Driveshafts deliver power as a rotary force, and in most applications there is a need to know the amount of power in the system. But getting measurements from a turning shaft requires some engineering ingenuity, so here Tony Ingham from Sensor Technology in Banbury runs through the basics.
However, there is a problem in that electrical leads connected to the strain gauges would wrap around the shaft as it turned and eventually snap. Fortunately, there are solutions to this, which we will look at later.
The circuitry is completed with a power supply, amplifier and a display, recorder or computer. These are usually mounted somewhere close to the driveshaft in a secure, static location.
Unfortunately, the slip rings are rather delicate; so much care must be exercised in their use. Also, they need to be set up with some precision so that a constant and even contact is maintained in operation.
Because of brush wear, slip rings need regular attention and they are not really suitable for long-term use, or for deployment in harsh working environments. It is also notable that the contact between the stationary brushes and rotating collars will create a degree of electrical noise which, particularly at higher speeds, will interfere with signal transmission. A final shortcoming of slip rings is that they create a drag force, which must be accounted for in signal measurements and frequently checked to make sure that it has not changed in value.
There is no physical contact between the coils, nor between shaft and transducer, yet power and signals are passed between them. This overcomes many of the drawbacks of slip ring systems; set up is easier, operation is more robust, there is no friction and higher speed operations can be accommodated.
However, every rotary transformer will have a maximum operating speed due to its inertia. They are also susceptible to noise and errors, especially if the coils become misaligned.
Installation and maintenance are straightforward because there is no slip ring to adjust, coils to align or cabling to accommodate. However, a battery is required to power the signals, which although small and long lasting, does require consideration.
The wave frequency is dependent upon the spacing of the teeth in the array and the direction of wave propagation is at right angles to the teeth. Therefore, any change in its length, caused by the dynamic forces of the shaft's rotation, alters the spacing of the teeth and hence the operating frequency. To measure the torque in a rotating shaft, two SAW sensors are bonded to a shaft at 45° to the axis of rotation. When the shaft is subjected to torque, a signal is produced, which is transmitted to the adjacent stationary pick-up via the RF couple.
Interestingly, SAWs were first detected by 19th century gentleman–scientist and Nobel Laureate Lord Rayleigh when he was investigating the cause and effects of earthquakes.
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