An eddy current, linear displacement system consists of a sensor or probe, an electronics module and an interconnecting coax cable. At the heart of the system is the impedance bridge circuit. One leg of the bridge is connected to the sensor coil via the sensor cable. The impedance of that circuit is balanced with electronic components. As a target engages the electromagnetic field generated by the sensor, the impedance of the sensor side of the bridge changes resulting in an output change in the system.


Coil vs. Cable Impedance
It is important to understand that the sensor and the interconnecting cable both provide inductance, capacitance, and resistance to that leg of the impedance bridge circuit. This circuit cannot distinguish between coil or cable induced impedance change.


System sensitivity is based on the amount of impedance change in the bridge circuit caused by target-field interaction. To maximize sensitivity, the ratio of coil impedance to cable impedance must be maximized. A simple way to do this is to keep the cable lengths as short as possible.


Cable Temperature
As impedance is a combination of inductance, capacitance, and resistance, anything that affects these properties affects impedance. Changes in the cable temperature can affect the system output.


Cable Movement
Physical movement of the cable can result in minor changes in the distance between the center conductor of the coax cable and the shield. This can result in capacitance changes in the cable that can affect the system output.


When A Long Sensor Cable Is Unavoidable
Kaman can optimize the oscillator frequency used to excite the coil allowing for a longer cable. This can be detrimental in some dynamic applications as it reduces the maximum allowable surface velocity of the target.


With longer cables, it is important to use a highly conductive target such as aluminum.


Restraining long cables to prevent vibration induced movement helps improve overall system stability.


Routing long cables to avoid temperature changes is also important to system stability. Routing the cable through conduit can help minimize temperature changes in the cable.


Cable lengths can be increased where required by the application. However, performance may be degraded and maximum cable lengths are determined by sensor and target type. Contact Kaman for application assistance.



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