ME2112-1 VIBRATION MEASUREMENT：me2112-1振动测量

1、.ME3112-1 VIBRATION MEASUREMENTSEMESTER 5SESSION 2009/2010Department of Mechanical EngineeringNational University of SingaporeINTRODUCTIONPurpose and ScopeThe purpose of this experiment is to introduce students to the use of certain measuring equipment commonly found in the applied mechanics laborat

2、ories. A cantilever beam is used as an object to illustrate the use of these equipments which include strainmeter, accelerometer, shaker and real time analyzer.Dynamic Measuring TechniquesObjective:To familiarize with the techniques in measuring dynamic quantities. The resonance frequencies and the

3、corresponding mode-shapes of a vibrating beam will be determined with several different techniques.Experimental Set-up:The schematic diagram for the experiment is shown below. Fig. 1 Schematic DiagramExperimental SetupDescription of Equipment: a. Electrodynamic exciter:Actual machinery vibrates

4、 either due to internal cause (unbalanced rotor, reciprocating parts etc.) or external cause (vibratory motion and/or acoustic energy transmitted from other machineries.) In the laboratory environment, the machinery or structure into vibratory motion is to externally force it with harmonic force.One

5、 of the most commonly used equipment in producing this force is the electrodynamic exciter. Fig 2 depicts a simplified cut away view of this equipment. It consists of a coil over a central core of permanent magnetic. The coil can be moved freely with respect to the magnet. When a sinusoidal signal i

6、s applied to the coil, because of electromagnetic interaction, the coil will move in a sinusoidal fashion with respect to the coil. If the coil is attached to any structure then the structure will in turn be forced to move sinusoidally.Electromagnet replaces the permanent magnet in more powerful mac

7、hine. Fig. 2 Small Electrodynamic Vibratorb. Transducers:Also known as sensors are used to sense certain measurable quantities of any structure.Accelerometer:This is commonly referring to the piezoelectric type of accelerator although accelerometers based on strain gauge are also available.Piez

8、oelectric accelerometers as shown in Fig.3 operate on the principle that a mass moves will stresses a piezoelectric crystal.Fig. 3 Piezoelectric accelerometer - thickness-compression typeAcceleration forces cause the amount of applied force on the crystal to vary cyclically and this, by the very nat

9、ure of the piezo electric effect, produces an electric output signal proportion to compression. Since this stress is itself directly related to acceleration, such a signal is proportional to the vibratory acceleration of the surface on which it is mounted. Since the electric output is charges, a cha

10、rge amplifier has to be used to convert the signal to voltage signal. This is by no mean the only possible way of constructing an accelerometer. Fig 4 shown five most commonly used modes of construction. Fig. 4 Piezoelectric accelerometer designsThe acceleration level at various points of the s

11、tructure can be measured by attaching the accelerometer one by one to all these points. These measurements will provide the overall relative motion of a structure hence it is mode-shape.c. StroboscopeStroboscope which generates short-duration light pulses at controllable rate is the equipment common

12、ly used to provide the first level of investigating vibratory motion. The stroboscope permits rotating and reciprocating objects to be viewed intermittently and produces the optical effect of slowing down or stopping motion. This will enable the resonant frequency and the mode-shape to be observed.

13、The actual amplitude of motion can also be ascertained if proper reference is introduced.PROCEDUREThe rigid end of a clamped-free beam is excited by a shaker. At the resonant frequency the beam undergoes violent vibration. The resonance frequency and the corresponding mode-shape can be viewed with a

14、 stroboscope, the accelerometer or the strain-gauge. Since the beam has more than one resonant frequency, determine the first five resonant frequencies and mode-shapes points by varying the frequency of excitation.Compare the values that you have obtained with theoretical results computed with the following formula.Fig. 5I = Mode No.E = Young's modulusI =  Area moment of inertia of beam about neutral axisL =  Span of beamM =  mass per unit length  li=1.875i=14.694i= 27.855i= 310.996i= 414.137i= 5