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Detecting X-axis position

11 Sep 2015  | Marian Stofka

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When you need to detect angular position related to Earth's gravity, you can use an Analog Devices three-axis microelectromechanical-system (MEMS) accelerometer. The ADXL335 has three analogue outputs that correspond to the X, Y, and Z axes of an orthogonal coordinate system (Reference 1). Because the Z axis is perpendicular to the footprint, or base, of the MEMS IC's package surface, you can use the accelerometer to detect tilt if you mount it on a PCB that's parallel to your product's base. The circuit in the figure lets you detect whether the tilt exceeds a preset limit. A digital output, in this example, drives an LED, but you can connect the signal to a microcontroller or another device.

When you orient the accelerometer IC horizontally relative to Earth, the LED is on. Whenever the Z axis of the device declines by a specific value, αT,of the angle, α, from the vertical direction, the LED turns off. The voltage difference at the Z-axis output, ZOUT, of the accelerometer, referenced to the power supply's midvoltage, VS/2, is VGZ=VGcosα, where VS is the power-supply voltage, VGZ is the voltage at the ZOUT pin, and VG is the terrestrial full-scale voltage. When the power-supply voltage is 3V, the terrestrial full-scale voltage is either 300 or –300 mV, depending on whether you orient the detector from the top down or from the bottom up. Op amp IC2 compares the voltage at the ZOUT pin to the reference voltage, VREF. If the positive voltage at the ZOUT pin is equal to or lower than the reference voltage, the output of IC2A goes high, and the output of IC2B remains high (Reference 2). Thus, the output of NAND gate IC3 becomes low, and the LED turns off. You can calculate the threshold tilt angle, αT, at which this action occurs from the equation cosαT=(VREF/VG).

Figure: The tilt on MEMS accelerometer IC1 produces a voltage, VGZ. When compared with VREF and 2VREF, VGZ produces a digital output at the NAND gate.

Resistors RS and RR set the voltage reference to 136.36 mV. Thus, the threshold tilt angle is 62.96°. Similarly, when the negative voltage at the ZOUT pin becomes lower in magnitude than the negative reference voltage, it indicates a tilt of 62.96° or more, the output of IC2B goes high, and the LED (Reference 3) also turns off. Theoretically, you can choose any other threshold angle within the interval of 0 to 90°. The practical limits with the 10-nF filtering capacitor, however, are 21.23 and 86.10°. The probability of a short-term false detection is 8×10–5. From the properties of the cosine function, the sensitivity of the tilt detector rises with rising tilt angle. To select another value of tilt within this interval, you calculate the appropriate reference voltage from the equation cosαT=(VREF/VG) and then change the value of the RR resistors as necessary.

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