Customer Solutions

Creating an Efficient Test System for an Applied MEMS Accelerometer

Author(s):

Kevin Speller, Applied MEMS

Industry:

Automotive

Product:

Compact FieldPoint, Data Acquisition, Distributed I/O, FieldPoint, GPIB & Instrument Control, High-Speed Digital I/O, LabVIEW, PXI/CompactPCI

The Challenge:

Improving an inefficient test and evaluation system for state-of-the-art Micromechanical Systems (MEMS) low-g servo accelerometers. The system uses manual tests during various phases of the product life cycle, form prototype characterization to production test, calibration, and user evaluation

The Solution:

Using NI software and hardware to test multiple parts simultaneously, and complile the data in a central database to enable easy comparisons by pat, test station, and/or test parameter.

Introduction
Applied MEMS Inc. manufactures a stable, robust, and state-of-the-art MEMS low-g servo accelerometer with a noise floor near 30 nano-g/vHz and a minimum dynamic range of >115 dB.
The accelerometer is coupled with a custom CMOS integrated circuit chip. We developed it for use in the demanding oil and gas exploration environment, where shock and extreme temperatures are common. We commercially produce the Si-Flex accelerometers in an automated high-volume, 6-inch wafer fabrication facility. Applications include seismic exploration and monitoring, inertial navigation, and vibration monitoring and analysis.

Solving Demanding Tests with NI Technology
We test our accelerometers at multiple test stations, some of which are described below:

• Accelerometer die probing
• Accelerometer die vacuum sealing
• Sensor/ASIC functional testing
• Noise floor testing
• Mechanical shock tolerance testing
• Accelerometer calibration
• Orthogonality testing
  

We use LabVIEW for all our test stations. We log all our test data to our network, and use ActiveX to create a Microsoft Excel spreadsheet of all the results from multiple stations. Each test station runs executables generated by the LabVIEW Application Builder. We use a Compaq Deskpro 1GHz, Pentium III, with Windows 2000/NT for most of our test stations.

We integrate multiple multifunction I/O, digital I/O, and counter/timer devices from National Instruments in a production test environment, use the data processing power of LabVIEW, and log data to Microsoft Excel.
The accelerometer die are probed at the wafer level. We use an NI PC-DIO-24 to switch relays to perform a variety of functional tests on the die. An NI PCI-GPIB board controls settings and acquires data from an LCR meter and precision multimeter. Dynamic data exchange links data to a main database for all test results.
We seal the accelerometer die under vacuum in a ceramic package to reduce the noise floor of the sensor. We use an NI PCI-MIO-16E-1 for measuring the vacuum level inside the sealed accelerometer package. An analog signal from the NI PCI-MIO-16E-1 electrostaticly impulses the proof mass of the accelerometer. Analog triggering starts the data acquisition of the sensor response. We compute the vacuum level inside the sealed accelerometer package using this data. The LabVIEW serial port VI acquires data from a bar-code scanner to enter part serial numbers. We link to data from other test stations using ActiveX technology.

An application-specific integrated circuit (ASIC) controls the accelerometer in a servo loop. We perform sensor/ASIC functional testing using an NI PCI-DIO-32HS for serial communication, data bit-stream acquisition, and multiplexing between parts. The bit-stream data is decimated, filtered, windowed, converted by the LabVIEW Fast Fourier Transform algorithm, averaged, and displayed. After parts load into the fixture, the test station automatically cycles through and tests each part. The LabVIEW serial port VI acquires data from a bar-code scanner to enter part serial numbers. ActiveX links data to test data from other test stations.
Seismic applications for the accelerometer demand a very low-noise sensor. We test the noise floor of each sensor in a seismic isolation chamber using an NI PCI-DIO-32HS to communicate with the ASIC and capture the digital sensor output bit stream. It then displays frequency and time-domain data. The hardware for this test is similar to the functional test with ASIC test hardware. However, this test occurs in a seismic isolation chamber that can provide noise levels below 30 nano-g/vHz in the seismic frequency band. ActiveX links data to test data from other test stations.

We test each accelerometer for mechanical shock tolerance to ensure robustness. A mechanical shock tester provides a 1,500 g, 0.5 MS shock measured with a high-g accelerometer. An NI multifunction I/O board captures a software-triggered signal from a high-g shock sensor. Using LabVIEW, we process, display, and store the data to disk.

The accelerometers are calibrated by rotating the parts through gravity on a precision stage. GPIB controls the stage, while the sensor output voltage at various angles determines the sensor scale factor and offset. An NI PCI-DIO-32HS writes coefficients and calibration data to the custom ASIC EEPROM serially. We use an NI PCI-6602 frequency counter for measuring the digital output bit stream and an NI PCI-DIO-96 for serial communication with the ASIC and multiplexing between sensors under test. We also use GPIB to acquire data from a multimeter, and the LabVIEW serial port VI to acquire data from a bar-code scanner to enter part serial numbers.

Orthogonality between sensing axes is particularly critical to seismic applications. With it, we gain improved vector fidelity and better seismic images. We test each completed sensor module with three orthogonally aligned accelerometers for cross-axis sensitivity. A precision linear shaker excites the test module, and GPIB controls and acquires data from a spectrum analyzer. LabVIEW controls the test, prompts the operator, and processes and stores the data. We use the computer parallel port for multiplexing.

Accelerometer Evaluation Systems
We can use the accelerometer in an analog or digital output mode. An NI DAQCard-AI-16XE-50 demonstrates the analog capabilities of the Si-Flex accelerometer. The DAQCard acquires data that LabVIEW processes. A 3-D time domain display shows the 3G full scale of the sensor. As the three orthogonally configured sensors move, a 3-D ball on the screen moves according to the acceleration input. A separate screen with AC-coupled, DC-coupled, and frequency-domain data demonstrates the analog output mode noise floor of less than 1 micro-g/vHz.

With the digital accelerometer evaluation system, users can evaluate the performance of the accelerometer in digital mode. The system can run from a laptop using the NI DAQCard-6533 or on a standard PCI bus with an NI PCI-DIO-32HS. It uses the NI DAQCard-6533 or NI PCI-DIO-32HS to acquire digital data from the accelerometer and perform serial communication with the ASIC. The data is decimated, filtered, windowed, averaged, and displayed. The system can display time or frequency-domain data records. The user can select record lengths ranging from 0.125 to 262 seconds for use in a wide variety of applications, including very low frequency, low-noise seismic measurements.

NI Technology Improves Production Throughput
Before our automated test stations existed, we manually tested our accelerometers individually, and comparison of data between test stations was difficult. Manual tests were too slow, labor intensive, and inconsistent. We have solved all these problems with our NI-based test stations. Using NI technology, we have achieved at least a 20-fold improvement in throughput, in addition to the added benefits of test accuracy, consistency, and data consolidation. Currently, we test multiple parts automatically, and the data collection process is automated. A central database compiles data, so we can make comparisons by part, test station, and/or test parameter. We are also working on additional improvements, which will soon double our current testing throughput.

For more information, contact:

 Kevin Speller

Applied Mems

12200 Parc Crest Drive

Stafford, TX 77477

Tel: (281) 552-3051

E-mail: kspeller@appliedmems.com.

361810a01.pdf

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