ASTM F2537-06
(Practice)Standard Practice for Calibration of Linear Displacement Sensor Systems Used to Measure Micromotion
Standard Practice for Calibration of Linear Displacement Sensor Systems Used to Measure Micromotion
SIGNIFICANCE AND USE
Linear displacement sensor systems play an important role in orthopedic applications to measure micromotion during simulated use of joint prostheses.
Linear displacement sensor systems must be calibrated for use in the laboratory to ensure reliable conversions of the system’s electrical output to engineering units.
Linear displacement sensor systems should be calibrated before initial use, at least annually thereafter, after any change in the electronic configuration that employs the sensor, after any significant change in test conditions using the sensor that differ from conditions during the last calibration, and after any physical action on the sensor that might affect its response.
Verification of sensor performance in accordance with calibration should be performed on a per use basis both before and after testing. Such verification can be done with a less accurate standard than that used for calibration, and may be done with only a few points.
Linear displacement sensor systems generally have a working range within which voltage output is linearly proportional to displacement of the sensor. This procedure is applicable to the linear range of the sensor. Recommended practice is to use the linear displacement sensor system only within its linear working range.
SCOPE
1.1 This practice covers the procedures for calibration of linear displacement sensors and their corresponding power supply, signal conditioner, and data acquisition systems (linear displacement sensor systems) for use in measuring micromotion. It covers any sensor used to measure displacement that gives an electrical voltage output that is linearly proportional to displacement. This includes, but is not limited to, linear variable differential transformers (LVDTs) and differential variable reluctance transducers (DVRTs).
1.2 This calibration procedure is used to determine the relationship between output of the linear displacement sensor system and displacement. This relationship is used to convert readings from the linear displacement sensor system into engineering units.
1.3 This calibration procedure is also used to determine the error of the linear displacement sensor system over the range of its use.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:F2537–06
Standard Practice for
Calibration of Linear Displacement Sensor Systems Used to
Measure Micromotion
This standard is issued under the fixed designation F2537; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2.1.3 core, n—central rod that moves in and out of the
sensor.
1.1 This practice covers the procedures for calibration of
linear displacement sensors and their corresponding power
NOTE 1—It is preferable that the sensors prevent the core from exiting
supply, signal conditioner, and data acquisition systems (linear
the sensor housing.
displacement sensor systems) for use in measuring micromo-
2.1.4 data acquisition system, n—system generally consist-
tion. It covers any sensor used to measure displacement that
ing of a terminal block, data acquisition card, and computer
givesanelectricalvoltageoutputthatislinearlyproportionalto
that acquire electrical signals and allows them to be captured
displacement. This includes, but is not limited to, linear
by a computer.
variable differential transformers (LVDTs) and differential
2.1.5 differential variable reluctance transducer (DVRT),
variable reluctance transducers (DVRTs).
n—a linear displacement sensor made of a sensor housing and
1.2 This calibration procedure is used to determine the
a core. The sensor housing contains a primary coil and a
relationship between output of the linear displacement sensor
secondary coil. Core position is detected by measuring the
system and displacement. This relationship is used to convert
coils’ differential reluctance.
readings from the linear displacement sensor system into
2.1.6 linear displacement sensor, n—an electrical sensor
engineering units.
that converts linear displacement to electrical output.
1.3 This calibration procedure is also used to determine the
2.1.7 linear displacement sensor system, n—a system con-
errorofthelineardisplacementsensorsystemovertherangeof
sisting of a linear displacement sensor, power supply, signal
its use.
conditioner, and data acquisition system.
1.4 This standard does not purport to address all of the
2.1.8 linear variable differential transformer (LVDT), n—a
safety concerns, if any, associated with its use. It is the
linear displacement sensor made of a sensor housing and a
responsibility of the user of this standard to establish appro-
core. The sensor housing contains a primary coil and two
priate safety and health practices and determine the applica-
secondary coils. When an ac excitation signal is applied to the
bility of regulatory limitations prior to use.
primary coil, voltages are induced in the secondary coils. The
magnetic core provides the magnetic flux path linking the
2. Terminology
primary and secondary coils. Since the two voltages are of
2.1 Definitions:
opposite polarity, the secondary coils are connected in series
2.1.1 calibrated range, n—distance over which the linear
opposing in the center, or null position. When the core is
displacement sensor system is calibrated.
displaced from the null position, an electromagnetic imbalance
2.1.2 calibration certificate, n—certification that the sensor
occurs. This imbalance generates a differential ac output
meets indicated specifications for its particular grade or model
voltage across the secondary coils, which is linearly propor-
and whose accuracy is traceable to the National Institute of
tional to the direction and magnitude of the displacement.
Standards and Technology or another international standard.
When the core is moved from the null position, the induced
voltage in the secondary coil, toward which the core is moved,
increases while the induced voltage in the opposite secondary
This practice is under the jurisdiction ofASTM Committee F04 on Medical and
coil decreases.
Surgical Materials and Devices and is the direct responsibility of Subcommittee
2.1.9 null position, n—the core position within the sensor
F04.15 on Material Test Methods.
housing where the sensor voltage output is zero (some sensors
Current edition approved Dec. 1, 2006. Published December 2006. DOI:
10.1520/F2537-06. do not have a null position).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
F2537–06
2.1.10 offset correction, n—removal of any offset in a cable to the linear range of the sensor. Recommended practice
sensor’s output so that at zero displacement, zero voltage is is to use the linear displacement sensor system only within its
recorded. linear working range.
2.1.11 percent error, n—the difference between a measure-
5. Apparatus and Equipment
ment of a reference standard and the actual length of the
5.1 Linear Displacement Sensor.
reference standard divided by the actual length of the reference
5.2 Power Supply, with output equal to that required by the
standard and the result converted to a percent.
sensor.
2.1.12 power supply, n—a regulated voltage source with
5.3 Signal Conditioner, Data Acquisition System, and Re-
output equal to that required by the sensor for proper operation.
lated Cables and Fittings.
2.1.13 sensor housing, n—central hole in a linear displace-
5.4 Test Method—Micrometer Fixture Calibration:
ment sensor that senses movement of the core within it.
5.4.1 Calibration Fixture,afixturethatprovidesameansfor
2.1.14 signal conditioner, n—electronic equipment that acts
fixing both a micrometer head and the linear displacement
to convert the raw electrical output from the linear displace-
sensor along a parallel displacement axis, and is capable of
ment sensor into a more useful signal by amplification and
applying displacement to the linear displacement sensor
filtering.
throughout its linear range. The alignment tolerance of the
3. Summary of Practice
calibration fixture must be measured.
3.1 Alinear displacement sensor is mounted in a calibration 5.4.2 Micrometer Head, a precision instrument with known
fixture such that it can be subjected to a precise, known error (that is, tolerance).The spindle of the micrometer must be
displacement. non-rotating and spring-loaded. The micrometer head shall be
3.2 Displacement is applied in steps over the full range of calibrated annually by the manufacturer or other qualified
the linear displacement sensor and electrical readings (for personnel.
example, voltages) are collected using the linear displacement
6. Hazards
sensor system.
6.1 Safety Hazards:
3.3 Each voltage reading is taken as the average of 100
readings over 0.1 s, decreasing the error of the reading. The 6.1.1 This practice involves electrical equipment. Verify
that all electrical wiring is connected properly and that the
error in the readings is recorded as the standard deviation in the
power supply and signal conditioner are grounded properly to
readings. This error should be constant and independent of
prevent electrical shock to the operator. Take necessary pre-
displacement. It should be noted that the error in the readings
cautions to avoid exposure to power signals.
is a summation of errors in each of the linear displacement
6.2 Safety Precautions:
sensor system components.
6.2.1 Examine the sensor housing for burrs or sharp edges,
3.4 The calibration factor (S) is calculated as the slope of
or both. Remove any protrusions that might cause harm.
the voltage versus displacement curve using linear regression.
6.2.2 The sensor can be permanently damaged if incorrectly
3.5 Linearity of the sensor is assessed.
handled. Consult the manufacturer’s guidelines for handling.
3.6 The percent error is determined for each calibration
6.2.3 The sensor can be permanently damaged if incorrectly
point collected. This percent error is evaluated together with
connected to the power supply, or if connected to a power
the tolerance of the micrometer head calibration.
supply with the wrong excitation level. Consult the manufac-
4. Significance and Use
turer’s guidelines for use.
4.1 Linear displacement sensor systems play an important 6.2.4 Follow all manufacturer’s recommendations with re-
role in orthopedic applications to measure micromotion during gard to safety.
simulated use of joint prostheses. 6.3 Technical Precautions:
4.2 Linear displacement sensor systems must be calibrated 6.3.1 If using a linear displacement sensor that permits the
for use in the laboratory to ensure reliable conversions of the core to leave the sensor housing, do not interchange cores with
system’s electrical output to engineering units. other linear displacement sensor housings.
4.3 Lineardisplacementsensorsystemsshouldbecalibrated 6.3.2 Replace the sensor if it, or any component of it, shows
before initial use, at least annually thereafter, after any change any signs of dents, bending, or other defects that may affect its
in the electronic configuration that employs the sensor, after performance.
any significant change in test conditions using the sensor that 6.3.3 Store all system components in dry, protective loca-
differ from conditions during the last calibration, and after any tions when not in use.
physical action on the sens
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