FI107356B - Pickup for measurement of vibrations - Google Patents

Description

107356

SENSOR FOR MEASUREMENT OF VIBRATIONS - GIVARE FÖR MÄTNING AV VIBRATIONER

The present invention relates to a sensor for measuring oscillations based on an acceleration sensor and outputting a signal of a generalized time derivative (fractional derivative) of the displacement greater than two. The invention utilizes the non-linear region (6) of the acceleration sensor and derives it by means of mass, damping and spring within the sensor (Fig. 1).

10 Traditionally, displacement, speed and acceleration sensors have been used to measure machine vibrations [1]. Patent applications [2] and [3] show that advanced acceleration time derivatives and generalized time derivative derivatives of the accelerator are useful for early detection of machine failures and for simplifying the interpretation of measurement results. The problem with this is that the signal from the displacement, velocity 15 or acceleration sensors, when using known solutions, has to be derivatized either by separate analog circuits or by performing calculations, whereby the accuracy of the results also diminishes. In addition, computational work consumes time and computational capacity, which is of particular importance when a condition monitoring system contains dozens or hundreds of sensors. The new solution to this problem is characterized by what appears in the patent claims section.

In the following, the invention will be described in detail with reference to the figures in which

Figure 1 shows a conventional piezoelectric accelerometer with 1 being mass, 2 being. , 25 piezoelectric element, 3 is spring and 4 is sensor body.

1073 ^ 6 i 2

Figure 2 shows the response of the accelerometer obtained during the vibration tests. 5 represents the linear region of the sensor and 6 shows the non-linear region of the sensor.

Fig. 3 shows the output of the acceleration sensor relative to the constant acceleration 5 of the accelerator, the ratio of the angular velocity ω and the angular velocity con = (k / m) U2, where k is the spring constant and m is the mass, as a function of [4], Kä; -rä 7 relates to a value of relative damping coefficient of 0.7.

Figure 2 shows the normally used linear operating range (5) of the acceleration sensor and the non-linear region (6) between the linear region and the resonance frequency of the sensor. Usually, the acceleration sensors use a value of relative damping factor between 0.65 and 0.7 [4] to obtain the widest possible linear range, as shown in the graph (7). In solving the problem, the non-linear region (6) of a conventional acceleration sensor will be utilized, which is normally an undesirable region for sensor operation. For example, the upper limit of the linear region of the piezoelectric acceleration sensor is approximately 1/3 of the resonance frequency of the sensor, leaving a large unused area between the linear region (5) and the sonar frequency. j i • · '20 The output of the acceleration sensor relative to the constant acceleration of the sensor with a constant amplitude can be calculated (Fig. 3) from the formula -, r - ί] 2 ω kjj J L <° n \ |

J

_____ j i i 4 3 107356

It can be seen from the expression (1) and Figure 3 that the acceleration sensor output depends on the relative damping factor when the calibration frequency and ωη remain the same. In the maintenance oscillation measurements, a ± 10% error is allowed [5, 6]. Here are some values of relative damping coefficient that can be used to construct sensors that measure x, xw and generalized derivative xi2A) directly so that the absolute error value is less than 1%. The calculations are based on equation (1) and the con value is 45000 Hz.

Measurement range Difference Calibration frequency 10 10 (24) 23000 - 37000 Hz 14000 Hz 15000 Hz 0.38

Sf 23601 - 29401 Hz 5800 Hz 17450 Hz 0.1 x (4) 30500 - 35200 Hz 4700 Hz 22875 Hz 0.1

The concept of a generalized derivative is discussed in more detail in patent application [3] and ΜΙ 5 in [7]. For example, the component x-2A of the spectrum of the generalized derivative x (24) is obtained from the component a of the acceleration spectrum. 2> a, - (: Uf, fAa, (2) • t • · 4 20, where / (is the frequency associated with component ai).

Maintenance measurements have particular benefit from accelerated transition time derivatives at higher frequencies, that is, above about 10000 Hz [2],: It has been shown above that sensors can be constructed in these frequency ranges that can directly measure for practical purposes 3c and - -—........................... .............. I ....... ......

10735ί I! χ (4). Had the starting point been the conventionally used transition, velocity, and acceleration signals, the xw signal would have had to be derivatized four, three, or twice, respectively, and the accuracy would have been clearly impaired. As a general rule of thumb in maintenance measurements, the signal can be derivatised at kb: -5 at most twice if reliable results are desired.

In the following, the calculations related to the construction of the 3c output transducer have been detailed in detail to three decimal places, with a calibration frequency <j) i 17450 Hz, ωη = 45000 Hz and a relative attenuation factor ζ of 0.1.

10

Frequency Sensor Accurate 3c (m / s3) 3c (m / s3) 23601 Hz 1.365 1.352 15 24601 Hz 1.409 1.410

25601 Hz 1.458 1.467 I

f 26601 Hz 1.512 1.524! 27601 Hz 1.573 1.582 28601 Hz 1.641 1.639 1 · Γ 20 29401 Hz 1.701 1.685

The sensor construction, based on the non-linear operating range of the accelerometer, has been discussed above. The signal derivation technique disclosed herein can also be applied to signals of other types of sensors. These include transition, m -:.: 25 side and pressure sensors.

5, 107356

REFERENCE PUBLICATIONS - ANFÖRDA PUBLIKATIONER

1. LAHDELMA, S., Methods and Instruments for Vibration Control. Helsinki 1988. Center for In-Line Training, Publication 83-88, Systematic Use and Condition Control. 7 sec.

2. Patent Application No. 922395, Class G 01H

3. Patent Application No. 934137, Class G 01H

4. THOMSON, W.T., Theory of Vibration with Applications. Prentice-Hall, Inc., Englewood Cliffs, New Jersey 1972. 467 s.

5. VDI2056 Beurteilungns maize fur mechanische Schwingungen von Maschinen. VDI-RICHTLINIEN, October 1964. 12 s.

6. ISO 2372 Mechanical vibration of machines with operating speeds from 10 to 200 rev / s -Basis for specifying evaluation standards. International Standard, Frist edition-1974-1 1-01. 9 sec.

: ·; 7. SKRIVASTAVA, H.M. & OWA, S., Univalent functions, fractional calculus, and their applications. Ellis Horwood Limited, Chichester 1989. 404 s.

• ·

Claims (1)

107356 .6 Patent claim A sensor for measuring oscillations, characterized in that it is based on an acceleration sensor and outputs a sigma l of a generalized time derivative (fractional derivative) of greater than two degrees. The invention utilizes the non-linear region (6) of the accelerometer a and the derivation is performed by means of a mass within the sensor, a wedge and a spring. i 4 i 4 ♦ | • · “1 · ·» <»« 4: i i j 107356 Givareförmätningavvibrationer, kännetecknat därav, attdengrundar sigpäen accelerationsgivare och fran den Es ut signalen av förskjutningens allmänna tidsderivata, bruten derivä. In uppfinningen uternttjas accelerationsgi-varens oljärä omräde (6) och deriveringen utföras med hjälp av mass, dämpning och ^ äder, lively är inom givaren. • - ·· t «