Accelerometers | Guestbook | |
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| Accelerometers for your Robot[The ADXL202 and the MEMSIC 2125] |
| Wednesday, 19 January 2005 | |||||||||||
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You've seen robots run on two wheels ,walk on two legs and balance just like us humans.I bet you've wondered whats in them that makes them "not tip over" The device that enables this is a tiny teeny IMEM [aMicro Electro-Mechanical System] which can measure anything between ±1g to ±20g [some of the high g accelerometers measure ±20g to ±250g ] The sensors:  What's a "g"? A "g" is a unit of acceleration having a value of 9.8 if you say 1g of acceleration .Its equivalent to: 1g = 1*9.8 = 9.8 m/s2 2g = 2*9.8 = 19.6m/s2 "g" in reality :
Where are Accelerometers used:
The following describes the two most popularly used accelerometers. Both these accelerometers work on the same principle ,but on different technologies. The Analog Devices ADXL202  Manufacturer ::Analog Devices Datasheet :: pdf Principle The ADXL202 has a movable mass inside it which responds to vibrations,tilt or jerks The sensor element is a differential capacitor whose output is proportional to acceleration. The beam is made up of many interdigitated fingers. Each set of fingers can be visualized as shown below   Images Courtesy Microchip.com The movement of the beam is controlled by the polysilicon springs holding the beam. These springs and the beam’s mass obey the laws of physics . The force (F) on a mass (m) subject to acceleration (a), according to Newton’s Second Law, is F = m a The deflection (x) of a restraining spring according to Hooke’s Law is proportional to the applied force, F = k x From the above two equations... a(acceleration) is proportional to x(displacement) This enables us to compute the acceleration. Once acceleration is obtained ,tilt can be determined. The MEMSIC 2125  Manufacturer :: Memsic Datasheet :: pdf Principle Unlike the Analog Devices ADXL which uses a movable beam, the Memsic accelerometers measure acceleration by monitoring a hot gas inside a chamber. The chamber is lined by a numerous temperature sensors which monitor the movement of the gas. The picture below illustrate this:  Images courtesy of Parallax Inc. (www.parallax.com), from the Stamps in Class Forum article "How to - Accelerometer (1) Fundamentals and Tilt" The first figure illustrates the accelerometer when it is held level.Note the position of the hot gas. The adjacent figure shows the accelerometer when tilted.The gas now shifts,changing the values of the sensors appropriately Output of an Accelerometer: Accelerometers like the ADXL and Memsic output their measurements in terms of PWM [Pulse Width Modulation] For example Take the ADXL202jqc which measures -2g to 2g According to the ADXL datasheet 0g would translate to a 50% duty cycle  a < than 0g readout with a lower than 50% duty cycle  a 0g readout with 50% duty cycle a > than 0g readout with a greater than 50% duty cycle
Accelerometer AXIS Both the above sensors measure 2 axis i.e the x and the y 
Selection of an axis depends on your project requirements. You can even use the sensors to measure a full 360 degree of tilt. by using two accelerometers oriented perpendicular to one another.When one sensor is reading a maximum change in output per degree, the other is at its minimum. 
Well thats it..these sensors do not need alot of interfacing components. The ADXL202 just need 2 resistors and capacitors to set certain parameters while the Memsic needs even fewer components. I'll be putting up a page on interfacing the ADXL202E with a PIC16F84A. Also a new section on Accelerometers has been added in the LINKS category. Questions?If you got any questions.please feel free to message me.You may click here or mail me at gerryseq(at)rediffmail.com Recommend this page to a friend Some of the images and content shown here are copyright of their respective creators and have been used with their permission.You may use the images ,but please ask permission before you do so or leave a "Image Courtesy" message {moscomment} |
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