Cruise control question

Brief little road trip to Austin today begged a question. As soon as I got out on the Interstate, I set the cruise control on 80 and settled in for the trip.

While still on the busy parts of the Interstate, various accelerations were called for, as usual. When I got to quieter cruising space, and let it settle back to cruise, it wanted to go 82.

More episodes of manuevering, involving bursts of acceleration, left the cruise settling at 83, even though I hadn’t touched it since setting it at 80.

What’;s going on here? Is this a malfunction, or a normal event associated with punching it above where the cruise is set?

It depends if you 'cancel’ed your cruise control ehile accelerating.
For instance:
If you did NOT cancel and accelerated (using your foot) and then pressed ‘resume’, it may have accelerated some. You do not need to press ‘resume’ in this case. The speed should automatically settle down at your setting as you ease you foot. Otherwise it may be a malfunction. (In my car ‘accel’ and ‘resume’ are the same button.)

If you DID press cancel, then you must press ‘resume’ once to get back to your set speed.

If you did all of the above then just use it as your excuse when a cop pulls you over.

If any cruise control manufacturers out there reading this… How about putting a couple of preset switches that can be programable, so we can switch between 100 to 80 to 60 and back again withought having to think too much about it???

I didn’t touch the cruise controls; just accelerated and then let it settle back down. No “cancel” and no need to hit “resume.”

Nice idea.

Is there any chance you bumped it, many times I’ve taken my foot off the accelartor and had the car speed up. This usually means that at some point while using my blinker to change lanes I turned cruise on. Also you said you never touched it, but I should mention that on at least some cars, breifly touching ACCEL will bring the cruise speed up one mile (I think), could you have tapped it?

It’s entirely possible that your cruise control isn’t that precise to begin with. There are a lot of ways to make a cruise control with varyind degrees of precision and complexity. It may do fine maintaining a steady speed but may not do a good job of returning to that exact speed. It may not have been anything you did, just the parameters of that cruise control.

In my casual observations vehicles with automatic transmissions aren’t regulated as well those with manual. My dodge truck does okay and my wife’s Ford has a little variation, both with auto tranny, but the '90 BMW I used to own (325 with manual transmission) had rock steady cruise control. I timed it on some long marked stretches of highway and it returned to speed as accurately as I could measure with a wristwatch.

May not work well with the current cruise control systems. Usually the cruise control is tuned PI controller which works as:

<Delta Acceleration> = KP * <Delta Speed> + KI * Integral <Delta Speed>

Sorry, donno how to write a equation here.

<Delta Acceleration> = More acceleration or braking
<Delta Speed> = Current Speed - Desired Speed

The system dynamics are linear for small <delta speeds> and the Contoller Parameters (KI and KP are tuned for that) , but higher <delta speeds> are not linear and the Controller performance is not guaranteed. Hence, if you put your cruise controller at 70 and brake to 30 and try to do resume - it may not work at all cos the controllor does’nt work after a certain <delta speed>.

Also, the controllers are tuned to provide a highly damped response. The last thing you’ll ever want is car with oscillating speed.

Back to the OP: I have seen the same problem specially with old Cruise Controls. I think this happens when you are accelerating and you set the Cruise Control, there is a slight lag between the actual speed and the speed that shows up on your dial - so maybe its that. Else, the Integral component will force any steady state error to 0.

Ummm, that could have had a lot more to do with the differnce between a Dodge/Ford and a BMW I would imagine.

Some auto trans with lock up converters have a little lag when the cruise control (CC) slows the vehicle considerably as the converter will seems to disengage and neeed to re-engage when the CC demands/adds more power. It might be a little overpowered and surge over the target speed too.

Typicall in an overdrive/lockup converter set up, the CC can’t maage the speed as precisely as desired.

All three of my cars (93 Honda Accord EX, '95 Nissan Altima GLE, '02 Hyundai Elantra GLS, all auto) have/had the same problem. The Accord was chipped, and the cruise control was only accurate to about +/-5mph, especially after hills. The Altima didn’t appear to register decreases in resistance… down hills, or smoother road surfaces, for example, and would accelerate and stay at the higher speed. The Elantra does exactly what the OP described- doesn’t return to the same (measured) road speed after cancelling or briefly accelerating manually.

As noted above, I think it has most to do with vehicle make, and the quality of materials and craftsmanship. I have a BMW whos cruise control is near perfect on all surfaces. You want 95.3 mph, you get it, every time. Hit the brakes to coast thru town and hit the resume button and bingo! 95.3 mph! Up hill, spot on! It will creep up a hair on steep downhills, but that is to be expected in my book.

Those Germans sure can build em! Our Ford pickup cruise control is a tool for keeping moving while you rest you right ankle, and a close approximation of any given speed is the best you can hope for.

Since BMWs have been cited twice in this thread as examples of a make with accurate cruise control, I’ll note here that the car in question is a BMW. And that makes me also think I’ve got a malfunction as I believe mine is the same model as gatopescado’s (740iL).

andy_fl, in my car, the only requirement for setting the speed is (>49kph). The only requirement for ‘resuming’ is (>39kph). ie, I can’t resume 120kph unless I am going above 39kph. The acceleration is fairly smooth all the way up to 120. Also, if I accelerate (using foot) and then let go (while in overdrive), it will switch automatically between 3 gears, using them to slow down - ie 3rd (no overdrive) 4th & 5th (overdrive).

I’m still not convinced that it is that difficult to place preset switches in the framework. Unless you are saying that it is, fundamentally, placing multiple cruise-controls in the system. If this is the case, can’t thre be some dynamic addition/subtraction to ‘remeber’ and automatically reset the parameters?

Wow. They have 80mph speed limits in Texas?

No, the speed limit is 70. If you don’t want to get run over, you set the cruise for 10 over the limit. As I’ve passed many a trooper doing 80, I think you have to have some other violation going on as well to get stopped for 10 over.

Not really on topic, but it might help to shed some light on the situation is this story in Car and Driver about the accuracy of speedometers by car make.

Interesting article Tuckerfan, thanks.

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