Types Of Accelerometers :
Accelerometers come in two axial types. The most common accelerometer measures acceleration only along a single axis. This type is often used to measure mechanical noise levels.
For example, you may use it on a compressor, and the results may help determine if the compressor is operating properly, or is in need of maintenance.
The second type is the tri-axial accelerometer. This accelerometer is capable of creating a three dimensional vector of acceleration in the form of orthogonal components. This type is used when it is necessary to determine the type of vibration (lateral, transverse, rotational, etc.) that a component is undergoing, or if the acceleration of the component in any direction must be known. Installation of this type of accelerometer must be done more carefully in order to get an accurate indication of the direction of acceleration. This type of accelerometer also has three separate outputs, each output needing its own signal conditioning.
Accelerometer Construction :
The active accelerometer contains a piezoelectric crystal bonded to the casing of the accelerometer. Bonded to the other side of the crystal is a small seismic mass. When the accelerometer is moved, this mass either compresses or stretches the crystal, inducing a charge on its surfaces. This charge is picked up by a small charge sensitive amplifier built inside the sensor. This amplifier is powered by an external constant current source, and modulates its output voltage with respect to the varying charge on the piezoelectric crystal. Thus, the accelerometer uses only two wires for both sensor excitation (current) and signal output (voltage).
This also means that the active accelerometer has a relatively low output impedance, usually on the order of a few kilo-ohms.
Advantages and Disadvantages of Accelerometer
The active accelerometer has advantages over the conventional passive element accelerometer. Because the active accelerometer has a relatively low output impedance, it is less sensitive to noise being picked up in its cabling. The built in signal conditioning circuitry further reduces noise pick-up by being placed as close as possible to the crystal. The net result is that the active accelerometer is easier to use, especially in very noisy environments.
Despite these advantages, there are some disadvantages which may prevent one from choosing an active accelerometer. Because the front end circuitry is provided inside the accelerometer, it has a fixed signal sensitivity independent of the excitation current. This circuitry also limits the usable temperature range of the accelerometer since most of the signal conditioning circuitry will fail above approximately 250 °F.
Accelerometer Parameters :
Active accelerometers have three major parameters to consider when selecting which model to use.
The first parameter is the sensor’s measuring range. This number represents the maximum acceleration that the sensor can reasonably measure, represented in +g’s. (One g is one Earth gravity.) Thus, a sensor with a measuring range of +5 g’s can measure 5 g’s of acceleration in either direction along its measuring axis.
The next parameter is sensitivity. This is the sensor’s “gain” given as a ration of volts per g. Thus a sensor with a sensitivity of 200 mV/g displaces 300 mV when undergoing 1.5 g of acceleration.
The final parameter is the accelerometer’s resonant frequency. This represents the frequency that the sensor rings at if bumped abruptly, much like a goblet rings when tapped. Because the sensor wants to naturally resonate at this frequency, it is very important to make sure that you do not use the accelerometer to measure any signals near the resonant frequency.
Power Source :
The power source to excite the accelerometer is a current source. Most accelerometers can operate on a current between 2 and 20 mA. While it may seem more economical to use a 2 mA source, it should be noted that the maximum frequency and amplitude that an accelerometer can drive is directly related to the current that is exciting it. Thus, long cable lengths need higher currents in order to drive high frequencies at large voltage ranges. Systems with short cables or low frequency measurements can tolerate being driven with lower currents.
In addition to the amount of current the source can supply, it must have a high enough voltage compliancy to power the sensor over its entire operating range. Voltage compliancy is a measure of the voltage range that a current source properly functions. For example, a 24 V compliant current source delivers a constant current over a range of 0 to 24 V across its terminals.
Most accelerometer systems use one of three types of current sources, resistor and a battery, diode and a battery, and active. These will be discussed in detail.
The simplest current source used for exciting accelerometers is a battery and resistor system – three 9 V batteries in series with a resistor.
While it is very simple and inexpensive, it is also very inadequate. The resistor maintains a constant current flow as long as there is no signal in the system. The moment a signal changes the voltage level at the output terminals, the current also changes in accordance with Ohm’s law. To put it bluntly, the battery and resistor current source is 0 V compliant by its very design! This dynamic changing of current betrays itself as non-linearity in the system’s response. As a result, this current source produces a lot of harmonic distortion.
Active Current Source:-
The necessarily low value of the resistor to supply the necessary current means that the dynamic impedance of this current source is also quite low, usually around 5 kilo-Ohms. (An ideal current source has an infinite dynamic impedance.)
Finally, the nominal current and peak output from this current source slowly changes over time as the batteries discharge.
The active current source is the best source to use for energizing accelerometers. Its compliancy can be very high, and does not degrade with either time or frequency. There are no batteries to replace, thus eliminating the need to service it. Depending on the complexity of the design, it can have a dynamic impedance on the order of a half a meg-Ohm and up.
The only drawback is that it is more expensive than the other current sources because of its greater complexity.
Sensor Grounding :
Proper grounding is essential to minimizing noise in the vibration measuring system. Improper grounding, however, can cause more noise problems than it can eliminate. There are two main causes of noise as a result of grounding, ground loops or floating nodes.
A ground loop occurs when both the sensor and the signal conditioning input are grounded. The loop forms between the sensor cable and the building ground. This huge loop is perfect for picking up any induced currents produced by machinery, lights, and more. Because the loop resistance is finite, the result is a large 50 or 60 Hz common mode noise signal on the signal conditioning’s input. Thus, a large common mode noise is picked up in the system. If the situation is bad enough, the common mode voltage can get high enough to saturate the signal conditioning’s front end, causing a multitude of noise problems.
Likewise, floating nodes which occur when neither the sensor nor the signal conditioning system are grounded, induced charges can build up on the sensor to the point of overloading the front end of the signal conditioning system.
The solution is simple. Either ground the sensor or the signal conditioning input, but not both.
The next stage, AC coupling, removes the DC voltage offset that is present on the accelerometer. This dramatically increases the resolution of the system, because the system no longer has to accommodate the offset. For example, an accelerometer with an output voltage swing of +2 V and an offset voltage of 10 V would require a digitizer with a 12 V input range, despite only using 4 V for the actual signal. The same sensor AC coupled would require a 4 V input range on the digitizer, increasing the resolution by a factor of three.
In addition, the AC coupling removes the long term DC drift that accelerometers have due to age and temperature effects.
The only care one needs to have with AC coupling is to make sure that the low frequency roll-off is lower than the range of frequencies desired.