Torque or Angle of Torsion Measuring Device

The present invention relates to a torque or angle of torsion measuring device with at least one pair of screening elements facing each other and concentrically arranged about a torque transmitting shaft, each element comprising a plurality of conductive zones, one of the elements of said pair being coupled to an end of the shaft, the other element being coupled to the other end of said shaft and at least two coils concentric to the shaft being provided in the vicinity of said elements and separated by these elements.

Such a device for converting an angle of torsion of a shaft into a digital code is known from the publication "Measurement Technique", Vol. 21, No. 7, July 1978, New York (U.S.), p. 930-934. However, the screening elements of the pair are provided with a different number of conductive zones and the device operates in accordance with the principle of the displacement of moire fringes for producing a digital output signal.

The patent GB No. 2 097 131 discloses a torque transducer comprising two pairs of wings, the wings of each pair facing each other and having a determined relative angular shift. The two pairs of wings permit a differential measurement. To each pair of wings is associated a coil in intimate contact with one of these wings. However, the relative angular shift of the wings of a pair causes a variation of the impedance of the associated coil such that in spite of the differential effect, the precision of the measurement is limited.

Further, there is known from the U.S. Pat. No. 4,356,732 a device in which the zones of the two elements are in alignment for a given initial torque. The coils are supplied with a high frequency signal and they induce eddy currents in the conductive zones of the elements. The energy which is necessary for creating these eddy currents is delivered by the coils which in turn produces a change of the impedance of these coils. The establishment of the eddy currents is in turn influenced by the relative angular shift of the elements when a torque is applied to the shaft, that is by the opening or closing of the non conductive regions between the conductive zones of the elements. Thus, the measurement of the variation of impedance of the coils permits to determine the relative angular shift of the elements and thus the torque applied to the shaft. However, like in the preceding case, in such a device, the precision of the measurement is limited because of the relatively weak variation of impedance of the coils, even for an angle of torsion as high as e.g. 20.degree. C. Further, the coils are sensitive to the variations of temperature so that the variation of impedance caused by the relative angular shift of the elements depends also on the temperature.

It is therefore an object of the present invention to realize a torque or angle of torsion measuring device which does not present the above mentioned disadvantages and which is of relatively simple electric construction. For solving this problem, the device according to the invention is characterized in that one of the coils is a transmitting coil, at least one other coil being a receiving coil for delivering a signal having an amplitude which depends on the relative angular shift of the screening elements when a torque is applied to the shaft. Because the device comprises a transmitting coil and one or more receiving coils separated by variable shielding elements, the signal from the transmitting coil is transmitted inductively to at least one receiving coil. The pair of shielding elements arranged between the transmitting and the receiving coil behaves like a shield the shielding effect of which is variable according to the relative angular shift between the elements. The preceding shows that the device according to the invention does not operate on the principle of creating eddy currents causing a variation of impedance of a coil. The transmitting coil is supplied by a constant high frequency current so that the temperature has no effect on the measurement. Likewise, the receiving coil is connected into a high impedance rectifier circuit delivering a signal which is practically independent from the temperature. This ensures an excellent temperature stability of the device, even without any differential arrangement. Moreover, the useful output signal of the device is not simply a relatively weak variation of a basis signal but a signal which varies within a large domain, i.e. of a factor of 1 to 10 between a relative position of the screening elements such that the transparent regions between the conductive, opaque zones of one element are covered by the conductive, opaque zones of the other element and a relative position of the screening elements such that the conductive, opaque zones of both elements overlap. Thus, the useful signal delivered by the device varies over a large range and it can reach high values.

FIG. 1 shows a diagram illustrating the principle of the device,

FIG. 2 shows an embodiment of the device of FIG. 1,

FIG. 3 shows a form of execution of the disks of the device of FIG. 1,

FIG. 4 shows schematically an arrangement of the device permitting a differential measurement,

FIG. 5 shows the relations between the relative angular positions of the sectors of the disks of the device of FIG. 4, and

FIG. 6 shows the connection of the coils of the device of FIG. 4.

FIG. 1 shows the principle of the device which comprises a set of coils T with a transmitting coil 1 and a receiving coil 2. The coils are separated by disks 3 and 4 facing each other and arranged at a small distance from each other. One of the disks is coupled to one extremity of a shaft which transmits a torsion couple to be measured and the other disk is coupled to the other extremity of this same shaft. Each disk comprises a plurality of conductive sectors 5 non transparent for electromagnetic waves separated by transparent sectors 6. These sectors are arranged according to a regular configuration on the internal face of the disks as shown in FIG. 3. The sectors may form a moire pattern on the disk. In the example of FIG. 1, the arrangement of the disks is such that in the absence of any torque acting on the shaft, the non transparent sectors 5 of one disk and the transparent sectors 6 of the other disk overlap. In this case, the shielding effect of the set T is maximum and the signal transmitted to the receiving coil 2 is minimum. The transmitting coil 1 is supplied by a high frequency generator 7 through a resistor 8 of high value such that the coil 1 is practically supplied by a constant high frequency current. Consequently, the current in the transmitting coil is not influenced by the variations of this coil as a function of the temperature and the electromagnetic field transmitted to the receiving coil is independent of the temperature. The receiving coil is connected by a resistor 9 of high value to a conventional detector circuit 10. The detector circuit has a high impedance and the intrinsic relatively weak variation of the receiving coil 2 as a function of the temperature has no practical influence on the signal at the output of the detector.

FIG. 2 shows an example of execution of the device of FIG. 1. The device comprises a torsion shaft 11 with a ball bearing 12 at each extremity. The torsion couple to be transmitted and measured is applied at one extremity of the shaft, i.e. at Me and it is transmitted to the other extremity at Ms. The disks 3 and 4 are secured respectively on connecting tubes 13 and 14, the tube 13 is secured at the torque receiving extremity Me of the shaft 11 and the tube 14 at the torque delivering extremity Ms of the same shaft. The ball bearings 12 which support the shaft 11 are mounted in a support having two parts 15A, 15B of insulating material. These parts 15A, 15B are coupled by means of a cylindrical metallic piece 16 forming shielding against the external electromagnetic influences. The transmitting coil 1 is mounted in a recess of the internal lateral side of the support 15A, in front and at close proximity to the disk 3, the central radius of the coil corresponding to the central radius of the disk. The receiving coil 2 is mounted in a recess of the internal lateral side of the support 15B, in front and at close proximity to the disk 4. The coils 1 and 2 are facing each other and they are centered on the shaft 11 as well as the disks 3 and 4. The whole is mounted in a frame not represented. The configuration of the device according to FIGS. 1 and 2 is stable with respect to the influences from the environment. The important criterions for the design of the shaft 11 are the stiffness and the permissible load. A relative twist of 0.5.degree. under normal load represents a good compromise between a great stiffness, that is a small oscillation of torsion of the whole, and a sufficient output signal. By increasing the number of the sectors of the disks, it is in principle possible to obtain a usable output signal for an smaller angular shifting.

FIG. 4 shows an example of execution of the device which permits a differential measurement of the torque or the angle of torsion. Such a measurement permits to improve the temperature stability and the stability with respect to the constraints from the environment. In this case, two pairs of sectors comprising disks 3,4 and 3',4' are provided. The disks 3 and 3' are coupled to the torque receiving extremity Me of the shaft 11 by means of a connecting tube 13' and the disks 4 and 4' are coupled to the torque delivering extremity Ms of the shaft 11 by means of a connecting tube 14'. A transmitting coil 17 is mounted in the centre, between the pairs of disks 3,4 and 3',4'. Receiving coils 18 and 19 are mounted opposite to the transmitting coil, at the external side of the disks 3 and 4'. The coils 17, 18 and 19 are concentric to the shaft 11. FIG. 5 shows that the sectors of the disks 3 and 4 are mutually shifted of a quarter of a pitch p as well as the sectors of the disks 3' and 4'. Likewise, the sectors of the disks 3 and 3' and the sectors of the disks 4 and 4' are mutually shifted of a quarter of pitch. Under these conditions and when a torque is applied to the shaft 11, the relative angular displacements of the sectors of the pairs of disks, represented by arrows, are such that the sectors of the disks 3 and 4 move away from each other while the sectors of the disks 3' and 4' draw nearer to each other, and inversely if the torque is applied in the other direction. FIG. 6 shows the bridge circuit utilized for the differential measurement. The receiving coils 18 and 19 form a measuring bridge with the resistors R1 and R2, an measuring instrument M being connected in the diagonal of the bridge. A high frequency signal generator 20 feeds the transmitting coil 17 through a resistor 8 of high value.

It is to be seen from the preceding that the device according to the invention is of simple design and that it has the advantage of being particularly stable with respect to the temperature and to be protected against the influences from the environment. It delivers a usable output signal the value of which being variable within a large domain.