1. Field of the Invention
The present invention relates to the field of torque measurement; and more particularly, to means and method for determining the amount of torque that occurs in a stationary or rotating shaft.
2. Description of the Prior Art
When torque is applied to a shaft, two principal lines of stress are induced along helical lines which are orthogonal to each other on the surface of the shaft. Various different methods of torque measurement have been available, but no method has been totally satisfactory. Two common methods of measuring torque, strain gage and optical, are well described in the literature. See "Sensor and Analyzer Handbook," by Harry Norton, Prentice Hall, 1982, pp 131-142.
Torque is most accurately measured by bonding strain gages in a cross arrangement along the helical lines of compression and tension. The strain gages are electrically configured in a balance-bridge and coupled to measuring electronics via slip rings or noncontacting rotary transformers. Generally, these cross arrangements are difficult to implement and usually require custom installation. In another variation, disclosed by Gurenko and Krutkis in Soviet patent 2,493,268, a light source is used to couple the gage signal to a stationary photodiode.
In optical torque transducers, light beams, code patterns and light sensors convert the differential angular displacement between two positions on a shaft, due to applied torque, into an output signal. Specifically, identical patterns made of light reflecting strips are arranged circumferentially around the shaft at two locations. The patterns are illuminated by laser diodes and the reflected light is sensed by photocell. The output of each photocell is a pulse train and the phase difference between them is a measure of the torque. In a similar device, by Kawamoto, U.S. Pat. No. 4,767,925, a pair of light emitting and receiving elements produce an output depending on the amount of light transmitted due to the relative rotation of two slotted disks. Levine in U.S. Pat. No. 4,433,585 discloses passing a beam of light through two diffraction gratings placed at different locations along a shaft and sensing the phase of the two resulting beams. These are not robust devices as they requires exact alignment. U.S. Pat. No. 5,001,937 to Bechtel et al. discloses an optically based torsion sensor that measures the phase displacement between two bands of alternating high and low reflectivity regions. A major drawback of this device is its dependence on the initial alignment of the two bands. In addition, minor differences in the rise time of detecting electronics will cause serious errors in measurement. U.S. Pat. No. 4,525,068 to Mannava et al. discloses a torque sensor utilizing optical Doppler measurements. Since Doppler measures velocity only, this device suffers from a very serious shortcoming in that it must infer torque from changes in instantaneous rotational velocity of two different sections of a shaft.
Two optical methods for measuring strain of an object are noteworthy. U.S. Pat. No. 4,939,368 to Brown discloses an optical grating to measure strain in a stationary object. The device is complicated in that it requires two frequencies of light and has no provision for measuring a moving object such as a rotating shaft U.S. Pat. No. 4,432,239 to Bykov discloses an apparatus for measuring the deformation of an object. The device utilizes an electrooptical frequency modulator to produce two components from an incident laser beam. A polarization splitter further splits the light into two different frequencies which illuminate a diffraction grating on a stationary object. This device is complicated and expensive, and has no provision for measurement of a moving object such as a rotating shaft.
The literature discloses a capacitive torque sensor consisting of two encoders either mounted perpendicular to the shaft at each end, or mounted circumferentially at two closely placed points along the shaft. See "Interest in Misfire Detection Technology Grows", Automotive Electronics Journal, Nov. 6, 1989, pg 12. Each encoder has two parts: a stator that consists of up to 256 radial fingers that are alternately charged; and a rotor that is mounted on the shaft. As the shaft turns, the rotor's potential switches between positive and negative at a frequency proportional to speed. A disk, in the center of the stator, electrically isolated from the charged fingers collects the signal. Like the optical torque sensor, the twist of the shaft is determined by measuring the phase difference between the two encoders. Also like the optical sensor this device requires exact alignment.
Finally, magnetic torque sensors comprise much of the prior art. The magnetic properties of most ferromagnetic materials change with the application of stress to such an extent that stress may be ranked with field strength and temperature as one of the primary factors affecting magnetic change. Magnetostriction is a measure of the stress sensitivity of a material's magnetic properties. Magnetic based torque sensors take advantage of the magnetostrictive properties of ferromagnetic metals, such as carbon steel. See "Noncontact Magnetic Torque Transducer," Sensors, 11/90, pp. 37-40. These sensors make a contactless measurement of changes of magnetic permeability in shaft materials, which are caused by torsional stress. In place of strain gages, magnetic flux is directed into the shaft and along the helical lines of compression and tension. A positive magnetostriction shaft experiencing torsion will exhibit increased permeability along the line of tension and decreased permeability along the line of compression. At low stress levels the permeability is nearly linear with stress, but varies dramatically at high stress. Another drawback of a magnetostrictive torque sensor is in the need for calibrating it individually with each shaft This requirement is obvious because the torque measurement is made by means of the magnetostrictive properties of the shaft material and cannot be predetermined in the manufacture of the sensor by itself. The variability in magnetostrictive properties is usually correlated with the variability of the mechanical hardness of the material. Hardness variability of shaft materials typically ranges from +10 percent to +40 percent. The shaft-to-shaft variability problem has been addressed in recent research by adding either a sleeve or coating of a well defined and magnetically soft material, such as nickel, permalloy, or ferromagnetic amorphous alloys. While this approach shows promise, installation can not be made in situ, and all magnetic materials, even the softest, can retain some magnetism leading to nonlinearities and drift.
There is a need for an accurate, simple, noncontact sensor for measuring torque in a stationary or rotating shaft.