Accurate thermal growth measurement with eddy current sensor

Eddy current sensors

Generating set manufacturer switches from laser to eddy current sensing to measure thermal growth in harsh environment.

A manufacturer of diesel generating sets is using non-contact, eddy current displacement sensors from Micro-Epsilon to measure the thermal expansion of its engine bed frames. The previous laser displacement sensors were proving to be unreliable due to the ambient temperature fluctuations and the presence of oil and dirt. Micro-Epsilon’s eddy current sensors were evaluated for this task and provided superior performance.

Mounted at pre-determined locations on the genset, the eddy current sensors are performing reliably in a number of important thermal growth tests, including relative displacement measurements between the engine block and the supporting bed frame.

The engine bed frame is a critical part of a genset, as it provides mechanical support for the engine, generator and other control systems. When the genset is running, the bed frame is subjected to high dynamic loads, which create a significant amount of heat due to the diesel engine combustion process. The bed frame must therefore cater for any expansion of the engine block, which is typically a few millimetres, relative to the bed frame.

Prior to purchasing eddy current sensors, the customer measured the thermal growth of engines using non-contact laser triangulation sensors. However, these proved problematic. Although the laser sensors were accurate enough, they were not reliable enough when operating in the harsh conditions that the engines were tested in, where dirt and oil are present. If the optical transmitter or receiver became dirty or smeared with oil, the accuracy of measurements would suffer and results could be unreliable.

The sensors also needed to be mounted close to the engine block at a distance of 10 to 30mm. The laser sensors were sensitive to temperature variations, particularly at the high end of the temperature scale. The customer therefore looked for an alternative displacement measurement sensor that was reliable and robust enough to cope with these harsh test conditions.

After discussing their application with Micro-Epsilon at a trade exhibition, the customer was given a demonstration of the eddyNCDT 3001 eddy current displacement sensor. This sensor is an extremely compact, high performance eddy current sensor with integrated electronics. Although the sensor is similar in size to a typical proximity or inductive sensor, its measuring performance is much greater, making it ideal for OEM applications.

All conditioning electronics on the eddyNCDT 3001 are completely integrated into the sensor housing. However, the compact M12 dimensions do not mean lower performance; the sensor’s temperature-compensated electronics provides high measurement stability (0.03% FSO) even in fluctuating ambient temperatures.

Most of its bed frame thermal growth tests require sensors that can measure to accuracies of 0.05mm, which means the eddyNCDT 3001 was sufficient in this case. As well as costing less than a laser sensor, the customer also liked how the eddy current sensor was packaged, including how compact, robust and integrated it all was. The sensor can also be cabled up directly to the customer’s test cell data acquisition unit, unlike the previous laser sensors, which required an external power supply, resulting in additional cabling across the test cell. The eddyNCDT 3001 plugs in directly with one cable for power and data.

The eddyNCDT 3001 is factory-calibrated for both ferrous and non-ferromagnetic materials, which eliminates the need for any onsite calibration. The robust IP67 sensor design, combined with the true eddy current measuring principle, enables measurements in harsh industrial environments where oil, pressure, dirt or dust may be present, as well as for offshore and marine applications. The sensor has a measuring range of 4mm and a frequency response of 5kHz (-3dB).

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