The output of linear displacement sensors based on the use of a balanced impedance bridge circuit, is influenced by environmental temperature changes. The magnitude of the change in output of this type of sensor system varies depending on what part of the system is experiencing the change in temperature.
It is not the intent of this tech note to instruct the reader in thermally stable circuit design. The intent here is to identify the indications, and provide a basic understanding of the source of thermal drift in the system output.
Thermal Changes in the Sensor
Fluctuations in sensor coil temperature will affect the output of inductive linear displacement sensors. This is primarily due to the affect the temperature has on the electrical properties of the wire used to manufacture the coil. System output changes due to changes in sensor temperature, lag very little. A simple charting of sensor temperature compared to system output will help to determine if change in sensor temperature is the cause.
Thermal Changes in the Sensor Cable
The affect is very similar to changes in coil temperature in that there is little lag time. The coil wire effect is typically 5-8 times that of the standard sensor cable, hence the magnitude of output change caused by changes in cable temperature will be less than that caused by the same change in sensor temperature.
Thermal Changes in the Electronics
It is always good practice to install electronic instrumentation in as thermally stable an environment as is possible. Appropriate component specification, testing, and documentation by the sensor manufacturer will ensure that thermal stability of the electronics will be sufficient to meet published performance specifications.
As ideal situations are not always obtainable, there can be changes in the system output caused by changes in the temperature of the electronics environment. The rate of change of the output due to electronics temperature change has a high degree of latency. This is due primarily from the electronics being self-heating and the enclosure providing some insulating of the electronics from the environmental temperature change.
Improving Thermal Stability
Performing temperature compensation calibration can decrease thermal sensitivity. This is accomplished on analog systems by discrete component tuning to a specific sensor/target pair. Digital sensors employ active temperature compensation, correcting the output for monitored changes in the sensor temperature. Temperature compensation calibration is especially beneficial with single coil sensors, and rapid temperature fluctuations.
Dual coil sensors, with an inactive coil incorporated into the sensor housing, provide common mode rejection when slow changes in sensor temperature are anticipated.
For more information refer to the related tech notes, or contact Kaman's Application Engineering group for assistance.