Lab5 Mems Lab Guide

THE UNC CHARLOTTE WILLIAM STATES LEE COLLEGE OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING AND ENGINEERING SCIENCE

3171L Instrument Lab Student Experiment Guide: MEMS Accelerometer Experiment

Fall 2010

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MEGR 3171: PWM Signals and Their Use in a Dual-axis MEMS Accelerometer 1.0 Objective The objective of this lab is to develop circuits using a MEMS accelerometer and Labview to demonstrate the application of accelerometers to real engineering problems. 2.0 Experiments 2.1 Equipment / Components Analog Devices ADXL213EB Two Axis Accelerometer Evaluation Board

Figure 2-1 ADXL213 Two Axis MEMS Accelerometer Evaluation Board The ADXL213 is a dual axis accelerometer on a single monolithic IC. The output signals are duty cycle modulated digital signals proportional to acceleration. The ADXL213 is capable of measuring both positive and negative accelerations to ±1.2 g. The accelerometer can measure static acceleration forces such as gravity, allowing the ADXL213 to be used as a tilt sensor. The accelerometer is mounted on an evaluation board shown in Figure 2-1. Table 2-1 ADXL213EB Pin Function Pin

Pin Function

A

A +V Supply (3 VDC to 5.25 VDC)

B

Self Test Input

C

Y-Axis Duty Cycle Out

D

X-Axis Duty Cycle Out

E

Ground

CAUTION

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The ADXL213EB is not reverse polarity protected. Reversing the +V supply and ground pins causes damage to the ADXL213.

Figure 2-2 ADXL213 Axis Orientation The acceleration output signal is PWM and can be determined by measuring the length of the positive pulse width (T1) and the period (T2). The output of each channel appears as in Figure 2-3.

Figure 2-3 ADXL213 Output Signal The nominal transfer function of the ADXL213 is Acceleration(g) = ((T1/T2) – Zero g Bias)/Sensitivity The zero g bias is 50% duty cycle and the sensitivity is 30% of g, therefore Acceleration(g) = ((T1/T2) – 0.50)/0.30 Therefore: For 1g : 1 = ((T1/T2) – 0.50)/0.30 and T1 = 0.80 T2 For 0g : 0 = ((T1/T2) – 0.50)/0.30 and T1 = 0.50 T2 For -1g : -1 = ((T1/T2) – 0.50)/0.30 and T1 = 0.20 T2

The duration of T2 is determined by the resistor R1 (RSET) T2 = RSET/125 M The accelerometers are pre-wired with 130 kW resistors, thus

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T2 = 130 x 103/125 x 106 = 1.04 x 10-3 seconds Therefore: For 1g : T1 = 832 msec For 0g : T1 = 520 msec For -1g : T1 = 208 msec

Small Breadboard

Figure 2-4 Small Breadboard NI DAQ Card Counter/timer

Figure 2-5 NI DAQ Board 2.2 Experiment Overview 2.2.1 Labview

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Develop a Labview program to read the accelerometer output and convert to g values. Treat this as a project. Establish your requirements and, with your knowledge of Labview, design a Labview program to accomplish this task. Hint, you may want to insert a low pass filter to clean up the signal. 2.3 Experiments 2.3.1 Experiment 1 : Accelerometer Rotation Experiment Labview Program Students will write a Labview™ program to measure the pulse width from the x and y signals from the MEMs accelerometer. This should comprise a while loop that runs continuously until terminated with the signals being measured one point at a time. When the program is terminated the data should be stored to a spreadsheet file Experiment Steps •

Build your Labview.

Insert the MEMs accelerometer into the small breadboard and connect the power supply and outputs. The two outputs from the accelerometer should be connected to the gates of counters 0 (Y) and 1 (X) of the DAQ board breakout block, Figure 2-6. Note: This experiment requires long flexible wires for smooth accelerometer rotation, therefore students should use strain gauge wires approximately 2 feet in length. •

Figure 2-6 NI DAQ Board Connection • •

Place the breadboard against the front face of the lab bench shelf. This establishes a smooth vertical plane for rotation, perpendicular to the gravity vector. Collect and store data. Use this to plot the results. Take data with and without a filter. 4

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Manually rotate the accelerometer about its center on an axis perpendicular to the gravity field and record the data.

Figure 2-7 Accelerometer Rotation •

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Students will plot measured data from the two axes against each and then perform a least squares fit to a circle. From this result they should be able to determine and estimate the change in counter output between perpendicular and parallel orientation of the accelerometer sensors relative to the gravity field. Using calibration data taken in the next section, students will calibrate the curve in terms of g’s.

2.2.2 Experiment 2: Tilt Sensor In this experiment students will simultaneously develop a tilt sensor and an airbag switch using the two separate axes of the accelerometer. As a first step, students will calibrate the accelerometer output. Experiment Steps •

Build your Labview calibration program.

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Insert the MEMs accelerometer into the small breadboard and connect the power supply and outputs.

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Rotate the breadboard to each of the orientations shown in Figure 2-8 and read and record your signal output.

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Figure 2-8 MEMS Tilt Orientations for Calibration

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Build a Labview program with indicator lights as the output. Use the x channel to measure tilt and activate the light at 30 degrees. Use the y channel as an airbag indicator. The light should come on at 0.5 g.

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