惯性导航原理的理解.docx

1、惯性导航原理的理解Part 1. AccelerometerTo understand this unit well start with the accelerometer. When thinking about accelerometers it is often useful to image a box in shape of a cube with a ball inside it. You may imagine something else like a cookie or a donut , but Ill imagine a ball:了解本单元,我们从加速度计开始。当考虑

2、加速度计通常想象成一个立方体形状的盒子里面有一个球很有助于理解。你也可以想像别的像饼干或者一个甜甜圈,但我会想象一个球:If we take this box in a place with no gravitation fields or for that matter with no other fields that might affect the balls position the ball will simply float in the middle of the box. You can imagine the box is in outer-space far-far aw

3、ay from any cosmic bodies, or if such a place is hard to find imagine at least a space craft orbiting around the planet where everything is in weightless state . From the picture above you can see that we assign to each axis a pair of walls (we removed the wall Y+ so we can look inside the box). Ima

4、gine that each wall is pressure sensitive. If we move suddenly the box to the left (we accelerate it with acceleration 1g = 9.8m/s2), the ball will hit the wall X-. We then measure the pressure force that the ball applies to the wall and output a value of -1g on the X axis.Please note that the accel

5、erometer will actually detect a force that is directed in the opposite direction from the acceleration vector. This force is often calledInertial Force or Fictitious Force. One thing you should learn from this is that an accelerometer measures acceleration indirectly through a force that is applied

6、to one of its walls (according to our model, it might be a spring or something else in real life accelerometers). This force can be caused by the acceleration , but as well see in the next example it is not always caused by acceleration.If we take our model and put it on Earth the ball will fall on

7、the Z- wall and will apply a force of 1g on the bottom wall, as shown in the picture below:In this case the box isnt moving but we still get a reading of -1g on the Z axis. The pressure that the ball has applied on the wall was caused by a gravitation force. In theory it could be a different type of

8、 force for example, if you imagine that our ball is metallic, placing a magnet next to the box could move the ball so it hits another wall. This was said just to prove that in essence accelerometer measures force not acceleration. It just happens that acceleration causes an inertial force that is ca

9、ptured by the force detection mechanism of the accelerometer.While this model is not exactly how a MEMS sensor is constructed it is often useful in solving accelerometer related problems. There are actually similar sensors that have metallic balls inside, they are called tilt switches, however they

10、are more primitive and usually they can only tell if the device is inclined within some range or not, not the extent of inclination.So far we have analyzed the accelerometer output on a single axis and this is all youll get with a single axis accelerometers. The real value of triaxial accelerometers

11、 comes from the fact that they can detect inertial forces on all three axes. Lets go back to our box model, and lets rotate the box 45 degrees to the right. The ball will touch 2 walls now: Z- and X- as shown in the picture below:The values of 0.71 are not arbitrary, they are actually an approximati

12、on for SQRT(1/2). This will become more clear as we introduce our next model for the accelerometer.In the previous model we have fixed the gravitation force and rotated our imaginary box. In last 2 examples we have analyzed the output in 2 different box positions, while the force vector remained con

13、stant. While this was useful in understanding how the accelerometer interacts with outside forces, it is more practical to perform calculations if we fix the coordinate system to the axes of the accelerometer and imagine that the force vector rotates around us.Please have a look at the model above,

14、I preserved the colors of the axes so you can make a mental transition from the previous model to the new one. Just imagine that each axis in the new model is perpendicular to the respective faces of the box in the previous model. The vector R is the force vector that the accelerometer is measuring

15、(it could be either the gravitation force or the inertial force from the examples above or a combination of both). Rx, Ry, Rz are projection of the R vector on the X,Y,Z axes. Please notice the following relation:R2 = Rx2 + Ry2 + Rz2(Eq. 1)which is basically the equivalent of thePythagorean theorem

16、in 3D.Remember that a little bit earlier I told you that the values of SQRT(1/2) 0.71 are not random. If you plug them in the formula above, after recalling that our gravitation force was 1 g we can verify that:12 = (-SQRT(1/2) )2 + 0 2 + (-SQRT(1/2)2simply by substituting R=1, Rx = -SQRT(1/2), Ry =

17、 0 , Rz = -SQRT(1/2) inEq.1After a long preamble of theory were getting closer to real life accelerometers. The values Rx, Ry, Rz are actually linearly related to the values that your real-life accelerometer will output and that you can use for performing various calculations.Before we get there let

18、s talk a little about the way accelerometers will deliver this information to us. Most accelerometers will fall in two categories（大多受加速度计可以分为以下两类）: digital and analog. Digital accelerometers will give you information using a serial protocol（协议） like I2C , SPI or USART, while analog accelerometers wi

19、ll output a voltage level within a predefined range that you have to convert to a digital value using an ADC (analog to digital converter) module. I will not go into much detail about how ADC works, partly because it is such an extensive topic and partly because it is different from one platform to

20、another. Some microcontroller will have a built-in ADC modules some of them will need external components in order to perform the ADC conversions. No matter what type of ADC module you use youll end up with a value in a certain range. For example a 10-bit ADC module will output a value in the range

21、of 0.1023, note that 1023 = 210 -1. A 12-bit ADC module will output a value in the range of 0.4095, note that 4095 = 212-1.Lets move on by considering a simple example, suppose our 10bit ADC module gave us the following values for the three accelerometer channels (axes):AdcRx = 586AdcRy = 630AdcRz =

22、 561Each ADC module will have a reference voltage, lets assume in our example it is 3.3V. To convert a 10bit adc value to voltage we use the following formula:VoltsRx = AdcRx * Vref / 1023A quick note here: that for 8bit ADC the last divider would be 255 = 2 8 -1 , and for 12bit ADC last divider wou

23、ld be 4095 = 212 -1.Applying this formula to all 3 channels we get:VoltsRx = 586 * 3.3V / 1023 = 1.89V (we round all results to 2 decimal points)VoltsRy = 630 * 3.3V / 1023 = 2.03VVoltsRz = 561 * 3.3V / 1023 = 1.81VEach accelerometer has a zero-g voltage level, you can find it in specs, this is the

24、voltage that corresponds to 0g. To get a signed voltage value we need to calculate the shift from this level. Lets say our 0g voltage level is VzeroG = 1.65V. We calculate the voltage shifts from zero-g voltage as follows:DeltaVoltsRx = 1.89V 1.65V = 0.24VDeltaVoltsRy = 2.03V 1.65V = 0.38VDeltaVolts

25、Rz = 1.81V 1.65V = 0.16VWe now have our accelerometer readings in Volts , its still not in g (9.8 m/s2), to do the final conversion we apply the accelerometer sensitivity, usually expressed in mV/g. Lets say our Sensitivity = 478.5mV/g = 0.4785V/g. Sensitivity values can be found in accelerometer sp

26、ecifications（规格）. To get the final force values expressed in g we use the following formula:Rx = DeltaVoltsRx / SensitivityRx = 0.24V / 0.4785V/g = 0.5gRy = 0.38V / 0.4785V/g = 0.79gRz = 0.16V / 0.4785V/g = 0.33gWe could of course combine all steps in one formula, but I went through all the steps to

27、 make it clear how you go from ADC readings to a force vector component expressed in g.Rx = (AdcRx * Vref / 1023 VzeroG) / Sensitivity(Eq.2)Ry = (AdcRy * Vref / 1023 VzeroG) / SensitivityRz = (AdcRz * Vref / 1023 VzeroG) / SensitivityWe now have all 3 components that define our inertial force vector

28、, if the device is not subject to other forces other than gravitation, we can assume this is the direction of our gravitation force vector. If you want to calculate inclination of device relative to the ground you can calculate the angle between this vector and Z axis. If you are also interested in

29、per-axis direction of inclination you can split this result into 2 components: inclination on the X and Y axis that can be calculated as the angle between gravitation vector and X / Y axes. Calculating these angles is more simple than you might think, now that we have calculated the values for Rx,Ry

30、 and Rz. Lets go back to our last accelerometer model and do some additional notations:The angles that we are interested in are the angles between X,Y,Z axes and the force vector R. Well define these angles as Axr, Ayr, Azr. You can notice from the right-angle triangle formed by R and Rx that:cos(Ax

31、r) = Rx / R , and similarly :cos(Ayr) = Ry / Rcos(Azr) = Rz / RWe can deduct fromEq.1that R = SQRT( Rx2 + Ry2 + Rz2).We can find now our angles by using arccos() function (the inverse cos() function ):Axr = arccos(Rx/R)Ayr = arccos(Ry/R)Azr = arccos(Rz/R)Weve gone a long way to explain the accelerom

32、eter model, just to come up to these formulas. Depending on your applications you might want to use any intermediate formulas that we have derived. Well also introduce the gyroscope model soon, and well see how accelerometer and gyroscope data can be combined to provide even more accurate inclination estimations.But before we do that lets do some more useful notations:cosX = cos(Axr) = Rx / RcosY = cos(Ayr) = Ry / RcosZ = cos(Azr) = Rz / RThis triplet is often calledDirection Cosine,