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ADXL202AE 数据表(PDF) 11 Page - Analog Devices |
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ADXL202AE 数据表(HTML) 11 Page - Analog Devices |
11 / 12 page REV. A ADXL202E –11– A DUAL AXIS TILT SENSOR: CONVERTING ACCELERATION TO TILT When the accelerometer is oriented so both its X and Y axes are parallel to the earth’s surface it can be used as a two axis tilt sensor with a roll and a pitch axis. Once the output signal from the accelerometer has been converted to an acceleration that varies between –1 g and +1 g, the output tilt in degrees is calculated as follows: Pitch = ASIN (Ax/1 g) Roll = ASIN (Ay/1 g) Be sure to account for overranges. It is possible for the acceler- ometers to output a signal greater than ± 1 g due to vibration, shock or other accelerations. MEASURING 360 OF TILT It is possible to measure a full 360 ° of orientation through gravity by using two accelerometers oriented perpendicular to one another (see Figure 5). When one sensor is reading a maximum change in output per degree, the other is at its minimum. 360 OF TILT 1g Y X Figure 5. Using a Two-Axis Accelerometer to Measure 360 ° of Tilt USING THE ANALOG OUTPUT The ADXL202E was specifically designed for use with its digital outputs, but has provisions to provide analog outputs as well. Duty Cycle Filtering An analog output can be reconstructed by filtering the duty cycle output. This technique requires only passive components. The duty cycle period (T2) should be set to <1 ms. An RC filter with a 3 dB point at least a factor of >10 less than the duty cycle fre- quency is connected to the duty cycle output. The filter resistor should be no less than 100 k Ω to prevent loading of the output stage. The analog output signal will be ratiometric to the supply voltage. The advantage of this method is an output scale factor of approximately double the analog output. Its disadvantage is that the frequency response will be lower than when using the XFILT, YFILT output. XFILT, YFILT Output The second method is to use the analog output present at the XFILT and YFILT pin. Unfortunately, these pins have a 32 k Ω output impedance and are not designed to drive a load directly. An op amp follower may be required to buffer this pin. The advantage of this method is that the full 5 kHz bandwidth of the accelerometer is available to the user. A capacitor still must be added at this point for filtering. The duty cycle converter should be kept running by using RSET <10 M Ω. Note that the acceler- ometer offset and sensitivity are ratiometric to the supply voltage. The offset and sensitivity are nominally: 0 g Offset = VDD/2 ADXL202E Sensitivity = (60 mV × VS)/g Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period, and Resolution of Duty Cycle Modulator Counter- ADXL202E Clock Counts RSET Sample Rate per T2 Counts Resolution T2 (ms) (k ) Rate (MHz) Cycle per g (mg) 1.0 124 1000 2.0 2000 250 4.0 1.0 124 1000 1.0 1000 125 8.0 1.0 124 1000 0.5 500 62.5 16.0 5.0 625 200 2.0 10000 1250 0.8 5.0 625 200 1.0 5000 625 1.6 5.0 625 200 0.5 2500 312.5 3.2 10.0 1250 100 2.0 20000 2500 0.4 10.0 1250 100 1.0 10000 1250 0.8 10.0 1250 100 0.5 5000 625 1.6 STRATEGIES FOR USING THE DUTY CYCLE OUTPUT WITH MICROCONTROLLERS Application notes outlining various strategies for using the duty cycle output with low cost microcontrollers are available from the factory. USING THE ADXL202E AS A DUAL-AXIS TILT SENSOR One of the most popular applications of the ADXL202E is tilt measurement. An accelerometer uses the force of gravity as an input vector to determine orientation of an object in space. An accelerometer is most sensitive to tilt when its sensitive axis is perpendicular to the force of gravity, i.e., parallel to the earth’s surface. At this orientation its sensitivity to changes in tilt is high- est. When the accelerometer is oriented on axis to gravity, i.e., near its +1 g or –1 g reading, the change in output acceleration per degree of tilt is negligible. When the accelerometer is perpen- dicular to gravity, its output will change nearly 17.5 mg per degree of tilt, but at 45 ° degrees it is changing only at 12.2 mg per degree and resolution declines. The following table illustrates the changes in the X and Y axes as the device is tilted ± 90° through gravity. Y X +90 0 –90 1g X Output Y Output (g) X Axis per per Orientation Degree of Degree of to Horizon ( ) X Output (g) Tilt (mg) Y Output (g) Tilt (mg) –90 –1.000 –0.2 0.000 17.5 –75 –0.966 4.4 0.259 16.9 –60 –0.866 8.6 0.500 15.2 –45 –0.707 12.2 0.707 12.4 –30 –0.500 15.0 0.866 8.9 –15 –0.259 16.8 0.966 4.7 0 0.000 17.5 1.000 0.2 15 0.259 16.9 0.966 –4.4 30 0.500 15.2 0.866 –8.6 45 0.707 12.4 0.707 –12.2 60 0.866 8.9 0.500 –15.0 75 0.966 4.7 0.259 –16.8 90 1.000 0.2 0.000 –17.5 BOTTOM VIEW Figure 4. How the X and Y Axes Respond to Changes in Tilt |
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类似说明 - ADXL202AE |
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