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Introduction to Piezoelectric Pressure Sensors
Piezoelectric Pressure Sensors measure dynamic pressures. They are generally not suited for static pressure measurements. Dynamic pressure measurements including turbulence, blast, ballistics and engine combustion under varying conditions may require sensors with special capabilities. Fast response, ruggedness, high stiffness, extended ranges, and the ability to also measure quasi-static pressures are standard features associated with PCB quartz pressure sensors.
The following information presents some of the design and operating characteristics of PCB pressure sensors to help you better understand how they function, which, in turn, helps you make better dynamic measurements.
This page describes two modes of operation for pressure sensors manufactured by PCB. Charge mode pressure sensors generate a high-impedance charge output. ICP® (Integrated Circuit Piezoelectric) voltage mode-type sensors feature built-in microelectronic amplifiers, which convert the high-impedance charge into a low-impedance voltage output. (ICP is a registered trademark of PCB Piezotronics.)
Piezoelectric pressure sensors are available in various shapes and thread configurations to allow suitable mounting for various types of pressure measurements. Quartz crystals are used in most sensors to ensure stable, repeatable operation. The quartz crystals are usually preloaded in the housings to ensure good linearity. Tourmaline, another stable naturally piezoelectric crystal, is used in some PCB sensors where volumetric sensitivity is required.
Figure 1 illustrates the cross-section of a typical quartz pressure sensor. This particular sensor is a General Purpose Series with built-in electronics.
Figure 1: Typical ICP Quartz Pressure Sensor
When the crystal is stressed, a charge is generated. This high-impedance output must be routed through a special low-noise cable to an impedance-converting amplifier, such as a laboratory charge amplifier or source follower. High insulation resistance must be maintained in the cables and connections.
The primary function of the charge or voltage amplifier is to convert the high-impedance output to a usable low-impedance voltage signal for recording purposes. Laboratory charge amplifiers provide added versatility for signal normalization, ranging, and filtering. PCB's charge amplifiers have additional input adjustments for quasi-static measurements, static calibration, and drift-free dynamic operation. Miniature in-line amplifiers are generally of fixed range and frequency.
Charge mode quartz pressure sensors may be used at higher temperatures than ICP® sensors, since the temperature limitation is determined by the temperature limit of the crystals rather than built-in electronics.
When considering the use of charge mode systems, remember that the output from the crystals is a high impedance charge. The internal components of the pressure sensor and the external electrical connector maintain a very high (typically 10 e13 ohm) insulation resistance. Consequently, any connectors, cables, or amplifiers used must also have a very high insulation resistance to maintain signal integrity.
Figure 2: Charge Mode System Schematic
Figure 3: Charge Mode System
Environmental contaminants on the connector, such as moisture, dirt, oil, or grease contribute to reduced insulation, resulting in signal drift and inconsistent results.
Use of special low-noise cable is required with charge mode pressure sensors. Standard, two-wire, or coaxial cable, when flexed, generates a charge between the conductors. This is referred to as triboelectric noise and cannot be distinguished from the sensor's charge output. Low-noise cables have a special graphite lubricant between the dielectric and the braided shield, which minimizes the triboelectric effect and improves the quality of the sensor's charge output signal.
ICP pressure sensors incorporate a built-in MOSFET microelectronic amplifier to convert the high-impedance charge output into a low-impedance voltage signal . An ICP sensor is powered from a constant-current source and can operate over long coaxial or ribbon cable without signal degradation. The low-impedance voltage signal is not affected by triboelectric cable noise or insulation resistance-degrading contaminants.
Power to operate ICP sensors generally takes the form of a low-cost, 24 to 27 VDC, 2 to 20 mA constant-current supply. Figure 4 schematically illustrates a typical ICP sensor system. PCB offers a number of AC or battery-powered, single or multi-channel signal conditioners, with or without gain capabilities for use with pressure sensors. Some data acquisition systems incorporate constant-current power for directly powering ICP sensors. PCB manufactures DC coupled signal conditioners for use during static calibration or quasi-static measurement applications.
Figure 4: ICP Sensor System Schematic
In addition to ease of operation, ICP® pressure sensors offer advantages over charge mode types in terms of cost of operation, since multi-channel ICP signal conditioners are often available for the cost of a single channel laboratory charge amplifier.
For some severe environments, use of a "hybrid" system, using a charge mode pressure sensor in conjunction with an in-line miniature charge or voltage amplifier and an ICP® power conditioner, can still offer cost savings. Figure 5 shows various complete two-wire ICP® system configuration.
Figure 5: Typical Sensor Systems
When a positive pressure is applied to an ICP pressure sensor, the sensor yields a positive voltage. The polarity of PCB charge mode pressure sensors is just the opposite: when a positive pressure is applied, the sensor yields a negative output. Charge output sensors are usually used with external charge amplifiers that invert the signal. Therefore, the resulting system output polarity of a charge output sensor used with a charge amplifier will produce an output that is the same as an ICP sensor. (Reverse polarity sensors are also available.)
Most PCB piezoelectric pressure sensors are constructed with either compression mode quartz crystals preloaded in a rigid housing, or unconstrained tourmaline crystals. These designs give the sensors microsecond response times and resonant frequencies in the hundreds of kHz, with minimal overshoot or ringing. Small diaphragm diameters ensure spatial resolution of narrow shock waves.
High-frequency response and rise time can be affected by mounting port geometry and associated electronics. (Limitations of driving long cables at high frequencies are discussed on page 82). Check all system component specifications before making measurements, or contact PCB for application assistance.
The quartz crystals of a piezoelectric pressure sensor generate a charge when pressure is applied. However, even though the electrical insulation resistance is quite large, the charge eventually leaks to zero. The rate at which the charge leaks back to zero is dependent on the electrical insulation resistance.
In a charge mode pressure sensor used with a voltage amplifier, the leakage rate is fixed by values of capacitance and resistance in the sensor, by low-noise cable, and by the external source follower voltage amplifier used. In the case of a charge mode pressure sensor used with a charge amplifier, the leakage rate is fixed by the electrical feedback resistor and capacitor in the charge amplifier.
In a pressure sensor with built-in ICP electronics, the resistance and capacitance of the crystal and the built-in ICP electronics normally determine the leakage rate.
When leakage of a charge (or voltage) occurs in a resistive-capacitive circuit, the leakage follows an exponential decay. A piezoelectric pressure sensor system behaves similarly. The value of the electrical capacitance of the system (in farads) multiplied by the value of the electrical resistance (in ohms) is called the Discharge Time Constant (in seconds).
DTC is defined as the time required for a sensor or measuring system to discharge its signal to 37% of the original value from a step change of measure. The DTC of a system relates to the low-frequency monitoring capabilities of a system. A long discharge time constant is useful because it allows quasi-static operation during calibration or measurement of certain long-duration pressure pulses.
In a charge mode system, the DTC is usually determined by the settings on an external charge amplifier. PCB Series 460 Charge Amplifiers feature a short, medium, and long time constant switch from which DTC is selected. It is assumed that the electrical insulation resistance is large; otherwise, drift occurs. Therefore, to minimize this drift, the pressure sensor connection point and cable must be kept clean and dry.
With ICP® sensors, there are three factors that must be considered when making low-frequency measurements.
These are:
The discharge time constant characteristic of the pressure sensor.
The discharge time constant of the AC coupling circuit used in the signal conditioner with the readout instrument. (If DC coupling is used, only the above [1] needs to be considered.)
ICP signal conditioner.
It is important that all factors be readily understood by the user to assure accurate low-frequency measurements.
In PCB pressure sensors featuring built-in ICP electronics, the discharge time constant of the sensor is set at a fixed value by the built-in circuit. Specifications for the ICP pressure sensors shown in this catalog list the DTC for each pressure sensor.
When an ICP sensor is subjected to a step function input, a quantity of charge, Δq, is produced proportional to the mechanical input. Output voltage is ΔV = Δq/C where C is the total capacitance of the sensing element, amplifier, and ranging capacitor. This voltage is then amplified by the MOSFET amplifier to determine final sensor sensitivity. After the initial step input, the charge signal decays according to the equation q = Qe(-t/RC) where:
q = instantaneous charge (pC)
Q = initial quantity of charge (pC)
R = bias resistor value (ohms)
C = total capacitance (pF)
t = time after to
e = base of natural log (2.718)
The equation used to determine signal decay is graphically represented in Figure 6:
Figure 6: Piezoelectric Signal Exponential Decay
The product of R and C represents the DTC (in seconds) of the sensor. Sensor time constants vary from just a few seconds to >2000 seconds for standard sensors. Special time constants can be supplied by altering the resistor value, R, in the sensor's built-in microelectronic amplifier.
It is important to determine the discharge time constant (DTC) of an ICP sensor system, as the DTC determines the system's low-frequency response. The DTC of a sensor system is usually dominated by the shortest DTC of either the sensor or signal conditioner.
The sensor DTC is fixed and is specified in this catalog for each sensor. In ICP signal conditioners featuring internal buffer amplifiers, the DTC is fixed by various internal components and is not affected by the input impedance of the readout instrument. PCB signal conditioners with capacitively coupled outputs have a DTC fixed by a shunting resistor across the capacitor. This fixes the DTC in the signal conditioner (usually at 10 seconds), unless the input impedance of the readout instrument is less than 1 megaohm. If the input impedance of the readout is less than 1 megaohm, the DTC is shortened.
After determining the shortest DTC in the system, the value can be used to determine the low frequency cutoff of the sensor system. The system time constant is analogous to a first order high pass RC filter. The theoretical lower-corner cutoff frequency (fc) is illustrated in Figure 7 and is calculated from the following relationships:
3 dB down: fc = 0.16/DTC
10% down: fc = 0.34/DTC
5% down: fc = 0.5/DTC
Figure 7: Transfer Characteristics of an ICP Sensor
It is sometimes desirable to measure a pressure lasting a few seconds in duration. This is especially true with high pressure sensor applications where static calibration takes place. (Before performing tests of this nature, it is important to DC-couple the entire monitoring system to prevent rapid signal loss. PCB Series 484 Signal Conditioners have AC and DC coupling modes of operation and are designed for such applications).
The general rule of thumb for such measurements is that the output signal loss and time elapsed over the first 10% of a DTC have a one-to-one relationship. If a sensor has a 500 second DTC, over the first 50 seconds, 10% of the original input signal decays. For 1% accuracy, data should be taken in the first 1% of the DTC. If 8% accuracy is acceptable, the measurement should be taken within 8% of the DTC, and so forth. Figure 8 graphically demonstrates this event.
Figure 8: Step Function Response
Left unchanged, the signal naturally decays toward zero. Decay back to zero (or baseline) takes approximately 5 DTC. Notice that after the original step impulse signal is removed, the output signal dips below the base line reference point (to + .01 TC). This negative value is the same value as that which has decayed from the original impulse. Further observation reveals that the signal, left untouched, decays upwards toward zero until equilibrium in the system is achieved.
The output characteristic of piezoelectric pressure sensor systems is that of an AC-coupled system, where repetitive signals decay until there is an equal area above and below the original base line. As magnitude levels of the monitored event fluctuate, the output remains stabilized around the base line with the positive and negative areas of the curve remaining equal. Figure 9 represents an AC signal following this curve. (Output from sensors operating in DC mode follow this same pattern but over an extended time frame associated with system discharge time constant values.)
Figure 9: Repetitive Pulse, AC Signal
For example, assume that a 0 to 2 volt output signal is generated from an AC-coupled pressure application with a one-second steady-state pulse rate and one second between pulses. The frequency remains constant, but the signal quickly decays negatively until the signal centers around the original base line (where area A = area B). Peak to peak output remains the same.
Precision mounting of pressure sensors is essential for good pressure measurements. Although some mounting information is shown in this catalog, always check the installation drawings supplied in the manual with the sensor, or contact PCB to request detailed mounting instructions. Use good machining practices for the drilling and threading of mounting ports, and torque the sensors to the noted values. Mounting hardware is supplied with PCB sensors. Various standard thread adaptors are available to simplify some sensor installations.
For free field blast applications, try to use "aerodynamically clean" mounts, minimizing unwanted reflections from mounting brackets or tripods.
The sensing crystals of many pressure sensors described in this catalog are located in the diaphragm end of the sensor. Side loading of this part of the sensor during a pressure measurement creates distortions in the signal output. See Figure 10.
Figure 10: Flush Mount Pressure Alignment
Also important is the avoidance of unusual side loading stresses and strains on the upper body of the sensor. Proper installation minimizes distortions in the output signal. A taut cable pulling at right angles to the electrical connector is an example of putting a side strain into the body. Another is the use of a heavy adaptor with cable attached to the small electrical connector in an environment with high transverse vibration.
In some types of applications, such as free-field blast measurements, a pressure sensor mounted in a thin plate can be subjected to side loading stresses when the pressure causes the plate to flex. Use of an O-ring mount minimizes this effect.
Flush mounting of pressure sensors in a plate or wall is sometimes desirable for minimizing turbulence, avoiding a cavity effect, or avoiding an increase in a chamber volume. Recess mounting is more desirable in applications where the diaphragm end of the pressure sensor is likely to be subjected to excessive flash temperatures or particle impingement.
Most PCB pressure sensors are supplied with seal rings for flush mounting. Certain models, such as Series 111, 112, and 113 can be provided with seal sleeves for recess mounting ports. Request seal sleeves when ordering.
Figure 11: Typical Recess Mount
Consider ordering enough spare seal rings or sleeves, particularly in applications that require frequent removal and reinstallation of the pressure sensor. Before reinstalling a pressure sensor, be sure to check the mounting port to be sure that an old, distorted seal ring is not still in the mounting hole. If you are using PCB pressure sensors and find that you have lost or misplaced the seals, call PCB and request that the needed items be sent out as no-charge samples.
Various mounting adaptors are available that often facilitate mounting of the pressure sensors. Call factory for details. Note that pressure sensors and adaptors with straight machined threads use a seal ring as a pressure seal. Pipe thread adaptors have a tapered thread, which results in the threads themselves creating the pressure seal.
Control of the location of the pressure sensor diaphragm is achieved with a straight thread/seal ring mount. Pipe thread mounts do not allow a precision positioning of the depth of the sensor since the seal is provided by progressive tightening of threads in the tapered hole until the required thread engagement is reached. However, pipe threads do offer a convenience of an easier machined port than straight threads. Pipe thread mounts are well suited for some general applications.
Automotive in-cylinder pressures, ballistic pressures, and free-field blasts are a few examples of applications that have a thermal shock accompanying the pressure pulse. The thermal shock can be in the form of a radiant heat, such as the flash from an explosion, heat from convection of hot gasses passing over a pressure sensor's diaphragm, or conductive heat from a hot liquid.
Virtually all pressure sensors are sensitive to thermal shock. When heat strikes the diaphragm of a piezoelectric pressure sensor that has crystals contained in an outer housing, the heat can cause an expansion of the case surrounding the internal crystals. Although quartz crystals are not significantly sensitive to thermal shock, the case expansion causes a lessening of the preload force on the crystals, usually causing a negative-signal output. To minimize this effect, various methods are used.
Certain PCB quartz pressure sensors feature internal thermal isolation designs to minimize the effects of thermal shock. Some feature baffled diaphragms. Other models designed for maximizing the frequency response may require thermal protection coating, recess mounting, or a combination to lessen the effects of thermal shock. Examples of coatings include silicone grease, which may also be used to fill a recess mounting hole, RTV silicone rubber, vinyl electrical tape, and ceramic coatings. The RTV and tape are used as ablatives, while the ceramic coating is also used to protect some diaphragms from corrosive gasses and particle impingements.
Crystals other than quartz are used in some PCB sensors. Though sensitive to thermal shock, tourmaline is used for shock tube and underwater blast sensors. In shock tube measurements, the duration of the pressure measurement is usually so short that a layer of vinyl tape is sufficient to delay the thermal effects for the duration of the measurement. In underwater blast applications, heat transfer through the water is not significant.
Note that thermal shock effects do not relate to the pressure sensor specification called "temperature coefficient" used in this catalog. The temperature coefficient specification refers to the change in sensitivity of the sensor relative to the static temperature of the sensor. Unfortunately, since the thermal shock effects cannot be easily quantified, they must be anticipated and minimized by one of the above mentioned techniques in order to ensure better measurement data.
Operation over long cables may affect frequency response and introduce noise and distortion when an insufficient current is available to drive cable capacitance.
Unlike charge mode systems, where the system noise is a function of cable length, ICP® sensors provide a high voltage, low impedance output well-suited for driving long cables through harsh environments. While there is virtually no increase in noise with ICP sensors, the capacitive loading of the cable may distort or filter higher frequency signals depending on the supply current and the output impedance of the sensor.
Figure 12: Long Extension Cable Schematic
Generally, this signal distortion is not a problem with lower frequency testing within a range up to 10 000 Hz. However, for higher frequency vibration, shock or transient testing over cables longer than 100 ft. (30 m.), the possibility of signal distortion exists.
The maximum frequency that can be transmitted over a given cable length is a function of both the cable capacitance and the ratio of the peak signal voltage to the current available from the signal conditioner according to:
where, fmax= maximum frequency (hertz)
C= cable capacitance (picofarads)
V= maximum peak output from sensor (volts)
Ic= constant current from signal conditioner (mA)
10 = scaling factor to equate units
Note that in this equation, 1 mA is subtracked from the total current supplied to sensor (Ic). This is done to compensate for powering the internal electronics. Some specialty sensor electronics may consume more or less current. Contact the manufacturer to determine the correct supply current.
When driving long cables, the equation above shows that as the length of cable, peak voltage output or maximum frequency of interest increases, a greater constant current will be required to drive the signal.
The nomograph below provides a simple, graphical method for obtaining the expected maximum frequency capability of an ICP measurement system. The maximum peak signal voltage amplitude, cable capacitance and supplied constant current must be known or presumed.
For example, when running a 100 ft. (30 m) cable with a capacitance of 30 pF/ft (98 pF/m), the total capacitance is 3000 pF. This value can be found along the diagonal cable capacitance lines. Assuming the sensor operates at a maximum output range of 5 volts and the constant current signal conditioner is set at 2 mA, the ratio on the vertical axis can be calculated to equal 5. The intersection of the total cable capacitance and this ratio result in a maximum frequency of approximately 10.2 kHz.
The nomograph does not indicate whether the frequency amplitude response at a point is flat, rising or falling. For precautionary reasons, it is good general practice to increase the constant current (if possible) to the sensor (within its maximum limit) so that the frequency determined from the nomograph is approximately 1.5 to 2 times greater than the maximum frequency of interest.
Note that higher current levels will deplete battery-powered signal conditioners at a faster rate. Also, any current not used by the cable goes directly to power the internal electronics and will create heat. This may cause the sensor to exceed its maximum temperature specification. For this reason, do not supply excessive current over short cable runs or when testing at elevated temperatures.
To more accurately determine the effect of long cables, it is recommended to experimentally determine the high frequency electrical characteristics.
The method illustrated in Figure 13 involves connecting the output from a standard signal generator into a unity gain, low-output impedance (<5 ohm) instrumentation amplifier in series with the ICP sensor. The extremely low output impedance is required to minimize the resistance change when the signal generator/amplifier is removed from the system.
Figure 13: Long Cable Testing
In order to check the frequency/amplitude response of this system, set the signal generator to supply the maximum amplitude of the expected measurement signal. Observe the ratio of the amplitude from the generator to that shown on the scope. If this ratio is 1:1, the system is adequate for your test. (If necessary, be certain to factor in any gain in the signal conditioner or scope.) If the output signal is rising (1:1.3 for example), add series resistance to attenuate the signal. Use of a variable 100 ohm resistor will help set the correct resistance more conveniently. Note that this is the only condition that requires the addition of resistance. If the signal is falling (1:0.75 for example), the constant current level must be increased or the cable capacitance reduced.
It may be necessary to physically install the cable during cable testing to reflect the actual conditions encountered during data acquisition. This will compensate for potential inductive cable effects that are partially a function of the geometry of the cable route.
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