THE OUTLOOK FOR USE OF NEW GENERATION VIBRO ACCELOMETERS FOR DIAGNOSING DINAMIC PARAMETERS OF THE FACILITIES OF AVIATION, POWER ENGINEERING AND OIL-GAS SYSTEMS
A basically novel type of a measuring means – triaxial with a one sensitive element (vector) piezoelectric vibro accelerometer. This domestic development for the first time provide possibilities for obtaining complete (true) data about vibration.
Due to considerable wear-off of the equipment in various industry spheres it becomes more and more important an issue of monitoring the dynamic parameters of various engines and mechanisms in order to evaluate the tolerable terms of their operation and also to prevent accidents to helicopters and airplanes or the destruction of engine rotors or oil- and gas-pipelines. However, one-axial vibro-pickups usually used for this purpose wide over the world fail to succeed in solving the problem of vibro-diagnostics which result in great expenses for repair of the equipment than could be significantly reduced by making the efficient system of vibration control.
The Russian company «Vector’s vibromeasuring» LLC has developed the production technology of new generation vibro-accelerometers, i.e. triaxial (vector) pickups with a single sensitive element. These pickups have the Russian and International patents and the Certificate of the Russian Federation Standardization Board and been tested by the leading domestic firms and also abroad ( in Switzerland, Sweden, Denmark and the USA).
The usage of vector vibro accelerometers in aviation, thermal and nuclear power engineering and in oil- and gas-industry systems opens basically new technological opportunities for vibrodiagnosing and aligning turbine rotors thus allowing to considerably reduce any losses in people`s, technical and financial resources as well as to reduce the amount, terms and costs of repair operations.
Advantages of major pickup parameters over the best world analogues:
- Reriable measurement of the vibroacceleration vector;
- Three measuring axes in the presence of single sensitive element, i.e. one point of reference;
- Absence of the phases difference between vibrations along three pickup axes;
- Wide frequency and temperature range, low sensitivity temperature dependence.
Areas of application:
- Aviation (engines especially);
- Thermal and nuclear industry (turbines balancing especially);
- Oil- and gaspumping (compressor stations) and fuel transportation;
- Foundations of high risk objects;
- Machinebuilding.
Application of vector pickups allows:
- To reliably define vibrocondition of different machines and mechanisms;
- To forecast vibration condition of different dynamic objects, resulting in correcting repair and operational standards;
- To reduce labour and financial expenses with the simultaneous increase of operational safety of the objects;
- To decrease the probability of industrial accidents.
New Generation Triaxial Vibro accelerometers for Application in Aircraft and Helicopter Safety Systems and for Vibrodiagnosing of Turbines in Power Engineering Systems.
At present to define the vibration conditions of different machines and mechanisms piezoelectric monoaxial vibro accelerometers are used.
With the help of these pickups it is possible to measure the projection of this vector on an axis of the pickup. But this projection allows to define neither the value nor the direction of the vibroacceleration vector a. It is evident that to solve this problem not a monoaxial but a triaxial pickup is required which enables to obtain the value and direction of acceleration vector a on the basis of its three projections. However, all widely known triaxial pickups are designed as three monoaxial ones incorporated into one housing such as, for example, "Bruel & Kjaer", type 4321.
But since in this case the pickup's three sensitive elements are placed in different points of space, that is there are three reference points with non-coincident coordinates, the laws of physics prohibit to consider their readings as acceleration vector a projections. Nevertheless, this consideration is neglected thus leading to distorted information on the aircraft, helicopters, power engineering turbines, etc. vibration level be obtained in course of the monitoring procedure.
The basic difference of the proposed triaxial pickup consists in the following: only one sensitive element (that is a single point of reference) is used - in the form of the rectangular parallelepiped, which has six faces covered by metal manufactured from piezoelectric crystal of suitable class of symmetry and orientation. This pickup's principle of operation is shown in Fig.2.
If the vibroacceleration vector a is directed along Z-axis, the charges occur only on the pair of faces which are perpendicular to Z-axis. If it is directed along Y-axis, then the charges occur only on the pair of faces which are perpendicular to Y-axis. And, finally, if the vibroacceleration vector a is directed along X-axis, the charges occur only on the pair of faces perpendicular to X-axis. In each of these three cases on the remaining four faces there will be no charges at all. It follows from the symmetry theory and the laws of tensor analysis. As a result theoretically transverse sensitivity of this pickup should be equal to zero. This can be represented in form of the following matrix.
| VIBRATION | |||||
|---|---|---|---|---|---|
| OUTPUT ELECTRIC SIGNAL | X |
Y |
Z |
% | |
X |
100 |
0 |
0 |
||
Y |
0 |
100 |
0 |
||
Z |
0 |
0 |
100 |
||
Practically this is within general requirements (<5%). The quality of the proposed pickup depends on the quality of the piezoelement, cable, connectors and the glue. But even when using the domestic production components there have been obtained the parameters which are substantially better аs compared to those of a widely known pickup by «Bruel & Kjaer» type 4506 and «Endevco» type 2228С (see the Table).
| Technical Specification | Proposed accelerometer type VTA 3 | Proposed accelerometer type VTA 7 | «Bruel & Kjaer» type 4506 | «Endevco» type 2228C |
|---|---|---|---|---|
Number of measuring axes |
3 |
3 |
3 |
3 |
Number of sensitive elements |
1 |
1 |
1 |
3 |
Main sensitivity (pickup + amplifier), mV/g
|
100 |
100 |
100 |
- |
Maximum transverse sensitivity, % |
5 |
5 |
5 |
5 |
Mounted resonance frequency along Z-axis, kHz |
51.5 |
25.5 |
10 |
21 |
Typical frequency range (5%) along Z-axis, Hz |
5 … 20000 |
5 … 20000 |
||
Typical frequency range ( ±5%) along Z-axis, Hz |
20 … 4000 |
|||
Typical frequency range (±10%) along Z-axis, Hz |
1 … 2000 |
|||
Operating temperature, °C |
–60…+250 |
–60…+250 |
–54 … + 121 |
–55 … + 175 |
Temperature coefficient of sensitivity |
0.1%/°C |
0.1%/°C |
1 ms-2/°C |
0.2%/°C |
Dimensions, mm |
Ø22x26 |
Ø38x28 |
17x17x14,5 |
- |
| Mass without cable, gram | 27 |
39 |
15 |
15 |
The main difference between proposed pickup and its well-known analogue ones is not confined to the fact that some parameters аre better and others аre worse but it lies in the following: only this pickup has a single point of reference which allows to obtain on the basis of three projections the value and the direction of the vibroacceleration vector a in this point. This means that only new generation pickups allows to obtain reliable data on the vibration state of different machines and mechanisms. When using any other pickups the link between the parameters of the mechanical vibration and the electric signals of the vibromeasuring system is extremely doubtful.
The pickup operates within a set that incorporates a tree-channel charge amplifier with a symmetrical (differential) high-ohmic input and а non-symmetrical low-ohmic output. The DC power supply unit for ±15 V, is attached to the amplifier.
Under production of pickups we make use of the supplies from the USA or Europe, for instance, we are purchasing the piezoelements at APC-company, USA, or “Ferroperm”, Denmark.
In March 2001 the comparative tests of the regular single axial pickup (type ABC-117, “Biofizpribor”, Lvov) and triaxial new age pickup (type VTA3, “REM”, Moscow) were conducted on the aviation engine (type АИ-9В) in real-life environment.
The results of the tests on one of the engine-operating mode are shown on Fig. 3. Single axial pickup was fixed on the engine parallel to the axis Z of the triaxial pickup and their indications coincided (a = 1.5 g). However the acceleration along axis X of the triaxial pickup was 5g and along axis Y – 5.5g. So, the magnitude of vibroacceleration, measured by the new age pickup was 7.6 g. At the same time the regular pickup indicated only 1.5 g, i.e. the indications were 5-time underrated.
So, the conducted tests confirmed the fact, that the usage of single axial pickups leads to the underrating of the magnitude of the viroaccelararion vector.
