GB2041519A - Gyroscopic apparatus - Google Patents

Abstract

Gyroscopic apparatus for determining a cardinal compass direction includes a two-axis rate gyroscope 50 supported on a turntable 53 with one sensitive axis Y horizontal. The other sensitive axis X is inclined at a small known angle to the horizontal. Pick-off means (XP. YP, Fig. 2, not shown) detect movements of the gyroscope about its sensitive axes and the pick-off outputs are passed to circuit means (30, 31, 32, 33, Fig. 3, not shown) which indicate the sense of any misalignment of the gyroscope from the required compass direction. The turntable 53 may then be rotated to bring the appropriate axis of the gyroscope into alignment with the required direction. <IMAGE>

Description

SPECIFICATION Gyroscopic apparatus This invention relates to gyroscopic apparatus, and in particular to such apparatus intended for North-seeking.

It is well-known to use gyroscopic apparatus for the determination of North by detection of the Earth's rotation. In general, however, the procedure is complex if simple equipment is used, and the determination of North to a given accuracy can take a long time.

It is known to mount North-seeking gyroscopic apparatus on a turntable such as a theodolite head, and to operate the apparatus such that an indication of the misalignment from North is given on a meter of other indicator. It is then necessary to rotate the turntable through the angle indicated, in the correct direction, to obtain the true North direction. Such a device obviously requires an accurately calibrated scale on the turntable, which adds to the cost of the turntable. It is also known to use a single-axis rate gyroscope mounted in a pendulous suspension so that its sensitive axis is maintained horizontal. This makes it unnecessary for the turntable to be perfectly level and stable, but does not overcome the problems set out above.

It is an object of the invention to provide gyroscopic apparatus for the determination of North which is simple in construction and easy to use without the need for calibration.

According to the present invention there is provided gyroscopic apparatus for the determination of a cardinal compass direction, which includes a rate gyroscope having two orthogonal sensitive axes, support means rotatable about a vertical axis and arranged to carry said gyroscope such that a first one of its sensitive axes is horizontal and pointing nominally in said cardinal compass direction and such that its other sensitive axis is inclined about the first sensitive axis at a known small angle to the horizontal, pick-off means responsive to movements of the gyroscope about said two sensitive axes, and circuit means responsive to the outputs of the pickoff means and arranged to provide an indication of the sense of any misalignment of said first axis from said cardinal compass direction.

The rate gyroscope may be supported by a pendulous suspension arranged to maintain the desired orientation of the two sensitive axes.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the relative alignments of the gyroscope axes and the cardinal compass directions; Figure 2 is a schematic isometric drawing of a two-axis rate gyroscope; Figure 3 is a circuit diagram of one form of circuit means; Figure 4 is a circuit diagram of an alternative circuit to that of Fig. 3; Figure 5 is a diagram of a pendulous suspension; and Figure 6 is a circuit diagram according to an alternative embodiment.

Referring now to Fig. 1, the main axes shown are the vertical axis V, the North-South axis NS and the East-West axis EW. In addition the diagram shown the X and Y axes of the two-axis rate gyroscope, these axes being mutually perpendicular. The Y axis is maintained accurately horizontal and nominally pointing in an East-West direction. The misalignment from that direction, which is usually small, is denoted by the angle y. The X axis of the gyroscope therefore also deviates from the North-South direction by the same angle y.In addition, however, the X axis is rotated about the Y axis so as to be inclined to the horizontal at a small known fixed angle ss. The gyro spin axis Z is inclined to the vertical by the same angle ss. Finally, the latitude of the location of the gyroscope is denoted by the angle a. This is, of course, a variable angle.

Fig. 2 illustrates the schematic form a conventional two-axis rate gyroscope. Referring to Fig. 2, a gyroscopic mass GM is rotated at speed by a motor (not shown) and is mounted in an inner gimbal IG pivoted about a horizontal axis, the Y axis, in an outer gimbal OG.

The outer gimbal is itself pivoted about an axis X perpendicular to the Y axis. The X axis carries a pickoff XP and a torquer XT, and the Y axis similarly carries a pickoff YP and a torquer YT. As is usual with a two-axis rate gyroscope the output of the pickoff XP is applied to an amplifier Al to provide both the X axis output XO and the input to the Y axis torquer YT. In a similar manner the pickoff YP is connected to a second amplifier A2 whose outpu.t.forms both the Y axis output YO and the input to the X axis torquer XT.With the rate gyroscope supported for rotation about a vertical axis with the alignment of the X and Y axes as described with reference to Fig. 1, and assuming the earth's angular velocity to be Sl, then the outputs of the two amplifiers are as follows: XO = u cos (a + ss) cos y and YO = u cos a sin y If the gyroscope is being rotated about the vertical axis, in order to correct the misali,n- ment of the Y axis from the East-West direction, at an angular velocity ss . This turning does not affect the output YO, since it is perpendicular to the Y axis.However, due to the inclination of the X axis to the horizontal, this turning is coupled to the X axis, and the output XO is now given by XO = S2 cos (a + ss) cos" + ft sin ss Fig. 3 is a circuit diagram of one form of circuit means to which the two outputs XO and YO from the gyroscope are applied.

The YO output is applied to a first signal processing unit 30 which divides the input by a constant factor. The other output XO is applied to a second signal processing unit 31 which subtracts a constant quantity from it.

The outputs of the two units are added together and applied to the input of a summing amplifier 32, arranged to have a long time constant. The output from the amplifier is applied to an indicating meter 33.

The constant dividing factor of unit 30 is arranged to be equal to Q cos a, such that the output of the unit 30 represents the quantity sin y. If the angle of misalignment is small then the output of the unit 30 is proportional to the angle y itself. The constant quantity which is subtracted from the XO input by unit 31 is arranged to be equal to 53 cos (a + p).

Since the angle y is small if the apparatus is aligned approximately, then cos y is approximately unit. Hence the output of unit 31 is approximately equal to y sin ss.

If the only input to the amplifier 32 was the output of unit 30, which is proportional to the angle y, then the output of the amplifier would also be proportional to the same angle.

However, the output of unit 31 is also present, and, if integrated, will provide a measure of the angle through which the apparatus is rotated in azimuth. The amplifier 32 acts as the integrator, and the two inputs are applied to it in such a way that one tends to offset he other.

In operation, the apparatus is left undisturbed until the output from amplifier 32 reaches a steady value. The output of the amplifier applied to meter 33 gives an indication, though not a measure, of the actual misalignment yO. This output is almost entirely due to the input YO to unit 30. because the input XO, in the absence of any rotation of the apparatus, is aimost entirely cancelled by the unit 31.

If the apparatus is now rotated about a vertical axis in such a direction as to reduce the misalignment, the input XO becomes XO = Qcos(a + ss)cosyO + Asinp The first term is continuously nulled by the unit 30 and the second term is integrated by the amplifier 32, giving an instantaneous output Yc representing the amount of correction applied. This is of opposite sense to an output to already present, and hence the actual amplifier output as indicated by the meter 33 is the difference (YO-Yc) Thus the apparatus may be rotated until this difference is zero, at which time the Y-axis is accurately aligned with the East-West direction.The X axis of the gyroscope, though not horizontal, is pointing accurately in the North-South direction. The approximations used above with regard to the values of zin y and cos y become more accurate as the misalignment is reduced to zero.

The change of value of Yc is instantaneous, and the alignment process is therefore rapid once the initial output of amplifier 32 has reached a steady value.

The units 30 and 31 may take many forms.

The simplest form of divider 30 is a potentiometer arranged to divide the input YO by the factor 52 cos a, which is constant at any particular latitude. The potentiometer would, ideally be variable to provide for different values of the angle a. Similarly, the unit 31 may be a summing amplifier arranged to combine a preset voltage of - Q cos (a + P) with the XO input of 52 cos (a + ss) cosy + i sin ss. In fact the amplifier 32 may be used for this purpose, and the value of the potentiometer representing unit 30 may be related to the value of the amplifier feedback resistor.

Fig. 4 illustrates such an arrangement.

In the circuit of Fig. 4, the potentiometer has a value of r where r = RE2 cos a R being the resistance of the feedback resistor of amplifier 32. The other two inputs are applied to the amplifier 32 through summing resistors of equal value R1, where R, = R sinp The angle ss is constant, being built into the apparatus. Fig. 4 represents perhaps the simplest form of units 30 and 31. More complex electronic circuitry may be used.

It will be clear from the above description that the apparatus must be mounted with its X and Y axes accurately positioned, and must be rotatable about a vertical axis. If the apparatus is mounted on a stable support, then all that is necessary is to provide a suitable turntable, such as the thoedolite head already referred to. However, this need not be calibrated, since no measure of the misalignment is provided. In cases where the stable support cannot be provided it is necessary to suspend the apparatus by means of a two-axis pendulous suspension. Such a suspension forms the subject-matter of our copending patent application Number 47332/78. Fig. 5 illustrates such a suspension in which the two-axis rate gyroscope 50 is suspended by a double crossleaf spring suspension. This comprises springs 51 defining the X axis and springs 52 defining the Y axis, mounted in a frame. A detailed description of this particular suspension will be found in the above-numbered specification. The entire suspension is mounted on a turntable 53.

The circuitry illustrated in Figs. 3 and 4 is suitable for use in the situation where the gyroscope is mounted in a pendulous suspension. It is equally applicable in the case of a stable mounting, but in that situation it is very important that the Y axis of the gyroscope must be accurately orthogonal to the turntable axis. Any error in this will introduce errors into the output of the amplifier 32. In addition, any deflection of the turntable whilst it is being rotated will introduce errors. These errors appear in the YO input to the circuitry, whereas the correction Yc introduced during rotation of the turntable appears only in the XO input. It is therefore possible to avoid the sources of error referred to by disconnecting the YO signal during rotation of the turntable.

Fig. 6 illustrates circuitry to take advantage of this fact, and also shows a simplified latitude correction arrangement. As before, the two inputs are applied through suitable summing resistors, but signal processing units are not included, another two inputs are selected by a switch SA so that only one of the inputs is applied to the amplifier 32 at once. The feedback resistor R is not connected directly to the amplifier output as before, but is connected through a potentiometer 61, and a switch SB. This switch, which is ganged with switch SA, has one contact connected to the output of the amplifier 32, and the other connected to a constant voltage V'.

With the two switches set as shown, the YO output alone is applied to the amplifier, with the resistance R connected through the potentiometer 61 to the output of the amplifier. The potentiometer is set to give the required latitude correction. The t2 term of the input is constant, and may be compensated for by suitable scaling of the summing resistor and the feedback resistor R. In this state the only input to the amplifier is that representing sin y, from which the misalignment yO is derived and incidated, charging the feedback capacitor of the amplifier in the process. When the amplifier output has reached a steady state the two switches may be operated to connect the other input to the amplifier which is converted into a pure integrator by the disconnection of the feedback resistor R.The voltage V' is such that, at the particular setting of the potentiometer 61, the latitude term is backedoff, leaving only the ft sin P term to be applied to and integrated by the amplifier.

The amplifier output discharges the feedback capacitor and hence reduces the indication on the meter 33.

The two switches should be such that their operation does not impart any mechanical impulse to the gyroscope which might introduce errors.

The latitude correction technique shown in Fig. 6 may also be used in the earlier embodiments.

The angle P by which the X axis of the gyroscope is offset from the horizontal may be chosen from a wide range. The angle must not be so small that its effect is lost. In the first embodiment described the smaller ss is made, then the greater must be the proportion of the XO input that must be fed to the summing amplifier. Hence the magnitude of any errors due to gyroscope inaccuracies would become greater. On the other hand, if P is made too large then the amplifiers shown in Fig. 3 associated with the gyroscope servo loops may be overloaded. Although P may have a value up to, say, 30 , a more usual value would be around 5 .

Claims (6)

1. Gyroscopic apparatus for the determination of a cardinal compass direction, which includes a rate gyroscope having two orthogonal sensitive axes, support means rotatable about a vertical axis and arranged to carry said gyroscope such that a first one of its sensitive axes is horizontal and pointing nominally in said cardinal compass direction and such that its other sensitive axis is inclined about the first sensitive axis at a known small angle to the horizontal, pick-off means responsive to movements of the gyroscope about said two sensitive axes, and circuit means responsive to the outputs of the pick-off means and arranged to provide an indication of the sense of any misalignment of the first axis rom said cardinal compass direction.

2. Apparatus as claimed in claim 1 in which the circuit means includes signal processing means operable to remove the constant components of the pick-of outputs from each axis, an integrating and summing amplifier operable to combine the processed signals for each axis, and indicator means responsive to the output of the amplifier.

3. Apparatus as claimed in claim 1 in which the circuit means includes an amplifier, switching means one at a time and to change the characteristics of the amplifier for each pick-off output, and indicator means responsive to the output of the amplifier.

4. Apparatus as claimed in any one of claims 1 to 3 in which the circuit means includes means arranged to compensate for variations in the latitude at which the apparatus is used.

5. Apparatus as claimed in any one of the preceding claims in which the support means includes a two-axis pendulous suspension on which the gyroscope is mounted.

6. Gyroscopic apparatus for the determination of a cardinal compass direction substantially as herein described with reference to the accompanying drawings.