Endevco Corporation
Best Practices
Application information for sensor mounting, cable
connections and routing, measurement system
configuration and general guidelines
Stephen Bill
Application Support Services
Endevco Corp.
June 2006
The information contained in this document is the property of Endevco Corporation and is confidential and/or copyright
material. This information and this document may not be used without the express authorization of Endevco. Any unauthorized
use or disclosure may be unlawful.
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Recommended references on these topics
Endevco Cable Connectors & Adaptors
Endevco Memorandum….Cable Applications/Installation Notes
TP 291…Accelerometer Selection Based on Applications
TP 308…Problems in High-Shock Measurements
TP 218…Effects of Mounting on Accelerometer Response
TP 312…Guide to Adhesively Mounting Accelerometers
TP 317…Thermal Isolation of Accelerometers
TP 319…Guide to Accelerometer Installation
TP 321…Acceleration Levels of Dropped Objects
TP 320…Isotron and Charge Mode PE Accelerometers (Pros & Cons)
Most of these references are available at www.endevco.com
TP denotes “Technical Paper”
Where to begin?
Sensor selection
TP 291..Accelerometer Selection Based On Applications
Prior to testing; Model, if possible, the structure to determine possible
amplitudes and resonance/anti-resonance frequencies
Determine the environmental situation for any testing (temp, pressure,
moisture, dirt, grease, spray, assumed vibration levels, etc.)
Determine the location of sensors and location I.D. (IDENTIFICATION)
Select the proper signal conditioner/amplifier/filter for the project
Ensure your supply of sensors, cables, instrumentation is clean, ready
and installation materials are present
Ensure proper electrical power grounds are used and where located
within the measurement system
You are now ready to start
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Mounting technique varies frequency response
Typical mounting could be: stud, screw, adhesive, adhesive pad, flat magnet, two
pole magnet, hand probe
Rule of thumb: The stiffer the interface to the structure the higher the frequency
response (stud is stiffest, hand probe is softest)
Graphical example: (Resonance peaks shown on graph)
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Mounting surface preparation
Mounting Surfaces and tapped holes should correspond to the
following specifications in order to ensure proper operation of stud
mounted accelerometers:
Surface flatness:
Surface roughness:
Hole perpendicularity:
Thread class:
Dynamic characteristics of
your mounting arrangement
will affect the test results.
o.ooo3” TIR
32 micro inch
+/- 0.5 degree
2 or 3
Rule of thumb: linear Range
of measurement is 20% of
Resonance peak.
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The accelerometer bottom surface
(good or bad?)
Stud mounting particulars:
As a general rule, rigidity of the accelerometer mounting over
the frequency range of interest is a prerequisite
Perpendicularity, direct contact, low relative dynamic mass, and compatible temperature
with the test structure are also desirable characteristics
Couplant, couplant, couplant…use this between the bottom of the accelerometer and the
structure or mounting block. Couplant can be light oil or a light grease
⎯ Place a small amount between the two surfaces (sensor & test object)
⎯ Finger tighten then loosen
⎯ Repeat two or three times
⎯ Then tighten with torque wrench to specified torque level amount on datasheet (this
is typically around 18 “lbs)
Note if sensor used has output signal connected to the accelerometer case or not!!!
The bottom surface of the
accelerometer should be flat,
smooth and clean
Is this accelerometer usable?
Machine this surface flat or replace this sensor….Do not use as is!!!
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Insulated mounting studs:
Insulated mounting studs provide electrical isolation of the
accelerometer from the structure ground. Ideal for elimination of
data-destructive ground loops. Insulated studs are vastly superior
to other methods of isolation which do not necessarily eliminate
effects of stray capacitance from signal lead to ground.
Widely accepted and field proven, they are recommended for use
when accelerometer case and cable shield are connected to
instrumentation ground.
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Cementing studs:
Ideal for application where the object surface cannot be drilled or
tapped, or where mounting surface irregularities make flat, rigid
mounts otherwise impossible.
Make accelerometer mounting possible without contamination of
sensor mounting threads which might occur if mounting directly to
test article.
Removal of the accelerometer is also facilitated by this method.
A variety of cements for various applications are available.
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Sensor Removal
Triaxial mounting blocks:
Vector coordinates from a single point on your test structure is
provided by use of Triaxial Mounting Blocks.
Block faces are precision machined to achieve orthogonal perfection
thus saving you time and money in designing your own Triaxial
Mounting Blocks.
Selection of materials is also based on careful consideration of end
use criteria.
Lightweight magnesium on Model 2961 ensures low dynamic mass.
Aluminum on Model 2950 is best comparison between mass and
economy.
Design of these blocks is precise to give reliable data.
When the majority of your sensors are triaxial in nature you may wish
to utilize triaxial accels.
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Sensor removal is extremely important tand should be performed correctly
Prerequisite…Use the Endevco removal tool (wrench) that is shipped with
the sensor KIT
If the sensor is adhesively mounted use a softening SOLVENT first
Before putting the sensor away ensure the bottom is very clean, not
scratched, clean the connector (wash it out with a volatile solvent)
Do not attempt to remove the sensor with any of the following tools
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Let’s deviate a moment and talk about the
measurement system setup:
What should the basic arrangement look like for measurements
Pay attention to where the Grounds are located
⎯ All grounds per channel should be at one location only
⎯ The grounds should be at the data collection end and not the sensor end
⎯ Avoid ground loops
Arrange your data system as per this block diagram:
Calibration
Source
Sensor
Object Under
Test
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Sensor
Cable
Cal/Oper
Junc Box
Signal
Conditioner /
Amplifier
straight thru
or insulated
based on
sensor ground
Coax
Filtering
If additional
Is needed
Tape Recorder
Or
Data Acquisition
System
Coax
Power Cord
Power Cord
Earth Ground
Power Cord
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Outlet
Grounded
Now you know that a measurement
channel should have only one ground
That ground should be at the
instrumentation end (not the sensor
end) of the data channel
configuration
Why?
⎯ Two or more grounds of a given channel
produces GROUND LOOPS
⎯ Ground Loops produce electrical noise
on the channel data resulting in errors
⎯ Which of these mounting studs or
mounting adaptors would you use to
prevent ground loops?
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We will be looking at cable issues
Where are the typical
weak links in a measurement system?
Electrical Noise (from ground loops as presented in previous
slide)
Mounting method (limiting measurement dynamic frequency
response)
Using a poorly cleaned, poorly attached or otherwise faulty cable
Insufficient data bandwidth (bandpass) due to lack of anti-aliasing
filters
Lack of use in any instrumentation to monitor the raw signal
waveform being measured and recorded
Incorrect data levels based on insufficient preparation for S/N
ratio
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Cables are the least expensive part of your measurement system
Cables are typically “abused” and overlooked regarding proper care and
maintenance
Cables come in many styles, lengths, terminations and shielding
We will be looking at “Best Practices” for cable usage, mounting, routing,
installation and removal
We will be looking at various accessories:
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Various cable choices
Sensor cable material and raw cable for long runs
Working with the
cabling and attached sensor
Prerequisite…Install your sensors per proper surface preparation,
mounting surface flatness, torque down, use of couplant, ground
isolation, and known “Best Practices”
Select routing path of sensor cabling and layout cable run(s)
Connect the cable to the sensor by rotating the cable nut onto the
sensor. Push the cable connector into the sensor then screw the
cable nut onto the sensor:
Tighten to 1.5”/lbs as rule of thumb
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Routing the cabling
Routing of sensor cables
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
Put a Dab of
Clear finger Nail
Polish at the
connection
Then Tie Down
Every 6” to 12”
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Be aware of objects in your way that are close by the sensor
You might select hole thru sensors so that with cable attached
you can orient the sensor to free the cable then torque sensor
down
Standard cable length is 10 feet. Special length cables are
available on special order
Cables & sensors are not made to be submerged in water nor
used in a water spray environment
Apply a Red RTV over the connector where the cable fits to the
sensor
Form a “DRIP LOOP” close to the sensor to direct water
(condensate) away from the sensor/cable connection
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Interesting Cable Issue (This is not an Endevco cable)
Helpful hints (cables)
Ensure cable tie down to prevent unwanted stress on
connector and prevent spurious electrical noise
Ensure screw-on cable connector is 1.5”/lbs (finger tight)
Use a supplied wrench for those cable assemblies that supply
a wrench for tightening and removal
Apply Dow-Corning Silastic 732RTV to connector threads and
outer joints to prevent moisture from entering connector
Clean cable…Dip Cable and connector in a volatile solvent
such as acetone, trichlorothane, or similar solvent
Check cable with an Ohm meter
Do not allow the cable to be stepped on, contaminated or
otherwise abused
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Dielectric material “Cold Flow” can be a problem with cables
Good metal-to-metal contact (at the connector) is critical
Fashion a tool to cut away the “Cold Flow” material
Ensure that the connector is “CLEAN”
Would you use this cable?
Why?
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Effect of Safety Wiring
What does a safety wire do to the ground circuit?
Based on the effect, could this cause a ground loop?
If so, how could you prevent the ground loop?
Endevco standard cables with safety wire (lock) holes are:
3090C, 3090DV, 6917B, 6917D, 6917DM1 & 6917M113
Subminiature cable assemblies
Example of cable connectors with safety wiring……
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Subminiature cable assemblies #2
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Low impedance cable assemblies
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Low impedance cable assemblies #2
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High impedance cable assemblies
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High impedance cable assemblies #2
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Final comments
Place the accelerometer on a flat prepared surface. If the test
object surface is not flat, make an adaptor mounting block that
has a flat surface for the sensor.
Don’t Drop the Accelerometer. It can be damaged due to high
“G” shock approaching 14,000G’s from a waist high fall.
Use clean, dry, well maintained instrumentation, sensors and
cables to complete your test projects.
Use RCC (Remote Charge Converter) in a hot environment PE
application to reduce electrical noise in your measurement.
Short Sensor
Cable
RCC
Signal Conditioning,
Filtering, Data
Acquisition Equipment
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PE
Accel
Coax Cable (Long Run)
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Confidentiality agreement
The information contained in this document is the property of Endevco Corporation and is
confidential 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 730-744), 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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