Cruise Control Technology

[SeaDuck79]

I'm talking Jethro Clampett dumb here, folks. Dumb, dumb, dumb.

With the Cruise Control on, driving over an incline as small as a freeway overpass causes the engine to downshift - despite the fact that it would not with the CC off (maintaining the same speed under the same conditions). I have seen this happen multiple times since buying this vehicle over a year ago. It also seems to think that it must take the fastest means possible to resume a set speed, even if it is already almost there. I had it downshift - on a flat road - at 68, when I told it to resume the set speed of 70. This is absolutely an insane waste of gas.

OK, now that I'm done ranting, I'll ask some who are perhaps more learned than I about a suggestion for improvement. Why can't the CC be more intelligent? The transmission already stores information about driving patterns (which include shift points), the CC servo stores the set speed, and the current speed, if not known, could certainly easily be accessed from other on-board computers.

So why couldn't a cruise control keep track of the difference between the set speed and the current speed, and recognize that it doesn't need to open the throttle all the way to move up 5 mph? Why couldn't the sensitivity be relaxed so that it could slip 1-2 mph, or see a modest increase in engine compression, on a small incline before deciding that it needs to downshift? This seems very doable with the current technology; all it requires is a bit more integration between the systems.

Carrying it a bit further into future possibilities, why not make the Nav systems topographical (i.e. 3D), to allow the CC to see the severity and length of a grade, and use that information to plot out when a downshift might really be most efficient prior to reaching that point?

Why, Lexus, is a usable, intelligent cruise control not a part of your otherwise fine collection of technology?

[SKperformance]

It downshifts according to load ,you may not see it but the computer is reacting to it.

Having a topographical map is alot of data and i don't know anyone who could store that much data unless they drive around with a server in their car.

It would be useless anyway as it uses engine load to determine downshifts and actual speed.

[Husker4theSpurs]

I noticed on my last trip that after I had the tranny recalibrated (due to hard shifting) it held it's speed on hills better and didn't downshift as often ... I could be wrong though.

[SW03ES]

Cruise control technology is very simple. The computer simply remembers your set speed and then speeds up, or slows down to maintain it. The technology is basically the same as its been for the last 15-20 years and its basically the same in all vehicles.

I think you're expecting too much. If you want that kind of technology you're going to need to look into a much more expensive vehicle than an RX330.

[stonecutter]

Cruise control technology is very simple. The computer simply remembers your set speed and then speeds up, or slows down to maintain it. The technology is basically the same as its been for the last 15-20 years and its basically the same in all vehicles.

I think you're expecting too much. If you want that kind of technology you're going to need to look into a much more expensive vehicle than an RX330.

←

Yes, the concept IS simple, but the implementation may not be, and certainly not unique. The computer remembers the set speed and maintains it, as you say, but how it goes about it is SeaDuck79's complaint. I'm not an automotive insider, but judging from other gizmos found on Lexus vehicles, I'd think it should be easy to program the CC so that throttle opening is proportional to (or at least dependent on) the difference between set and actual speeds. From the description, it sounds like the CC operates as a simple "bang-bang" control system, all or nothing on the throttle.

[SeaDuck79]

It downshifts according to load ,you may not see it but  the computer is reacting to it.

Having a topographical map is alot of data and i don't know anyone who could store that much data unless they drive around with a server in their car.

It would be useless anyway as it uses engine load to determine downshifts and actual speed.

←

Are you saying that the CC uses different data than the tranny to determine engine load? Because under identical load and road conditions, the CC is far more sensitive than my foot.

I know the topo map idea is for the future - maybe someday! First things first, though. The other stuff I suggested is already doable, with information that is already being stored and used.

[SeaDuck79]

Cruise control technology is very simple. The computer simply remembers your set speed and then speeds up, or slows down to maintain it. The technology is basically the same as its been for the last 15-20 years and its basically the same in all vehicles.

I think you're expecting too much. If you want that kind of technology you're going to need to look into a much more expensive vehicle than an RX330.

←

Yes, the concept IS simple, but the implementation may not be, and certainly not unique. The computer remembers the set speed and maintains it, as you say, but how it goes about it is SeaDuck79's complaint. I'm not an automotive insider, but judging from other gizmos found on Lexus vehicles, I'd think it should be easy to program the CC so that throttle opening is proportional to (or at least dependent on) the difference between set and actual speeds. From the description, it sounds like the CC operates as a simple "bang-bang" control system, all or nothing on the throttle.

←

Thank you - someone else understands! The issue is why it STILL does what it does, when there's such a much better way of doing it using existing technology and information on the vehicle. Yes, CC technology hasn't markedly improved in years, but WHY NOT? I think I'll ask Lexus, for whatever good it will do.

[wwest]

The cruise control computer has no eyes.

When you are in control and want to maintain a constant speed up an incline YOU can SEE how steep and long the climb will be.

The cruise control doesn't "know" of the incline until the engine starts to "lug", load up, due to the extra effort needed to move 2 tons up an incline. The firmware may even have a delay built in for the instances where the "incline", engine load, is only short term.

So of course the transmission doesn't downshift with you driving. You saw the upcoming incline and were able to add throttle predictively, the CC has not choice but to do it reactively, AFTER encountering the incline.

Same with going downhill, reaching the crest, you know to start releasing the throttle, "IT" doesn't know to do that until the vehicle actually starts gaining speed.

And almost all modern day vehicles will downshift very quickly if/when the throttle is cracked open for even the slightest level of acceleration. Today's transmissions and transaxles want to remain in O/D gear ratio to conserve fuel and lower emissions. That REQUIRES that the torque converter lockup clutch be engaged as much as reasonably possible.

The lockup clutch is not of enough heavy duty to sustain the engine's high level of torque required to accelerate a 2 ton vehicle. The transaxle/transmission MUST be programmed to drop out of O/D with even a small level of additional torque required.

BUt here again you are denigrating the ECU for not having human "feel" and reaction times, etc.

[SeaDuck79]

The cruise control computer has no eyes.

When you are in control and want to maintain a constant speed up an incline YOU can SEE how steep and long the climb will be.

The cruise control doesn't "know" of the incline until the engine starts to "lug", load up, due to the extra effort needed to move 2 tons up an incline. The firmware may even have a delay built in for the instances where the "incline", engine load, is only short term.

So of course the transmission doesn't downshift with you driving. You saw the upcoming incline and were able to add throttle predictively, the CC has not choice but to do it reactively, AFTER encountering the incline.

Same with going downhill, reaching the crest, you know to start releasing the throttle, "IT" doesn't know to do that until the vehicle actually starts gaining speed.

And almost all modern day vehicles will downshift very quickly if/when the throttle is cracked open for even the slightest level of acceleration. Today's transmissions and transaxles want to remain in O/D gear ratio to conserve fuel and lower emissions. That REQUIRES that the torque converter lockup clutch be engaged as much as reasonably possible.

The lockup clutch is not of enough heavy duty to sustain the engine's high level of torque required to accelerate a 2 ton vehicle. The transaxle/transmission MUST be programmed to drop out of O/D with even a small level of additional torque required.

BUt here again you are denigrating the ECU for not having human "feel" and reaction times, etc.

←

I'm sorry, west, I appreciate your knowledge, but when exactly were you in the car with me during this testing? I have a very steady foot on the gas, and I understand that my complaint would hold no water if I didn't compare my foot to the CC under IDENTICAL conditions. I held the gas steady - didn't pre-accelerate up the hill or let off the gas, I held it steady like the CC would. Multiple tests, and all had the same results. Without the CC, it did NOT downshift nearly as often or as soon as with it. The transmission should react the same way whether my foot or the CC is controlling throttle pressure. My contention remains that something in the CC workings is the problem - not a broken part, but a design flaw.

[wwest]

Sorry, but your statement was....

"maintaining the same speed under the same conditions..."

That says to me, your true intent or not, that you were modulating the throttle in order to maintain the same speed regardless of flat, uphill, downhill, etc.

And you didn't have to pre-accelerate, all you had to do, did, was see the upcoming incline and thereby be fully prepared to apply more throttle just as soon as the car reached the incline.

The CC, on the other hand, was flying blind, and had to wait for the speed to decay, however so slightly, before applying more throttle.

I am in some agreement with you in that I often notice that upon cresting a hill the CC causes the vehicle to overspeed slightly and then the slowdown, when it comes, is always a bit abrupt, somewhat discomforting.

But having designed a few servo loops myself, some in hardware and some implemented in software, "instantly" reacting servoloops controling mechanical devices often go into oscillation, and are extremely hard to "tune". When controlling a 2 ton mechanical device it is best to crank in just a little bit of correction each few milliseconds depending on the overall mechanical time constants involved.

[SeaDuck79]

Sorry, but your statement  was....

"maintaining the same speed under the same conditions..."

That says to me, your true intent or not, that you were modulating the throttle in order to maintain the same speed regardless of flat, uphill, downhill, etc.

And you didn't have to pre-accelerate, all you had to do, did, was see the upcoming incline and thereby be fully prepared to apply more throttle just as soon as the car reached the incline.

The CC, on the other hand, was flying blind, and had to wait for the speed to decay, however so slightly, before applying more throttle.

I am in some agreement with you in that I often notice that upon cresting a hill the CC causes the vehicle to overspeed slightly and then the slowdown, when it comes, is always a bit abrupt, somewhat discomforting.

But having designed a few servo loops myself, some in hardware and some implemented in software, "instantly" reacting servoloops controling mechanical devices often go into oscillation, and are extremely hard to "tune". When controlling a 2 ton mechanical device it is best to crank in just a little bit of correction each few milliseconds depending on the overall mechanical time constants involved.

←

Well, I can see why you said what you said, but I think we're still misunderstanding one another. What I did in my manual tests was attempt to emulate the conditions under which the cruise control was operating. Since it's function is to maintain a constant speed, that's what I did, too.

Rather than reiterate the rest of the story, which I think you probably understand, let me ask you a question, based on your statements about servo operation.

Are you saying that what I'm expecting to be done (tunability of the CC servo to decrease sensitivity) is not physically possible or practical? Is there another technology that would or should take the place of the current one to act as a speed control - one that would better emulate manual driving conditions and not diminish fuel economy in the process?

[wwest]

None today that I can think of.

Artificial vision system somewhere in the future.

Lived in FW, from ~67 until 87, lots of work for Weyerhauser.

[COZ]

Seaduck. I completely agree with you. The CC on this thing is dumb and also I have the same issues with my Camry. I figured the upgrade to Lexus would change things. The CC is very basic and is the same technology used on most cars. Things that could make it better… Relax the speed sensitivity tolerance. Allow for more speed decay before accelerating. When the CC accelerates it could do it gradually instead of "punching it". I'm convinced that if Lexus made it priority they could have a "smart" CC instead of cheaping out with a CC similar to everyone else’s.

I've only owned My RX for a few weeks and tuning the CC is on my list of things to do.

[wwest]

"Punching it...."

Remember that the automatic transmission/transaxle is heavily biased toward wanting to be in O/D, with the lockup clutch engaged, in order to achieve the best possible fuel economy.

If any level of additional torque is required of the drivetrain the lockup clutch must be quickly released in order to bring the torque multiplication aspect of the torque converter into play. And again, to help achieve the best fuel economy under acceleration.

So even the slightest additional throttle opening will often result in the transmission/transaxle ECU commanding a downshift out of O/D.

Also keep in mind that even absent being in O/D modern day engines are running right on the "cusp", the leanest fuel/air mixture ratio possible using the downstream oxygen sensor for long term, slow reacting, feedback and the engine knock sensors for instantaneous feedback when the mixture gets too lean, the timing too early, or the engine is "lugging".

Engine knock or pinging can pretty severely damage an engine if not quickly corrected or if allowed to often occur even on an intermittent basis.

Engines knock or ping because the fuel/air mixture "flamefront" is expanding too rapidly for the cylinder volume or in the alternative the piston itself cannot move downward as fast as possible to accomodate the rapidly expanding flamefront.

The latter is typically the result of "lugging" the engine.

Most of us who often/still drive stick shifts know by experience and instinct to downshift a manual transmission before we get into the "lugging" range of the engine.

But what about modern day automatic transaxles/transmissions...??

They LEARN.

Over time, as you drive the car, the engine/transmissaxle ECU will build a "map" of parameters which it then uses to avoid engine operational circumstances that are detrimental. The ECU literally "wants" to achieve the absolute best in fuel economy but not at the detriment of mechanical failure of the drivetrain, short (knock/pinging) or long (transmissaxle clutch wear) term.

Think about what happens if you fuel a car that requires premium fuel with regular. The ECU detects pinging in some areas of the previously learned and stored parametric mapping and adusts, accordingly, the operational charactoristics, parameter mapping, of the engine ignition timing, Fuel/air mixture vs RPM & loading, and transmission shift points.

But that's the simple part...

How does it know to re-adjust all of those parameters once you refuel with premium?

Because it pushes the fuel economy "envelope", continuously!

In order to continuously, ALWAYS, operate with the very best parametric mapping for the best fuel economy it continues to "experiment" with the parameters. So, not long after you refuel with preimum "it" will have discovered that the previous, more conservative parameters are no longer valid and build a new set.

[COZ]

Don't know how we went from the cruise control to a long winded explanation about fuel economy. What I do know is that if my foot can push down on the accelerator enough to smoothly speed up with out the tranny donwshifting then a CC can also perform the same simple task.

Its all about the tolerances programmed into the computer. Here is an excerpt from the Wiring manual.

Overdrive Control Function

The overdrive control may be cancelled if the vehicle travels on the slope during cruise control travelling. After the overdrive control has been cancelled, if the vehicle speed exceeds the overdrive return speed (Set speed minus 2 km/h) and it is decided that the slope is finished, the vehicle returns to the overdrive control mode again.

2 Km isn't a lot of speed. They could let it drop 4 or 5km before dropping out of overdrive. Better yet, the computer could also sense the rate of speed decay and use that to determine if it is even necessary to drop out of overdrive. If speed decays slowly then apply more power slowly. If speed decays quicly, then drop it out of overdrive to maintain speed. Unfortunatly for us there is no easy way to tweak this as its all done in the ECM. Until I figure out if I can tweak it I guess when I see a hill I'll just smoothly step on the accelerator and cancel CC so that there is no jolt from sudden deceleration or from the CC shifting the car out of overdrive thereby letting my wife remain peacefully asleep.

[COZ]

Some things I have discovered about this cruise control since I bought it in August. First, it is very sensitive to tire pressure. Low tire pressure equals higher rolling resistance. I inflated the tires to 32-33 PSI cold and the CC responded as it should with no more downshifting all the time to maintain speed on hills. Second, the CC works more smoothly at speeds less than 78 MPH. Cruising around at 80 it would occasionally kick down into third gear when going up a mild hill.

[soggy60]

I agree the RX330 CC is too 'agressive'.

We had a RX300 ( and other vehicles w/ CC), the 'level of effort' seemed to be tied to the delta between actual speed & 'set point'.

The RX300 ( and others) would gently accel if the delta is small; if the delta was larges, CC would ease into 'flooring it' and ease out as setpoint is approached.

The 330 seems to floor it for any delta, and doesn't come out until delta=0.

[COZ]

After driving a lot of highway miles lately with the CC. I found that using the CC on roads with mild hills and speeds above 78MPH would cause agressive acceleration. At or below 78 MPH I noticed it would downshift out of overdrive on bigger hills but it was almost imperceptable. Above 78 MPH it would kick down into 3rd gear, which was very noticable. Keeping the tire pressure around 35 PSI also seemed to make a big difference in the way the car drove and the CC behaved. I was also able to get 24.6 MPG :D