The ADXL1004 device can, for example, warn you when a bearing is reaching end-of-life, when a part is off-balance, or when vibration might cause metal-fatigue. See the ADXL1005 datasheet for more information. (Additionally, see the ADXL1004's product overview for a ±500 g variant.)
ADXL1005 evaluation board from Analog Devices
About the IC Mechanical Construction
There are three particularly relevant quotes that describe the sensor design:
- “...a polysilicon surface-micromachined structure built on top of a silicon wafer.”
- “Polysilicon springs suspend the structure over the surface of the wafer”
- “Acceleration deflects the structure and unbalances the differential capacitor.”
More information is provided on page 9 of the datasheet.
This is a case where a picture could replace a thousand words, and I personally would like to see a small one included in this section. The website mems-exchange.org provided the following surface micromachined MEMS structure that should have some similarities to what is under the epoxy package of the ADXL1005.
Polysilicon resonant structure with surface micromachining from mems-exchange.org
Device Package and Mounting
The device is offered in a 5mm x 5mm x 1.8mm ROHS-compliant 32-pin “Lead-Free Chip-Scale Package” (LFCSP) with an exposed pad that should be tied to ground with multiple thermal vias for heat dissipation.
The datasheet indicates that “careful attention to PCB thermal design is required.” That statement usually indicates that multiple vias should be used to tie the pad to large sections of copper, and for high-temperature and harsh environments, you should consult with a thermal engineer.
This sensor is a single-axis in-plane accelerometer -- you can visualize the direction in which it measures acceleration as a line that passes from pin 24 to pin 1; positive accelerations are towards pins 1, negative accelerations are towards pins 24.
To accurately measure accelerations, the sensor must be mounted securely to a small, thick PCB between two or more closely spaced mounting holes. The PCB should be rigidly attached to the measurement surface with a method (tape, magnet, bolt, etc…) that is appropriate for the frequency range of measurement. For a better understanding of which type of mount is appropriate for your design, consult a mechanical engineer.
Image of various accelerometer mounting options and frequency response from Crystal Instruments
Device Characteristics and Limitation
According to the datasheet, the ADXL1005 is capable of withstanding external shocks of up to 10,000 g’s and can survive a fall of up to 1.2 m—which allows a designer to accidentally knock the component from their testbench to a linoleum floor with several inches to spare.
Thermally, the IC can perform at temperatures that range from -40 °C to 125 °C. Thermal noise does negatively affect the measurements, so at excessively high temperatures, consult with a thermal engineer about proper heat dissipation.
This accelerometer can operate at 3.0 to 5.25 VDC. It provides ratiometric output, which means the acceleration scale is tied to the supply voltage. This also means that any power supply instability will be transferred directly to the output. You should use proper decoupling techniques and consider tying your ADC reference to the Vdd supply pin of the ADXL1005.
Since the ADXL1005 provides ratiometric output, supply voltage variations are especially problematic. Use proper decoupling techniques and if possible, tie your ADC reference to the ADXL1005’s Vdd pin.
The device has a 1 mA measure-mode current consumption and a 225 µA standby mode. Both modes have overrange (OR) detection, which disables the internal clock to try to protect the sensor at accelerations approximately ≥ ±200 g. The OR pin can be connected to a microcontroller to notify of the overrange condition.
Functional block diagram from the ADXL1005 datasheet
An on-demand self-test can be activated at any time by setting the self-test (ST) pin to logic high. This 300 µs test electrostatically moves the mass inside the accelerometer and processes the data as if an actual acceleration occurs. By comparing the difference between the pre-test and during-test output voltages to known values in the chart on page 10, designers can determine if the device is performing as expected.
Mechanical Resonance and Clock
The resonant frequency of the sensor is approximately 42 kHz, and the internal clock frequency is approximately 200 kHz. When used to measure vibrations at less than 10 kHz, pass the sensor output through a low-pass filter before it reaches your ADC to mitigate the effects of these high-frequency noise sources. When measuring frequencies at or above the resonant frequency (which is possible), consult the datasheet for additional information.
Evaluation Board Available
Image of Analog Devices ADXL1005Z evaluation board from Analog.com
Analog does provide an evaluation board to allow experimentation with the ICs in this family.
This is a large-range analog accelerometer that is a possible choice for monitoring vibrations in machinery and HVAC systems. If you choose to use it in your next design, the mechanical and thermal design considerations will provide you the opportunity to work with your colleagues in other departments. If everybody on the team does their job correctly, you can create a product that saves a power-shaft before a bearing seizes, or allows you to replace the bearings on a slant-bed lathe before excessive runout shuts down a production line.
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