Use of Accelerometers to Determine Characteristics of a Vibrating System


ME 1041 Lab # 5


































Name: Andy McDevitt
Prof: M.J.Rudert
Due: 2-28-00
TA: Mike Ramsay
Table of contents:

1. Objective pg.
2. Introduction pg.
3. Procedure pg.
4. Results pg.
5. Questions pg.
6. Conclusion pg.
7. Appendix A. (Graphs)
8. Appendix B. (Calculations)








Objective:
To determine the response of a free vibrating system as well as characteristics of such.The objective of the free vibration experiment is to determine the damped natural frequency, log decrement and damping constant of two different masses in free vibration and compare theoretically. The objective of the impulse experiment is to determine the maximum gs, or shock, of two different masses.

Introduction:
The accelerometer being used in this laboratory is a Quartz Accelerometer with a built-in amplifier. More specifically it is a series 302A accelerometer that is used since it is used specially for measurement of vibration and shock of systems. The acceleration of the shock or vibration motion is measured from 1g to 500g over a wide frequency range. The accelerometer used gives exceptionally low-frequency response and tests long duration shocks up to 20 milliseconds, which will be seen to be very advantageous.
How exactly does the Accelerometer work? The accelerometer is composed of three important parts; P-channel MOSFET transistor and amplifier, Piezoelectric sensor and Preload sleeve.







The Piezoelectric sensor is composed of two crystalline quartz plates. Located between the two plates is a positive conductor and a negative conductor, the negative conductor being the housing. The preload sleeve puts the plates in compression, which in turn shifts the quartz into a linear output range for positive and negative accelerometers. Quartz is used since it has a high voltage sensitivity, which means there is a small capacitance.
Functionally, when a quartz crystal is stressed it causes electrons to move to one side of the crystal. The quartz crystal then functions as a capacitor and holds the electrons. The electrons that migrate to the one side create a voltage proportional to the stress that caused this to occur.
The amplifier is used to reduce noise to a negligible value if placed closely to the sensor. Sometimes known as a impedance converter, the transistor has high input impedance and low output impedance.








Procedure:

Equipment: Quartz accelerometer series 302A
Power unit (included in accelerometer)
Digital Plotter
Cantilever beam with concentrated mass





Experiment #1

Mount cantilever beam to the side of the lab table. Now select a desired concentrated mass. Attach the accelerometer to the top of the mounted mass. With the power unit, connect the wire to the top of the accelerometer. Connect the other wire from the power unit to the digital plotter. The experiment is now ready to run. Lift the mass to a desired height and release. While the mass is in free vibration manually trigger the digital plotter and plot the results. Repeat this procedure until an accurate portrayal of the free vibrating mass is plotted.

Experiment #2
Repeat experiment #1 using a different concentrated mass.
Results:

Experiment #1: (Stainless Steel Encased) t = 1.395 sec.

TABLE # 1

TRIAL ICE (F) WATER (F)
1 32.2 209.0
2 32.1 208.5
3 32.0 208.4
4 31.2 207.6
5 30.6 207.8
Average 31.62 208.26



Experiment #2: (Bare Wire) t = 0.062 sec.

TABLE # 2

TRIAL Vi (mV) Vf (mV)
1 0.009 5.20
2 0.004 5.19
3 0.005 5.21
4 0.004 5.21
5 0.005 5.20
Average 0.0054 5.2002



Experiment #3: (RTD) t = 0.408 sec.

TABLE # 3

TRIAL Vi (mV) Vf (V)
1 -1.7 0.60
2 -1.7 0.66
3 -1.7 0.55
4 -1.7 0.48
5 -1.7 0.43
Average -1.7 0.544

Discussion:

Experiment 1
The data that was recorded for this experiment portrayed acceptable values of ice water and boiling water. The average of the values for the ice water was calculated to be 31.62 F. The theoretical value of freezing water is 32F. The average value for the boiling water came to be 208.26F. The temperature of the ice water was very accurate. As for the boiling water, there lie some possible sources of error. The altitude at which the experiment was performed could cause the temperature at which the water boiled to be slightly lower. If the water were not 100% water then that could also be a possible source of error. This experiment may be useful in calculating the temperature of coolant in a car to make sure the engine is keeping it at an efficient temperature and to also check if the gauge inside the car is giving an accurate reading. When comparing the experimental data to theoretical data one can see that there is obviously something affecting the temperature that the water is boiling at.

Experiment 2
In experiment two, the data that was recorded supported the data in experiment one. The initial voltage averaged to be 0.0054 mV. The final voltage after being transferred to the boiling water averaged a value of 5.2002 mV. When taking these values