WO1998023938A1

WO1998023938A1 - A quality control system for testing mechanical devices

Info

Publication number: WO1998023938A1
Application number: PCT/TR1997/000021
Authority: WO
Inventors: Niels Kjaergaard; Carsten L. Fog; Lars Alkestrup; Tugrul Durakbasa; Hakan Serafettinoglu; Evren Albas; Faruk Bayraktar; Nazim Kadrizade; Kenan ATAÇ; Temel Belek
Original assignee: Arçelik A.S.; Brüel And Kjaer A/S
Priority date: 1996-11-29
Filing date: 1997-11-28
Publication date: 1998-06-04
Also published as: AU5237698A; TR199901198T2

Abstract

The present invention relates to a quality control system for testing devices, such as mechanical devices with moving parts, such as electro motors, comprising a fixture adapted to hold and operationally engage with the device, at least one energy flow transducer being positioned at the fixture so as to be operational communication with the device when the device is operationally engaged with the fixture, the transducer sensing acceleration of and force exerted by the device, and providing electrical output signals in response to and as a function of sensed acceleration and forces, respectively, and a measuring apparatus for receiving the output signals from the at least one energy flow transducer, and for determining parameters derived from sensed forces and accelerations.

Description

A QUALITY CONTROL SYSTEM FOR TESTING MECHANICAL- DEVICES

FIELD OF INVENTION

The present invention relates to a quality control system for testing devices, such as mechanical devices with moving parts, such as ele^~ctro motors.

GENERAL BACKGROUND

In modern manufacturing facilities, consumer products are produced in very large numbers by automated assembly lines. Various test systems are known for testing products manu- factured in large numbers:

For example, it is known that measurement of vibration generated or sound emitted by a manufactured device during operation can be useful for detection of malfunction of the device .

Sound measurements are typically performed using a microphone to pick up sound emitted by a device under test. Such sound measurements are susceptible to background noise and, typically, the background noise level of a manufacturing facility is so high that it prevents useful sound measurements. It is known to substitute the microphone by a sound intensity probe to reduce the influence ^"of background noise, however, in many cases the reduction is not sufficient for measurements to become useful.

Further, complicated acoustical measurements, such as measurements of the spectrum of sound emitted from a small electro motor, is difficult to perform in a room in which reflections from the floor and walls are generated. Such reflections distort the sound measurements so that it becomes almost impossible to reproduce measurements, because even small movements of the microphone or the device may change the measured spectrum completely. To overcome these difficulties, it is known to perform measurements' of sound emitted from a device under test in an anechoic chamber in which the background noise is very low and no reflections from the floor or walls occur. However, it is very expensive to provide an anechoic chamber and—very expensive to move devices manufactured in large numbers to such a facility for testing.

Measurement of vibration generated by a device under test is, typically, performed utilising an accelerometer attached to a specific part of the device. However, -the vibrations sensed by the accelerometer are, typically,-^"very dependent upon the specific positioning of the accelerometer, as the pattern of vibrations of the device varies across the surface of the device. Thus, in order to be able. o compare measurements, it is vital to position the accelerometer very accurately on the device to be tested. Still, substantial differences in measurements of vibration may occur, e.g. because of small differences from one device to another due to the manufacturing process, such as small differences in thickness of the device housing, small bumps or dents in the housing, etc.

These kinds of small differences change the vibration pattern of the devices locally.

Quality control systems in which an operator analyses and classifies the device under test are known. For example, electro motors may be evaluated by an operator listening to them while they are operating. From the sound generated by a running motor, the operator decides whether the motor is a good motor having been manufactured without failures, or it is a motor with a failure. Certain failures may generate sounds of specific characteristics that can be recognized by the operator. i

Likewise, the operator may put a hand on the device to feel the -vibrations in the device. If the device_^ vibrates in a way that is different from a typical device, the operator may decide that it has a failure. However; classifications by operators are inaccurate and inconsistent.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome the above-mentioned disadvantages of the prior art .

It is an object of the present invention to provide a quality control system and a method that is not sensitive to background acoustic noise.

It is another object of the present invention to provide a quality control system and a method that is not sensitive to reflections of sound and vibrations from the environment.

It is yet another object of the present invention to provide a quality control system and a method that is not sensitive to positioning of transducers on the device.

It is a further object of the present invention to provide a quality control system and a method for automatic classifica- tion of the devices.

It is a still further object of the present invention to provide a quality control system and a method that is fully automatic .

According to the invention, the above-mentioned and other objects are fulfilled by a quality control system for testing devices, comprising a fixture adapted to hold and operationally engage with the device. The fixture has at least one energy flow transducer for sensing acceleration of and force exerted by the device and it provides electrical output signals in response to and as a function of sensed accele- rations and forces, respectively. The at least one transducer is positioned so as to be in operational communication with the device when the device is operationally engaged with the fixture. Further, the system comprises a measuring apparatus connected to the _outputs of the at least one energy flow transducer, for determining parameters derived from sensed forces and accelerations .

Further, a method of testing devices is provided, comprising the step_s of positioning the device in a _fixture adapted to hold and engage with the device, s_ensing acceleration of and force exerted by the device on at least one energy flow transducer positioned in operational communication with the device, and determining parameters derived -f-rom sensed forces and accelerations.

Devices tested by the quality control system may be any device that generates mechanical energy during operation, such as mechanical devices with moving parts, such as compressors for refrigerators, electro motors, household machines, electric razors, fractional horse power electro motors, combustion engines, etc, etc.

According to an important aspect of the present invention, mechanical energy generated by the operating device is sensed by one or more energy flow transducers. The term energy flow transducer designates a force transducer and an accelero- meter. The force transducer and the accelerometer may be coupled in series, or they may be placed in parallel with a small distance apart to minimize influence on the measurement results from phase difference of the signals from the force transducer and the accelerometer. The force transducer senses force acting on the transducer along a specific axis of the transducer and converts the sensed for-ee into a first elec^¬ trical signal that is a function of the sensed force. Likewise, the accelerometer senses acceleration along the specific axis of the transducer and converts the sensed accele- ration into a second electrical signal that is a function of the sensed acceleration. The energy flow transducer thus comprises a force transducer sensing along an axis substantially parallel to the sensing axis of an accelerometer. The second electrical signal (acceleration) may be integrated whereby a velocity signal irs provided. -Force multiplied by velocity provides the rate of energy transferred from the device to the energy -flow transducer and, thus, the rate of energy transferred^" from the device to the transducer along the specific axis ^"of the transducer may be determined.

Power is the- rate at which work is done by one system on another. When a first system is acting on a second system with a force F_{i t} the second system gets a resulting velocity V -r The time averaged power transmitted is then defined by

P= <P₁> = <F_iV_i > _c (1)

where <>_c denotes the time averaging.

The force F_j_ is measured by a force transducer and the velocity V_i is measured by an accelerometer followed by integration of the acceleration signal to provide the velocity

It is an important advantage of the present invention that the energy flow transferred to the transducer depends sub- stantially solely on the energy generated by the device.

It is another important advantage of the present invention that the energy flow transferred to the transducer may be made substantially independent of the exact positioning of the transducer in relation to the device.

It is yet another important advantage of the present invention that the energy flow transferred to the transducer may be made substantially independent of the background acoustical noise and vibration level during testing.

It is -still another important advantage of the present invention that the energy flow transferred to the transducer may be made substantially independent of reflections of sound and vibrations from the environment.

The fixture -may^"be of any shape adequate for holding and operationally engaging with the device to be tested. It may comprise a horizontal base-, e.ςf. made of aluminium, for the device to be positioned on. Further, means for fixing the^-" device to the fixture during the test, such as pneumatic activators, electro magnets, permanent magnets, nuts and bolts, etc, may be provided. Still further, the fixture may be provided with a number of holes for mounting fittings, which secure an accurate and safe positioning of the device during the test. The actual configuration of the fittings may be designed for positioning the device at a predetermined position and/or in such a way that the device is attached to the fixture at one or more well-defined contact points.

The fixture may be supported by one or more vibration absorbers, one or more air springs or a combination thereof.

It is preferred to use air springs with a mechanical input impedance having a very small real part thus dissipating very little vibrational energy flow. The vibrational energy flow is therefore mainly transmitted from the fixture to the environment through the at least one energy flow transducer each of which is positioned at the fixture in such a way that it is in operational communication with the device when the device is operationally engaged with the fixture. For example, an energy flow sensor may be positioned at the centre of three air springs supporting the fixture.

As already mentioned, the energy flow transducer senses energy flow in one direction along a specific axis of the transducer. Energy flow may be sensed in a plurality of directions by utilisT g a plurality of energy flow sensors positioned with their specific axes along the directions in question. If fx^" two energy flow transducers are used, accele- ration and force in two dimensions may be determined, and if three energy flow transducers are used, acceleration and force in three dimensions may be determined- . If three energy flow transducers are positioned perpendicular to each other, the accelerations and forces in the three perpendicular dimensions may be- etermined.

According to the invention, the fixture may be made of any material suitable for holding and engaging the device and for transmitting the_mechanical vibrations.

According to a preferred embodiment of the invention, the fixture is a pyramid-shaped element made of any material suitable for holding the device and for transmitting mechanical vibrations. Presently, it is preferred to use a regular pyramid-shaped element made of solid aluminium, however, pyramid-shaped elements with any number of inclined surfaces with a corresponding polygonal base may be utilized.

The four surfaces of the preferred regular pyramid-shaped element are identical, and for use in connection with small electro motors, the size of the six edges of the pyramid- shaped element are preferred to be approximately .- 6 cm. Even though the preferred regular pyramid-shaped element is made of solid aluminium, also materials such as iron, copper, brass or plastic may be utilized.

The preferred regular pyramid-shaped element is solid, but the pyramid-shaped element may be hollow.

A device to be tested may be positioned at a first pyramidal surface of the pyramid-shaped element and each of the at least one energy flow transducers may be in operational communication with corresponding pyramidal surfaces of the pyramid-shaped element whereby forces and acceleration may be sensed along a plurality of directions.

When used for energy flow measurement, the regular pyramid- shaped element may be turned up side down so that the ^" polygonal base of the pyramid-shaped element may carry the device during the testing and one energy flow transducer may be connected to each of the three inclined surface^'s ^"of the pyramid-shaped element. The device may be held in ^"position using a magnet.

An energy flow transducer may be positioned in operational communication with each of the three inclined surfaces of the .pyramid-shaped element, whereby simultaneous measurements of energy flow in three dimensions may be provided.

During the test, the at least one energy flow transducer may be positioned in direct contact with the pyramid-shaped element so that the force is exercised by the surface of the pyramid-shaped element directly on the force transducer.

The interconnection between the fixture and the at least one energy flow transducer may also be provided by stingers that transmit mechanical vibrations. A stinger is a wire made of a material suitable for transmission of mechanical vibrations, such as metal, plastic, etc.

It is an important advantage in use of a stingers that transfer of torque to the energy flow transducer in question is eliminated.

The measurement apparatus is adapted to receive output signals from the at least one energy transducer and to determine parameters from the -output signals.

The parameters determined by the measurement apparatus may be spectra of selected output signals of the at least one energy flow transducer, such as energy flow spectra or quasi energy flow-spectra.

The measurement apparatus may comprise a charge amplifier for each output signal, ie two charge amplifiers for each energy flow transducer, or six charge amplifiers for a measurement apparatus adapted to receive signals.. from three energy flow transducers. The amplified signals ^"from the flow transducers ^" may be transmitted to a dual channel frequency analyzer via a multi channel stereo switch box .multiplexing the^_signals, so as the dual channel frequency analyzer only receives signals from one energy flow transducer at a time.-

The measurement apparatus may comprise _processor means adapted to calculate the energy flow spectrum or the quasi energy flow spectrum of the output signals^" of the at least one energy flow transducer connected to the at least one spectrum analyzer.

The signals may be transmitted from the multi channel stereo switch box to the dual channel frequency analyzer via an IEEE GPIB (General Purpose Interface Bus) connection, a serial RS232 connection or other similar connection.

The parameters may comprise any set of parameters characteristic for the type of device being tested, such as force, acceleration, velocity, energy flow (force times velocity), quasi energy flow (force times acceleration), etc, in one or more directions at specific frequencies.

Further, the parameters may comprise the frequency spectrum (amplitude and/or phase) , the auto correlation, the cross correlation of any combination, etc, of the above-mentioned and other parameters .

The quasi energy flow spectrum has been proven to be extremely useful for evaluation of tested devices, as the signal to noise ratio at high frequencies is higher than for the energy flow spectrum. The power generated by the device is calculated by multiplying force by velocity, while the quasi energy flow spectrum is calculated by multiplying force by acceleration. As velocity may be derived by integrating acceleration, which in the frequency domain corresponds to dividing acceleration by s , it is seen that the energy flow spectrum decreases 10 dB/decade faster than the quasi energy flow spectrum at high frequencies and, thus, that the quasi energy flow spectrum has a better signal to noise ratio at high-frequencies than the energy flow spectrum.

In the following, the quasi energy flow spectra may be used where the energy flow spectra may be used, and vice versa, as ^"these two measures are _^derived on the basis of approximately the same information.

Preferably, the quality control system comprises moving means for positioning the device on the fixture in operational engagement therewith.

The moving means may comprise a robot that may be positioned at a transporting means, such as a conveyor belt, etc, for transporting the devices to be tested from manufacturing facilities to the quality control system. The robot may comprise an arm with holding means adapted to transfer the next device to be tested from the transporting means and to position it on the fixture, where it is released from the holding means.

The quality control system may further comprise power connecting means for connecting the Sevice to a power source. The power connecting means may comprise connections for the type of power needed for energizing the devices to be tested, such as electrical power for, e.g., electro motors, hydraulic fluid under pressure for, e.g., hydraulic machines, compressed air for, e.g., pneumatic tools, etc. The robot may move the device from the transporting means to the power connecting means for connecting the device to the appropriate power source before the device is moved to the fixture. The physical connection of the power may be achieved by a hydraulically controlled prston moving -power connectors from a resting position to an engaging position at corresponding connectors of the device.

It is preferred to power the device before positioning the "device on the fixture. When a device, such as an electrp motor, a compressor, etc, is powered, a starting _torque is generated. If the motor is positioned on the fixture ^"before it is powered, the starting torque will be transferred to the fixture- and it is necessary to design and build the fixture to withstand- the starting torque. This is not necessary if the device is powered up while it is held by the robot, because then the starting torque is transferred to the robot.

The quality control system may also comprise classification means for classifying the tested device in accordance with the determined parameters into one class of a set of prede- fined classes. For example, the parameters may be a set of magnitudes of the energy flow spectrum or quasi energy flow spectrum within a corresponding set of frequency ranges determined from the output signals of one energy flow sensor. Each predefined class may be defined by a set of upper and lower limits for each frequency^" range of the set of frequency ranges . A device may then be classified as belonging to a certain class if, for each frequency range of the set of frequency_ ranges , its energy flow spectrum magnitude is within the corresponding upper and^* lower limits of the class.

Each class may correspond to a specific type of failures of the device. For example, shaft imbalance, wheel imbalance, crookedness, imperfections of teeth in cogs, tight bearing, loose bearings, etc, may cause the device to vibrate in different characteristic ways, whereby a characteristic energy flow spectrum is generated for each type of failure. The type of failure of the device may then be detected by comparing its energy flow spectrum with various classes of energy flow spectra.

The upper and lower limits of a specific class of devices may be determined by ^"testing a set of devices known to belong to that class. For example, the upper limits may be determined as the average of energy flow spectra within the corresponding frequency range ^"plus three times the standard deviation. Likewise, the lower limits may be determined as the average of energy flow spectra within the corresponding frequency range minus three times the standard deviation.

It is presently preferred to specify classes solely by upper limits .

Preferably, parameters are determined from output signals from more than one sensor, e.g. by adding energy flow or quasi energy flow from two or three energy flow sensors. Adding signals from more than one transducer increases the signal to noise ratio and also increases classification accuracy.

Having classified the devices, the moving means may be controlled to remove the devices from the fixture to the power connecting means for disconnecting the power to the device. From the power connecting "means the devices may be moved to a position selected from a set of predetermined positions.

Each position of the set of predetermined positions may correspond to a specific predefined class and, after classification, the moving means may position the device at the position corresponding to the class of the device. For example, if the device has no failures, the moving means may position the device at a conveyor belt transporting the device to the shipping department, or, if the device has a failure, the moving means may position the device at a conveyor belt transporting the device to the repair department. The quality control system may also comprise storage means for storing parameters determined by the measurement apparatus. The data may be retrieved at the repair department for easier repair of the faulty devices, or the data may be used for statistical purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, a preferred embodiment of the quality control system will be described with reference to the drawings, wherein

Fig. 1 shows a general layout of a preferred embodiment of the quality control sjystem ac-cording to the invention,

Fig. 2 is a flow diagram of the quality control system of fig- 1,

Fig. 3 shows a cross section of a fixture according to the invention with an energy flow transducer and a motor under test ,

Fig. 4 shows a top v ew of a pyramid-shaped element fixture and three energy flow transducers,

Fig. 5 shows a cross section of an energy flow transducer,

Fig. 6 shows a reference energy flow frequency spectrum from a motor without faults,

Fig. 7 shows a frequency spectru 'from a motor with a fault in the collector,

Fig. 8 shows a .frequency spectrum from a motor with an un- balance problem,

Fig. 9 shows a frequency spectrum from a motor with a fault in the bearings, and Fig. 10 shows a frequency spectrum from a motor where the _.shaft has been misaligned^--

DETAILED DESCRIPTION OF THE DRAWINGS -

The preferred embodiment of the quality^" control system will -5- now be described with reference to fig. l^"and 2.

Fig. 1. shows the general layout of the quality control system and fig. 2 is a flow_ diagram displaying the single steps of making measurements using a preferred embodiment of the quality control system.

0 When the electro motors 10 have been manufactured in the production department, they are positioned on a conveyor belt 1 and transported to the quality control system illustrated in fig. 1. The quality control system is controlled by a PLC 7, comprising control of a product transfer robot 2, an 5 energy flow measurement system 3 with an energy flow transducer 11, a conveyor belt 8 for transporting the tested devices to a packing department, and a conveyor belt 9 for transporting the tested devices to a repair department.

- At start-up of the system, the robot 2 is positioned in a 0 start position (step 20) . When a motor 10 arrives on the conveyor belt 1 to the measurement set-up from the production department it passes a bar code reader 13 that reads a bar code attached to the motor 10 (step 21) . The information extracted from the bar code is transferred to the PLC 7 5 making it possible for the PLC 7 to identify the type of the motor 10. Included in the bar cod£ information is also a serial number identifying the individual motor.

The PLC 7 then issues a signal "Take the motor" to the product transfer robot 2 and the robot 2 takes the motor 10 from a pallet on the conveyor 1 (step 22), connects it to a connector for power supplying the motor 10 (step 23), turns on the power to the motor 10, and, when the motor 10 is running at a steady speed (step 24) , the motor 10 is positioned on the test fixture 12 (step 25) . The energy flow t ansducer 11 picks up the vibrational energy flow (step 26) and transmits them -to a dual channel frequency analyzer 4^". The energy flow frequency spectrum is determined and the spectrum is transmitted to a PC 5.

The PC 5 compares the energy flow frequency spectrum to a reference spectrum, - and if they are within the set tolerances, the quality of the motor 10 is accepted and the motor 10 is- classified accordingly, otherwise the motor 10 is classified as being faulty (step 27). From the energy flow frequency spectrum of a faulty motor 10, it is possible to determine and distinguish between a number of_ different faults. This will be explained in greater detail below. Together with the serial number of the motor 10, the PC stores information on the type of fault, if any, in a data storage unit 6.

After the measurement has been finished, the PLC 7 issues a "Take the motor" signal, which makes the robot 2 remove the motor 10 from the fixture 12 (step 28) , turn off the power to the motor 10, and disconnect the power connector to the motor 10 (step 29) .

If the motor 10 has been classified as being OK, the robot 2 positions the motor 10 on the conveyor belt 8 leading to the packing department (step 31) , otherwise the motor 10 is positioned on the conveyor belt 9 leading to the repair department (step 32). In the repair department, information on the fault can be retrieved using a bar code reader and the information stored in the data storage unit 6, and the motor 10 can be repaired accordingly.

Finally the robot 2 is moved to the start position (step 33) and is ready for the next motor 10. In fig. 3 a preferred embodiment of an energy flow test table^" 3 is shown.- The energy flow test table 3 comprises a platform 44 on which the robot 2 can position the motor 10 during the test. The platform 44 is connected to three air springs ^"40 via three rods 45. The air—springs 40 are positioned on the fixture 12. Also positioned on the fixture 12 is a vibrational energy absorber 41, whereon an energy flow transducer 42 is positioned. -The energy flow transducer 42 is connected to the motor 10 o platform 44 via a stinger 43.

The air springs 40 and the vibrational energy- absorber 41 insulates-the motor 10 and the energy flow transducer 42 froπr external vibration, thereby improving the signal to noise ratio of the measurements carried out by the energy flow transducer ^"42. When the motor 10 is connected to the power supply and is in operation, the vibrations or the mechanical- energy from the motor 10 are transmitted via the stinger 43 to the energy flow transducer 42, thereby achieving a point measurement of the mechanical energy.

The energy flow transducer 42 comprises an accelerometer and a force transducer connected in series. The measurements made by the accelerometer and the force transducer is converted into voltage signals and sent to the dual channel frequency analyzer 4, where the frequency spectrum of the signals is determined. The frequency spectrum is then transmitted to the PC 5 and compared to a reference mask of a motor without failures .

Fig. 4 shows a top view of a pyramid-shaped element fixture 46 and without a motor positioned'upon it. The pyramid-shaped fixture 46 comprises four surfaces of equal size. On the top surface, a magnet is arranged to receive and hold a motor. On each of the other three sides or surfaces of the pyramid- shaped fixture 46, one end of a stinger 47 is connected. The stingers 47 relay the vibrational energy from a motor positioned on the pyramid-shaped fixture 46 to the three energy flow transducers 48 positioned at the other ends of the stingers 47. The energy flow transducers 48 are mounted on vibrational energy absorbers- 49 isolating the pyramid-shaped fixture 46 from the surroundings.

Fig. 5 shows a cross^" section of an energy flow transducer 63, where a force transducer and an accelerometer have been build into the same ^"housing 64. The force transducer part 50 of the energy flow transducer 63 comprises a piezoelectric element 53, an insulating plate 55, a seismic mass 65, and a clamping ring 54. The piezoelectric element 53^" is positioned between the seismic mass 65 and an- insulating plate 55 and clamped to the seismicr mass 65 by the clamping ring 54. A silicone 0- ring 52 insulates the seismic mass 65 from the housing 64. The transducer 63 ^"may-be mounted on or connected to an object via a connector 59. When- the force transducer part 50 is subject to- external forces, shear forces will be created on the piezoelectric element 53, which in turn will generate an electrical charge in the piezoelectric element 53. The electric charge is a measure of the force acting on the transducer .

The accelerometer part 51 of the energy flow transducer 63 comprises a seismic mass 57, a clamping ring 56, a base 66, and a piezoelectric element 58. The piezoelectric element 58 is positioned between the base 66, to which it is in conducting engagement, and the seismic mass 57, and the clamping ring 56 is arranged ^"to clamp the piezoelectric element 58 and the seismic mass 57 to the base 66.

When the transducer 63, and thereby the accelerometer part 51, is subject to accelerations, ttie movement of the seismic mass 57 in relation to the base 66, will create shear forces on the piezoelectric elements 58, which in turn will generate an electrical charge in said piezoelectric element 58. The electric charge is a measure of the acceleration acting on the transducer 63. The outputs from the force transducer part 50 and the accelerometer—part 51 are^~transmitted to receivers via. the connectors 61 and 62 respectively.

Fig. 6 shows a spectrum of the energy flow of an_electro -5 motor. The spectrum is shown as a bar chart with each bar representing the energy flow at a 1/3 octave frequency band. The energy flow frequency spectrum is mea-sured at frequencies up to 10 kHz, and the scale on the frequency axis is logarithmic .

0 From a^{" '}large number of reference- otors , a corresponding large number of energy flow spectra can be determined. By averaging the spectra, a reference spectrum can be determined as the mean value of the values in each- frequency band. The reference spectrum has a specific profile common to all 5 motors manufactured without faults, and this reference spectrum sets the standard that should be achieved by all motors.

. tolerance mask is calculated for the energy flow spectra by using the mean value for each frequency band plus/minus a value estimated from the standard deviation. The tolerance 0 mask thereby defines an upper limit and a lower limit for the vibrational energy flow for each frequency band.

/ / / / / / / / /

Fig. 7 shows an example of a tolerance band. The bars 72 going from the base line and upwards define the lower limit 5 for the energy flow, while the bars 71 going from the top and downwards define the upper limit for the energy flow. The area 73 between the two lines of bars 71, 72 is the acceptance area or the mask 73.

The black lines 74 in the figure display the energy flow 0 spectrum for a measurement of a motor. Each of the black lines 74 displays the amplitude of the energy flow determined by the dual channel frequency analyzer 4 and transmitted to the PC 5. If all the -amplitudes 74 in the energy flow spectrum are within the mask 73, the motor is classified as OK, ie without faults. If, however, one or more of the amplitudes _74 are outside the mask 73 the motor is faulty.

Motor failures can be detected and classified in accordance with the determined, difference between the frequency spectrum of the motor during the test and o the reference motor. The quality control system may be calibrated, by testing a large number of motors with known faults, ^"and for each type^~"bf fault a number of parameters^~may be derived describing the characteristics of the faults.

A neural- network may be alternatively be utilized in the process of determining if a motor-is faulty, and if the motor is faulty, the network may specify the kind of fault. The data from the above mentioned large number of reference motors may be read by the neural network program, and the program may learn to distinguish between the data from a good reference motor from the data from a faulty motor. Further more, the data from measurements in the daily production may be feed to the computer program, in order to let the program gain more experience . The neural network program may be a program such as QC-Brain developed by Brϋel & Kjasr, Denmark.

In fig. 7, a measurement result relating to- a motor with a fault in the collector is displayed. The fault is detected as the amplitude 74 for the frequencies around 1.2 kHz and around 2.5 kHz are outside the mask 73. From the calibration of the quality control system, it _*is known that if the amplitudes 74 are outside the mask around 1.2 kHz and around 2.5 kHz, the fault is located in the collector.

In fig. 8 amplitudes 74 in the frequency area of 125-250 Hz are outside the mask 73. From the calibration of the quality control system it is known that this indicates a motor with an unbalance of the shaft. In fig. 9 there, are two areas with amplitudes 74 outside the mask 73. The amplitudes 74 for—frequenciei^~above 4 kHz are outside .the mask 73, indicating a motor having a fault in the bearings. Also the parameters 74 for^~frequencies around 315 Hz are outside the mask 73, indicating a motor with an unbalance -of the shaft. A motor with a fault in the bearings will very often also have problems with an unbalance of the shaft, and it is therefore not surprising that the frequency spectrum indicates both faults. It is obvious that it is possible ^"to distinguish between the spectrum for a motor having a fault in the bearings, as shown _in fig. 9, and the spectrum for a motor with an unbalance of the shaft, as- shown in fig 8, even though the spectrum for a motor having a fault in the bearings also includes parameters indicating that the motor has an unbalance of the shaft.

Also in fig. 10 there are two areas with amplitudes 74 outside the mask 73. The amplitudes 74 for frequencies around 315 Hz are outside the mask 73, indicating a motor with an unbalance of the shaft, but also the amplitudes between 80 Hz and 125 Hz are outside the mask 73, indicating a motor with misalignment problems. A motor with misalignment problems will very often also have problems with an unbalance of the shaft, and it is therefore not surprising that the frequency spectrum indicates both faults.

Claims

1. A quality control system for testing devices, comprising

a fixture adapted to hold and operationally engage with the device,

5 at least one energy flow transducer being-positioned at the fixture so as- to be in operational communication with the device when the device is operationally engaged with the fixture,/ the transducer

sensing acceleration of and force exerted by the device, l.Q and

providing electrical output signals in response to and as a function of sensed accelerations and forces, respectively, and

a measuring apparatus for receiving the output signals from 15 the at least one energy flow transducer, and for determining parameters derived from sensed forces and accelerations.

^"2. A system according to claim 1, further comprising moving means for -positioning the device on the fixture in operational engagement therewith.

0 3. A system according to claim 2, wherein the moving means are adapted to position the device at a power connecting means, and the quality control system comprises means for connecting the device to a power srource .

4. A system according to claim 3, wherein the power connect- 5 ing means are adapted to connect the device to the power source before the moving means positions the device on the fixture .

5. A system according to any of the preceding. claims, comprising at least two energy flow transducers positioned so as to allow determination of acceleration and force in two dimensions .

6. A system according to any of the preceding claims, comprising at least three energy flow sensors positioned so as to allow determination of^" acceleration and force in three dimensions .

7. A system according to any of the preceding claims, wherein at least one energy flow transducer is connected to the^" fixture via a stinger so as to eliminate transfer of torque to the energy flow transducer in question.

8. A system according to any of the preceding claims, wherein the measurement apparatus comprises at least one dual channel spectrum analyzer, the channels of the spectrum analyzer being connected to the at least one energy flow transducers, respectively.

9. A system according to claim 8, wherein the measurement apparatus further comprises processor means adapted to calcu- late the energy flow spectrum" of the output signals of the at least one energy flow transducer connected to the at least one spectrum analyzer.

10. A system according to claim 8 or 9, wherein the processor means are adapted to calculate the quasi energy flow spectrum of the output signals of the energy flow transducer connected to the at least one spectrum analyzer.

11. A system according to any of claims 4-10, wherein the moving means are adapted to remove the device from the fixture and position the device at a position selected from a set of predetermined positions.

12. A system according to any of the preceding claims, further comprising classification means adapted to classify the device in accordance with determined parameters into one class of a set of predefined classes.

13. A system according to claim 12, wherein the^_classifica- tion means are adapted to classify the device in accordance with the determined parameters into one class of a set of predefined classes defined by predetermined ranges- of values calculated from energy flow spectrum values determined from the output signals of selected energy flow transducers within specific corresponding frequency ranges.

14. A system according to claim 12, wherein the classification means are adapted to classify the device in accordance with determined parameters into one class of a set of prede- fined classes defined by predetermined ranges of values calculated from quasi energy flow spectrum values determined from the output signals of selected energy flow transducers within specific corresponding frequency ranges.

15. A system according to any of claims 12-14, wherein each position of the set of predetermined positions corresponds to a specific predefined class, the moving means being adapted to position the device at the position corresponding to the class of the device.

16. A system according to any of the preceding claims, where- in the fixture comprises a pyramid-shaped element, the device being positioned at a first pyramidal surface of the pyramid- shaped element and each of the at least one energy flow transducers being in operational communication with corresponding pyramidal surfaces of the pyramid-shaped element whereby forces and acceleration may be sensed along a plurality of directions.

17. A system according to claim 16, wherein the pyramid- shaped element is a regular pyramid-shaped element.

18. A system according to claim 16 or 17^ wherein the length of a side of the-pyramid-shaped element is approximately ^"" 6 cm.

^•19. A system according to any of claims 16-18, wherein the pyramid-shaped element is made of Al .

20. A system according to any of claims 16-19.- wherein the ^' pyramid-shaped element is solid.

21_. A fixture siaped as a pyramid for measurement of energy flow from a device and having a first pyramidal surface adapted to hold and engage with the device, and second pyra- -midal surfaces, each of ^which is^"~adapted to be in^" operational communication with a vibration transducer so as to allow sensing of vibrations along a plurality of directions.

22. A fixture according to claim 21, having the shape of a regular pyramid.

23. A fixture according to claim 21 or 22, having a side length of approximately 6 cm.

24. A fixture according to any of claims 21-23, made ^"of Al .

25. A fixture according to any of claims 21-24, that is solid.

26. A method of testing a device, comprising the steps of

positioning the device in a fixture adapted to holding and engaging with the device,

sensing acceleration- of and force exerted by the device using at least one energy flow transducer in operational communication with the device, and determining parameters derived from sensed forces and accelerations .

27. A method according to claim 26, furthe^"r comprising the step of ^"connecting the device to a power source before posi- tioning the device in the fixture, so that the device is operating during testing.

28. A method according to claim 26 or 27, wherein the_.s_ensing step comprises sensing of acceleration and force^~in at least two dimensions using at Least two energy flow transducers positioned in operational communication with the device.

29. A method according to any of claims_ 26-28, wherein the sensing step comprises sensing of acceleration and force in at least three dimensions using at least three energy flow transducers positioned in operational communication with the device.

30. A method according to any of claims 26-29, wherein the sensing step comprises transferring acceleration and force from the pyramid-shaped element to at least one energy flow transducer via a stinger, so as to eliminate transfer of torque from the pyramid-shaped element to the at least one energy flow transducer.

31. A method according to any of claims 26-30, wherein the determination step further comprises the step of determining spectra of selected output signals of the at least one energy flow transducer.

32. A method according to claim 31, wherein the determination step further comprises the step of determining energy flow spectra of selected output signals from the at least one energy flow transducer.

33. A method according to claim 30 or 31, wherein the determination step further comprises the step of determining quasi energy flow spectra of selected output signals from the at least one energy flow transducer.

34. A method according to any of claims 26-337 further comprising the step of removing the device from the fixture and positioning the device at a position selected from a set of predetermined positions.

35. A method according to any of claims 26-34, further comprising the step of classifying the device in accordance with determined parameters into one class of a set of predefined classes.

36-. A method according, to claim 35, wherein the.predef ned classes are defined by predetermined ranges of values calculated from energy flow spectrum values determined from the output signals of selected energy flow transducers within specific corresponding frequency ranges.

37. A method according to claim 35, wherein the predefined classes are defined by predetermined ranges of values calculated from quasi energy flow spectrum values determined from the output signals of selected energy flow transducers within specific corresponding frequency ranges.

38. A method according to any of claims 35-37, wherein each p^"osition of the set of predetermined positions corresponds to a specific predefined class and, after classification, the moving means position the device at the position correspon- ding to the class of the device.

39. A method according to any of claims 26-38, wherein the fixture comprises a pyramid-shaped element, the step of positioning the device on the fixture comprising positioning the device on a first pyramidal surface of the pyramid-shaped element, and wherein the sensing step comprises sensing forces and acceleration along a plurality of directions using a plurality of energy flow transducers positioned in operational communication with corresponding pyramidal surfaces of the pyramid-shaped element .