GB2082801A - Inertial platform - Google Patents

Abstract

An inertial platform comprises a frame 10 secured to a vehicle, an outer gimbal 11 supported by the frame, and an inner gimbal 15 supported by the first gimbal. The two gimbal axes 12, 16 are perpendicular, and each have a pick-off 13, 17 and a torque motor 14, 18. Gyroscopic means 19, 20 are carried on the inner gimbal and have three mutually perpendicular sensitive axes. Circuit means are provided which ensure that, in operation, the inner gimbal is maintained in an attitude in which first and second sensitive axes are horizontal and the third sensitive axis is vertical. Accelerometers 21, 22 are carried on the inner gimbal. <IMAGE>

Description

SPECIFICATION Inertial platforms Many types of inertial platform exist, each different type being intended to operate under a particular set of conditions. At one extreme is the platform having three, or probably four, gimbals and carrying on the inner gimbal the gyros necessary for stabilisation and a set of accelerometers. This type of platform is very complex and expensive from a mechanical viewpoint, requiring slip-ring connections, and gimbal bearings, and being of increased size and weight. The other extreme is the so-called "strapdown" system in which the gyros and accelerometers are fixed rigidly to the vehicle frame. This arrangement is more robust and is simpler mechanically, but the computing complexity is considerably greater.

Neither of the two alternatives described above is particularly cheap, and there is a requirement for 'a simple low-cost inertial platform. It is an object of the present invention to provide such a platform.

According to the present invention there is provided an inertial platform which includes a frame arranged to be secured to a vehicle, an outer gimbal supported from the frame about a first gimbal axis, an inner gimbal supported from the outer gimbal about a second gimbal axis perpendicular to the first gimbal axis, a pickoff and a torque motor on each gimbal axis, gyroscopic means mounted on the inner gimbal and having three mutually perpendicular sensitive axes, circuit means responsive to outputs from the pickoffs and from the gyroscopic apparatus when the platform is in operation to maintain the inner gimbal in an attitude in which first and second sensitive axes of the gyroscopic means are horizontal and the third sensitive axis is vertical, and a number of accelerometer carried on the inner gimbal and having sensitive axes parallel to some or all of the sensitive axes of the gyroscopic means.

The invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a platform, showing an arrangement of the hardware; Figure 2 is a block circuit diagram of the circuit means and other components of the platform; and Figure 3 illustrates the operation of the circuit means.

Referring now to Figure 1, a frame fixed rigidly to the vehicle and shown schematically at 10 supports an outer gimbal 11 for rotation about an axis 12. The outer gimbal is provided with a pickoff 13 and a torque motor 14 for controlling its attitude relative to the frame 10. The outer gimbal 11 supports an inner gimbal 1 5 for rotation about an axis 1 6 which is perpendicular to the axis 12. The inner gimbal is provided with a pickoff 1 7 and a torque motor 18 for controlling its attitude relative to the outer gimbal 11.

The inner gimbal 1 5 forms a platform on which are mounted gyroscopes, or gyros, having three sensitive axes. As shown in Figure 1, a first gyro 19 has two mutually perpendicular sensitive axes both of which are parallel to the plane of the inner gimbal 1 5. A second gyro 20 has its single sensitive axis arranged perpendicular to the two sensitive axes of gyro 1 9. Each gyro has the usual pickoff and torquer on each sensitive axis.

Also carried on the inner gimbal are the accelerometers required to provide outputs from the platform. Two accelerometers 21 and 22 are shown, having their sensitive axes aligned with those of the two-axis gyro 1 9.

The various electrical connections to and from the platform are shown schematically in Figure 2 in which a block diagram of the necessary circuit means to which these connections are supplied.

Pickoff signals from the 2-axis gyro 1 9 are applied to separate servo amplifiers 30 and 31.

The outputs from these amplifiers are applied to the torque motors 14 and 1 8 on the inner and outer gimbal axes respectively. The outputs from the corresponding gimbal pickoffs 13 and 1 7 are applied to a central processor 32 which derives pitch and roll outputs. The processor also provides outputs to the torquers on the two axes of the gyro 19.

The second gyro 20 has its pickoff and torquer connected in a conventional capture loop. The pickoff output is connected through a capture amplifier 33 to an integrating encoder 34. The output from the encoder is applied both to the processor 32 and to the torquer of the gyro 20.

The two accelerometers are connected in a similar way to gyro 20. The output of accelerometer 21 is applied through a capture amplifier 35 to an integrating encoder 36. The encoder output is applied to the processor 32 and to the force coil of the accelerometer. Similarly, the pickoff output from accelerometer 22 is applied through a capture amplifier 37 to an integrating encoder 38. The output of the encoder is applied to the processor 32 and to the force coil of the accelerometer 22.

The function of the processor 32 is illustrated schematically in Figure 3. This drawing shows the inputs to and outputs from the processor shown in Figure 2, and indicates the functions to be performed. Many of these are conventional to the inertial platform field and need not be described in detail. The function may be performed by hardware or by software.

Referring to Figure 3, the pickoff output from the gyro 20 of Figure 2 is applied to a capture amplifier 37 and integrator 38, and this integrator produces an output which represents increments of azimuth angle of the platform relative to some datum. This signal is applied via an input A to the processor to a further integrator 40, the output of which represents the absolute platform azimuth angle, or heading h. A correction signal is applied to the integrator 40 as will be described later.

The two accelerometers 21 and 22 of Figure 2 also have capture loops containing integrators, and the outputs of these integrators are applied to inputs B and C of the processor. Assuming the axes 12 and 1 6 of Figure 1 to represent the X and Y direction respectiveiy then the output of integrator 36 applied to input B represents increments of X velocity, whilst the output of integrator 34 applied to input C represents increments of Y velocity. Further integrators 41 and 42 produce outputs representing total velocity in the X and Y directions. These are modified by azimuth resolver 43 which produces output representing North and East velocities. The resolver 43 has an input from the integrator 40, and thus effects a continuous transformation on the X and Y velocities applied to it in dependence upon the heading of the platform.

The North and East velocities from resolver 43 are applied to a conventional Schuler loop circuit 44 which produces output signals representing latitude LA and longitude LO, as well as the North and East velocities NV and EV. Inputs CR to the Schuler loop circuit 44 may provide corrections for gyro drift, instrument bias etc, and an output from the circuit provides the azimuth rate corrections for the integrator 40 already mentioned. These corrections are conventional, and relate to the earth's rate and velocity. Outputs from the processor are required for the torquer of the accelerometer 19 of Figure 1, and these are obtained at D and E. These are derived from the Schuler loop circuit 43 by way of a further azimuth resolver 45, performing the reverse function to the resolver 43. Resolver 45 derives from the corrected Schuler loop output the necessary X and Y gyro torquing signals.

Signals from the two gimbal pickoffs 13 and 17 are applied to inputs F and G of the processor These are applied to a third resolver 46 along with a fixed quantity representing the angle in the horizontal plane between the platform XY axes and the pitch-roll axes from integrator 4. This resolver converts the X and Y outputs from the pickoffs into pitch and roll angle outputs from the processor.

The three resolvers and the Schuier loop circuit perform fairly simple transformation operations of a type which are well-known in the inertial navigation field. Such transformations are explained in detail in a number of reference books and will not therefore be described further.

As already stated, the functions of the processor may be provided by circuitry or by programming. The desired results may be obtained by different methods to those described.

The three sensitive gyro axes may be provided by three separate single-axis gyros, or by two two- f axis gyros with one redundant axis. A third accelerometer could be provided if required. The two gimbal pickoffs 13 and 17 are only necessary if the platform is required to give outputs indicating pitch and roll angle. The processor could be used to resolve the X and Y velocity increments rather than operating on the integrated increments.

Claims (5)

1. An inertial platform which includes a frame arranged to be secured to a vehicle, an outer gimbal supported from the frame about a first gimbal axis, an inner gimbal supported from the outer gimbal about a second gimbal axis perpendicular to the first gimbal axis, a pickoff and a torque motor on each gimbal axis, gyroscopic means mounted on the inner gimbal and having three mutually perpendicular sensitive axes, circuit means responsive to outputs from the pickoffs and from the gyroscopic apparatus when the platform is in operation to maintain the inner gimbal in an attitude in which first and second sensitive axes of the gyroscopic means are horizontal and the third sensitive axis is vertical, and a number of accelerometers carried on the inner gimbal and having sensitive axes parallel to some or all of the sensitive axes of the gyroscopic means.

2. A platform as claimed in Claim 1 in which the gyroscopic means includes a first gyroscope providing the first and second sensitive axes, and a second gyroscope providing the third sensitive axis.

3. A platform as claimed in Claim 1 in which the gyroscopic means includes three single-axis gyroscopes, each providing a separate one of the sensitive axes.

4. A platform as claimed in any of Claims 1 to 3 in which the circuit means is operable to provide navigational output signals.

5. An inertial platform substantially as herein described with reference to the accompanying drawings.