Transcript Document

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Practical considerations of accelerometer
noise
By: Bruce Lent
IMAC XXVIII
The information contained in this document is the property of [name of Meggitt company] and is proprietary and/or
copyright material. This information and this document may not be used without the express authorization of [name of
Meggitt company]. Any unauthorized use or disclosure may be unlawful.
Information contained in this document is subject to U.S. Export Control regulations, specifically the (choose as
appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations. Each recipient of this
document is responsible for ensuring that transfer or use of any information contained in this document complies with all
relevant (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations.
Topics covered
Importance of noise level
Charge mode accelerometers
Voltage mode accelerometers
Piezoresistive
Important terms
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Voltage (IEPE) /Charge Mode Comparison)
Accelerometer – cable – remote charge amplifier
Integrated accelerometer/charge Amplifier (IEPE)
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Importance
Determine the minimum discernible
signal with reasonable resolution.
Low level signals
Wide dynamic range
High level shock testing
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Low – level signals 3 X the noise level
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High level signals – shock pulse
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Noise terminology
Resolution:
Broadband noise over a specified bandwidth in terms of mg rms
or µg rms
Residual noise: Is the same as resolution but usually used for electronic
amplifiers
Threshold: Noise expressed in terms of mg rms or µg rms
Threshold = maximum noise/sensitivity
Spectral Noise: Noise level over a limited portion of the total
bandwidth expressed in V/√Hz (may be volts or g)
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Spectral noise
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Charge mode noise sources
The information contained in this document is the property of [name of Meggitt company] and is proprietary and/or
copyright material. This information and this document may not be used without the express authorization of [name of
Meggitt company]. Any unauthorized use or disclosure may be unlawful.
Information contained in this document is subject to U.S. Export Control regulations, specifically the (choose as
appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations. Each recipient of this
document is responsible for ensuring that transfer or use of any information contained in this document complies with all
relevant (choose as appropriate) International Traffic in Arms Regulations and / or Export Administration Regulations.
Charge mode noise sources
1/F noise is produced by impurities in the PE material
Mechanical and thermal noise
Loss factor of the PE material
Long cable length adds capacitance
Triboelectric noise – low noise cable required
Electronic amplifiers are the largest noise contributors
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Capacitance Sources
The Accelerometer
This value is measured during calibration and listed on the data sheet
The Cable
Capacitance is specified in pC/foot (meters)
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Routing the cabling
First we will discuss “TRIBOELECTRIC EFFECT”:
This is the self generation of electrical noise from within the cable due to the
flexing of the cable itself.
“LOW NOISE” treatment is and has been the solution to minimizing this effect
Mounting Solution Below
Then Tie Down
Every 6” to 12”
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Put a Dab of
Clear finger Nail
Polish at the
connection
Basic Low Noise Cable
Low Noise application
Wrapped – Teflon tape coated with
carbon material on both sides.
Dipped (Smaller gauge wire) – Wire is
passed through a reservoir containing
the low noise dispersion material.
The low noise film is then fused to the cable by
heat.
Cable Length
Myth: Charge mode accelerometer cables must be short
Fact: This is dependent on resolution requirements and ambient EMI
conditions. EMI problems are usually resolved using differential
accelerometers.
In the following example, we will see how good performance can be
obtained with a 500 foot cable
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Cable Length Facts
As the input shunt capacitance of a charge amplifier is increased the
input noise is increased.
As cable length is increased, the shunt capacitance is typically increased
by 30pF/foot (check cable specifications for exact amount).
Rule of Thumb:
Input noise increases by 0.008 pC rms per 1000 pF of source capacitance.
Source capacitance includes the total cable capacitance and the
accelerometer’s capacitance.
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Cable Noise Calculation
Given:
Cable length = 500 feet
Cable capacitance = 30 pF/foot
Accelerometer capacitance = 350pF
Accelerometer sensitivity = 40 pC/g
In pC rms = 0.03 [amplifier residual noise] +(0.008 x 15.35)
= 0.1528 pC rms
Since noise is random, multiply the above rms value by the crest factor of three to
obtain the peak value.
Pc peak noise= 3 x 0.1528pC rms = 0.4584 pC peak
Equivalent noise = 0.4584/40 pC/g = 0.0115g
Since a 3 to 1 signal to noise ratio is necessary for a measurement, multiply
0.0115g x 3 = 0.344g to obtain the minimum measurable acceleration.
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Piezoresistive accelerometers
Generally low noise 1 – 5 µV
Often not specified because of their high level applications
Electronic amplifiers are the major contributor
Low impedance – cable noise not a problem
Page 3 ©Endevco. Proprietary. 6 August 2009
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The information contained in this document is the property of Endevco and is proprietary and/or copyright material.
This information and this document may not be used or disclosed without the express authorization of Endevco]. Any
unauthorized use or disclosure may be unlawful.
The information contained in this document may be subject to the provisions of the Export Administration Act of 1979
(50 USC 2401-2420), the Export Administration Regulations promulgated thereunder (15 CFR 730-774), and the
International Traffic in Arms Regulations (22 CFR 120-130). The recipient acknowledges that these statutes and
regulations impose restrictions on import, export, re-export and transfer to third countries of certain categories of data,
technical services and information, and that licenses from the US Department of State and/or the US Department of
Commerce may be required before such data, technical services and information can be disclosed. By accepting this
document, the recipient agrees to comply with all applicable governmental regulations as they relate to the import,
export and re-export of information.'
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