On Apr 23, 6:22 am, dbd <d...@ieee.org> wrote:> On Apr 22, 10:27 am, Vladimir Vassilevsky <antispam_bo...@hotmail.com> > wrote: > > > Randy Yates wrote: > > > When I was working on an IMU system 10 years ago, they were coming out > > > with (I think I have the terminology correct) "ring laser gyros." Aren't > > > they supposed to be a lot more accurate? > > > Unfortunately, the best laser gyros are orders of magnitude less > > accurate then the best mechanical ones. The mode locking and the noise > > are the problems. > > > VLV > > Whatever the past status of the -best- has been, the ring laser gyros > have successfully been winning design-ins in naval navigation: > > http://www.sperrymarine.northropgrumman.com/Admin/Downloads/354/Produ...http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/9015/28647/0128... > > and space navigation: > > http://www.spaceandtech.com/digest/flash2002/flash2002-033.shtml > > So maybe size, cost and robustness matter too. > > Dale B. Dalrymplehttp://dbdimages.comWhat about this... http://www.youtube.com/watch?v=V40ScvJeFxg Wang King
Rate Gyros and Accelerometers
Started by ●April 22, 2007
Reply by ●April 22, 20072007-04-22
Reply by ●April 22, 20072007-04-22
On Apr 22, 1:14 am, gyansor...@gmail.com wrote:> When would you use one versus ther other eg suppose you need to it > detect when a robot is falling over. I assume the rate gyro gives > velocity which can be integrated to get position. Accelerometer has to > be integrated twice to get position - I expect you could use a Kalman > filter or some such. I am just confused when one versus the other is > used.I saw an article somewhere which says you need both. I just want > to be able to detect when a mass is falling over in one degree of > freedom only - like one of those self-balancing wheeled things - > Segway. > > Thanks > > Wang KingHere is a good article on why you need both sensors and how to combine the measurement of both. Its a simillar to a kalman filter but a lot less complicated http://www.xbow.com/Support/Support_pdf_files/IMUAppNote.pdf
Reply by ●April 22, 20072007-04-22
Randy Yates wrote:> Tim Wescott <tim@seemywebsite.com> writes: > >> gyansorova@gmail.com wrote: >>> When would you use one versus ther other eg suppose you need to it >>> detect when a robot is falling over. I assume the rate gyro gives >>> velocity which can be integrated to get position. Accelerometer has to >>> be integrated twice to get position - I expect you could use a Kalman >>> filter or some such. I am just confused when one versus the other is >>> used.I saw an article somewhere which says you need both. I just want >>> to be able to detect when a mass is falling over in one degree of >>> freedom only - like one of those self-balancing wheeled things - >>> Segway. >>> Thanks >>> >> A gyro detects rotational velocity. An accelerometer detects linear >> acceleration. Linear and rotational motion are two very different >> things, so you usually use a gyro where you are interested in knowing >> about how things turn, and an accelerometer when you are interested in >> their motion in one direction. >> >> A rate gyro is just a gyro that is arranged to directly read out >> rate. But all pure gyros* drift, so any gyro, by itself** will >> ultimately only be good for telling you about rotational rates. >> >> * A gyro compass is more than a gyro: it's a high-quality, cleverly >> mounted gyro with a cleverly located weight on the frame that makes it >> point north. In a way it's a purely mechanical inertial navigation >> system that only works in one dimension. >> >> ** Gyros are a necessary, but not complete, part of an inertial >> navigation system. A complete INS needs at least three axes of gyro >> output and three axes of accelerometer output. Unless the gyros and >> accelerometers are really, really good an INS also needs help from >> external readings. These days people couple GPS receivers with cheap >> inertial measurement units; the GPS works well at low frequencies, the >> IMU works at higher frequencies, and they complement each other >> nicely. You still need an IMU that's well out of consumer price range >> to be really useful, though. > > When I was working on an IMU system 10 years ago, they were coming out > with (I think I have the terminology correct) "ring laser gyros." Aren't > they supposed to be a lot more accurate?They have a lower limit of sensitivity. Optical imperfections in the prisms create back scattering, mingling the counter-rotating beams. Quality always comes at a price. It's true that the best ring laser "gyros" are better than mechanical ones. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●April 22, 20072007-04-22
Vladimir Vassilevsky wrote:> > > Randy Yates wrote: > > >> When I was working on an IMU system 10 years ago, they were coming out >> with (I think I have the terminology correct) "ring laser gyros." Aren't >> they supposed to be a lot more accurate? > > Unfortunately, the best laser gyros are orders of magnitude less > accurate then the best mechanical ones. The mode locking and the noise > are the problems.They've made the mechanical ones a lot better, then. You are right about reasonable run-of-the-mill laser "gyros". (I use quotes because although they do the same job, nothing gyrates and there are no gyroscopic forces involved. A analogy is the rubber industry's "white carbon black".) Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●April 23, 20072007-04-23
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in news:lZudnSKjy8pabrfbnZ2dnUVZ_i2dnZ2d@centurytel.net:> It depends on how they're used. An accelerometer can give angular > position (such as pitch and roll angles). The acceleration is > "gravity".The accelerometer, depending on its orientation, would yield orientation with respect to gravity, tangential and centripetal accelerations, and plain old linear acceleration. Breaking these out can be complex. -- Scott Reverse name to reply
Reply by ●April 23, 20072007-04-23
"Scott Seidman" <namdiesttocs@mindspring.com> wrote in message news:Xns991B548AB6B33scottseidmanmindspri@130.133.1.4...> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in > news:lZudnSKjy8pabrfbnZ2dnUVZ_i2dnZ2d@centurytel.net: > >> It depends on how they're used. An accelerometer can give angular >> position (such as pitch and roll angles). The acceleration is >> "gravity". > > The accelerometer, depending on its orientation, would yield orientation > with respect to gravity, tangential and centripetal accelerations, and > plain old linear acceleration. Breaking these out can be complex.ttocS, All correct and more complete. Fred
Reply by ●April 23, 20072007-04-23
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in news:h9adnS9V3vZiV7HbnZ2dnUVZ_vyunZ2d@centurytel.net:> > "Scott Seidman" <namdiesttocs@mindspring.com> wrote in message > news:Xns991B548AB6B33scottseidmanmindspri@130.133.1.4... >> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote in >> news:lZudnSKjy8pabrfbnZ2dnUVZ_i2dnZ2d@centurytel.net: >> >>> It depends on how they're used. An accelerometer can give angular >>> position (such as pitch and roll angles). The acceleration is >>> "gravity". >> >> The accelerometer, depending on its orientation, would yieldorientation>> with respect to gravity, tangential and centripetal accelerations, and >> plain old linear acceleration. Breaking these out can be complex. > > ttocS, > > All correct and more complete. > > Fred > >Always looking at this sort of stuff, as the physiological system I work with has an angular velocitometer working in parallel with linear accelerometers. To track how we're stimulating this system, I need to be able to measure all this with instrumentation. As an aside, 15 years ago, angular velocitometers were expensive propositions. Today, I can get a 3D micromachined gyro on a chip for about 20 bucks, and it uses the same basic tuning fork technology of the devices I paid thousands of dollars for in the past. -- Scott Reverse name to reply
Reply by ●April 23, 20072007-04-23
Vladimir Vassilevsky wrote:> > > Randy Yates wrote: > > >> When I was working on an IMU system 10 years ago, they were coming out >> with (I think I have the terminology correct) "ring laser gyros." Aren't >> they supposed to be a lot more accurate? > > Unfortunately, the best laser gyros are orders of magnitude less > accurate then the best mechanical ones. The mode locking and the noise > are the problems. > > VLVSomeone should take the time to check -- my understanding is that ring laser gyros that work in free space are pretty darn good. I have experience with fiber-optic laser gyros, or at least their specifications. For a given volume & weight I've always seen better specifications from mechanical gyros, so when they can be found I have used mechanical ones. For a good long time the defense department was actively pushing FOGs because they were going to be ever so cool compared to those nasty greasy mechanical things. To date, that promise hasn't materialized, but because of their pushing there are a lot of systems using FOGs, and not very many vendors selling good mechanical gyros. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
Reply by ●April 23, 20072007-04-23
Tim Wescott <tim@seemywebsite.com> wrote in news:rdSdnR- 5h58iR7HbnZ2dnUVZ_rGinZ2d@web-ster.com:> Someone should take the time to check -- my understanding is that ring > laser gyros that work in free space are pretty darn good.http://www.analog.com/UploadedFiles/Data_Sheets/ADIS16120.pdf This is an Analog Devices MEMS gyro. It uses an orthogonal tuning fork. -- Scott Reverse name to reply Hak mir nisht ken tshaynik
Reply by ●April 23, 20072007-04-23
Scott Seidman wrote:> Tim Wescott <tim@seemywebsite.com> wrote in news:rdSdnR- > 5h58iR7HbnZ2dnUVZ_rGinZ2d@web-ster.com: > >> Someone should take the time to check -- my understanding is that ring >> laser gyros that work in free space are pretty darn good. > > http://www.analog.com/UploadedFiles/Data_Sheets/ADIS16120.pdf > > This is an Analog Devices MEMS gyro. It uses an orthogonal tuning fork. >Right. And its performance is several orders of magnitude worse than the mechanical gyros I work with. Its value comes about because it is small and cheap. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ Do you need to implement control loops in software? "Applied Control Theory for Embedded Systems" gives you just what it says. See details at http://www.wescottdesign.com/actfes/actfes.html
