Sensor Considerations and Noise

Sensor Considerations

Slip Rings can transmit virtually any type of sensors. No signal transformation or change occurs to the signal inside the slip ring. However, some considerations need to be given to various sensors for accurate signal transmission.

Thermocouples

Thermocouples are commonly used on rotating hardware due to their simplicity and robustness. Any type ofthermocouple can be transmitted through a slip ring. However, the user must be aware that the thermocouple alloy material does not continue in the slip ring and that the rotating reference junction is created where the thermocouple alloys are terminated to the slip ring input PCB. Because thermocouples only measure the difference between the hot junction and rotating reference junction, the temperature at the reference junction must also be measured. This is commonly done using a rotating RTD or thermistor located on the input PCB. Aerodyn PCBs are designed to accommodate these sensors. These sensors must be considered in the selection of the overall channel count for the application. Inside the slip ring, Aerodyn uses the same alloy for ring and brush contacts so that no thermoelectric potential is created at this location. Thermocouple signals are then read as millivolts at the output of the slip ring, and correction for rotating reference temperature must be performed. Consult Aerodyn for assistance with this algorithm.

Various configurations of strain gages are used on rotating hardware. These include quarter,half, and full bridges; static and dynamic applications; and constant current or constant voltage excitation. All these can be transmitted through the slip ring. These configurations require either two-, three-, or four-wire channel configurations in the slip ring.

Quarter bridge dynamic sensors are very commonly used for airfoil vibratory measurements.These are two-wire sensors usually with constant current excitation.Common excitation levels are 10mA to 25mA. Aerodyn slip rings can transfer up to 250 mA per contact.

Frequency bandwidth is not limited with slip rings up to megahertz ranges.

Common RTDs used are 1000 ohm or 100 ohm thin film, platinum element sensors.These can be configured in two-,three-, or four-wire configurations. Aerodyn slip rings have relatively low series resistance, typically .5 ohm or less per contact,enabling two-wire use of RTDs. If higher accuracy is needed, Aerodyn would recommend a four-wire configuration.

For rotating reference temperature measurement, Aerodyn typically uses 1000 ohm RTDs placed at the same radius as the wire terminations. These are then monitored through the slip ring in a four-wire configuration.

Pressure transducers and strain gage-based sensors (such as shaft torque, bending moment, and tension) are common full bridged transducers transmitted by slip rings. These are typically four-wire configurations and no special considerations are needed.

Accelerometers, proximity sensors, and many other electrical-based transducers have been successfully transmitted through slip rings. Consult Aerodyn for any special considerations for these types of sensors.

Noise

Aerodyn Engineering’s slip rings transfer very clean and quiet signals from rotating sensors to stationary data systems. Our gold alloy contacts provide very quiet signals, long life, and high contact density. Dynamic noise of typical Aerodyn slip rings is less than 1 m at any discreet frequency. Each slip ring is spun up to operating speed and each channel is tested for a clean signal in our test facility.Frequency bandwidth of slip rings is typically not limited until MHz range. Cross talk, channel to channel, is typically -55 dB or greater.