Vibrating rate sensor tuning

Abstract: A method of tuning a vibrating rate sensor (10) the step of balancing and mode alignment of the vibrating portion (34) of said sensor (10) by material removal from said portion (34) after the drive and pick-off transducers (30, 32, 18, 20) are fixed to said portion (34).

Description: The present invention relates to a method of tuning a vibrating rate sensor and relates particularly, but not exclusively, to a method of balancing of a vibrating portion of a vibrating rate sensor. Figure 1 illustrates diagrammatically a conventional single axis rate sensor of the type described in GB-A-2061502 and EP-A-0153189. The cylinder of the vibrating rate sensor is driven into resonance by applying an oscillating electronic signal to the drive transducers. The vibration pattern, viewed from the top, of the resonance mode is shown in Figure 2. It will be seen that there are anti-nodes of maximum radial vibration and nodes of minimal radial vibration present. The primary loop maintains this resonance by ensuring a 90 degree phase shift between the primary drive transducer and primary pick-off transducer. When the above mentioned rate sensor is rotated, the resonance mode rotates with respect to the cylinder, and a signal can be detected on the secondary pick-off transducer. This signal is amplified and fed back to the secondary drive transducer in order to null it to zero. The strength of this nulling signal is proportional to rotating rate. The signal is demodulated and outputted as a DC rate signal. It should be noted that the vibration pattern can have a different shape to that shown in Figure 2. However, the vibration pattern must have nodes and anti-nodes occurring cylindrically around the perimeter of the cylinder. The cylinder does not have to be driven exactly at the resonance - but in practice it is desirable so to do. The resonant body neither has to be circular, nor cylindrical. Hemispheres and rings can be used. However, it is desirable that the oscillating modes seen by primary and secondary electronic circuits (referred to as the primary and secondary resonance) have the same, or nearly the same resonant frequency. EP-A-0171378 discloses an angular rate sensor in which the vibrating portion is a tuning fork having a pair of vibrating tines driven in complementary resonant vibration by a pair of coils positioned about the free ends of the tines. GB-A-2061502 describes a metal cylinder, closed at one end and supporting by a stem, which is driven into resonance using piezo-electric elements stuck onto the metal cylinder as shown in Figure 3. Piezo-electric elements are also used to pick-off the movement. EP-A-0153189 describes a similar device of unitary construction made from piezo-electric material where the drive and pick-off pieces are formed by polarising the material between electrode regions as shown in Figure 4. GB-A-2061502 illustrates that, because of the difficulty of mounting a secondary pick-off transducer exactly at a nodal position, it is more convenient to provide a pair of secondary pick-off transducers with their two outputs combined together in such a manner as to provide a zero output in the absence of the rotation. The two secondary drive transducers are required opposite the pair of secondary pick-off transducers. The outputs of the secondary pick-off transducers are adjusted to give maximum rejection of the applied radial vibration. Then the signal applied to the secondary drive transducers are adjusted. EP-A-0153189 illustrates that the device described in GB 2061502 can be made entirely from a unitary piece of piezo-electric material, and that the transducing action of electrical input to the mechanical output can be made to take place within the resonator itself. One of the problems associated with the above two described sensors is that it is difficult to machine piezo-electric cylinders such that the primary and secondary modes of vibration have identical resonant frequencies. If the true modes of vibration are not aligned to the ideal positions of the drive and pick-off transducers, this leads to bias signals being present. Thus, the secondary pick-off transducers will in general see an output even when no rotation rate is applied. This is the case even when the transducer has been positioned accurately. The arrangement shown in GB-A-2061502 will find it difficult to achieve the desired performance targets. Another drawback of the arrangement given in GB-A-2061502 is that it is not efficient in the number of bond wires and/or transducing elements. Thus one more bond wire is required in the secondary pick-off transducer and one more in the secondary drive transducer. One of the disadvantages of EP-A-0153189 is that it is difficult to machine cylinders accurately enough and it is not possible to position electrodes accurately enough to achieve the performance required for many practical applications. There therefore exists a requirement for a method of increasing the performance capabilities of a vibrating rate sensor. The present invention attempts to overcome the problems associated with the above mentioned prior art sensors by providing a method of mechanically balancing the cylinder by the removal of material, and then trimming the effective transducer positions to minimise bias. The present invention is applicable to both the devices described in the above mentioned patents, together with other similar rotation rate sensors based on resonating hemispheres, cylinders, and rings. Accordingly, the present invention provides a method of mechanically balancing a hemispherical, cylindrical or ring-like vibrating portion of a vibrating rate sensor of the type having primary and secondary drive transducers, and primary and secondary pick-off transducers, characterised by including the steps of: (a) applying a sinusoidal signal to the primary drive transducers, (b) adjusting the frequency of the signal until a desired vibration mode is excited and the phase of the primary pick-off transducers is at 90 degrees with respect to the primary drive transducers, (c) measuring the primary vibratory mode frequency (f np ) (d) repeating steps (a) and (b) for the secondary drive transducers, (e) measuring the frequency of the secondary vibratory mode (f ns ) (f) removing a small amount of material from a portion of the vibrating portion adjacent either the drive transducer or the pick-off transducer which corresponds to the mode having the lowest resonant frequency so as to reduce the frequency difference to within a desired amount mechanically to balance the vibrating portion. It will be appreciated that by balancing the vibrating portion after it has been machined and after the transducers have been positioned thereon, it will be possible to balance the vibrating portion without having to accurately machine it or accurately position transducers thereon. Preferably the method includes the step of repeating steps (a) to (f) iteratively until the frequency difference is within a desired amount. The method may be applied to a vibrating rate sensor having the primary drive transducers split asymmetrically on one side of the vibrating portion thereof and complete on another side thereof. Alternatively, the primary drive transducers may be split symmetrically on one side of the vibrating portion thereof and complete on another side thereof. In a further alternative, the primary drive transducers may be complete on both sides of the vibrating portion but angularly displaced so as to facilitate swinging of the effective transducer position either side of a mean position. The present invention will now be more particularly described by way of example only with reference to the following drawings in which, Figure 1 is a schematic diagram of a conventional single axis rate sensor of the type described in GB 2061502 and GB 2154739, Figure 2 is a diagrammatic representation of the vibration pattern produced in the vibrating cylinder of a single axis rate sensor, Figure 3 is a perspective view of a vibrating cylinder structure for inclusion in a typical vibrating rate sensor, Figure 4 illustrates a conventional device similar to that shown in Figure 1 but of unitary construction made from piezo-electric material where the drive and pick-off transducers are formed by polarising the material between electrode regions, Figure 5 is a schematic representation of a vibratory rate sensor capable of being tuned in accordance with the method of the present invention, Figures 6 to 8 illustrate in more detail the transducer position on the cylinder shown in Figure 5. Referring now to the drawings in general but particularly to Figure 5 which illustrates a vibrating rate sensor of the type shown in Figure 1 and described above, but having the additional features of primary and secondary pick-off transducers 18, 20 being connected to charge amplifiers 22, 24 and an inner transducer 26 and secondary drive transducers 28 being connected to earth. As in EP-A-0153189 the primary pick-off transducers 18 are connected together, as are the secondary pick-off transducers 20. The primary drive transducers 30, 32 are split on one side of the cylindrical vibrating portion 34 only, and connected together via a resistor network 36, 38. Although the vibrating portion 34 has been described as cylindrical, the method of the invention can also be utilised with hemispherical or ring-like vibrating portions. Figure 6 to 8 illustrate a number of alternative split drive transducer positions. In order mechanically to balance the vibrating portion 34 of the vibrating rate sensor 10, the following steps are undertaken. (Step A) A sinusoidal signal is applied to the primary drive transducers 30, 32; (Step B) The frequency of the signal is adjusted until a desired vibration mode is excited and the phase of the primary pick-off transducers 18 is at 90 degrees with respect to the primary drive transducers 30,32; (Step C) The frequency (f np ) of the primary vibratory mode is measured; (Step D) Steps A and B are repeated for the secondary drive transducers 28; (Step E) The frequency of the secondary vibratory mode (f ns ) is measured; (Step F) A small amount of material is removed from a portion of the vibrating portion 34 adjacent either the drive transducer or the pick-off transducer whichever corresponds to the mode having the lowest resonant frequency so as to reduce the frequency difference to within a desired amount mechanically to balance the vibrating portion. Steps A to F may be repeated iteratively until the frequency difference is within a particularly desired amount. It should be noted that in practice it is difficult (or time consuming) to reduce the component of the secondary pick-off signal which is in quadrature with respect to primary pick-off signal to zero by material removal. Figures 6 to 8 illustrate different arrangements for the transducer configuration on the cylindrical vibrating portion 34. Figure 6 illustrates the primary drive transducers 30, 32 split asymmetrically on one side of the cylindrical portion 34, but complete on the other. Figure 7 illustrates the primary drive transducers 30,32 split symmetrically on one side of the portion 34, but complete on the other. Figure 8 illustrates the primary drive transducers 30,32 complete on both sides of the portion 34. However these transducers are angularly displaced such that by varying the drives on the transducers, it is possible to swing the effective transducer position either side of the mean position. Irrespective of which arrangement is used, the quadrature component of the secondary pick-off transducers 20 with respect to the primary pick-off signal can be reduced to zero. The split transducer patterns shown in Figure 6 to 8 are more efficient in terms of bond wires and electrodes than the equivalent scheme given in EP-A-0153189.

Claims: A method of mechanically balancing a hemispherical, cylindrical or ring-like vibrating portion (34) of a vibrating rate sensor (10) of the type having primary (30,32) and secondary (28) drive transducers, and primary and secondary pick-off transducers (18,20), characterised by including the steps of; (a) applying a sinusoidal signal to the primary drive transducers (30,32), (b) adjusting the frequency of the signal until a desired vibration mode is excited and the phase of the primary pick-off transducers (18) is at 90 degrees with respect to the primary drive transducers (30,32), (c) measuring the primary vibratory mode frequency (f np ) (d) repeating steps (a) and (b) for the secondary drive transducers (28), (e) measuring the frequency of the secondary vibratory mode (f ns ) (f) removing a small amount of material from a portion of the vibrating portion (34) adjacent either the drive transducer or the pick-off transducer which corresponds to the mode having the lowest resonant frequency so as to reduce the frequency difference to within a desired amount mechanically to balance the vibrating portion. A method as claimed in claim 1, in which steps (a) to (f) are repeated iteratively until the frequency difference is within a desired amount. A method according to claim 1 or claim 2, in which the vibrating rate sensor (10) having the vibrating portion (34) being balanced, has the primary drive transducers (30,32) split asymmetrically on one side of the vibrating portion (34) and complete on another side thereof. A method according to claim 1 or claim 2, in which the vibrating rate sensor (10) having the vibrating portion (34) being balanced, has the primary drive transducers (30,32) split symmetrically on one side of the vibrating portion (34) and complete on another side thereof. A method according to claim 1 or claim 2, in which the vibrating rate sensor (10) having the vibrating portion (34) being balanced, has the primary drive transducers (30,32) complete on both sides of the vibrating portion (34) but angularly displaced so as to facilitate swinging of the effective transducer position either side of a mean position.

Inventors: MCINNES JAMES BRITISH AEROSPAC, VARNHAM MALCOLM P BRITISH AERO, MCINNES, JAMES, VARNHAM, MALCOLM P., MCINNES, JAMES, BRITISH AEROSPACE SYSTEMS, VARNHAM, MALCOLM P., BRITISH AEROSPACE SYSTEMS
Applicants: BRITISH AEROSPACE, BRITISH AEROSPACE PUBLIC LIMITED COMPANY
Application year: 1993
Main classification: G01C19
IPC codes: G01P9, G01C19