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Digital Rotor
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| Photo 1. Accumetrics AT-5000 Series digital telemetry exemplifies the simplest and easiest to use digital rotor telemetry available today. |
Sensors often must be mounted within a structure. For example, strain gauges on turbine or fan blades are the only way to fully assess vibration problems and understand dynamic characteristics in machines ranging from turbines to windmills. Thermocouples embedded in rotor metal may be the only way to reliably detect overheating in equipment as diverse as electric motors and aircraft wheels, and accurate torque measurements still require strain gauges to be mounted directly on shaft sections. Sensors are sometimes mounted on rotors for short-term testing to solve mechanical design problems, validate the performance of a new design, or calibrate analytic models. Continuous monitoring of rotor parameters can also serve as part of protective relay systems or provide data for preventive maintenance strategies.
When electrical sensors are mounted on rotating structures, the major challenge is getting signals off the rotor. Direct wire connection is not possible. Sometimes slip rings are used, but not without problems. The sliding contacts have a limited operating life and produce electrical noise as they wear, often obscuring or corrupting the meaningful information from the low-level sensor signals. Moreover, slip rings usually are practical only when sensor wires can be routed to the end of a shaft where commercial instrumentation slip rings can be installed.
FM Telemetry
Radio telemetry has been used since the 1960s as a noncontact means of getting sensor signals off rotating structures. Miniature radio transmitters, often powered by batteries, can be mounted at convenient locations on the rotors and transmit signals to a nearby receiving antenna. Conventional rotor telemetry systems use analog frequency modulation (FM) transmission, usually operating in the FM broadcast band. These kinds of systems are still available, but there are disadvantages. It is often difficult to find frequencies that are free of interference from radio broadcast stations. If the temperature of the rotating environment changes, the transmitter frequencies can drift into the frequencies of other stations. As the rotors turn, there are sometimes “dropouts,” angular positions where the received signals become too weak due to the cancellation effects of multipath transmission. As an analog process, the FM transmission technique is often subject to drift phenomena that will shift the calibration factors of the measurement.
Photo 2. In some cases, components can be simply taped onto rotating shafts using special high-strength tape.
More complex modulation techniques are sometimes used to minimize these drawbacks. FM-FM, where the signal is first frequency-modulated on a low-frequency carrier and then on the radio frequency carrier, works for low-bandwidth (slowly changing) sensor signals. Pulse width modulation-FM first modulates samples of the signal as the time duration of short pulses, and then frequency-modulates the pulses. The problem here is that when the SNR of the recovered signals gets low enough, the receiver cannot accurately detect the precise frequency. Furthermore, multiplexing highly dynamic signals in FM telemetry systems is very difficult; to transmit data from multiple signals channels, a separate transmitter and receiver for each sensor channel are often required.
Principles of Digital Telemetry
In the more demanding field of spacecraft and aircraft communications, FM has for the most part given way to digital telemetry techniques. The fundamental advantage is that the receiver need only distinguish a one from a zero to fully preserve the integrity of the data. Understanding digital telemetry requires a grasp of the fundamental concepts of digital data acquisition.
Time-varying sensor signals may be acquired and processed in computers by first sampling the waveform and then digitizing it. Sampling is performed at a rate that is determined by the signal bandwidth. In most digital acquisition systems, signals are sampled at a rate anywhere between four and ten times per cycle of the highest frequency of interest. Higher frequencies are removed with low-pass filters. The sampled representation of the signal is then converted into a digital code. The resolution of the conversion is determined by the number of bits. Twelve bits, for example will resolve a signal range into 212 = 4096 discrete steps, giving a resolution of ~0.025% of range.
In digital telemetry systems, as with many other digital processing systems, multiple signals are often combined by a multiplexer before digitizing. This allows many sensors to be combined into one composite digital data stream before transmission, and for a single transmitter and receiver pair to be used for multiple sensor channels. Once in digital form, the bits are organized for serial transmission. This organization must include provisions for synchronizing the receiver to the digitizing process. “Frame synchronizing” bits are usually combined with the data to allow the receiver to recognize the repetitive sampling process (called a frame) and proc ess the information accordingly.
Figure 1. The AT-7000 modular digital telemetry system offers the flexibility to configure any mix of sensor types, as shown in this 16-channel application.
There are several ways to modulate digital data on an RF carrier. Common techniques include frequency-shift keying, where the carrier shifts between two discrete frequencies depending on the state of the digital data. Phase-shift keying modulates digital information by shifting the phase on the carrier. on/off keying simply turns the carrier on and off based on the state of digital data. In simple modulation techniques, one of these keying processes is used once per bit period. There are also more complex modulation techniques that involve more than one transition of the digital state in each bit period.
One of the most challenging requirements of rotor telemetry is that of providing power to the rotating transmitter modules, a proposition often made more difficult by the fact that common sensors such as strain gauges also need electrical excitation. Batteries can be used for short-term tests, assuming installation and replacement is feasible. In most other cases, induction power is preferred. Power is supplied from an RF source in the nonrotating equipment and coupled to the rotor through a rotary transformer consisting of a primary winding or loop that surrounds the rotor and a secondary winding mounted on the rotor. These windings are configured to provide continuous power to the rotating system, independent of rotor speed or position.
Examples of Digital Telemetry
Digital telemetry is used in applications ranging from simple one-channel configurations to complex rotor measurement systems with more than 600 channels. A few examples will serve to illustrate this point.
Figure 2. For long-term multichannel rotor telemetry, a clamp-on shaft collar is often used.
Accumetrics’s AT-5000 Series (see Photo 1) digital telemetry exemplifies the simplest and easiest to use digital rotor telemetry available today. This battery-operated single-channel system uses a very small transmitter module (1 by 0.875 by 0.3 in.) that can be easily mounted on shafts and other rotating components where space is limited. Designed to operate up to 400 hr. on a miniature 3 V lithium cell, the transmitter consumes only 6 mW of DC power but nonetheless offers considerable functionality. The miniature transmitter’s filtering and sampling circuitry provides 7800 sps, 12 bit digital conversion, and RF transmission. The unit is available with either internal or external transmitting antennas and has been used on shafts ranging from 1 in. to almost 4 ft. in diameter. In many cases, AT-5000 transmitter and battery components are mounted in clamp-on collars custom designed for a particular sized shaft. They can also be simply attached by means of special high strength tape (see Photo 2), allowing quick and easy installation on almost any size of rotating component without the need to machine specialized housings or collars.
Some of the more interesting recent applications of the AT-5000 include:
• Measuring torque on the driveshaft of a 60,000 hp water pump to confirm pump efficiencies
• Measuring torque on the halfshafts and driveshafts of new automobile models currently under development
• Measuring the tire bead temperatures on C130 aircraft during taxiing and landing operations
In other applications, it is necessary to acquire signals from many rotor-mounted sensors. Digital telemetry facilitates multiplexing large numbers of sensors into high-speed data streams. This benefit eliminates the need for many different sets of transmitters and receivers operating at different frequencies, as are often required by older FM technology. Of course, each multichannel application requires a different mix of sensors, so these must be flexible enough to adapt to the application.
The AT-7000 system architecture is designed for multichannel rotor telemetry. Modular in design, these systems can be assembled with a collection of rotor-mounted modules that combine the correct number of sensor channels for the application. Figure 1 shows an example of a 16-channel system that combines eight thermocouples with four strain gauges and four pressure transducers. The building blocks for this system are miniature data acquisition modules. The temperature acquisition module multiplexes and digitizes eight thermocouples along with cold junction compensation sensors. Each dynamic acquisition module conditions, multiplexes, and digitizes four sensors that can be either full bridges (pressure transducers) or, with an external bridge completion module, quarter-bridge strain gauges. The operating sensor bandwidths and sample rates for all these data acquisition modules are controlled by the master control and digital telemetry module (MCDTM) and may be customized to the needs of each application.
Photo 3. In some cases, the electronics packaging may be on the centerline of rotation, as in this truck wheel application. (Courtesy of Accuride Corp., Henderson, KY.)
All the data are combined over a digital bus into a high-speed digital data stream and then modulated in the MCDTM for transmission off the rotor. All the modular components are prepackaged into a rotor-mounted system designed for the application. Often, a clamp-on shaft collar is used (see Figure 2), or the electronics packaging may be on the centerline of rotation as in the truck wheel system shown in Photo 3.
The AT-7000 modular telemetry system has been used to:
• Measure blade flex on a power generating windmill
• Monitor the cage and winding temperatures in a hydroelectric generator
• Measure strain, pressure, and temperature on the connecting rod of an operating diesel engine
• Measure wheel forces and temperatures on automotive and railway wheels
Applying Rotor Telemetry Systems
Digital telemetry offers much greater noise immunity and stability than was possible with the older FM telemetry technology. Digital systems typically require neither tuning nor calibration, making them much easier to use.
Among the first issues to be addressed in planning a telemetry application is where on the rotor to place the components. It is often desirable to mount them near the sensor location. In some applications this is very straightforward. In others, there is no space available. In installations where the sensor’s location is well above the 125ºC maximum limit for most telemetry components, the sensor leads must be routed to a more practical site.
The rotational speed and mounting diameter determine the centrifugal load applied to telemetry components. Most will typically withstand 10,000–20,000 g’s, but some high-speed applications are characterized by much higher g forces and require special packaging.
Providing power to operate the telemetry components is often an important issue and typically requires close collaboration between the user and the equipment manufacturer. Battery power is generally the easiest to apply, but has obvious limitations. For continuous duty or long-term testing, induction is preferred. Induction power, however, requires that the rotor and stationary coils be relatively closely coupled. Typical gaps between the coils range from 0.25 to 0.50 in. (6–12 mm). 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. 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 not so easily as can be accomplished with typical benchtop instruments. Careful planning and coordination are the watchwords.
Summary
Digital rotor telemetry has made collecting sensor signals from rotating components a great deal easier and more reliable. Applications today range from the quick and easy test to the machinery health monitor designed to operate for the life of the machine. It is a powerful tool that has created new opportunities to design better rotating machinery and to operate within safer and more efficient bounds.
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