Macro Sensors LVDT Press Release: The hot and cold of LVDTs

Harold Schaevitz
Macro Sensors
Pennsauken, NJ

Designers rarely consider how changes in temperature might affect linear variable differential transformers (LVDTs). Yet a temperature change large enough can skew LVDT readings.

This effect can take the form of a shift in LVDT output level with respect to its core displacement. This creates a scaling error. It's the same as multiplying the calibrated output signal by a scale factor slightly larger or smaller than unity.

Or it can take the form of a change in the null-reference output position, the starting or reference point used for calibrating the LVDT output. This type of error is known as a zero-shift or scale-shift error. The effect is the same as adding or subtracting a small constant value to or from the calibrated output voltage.

Rising ambient temperatures produce a higher resistance in the copper wire used for the primary and secondary coils of the LVDT. The high primary-coil resistance boosts winding impedance and forces primary current to drop. This affects output and sensitivity levels, generating a scaling error.

A change in the winding resistance of the secondary does not have as much impact if the load that has a high impedance is used. But resistance changes in the secondary will factor into the transfer of power to a low-impedance load. For example, if the load impedance is 50X greater than the secondary-winding resistance, a 50% rise in secondary resistance would only reduce output voltage by approximately 1%.

Of course, changes in temperature make materials expand or contract in accordance with their thermal coefficient of expansion.

LVDT Installation Factors
Error caused by support expansion and an extension rod not square to the contact surface.

Different expansion coefficients between various metals can affect the accuracy of an LVDT installation, as evident in this layout. A brass-core extension rod 10-in. long is fastened to an LVDT mounted on a steel plate 12 in. from the plate's attachment point on a machine. The difference in expansion coefficients between steel and brass creates an offset error of almost .0004 in. with a 30°C rise in temperature. Additional errors can creep into the measurement if the LVDT support and extension rod guide are not square to the the surface being measured. The null or zero position of the LVDT shifts as the supports expand or contract.

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Company Information:
Name: Macro Sensors
Address: 7300 US Route 130 North, Bldg. 22
City: Pennsauken
State: NJ
ZIP: 08110 1334
Country: USA
Phone: 856-662-8000
FAX: 856-317-1005
http://www.macrosensors.com



Press Release: The story @ Machine Design, May 10, 2007	The Hot and Cold of LVDTs Changes in temperature can cause subtle errors in LVDT readings.
Published: May 10, 2007 (above: May 10, 2007 Issue, Machine Design)	Harold Schaevitz Macro Sensors Pennsauken, NJ Designers rarely consider how changes in temperature might affect linear variable differential transformers (LVDTs). Yet a temperature change large enough can skew LVDT readings. This effect can take the form of a shift in LVDT output level with respect to its core displacement. This creates a scaling error. It's the same as multiplying the calibrated output signal by a scale factor slightly larger or smaller than unity. Or it can take the form of a change in the null-reference output position, the starting or reference point used for calibrating the LVDT output. This type of error is known as a zero-shift or scale-shift error. The effect is the same as adding or subtracting a small constant value to or from the calibrated output voltage. Rising ambient temperatures produce a higher resistance in the copper wire used for the primary and secondary coils of the LVDT. The high primary-coil resistance boosts winding impedance and forces primary current to drop. This affects output and sensitivity levels, generating a scaling error. A change in the winding resistance of the secondary does not have as much impact if the load that has a high impedance is used. But resistance changes in the secondary will factor into the transfer of power to a low-impedance load. For example, if the load impedance is 50X greater than the secondary-winding resistance, a 50% rise in secondary resistance would only reduce output voltage by approximately 1%. Of course, changes in temperature make materials expand or contract in accordance with their thermal coefficient of expansion. LVDT Installation Factors Different expansion coefficients between various metals can affect the accuracy of an LVDT installation, as evident in this layout. A brass-core extension rod 10-in. long is fastened to an LVDT mounted on a steel plate 12 in. from the plate's attachment point on a machine. The difference in expansion coefficients between steel and brass creates an offset error of almost .0004 in. with a 30°C rise in temperature. Additional errors can creep into the measurement if the LVDT support and extension rod guide are not square to the the surface being measured. The null or zero position of the LVDT shifts as the supports expand or contract. ---------------------------------------------------------------- Company Information: Name: Macro Sensors Address: 7300 US Route 130 North, Bldg. 22 City: Pennsauken State: NJ ZIP: 08110 1334 Country: USA Phone: 856-662-8000 FAX: 856-317-1005 http://www.macrosensors.com