McNab Wireless Rotary Shaft Horsepower / Torque Meters
Marine Systems > Wireless Rotary Shaft Horsepower / Torque Meters
McNab Wireless Rotary Shaft Horsepower and
Torque Meters are for use in marine environments on ship propeller
shafts to measure shaft horsepower, torque, and RPM (rotations per
minute). They feature a brushless
construction and the unique data-coupling system, eliminates the need
for flimsy wire antennas or complex rotary transformer housings.
MARK II BASIC Brushless Shaft Horsepower Systems
The required spacing is 9/32 +/- 1/64 between shaft and torque meter. See 64003 for configuration. This means of providing power to the shaft eliminates the need for slip rings/ brushes and provides power at a ship’s normal 60Hz as opposed to other methods of putting power on the shaft, which typically operate at higher frequencies and could be a possible source for ship signature analysis by others.
Strain Gage Torque Sensors (64000)
Four strain gages, two on each side of the shaft, mounted 180° apart circumferentially, and electrically configured in a bridge network, sense the torque on the shaft. Mounting the gages on opposing sides of the shaft virtually eliminates any bending influences that might otherwise affect the true degree of torque on the shaft (3). A redundant set of gages is mounted alongside the active set so that if for any reason either or both sets of the first gages fail to function, the second gages may be put into service with minimum downtime. Direct shaft mounting of strain gages minimizes possible shaft compliance errors and shock and vibration influences that might otherwise occur by using fixtures to mechanically amplify the torque sensed by the gage.
No shaft modifications are required for gage mounting, which is performed by an experienced McNab service team. A better than 10:1 ratio between the typical strain on the shaft and gage strain limits insures against stress induced gage damage or deterioration. After mounting, the gages are hermetically sealed for protection against moisture and humidity.
The transmitter: 1) provides and AC excitation frequency to the strain gage bridge sensors, 2) receives an output signal from the sensor that is proportional to the torque on the shaft, and 3) conditions the signal to be coupled through capacitance to the hull mounted receiver. Capacitance signal transfer keeps the signal transmission level down to less than 10 milliwatts, to avoid interference with other electronic equipment.
The transmitter supplies an avenge 1400 Hz signal to the strain gage torque sensors. Minimal engineering standards call for a 2:1 ratio between sampling rate and shaft vibration, which on a vessel shaft is approximately 10 Hz maximum. The 1400 Hz excitation frequency has been incorporated in the system to far exceed that ratio, and to eliminate said bypass problems (4). As torque induced strain is developed on the shaft, the impedance of the bridge sensors changes. This impedance change alters the frequency of the 1400 Hz excitation frequency an amount proportional to the sensed strain, producing a torque signal proportional to the torque on the shaft. Transmitter electronics provide electrical isolation for protection against pager loading and prevent transmission loading.
The torque signal is brought off the shaft to the stationary receiver via FM/FM telemetry at 10.7 MHz using the principle of capacitive coupling: a protective metallic shield covering the sensors and transmitter transfers the low power signal off the shaft to a closely located metallic foil attached to the receiver.
The protective cover prevents EMI/RFI emissions from affecting the shaft mounted components, serves in the capacitance coupling of the signal from the shaft to the hull mounted receiver, and protects the shaft mounted components against physical damage. Better than 20 dB of shielding between EMI/RFI radiation and the shaft mounted components is provided by the cover. The cover is supplied as part of the system and is machined to match the shaft diameter down to 0.003 inches. Overall length of the system along the shaft is less than 20 inches.
The receiver accepts and decodes the signal transmitted off the shaft aid provides a differential DC voltage output proportional to the sensed torque for input to the indicator panel. After signal reception, the modulated signal is sent through the LC bandpass filter and then a ceramic bandpass filter to clean unwanted frequencies from the signal. The signal is next demodulated and further filtered by a Phase Locked Loop (PPL) circuit. The demodulated signal is at the same low frequency as the signal before the 10.7 MHz modulation performed by the shaft installed transmitter.
A second Phase Locked Loop circuit, configured as a tracking filter, creates a synthetic signal at the frequency identical to the demodulated signal: the synthetic signal is free of noise and has been amplified. This PPL principle is used on space flights, where by necessity, low level transmissions must be decoded accurately and consistently.
The filtered torque signal is then sent to an indicator circuit, which also produces an isolated differential DC output signal. Integration smoothes any spontaneous AC interference and the differential output reduces possible ground loop problems and effectively doubles the voltage output per unit torque, providing greater resolution at longer cable runs to the indicator panel without signal loss.
Signal to Noise Ratio
Signal to noise ratio reflects a receiver’s ability to differentiate between the desired signal and the unwanted interference caused by EMI/RFI noise in the environment (i.e., fluorescent lighting, motors, generators, and communication equipment all produce noise). The receiver circuitry described in the previous paragraph provides a high modulation index: 150,000:10 million versus 1500:10 million typical in many receiving systems, resulting in a signal to noise ratio of at least 40 dB (noise levels must be 10,000 tines greater than the torque signal to affect reception).
The torque outputs from the receiver unit are sent through ship cabling to an indicator. This displays torque in either ft/lbs or ft/tons.
The system is shipped pre-calibrated. No further calibration is required after the system has been zeroed. Zero calibration of the system follows the guidelines laid down in the SNAME Code for Sea Trials, 1973 (5). Verification of span calibration is performed according to universally accepted methods (6, 7, 8) and may be done without husk covers being removed. Calibration units are traceable to the National Bureau of Standards. A sea trial torque meter is not required a back-up to assure system accuracy and repeatability.
The system meets the military specification requirements for shock vibration, accuracy, and stability, environmental, airborne and structure borne noise, and EMI/RFI interference. The sensors and all electronic components are hermetically sealed for protection against moisture and humidity. All housings have been designed to stand up to the toughest marine environmental conditions.
Through the use of both power and signal telemetry between the shaft and hull mounted components, brushes and slip rings have been avoided, eliminating contacting surfaces susceptible to wear, thus reducing maintenance requirements. Exciting the torque sensor strain gage bridge with an AC signal unique to McNab and the subsequent AC coupled signal conditioning eliminates drift problems inherent to DC excitation and signal conditioning, providing unparalleled long-lasting stability.
All spares are available off-the-shelf. The military torsionmeter specification (9), prohibiting parts that trust be “pair-matched” to ensure proper system operation, as is required by other designs. It is not required to return the sensors or other components to the manufacturer in order to meet preventive maintenance requirements.
The following appendices are provided to highlight important system characteristics: