Piezoelectric vibration sensors

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Concept and functional features of piezoelectric sensors, the scope of its application. Designing with piezoelectric sensors. Piezo-vibration sensor Parallax 605–00004 and Bosch 608–00112: overview, technical characteristic, accessories, installations.

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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 Mea...