Accelerometers - Revision history

Lynch at 14:42, 18 February 2009

2009-02-18T14:42:16Z

Lynch at 14:40, 18 February 2009

2009-02-18T14:40:31Z

Lynch at 14:40, 18 February 2009

2009-02-18T14:40:13Z

Lynch at 14:39, 18 February 2009

2009-02-18T14:39:45Z

MeganBerry: /* Two Axis Accelerometer Analog Devices ADXL203 */

2008-12-17T20:28:54Z

Two Axis Accelerometer Analog Devices ADXL203

MeganBerry: /* Two Axis Accelerometer Analog Devices ADXL203 */

2008-12-17T20:26:49Z

Two Axis Accelerometer Analog Devices ADXL203

MeganBerry: /* Two Axis Accelerometer Analog Devices ADXL203 */

2008-12-17T20:06:10Z

Two Axis Accelerometer Analog Devices ADXL203

MeganBerry at 18:48, 17 December 2008

2008-12-17T18:48:21Z

MeganBerry at 18:41, 17 December 2008

2008-12-17T18:41:11Z

MeganBerry: /* Two Axis Accelerometer Analog Devices ADXL203 */

2008-12-17T17:02:28Z

Two Axis Accelerometer Analog Devices ADXL203

← Older revision		Revision as of 14:42, 18 February 2009
Line 1:		Line 1:
	'''[Note: This page describes particular accelerometers, but you should expect to get your own (and not necessarily these models) as the Mechatronics Lab does not generally stock significant numbers of these. You can buy the accelerometer chips by themselves, but they usually come in surface mount packages that may be difficult to work with. Instead, it is easier to buy evaluation boards that break out the pins for you. Accelerometers with analog voltage outputs are recommended for easy interfacing to ~~the~~ PIC. These boards often have low-pass filters on the output, too; if so, you may need to change the capacitor to get the cutoff frequency you want. Expect to pay $30-$50 or so for two-axis and three-axis accelerometers from vendors such as [http://sparkfun.com Sparkfun], [http://www.digikey.com Digikey], and [http://www.parallax.com Parallax], or directly from the manufacturers such as [http://www.analog.com Analog Devices], where you may also be able to get samples. Choose the range best suited for your application (e.g., 1.7-2 g's if you're simply measuring orientation in a gravity field, or perhaps up to 5-20 g's if you are actively vibrating). Some accelerometers offer you a choice of operating ranges.]'''		'''[Note: This page describes particular accelerometers, but you should expect to get your own (and not necessarily these models) as the Mechatronics Lab does not generally stock significant numbers of these. You can buy the accelerometer chips by themselves, but they usually come in surface mount packages that may be difficult to work with. Instead, it is easier to buy evaluation boards that break out the pins for you. Accelerometers with analog voltage outputs are recommended for easy interfacing to a PIC. These boards often have low-pass filters on the output, too; if so, you may need to change the capacitor to get the cutoff frequency you want. Expect to pay $30-$50 or so for two-axis and three-axis accelerometers from vendors such as [http://sparkfun.com Sparkfun], [http://www.digikey.com Digikey], and [http://www.parallax.com Parallax], or directly from the manufacturers such as [http://www.analog.com Analog Devices], where you may also be able to get free samples. Choose the range best suited for your application (e.g., 1.7-2 g's if you're simply measuring orientation in a gravity field, or perhaps up to 5-20 g's if you are actively vibrating). Some accelerometers offer you a choice of operating ranges.]'''

	[[image:mems accelerometer.png\|right]]		[[image:mems accelerometer.png\|right]]

@@ Line 5: / Line 5: @@
 Accelerometers measure linear acceleration and also gravity; the two are indistinguishable.  Thus they unavoidably function as tilt sensors as well.   Inexpensive 1, 2, and 3-axis accelerometers are available which are constructed with MEMS techniques.
 MEMS gyroscopes are also available, for measuring angular velocity, but are more expensive.
 <br clear=all>
 [[Image:carrier.jpg|right]]
 The LIS2L02AS4-TR accelerometer gives you a choice of +/-2g or +/-6g full scale.  It needs only a single +5V supply.  Its bandwidth is from DC to 1.5KHz.  The output is usually low-pass filtered in applications which do not need the full 1.5KHz bandwidth, using an external capacitor.

@@ Line 1: / Line 1: @@
-[[image:mems accelerometer.png|right]]
 '''[Note:  This page describes particular accelerometers, but you should expect to get your own (and not necessarily these models) as the Mechatronics Lab does not generally stock significant numbers of these.  You can buy the accelerometer chips by themselves, but they usually come in surface mount packages that may be difficult to work with.  Instead, it is easier to buy evaluation boards that break out the pins for you.  Accelerometers with analog voltage outputs are recommended for easy interfacing to the PIC.  These boards often have low-pass filters on the output, too; if so, you may need to change the capacitor to get the cutoff frequency you want.  Expect to pay $30-$50 or so for two-axis and three-axis accelerometers from vendors such as [http://sparkfun.com Sparkfun], [http://www.digikey.com Digikey], and [http://www.parallax.com Parallax], or directly from the manufacturers such as [http://www.analog.com Analog Devices], where you may also be able to get samples.  Choose the range best suited for your application (e.g., 1.7-2 g's if you're simply measuring orientation in a gravity field, or perhaps up to 5-20 g's if you are actively vibrating).  Some accelerometers offer you a choice of operating ranges.]'''
+[[image:mems accelerometer.png|right]]
 Accelerometers measure linear acceleration and also gravity; the two are indistinguishable.  Thus they unavoidably function as tilt sensors as well.   Inexpensive 1, 2, and 3-axis accelerometers are available which are constructed with MEMS techniques.

@@ Line 1: / Line 1: @@
 [[image:mems accelerometer.png|right]]
+'''[Note:  This page describes particular accelerometers, but you should expect to get your own (and not necessarily these models) as the Mechatronics Lab does not generally stock significant numbers of these.  You can buy the accelerometer chips by themselves, but they usually come in surface mount packages that may be difficult to work with.  Instead, it is easier to buy evaluation boards that break out the pins for you.  Accelerometers with analog voltage outputs are recommended for easy interfacing to the PIC.  These boards often have low-pass filters on the output, too; if so, you may need to change the capacitor to get the cutoff frequency you want.  Expect to pay $30-$50 or so for two-axis and three-axis accelerometers from vendors such as [http://sparkfun.com Sparkfun], [http://www.digikey.com Digikey], and [http://www.parallax.com Parallax], or directly from the manufacturers such as [http://www.analog.com Analog Devices], where you may also be able to get samples.  Choose the range best suited for your application (e.g., 1.7-2 g's if you're simply measuring orientation in a gravity field, or perhaps up to 5-20 g's if you are actively vibrating).  Some accelerometers offer you a choice of operating ranges.]'''
 Accelerometers measure linear acceleration and also gravity; the two are indistinguishable.  Thus they unavoidably function as tilt sensors as well.   Inexpensive 1, 2, and 3-axis accelerometers are available which are constructed with MEMS techniques.

@@ Line 83: / Line 83: @@
 If the object of interest is oriented with both accelerometers parallel to gravity, each accelerometer will have at least one output that is either very high (3.5V) or very low (1.5V). This allows orientation to be determined for the third axis without the need for a third accelerometer. The other outputs from the accelerometers will be roughly 2.5V but will be very sensitive to tilting out of plane. These outputs can therefore be used to determine the precise angle of the object with respect to gravity.
 <br clear=all>
@@ Line 95: / Line 94: @@
 To determine which side is upwards, the controller can compare the four accelerometer outputs. When any side is facing upwards, one of the four accelerometer inputs will be either very high voltage (greater than 3.25V) or very low voltage (less than 1.75V). The controller checks the accelerometer output values until one of these conditions is met and references the correct side as being upward. Sample code for this calculation is given here. This code assumes that two accelerometers are being used so that orientation can be determined if any of the six primary sides is facing upwards: +X, -X, +Y, -Y, +Z, -Z.
+<br clear=all>
+[[Image:ADCvalues.JPG|left]]

@@ Line 73: / Line 73: @@
 <br clear=all>
 [[Image:AccelYZ.JPG|right]]
+Tilt of a Two Dimensional Object
+If you only need to sense the tilt of a planar object, one accelerometer is sufficient. The accelerometer should be mounted so that the circuit is perpendicular to gravity, either face up or face down, for the standard orientation of your object. This mounting will give the greatest change in voltage output per degree of tilt. In this position, both the X and Y output should read roughly 2.5V. Tilting -45 to +45 degrees about either axis should vary that output between 1.75V and 3.25V. In this range, the change in voltage is roughly linear and equal to .0167 V/degree. For angles of tilt much greater than 45 degrees in either direction the accelerometer is less sensitive and the linear relationship no longer holds. If the angle of tilt that you are expecting to measure is much greater than 45 degrees consider either a second accelerometer or a look-up table.
 Orientation of a Three Dimensional Object
-Because the accelerometer gives the same response whether the circuit side is up or the circuit side is down, a second accelerometer is needed to distinguish between these two orientations. The second accelerometer should be oriented out of plane with the first accelerometer, preferably at a 90 degree angle. If either accelerometer is perpendicular to gravity, the outputs from that accelerometer will be roughly 2.5V. However, the second accelerometer which will have a low output for one orientation and a high output for the other orientation.
+Because the accelerometer gives the same response whether the circuit side is up or the circuit side is down, a second accelerometer is needed to distinguish between these two orientations. The second accelerometer should be oriented out of plane with the first accelerometer, preferably at a 90 degree angle. If either accelerometer is perpendicular to gravity, the outputs from that accelerometer will be roughly 2.5V. However, the second accelerometer will have a low output for one orientation and a high output for the other orientation.
 If the object of interest is oriented with both accelerometers parallel to gravity, each accelerometer will have at least one output that is either very high (3.5V) or very low (1.5V). This allows orientation to be determined for the third axis without the need for a third accelerometer. The other outputs from the accelerometers will be roughly 2.5V but will be very sensitive to tilting out of plane. These outputs can therefore be used to determine the precise angle of the object with respect to gravity.

@@ Line 63: / Line 63: @@
 == Two Axis Accelerometer Analog Devices ADXL203 ==
-This accelerometer from Analog Devices is very useful due to its availability from circuit suppliers as an evaluation board. Although the component itself is small it comes mounted to a board which allows it to be plugged into any breadboard for testing and prototyping. The range of the accelerometer is ±1.7 times gravity which is useful when the primary value of interest is the angle of a plane with respect to gravity.
 <br clear=all>
 [[Image:AccelXY.JPG|right]]
+This accelerometer from Analog Devices is very useful due to its availability from circuit suppliers as an evaluation board. Although the component itself is small it comes mounted to a board which allows it to be plugged into any breadboard for testing and prototyping. The range of the accelerometer is ±1.7 times gravity which is useful when the primary value of interest is the angle of a plane with respect to gravity.
-The accelerometer takes in a 5V power signal and outputs two voltages which are the x axis and y axis outputs. When the accelerometer is perpendicular to gravity both the x and y outputs read a voltage of about 2.5V. Rotation about one axis, Z axis in the example images, will produce a varying signal from the X output and a constant 2.5V signal from the Y output. Rotation about a second axis, X axis in the example images, will produce a varying signal from the Y output and a constant 2.5V signal from the X output. Rotation about the final axis, Y axis in the example images, will produce varying signals for both outputs.
+The accelerometer takes in a 5V power signal and outputs two voltages which are the x axis and y axis outputs. When the accelerometer is perpendicular to gravity both the x and y outputs read a voltage of about 2.5V. Rotation about one axis, Z axis in the example images, will produce a varying signal from the Y output and a constant 2.5V signal from the X output. Rotation about a second axis, X axis in the example images, will produce a varying signal from the X output and a constant 2.5V signal from the Y output. Rotation about the final axis, Y axis in the example images, will produce varying signals for both outputs.
-Orientation of a Three Dimensional Object
-Because the accelerometer gives the same response whether the face is up or the face is down, two accelerometers are needed to distinguish between these two orientations. The second accelerometer should be oriented out of plane with the first accelerometer, preferably at a 90 degree angle. With this placement of the two accelerometers the face down or face up orrientation of the first accelerometer can be determined from the outputs of the second accelerometer.
 <br clear=all>
 [[Image:AccelYZ.JPG|right]]
-<br clear=all>
-[[Image:AccelXZ.JPG|right]]
+Orientation of a Three Dimensional Object
+Because the accelerometer gives the same response whether the circuit side is up or the circuit side is down, a second accelerometer is needed to distinguish between these two orientations. The second accelerometer should be oriented out of plane with the first accelerometer, preferably at a 90 degree angle. If either accelerometer is perpendicular to gravity, the outputs from that accelerometer will be roughly 2.5V. However, the second accelerometer which will have a low output for one orientation and a high output for the other orientation.
+If the object of interest is oriented with both accelerometers parallel to gravity, each accelerometer will have at least one output that is either very high (3.5V) or very low (1.5V). This allows orientation to be determined for the third axis without the need for a third accelerometer. The other outputs from the accelerometers will be roughly 2.5V but will be very sensitive to tilting out of plane. These outputs can therefore be used to determine the precise angle of the object with respect to gravity.
+<br clear=all>
+[[Image:AccelXZ.JPG|right]]
 Using the Output
-To read the x axis and y axis outputs, which are analog voltage signals, you can use the analog to digital converter built into the PIC. This converter takes in the analog signal and converts it to either a 10 bit or 8 bit value which can be understood by the PIC. A 10 bit value will have greater resolution. With 10 bit resolution a signal of 5V would have a decimal value of 1023 (binary 1111111111) and a signal of 0V would have a decimal value of 0. All other voltage values can be determined by a linear relationship between these two points. For example, 2.5V would have a decimal value of 511.
+To read the x axis and y axis outputs, which are analog voltage signals, you can use the analog to digital converter built into the PIC. Sample code to read in the data from two accelerometers can be found here. This converter takes in the analog signal and converts it to either a 10 bit or 8 bit value which can be understood by the PIC. A 10 bit value will give better resolution, with every binary increment being equal to an increase of 0.005V in the analog signal. A 8 bit value will have slightly lower resolution, with every binary increment being equal to an increase of 0.02V in the analog signal.
+The analog voltage is converted to a binary value with the specified bit value and can also be read as a decimal value. The analog voltages along with their resultant binary and decimal values for 10 bit and 8 bit resolution are given in the table below. All other intermediate values follow a linear relationship between analog voltage and binary value.
-To determine which side is upwards, the controller can compare the four accelerometer outputs. When any side is facing upwards, one of the four accelerometer inputs will be either very high voltage (greater than 3.25V) or very low voltage (less than 1.75V). The controller checks the accelerometer output values until one of these conditions is met and references the correct side as being upward.
+To determine which side is upwards, the controller can compare the four accelerometer outputs. When any side is facing upwards, one of the four accelerometer inputs will be either very high voltage (greater than 3.25V) or very low voltage (less than 1.75V). The controller checks the accelerometer output values until one of these conditions is met and references the correct side as being upward. Sample code for this calculation is given here. This code assumes that two accelerometers are being used so that orientation can be determined if any of the six primary sides is facing upwards: +X, -X, +Y, -Y, +Z, -Z.

@@ Line 75: / Line 75: @@
 <br clear=all>
-[[Image:AccelYZwiki.jpg|right]]
+[[Image:AccelYZwiki.JPG|right]]
 <br clear=all>
-[[Image:AccelXZwiki.jpg|right]]
+[[Image:AccelXZwiki.JPG|right]]