加速度计和陀螺仪的区别
加速度计和陀螺仪的区别
这是一篇 diydrones.com 网站上的文章，介绍为什么在无人机自主导航中需要用到加速度计、陀螺仪、磁罗盘计和GPS等传感器（原文地址）。http://www.diydrones.com是个开源的讨论UAV（无人机自主飞行导航器）的社区网站。
An accelerometer measures acceleration. A 3-axis accelerometer will tell you the orientation of a stationary platform relative to earth’s surface, once that platform starts moving, however, things get more complicated. If the platform is in free-fall, it will show zero acceleration. If it is accelerating in a particular direction, that acceleration will simply be added to whatever acceleration is being provided by gravity, and you will not be able to distinguish. A 3-axis accelerometer in an aircraft in a properly coordinated turn with a 60 degree angle of bank, for instance, will show 2 G “vertical” acceleration in the aircraft, despite the fact that the aircraft is tilted 60 degrees relative to the horizon. So, accelerometers alone can’t be used to keep in an aircraft in a particular orientation.
译文：加速度计用于测量加速度。借助一个三轴加速度计可以测得一个固定平台相对地球表面的运动方向，但是一旦平台运动起来，情况就会变得复杂的多。如果平台做自由落体，加速度计测得的加速度值为零。如果平台朝某个方向做加速度运动，各个轴向加速度值会含有重力产生的加速度值，使得无法获得真正的加速度值。 例如，安装在60度横滚角飞机上的三轴加速度计会测得2G的垂直加速度值，而事实上飞机相对地区表面是60度的倾角。因此，单独使用加速度计无法使飞机保 持一个固定的航向。
飞机的横滚角
A gyro measures rate of rotation around a particular axis. If a gyro is used to measure the rate of rotation around the aircraft roll axis, it will measure a non-zero value as long as the aircraft is rolling, but measure zero if the roll stops. So, a roll gyro in an aircraft in a coordinated turn with a 60 degree bank will be measure a rate of zero, same as an aircraft flying straight and level. You can approximate the current roll angle by integrating the roll rate over time, but you can’t do so without some error creeping in. Just to make life more interesting, gyros drift with time, so additional error will accumulate over a period of minutes or even seconds, and eventually, you’ll have a totally inaccurate idea of your current roll angle relative to the horizon. So, gyros alone can’t be used to keep in an aircraft in a particular orientation.
译文：陀螺仪测量机体围绕某个轴向的旋转角速率值。使用陀螺仪测量飞机机体轴向的旋转角速率时，如果飞机在旋转，测得的值为非零值，飞机不旋转时，测量的值为零。因此，在60度横滚角的飞机上的陀螺仪测得的横滚角速率值为零，同样在飞机做水平直线飞行时，角速率值为零。可以通过角速率值的时间积分来估计当前的横滚角度，前提是没有误差的累积。陀螺仪测量的值会随时间漂移，经过几分钟甚至几秒钟定会累积出额外的误差来，而最终会导致对飞机当前相对水平 面横滚角度完全错误的认知。因此，单独使用陀螺仪也无法保持飞机的特定航向。
飞机的俯仰角
飞机横滚角
飞机偏航角
So, in a nutshell: Accelerometers are right in the long term but wrong (noisy) in the short term. Gyros are right in the short term but wrong (drifiting) in the long term. You need both–each to calibrate the other–to be right all the time.
译文：一言以蔽之，加速度计在较长时间的测量值（确定飞机航向）是正确的，而在较短时间内由于信号噪声的存在，而有误差。陀螺仪在较短时间内则比较准确而较长时间则会有与漂移而存有误差。因此，需要两者（相互调整）来确保航向的正确。
But even that only works in for pitch and roll. For yaw, which is orthogonal to gravity, the accelerometers can’t help you, so you need something else to correct your drifting yaw gyro. That’s either magnetometers (electronic compasses, which are vulnerable to magnetic interference and intertial forces) or GPS.
译文：即使使用了两者，也只可以用于测得飞机的俯仰和横滚角度。对于偏航角度，由于偏航角和重力方向正交，无法用加速度计测量得到，因此还需要采用其他设备来校准测量偏航角度的陀螺仪的漂移值。校准的设备可以使用磁罗盘计（电子磁罗盘，对磁场变化和惯性力敏感）或者GPS。
GPS has a relatively slow update rate (1 to 10 Hz) and is subject to short term errors. It is possible to use GPS alone to keep a very stable and slow flying airframe on a particular ground track on a calm day.
译文：GPS数据更新较慢（1Hz到10Hz），并且短时间内存在误差。可以只用GPS就可在地磁平稳的时间内，在地面跟踪较为稳定和慢速的飞行 器。
An inertial measurement unit (IMU) combines (fuses) information from two or more sensors, such as gyros, accelerometers, magnetometer, and/or GPS, to determine orientation and velocity vector relative to the earth. The computations are fairly complex, and special filtering is often required to eliminate the measurement noise these silicon devices are subject to, so a “low cost” off-the-shelf IMU with decent specs can easily cost $1000 to $5000 US.
译文：惯性导航单元（IMU）组合（融合）来自两个或以上的传感器（例如陀螺仪、加速度计、磁场计和/或GPS）信息用于飞机相对地球的航向矢量和 速度矢量。这种融合算法相当复杂，同时还需要对这些电子器件固有的测量噪声进行特殊滤波，因此市场上具有还算过得去的参数，“廉价”的IMU的价格也要 1000至5000美元。
Infra-red horizon sensing “copilots” are inexpensive and work fairly well as long as they have a clear view of the horizon. Unfortunately, mountains, clouds, haze, buildings, etc., will confuse them. [Chris here: I think IR stabalization is more effective than people give it credit for, and is actually rarely confused in standard usage. Both Paparazzi and AttoPilot are built on it, and they work great. See this interview for more.]
译文：红外水平感应辅助导航仪价格便宜，只要有水平清晰的视觉，它工作良好。不幸的是，山峰、云层、烟雾和建筑等会影响其水平视觉。【Chris： 我认为红外导航仪的稳定性比人们认为的要稳定的多，并且在按标准情况下使用时很少会收到影响。Paparazzi和AttoPilot都使用了它，并工作 良好。可以参考this interview获取更多资料。】
In the end, the techniques used to stabilize a UAV will be very much dependent on the intended use, budget, and how comfortable one might be working with sensor fusion, Kalman filters, etc.
最后，用于UAV的导航器的设计技巧（设计方案）依赖于使用目的、经费预算和传感器数据融合计算、卡尔曼滤波的便利性等方面。
