Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote:> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates > <yates@digitalsignallabs.com> wrote:>>> Just shaking an accelerometer will cause a bias shift. This is called >>> "vibration rectification".>>Is that because it produces accelerations that exceed the >>accelerometers' maximum?> Due to nonlinearities while operating well within the maximums.> Of course if you rail the thing you'll see gross errors.Some time ago, I was reading about laser gyros. Two beams going opposite directions, either in air or glass fiber, and then into an interferometer. They work well at high rotations rates, but can mode lock at low rates such that they ignore slow rotations. -- glen
Gravity Vector Tracking
Started by ●May 1, 2013
Reply by ●May 3, 20132013-05-03
Reply by ●May 3, 20132013-05-03
>Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote: >> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates >> <yates@digitalsignallabs.com> wrote: > >>>> Just shaking an accelerometer will cause a bias shift. This is called >>>> "vibration rectification". > >>>Is that because it produces accelerations that exceed the >>>accelerometers' maximum? > >> Due to nonlinearities while operating well within the maximums. > >> Of course if you rail the thing you'll see gross errors. > >Some time ago, I was reading about laser gyros. Two beams going >opposite directions, either in air or glass fiber, and then into >an interferometer. > >They work well at high rotations rates, but can mode lock at low >rates such that they ignore slow rotations. > >-- glen >Otherwise known as a FOG (fiber optic gyroscope). Pretty common, HG1700 by honeywell. MEMS are getting close to being as good as these, but not yet.
Reply by ●May 3, 20132013-05-03
>On Fri, 03 May 2013 00:01:36 -0400, Randy Yates ><yates@digitalsignallabs.com> wrote: > >>Randy Yates <yates@digitalsignallabs.com> writes: >> >>> Looking for suggestions or pointers on how to track the gravity vector >>> in 6D data. >> >>Tim, Vlad, Peter, Spehro, and SG: Thank you for all the input you've >>provided so far. >> >>By the way, we're using the ST LSM330 as the gyro and low-G >>accelerometer, and an Analog Devices ADXL377 for the high-G >>accelerometer. > >Do you have any sensors other than the gyros and accelerometers to >maintain alignment? We use various devices that use the MEMS gyros >and accelerometers to instrument cars on track to get lateral >g-loading, acceleration/deceleration, estimate hp, etc., etc. They >work really well for that, but most also use gps to remove drift/bias. >GPS by itself doesn't get enough resolution to get detailed info, but >together you can get very repeatable, reliable info even with the >cheapie MEMS sensors. > >Back in my radar days we used IMUs with periodic GPS realignment >updates (with a Kalman predictive filter) to estimate the aircraft >position in order to keep the antenna accurately aimed at a ground >target. That was in the late 80s, so it's not like it's new >methodology or technology. > >I don't know how your application compares, but if there's any way to >get additional alignment data, even just once in a while, it can go a >long way toward keeping the drift/bias under control. > > > >Eric Jacobsen >Anchor Hill Communications >http://www.anchorhill.com >Most good Navigation systems use a blended solution, Inertial and GPS data with a Kalman. Though, many launch vehicles are Intertial only during flight. Though more are going towards a blended solution, sometimes dynamics of the vehicle force GPS tracking issues.
Reply by ●May 3, 20132013-05-03
On Fri, 03 May 2013 21:46:44 +0000, glen herrmannsfeldt wrote:> Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote: >> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates >> <yates@digitalsignallabs.com> wrote: > >>>> Just shaking an accelerometer will cause a bias shift. This is called >>>> "vibration rectification". > >>>Is that because it produces accelerations that exceed the >>>accelerometers' maximum? > >> Due to nonlinearities while operating well within the maximums. > >> Of course if you rail the thing you'll see gross errors. > > Some time ago, I was reading about laser gyros. Two beams going opposite > directions, either in air or glass fiber, and then into an > interferometer. > > They work well at high rotations rates, but can mode lock at low rates > such that they ignore slow rotations.Most laser gyros have a dither element to prevent mode locking. It vibrates at a higher speed than the laser bandwidth, changing the length of the path slightly, which breaks the mode lock. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●May 3, 20132013-05-03
On Fri, 03 May 2013 17:30:42 -0500, jacobfenton wrote:>>Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote: >>> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates >>> <yates@digitalsignallabs.com> wrote: >> >>>>> Just shaking an accelerometer will cause a bias shift. This is >>>>> called "vibration rectification". >> >>>>Is that because it produces accelerations that exceed the >>>>accelerometers' maximum? >> >>> Due to nonlinearities while operating well within the maximums. >> >>> Of course if you rail the thing you'll see gross errors. >> >>Some time ago, I was reading about laser gyros. Two beams going opposite >>directions, either in air or glass fiber, and then into an >>interferometer. >> >>They work well at high rotations rates, but can mode lock at low rates >>such that they ignore slow rotations. >> >>-- glen >> > Otherwise known as a FOG (fiber optic gyroscope). Pretty common, HG1700 > by honeywell. MEMS are getting close to being as good as these, but not > yet.AFAIK the superest-zoot laser gyros are still free-air, with FOGs coming in second, then quartz MEMS. And quartz MEMS, it turns out, have UGLY random-walk characteristics compared to old-fashioned rotating mass gyros. Ugly as in "looks bad when you use it to stabilize an imaging platform". I don't know if that's been overcome (my information is over ten years old), but I do know that two decades ago the military was all hot to change everything to MEMS gyros, yet one decade ago they were having serious second thoughts as the MEMS gyros seemed to be running out of poop while the mechanical gyros just kept on delivering their promised performance. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Reply by ●May 4, 20132013-05-04
On Thu, 02 May 2013 19:07:09 +0200, SG wrote:> Am 01.05.2013 21:54, schrieb Randy Yates: >> Looking for suggestions or pointers on how to track the gravity vector >> in 6D data. > > I'm assuming you mean 6 degrees of freedom, 3 rotation rate sensors and > 3 accelerometers. > > Yes, measuring the gravity vector is simple in this case unless your > sensors are in a free fall for a longer time. ;-) These days the sensor > chips can do this themselves. It's called "sensor fusion". The result of > sensor fusion is a separation of "linear acceleration" and "gravity". > Without rotation rate sensors you can only lowpass filter the > accelerometer outputs. This will make your gravity vector thingy respond > very slowly. This is where the rotation rate sensors become interesting. > They allow you to map object coordinates to a more or less stable world > coordinate system (there is a bit of drift but it does not matter for > this application). You just track the orientation using the gyroscopes > (check out Rodrigues' rotation formular for this). Then, you map the > accelerometer data into this "more stable world coordinate system", do > the lowpass filtering there and transform it back again. What you get is > a vector in object coordinates that always points straight to the sky. > If you want a vector that points towards the ground you just have to > negate it. Done. > > HTHThat's actually only valid if your sensor package has a zero average acceleration. Once your acceleration gets within the bandwidth of the low-pass filter, then it starts to corrupt the measurement. You can see this with the gyro-and-tilt-switch vertical gyro that I was mentioning before: it works great on a tank going in a straight line, but on a helicopter orbiting a fire you eventually get a tilt to your vertical reference. For that matter, if you hook your suggested algorithm up to a strategic- grade IMU and set the time constant to a week, then it'll filter out the rotation of the earth and your "vertical" reference will be parallel to the earth's rotational axis. So yes, it is simple as long as you make some unstated simplifying assumptions and then don't try anything that violates them. -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Reply by ●May 4, 20132013-05-04
Am 04.05.2013 07:30, schrieb Tim Wescott:> On Thu, 02 May 2013 19:07:09 +0200, SG wrote: > >> Am 01.05.2013 21:54, schrieb Randy Yates: >>> Looking for suggestions or pointers on how to track the gravity vector >>> in 6D data. >> >> I'm assuming you mean 6 degrees of freedom, 3 rotation rate sensors and >> 3 accelerometers. >> >> Yes, measuring the gravity vector is simple in this case unless your >> sensors are in a free fall for a longer time. ;-) These days the sensor >> chips can do this themselves. It's called "sensor fusion". The result of >> sensor fusion is a separation of "linear acceleration" and "gravity". >> Without rotation rate sensors you can only lowpass filter the >> accelerometer outputs. This will make your gravity vector thingy respond >> very slowly. This is where the rotation rate sensors become interesting. >> They allow you to map object coordinates to a more or less stable world >> coordinate system (there is a bit of drift but it does not matter for >> this application). You just track the orientation using the gyroscopes >> (check out Rodrigues' rotation formular for this). Then, you map the >> accelerometer data into this "more stable world coordinate system", do >> the lowpass filtering there and transform it back again. What you get is >> a vector in object coordinates that always points straight to the sky. >> If you want a vector that points towards the ground you just have to >> negate it. Done. >> >> HTH > > That's actually only valid if your sensor package has a zero average > acceleration. Once your acceleration gets within the bandwidth of the > low-pass filter, then it starts to corrupt the measurement. You can see > this with the gyro-and-tilt-switch vertical gyro that I was mentioning > before: it works great on a tank going in a straight line, but on a > helicopter orbiting a fire you eventually get a tilt to your vertical > reference. > > For that matter, if you hook your suggested algorithm up to a strategic- > grade IMU and set the time constant to a week, then it'll filter out the > rotation of the earth and your "vertical" reference will be parallel to > the earth's rotational axis. > > So yes, it is simple as long as you make some unstated simplifying > assumptions and then don't try anything that violates them.Good points.
Reply by ●May 6, 20132013-05-06
>On Fri, 03 May 2013 17:30:42 -0500, jacobfenton wrote: > >>>Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote: >>>> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates >>>> <yates@digitalsignallabs.com> wrote: >>> >>>>>> Just shaking an accelerometer will cause a bias shift. This is >>>>>> called "vibration rectification". >>> >>>>>Is that because it produces accelerations that exceed the >>>>>accelerometers' maximum? >>> >>>> Due to nonlinearities while operating well within the maximums. >>> >>>> Of course if you rail the thing you'll see gross errors. >>> >>>Some time ago, I was reading about laser gyros. Two beams goingopposite>>>directions, either in air or glass fiber, and then into an >>>interferometer. >>> >>>They work well at high rotations rates, but can mode lock at low rates >>>such that they ignore slow rotations. >>> >>>-- glen >>> >> Otherwise known as a FOG (fiber optic gyroscope). Pretty common, HG1700 >> by honeywell. MEMS are getting close to being as good as these, but not >> yet. > >AFAIK the superest-zoot laser gyros are still free-air, with FOGs coming >in second, then quartz MEMS. > >And quartz MEMS, it turns out, have UGLY random-walk characteristics >compared to old-fashioned rotating mass gyros. Ugly as in "looks bad >when you use it to stabilize an imaging platform". > >I don't know if that's been overcome (my information is over ten years >old), but I do know that two decades ago the military was all hot to >change everything to MEMS gyros, yet one decade ago they were having >serious second thoughts as the MEMS gyros seemed to be running out of >poop while the mechanical gyros just kept on delivering their promised >performance. > >-- >My liberal friends think I'm a conservative kook. >My conservative friends think I'm a liberal kook. >Why am I not happy that they have found common ground? > >Tim Wescott, Communications, Control, Circuits & Software >http://www.wescottdesign.com >MEMS are still seeing use in military, especially since everything is guided these days, artillery is a good example, they are all guided now.
Reply by ●May 6, 20132013-05-06
On Mon, 06 May 2013 12:48:27 -0500, jacobfenton wrote:>>On Fri, 03 May 2013 17:30:42 -0500, jacobfenton wrote: >> >>>>Spehro Pefhany <speffSNIP@interlogdotyou.knowwhat> wrote: >>>>> On Thu, 02 May 2013 21:29:54 -0400, Randy Yates >>>>> <yates@digitalsignallabs.com> wrote: >>>> >>>>>>> Just shaking an accelerometer will cause a bias shift. This is >>>>>>> called "vibration rectification". >>>> >>>>>>Is that because it produces accelerations that exceed the >>>>>>accelerometers' maximum? >>>> >>>>> Due to nonlinearities while operating well within the maximums. >>>> >>>>> Of course if you rail the thing you'll see gross errors. >>>> >>>>Some time ago, I was reading about laser gyros. Two beams going > opposite >>>>directions, either in air or glass fiber, and then into an >>>>interferometer. >>>> >>>>They work well at high rotations rates, but can mode lock at low rates >>>>such that they ignore slow rotations. >>>> >>>>-- glen >>>> >>> Otherwise known as a FOG (fiber optic gyroscope). Pretty common, >>> HG1700 by honeywell. MEMS are getting close to being as good as these, >>> but not yet. >> >>AFAIK the superest-zoot laser gyros are still free-air, with FOGs coming >>in second, then quartz MEMS. >> >>And quartz MEMS, it turns out, have UGLY random-walk characteristics >>compared to old-fashioned rotating mass gyros. Ugly as in "looks bad >>when you use it to stabilize an imaging platform". >> >>I don't know if that's been overcome (my information is over ten years >>old), but I do know that two decades ago the military was all hot to >>change everything to MEMS gyros, yet one decade ago they were having >>serious second thoughts as the MEMS gyros seemed to be running out of >>poop while the mechanical gyros just kept on delivering their promised >>performance. >> >>-- >>My liberal friends think I'm a conservative kook. My conservative >>friends think I'm a liberal kook. Why am I not happy that they have >>found common ground? >> >>Tim Wescott, Communications, Control, Circuits & Software >>http://www.wescottdesign.com >> > MEMS are still seeing use in military, especially since everything is > guided these days, artillery is a good example, they are all guided now.MEMS has a definite place in a tactical guided missile, there's no question of that. In fact, the deficiencies of MEMS sensors seem to fall into areas that don't matter much in such uses. But they sure don't make video look good when you use them to stabilize a camera! -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
