Instrumentation Monthly Test | Measurement | Control
In the 43 years it has been making accelerometers, DJB Instruments UK Ltd has seen many changes in technology, analysis techniques and the increased reliance on test software. Although the applications for accelerometers has increased the understanding of how they should be selected, mounted, connected and handled has reduced. Today many engineers see them as a simple sensor, however this is far from the truth, in the world of dynamic vibration measurement, nothing is simple and errors of up to 35% can be introduced before a single piece of data is analysed.
It remains the responsibility of the engineer to collect accurate data. Acquisition software on PC’s can’t determine good data from bad, so the engineer must ensure accuracy starts at the sensor.
For the last 4 years DJB have been leading the way in providing specific accelerometer training to customers and this article summarises some key information for use as a reference guide:
Terminology and Types of Accelerometer:
| Piezoelectric – | Charge Output devices, require a charge amplifier or an IEPE inline converter. |
| Pros – |
Great for high temperature use – 260°C Purely mechanical assembly, robust and can withstand more severe environments. Wideband frequency response, 0.1Hz to 15kHz or more |
| Cons – | Very sensitive to noise issues caused by dirt on connectors, cable type and length. |
IEPE (Integrated Electronic Piezoelectric) – Also known by a variety of other names, but all refer to an electronic amplifier circuit built into the accelerometer.
| Pros – |
Signal conditioning built into many acquisition hardware systems, no need for a charge amplifier. Less sensitive to cable type or noise effects. Wideband frequency response, 0.1Hz to 15kHz or more |
| Cons – |
Lower temperature range, 125°C standard, 185°C High temperature More susceptible to damage due to electronic components. Susceptible to ESD damage. |
Fixed Sensitivity and range
MEMS (Micro Electromechanical System) – Piezoresistive or Variable Capacitance
| Pros – | DC low frequency response |
|
Cons – |
Restricted temperature operating range Restricted frequency range (DC to 1kHz typical) Sensitive to damage. |
Accelerometer Selection – Points to consider:
There are many factors to consider when selecting an accelerometer for testing purposes and it’s not necessarily as simple as using the first one that comes to hand or one stored in a drawer for the last 3 years. The following is a brief list of topics that need consideration:
- Mass/size – Mass loading is the effect of adding mass to a test structure and in doing so changing the natural frequency, this must be minimised. (For practical demo visit DJB’s Youtube page)
- Sensitivity – IEPE accelerometers are fixed sensitivity and have a fixed measurement range, due to their nominal output of 5-7VDC, anything above this will cause an overload of the onboard electronics.
- Frequency range – All accelerometers have a flat frequency response range over which the sensitivity does not vary by more than ±5%, ensure this range matches the test range.
- Operating Temperature – Ensure the test conditions are within the operational range of the accelerometer.
- Connector orientation – Top or side entry connectors are available, using the correct option will reduce stress on the cable connection, this is critical to ensure no data loss.
- Mounting method – This is a huge topic and an area where significant issues can be caused:
o Stud mount – screw it down – gives excellent high frequency responses and is widely considered the best solution IF you can drill a hole in your test item.
o Adhesive mount – use as little adhesive as possible, too much and you will dampen the vibration response and cause errors in your data. Cyanoacrylate (super glue) is commonly used. BE CAREFUL when removing them, do NOT knock them off, this can damage IEPE electronics, twist them off with a spanner or use the teardrop tool provided.
o Petro Wax – great for low frequency modal testing or when you want to move accelerometers often, use as little wax as possible, only use at normal ambient temperatures and be aware of the frequency limitations.
DJB recommend annual calibration, if you are testing at high or low temperature, consider having your accelerometers calibrated at your test temperature. Ensure your calibration frequency range covers your testing frequency range so you can prove accuracy. Ensure your accelerometers are checked for CROSS-AXIS during your calibration, this is very important and a feature which is best controlled by DJB’s patented unique Konic Shear design.
Cables: Low Noise or Non-Low Noise? That is the question. (See DJB’s Youtube page for a practical demo.)
- To control triboelectric noise caused by cable movement, all charge accelerometers MUST be used with low noise cables, otherwise data accuracy will be compromised.
- IEPE accelerometers can be used with almost any type of cable including non-low noise types.
These are some of the key points but there is a lot more detail that should be considered for each of these, if you would like to know more or see how accelerometers work and are built consider attending one of DJB’s full training courses at their UK factory and remember ‘If in doubt ask!’, DJB engineers are available to guide you on the correct choice for your test.
If you would like further information please visit DJB Instruments UK Ltd website www.djbinstruments.com or email the sales team on sales@djbinstruments.com
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