Operation
The JEWELL LCA, LCF and LCI accelerometers
and inclinometers are precision inertial instruments. They utilize
closed loop sensor technology to produce a highly accurate output
at a relatively low price for the specified performance. The inertial
sensor output is an analog voltage proportional to applied acceleration
and tilt from DC through a specified frequency.
The
sensing element in a JEWELL inertial instrument is the torquer,
a meter mechanism designed specifically for sensor use. JEWELL has
produced inertial instrument torquers and complete acceleration
sensing assemblies for many years. Hundreds of thousands of acceleration
sensors have been manufactured. JEWELL sensors are used throughout
the world for detecting acceleration and tilt from less than one
m G (one m Radian) to more than 50G.The
torquer mechanism is typically the most expensive subassembly in
a servo sensor. Torquers are sophisticated meter movements, and
JEWELL as a meter manufacturer can produce torquers very efficiently.
We, therefore, often have a cost advantage when compared to other
traditional technology inertial instrument producers.A
torquer used to sense acceleration or tilt is intentionally unbalanced
in its plane of allowable angular motion. When acceleration or tilt
is present, a torque proportional to the mechanism unbalance and
the physical input is developed. The torque results in an angular
motion sensed by an optical position detector. The position detector
output is compared to a reference voltage, and the difference is
an error signal that is the input to a servo amplifier. The servo
amplifier output current is applied to the torquer in opposition
to the acceleration or tilt torque. At a constant inertial input,
the torquer mechanism angular position is minutely different from
the zero G position. The servo amplifier output current is directly
proportional to the applied acceleration or sine of the input tilt
angle. An analog voltage is produced by measuring the servo current
with a load resistor.Although
the LCA, LCF and LCI models are equivalent in concept and operation,
there are important mechanical differences. LCF and LCI Series moving
systems are suspended by torsion (taut band) flexures. LCA Series
moving systems are suspended by pivot and jewel bearings. Pivot
and jewel suspension units can have wider bandwidths and can be
used for higher range accelerations. The torsion flexure has superior
repeatability, and is most often used for tilt sensing and low range
accelerations. LCF units have the torsion flexure surrounded by
silicone fluid, and can withstand higher shock and vibration than
the LCA or LCI. The LCF can also provide accurate low frequency
output information during the time that the shock and vibration
are present.Note
that an accelerometer and an inclinometer are the same device. The
distinction is one of application, not operation. Accelerometer
users typically sense changes in velocity and characterize outputs
and errors in G. Inclinometer users sense changes in angular position
and think of outputs and errors in units of angular measurement.
An inertial instrument responds to both earth’s gravity and
acceleration.
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Specifications
The following information describes
how JEWELL engineers interpret performance characteristics and error
sources often listed in inertial instrument specifications. The
JEWELL interpretation is generally consistent with IEEE accelerometer
test conventions. Areas where standard unit performance specifications
can be improved, and inputs or outputs modified, are noted.
Range
and Scale Factor
The
range of an accelerometer or inclinometer is the input from + to
- over which the transducer is expected to yield the specified output.
A ±2G accelerometer has an input range of 4G. The scale is one half
the range, 2G in the example given.JEWELL
inertial sensors operating at ±15 volt bipolar input voltage have
an operating overrange capability of 50% to 100%. A 30° inclinometer,
for example, will operate without meaningful performance degradation
to 90°. The sine of 90° (1.00) being twice the sine of
30° (0.50). Beyond 100% overrange, the output voltage will
begin to clip. The overrange capability for unipolar input power
units is typically limited to less than 25% by the available input
or output voltage. Units with 4-20mA output are limited by the voltage
to current converter at less than 5% overrange. Exceeding the acceleration
or tilt range will not harm the unit, but the output will not be
accurate.The
scale factor for standard units is set to 5.00 volts DC +/–
0.5%. The setting accuracy can be improved to +/– 0.10%. Other
voltage outputs at full scale (1 volt, 2 volts, or 10 volts for
example) are also available.The
scale factor temperature sensitivity is primarily a function of
the torquer magnet temperature characteristics. Higher temperature
causes lower magnet flux density and, therefore, an increasing accelerometer
scale factor. The magnet thermal sensitivity curve is not linear.
Improvements of approximately 2 to 1 are possible, but applications
requiring improved scale factor temperature sensitivity must be
evaluated on an individual basis as a result of the nonlinear thermal
performance of the magnet.
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Natural
Frequency and Damping
The accelerometer or inclinometer dynamics can be treated
as second order for most JEWELL designs.The
natural frequency is the frequency at which the phase of the accelerometer
or inclinometer output lags the input by 90°. The standard
LCA natural frequency is between 70 Hz and 100 Hz. An LCA can be
supplied with decreased or increased natural frequency from 50 Hz
to 250 Hz. In an LCA, the dynamic output is the servo characteristic.
The standard LCI Series natural frequency is approximately 5 Hz.
An LCI can be manufactured with optional natural frequency response
from 1 Hz to 10 Hz. The LCI dynamics are modified by an output filter.
The standard LCF has a natural frequency of 30 Hz, and the optional
natural frequency range is 20 Hz to 60 Hz. The dynamic output of
an LCF is the servo characteristic.LCA
and LCI damping is nearly entirely electrical, coming from a lead/lag
network in the servo feedback. The LCA damping ratio is approximately
0.6, yielding some amplitude peaking at frequencies near the natural
frequency. The LCI damping ratio is closer to 0.7. Damping in an
LCF is provided by a combination of electrical lead and fluid viscosity.
The ambient temperature damping ratio is nominally 0.8, and thermal
changes in fluid viscosity make the LCF damping ratio temperature
sensitive. Changes to the published damping ratios can be accomplished,
but the changes must be evaluated for individual applications.The
bandwidth of an accelerometer or inclinometer is defined as the
frequency range below the frequency at which the amplitude of the
output is 3dB down relative to the input.
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Bias
The bias is the accelerometer output when no acceleration
is applied. Bias results primarily from residual suspension torque.
The output from a stationary accelerometer or inclinometer on a
flat surface is a combination of bias, misalignment and noise. Bias
errors can be reduced from the standard values, but the specific
amount is range and type dependent.The
bias temperature sensitivity is caused by temperature induced changes
to internal alignments. LCI bias temperature sensitivity can generally
be improved by 2 to 1 as an option. LCA and LCF bias temperature
sensitivities cannot easily be improved.
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Input
Axis Misalignment
The input axis misalignment is the geometric sum of the
pivot (output) axis and pendulous axis misalignments relative to
the base and a reference side of the accelerometer. The standard
alignment is a function of sensing range. Alignment of +/–
0.125° is the practical minimum available as an option. The
alignment specification in the data sheets applies to both axes.Cross
axis acceleration or tilt is sensed by an inertial instrument as
a function of the sine of the misalignment angle.
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Nonlinearity
Nonlinearity is the largest deviation in the accelerometer
output curve over its specified input range when the output is compared
to a least squares best fit straight line. Improvements to nonlinearity
are generally not practical. For most inclinometers and accelerometers
of 5G full scale or less, nonlinearity errors are small enough to
be ignored. The nonlinearity specified in the data sheets is somewhat
arbitrary. Most production units are not subjected to a linearity
test as servo devices are almost never nonlinear beyond specification.
The typical true nonlinear error is less than 0.002 volt in ±5 volts
for acceleration or tilt inputs less than 1G.
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Resolution
and Threshold
Threshold is the smallest input change that will result
in a meaningful (50% of expected) output change at zero. Resolution
is the smallest input change anywhere in the operating range that
will result in a meaningful output change. Resolution and threshold
errors are not always the same for accelerometers and inclinometers,
but in either case are so small that they can be treated together
and essentially ignored. The resolution for standard units is, in
fact, infinitely small. Specifying threshold and resolution errors
is more a test definition than a performance indicator.
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Repeatability
Repeatability defines an area of output uncertainty within which
the sensor may yield different outputs for identical inputs. The
uncertainty is usually a function of moving system suspension friction
and position errors. Flexure suspension units, the LCF and LCI,
have relatively small repeatability errors. Pivot and jewel LCA
units have 0.001G to 0.005G of uncertainty. Observed repeatability
is application dependent. Pivot and jewel units have better than
expected "real life" repeatability when some vibration is present.
The vibration energy helps reduce the effect of bearing friction.
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Noise
Noise is the dynamic output from the accelerometer when
no acceleration input is present. Measuring the true accelerometer
noise output is a challenging task since there is almost always
some environmental vibration present. Environmental noise of 0.001G
in an office, and 0.005 G to 0.010 G in a factory, would not be
surprising. The published noise specification reflects what a typical
user might find when looking at accelerometer AC output in a laboratory
or office, not the actual transducer noise. The accelerometer or
inclinometer noise floor is at least ten times lower than specified.
Noise tests results of less than 20m volts are typical. The
true output noise level is not likely to be a significant error
to most users.
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Input
Voltage
The input voltage specified is the range of voltages
over which the unit is expected to operate within specification.
If the input voltage is increased, even to +/– 20 volts, an
accelerometer or inclinometer will continue to operate. At higher
input voltages, additional current is required. The internal heat
dissipation increases, and component life begins to decrease. Most
units will continue to operate at input voltages as low as ±10 volts.
The output voltage clipping level and overrange capability are decreased
at lower input voltages.A
standard unit can be operated from a 24 volt single ended supply
by splitting the 24 volts with two resistors and taking the output
as ±5 volts referenced to the resistor common.Alternate
input power choices including 5 volts, 12 volts and 28 volts single
ended as well as ±5 volts bipolar are available for LCA and LCF
Series units. The LCI is available in a +/– 5 volt bipolar
version, but normally cannot be produced for unipolar power applications.
For quantities of greater than 100 pieces, the LCI can be modified
to operate from a unipolar supply.
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Input
Current
The input current is the current the accelerometer or
inclinometer will draw from a power supply set to +/– 15 volts.
The positive and negative currents are not balanced. Applied acceleration
and tilt increase the current requirement by 6mA/G for the LCI,
0.5mA/G for the LCA and 3mA/G for the LCF.It
is possible to reduce the LCF current requirement to 5 mA, but applications
must be evaluated on an individual basis.
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Output
Impedance
The standard output configuration is an operational amplifier
with 100 ohms in series with the output. Other configurations including
direct output across the load resistor, or use of an external load
resistor, are possible.
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Operating
Temperature Range
The operating temperature range defines the temperature
extremes over which the accelerometer or inclinometer will work
without temperature induced failure or a permanent change in some
output characteristic. A unit will continue to operate at temperatures
somewhat higher or lower than stated in the standard specifications.
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Survival
Temperature Range
The survival temperature range defines the temperature
extremes a unit can be exposed to without damage when it is not
powered. The actual survival temperature range is slightly greater
than that specified.
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Shock
and Vibration
Shock indicates the highest shock level that the accelerometer
or inclinometer can be exposed to without causing a permanent change
to the unit. The specified shock cannot be applied an infinite number
of times.Random
vibration limits are specified for 3 hour exposure to white noise
in the bandwidth 20 Hz to 2000 Hz.The
highest continuous acceleration level that can be applied without
damage is 30G.
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Seal
Sealing specifies the design technique selected to prevent
moisture, dust, or other external contaminant from entering the
sensor housing. The epoxy seal utilized is not hermetic by a military
leak rate definition, but is effective for normal transducer use.
Water or other contaminants will not enter the housing.
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