WIDE BANDWIDTHACCELEROMETERS FOR
CONDITION MONITORINGWIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING

Machine condition monitoring and predictive maintenance relies upon early detectionof changes in the vibration spectrum that are indicative of defects or wear. As market
trends lead toward the monitoring of higher-volume and smaller systems, the signal
bandwidth of interest also increases. It is now also possible to embed accelerometers
directly into equipment for condition monitoring. This white paper will illustrate
the requirement of wide bandwidth monitoring with a discussion of possible
sources along with both simulation and experimental results of defect conditions.
Most machinery that is driven by large electric motors
will have an input drive running at a speed of between
900 and 3600 rpm (15 to 60 Hz). This fundamental
frequency can easily be monitored using
accelerometers with relatively low bandwidth, but theinformation that this signal yields may be limited toimbalance condition only.

analyze. The basic gear-mesh frequency is given by theproduct of the number of teeth and the running speedin Hz. The magnitude of sidebands or harmonics of thisfrequency can indicate the onset or severity of various
defects. Gears will also possess their own natural
frequencies, which may be excited by impulsive defects
or by strong vibration at higher frequencies.

Rolling-element bearings are a common cause ofmachine failure. As bearing defects begin to occur, thevibration level produced in the early stages of failure
may be very low and could be masked by vibrationfrom other sources. Frequency-domain analysis is very
useful for revealing small signals that may be hidden inthe time-domain. Bearing characteristic frequencies are
generally many times higher than the running speed(around 50% of the number of balls times the runningspeed) and may be present even when equipment is ingood condition. Bearing defects tend to be impulsive,
which produces a series of harmonics spaced at thecharacteristic frequency, extending to very highfrequencies.

Blades and vanes again generate vibration at a multipleof running speed, even when in good condition. The
blade passing frequency is given by number of blades
times the running speed. A missing blade will generatea large number of harmonics of the blade-passfrequency (as well as increasing imbalance at 1 x
running frequency).
Mechanical looseness is also very likely to cause many
harmonics of running speed to appear in the spectrum.
In general terms, looseness can cause discontinuities inmotion, and sudden changes in position or
displacement can produce very high levels ofacceleration.

Gears represent another case where high multiples ofrunning speed/frequency will be generated, although
gearboxes can be significantly more complex toTE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING

PAGE 2WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORINGWIDE BANDWIDTH SIMULATIONS
To illustrate the general nature of mechanical discontinuities, a numerical simulation is presented below. Part of amechanism is assumed to displace nominally ±5 mm at a frequency of 50 Hz. To create a simulated discontinuity,
we clip the displacement, on positive excursions only, at 4.98 mm, as highlighted in the zoomed trace at right:0.006

0.004Displacement (m)0.002

0-0.002-0.004-0.006

00.01

0.02

0.03

0.04

0.05

Time (s)FIGURE 1. DISPLACEMENT CURVE WITH CLIPPING

0.0052

0.005

Displacement (m)0.00480.00460.00440.0042

0.004
0.002

0.003

0.004

0.005
Time (s)

0.006

0.007

0.008

FIGURE 2. ZOOMED REGION OF DISPLACEMENT CURVE WITH CLIPPINGTE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING

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WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING
The corresponding acceleration plot shows the underlying sinusoidal acceleration component at ±50.3 g but withimpulses generated by the start and end of mechanical clipping (with -237 g peaks):
10050

Acceleration (g)

0-50

-100

-150-200-250

-300

0

0.01

0.02

0.03

0.04

0.05

Time (s)FIGURE 3. ACCELERATION CURVE CORRESPONDING TO CLIPPED DISPLACEMENT CURVE

Now, the corresponding FFT result, first with full bandwidth, then a zoom of first 2 kHz:
-20-30Magnitude (dBV @ 1 mV/g)

-40

-50

-60

-70

-80

-90-100

01000

2000

3000

4000
5000
6000Frequency (Hz)

7000

8000

9000

10000

140016001800

2000

FIGURE 4. FFT RESULT AT FULL BANDWIDTH-20-30

Magnitude (dBV @ 1 mV/g)-40-50-60-70-80-90-100020040060080010001200
Frequency (Hz)FIGURE 5. ZOOMED FFT SHOWING HARMONIC STRUCTURETE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING

PAGE 4

WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING
The 0.020 mm “defect” has introduced significant signal energy over a very wide bandwidth, as a series ofclosely-spaced harmonics of the running speed. In this simulation, the magnitude at 10 kHz is only a few dB below
the level around 1500 Hz, and signal energy would continue out well beyond 10 kHz.
Now, what happens if we limit the bandwidth of the sensing device (accelerometer)? In the simulation below, adigital low-pass filter at 5 kHz was applied to the same data:
10050

Acceleration (g)

0-50

-100

-150

-200-250

-300

0

0.01

0.02

0.03
Time (s)

0.04

0.05

0.06

FIGURE 6. ACCELERATION RESULT WITH LIMITED BANDWIDTHIn this case, instead of the negative-going peaks reaching -237 g, the bandwidth restriction has reduced these to-158 g (a drop of one-third).
Although this numerical simulation is not intended to represent any specific real-world case, the general principles
are valid: abrupt mechanical discontinuities in motion cause short acceleration impulses. The impulses produce
broad-bandwidth energy in the spectrum, and a sensor which is limited in bandwidth will not capture the full
extent of the transient events.
Even very large, slowly-rotating structures (such as wind turbines) may be coupled to gear sets with multiple
stages and very high tooth count. Signal energy above 10 kHz is considered useful for early detection of defects,
as it can be well clear of lower-order vibration at higher amplitude levels which may be present in normal running
conditions.
Individual events which create fracture or spalling can release energy over a very wide bandwidth (dynamic strain
measurements have shown ultrasonic energy out to 1 MHz). Although conventional accelerometers cannot beexpected to capture such a wide bandwidth, audio-band signals may indicate some likelihood of damage.

TE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORINGPAGE 5WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING
5432Voltage (V)10-1-2-3-4-5-2-1012345678Time (ms)FIGURE 7. TIME TRACE METAL TO GLASS IMPACT WITHOUT FRACTURE
10864Voltage (V)20-2-4-6-8-10-2-10123Time (ms)45678FIGURE 8. TIME TRACE METAL TO GLASS IMPACT WITH FRACTURETwo time traces are shown above, on the left is a hard metal-to-glass impact which caused no visible damage; onthe right, a metal-to-glass impact which caused limited local spalling of the glass (in an area less than 1 mmdiameter). Their spectra are overlaid below.
-20

-30

Magnitude (dBV)

-40

-50

-60

-70

-80

-90

02000

4000

6000

8000

10000
12000
Frequency (Hz)

14000

16000

18000

20000

FIGURE 9. FFT COMPARISON WITH AND WITHOUT FRACTURETE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORINGPAGE 6

WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING
The blue curve is impact with no spalling, orange is with spalling. There is good separation generally in the range 5
kHz to 15 kHz, and particularly from 12 – 15 kHz.
While these are transient (time-limited) events with relatively high energies, we may consider some forms ofmachine wear to be a continuous series of microscopic spalling events. Piezoelectric accelerometers, offering
excellent resolution over very wide bandwidth, are an suitable approach to early detection of high-frequency
noise associated with machine wear.

ACCELEROMETER TECHNOLOGY COMPARISION
In a capacitive MEMS accelerometer, the inertial movement of one array of fingers relative to a fixed array issensed by measuring change in capacitance between the arrays. This is usually done by allowing the change incapacitance to modify a frequency, well above the intended detection bandwidth, which can then bedemodulated to obtain a final analog output. Since the device is constantly excited, the power consumption is
generally significantly higher than that of a piezoelectric device. Until relatively recently, capacitive MEMS devices
have been limited to low-frequency operation, although there is a current trend towards higher-bandwidth
devices.
In comparison with MEMS technology, piezoelectric accelerometers can achieve significantly higher bandwidthand superior noise performance (signal resolution). The sensing element is typically piezoelectric ceramic material(or “crystal”) loaded in shear by an inertial mass. Due to the high stiffness and high charge sensitivity of thecrystal and the relatively low mass required to achieve typical detection ranges for condition monitoring, theresonance frequency will be minimum >30 kHz, with some designs >50kHz. This allows the passband to extend
well beyond 10 kHz. The piezoelectric sensing element itself requires no power, and signal conditioning (charge tovoltage conversion) can be provided within a device with very low current requirement.

TE CONNECTIVITY WIDE BANDWIDTH ACCELEROMETERS
Packaged Industrial Accelerometers
ModelAcceleration Range

Frequency Response

Number of Axes

8021

±10 g to ±500 g

0.5 – 15000 Hz (±3 dB)

Single Axis

8711±5 g to ±500 g

0.3 – 18000 Hz (±3 dB)

Single Axis

8911 for POC*

±25 g to ±500 g

2 – 15000 Hz (±3 dB)

Wireless

Embedded OEM Accelerometers
Model

Acceleration Range

Frequency Response

Number of Axes

820M1

±25 g to ±500 g

2 – 15000 Hz (±3 dB)

Single Axis

830M1

±25 g to ±500 g

2 – 15000 Hz (±3 dB)

Triaxial*Proof of Concept (POC)
This device has not been authorized as required by the rules of the Federal Communications Commission or other regulatory authorities.
This device is not, and may not be, offered for sale or lease, or sold or leased, until authorization is obtained.
POC product is availability for evaluation purposes only.

TE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING

PAGE 7

WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORINGABOUT TE CONNECTIVITYTE Connectivity is a $12 billion global industrial technology leader creating a safer, sustainable,
productive and connected future. Our broad range of connectivity and sensor solutions, provenin the harshest environments, enable advancements in transportation, industrial applications,medical technology, energy, data communications and the home. With approximately 80,000
employees, including more than 7,500 engineers, working alongside customers in approximately
140 countries, TE ensures that EVERY CONNECTION COUNTS. Learn more at www.te.com.

AUTHOR
Richard Brown, Field Application Engineer, TE Connectivity

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01/2021

TE CONNECTIVITY SENSORS / WIDE BANDWIDTH ACCELEROMETERS FOR CONDITION MONITORING