Mounting instructions. Acceleration Transducer B12. B 26.B12.10 en

Transcription

1 Mounting instructions Acceleration Transducer B12 B 26.B12.10 en

2

3 B12 3 Contents Page Safety instructions Scope of supply Application Construction and mode of operation Installation Mounting Electrical connections Calibration Notes on use Cable effects Interference Appendix Basic theory Amplitude frequency relationship Temperature sensitivity of damping Signal delay Phase displacement Transit time Relationship between vibration displacement amplitude, frequency and acceleration Technical Data Dimensions

4 4 B12 Safety instructions Appropriate use B12 type acceleration transducers are used for measuring vibration and impact accelerations as well as constant accelerations. Use for any additional purpose shall be deemed to be not in accordance with the regulations. In the interests of safety, the transducer should only be operated as described in the Mounting Instructions. It is also essential to observe the appropriate legal and safety regulations for the application concerned during use. The same applies to the use of accessories. The transducer is not a safety element within the meaning of its use as intended. Proper and safe operation of this transducer requires proper transportation, correct storage, assembly and mounting and careful operation and maintenance. General dangers of failing to follow the safety instructions The B12 acceleration transducer corresponds to the state of the art and is fail-safe. The transducers can give rise to residual dangers if they are inappropriately installed and operated by untrained personnel. Everyone involved with the installation, commissioning, maintenance or repair of a force transducer must have read and understood the Mounting Instructions and in particular the technical safety instructions. Remaining dangers The scope of supply and performance of the transducer covers only a small area of force measurement technique. In addition, equipment planners, installers and operators should plan, implement and respond to the safety engineering considerations of force measurement technique in such a way as to minimise residual dangers. Prevailing regulations must be complied with at all times. There must be reference to the residual dangers connected with force measurement technique.

5 B12 5 In these mounting instructions residual dangers are pointed out using the following symbols: Symbol: DANGER Meaning: Highest level of danger Warns of a directly dangerous situation in which failure to comply with safety requirements will lead to death or serious physical injury. Symbol: WARNING Meaning: Dangerous situation Warns of a potentially dangerous situation in which failure to comply with safety requirements can lead to death or serious physical injury. Symbol: CAUTION Meaning: Possibly dangerous situation Warns of a potentially dangerous situation in which failure to comply with safety requirements could lead to damage to property, slight or moderate physical injury. Symbol: NOTE Refers to the fact that important information is being given about the product or its use. Symbol: Meaning: CE mark The CE mark signals a guarantee by the manufacturer that his product meets the requirements of the relevant EC directives (see Declaration of conformity at the end of this document).

6 6 B12 Prohibition of own conversions and modifications The transducer must not be modified from the design or safety engineering point of view except with our express agreement. Any modification shall exclude all liability on our part for any damage resulting therefrom. Qualified personnel The transducer is only to be installed by qualified personnel strictly in accordance with the technical data and with the safety rules and regulations which follow. It is also essential to observe the appropriate legal and safety regulations for the application concerned. The same applies to the use of accessories. Qualified personnel means persons entrusted with the installation, fitting, commissioning and operation of the product who possess the appropriate qualifications for their function. Conditions on site Protect the transducer from damp and weather influences such as rain, snow, etc. Maintenance The B12 acceleration transducer is maintenance free. Accident prevention Although the specified nominal force in the destructive range is several times the full scale value, the relevant accident prevention regulations from the trade associations must be taken into consideration.

7 B Scope of supply 1 Acceleration transducer B12 1 Mounting instruction 2 Application The Type B 12 range of acceleration transducers can be used for measuring vibration and shock accelerations as well as constant acceleration. They are both small and light which makes them ideal for mounting on small test subjects. They are robust and wear free and can be mounted in any attitude. The two B 12 transducers that are available are of identical external dimensions and differ only in their characteristic frequency and measuring range. In each case the working frequency range lies between 0 Hz and half the characteristic frequency. The lower the characteristic frequency, the higher the sensitivity due to the nature of the systern. Thus, versatility of application is an inherent feature. The B 12/200 transducer has a nominal acceleration of 200 m/s 2 and, because of its excellent absolute zero stability, is particularly suitable for measuring constant and slowly changing accelerations. It can also be used as a clinometer. The smallest value of acceleration that can be detected is 0.02 m/s 2 (when using a suitable amplifier). The B 12/500 is included in the range for accelerations up to 1000 m/s 2 and have working frequency ranges up to 250 Hz. The amplitude frequency response of the transducers is measured on a vibratory table at the works. A copy of the curve is supplied with each accelerometer.

8 8 B12 3 Construction and mode of operation The mechanism of the B 12 acceleration transducer is an above resonance mechanical oscillating systern which is housed in a cylindrical steel body (Fig. 1). In the housing, a fluid damped mass is suspended between two diaphragm springs. The transducer is securely attached to the test subject by means of a screw stud and the body transfers the acceleration to be measured to the mass via the springs. The resulting relative displacement between mass and body is proportional to the absolute acceleration (the physical relationships on which the measuring system is based are explained in more detail in section 7). The mass is in the form of an armature for a differential choke systern and its displacement produces opposite changes in the inductance of the coils. Together, the coils form an inductive semi bridge which is expanded to a Wheatstone bridge in the amplifier. The balanced bridge becomes unbalanced by the displacement of the mass and in this way produces a measuring signal which is proportional to the relative displacement of the mass and therefore to the acceleration. The attachment of the transducer to the test subject is facilitated by a 10 mm a/f hexagon and an M 6 screw stud 6.5 mm long. The stud is drilled and incorporates the filler for the damping oil. At the opposite end to the mounting stud are the four pins of the HS 31 P plug for the electrical connections. The centre of gravity of the mechanism is approximately in the middle of the cylindrical part of the body and the direction of measurement is along the body axis. Fig. 1 Construction

9 B Installation 4.1 Mounting The transducer must be attached to the test subject by means of the screw stud so that the acceleration being measured is along the axis of the body. The physical attitude of mounting can be adapted to suit the particular conditions. The tightening torque must only be applied via the hexagon (10 a/f) and must not exceed 3.5 Nm. Fixing of the transducer by means of magnetic clamps is not recommended because of possible interference (see 6.2). 4.2 Electrical connections The electrical connections can be made either by socket or by the direct soldering of wires. When an HK 31 S connector is used it makes the body splashproof (degree of protection IP 54 to DIN 40050). In order to obtain good results it is advisable to use HBM connecting leads, a choice of which is given in the table below. Table: Connecting leads

10 10 B12 For extension purposes, Cable 0302 xx with Amphenol socket and plug is available in lengths of 6 m, 10 m and 20 m (xx = required length). The special Cable 5/00 4 can be used without plugs and connector sockets; for lengths over 100 m we recommend Cable 8/00 4 of larger cross section. For certain applications, such as very high accelerations, small test subjects or the need for a low transducer mass, it is advisable to solder flexible tails directly to the connector pins. In order not to overheat the internal soldering, use a low melting temperature and ensure good dissipation of the heat. lf, because of possible interference (see section 5), it is desired to take measurements without a chassis ground, the grounding pin can be cut off (it is the thicker one). lf the measuring conditions are uncertain it is best to cut off only half of the pin first and to take test measurements with the connector socket pushed on only half way. lf it is found that the chassis ground is still required, the plug will still be operative when the connector is pushed fully into place. The B 12 transducers can be used with any HBM 5 khz carrier frequency amplifier or the MC 2 measuring converter, although the relevant cut off frequencies must be noted. Fig. 2 Electrical connections

11 B Calibration In order to calibrate the measuring system it is first necessary to set up the amplifier according to the operating instructions. Acceleration transducers B 12/200 and B 12/500 are calibrated quite simply by rotating them through 180 o from the vertical (Fig. 3). When the transducer is in the vertical position, the mass is in a defined, fixed position (1) and slight inclinations of the measuring axis have no effect. The amplifier should be adjusted to zero while the transducer is in this position. When the transducer is rotated through 180 o to the opposite fixed position (2), acceleration 2g acts on the mass. Set the appropriate value on the amplifier (it varies according to place but an average value is 19.62m/s 2 ) and the measuring system is calibrated. Fixed position 1 Fixed position 2 Display setting: 0 Display setting: 2g Fig. 3 Calibration For measurements of the highest accuracy, all B 12 transducers should be calibrated within the range of the anticipated reading. When measurements are taken over extended periods of time, regular recalibration is advisable.

12 12 B12 6 Notes on use 6.1 Cable effects With inductive acceleration transducers the cable effects depend on the particular type of cable employed. When HBM cables are used there is no detectable effect on the sensitivity until they are more than 100 m long. The effect of a long cable can be compensated for by connecting it before the calibration is carried out (see section 4). 6.2 Interference Although the carrier frequency method employed with inductive pickups is largely insensitive to electrical interference, it is still possible for such interference to affect readings if it is of sufficient intensity. Unwanted interference can be picked up from power lines running parallel to the transducer leads, from contactors and electric motors or as a result of potential differences in the earthing system when multiple earthing is employed. Under unfavourable conditions, signal deviations of almost 10% referred to the rated value have been measured in a magnetic field of 2 mt. Consequently, it is essential to deal with the effects of magnetic fields and to employ magnetic screening of the transducers if necessary. Since the effects of interference depend on the specific situation and details of the arrangement, no generally applicable rules can be given. The most important preconditions for satisfactory measurements are HF screening of the measuring system through the use of HBM cables and the avoidance of earth loops with multiple earthing.

13 B Appendix 7.1 Basic theory The principle of the acceleration transducer is based on a seismic longitudinal vibration system with a fixed mass and relative damping between mass and body, as shown diagrammatically in Fig. 4. Systems of this type are in principle also suitable for measuring vibration displacement and velocity. For a clear illustration of the conditions, the measuring system here is considered in conjunction with an imaginary reference system at absolute rest. Acceleration of the body (a = d 2 s/dt 2 ) is transferred via the spring to the mass which it sets in motion. The mass moves the absolute distance y (measured against the imaginary reference system) and the relative distance r (measured against the body; see Fig. 4). Fig. 4 Principle of operation In a specific frequency range below the natural frequency the relative displacement of the mass r = (y s) is proportional to the applied acceleration. The coils convert the relative displacement of the mass into an electrical signal which is transmitted to the amplifier for processing. The relation between the acceleration to be determined and the relative displacement of the mass measured can be derived mathematically from the force equilibrium condition of the oscillatory system (Σ F = 0).

14 14 B12 When an external force is acting on the body, the system can be described with a 2nd order inhomogeneous linear differential equation: Equation 1 The characteristics of the oscillatory system can be adapted to particular requirements through appropriate sizing of the mass, spring and damping. An above resonance system as used for measuring acceleration has a stiff spring with a low damping constant and low mass. In the system equation, predominates and gives approximately: ω 0 2. r = a The relative displacement is proportional to the acceleration. The transfer sensitivity reduces by 1/ω 0 2.

15 B Amplitude frequency relationship The ratio of the amplitudes of relative displacement r and acceleration a in relation to the frequency is of special interest in assessing the accuracy of acceleration measurement (amplitude frequency response). For sinusoidal events the relationship can be written mathematically as follows: Equation 3 For the purpose of illustration the amplitude frequency response (resonance curve) is shown in Fig. 5 with various damping factors as parameters. Exact measurements can be expected when the characteristic curve is a straight line. With a damping factor of 0.6 to 0.7 this is the case in the range up to 50% of the characteristic frequency (ω/ω 0 = 1). Varying the damping factor restricts the effective working frequency range. Working frequency range Fig. 5 Amplitude response * D for B12: 0,6 0,1

16 16 B Temperature sensitivity of damping Temperature variations lead to a change in the damping factor D. The effect of operation temperature on the damping is shown in Fig. 6. The reference temperature of the B 12 transducers is 23 o C. Fig. 6 Temperature sensitivity of damping * D for B12: 0,6 0,1 7.4 Signal delay With any transducer the output signal is delayed slightly from the input acceleration (Fig. 7). With sinusoidal motions of the test subject and transducer the delay can be represented by the phase displacement angle ϕ. The transit time t is introduced for irregular accelerations. t is the time difference between specific features of the acceleration curve (e.g. minima, zero crossings, etc.). Input variable Measured signal Phase displacement Transit time Fig. 7 Signal delay

17 B Phase displacement The phase displacement angle ϕ can be calculated from the differential equation of the oscillatory system and is as follows: Fig. 8 shows the relation between the phase displacement angle and the normalized angular frequency for different damping factors (phase frequency response). Fig. 8 Phase frequency response * D for B12: 0,6 0, Transit time Fig. 9 shows the relation between the normalized transit time for various damping factors and the normalized angular frequency. The curves are derived from the phase frequency response (Fig. 7) to τ = ϕ 360 o. ω ω o They are universally applicable to acceleration transducers employing damped mechanical oscillating systerns.

18 18 B12 When the acceleration transducers are operated with the usual damping factor of 0.6 for the reference temperature (23 o C), the transit times in the working frequency range are approximately equal. The acceleration is then accurately reproduced. The transit times for the B12 transducers are given in the table below. Working frequencyrange Fig. 9 Transit times * D for B12: 0,6 0,1 Normalized Transit time t [ms] transit time B12/200 B12/500 r = 0,25 1,25 0,5 r = 0,2 1 0,4 7.5 Relationship between vibration displacement amplitude, frequency and acceleration For sinusoidal events, the relation between vibration displacement amplitude (s), frequency (f) and acceleration (a) can be described by the following equation: When the maximum values of the vibration displacement amplitude and frequency are known, the equation can be used to ascertain the type of acceleration transducer to be used.

19 B Technical Data Type Measured quality constant and varying acceleration, vibration acceleration Direction of measurement selectively along the body axis, anywhere in the field of gravity Mechanical principle of measurement subcritical mechanical oscillating Types B12/200 B12/500 Characteristic frequency appr. Hz Working frequency range Hz Damping factor D at reference temperature 0.6 0,1 Nominal acceleration m/s Sensitivity mv/v Sensitivity tolerance mv/v 8 Nominal sensitivity mv/v Nominal output signal span mv/v Temperature effect per 10K in the nominal temperature range on the sensitivity *), related to the actual value, typ. % 0.2 on the zero signal, related of the nominal sensitivity, typ. % B12 Linearity deviatation inclusive hysreresis related to the nominal output signal span % 2 Lateral sensitivity **) (direction factor) % 3 Relative lateral force limit related to the nominal acceleration % 100 Electrical principle ind. system with differential chokes * ) The characteristic value is the actual output signal at nominal acceleration ** ) The lateral sensitivity is the output signal when loaded perpendicular to the measuring axis, referred to the output signal for the same load along the measuring axis of the accelerometer

20 20 B12 Types B12/200 B12/500 Nominal excitation voltage (r.m.s) V 2.5 5% Operating range of the excitation voltage V (r.m.s) Carrier frequency khz 5 Input resistance at reference temperature (between points 2 and 3) ca. 40 Input inductance at reference temperature (between points 2 and 3) mh ca. 10 Reference temperature o C +23 Nominal temperature range o C Operating temperature range o C Storage temperature range o C Weight g ca. 17 Mounting M6 screw stud 9 Dimensions

21 B12 21

22 22 B12

23 B12 23

24 OTTINGER ALDWIN ESSTECHNIK HBM Mess- und Systemtechnik GmbH Postfach , D Darmstadt Im Tiefen See 45, D Darmstadt Tel.: +49/ 61 51/ ; Fax: +49/ 61 51/ ; e mail: TSC@hottinger baldwin.com Modifications reserved. All details describe our products in general form only. They are not to be understood as express warranty and do not constitute any liability whatsoever. IM D POD