viable solution, it is only a partial answer to the overall interface-circuit requirement. The JFET must be properly biased, and often needs added temperature-compensation to ensure accuracy and stability if the ambient environment is not controlled or stable. 

For this reason, many designers instead prefer to use a high-impedance op amp such as the TLV2771 series from Texas Instruments (Figure 2). This CMOS-input op amp features 360-mV input offset voltage, 17 nV/√Hz input noise voltage, and 2-pA input bias current—specifications  that are a good match when using a piezo transducer for high-accuracy, high-resolution measurements in medical, scientific, and industrial applications. 

1215 Piezo Sensors Thrive Under Stress Take Strain Off Users Minds In Article 2
Figure 2: The TLV2775 CMOS-input op amp from Texas Instruments is well-suited for use in signal-conditioning front-end circuits for high-impedance sources such as piezo transducers. (Source: Texas Instruments)

Although the circuit using this op amp still requires a handful of support components, the overall circuit design is simpler and has a much less critical physical placement than one based on the JFET alone. This is important because high-impedance circuits can be very sensitive to subtle design and layout issues.

Taking integration further, another alternative is to use devices that employ MEMS technology to put a piezo sensor and its associated circuitry in a single package. The Freescale MPL3115A2 (Figure 3) is a low-power, high-accuracy altimeter, barometer, and thermometer with digital output, all packaged in a 3 mm × 5 mm × 1.1 mm device. The complete device includes a pressure-sensing element, analog and digital signal processing, and an I2C interface, and can function over the range of 50 to 110 kPa with 20-bit resolution. 

1215 Piezo Sensors Thrive Under Stress Take Strain Off Users Minds In Article 3
Figure 3: The Freescale MPL3115A2 piezo-based pressure sensor includes analog front end, A/D converter, and serial interface along with a temperature sensor; it uses the classic multi-element Wheatstone bridge configuration for high sensitivity and accuracy. (Source: Freescale Semiconductor)

As a consequence of the internal A/D function and I2C interface, it minimizes the conversion and computational burden on the associated microcontroller. This particular unit targets relatively slow-changing pressure signals—such as those due to changes in weather or altitude—so there is no need for fast conversions and updating that would incur a larger dissipation penalty. The internal 12-bit thermometer in the MPL3115A2 can be used for basic temperature readings, of course, as well as for implementing temperature correction of sensor readings, if needed. 

Instead of incorporating only a single piezo element, this component uses a multi-element Wheatstone-bridge configuration. This classic sensor topology enables more accurate sensing of the relatively small changes in pressure, and with repeatability and precision. Devices such as this highly integrated sensor with integral electronics are also being used in applications as diverse as internal tire-pressure readings (mandated by the latest vehicle-safety standards). 

Piezo Sensors Helping With Energy Harvesting

In recent years, piezo sensors have taken on a role that is quite different from their traditional one of pressure sensing for test and measurement applications. Instead, the piezoelectric effect is used for energy harvesting designs in systems located where no independent power source is available, or battery replacement is impractical or undesirable. 

Their application as a source of energy to be harvested is unlike their use as pressure transducers, where the challenge is to capture the minute electrical signals with precision and accuracy. Instead, this challenge is to capture, store, and then release, as needed, the electrical energy that the piezo device produces from random ambient vibration. These vibrations are available to some extent in almost every installation including roadways, bridges, floors, motors, and even shoes. Accuracy, precision, linearity, and temperature-induced drift—the traditional metrics of data acquisition performance—are not relevant here. Instead, it’s all about efficiency.

Capturing the piezo device output energy due to the vibration may seem like a modest challenge, but it is not. The random energy bursts are tiny and can easily be lost to circuit leakage across low-impedance paths, or dissipation within the capture/storage electronic circuitry. Any energy captured from the piezo sensor must be carefully and immediately directed to a storage element such as a battery or capacitor, so it is not lost. Finally, the stored energy must be carefully allocated and metered to the load—the actual circuit the harvester is powering—while making sure that the storage element is not drained so far down that it will take too long to replenish (re-filling the storage element from a low-energy state has other efficiency and loss implications).

Despite these issues, vendors of analog and power management ICs have recognized the opportunity that piezo-based sensors can offer as a low-cost, reliable, long-term source of free energy. For example, the LTC3588 from Linear Technology Corp. (Figure 4) is designed to harvest low-energy, random outbursts from piezo devices. It integrates a low-loss, full-wave bridge rectifier with a high-efficiency buck converter to form a complete energy harvesting solution. 

1215 Piezo Sensors Thrive Under Stress Take Strain Off Users Minds In Article 4
Figure 4: Using piezo devices in energy harvesting requires a very different interface than when they are being used as sources of pressure-related data. The LTC3588 from Linear Technology captures the random vibration-induced energy outbursts, directs them to a storage element such as capacitor, and then manages the power drawn by the load. (Source: Linear Technology)

The LTC3588 address the “which came first—the chicken or the egg?” dilemma, which occurs when attempting to harvest energy from a random source. The quandary is this—the circuit and storage element can’t capture and retain the harvested energy until the associated IC has enough power itself to operate; yet the system can’t accumulate that power until the circuitry is functioning. Further, while the energy storage element is trying to “fill,” the load may be draining it of power, so there is no net accumulation. It is analogous to a hole in a bucket so big that the supply hose can’t make any headway on filling it up.

The solution, built into the LTC3588, is an ultralow-quiescent-current undervoltage lockout (UVLO) mode with a wide hysteresis window. This allows charge to accumulate on an input capacitor but not be used until the buck converter can efficiently transfer a portion of the stored charge to the output. For the load rail, users can select one of four output voltages (1.8 V, 2.5 V, 3.3 V and 3.6 V) via pin selection, and the IC can deliver up to 100 mA from the storage battery/capacitor, which is sufficient for the low duty-cycle/moderate current requirement of many IoT devices. In keeping with the very small physical package of many of the harvesting-powered data collection and reporting applications it serves, the LTC3588 is available in tiny 10-Lead MSE and 3 mm × 3 mm DFN packages.

Despite their maturity in countless established applications, piezo-based sensors are still finding new uses. These range from basic pressure-related sensing situations such as tire-pressure monitoring, to newer ones such as energy harvesting in IoT applications. Spurred by advances in materials science, ongoing research and development work is providing new piezo crystal materials, which provide different combinations of desired performance and cost attributes to expand their applications. When combined with suitable electronic components and circuits, piezo sensors and transducers offer designers an attractive option to meet many project requirements in output, sensitivity, linearity, ruggedness, repeatability, and cost.

 

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