GB2197473A - Method and apparatus for vibration testing equipment - Google Patents

Abstract

A vibration test is performed on equipment such as a helicopter (11), while it is standing on the ground or is suspended from a crane or beam, by mounting an out-of-balance motor (12) at a position on the helicopter, and running the motor at a set rate corresponding to a frequency (fo) at which vibrations are expected to occur; accelerometers (21) mounted at selected positions on the helicopter sense vibrations occurring at those positions for analysis, e.g. by a device 22 at some distance from the helicopter, coupled to the sensors through an optical or radio link. <IMAGE>

Description

SPECIFICATION Method and apparatus for vibration testing equipment This invention relates to a method and apparatus for testing a helicopter or other vehicle or equipment on which vibrations may occur in use. Such vibrations can affect the efficiency of performance of the equipment, particularly if it is a helicopter with large rotating masses, but it is often very difficult to identify the sources of out-of-balance forces or vibrations so that it is not easy to balance them or introduce such compensation.

As suggested above, the invention has particular application to the testing of helicopters and, in the following description, the invention will be described as related to the testing of helicopters, but it will be borne in mind that the method is equally applicable to other vehicles and, indeed, other machines and, in particular, rotating machines.

In the past, vibration testing has involved mounting the equipment on a vibration table, and such a table is very expensive if it is to support equipment as large as a helicopter.

According to the present invention, in a method of testing a helicopter or other vehicle or equipment, a vibrator, e.g. an out-of-balance motor device, is run while secured at a selected position on the equipment and the consequent vibrations at one or more selected positions of the equipment are observed or measured, or sensed.

Sometimes, the vibrations occur, not due to the rotating parts of the equipment being outof-balance, but due to loose connections or components which are improperly supported, and vibrations arising from those causes can be detected, even if rotary components of the equipment are stationary.

Thus, if an out-of-balance motor device is secured at one or more positions on the equipment in turn, and then run, vibrations may be induced which can be observed visually as being due to loose fastenings, or such vibrations may be detected by accelerometers or other instruments, and analysed by use of an electronic vibration analysis device, for example, the device described in British Patent no. 2116804 arid European patent publication no. 112031.

It may be a matter of experience in deciding what are the best positions on the equipment on which the out-of-balance motor device can be secured in order to show up faults that can be corrected.

The vibrator could be an electric motor driving an out-of-balance disc at one or each end of the shaft and run at a selected speed while the frame is bolted to the equipment. One form of out-of-balance electric motor of that kind has semi-circular discs at each end of the rotor shaft which can be angularly adjusted in position so that the degree of out-of-balance can be adjusted, and that gives another possibility for varying the test to be applied to the equipment.

Another possible out-of-balance motor device is a wind-driven bali race with a single ball.

Although it is possible that effective results can be achieved with the equipment standing on the ground, it has been discovered that the test is particularly effective if the helicopter. or other equipment is suspended from a crane or an overhead beam during the test so that reaction from the ground is avoided, except through the suspension means.

If an electrical vibration analysis device is used for the test, that can be coupled to the equipment during the test, perhaps by optical or radio link, while the test is being performed and the results can be recorded so that they are available for analysis at the end of the test.

it is also possible that, if the equipment is a helicopter or other vehicle, a tester can sit in the vehicle during the test while it is suspended and make observations.

The method according to the invention includes the subsequent adjustment of the equipment to reduce any vibrations which have been detected during the test.

The invention includes apparatus for performing the test including an out-of-balance motor device, or other vibrator, means for driving the vibrator at a set rate, and sensors for sensing vibration during the test. The apparatus may be in combination with a vibration analayser.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example with reference to the accompanying drawing, of which the single Figure is a block diagram showing a vibration test being carried out on a helicopter using apparatus in accordance with the invention.

The helicopter is shown diagrammatically at 11, and a vibrator 12 is mounted at a selected point on the helicopter. Tests may be performed with the vibrator on a part of the fuselage, on a part of the engine, on a rotor bearing housing, on the undercarriage, or elsewhere.

The vibrator 12 is a smail AC motor with offset cams at each end of the motor spindle.

The offset of the cams can be adjusted between tests.

The vibrator motor is driven from a command signal consisting of a series of pulses at a frequency which is a multiple of a particular frequency at which a test is to be run, because that frequency is a frequency of interest. For example, the frequency might be related to the rotation speed of the helicopter rotor, or a frequency at which a component of the structure is resonant. The command signal Nfo is fed to a phase comparator 13 in a phase-locked loop including an amplifier 14 of the output from the comparator 13, and an inverter 1 5 for supplying an inverted version of the amplifier output to energise the vibrator motor 12. The vibrator is effectively driven by a pulse width modulator, of which the duty cycle represents the motor drive.A feedback circuit is completed by a tachometer generator 16 driven from the vibrator motor, and whose output is formed into a series of pulses at 17 and fed through a divider 18 to the second input to the phase comparator 13. The error signal from the phase comparator will depend on any difference between the speed commanded by the command signal and the actual speed of the vibrator motor and will be used to adjust the energisation of the motor to maintain the motor speed at the desired speed.

Accelerometers, one of which is indicated diagrammatically at 21, are mounted on the helicopter structure, and the output from the accelerometers is fed to an electronic analyser to show the result on the helicopter at the accelerometer locations due to operation of the vibrator.

The accelerometer outputs can be analysed by an electronic vibration analysis device indicated generally at 22, which may be of the kind disclosed in European patent publication no. 112031, and which may be arranged to provide the command signal Nfo.

The vibrator also provides, at 23, a "once per revolution" signal for use in the analyser.

A microprocessor 24 handles communication between the analyser and the vibrator and its drive circuit. There may be optical coupling between the vibrator and its drive circuit on the one hand, and the microprocessor and analyser on the other hand to provide isolation from the AC source for driving the vibrator.

The microprocessor and analyser may be, perhaps, 10 m from the structure under test.

Whereas, in the past, a vibration test on a helicopter or other large piece of equipment has involved the provision of a large and expensive vibratory table, on which the helicopter has been mounted, the present invention enables the tests to be performed without the need to provide the expensive vibratory table.

Thus, the tests might be performed with the helicopter standing on the ground, or possibly, even, suspended from a crane.

The method of testing can be applied, for example, to components on drill rigs at sea, or in other places where it is impractical to have a vibration testing table. The tests may not only provide information about vibration of the object being tested, but might also be used to show flaws and cracks in the object.

The rotary adjustable vibrator and the control circuit for driving it may be supplied as a piece of equipment for use in conjunction with known electronic analysing equipment.

Claims (12)

1. A method of testing a helicopter or other equipment in which a vibrator is run while secured at a selected position on the equipment, and the consequent vibrations at one or more selected positions on the equipment are sensed.

2. A method as claimed in Claim 1, in which the vibrator comprises an out-of-balance motor device.

3. A method as claimed in Claim 1 or Claim 2, including electrical means for causing the vibrator to run at a controlled preset rate.

4. A method as claimed in any of the preceding claims, in which accelerometers or other instruments mounted at the selected positions on the equipment provide output signals representative of induced vibrations at those positions.

5. A method as claimed in any of the preceding claims, performed while the equipment is standing on the ground.

6. A method as claimed in any of Claims 1 to 4 performed while the equipment is suspended.

7. A method as claimed in any of the preceding claims, in which the sensed vibrations are analysed in an electronic vibration analysis device.

8. A method as claimed in Claim 7, in which, during the test the analysis device is coupled to the sensors through an optical or radio link.

9. A method of vibration testing a helicopter or other equipment performed substantially as herein specifically described with reference to the accompanying drawing.

10. Apparatus for performing a vibration test on a helicopter or other equipment comprising an out-of-balance motor or other vibrator capable of being mounted on the equipment, means for driving the vibrator at a set rate, and sensors for mounting on the equipment for sensing vibrations during the test.

11. Apparatus as claimed in Claim 10, in which the driving means comprise a phaselocked loop feedback system for maintaining the rate of the vibrator in synchronism with a command signal defining the set rate.

12. Vibration testing apparatus arranged substantially as herein specifically described with reference to the accompanying drawing.