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Mechanical measurements include the following:
Principals of Operation
There are two type of strain gauges,
- Thin film strain gauges which are normally made of stainless
steel or aluminum and which work by the principal of metals changing their
resistivity with strain,
- Vibrating wire strain gauges.
Load cells are normally a structure with a known elasticity which
is then strain gauged. The load is then proportional to the strain gauge
output.
The thin film type is the most common, with vibrating wire mode
systems used for large structures. Of these thin film strain gauges, there are
4 types:
- Six wire full bridge
- Four wire full bridge
- Half bridge
- Quarter bridge
The Six wire full bridge system is the most accurate, since any
temperature effects are canceled out at the sensor end, and any voltage drop
across the excitation cables are also canceled out.
Interfacing to NP4000
Uses should use the NPxxSG strain gauge module (where xx=16 or
24). The modules are numbered as follows
NPxxSG-1 . . . . Full
NPxxSG-2
. . . . Half
NPxxSG-3 . . . . Quarter
The quarter bridge strain gauge is set up for 120 Ohms strain
gauge. Please check factory for non-standard strain gauges
Connections are as follows
|
Full - 6 wire |
Full |
Half |
Quarter |
 |
1 - Supply Ref..
2 - Positive input
3 - Negative
input
4 - Supply Ref. |
1 - Positive supply
2 - Positive input
3 - Negative
input
4 - Negative supply |
1 - Positive supply
2 - input
3 - Not connected
4
- Negative supply |
1 - Positive input/supply
2 - Negative input/supply
3
- Link to 4
4 - Link to 3 |
Excitation Voltage
The NP4000 instrument excites the strain gauge by 2.5V. This is a
low voltage, and is chosen to cause minimal heating of the sensor (particularly
important with quarter bridge gauges). For best results a higher external
excitation voltage should be used together with a six wire strain gauge.
Output and Range
The typical output range of a thin film strain gauge is 2
millivolts per volt (2mV/V), with a typical strain of ?. This is in-line with
the strain of typical steel and concrete structures. With excitation at 2.5V,
and a DC offset accuracy of 1uV, the typical noise equivalent strain is ?
microstrain.
Sources of Supply
- Strainsert Company
Manufactures internally gaged force sensing transducers, including load cells,
clevis pins,
bolts, studs, tension links, and load indicators.
- Columbia Research
Labs, Inc.
Manufacturers of piezoelectric accelerometers and
transducers, inertial sensors,
strain sensors, and displacement sensors.
- Kistler Instrument Corporation
(www.kistler.com)
Manufacturers of high quality piezoelectric,
piezoresistive and capacitive sensors,
transducers, accelerometers and
software for measuring pressure, force, acceleration, strain, velocity and
torque
Principals of Operation
Accelerometers measure acceleration by the principal of inertial
measurement. This is where the force on a seismic mass is measured, and by
Newtons 3rd law F=ma, the acceleration can be deduced. The force measurement is
made by a variety of means listed below
The main types of accelerometers as as follows:
- Servo system.
This is an expensive but accurate system
whereby a magnetic field is generated by external electronics in order to keep
a seismic mass in a fixed location. The acceleration is proportional to the
current in the magnetic coils.
- Silicon Microstructure
This is a recent low cost technique
that can yield good results. Silicon is machined by differential etching
process to form a load cell beam. The resistance of the load beam or diaphragm
is then proportional to the applied acceleration
- Piezoresistive
The resistance of a piezoelectric sensor is
measured and is proportional to the strain caused by a small seismic mass under
the applied acceleration.
- piezoelectric
The voltage of a piezoelectric sensor is
measured and is proportional to the strain caused by a small seismic mass under
the applied acceleration. This system is not very good for very low
accelerations
- Strain Gauged
As Piezoresistive, but using a standard
strain gauge.
Applications
Some uses of accelerometers are as follow:
- Structural Monitoring
For large structures the frequencies
are low (0.1 to 10Hz), and the acceleration is often turned into displacement
by double integration. The displacement under different environmental
conditions is used to access the safety and operational use of the
structure
- Condition Monitoring
The vibration of a piece of machinery
under operational conditions is monitored. Any adverse oscillations can be used
to ascertain likely failure in the machine
- Impulse Testing
A system is hit, usually with a hammer, and
the resulting vibrations can be used to locate cracks
Interfacing to NP4000
Normally a standard voltage input module can be used. Check the
range of the module against the output range of the accelerometer. Most
non-signal conditioned accelerometers have an output range of a few millivolts.
Signal conditioned modules may have voltage or current mode output. In either
case, pins 2 and 3 will be the inputs.
|
1 - Case
2 - Positive input
3 - Negative input
4
- Not connected |
Sources of Supply
- Lammerholm Flemming (www.lamerholm.com)
Manufactures
Piezoresistive, piezoelectric, and silicon micromachined sensors, mainly for
the automotive industry
- Monitran
Manufactures rugged Piezoresistive and
piezoelectric sensors, mainly for condition monitoring applications. A
particular feature is the use of stainless steel cabling, which enables the
sensors to be placed in harsh environments
- Kistler
Manufacturers
of high quality piezoelectric, piezoresistive and capacitive sensors,
transducers, accelerometers and software for measuring pressure, force,
acceleration, strain, velocity and torque.
RECOMMENDED for very low
frequency measurement for structural monitoring.
- Silicon
Microstructures
high-performance, high-volume, low-cost, silicon-based
pressure sensors, accelerometers, and
special purpose sensors and
microstructures.
- ENDEVCO
- EuroSensor
Medium quality signal conditioned silicon
micromachined sensors.
- Analogue Devices
Very
low cost silicon (chip) sensors for OEM applications
- Sundstrand/Allied signal
Very high quality (and expensive)
servo type accelerometers
- Lucas Aerospace
Manufactures servoed accelerometers for
structural monitoring. These devices have bearings in them which have been
known to seize, hence they are not recommended by us.
- Silicon Designs
Inc.
Manufactures small size capacitance accelerometers for OEM
applications
Principals of operation
Displacement sensors, sometimes called proximity sensors provide
an output that is proportional to the distance between two objects. For
industrial control systems, sensors that switch at a particular distance
(proximity switches) are commonly used as a non-contact sensing system.
The main methods of measuring displacement are as follows:
Applications
Interfacing
Generally, pre-signal conditioned sensors should be
used. For the LVTD, an AC signal is required, and there are several single chip
devices that can provide this excitation. The signal output is then either a
voltage or current loop, can be wired into the appropriate voltage or current
input module (NPAIxxV or NPAIxxC) Potentiometer sensors should use the NPAIxxR
interface module. Optical grating systems normally have their own digital
interface. Contact Keynes Controls for further assistance.
Sources of Supply
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