A number of different forms of torquemeters have been developed for indicating the torque being transmitted through a shaft. One conventional form of such a torquemeter utilizes a strain gage mounted to the shaft. Such torquemeters provide excellent means for indicating such torque transmission and have found widespread commercial use.
In some applications, however, it is desirable to avoid any change in the mechanical characteristics of the load-bearing shaft network. One such application is in turbo machines and the like, wherein the shaft characteristics are accurately designed to optimize the shaft dynamics for efficient operation at the conventional high speeds at which such equipment is operated.
Additionally, nonintrusive torquemeters are advantageously applied in other forms of machinery wherein it is desirable to avoid repositioning of the drive elements or affect the shaft balance, shaft overload capacity, and/or shaft stiffness.
Such nonintrusive shaft torquemeters commonly are arranged to utilize the magnetic anisotropy induced in the shaft by shear stresses resulting from the applied torque.
The presently available nonintrusive shaft torquemeters of this type have the serious disadvantage of limited measurement accuracy because of spurious signals developed for a number of reasons. Such spurious signals are generated by residual magnetic nonuniformity within the shaft, the effects of mechanical vibration of the shaft, and the effects of shaft runout and associated air gap variations. Another problem which has arisen in a number of the prior art structures is the residual variations in output resulting from variations in the shaft position defining the zero point. Such variations in the zero point positioning cause significant measurement uncertainties.
Another serious problem of the known nonintrusive shaft torquemeters is the adverse effect on accuracy caused by axial and radial shaft temperature gradients. Such temperature gradients commonly exist in shaft systems wherein power consuming and power producing devices are located at different points along the shaft.
The use of spaced windings and associated magnetic structures around the circumference of the shaft has been one attempted solution but has been found not to completely integrate out the signal variations caused by the above discussed anomalies and bending of the shaft in use.