ASTM F2537-06(2011)
(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.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 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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Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: F2537 − 06 (Reapproved 2011)
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
the sensor housing.
supply, signal conditioner, and data acquisition systems (linear
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
coils’ differential reluctance.
system and displacement. This relationship is used to convert
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 The values stated in SI units are to be regarded as 2.1.8 linear variable differential transformer (LVDT), n—a
standard. No other units of measurement are included in this
linear displacement sensor made of a sensor housing and a
standard.
core. The sensor housing contains a primary coil and two
secondary coils. When an ac excitation signal is applied to the
1.5 This standard does not purport to address all of the
primary coil, voltages are induced in the secondary coils. The
safety concerns, if any, associated with its use. It is the
magnetic core provides the magnetic flux path linking the
responsibility of the user of this standard to establish appro-
primary and secondary coils. Since the two voltages are of
priate safety and health practices and determine the applica-
opposite polarity, the secondary coils are connected in series
bility of regulatory limitations prior to use.
opposing in the center, or null position. When the core is
2. Terminology
displaced from the null position, an electromagnetic imbalance
occurs. This imbalance generates a differential ac output
2.1 Definitions:
voltage across the secondary coils, which is linearly propor-
2.1.1 calibrated range, n—distance over which the linear
tional to the direction and magnitude of the displacement.
displacement sensor system is calibrated.
When the core is moved from the null position, the induced
2.1.2 calibration certificate, n—certification that the sensor
voltage in the secondary coil, toward which the core is moved,
meets indicated specifications for its particular grade or model
increases while the induced voltage in the opposite secondary
and whose accuracy is traceable to the National Institute of
coil decreases.
Standards and Technology or another international standard.
2.1.9 null position, n—the core position within the sensor
housing where the sensor voltage output is zero (some sensors
ThispracticeisunderthejurisdictionofASTMCommitteeF04onMedicaland
do not have a null position).
Surgical Materials and Devices and is the direct responsibility of Subcommittee
F04.15 on Material Test Methods.
2.1.10 offset correction, n—removal of any offset in a
Current edition approved June 1, 2011. Published June 2011. Originally
sensor’s output so that at zero displacement, zero voltage is
approved in 2006. Last previous edition approved in 2006 as F2537–06. DOI:
10.1520/F2537-06R11. recorded.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
F2537 − 06 (2011)
2.1.11 percent error, n—the difference between a measure- tional to displacement of the sensor. This procedure is appli-
ment of a reference standard and the actual length of the cable to the linear range of the sensor. Recommended practice
referencestandarddividedbytheactuallengthofthereference is to use the linear displacement sensor system only within its
standard and the result converted to a percent. linear working range.
2.1.12 power supply, n—a regulated voltage source with
5. Apparatus and Equipment
outputequaltothatrequiredbythesensorforproperoperation.
5.1 Linear Displacement Sensor.
2.1.13 sensor housing, n—central hole in a linear displace-
5.2 Power Supply, with output equal to that required by the
ment sensor that senses movement of the core within it.
sensor.
2.1.14 signal conditioner, n—electronic equipment that acts
5.3 Signal Conditioner, Data Acquisition System, and Re-
to convert the raw electrical output from the linear displace-
ment sensor into a more useful signal by amplification and lated Cables and Fittings.
filtering.
5.4 Test Method—Micrometer Fixture Calibration:
5.4.1 Calibration Fixture,afixturethatprovidesameansfor
3. Summary of Practice
fixing both a micrometer head and the linear displacement
3.1 Alinear displacement sensor is mounted in a calibration
sensor along a parallel displacement axis, and is capable of
fixture such that it can be subjected to a precise, known
applying displacement to the linear displacement sensor
displacement.
throughout its linear range. The alignment tolerance of the
3.2 Displacement is applied in steps over the full range of calibration fixture must be measured.
5.4.2 Micrometer Head, a precision instrument with known
the linear displacement sensor and electrical readings (for
example, voltages) are collected using the linear displacement error(thatis,tolerance).Thespindleofthemicrometermustbe
non-rotating and spring-loaded. The micrometer head shall be
sensor system.
calibrated annually by the manufacturer or other qualified
3.3 Each voltage reading is taken as the average of 100
personnel.
readings over 0.1 s, decreasing the error of the reading. The
errorinthereadingsisrecordedasthestandarddeviationinthe
6. Hazards
readings. This error should be constant and independent of
6.1 Safety Hazards:
displacement. It should be noted that the error in the readings
6.1.1 This practice involves electrical equipment. Verify
is a summation of errors in each of the linear displacement
that all electrical wiring is connected properly and that the
sensor system components.
power supply and signal conditioner are grounded properly to
3.4 The calibration factor (S) is calculated as the slope of
prevent electrical shock to the operator. Take necessary pre-
the voltage versus displacement curve using linear regression.
cautions to avoid exposure to power signals.
3.5 Linearity of the sensor is assessed.
6.2 Safety Precautions:
3.6 The percent error is determined for each calibration 6.2.1 Examine the sensor housing for burrs or sharp edges,
point collected. This percent error is evaluated together with or both. Remove any protrusions that might cause harm.
the tolerance of the micrometer head calibration. 6.2.2 The sensor can be permanently damaged if incorrectly
handled. Consult the manufacturer’s guidelines for handling.
4. Significance and Use
6.2.3 The sensor can be permanently damaged if incorrectly
4.1 Linear displacement sensor systems play an important connected to the power supply, or if connected to a power
role in orthopedic applications to measure micromotion during
supply with the wrong excitation level. Consult the manufac-
simulated use of joint prostheses. turer’s guidelines for use.
6.2.4 Follow all manufacturer’s recommendations with re-
4.2 Linear displacement sensor systems must be calibrated
gard to safety.
for use in the laboratory to ensure reliable conversions of the
system’s electrical output to engineering units. 6.3 Technical Precautions:
6.3.1 If using a linear displacement sensor that permits the
4.3 Lineardisplacementsensorsystemsshouldbecalibrated
core to leave the sensor housing, do not interchange cores with
before initial use, at least annually thereafter, after any change
other linear displacement sensor housings.
in the electronic configuration that employs the sensor, after
6.3.2 Replacethesensorifit,oranycomponentofit,shows
any significant change in test conditions using the sensor that
any signs of dents, bending, or other defects that may affect its
differ from conditions during the last calibration, and after any
performance.
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