Digital rotor telemetry

The major challenge, when electrical sensors are mounted on rotating structures, is getting the signals off the rotor. Direct wire connection is not possible. Sometimes slip rings are employed but the sliding contacts are limited in operating life and produce electrical noise as they wear, often obscuring or corrupting the meaningful information from the low level sensor signals.

By John Reschovsky, president, Accumetrics Associates, Inc., Schenectady, NY December 1, 2001

T he major challenge, when electrical sensors are mounted on rotating structures, is getting the signals off the rotor. Direct wire connection is not possible. Sometimes slip rings are employed but the sliding contacts are limited in operating life and produce electrical noise as they wear, often obscuring or corrupting the meaningful information from the low level sensor signals. Digital Telemetry replaces Analog Since the 1960’s, traditional rotor telemetry systems used analog FM (frequency modulation) transmission usually operating in the FM broadcast band. Although they have been useful tools for many machine tests, the technique has many disadvantages, including interference and noise, tem-perature-related frequency drift, ‘drop-outs,’ and calibration shift. In the more demanding applications, FM has generally given way to digital telemetry techniques. The fundamental advantage is that the receiver needs only to distinguish a ‘one’ from a ‘zero’ to fully preserve the full integrity of the data. In digital telemetry systems, multiple signals are often combined by a multiplexer prior to digitizing. This allows many sensors to be combined into one composite digital data stream prior to transmission eliminating the need for many trans-mitters and receivers in many multi-channel applications. Once in digital form, the bits are organized for transmission in serial fashion. This organization must include provisions for synchronizing the receiver to the digitizing process. Normally ‘frame synchronizing’ bits are combined with the data to allow the receiver to repetitive process the infor-mation appropriately. The modulation of digital data on a radio frequency carrier may be done in several ways, in-cluding frequency shift keying (FSK), phase shift keying, and on-off keying (OOK) simply turns the carrier on and off based on the state of digital data. In simple modulation techniques, one of these keying processes is employed once per bit period. More complex modulation techniques are sometimes employed that involve more than one transition of the digital state in each bit period. Key Concepts in Applying Rotor Telemetry Systems Among the first issues to be addressed in planning a telemetry application is where the telemetry components may be located on the rotor. It is often desirable to mount these components close to the sensors but in some applications, space constraints or environmental conditions such as tem-perature force the user to route the sensor leads to a more practical location for telemetry components. The rotational speed and mounting diameter determines the centrifugal load applied to telemetry components. Most telemetry components will typically withstand 10,000 to 20,000 Gs. In some high-speed applications, telemetry must withstand much higher G forces and special packaging considerations is needed. Providing power to operate the telemetry components is often an important issue. Battery power is generally the easiest to apply, but has obvious limitations. For continuous duty or long term test-ing, induction power is the preferred approach. This requires that the rotor and stationary coils be relatively closely coupled, typically between 0.25 and 0.50-inches. In applications where the rotating member experiences significant radial or axial motion relative to stationary mounting surfaces, application of induction power may be extremely difficult. Finally, it is important to carefully consider the sensors to be employed. Telemetry transmitters are typically designed to be interfaced with a particular class of sensor. Changing amplifier gains or balancing bridges can be done in rotor telemetry applications, but it is not as easy as with typical ‘bench top’ instruments. Therefore, careful planning and coordination is generally required.

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