Machinery vibration analysis is used to check the health of machinery as a routine basis and also to understand the cause of vibration. Vibration is a symptom of an internal defect, it is a very sensitive and early predictor of developing defects.

Every machine has its own ‘vibration signature’, this is the frequency spectrum of the vibration signal and it will show defects as imbalance, misalignment, resonance, defective bearings, gears, etc. The amplitude of the signal will show the severity of the problem.

A vibration signal can be acquiered using an accelerometer, an ADC, a microcontroller and a serial connection to a PC. This signal can then be registered and analyzed at the PC, using FFT algorithms. Using a portable PC means to have a portable FFT machinery vibration analyzer.

• Easy to use.
• Small size.
• Low cost.
• Able to sense accelerations as small as a few mili G and cover a range of various G.
• Battery operated.

The system has the following parts:

• Accelerometer.
• Amplifier an filter.
• ADC.
• Microcontroller.
• Serial interface.
• PC.

The vibration sensor used is the CXL10HF1Z, a single axis accelerometer made by CrossBow.

The signal from the accelerometer is introduced to the positive input of an instrumentation amplifier (AD620). A reference signal coming from a voltage diveder is fed to the negative input of the amplifier. This reference voltage is used to reduce the Zero-G voltage coming from the accelerometer.

The filter is simply a RC network, with a cutoff frecuency of 160Hz. This is done to limit the frecuency of the input signal to avoid aliassing errors.

The ADC used is the MAX189, it has a resolution of 12 bits and a serial interface, that will allow to meassure very small signals, a few mili G, and will cover a ±10G span.

A small microcontroller is used to communicate with the ADC an read the vibratin signal. The microcontroller used is the Philips 87LPC762, that has a few I/O ports, timers and UART, so it is very simple to send the vibration information to a PC.

The serial interface is made witha a MAX232 and the corresponding capacitors. The MAX232 changes TTL voltages to RS-232 voltages, so the microcontroller can communicate with the PC’s serial port using the same signal levels.

The computer program communicates with the acquisition system using the serial port. It receives the digitalized vibration signal and plots it in a G vs. time chart. After reading the signal, the program calculates the FFT and plots a chart of G vs frequency.

It is also possible to save the information to a file and to retrieve information from a file.

A copy of the frecuency spectrum of the signal can be sent to the printer for reporting.