Path: EDN Asia >> Design Ideas >> IC/Board/Systems Design >> Detecting X-axis position
IC/Board/Systems Design Share print

Detecting X-axis position

11 Sep 2015  | Marian Stofka

Share this page with your friends

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.

1 • 2 Next Page Last Page


Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.


Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming


News | Products | Design Features | Regional Roundup | Tech Impact