How do icp accelerometers work

When I used to offer support to customers of our enDAQ sensors formerly known as Slam Stick vibration data loggers that have an embedded triaxial piezoelectric accelerometer, I found myself frequently fielding questions that could be answered with the plot shown in Figure 4. Picture the following scenario:. You may have a test where the true acceleration level is 10g and you are using a g accelerometer - you have plenty of clearance to prevent clipping! That thing will be experiencing and reporting an acceleration of g.

Its resonant frequency or the resonance of the mounting you used amplified the acceleration levels to such a degree that it made the accelerometer useless. The only way to prevent this amplification is to a avoid using an accelerometer with a resonance within the frequency range of your test environment or b mechanically filter the frequency through the use of double sided tape, duct seal putty, or even vibration isolators.

Some accelerometers will also be available with internal damping to reduce this amplification at resonance - but this typically reduces the bandwidth of the accelerometer.

Another drawback of the amplification that occurs at high frequencies and resonance, is that the amplification can cause the accelerometer to experience a shock amplitude that exceeds its measurement range.

When the input acceleration levels including those amplified by mechanical resonance exceed the sensor's measurement range - this will saturate the internal charge amplifier. Figure 5 provides a few plots from testing one of our engineers did on an AR15 rifle with two enDAQ sensors : one with a piezo resistive accelerometer and the other with a piezo electric accelerometer.

The data from the piezoresistive accelerometer indicated that the shock levels should be "only" around g. But the vibration from the gun shot had significant high-frequency content in it that excited the piezoelectric accelerometer's resonance. This caused the output of the accelerometer to saturate its internal charge amplifier which results in an exponential decay that lasted about one second.

This exponential decay is the key indicator of amplifier saturation. Either mechanical damping is needed, or a different sensor type or measurement range. PCB Piezotronics has a great white paper with more information on this exponential decay that occurs from charge amplifier saturation. Piezoelectric accelerometers are very popular, therefore they are available in a variety of measurement ranges and from a variety of vendors.

I broke out the list of vendors by whether or not they support e-commerce buy online. I understand the value of the "request a quote" process for large orders to establish that relationship with the potential customer, but when you just need a couple accelerometers for a test - you just want to buy it without speaking to anyone!

The following two companies have an e-commerce platform to allow you to directly purchase most of their accelerometers right online. These next four companies all have an extensive sensor suite and make good products, but you'll need to speak with a sales rep to get pricing and to order your accelerometers.

Another option that is often overlooked is to buy an all-in-one system that incorporates the accelerometer s , electronics, and power source all in one package. This can be very useful for:. This capacitor must charge through the input resistance of the readout instrument and, if a DC readout is used, the output voltage will appear to drift slowly until charging is complete.

However, as the frequency of the measurand increases, the system eventually becomes nonlinear. This is due to the following factors: 1. Mechanical Considerations 2. Cable Characteristics Each of these factors must be considered when attempting to make high frequency measurements. The mechanical structure within the sensor most often imposes a high frequency limit on sensing systems. That is, the sensitivity begins to rise rapidly as the natural frequency of the sensor is approached.

It can be seen that the sensitivity rises as frequency increases. Pressure and force sensors respond in a similar manner. Mounting also plays a significant role in obtaining accurate high-frequency measurements. Be certain to consult installation procedures for proper mounting. In general, voltage amplified systems respond to frequencies on the order of 1 MHz, while most charge amplified systems may respond only to kHz.

This is typically due to limitations of the type of amplifier, as well as capacitive filtering effects. For such cases, consult the equipment specifications, or call PCB for assistance.

Operation over long cables may affect frequency response and introduce noise and distortion when an insufficient current is available to drive cable capacitance. Generally, this signal distortion is not a problem with lower frequency testing up to 10 kHz. However, for higher frequency vibration, shock, blast or transient testing over cables longer than 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 cable capacitance and the ratio of the peak signal voltage to the current available from the signal conditioner, according to:.

This is done to compensate for powering 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, Equation 7 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.

For example, when running a ft. 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 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. 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 determine the high frequency electrical characteristics involved with long cable runs, two methods may be used.

The extremely low-output impedance is required to minimize the resistance change when the signal generator and amplifier are removed from the system. The alternate test method, also shown in Figure 13, incorporates a sensor simulator which contains a signal generator and sensor electronics conveniently packaged together.

Observe the ratio of the amplitude from the generator to that shown on the scope. If this ratio is , 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 e. Use of a variable 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 e. 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.

These are: 1. The discharge time constant characteristic of a sensor a fixed value unique to each sensor. The time constant of the coupling circuit used in the signal conditioner.

If DC coupling is used, only 1 needs to be considered. It is important that both factors are readily understood by the user to avoid potential problems. The discharge time constant is the more important of the low frequency limits, because it is the one over which the user has no control.

While the sensing element will vary widely in physical configuration for the various types and ranges of pressure, force, and acceleration sensors, the basic theory of operation is similar for all. Note: A ranging capacitor, which would be in parallel with the resistor, is used to reduce the voltage sensitivity and is not shown.

From this equation, the smaller the capacitance, the larger the voltage sensitivity. While this is true, there is a practical limit where a lower capacitance will not significantly increase the signal-to-noise ratio. In this case, only the feedback capacitor located between the input and output of the amplifier determines the voltage output, and consequently the sensitivity of the sensor.

While the principle of operation is slightly different for quartz and ceramic sensors, the schematics Figure 6 indicate that both types of sensors are essentially resistor-capacitor RC circuits. Since the capacitance fixes the gain and is constant for a particular sensor, the resistor is used to set the time constant.

Typical values for a discharge time constant range from less than one second to up to seconds. Figure 15B is a Bode plot of the low-frequency response. This filtering characteristic is useful for draining off low-frequency signals generated by thermal effects on the transduction mechanism. If allowed to pass, this could cause drifting, or in severe cases, saturate the amplifier. Piezoelectricity Piezoelectricity is the collection or flow of electrons on opposite sides of a material due to the application of a mechanical stress on the material.

Piezoelectric materials allow us to build sensors that: Have an extremely linear relationship between electrical output and mechanical input. Provide electrical output to very fast, dynamic, events. The speed of this response is expressed as either "rise time" measured in micro-seconds or "frequency" measured in Hz. Measure mechanical inputs that are both very small and very large.

This is called "dynamic range. While piezoelectric sensors hold the aforementioned advantages over other sensor technologies, some of the disadvantages are: While they are very good dynamic sensors, piezoelectric sensors will not measure pure static events. Sometimes they can measure what we call "quasi-static" events, but not purely static events.

Some of the piezoelectric materials have more "pyroelectric" output than others. This means that they generate electricity due to a change in temperature. The electricity these sensors generate is not like the electricity that we use to power our lights or electronic devices. These sensors can generate very high voltages, but almost no current, or amperage.

A sample calibration certificate is shown in Figure 6. Back-to back calibration is performed with the test accelerometer mounted onto a reference accelerometer. This technique provides a quick and easy method for determining the sensitivity of an accelerometer over a wide frequency range. The reference accelerometer is an extremely accurate device with specifications traceable to a recognized standards laboratory. It is possible to vibrate both accelerometers and compare output data by securely mounting the test accelerometer to the reference standard accelerometer.

The sensitivity of the reference accelerometer is known so the sensitivity of the test accelerometer can be calculated. Sensorline SM In the interest of constant product improvement, specifications are subject to change without notice.

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