Piezoelectric vibration sensors

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Piezoelectric vibration sensors

Introduction

The piezoelectric sensor is used for flex, touch, vibration and shock measurement. Its basic principal, at the risk of oversimplification, is as follows: whenever a structure moves, it experiences acceleration. A piezoelectric shock sensor, in turn, can generate a charge when physically accelerated. This combination of properties is then used to modify response or reduce noise and vibration. Why is that important? Because vibration and shock can shorten the life of any electronic and electromechanical system. Delicate leads and bond wires can be stressed, especially after exposure to long term vibration. Solder joints can break free and PCB traces can ever so slightly tear from impact and impulse shock, creating the hardest type of system failure to debug; an intermittent failure. This article discusses piezoelectric shock and vibration sensors and sensor technology, focusing on available products (all parts mentioned here can be found on the Digi-Key website - links are provided), as well as design issues and design techniques.

1. How it works

The piezoelectric effect was discovered by Pierre and Jacques Curie in the latter part of the 19th century. They discovered that minerals such as tourmaline and quartz could transform mechanical energy into an electrical output. The voltage induced from pressure (Greek: piezo) is proportional to that applied pressure, and piezoelectric devices can be used to detect single-pressure events as well as repetitive events. Still, the ability of certain crystals to exhibit electrical charges under mechanical loading was of no practical use until very-high-input impedance amplifiers enabled engineers to amplify the signals produced by these crystals. Several materials can be used to make piezoelectric sensors, including tourmaline, gallium phosphate, salts, and quartz. Most electronic applications use quartz since its growth technology is far along, thanks to development of the reverse application of the piezoelectric effect; the quartz oscillator. Sensors based on the piezoelectric effect can operate from transverse, longitudinal, or shear forces, and are insensitive to electric fields and electromagnetic radiation. The response is also very linear over wide temperature ranges, making it an ideal sensor for rugged environments. For example, gallium phosphate and tourmaline sensors can have a working temperature range of 1,000?C. The physical design of the piezoelectric sensor depends on the type of sensor you wish to create. For example, the configuration of a pressure sensor, or a shock (impulse) sensor, would arrange a smaller, but well-known mass of the crystal in a transverse configuration, with the loading deformation along the longest tracks to a more massive base. This assures that the applied pressure will load the base from only one direction.

An accelerometer based on the piezoelectric effect, would use a known mass to deform the sensing crystal part in either a positive or negative direction depending on the excitation force . It should be noted that you need a known modulus of elasticity in the sensor substrate.

Because the modulus of elasticity is known for a substrate material, the unconstrained mass is allowed to move with vibration making this type of piezoelectric sensor ideal for detecting shock and vibration.

2. Designing with piezoelectric sensors

piezoelectric sensor piezo vibration

Piezoelectric sensors require some precautions when connecting to sensitive electronic components. First and foremost, the voltage levels created by hard shock can be very high, even around 100-V spikes. More than likely, an op amp will be used to interface these sensors to an A/D converter, either discrete or on a microcontroller. One tip is to choose a high-input-impedance op amp to minimize current. One possible candidate is the Linear Technology JFET input dual op amp. It has 10№І ? input resistance and a 1 MHz gain bandwidth product, good enough to easily handle the vibration ranges of piezoelectric sensors. Another suitable part is the TLV2771 from Texas Instruments. This rail-to-rail low-power op-amp also has a 10№І ? differential input resistance and a 5 MHz unity-gain bandwidth. Signal conditioning in a single stage can prepare the input from the shock sensor directly into an A/D converter.

Op-amp circuits can be designed to operate in voltage mode or charge mode. Charge mode is used when the amplifier is remote to the sensor. Voltage mode is used when the amplifier is very close to the sensor. Another tip is to attenuate the input signal and use the op amp's gain to bring into the desired range. Be aware that you may need snubbing protection on the inputs of the op amp, especially if the design could be subjected to harsh hits. Also note that you may think that a pressure sensor would generate only a positive voltage, but, in reality, the signal from the sensor can ring and introduce negative voltage spikes. This means that you may need to squelch negative voltage levels on the op-amp inputs, especially if using only a single rail power supply on the op amp.

3. Vibration sensor Parallax 605-00004

Many off the shelf piezoelectric sensors are readily available to use in your designs. A case in point is the Parallax 605-00004, which is a piezo vibra tab sensor capable of acting as a switch, or as a vibration sensor . A polymer film laminate uses crimped contacts and features a sensitivity of 50 mV/g. Figure 5. The flexible through-hole LDTO polymer film piezoelectric sensors can be hard mounted or free floating to detect strain, shock, or vibration. You should be aware that adding mass to a piezoelectric sensor can change its resonant frequency as well as change its baseline sensitivity. Many piezoelectric sensors like the 605-00004 are characterized to be used this way and provide supporting tables and graphs.

Another part worth considering is the Measurement Specialties 0-1002794-0 cantilever piezo film sensor. This is also a vibra tab sensor capable of hard mounting to a surface, floating in an axis of inertia, or mass loaded to prebias and calibrate. The output voltage swings can directly trip a FET or CMOS input, and a multiaxis response can be obtained by offsetting the mass center.

The LDT0 is a flexible component comprising a28 µ m thick piezoelectric PVDF polymer film with screen-printed Ag-ink electrodes, laminated to a0.125 mm polyester substrate, and fitted with two crimped contacts. As the piezo film is displaced from the mechanical neutral axis, bending creates very high strain within the piezo polymer and therefore high voltages are generated. When the assembly is deflected by direct contact, the device acts as a flexible «switch», and the generated output is sufficient to trigger MOSFET or CMOS stages directly. If the assembly is supported by its contacts and left to vibrate «in free space» (with the inertia of the clamped/free beam creating bending stress), the device will behave as a form of accelerometer or vibration sensor. Adding mass, or altering the free length of the element by clamping, can change the resonant frequency and sensitivity of the sensor to suit specific applications. Multi-axis response can be achieved by positioning the mass off center.

Four different experiments serve to illustrate the various properties of this simple but versatile component.

1. LDT0 as Vibration Sensor - with the crimped contacts pushed through a printed-circuit board, the LDT0 was soldered carefully in place to anchor the sensor. A charge amplifier was used to detect the output signal as vibration from a shaker table was applied (using a charge amplifier allows a very long measurement time constant and thus allows the «open-circuit» voltage response to be calculated). Small masses (approximately 0.26g increments) were then added to the tip of the sensor, and the measurement repeated. Results are shown in Table 1 and the overlaid plots in Fig 1. Without adding mass, the LDT0 shows a resonance around 180 Hz. Adding mass to the tip reduces the resonance frequency and increases «baseline» sensitivity.

2. LDT0 as Flexible Switch - using a charge amplifier to obtain «open-circuit» voltage sensitivity, the output was measured for controlled tip deflections applied to the sensor (supported by its crimped contacts as described above). 2 mm deflection was sufficient to generate about 7 V. Voltages above 70V could be generated by bending the tip of the sensor through 90° (see Table 2).

3. LDT0 Electrical Frequency Response - when the source capacitance of around 480 pF is connected to a resistive input load, a high-pass filter characteristic results. Using an electronic noise source to generate broad-band signals, the effect of various load resistances were measured and the -3 dB point of the R-C filter determined (see Table 3).

4. LDT0 Clamped at Different Lengths - using simple clamping fixture, the vibration sensitivity was measured (as in (1) above) as the clamp was moved to allow different «free» lengths to vibrate. The sensor may be «tuned» to suit specific frequency response requirements (see Table 3).

This Piezo Film Vibra Tab Sensor is the LDT0 Solid State Switch/Vibration Sensor manufactured by Measurement Specialties. The LDT0 is a piezoelectric film device capable of acting as switch or vibration sensor. Characteristics of this device allow even more possibilities for use.

Quick Start Circuit

The circuit above allows you to start using the LDT0 as a switch or shock detector. You can test for functionality by checking the pin for a HIGH signal on the connected I/O pin when the sensor is tapped, flicked or snapped.

Sensitivity.

As a vibration sensor the LDT0 has a sensitivity of 50 mV/g. As mass is added to the device, sensitivity decreases, as does it's resonant frequency. Please see the manufacturer's datasheet for further details about how adding mass to the device affects these characteristics.

The LDT0 is a flexible film piezoelectric device laminated to a polymer substrate and includes two crimped contacts for mounting and electrical connections. As the device is bent or displaced from its neutral axis, a very high strain is generated by the piezo-polymer and high voltage is generated. This device can generate voltages of ~70 volts. Always be sure to clamp, buffer or filter the signal going to the I/O pin to keep it within acceptable voltage/current limits.

Module Dimensions

4. Piezo-vibration sensor Bosch 608-00112

Overview

Vibration sensors of this type are suitable for the detection of structure-borne acoustic oscillations as can occur for example in case of irregular combustion in engines and on machines. Thanks to their ruggedness, these vibration sensors can be used even under the most severe operating conditions. On account of its inertia, a mass exerts compressive forces on a ring-shaped piezo-ceramic element in time with the oscillation which generates the excitation.

Within the ceramic element, these forces result in charge transfer within the ceramic and a voltage is generated between the top and bottom of the ceramic element. This voltage is picked-off using contact discs - in many cases it is filtered and integrated - and made available as a measuring signal. In order to route the vibration directly into the sensor, vibration sensors are securely bolted to the object on which measurements take place. Every vibration sensor has its own individual response characteristic which is closely linked to its measurement sensitivity. The measurement sensitivity is defined as the output voltage per unit of acceleration due to gravity (see characteristic curve). The production-related sensitivity scatter is acceptable for applications where the primary task is to record that vibration is occurring, and not so much to measure its severity. The low voltages generated by the sensor can be evaluated using a high-impedance AC amplifier.

Installation instructions

The sensor's metal surfaces must make direct contact. No washers of any type are to be used when fastening the sensors. The mounting-hole contact surface should be of high quality to ensure low-resonance sensor coupling at the measuring point. The sensor cable is to be laid such that there is no possibility of sympathetic oscillations being generated. The sensor must not come into contact with liquids for longer periods.

Drawing

Conclusion

Piezoelectricity is the electric charge that accumulates in certain solid materials (notably crystals, certain ceramics, and biological matter such as bone, DNA and various proteins) in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure. It is derived from the Greek piezo or piezein (рйЭжейн), which means to squeeze or press, and electric or electron (Юлекфспн), which stands for amber, an ancient source of electric charge. Piezoelectricity was discovered in 1880 by French physicists Jacques and Pierre Curie. The piezoelectric effect is understood as the linear electromechanical interaction between the mechanical and the electrical state in crystalline materials with no inversion symmetry. The piezoelectric effect is a reversible process in that materials exhibiting the direct piezoelectric effect (the internal generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the internal generation of a mechanical strain resulting from an applied electrical field). For example, lead zirconate titanate crystals will generate measurable piezoelectricity when their static structure is deformed by about 0.1% of the original dimension. Conversely, those same crystals will change about 0.1% of their static dimension when an external electric field is applied to the material. The inverse piezoelectric effect is used in production of ultrasonic sound waves. Piezoelectricity is found in useful applications such as the production and detection of sound, generation of high voltages, electronic frequency generation, microbalances, and ultrafine focusing of optical assemblies. It is also the basis of a number of scientific instrumental techniques with atomic resolution, the scanning probe microscopies such as STM, AFM, MTA, SNOM, etc., and everyday uses such as acting as the ignition source for cigarette lighters and push-start propane barbecues.

Piezoelectric sensors have proven to be versatile tools for the measurement of various processes. They are used for quality assurance, process control and for research and development in many different industries. Although the piezoelectric effect was discovered by Pierre Curie in 1880, it was only in the 1950s that the piezoelectric effect started to be used for industrial sensing applications. Since then, this measuring principle has been increasingly used and can be regarded as a mature technology with an outstanding inherent reliability. It has been successfully used in various applications, such as in medical, aerospace, nuclear instrumentation, and as a pressure sensor in the touch pads of mobile phones. In the automotive industry, piezoelectric elements are used to monitor combustion when developing internal combustion engines. The sensors are either directly mounted into additional holes into the cylinder head or the spark/glow plug is equipped with a built in miniature piezoelectric sensor.

The rise of piezoelectric technology is directly related to a set of inherent advantages. The high modulus of elasticity of many piezoelectric materials is comparable to that of many metals and goes up to 106 N/mІ citation needed. Even though piezoelectric sensors are electromechanical systems that react to compression, the sensing elements show almost zero deflection. This is the reason why piezoelectric sensors are so rugged, have an extremely high natural frequency and an excellent linearity over a wide amplitude range. Additionally, piezoelectric technology is insensitive to electromagnetic fields and radiation, enabling measurements under harsh conditions. Some materials used (especially gallium phosphate [2] or tourmaline) have an extreme stability even at high temperature, enabling sensors to have a working range of up to 1000°C. Tourmaline shows pyroelectricity in addition to the piezoelectric effect; this is the ability to generate an electrical signal when the temperature of the crystal changes. This effect is also common to piezoceramic materials. One disadvantage of piezoelectric sensors is that they cannot be used for truly static measurements. A static force will result in a fixed amount of charges on the piezoelectric material. While working with conventional readout electronics, imperfect insulating materials, and reduction in internal sensor resistance will result in a constant loss of electrons, and yield a decreasing signal. Elevated temperatures cause an additional drop in internal resistance and sensitivity. The main effect on the piezoelectric effect is that with increasing pressure loads and temperature, the sensitivity is reduced due to twin-formation. While quartz sensors need to be cooled during measurements at temperatures above 300°C, special types of crystals like GaPO4 gallium phosphate do not show any twin formation up to the melting point of the material itself. However, it is not true that piezoelectric sensors can only be used for very fast processes or at ambient conditions. In fact, there are numerous applications that show quasi-static measurements, while there are other applications with temperatures higher than 500°C. Piezoelectric sensors can also be used to determine aromas in the air through measurements of resonance and capacitance simultaneously.

Computer controlled electronics vastly increase the range of potential applications for piezoelectric sensors. Piezoelectric sensors are also seen in nature. The collagen in bone is piezoelectric, and is thought by some to act as a biological force sensor A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional voltage source and filter network. The voltage V at the source is directly proportional to the applied force, pressure, or strain. The output signal is then related to this mechanical force as if it had passed through the equivalent circuit. Frequency response of a piezoelectric sensor; output voltage vs applied force A detailed model includes the effects of the sensor's mechanical construction and other non-idealities. The inductance Lm is due to the seismic mass and inertia of the sensor itself. is inversely proportional to the mechanical elasticity of the sensor. C0 represents the static capacitance of the transducer, resulting from an inertial mass of infinite size. is the insulation leakage resistance of the transducer element. If the sensor is connected to a load resistance, this also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency. In the flat region, the sensor can be modeled as a voltage source in series with the sensor's capacitance or a charge source in parallel with the capacitance For use as a sensor, the flat region of the frequency response plot is typically used, between the high-pass cutoff and the resonant peak. The load and leakage resistance need to be large enough that low frequencies of interest are not lost. A simplified equivalent circuit model can be used in this region, in which Cs represents the capacitance of the sensor surface itself, determined by the standard formula for capacitance of parallel plates. [7] [8] It can also be modeled as a charge source in parallel with the source capacitance, with the charge directly proportional to the applied force, as above. Based on piezoelectric technology various physical quantities can be measured; the most common are pressure and acceleration. For pressure sensors, a thin membrane and a massive base is used, ensuring that an applied pressure specifically loads the elements in one direction. For accelerometers, a seismic mass is attached to the crystal elements. When the accelerometer experiences a motion, the invariant seismic mass loads the elements according to Newton's second law of motion.

The main difference in the working principle between these two cases is the way forces are applied to the sensing elements. In a pressure sensor a thin membrane is used to transfer the force to the elements, while in accelerometers the forces are applied by an attached seismic mass. Sensors often tend to be sensitive to more than one physical quantity. Pressure sensors show false signal when they are exposed to vibrations. Sophisticated pressure sensors therefore use acceleration compensation elements in addition to the pressure sensing elements. By carefully matching those elements, the acceleration signal (released from the compensation element) is subtracted from the combined signal of pressure and acceleration to derive the true pressure information. Vibration sensors can also be used to harvest otherwise wasted energy from mechanical vibrations. This is accomplished by using piezoelectric materials to convert mechanical strain into usable electrical energy. Two main groups of materials are used for piezoelectric sensors: piezoelectric ceramics and single crystal materials. The ceramic materials (such as PZT ceramic) have a piezoelectric constant / sensitivity that is roughly two orders of magnitude higher than those of the natural single crystal materials and can be produced by inexpensive sintering processes. The piezoeffect in piezoceramics is «trained», so unfortunately their high sensitivity degrades over time. The degradation is highly correlated with temperature. The less sensitive 'natural' single crystal materials (gallium phosphate, quartz, tourmaline) have a much higher - when carefully handled, almost infinite - long term stability. There are also new single crystal materials commercially available such as Lead Magnesium Niobate-Lead Titanate (PMN-PT). These materials offer greatly improved sensitivity (compared with PZT) but suffer from a lower maximum operating temperature and are currently much more expensive to manufacture. A piezoelectric transducer has very high DC output impedance and can be modeled as a proportional voltage source and filter network. The voltage V at the source is directly proportional to the applied force, pressure, or strain. The output signal is then related to this mechanical force as if it had passed through the equivalent circuit. Frequency response of a piezoelectric sensor; output voltage vs applied force A detailed model includes the effects of the sensor's mechanical construction and other non-idealities. The inductance Lm is due to the seismic mass and inertia of the sensor itself. Is inversely proportional to the mechanical elasticity of the sensor. C0 represents the static capacitance of the transducer, resulting from an inertial mass of infinite size. Is the insulation leakage resistance of the transducer element. If the sensor is connected to a load resistance, this also acts in parallel with the insulation resistance, both increasing the high-pass cutoff frequency. In the flat region, the sensor can be modeled as a voltage source in series with the sensor's capacitance or a charge source in parallel with the capacitance.

For use as a sensor, the flat region of the frequency response plot is typically used, between the high-pass cutoff and the resonant peak. The load and leakage resistance need to be large enough that low frequencies of interest are not lost. A simplified equivalent circuit model can be used in this region, in which Cs represents the capacitance of the sensor surface itself, determined by the standard formula for capacitance of parallel plates. It can also be modeled as a charge source in parallel with the source capacitance, with the charge directly proportional to the applied force, as above.

Reference

1. http://en.wikipedia.org/wiki/Piezoelectricity

2. http://en.wikipedia.org/wiki/Piezoelectric_sensor

3. Technical manual for piezo vibration sensor Parallax 605-00004

4. Technical manual for piezo vibration sensor Bosch 608-00112

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