Prior technology for machinery data acquisition used slip rings, FM radio communication, or non-real-time digital communication. Slip rings are often noisy, require much space that may not be available, and require access to the shaft, which may not be possible. FM radio is not accurate or stable, and is limited in the number of channels, often with channel crosstalk, and intermittent as the shaft rotates. Non-real-time digital communication is very popular, but complex, with long development time, and objections from users who need continuous waveforms from many channels.
This innovation extends the amount of information conveyed from a rotating machine to a data acquisition system while keeping the development time short and keeping the rotating electronics simple, compact, stable, and rugged. The data are all real time. The product of the number of channels, times the bit resolution, times the update rate, gives a data rate higher than available by older methods. The telemetry system consists of a data-receiving rack that supplies magnetically coupled power to a rotating instrument amplifier ring in the machine being monitored. The ring digitizes the data and magnetically couples the data back to the rack, where it is made available.
The transformer is generally a ring positioned around the axis of rotation with one side of the transformer free to rotate and the other side held stationary. The windings are laid in the ring; this gives the data immunity to any rotation that may occur.
A medium-frequency sine-wave power source in a rack supplies power through a cable to a rotating ring transformer that passes the power on to a rotating set of electronics. The electronics power a set of up to 40 sensors and provides instrument amplifiers for the sensors. The outputs from the amplifiers are filtered and multiplexed into a serial ADC. The output from the ADC is connected to another rotating ring transformer that conveys the serial data from the rotating section to the stationary section. From there, a cable conveys the serial data to the remote rack, where it is reconditioned to logic level specifications, de-serialized, and converted back to analog. In the rotating electronics are code generators to indicate the beginning of files for data synchronization.
An alternative method would be to use two symmetrical coils. Since the two coils are rotationally symmetrical, rotation does not influence the magnetic coupling from the primary to the secondary. Since the secondary coil is electrostatically shielded, environmental noise pickup is intrinsically low. Since the transformer is air-core, the uncompressed bandwidth can be high — 50 MHz, 200 MHz, or higher.
The rotating coil is the primary component of the transformer and is in the shape of a thin ring, containing a few turns of wire. The plane of the ring is perpendicular to the axis of rotation. Radially, just beyond the rotating primary coil, is the secondary coil in the shape of a ring, and lying close to the primary. The secondary coil is a single turn of coaxial cable with the center conductor connected to the shield of the cable where it leaves the coil. The binary data are fed into both ends of the primary coil through an impedance matching resistor, with one end receiving the data inverted. This double-ended (full-bridge) approach reduces propagation delay distortions and increases signal strength. The secondary coil has an impedance matching resistor at the end of the cable. Use of a coaxial cable reduces capacitive coupling, but freely allows magnetic coupling. To enhance the coupling, ferrite cloth can be laid into a groove and the primary coil wound on top of it. Similarly, ferrite cloth can be formed around the secondary coil. Copper rings can be placed on either side of the coil set to reduce outside influences.
This work was done by Elmer Griebeler, Nuha Nawash, and James Buckley of Glenn Research Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18575-1/7-1.