Rate Gyros and Accelerometers

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 King

<gyansorova@gmail.com> wrote in message 
news:1177218883.487989.201610@d57g2000hsg.googlegroups.com...
> 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.

Wayne,

It depends on how they're used.  An accelerometer can give angular position 
(such as pitch and roll angles).  The acceleration is "gravity".

Rate gyros, on the other hand, are useful for damping in a control loop.

So, if one is controlling pitch angle, the accelerometer gives position and 
the angular rate allows one to damp the result of corrections that might 
otherwise occur if only position were sensed.  This helps in a bang-bang 
control and probably in others.

Fred 

On Apr 22, 5:32 pm, "Fred Marshall" <fmarshallx@remove_the_x.acm.org>
wrote:
> <gyansor...@gmail.com> wrote in message
>
> news:1177218883.487989.201610@d57g2000hsg.googlegroups.com...
>
> > 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.
>
> Wayne,
>
> It depends on how they're used.  An accelerometer can give angular position
> (such as pitch and roll angles).  The acceleration is "gravity".
>
> Rate gyros, on the other hand, are useful for damping in a control loop.
>
> So, if one is controlling pitch angle, the accelerometer gives position and
> the angular rate allows one to damp the result of corrections that might
> otherwise occur if only position were sensed.  This helps in a bang-bang
> control and probably in others.
>
> Fred

I once made a self balancing machine like an inverted pendulum but on
a table and it used a solid-state rate-gyro. Others doubted this would
work - they said that I need a stabilised platform like you have in a
rocket. Anybody know why? It did work and I noticed that the Segway
uses a similar method.
What you say above looks live derivative feedback - why would you go
to the trouble of having an extra sensor when phase-lead would do the
same thing?

Wang King.

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 

From a DSP point of view that's baloney. Since you posted to a DSP group
I assume you wish to look at this as a signal processing problem and not
an abstract physics problem. 

	What you have is two different devices for measuring position. Each of
those devices has a characteristic frequency response. You may say that
one device is measuring "acceleration" and one is measuring "velocity"
but those are just abstractions.  In order for you to get accurate
measurements of position you need to filter the data stream from your
input devices. The filter you use you might call one "integration" and
the other "double integration" but again these are just abstractions. 
	Your question is which device is more accurate at measuring position.
Well, abstractions are not likely to supply a good answer. The answer
depends on how well you can determine the frequency response of the
system as a whole and thus produce a filter that will give you a
perfectly flat response which will give you a perfectly accurate measure
of position. Of course to accomplish this to perfection you may need to
remove all noise from the system and restrict the response of the system
to a very narrow range of frequencies. That is to say, the design of 
and the weight distribution of your upside down pendulum (or Segway) and
the bearing on which you rest your pendulum (or Segway) may have much
more to do with the success of your measurements than the measuring
device you choose.   

-jim



>- 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 King

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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.


>>It depends on how they're used.  An accelerometer can give angular position
>>(such as pitch and roll angles).  The acceleration is "gravity".
>>Rate gyros, on the other hand, are useful for damping in a control loop.
>>

> I once made a self balancing machine like an inverted pendulum but on
> a table and it used a solid-state rate-gyro. Others doubted this would
> work - they said that I need a stabilised platform like you have in a
> rocket. Anybody know why?

Because the integration of the angular velocity measured by the 
rate-gyro will not give you the accurate absolute angle - there will be 
always a runaway due to the zero drift and other inaccuracies. The solid 
state hyros are horrid in that sense - you can expect the error as much 
as several hundred degrees per hour.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

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.

-- 

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

Vladimir Vassilevsky wrote:
> 
> 
> 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.
> 
> 
>>> It depends on how they're used.  An accelerometer can give angular 
>>> position
>>> (such as pitch and roll angles).  The acceleration is "gravity".
>>> Rate gyros, on the other hand, are useful for damping in a control loop.
>>>
> 
>> I once made a self balancing machine like an inverted pendulum but on
>> a table and it used a solid-state rate-gyro. Others doubted this would
>> work - they said that I need a stabilised platform like you have in a
>> rocket. Anybody know why?
> 
> Because the integration of the angular velocity measured by the 
> rate-gyro will not give you the accurate absolute angle - there will be 
> always a runaway due to the zero drift and other inaccuracies. The solid 
> state hyros are horrid in that sense - you can expect the error as much 
> as several hundred degrees per hour.
> 
A rate gyro is sufficient for telling you that a pendulum (either 
inverted or suspended) has come to rest, because the derivative of the 
rate change goes to zero.  As long as your system is stable, the 
pendulum will come to rest pointing up and down.

Even the best of gyros there will be some horizontal offset when you 
finish.  So you need to have some alternate means of sensing position, 
and you need to know what to do with those position measurements to get 
the whole gizmo to go to the correct position.

-- 

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

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?
-- 
%  Randy Yates                  % "Midnight, on the water... 
%% Fuquay-Varina, NC            %  I saw...  the ocean's daughter." 
%%% 919-577-9882                % 'Can't Get It Out Of My Head' 
%%%% <yates@ieee.org>           % *El Dorado*, Electric Light Orchestra
http://home.earthlink.net/~yatescr

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

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/Product%20Brochure.pdf
http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/9015/28647/01282514.pdf

and space navigation:

http://www.spaceandtech.com/digest/flash2002/flash2002-033.shtml

So maybe size, cost and robustness matter too.

Dale B. Dalrymple
http://dbdimages.com