Nav Gyroscope: Where is it?

I'm not sure what you mean by "nav gyro." As far as I know, there's no such thing in the Prius Nav system; it just figures out the direction you're headed by looking at successive GPS positions. I could be wrong, but this was told to me on another thread.

 

Since the Nav system continues to function even if the car is in a long tunnel and has lost satellite reception, my assumption is that the Nav computer receives additional data from
a)the odometer - for distance traveled and
b)the steering wheel - for directional information.

When the satellite reception is re established the Nav computer will correct the reported position.

 

The nav system gets a speed input, like the odometer, and it has a gyroscope to detect turns. This is mounted in the central console between the seats, along with the yaw sensor for the VSC (I believe they're separate devices, but they could be shared). It also has an input which tells it the car is in reverse (useful because the odometer pulse is directionless).

 

It's called "dead reckoning". Based on the direction and speed
you were last known to be traveling before the GPS signals went
away, the nav system can look at vehicle speed and make a pretty
good guess as to where you are now. No gyro needed. It isn't
perfect -- for example if you went into a tunnel that then *curved*
it would likely get it very wrong until you came out again!
.
_H*

 

thats if your not too busy being killed by recycled concrete.

 

No Kidding! Traveling on Lower South Wacker (Chicago) was enough to royally confuse the system. After curving from W to W to SE to W, and then coming up above ground, the NAV continued to believe I was driving on side streets at 50+ MPH instead of on the freeway.

I had to turn it off for quite awhile before it was able to accurately place the car on the correct roadway!

 

The owners manual and the secret maintenance menus both mention that the Prius has a gyroscope to maintain heading while out of GPS range.

One of the secret menus gives the gyroscope output in volts and concluded bearing.

nb: I'm speaking from my shiny green 2007 Prius; your model may vary.

toyoprius: did you find it? and where?

 

I would have thought it was in the satellites. Love to be proven wrong.

My hand held GPS uses the maps when the signal is low for a few seconds. It seems to assume speed and road remain the same for a few seconds after loss of signal.

 

Here in Italy we have one 8 mile long tunnel under the Alps with two curves. The GPS has no problem in keeping track of it.

 

Of course, it also has the map information to rely on, as well as the gyroscope. It usually assumes you're following a road on the map, until the other data insists that isn't the case.

 

I don't think my HP hw6515 PDA has a gyro in it but it has a phone and GPS in it and it works the same. I believe it is programed to follow roads unless it gets a triangulation of signals telling it it isn't on a road. That isn't saying the Prius hasn't got a gyro in it.

 

The NAV system uses GPS to fix the car's position, and uses a series of fixes to determine course and speed. The accuracy of the GPS is not good enough to consistently stay on the road, so the NAV system looks at the map and snaps to the nearest parallel road. This is why it sometimes shows you driving down a side street, instead of the street your are actually on. Once it gets a good GPS fix, the NAV system will correct the problem and jump back to the correct street. You usually see this behavior right after start-up, or after coming out of a poor reception area (tunnel, big buildings, mountains...). As a back-up, the NAV system dead reckons when it looses the GPS signal. To do this it uses a solid-state gyro to determine direction and distance taken from the wheels, and calculates where you should be based on where you were and how far you drove since the last fix. It's better than nothing, but it will get lost pretty fast. It's really intended to bridge short gaps in GPS coverage, not to work stand-alone for any length of time.

Tom

 

Okay, I was wrong -- the system *does* have a gyroscope of some
sort, although it probably gets used as part of a suite of
dead-reckoning techniques. Without the satellite input, any
system will drift after a while, of course.
.
However, it looks like the gyro isn't a separate unit, but is
rather part of the Nav ECU itself. The service manuals say, in cases
of "gyro error" or the map pointer showing rotation when the
vehicle is sitting still, to replace the nav ECU under the seat.
That would imply that the yaw sensor under the center console
isn't involved, nor is there a separate gyro there. Has anyone
had things apart enough to really tell? Would it conceivably
be different between '04 and later years?
.
My Garmin standalone appears to do a little dead reckoning based
on speed and direction of travel at the moment of sat signal
loss, but naturally that's limited. I've watched it follow me a
little way into a tunnel before giving up, heh...
.
_H*

 

Dead-reckoning, for anyone who doesn't already know, involves taking an educated guess as to where you are based on your last known position and any information about how far and in what direction you have traveled since the last known position. The better your information about distance and direction, the better the results. If you have completely accurate information about distance and direction your dead-reckoning will be perfect. This is how inertial navigation works for missiles, airplanes, and submarines. They start with a known position, then use a very sophisticated set of gyros and accelerometers to sense speed and direction changes. It works well, but the equipment is expensive and delicate. With GPS, there is much less need for this sort of equipment. The dead-reckoning system on the Prius is obviously much less sophisticated, but having a yaw sensor gives it more information for producing a better dead-reckoning position.

Tom

 

Ah, it's a solid state gyro equivalent, probably something like this (though it's probably not this specific part). I didn't think it would be a real gyro like aircraft use; these are thousands of dollars. I imagine it's just mounted on the nav unit's circuit board.

It only needs to measure one axis -- the car turning left/right. That combined with the odometer and the maps can give it clue as to where it is when it loses the satellites.

 

Solid state gyros use vibrating crystals. The mass of the crystal moves back and forth along one axis; it's movement causes momentum that tries to keep the crystal moving along that same axis. Off axis rotation causes stress to the crystal which is measured as an electrical signal. You typically use one crystal for each axis of measurement, so in this case you only need one.

Tom