TECHNICAL FIELD

The invention is related to condition monitoring of machines, especially bearings in machines, in particular in relation to bearings of idler rollers of for example conveyor belts.

BACKGROUND

Bearings are a very important component in rotating machinery. If a bearing fails, then the complete functionality of the machinery usually also fails. In some applications it might be very difficult or just extremely expensive to replace a failed bearing outside regular scheduled maintenance. Such applications might be continuous manufacturing lines, including conveyors of for example mines and cement plants.

A trough type conveyor will traditionally comprise a conveyor belt that is typically supported by three idler rollers for approximately every meter in length. Each idler roller will usually comprise two bearings each, giving six bearings per meter of conveyor. Other types of conveyors, such as pipe conveyors, will also comprise idler rollers for support. Conveyors in mines and cement plants can easily be several kilometers long, resulting in more than ten thousand bearings being incorporated in a single conveyor system. If a bearing of an idler roller fails, then most likely the idler roller will seize. A seized idler roller can fray and cut the conveyor belt. Conveyor belt damage can result in expensive belt repair/replacement, risk of workers' safety, and/or unplanned downtime if no parallel belt or buffer stock exists. Some plants can have as many as 5 to 8% of the conveyor idlers failing in a month. Condition monitoring is done in an attempt to predict when a bearing needs to be replaced before it fails, suitably enabling replacement in an orderly and scheduled manner. It is difficult to overcome the logistical problem of monitoring possibly more than ten thousand bearings of a single conveyor system. Traditionally condition monitoring has been done with a worker walking along the conveyor belt listening and looking for failing or failed conveyor idler rollers. It is hard to detect failing idlers over commonly a very high background noise level. A more advanced method of condition monitoring is to use a thermographic camera to detect failing idlers. Unfortunately it is very time consuming and only catches failing idlers at a very late stage of failure. Thus there seems to be room for improvement in the ways of assessing the condition of a bearing, especially a bearing of an idler roller.

SUMMARY

An object of the invention is to define a method and means to improve the monitoring of a condition of a bearing, especially a bearing of an idler roller. The aforementioned object is achieved according to the invention by the use of an acoustic sensor unit according to the invention comprising a parabolic dome with a high pass filter characteristic. Industrial environments are traditionally noisy, this makes it difficult to do condition monitoring on machines and/or bearings in such environments. One method according to the invention uses acoustic signals to determine bearing condition. To reduce the noise from frequencies that are not of interest to analyze for determining bearing condition, these unwanted frequencies are filtered away as early as possible, even before conversion between acoustic pressure waves to electronic signals. This is accomplished by a parabolic dome having a high pass frequency filter characteristic, thus dampening frequencies below a cut off frequency and amplifying frequencies above. The cut off frequency is suitably in the range of 3 kHz to 6 kHz, and preferably about 5 kHz. In addition, due to the filtering characteristics of the parabolic dome, the electrical signal can advantageously just be enveloped for some further processing. The aforementioned object is further achieved according to the invention by an acoustic sensor unit comprising a parabola shaped dome and an acoustic sensor mounted therein. The acoustic sensor is mounted within the parabola shaped dome such that it coincides with a focus of the parabola shaped dome. According to the invention the parabola shaped dome is dimensioned to have a high pass filter characteristic on acoustic pressure wave signals entering the parabola shaped dome. Suitably the high pass filter characteristic has a cut-off frequency somewhere between 3 kHz and 8 kHz, such as somewhere between 4 kHz and 6 kHz. Preferably the high pass filter characteristic has a cut-off frequency around 5 kHz.

The parabola shaped dome of the acoustic sensor unit suitably has an acoustic entrance opening with a diameter in the range of 75 mm to 150 mm, has depth from the acoustic entrance to the back in the range of 50 mm to 70 mm, and has a focal point in the range of 8 mm to 12 mm, or it has an acoustic entrance opening with a diameter in the range of 90 mm to 115 mm, has depth from the acoustic entrance to the back in the range of 55 mm to 65 mm, and has a focal point in the range of 9 mm to 11 mm. Preferably the parabola shaped dome has an acoustic entrance opening with a diameter of 100 mm to 106.5 mm, has depth from the acoustic entrance to the back of 60 mm, and has a focal point of 10.42 mm. Suitably the parabola shaped dome is made of plastic with a flat inner surface.

The aforementioned object is also achieved according to the invention by a machine condition monitor unit comprising user input means in the form of a keyboard, user output means in the form of a display and a digital signal processor arranged to analyze electrical signals received from an acoustic sensor in view of bearing diagnostics. According to the invention the machine condition monitor unit is arranged to be coupled to any acoustic sensor unit described above. Suitably the bearing condition monitor unit envelopes a signal received from the acoustic sensor unit. Other advantages of the invention will become apparent from the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail for explanatory, and in no sense limiting, purposes, with reference to the following figures, in which:

Fig. 1A shows a view of a condition monitoring unit according to the invention in a typical application monitoring a conveyor, Fig. 1 B shows a cross section of a conveyor,

Figs. 2A & 2B show two different embodiments of the acoustic sensor unit according to the invention, Fig. 3 shows a side view of the acoustic sensor unit according to the invention,

Fig. 4 shows a functional block diagram of a condition monitoring unit according to the invention,

DETAILED DESCRIPTION

In order to clarify the inventions, some examples of its use will now be described in connection with Figures 1A to 4 Figure 1A shows a view of a condition monitoring unit 100 according to the invention in a typical application monitoring a conveyor 1 10. A conveyor 1 10 will convey goods, such as gravel, on a conveyor belt 1 19 from a first place to a second place. The conveyed distance can be in the range of kilometers. The illustrated conveyor section 110 is of a trough conveyor and will typically require three idler rollers 112, 114, 116 approximately every meter along the length of the conveyor 110. One idler roller 114 is located directly underneath the belt 119 and one idler roller 112, 116 is located on each side, bending up the belt, to thereby create a trough. Figure 1B illustrates a cross section of such a conveyor.

Each idler roller 112, 114, 116 along the whole conveyor will typically comprise two bearings, one on each end of an idler roller 112, 114, 116. A complete conveyor will then easily comprise more than a thousand bearings. It is these bearings that the condition monitoring unit 100 according to the invention is monitoring the condition of. Typically a worker will walk along the length of the conveyor 110 and point the condition monitoring unit 100, or at least an acoustic sensor of the condition monitoring unit, towards the idler rollers 112, 114, 116 carrying the bearings of interest. It has been found that by analyzing sound pressure signals within the acoustic frequency range, an assessment of the condition of a bearing can be made. Acoustics cover frequencies from zero Hertz up to several mega Hertz and is usually subdivided into infrasound covering 0 Hz to about 20 Hz, sound covering about 20 Hz to 20 kHz and ultra sound being above 20 kHz up to several mega Hz.

For diagnosing bearings it has been determined that frequencies above 3.5 kHz to 5 kHz up to about 40 kHz are valuable. According to the invention, to enhance this, an acoustic parabolic dome is designed to have a lower cut-off frequency somewhere between 3.5 kHz to 5 kHz and an amplification of frequencies above this, for an acoustic sensor, such as a microphone, placed in the acoustic focus point within the parabolic dome. Such a parabolic dome will thus enhance the frequencies that are essential for further signal processing to reach a diagnosis and suppress frequencies outside this range. To attain this high pass characteristic, the parabolic dome will be relatively deep in relation to its width, and thus also provide physical protection to an acoustic sensor.

Figures 2A and 2B illustrate two different embodiments of the acoustic sensor unit 202, 206 according to the invention. Figure 2A illustrates a first embodiment of the acoustic sensor unit 202 which is attached directly 203 with a digital signal processing unit 200 comprising both input and output. This makes a compact unit that is easily handheld and directed towards for example a conveyor. Figure 2B illustrates a second embodiment of the acoustic sensor unit 206 which is in-directly attached 207 to a digital signal processing unit 200. This enables the digital signal processing unit 200 to for example be attached to a belt, making a very light and easily maneuverable acoustic sensor unit 206. A user would then most likely rely on an acoustic/audio feedback from the digital signal processing unit 200 for alarms etc.

Figure 3 illustrates a side view of the acoustic sensor unit according to the invention. The acoustic sensor unit comprises an acoustic sensor 350 such as a microphone, for example an ICP Array Microphone, PCB-130D20, or alternatively one from Knowles. The acoustic sensor 350 should preferably have a frequency range of 20 Hz to 80 kHz, at least 20 Hz to 40 kHz, the mentioned microphones are tested to comply with this. The acoustic sensor 350, which converts sound wave pressure to electrical signals, suitably has an electrical connector 356 or alternatively an electrical cable attached directly to it.

The acoustic sensor unit further comprises a parabolic dome 360. The parabolic dome 360 is designed to give the best balance between directional focus and amplification of signals in the range of interest. A shallow depth tends to give a too tight focus area, to only pick up signals from directly where a parabola dome is pointed at. The shape is also designed to filter out low frequency noise and to amplify higher frequencies within our band of interest. The frequency band of interest is 3.5 kHz to 40 kHz and mostly so within the frequency range of 5 kHz to 40 kHz. The parabolic dome according to the invention will thus have a high pass characteristic with a cut off frequency somewhere in the range of 3.5 kHz to 5 kHz. As an example, one embodiment has a depth 364 of 60 mm a diameter 362 of 100 mm and a focal point 354 of 10.42 mm, where of course the membrane 352 of a microphone would be located. In another embodiment a diameter 362 of 106.5 mm is specified. The acoustic sensor unit will also comprise an extended cover/protection 366 which suitably acts as a connector between a microphone 350 and the parabolic dome 360, and possibly also as a handle.

The material used for the parabolic dome 360 needs to be hard, flat and robust. The inner surface needs to be very flat to reflect the sound waves/pressure correctly. The parabolic dome must also be strong and robust enough to give the acoustic sensor a physical protection. A preferred material is ABS plastic. Metal is not encouraged as it can create a ringing effect with sounds.

Figure 4 illustrates a functional block diagram of a typical condition monitoring unit according to the invention. There will be an acoustic sensor 450, typically a microphone, with a wide enough bandwidth to be able to sense sound pressure in the frequency range of interest. Typically an acoustic sensor 450 for the purposes of the invention should have a bandwidth of 20 Hz to at least 40 kHz. The acoustic sensor 450 is coupled to a pre-processing block 472, which will suitably comprise amplification, analog to digital conversion, filtering and possibly automatic normalization. It is possible to implement the invention completely in the analog domain, but normally it is a desire to do as much processing as possible in the digital domain, thus putting the analog to digital conversion as close as possible to any analog input. For most further digital signal processing 474 it is advantageous that the signal is enveloped 473. The core digital signal processing 474 will have some user interface input 476, such as a keyboard, for changing parameters and limits. There will also be an output 478, such as a screen, to show the results of the analysis. In some applications it is desirable to have an audio output 480, for example in the form of headphones.

The invention is not restricted to the above-described embodiments, but may be varied within the scope of the following claims.

FIGURE 1A shows a view of a condition monitoring unit according to the invention in a typical application by a conveyor,

100 A condition monitoring unit according to the invention

110 Conveyor

112 Right side idler roller,

114 Bottom idler roller,

116 Left side roller,

19 Conveyor belt.

FIGURE 1 B shows a cross section of a conveyor,

112 Right side idler roller,

114 Bottom idler roller,

116 Left side roller,

119 Conveyor belt.

FIGURE 2A and 2B show two different embodiments of the acoustic sensor unit according to the invention,

200 Digital signal processing unit with keyboard input and display output,

202 Parabola of acoustic sensor unit with direct attachment to digital signal processing unit.

203 Connector of acoustic sensor unit,

206 Parabola of acoustic sensor unit with indirect connection to digital signal processor unit, suitably comprising a carrying handle,

207 Indirect connector.

FIGURE 3 shows a side view of the acoustic sensor unit according to the invention.

350 Microphone, 352 Microphone membrane,

354 Distance from bottom of parabola to microphone membrane

356 Microphone electrical connector,

360 Parabola,

362 Diameter of parabola opening,

364 Distance from bottom of parabola to opening of parabola,

366 Extension of parabola, microphone protection, microphone connection to parabola. FIGURE 4 shows a functional block diagram of a condition monitoring unit according to the invention,

450 Acoustic sensor unit/microphone,

472 Amplification, AID conversion, Band Pass Filter,

473 Enveloping,

474 Further digital signal processing,

476 Input/keyboard,

478 Output/Screen,

480 Optional output/headphones,