ADCMXL3021 datasheet(14/50 Pages) AD

14 / 50 page

ADcmXL3021

Data Sheet

Rev. A | Page 14 of 50

THEORY OF OPERATION

The ADcmXL3021 is a triaxial, vibration monitoring subsystem

that includes wide bandwidth, low noise MEMS accelerometers, an

analog-to-digital converter (ADC), high performance signal

processing, data buffers, record storage, and a user interface that

easily interfaces with most embedded processors. See Figure 33

for a basic signal chain. The subsystem is housed in an aluminum

module that is mounted using four screws (accepts screw size

M2.5) and is designed to be mechanically stable beyond 40 kHz.

The combination of this mechanical mounting and oversampling

ensures that aliasing artifacts are minimized.

MEMS

SENSOR

SIGNAL

PROCESSING

BUFFER/RECORDS

USER

INTERFACE

ADC

Figure 33. Basic Signal Chain

The ADcmXL3021 has a high operating input range of ±50 g

and is suitable for vibration measurements in high bandwidth

applications, such as vibration analysis systems that monitor

and diagnose machine or system health. User configurable

internal processing supports both time domain and frequency

domain calculations.

The low noise and high frequency bandwidth enable the

measurement of both repetitive vibration patterns and single

shock events caused by small moving parts, such as internal

bearings. The high g range provides the dynamic range to be

used in high vibration environments, such as heating, ventilation,

and air conditioning systems (HVAC), and heavy machine

equipment. To achieve best performance, be aware of system

noise, mounting, and signal conditioning for the particular

application.

Proper mounting is required to ensure full mechanical transfer

of vibration to accurately measure the desired vibration. A

common technique for high frequency mechanical coupling is

to use a combination of a threaded screw mount system and

adhesive where possible. For lower frequencies (below the full

capable bandwidth of the sensor), it is possible to use magnetic or

adhesive mounting. Proper mounting techniques ensure

accurate and repeatable results that are not influenced by

measurement system mechanical resonances and/or damping at

the desired frequencies, and represents an efficient and proper

mechanical transfer to the system being monitored.

CORE SENSORS

The ADcmXL3021 uses three ADXL1002 MEMS accelerometers,

with sensing axes configured to be mutually orthogonal to each

other. Figure 34 is a simple mechanical diagram that shows how

MEMS accelerometers translate linear acceleration to

representative output signals.

The moving component of the sensor is a polysilicon surface-

micromachined structure built on top of a silicon wafer.

Polysilicon springs suspend the structure over the surface of

the wafer and provide a resistance against acceleration forces.

Deflection of the structure is measured using differential capacitors

that consist of independent fixed plates and plates attached to

the moving mass. Acceleration deflects the structure and

unbalances the differential capacitor, resulting in a sensor

output with an amplitude proportional to acceleration. Phase

sensitive demodulation determines the magnitude and polarity

of the acceleration.

MOVABLE

FRAME

UNIT

FORCING

CELL

UNIT SENSING

CELL

MOVING

PLATE

FIXED

PLATES

PLATE

CAPACITORS

ANCHOR

Figure 34. MEMS Sensor Diagram

SIGNAL PROCESSSING

The signal chain of the ADcmXL3021 includes wideband

accelerometers to monitor three axes, a low-pass analog filter

with a 13.5 kHz cutoff frequency, an oversampling ADC (sampling

at 220 kSPS per axis), a microcontroller, and discrete components

to provide a flexible vibration monitor subsystem that supports

multiple processed output modes. There are four modes of

operation. One mode of operation is the full rate real-time

streaming (RTS) output. The three other modes include system

level signal processing: manual FFT mode (MFFT), automatic

FFT mode (AFFT), and manual time capture (MTC) mode

MTC mode supports 4096 consecutive time domain samples to

which averaging, finite impulse response (FIR), and windowing

signal processing can be enabled, along with the calculation of

statistics, alarm configuring, and monitoring. In MTC mode, the

raw time domain data is made available in register buffers for all

three axes for the user to access and externally process.

In both FFT modes, manual fast Fourier transform (MFFT) and

AFFT modes support the process of calculating an FFT of the

current time domain record.

Finally, a continuous RTS mode bypasses all device digital

computations and alarm monitoring, outputting real-time data

over the SPI in burst data output format (see Figure 5).

ADCMXL3021 Datasheet(PDF) 14 Page - Analog Devices

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