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Old 11th July 2014, 15:06   #1
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Post Understanding Cruise Control

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Cruise control -

A control mechanism for automatically regulating the speed of a vehicle. Cruise control allows the driver to maintain the speed of a car without continually holding down the accelerator pedal.

Cruise control is an invaluable feature on cars. Without cruise control, long road trips would be more tiring for the driver at least.

Conventional cruise control system operating methods.

The cruise control system actually has a lot of functions other than controlling the speed of your car. For instance, the cruise control pictured below can accelerate or decelerate the car by 1 mph with the tap of a button. Hit the button five times to go 5 mph faster. There are also several important safety features -- the cruise control will disengage as soon as you hit the brake pedal, and it won't engage at speeds less than 25 mph (40 kph).

The system pictured below has five buttons: On, Off, Set/Accel, Resume and Coast. It also has a sixth control -- the brake pedal, and if your car has a manual transmission the clutch pedal is also hooked up to the cruise control.

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  1. The on and off buttons don't actually do much. Hitting the on button does not do anything except tell the car that you might be hitting another button soon. The off button turns the cruise control off even if it is engaged. Some cruise controls don't have these buttons; instead, they turn off when the driver hits the brakes, and turn on when the driver hits the set button.
  2. The set/accel button tells the car to maintain the speed you are currently driving. If you hit the set button at 45 mph, the car will maintain your speed at 45 mph. Holding down the set/accel button will make the car accelerate; and on this car, tapping it once will make the car go 1 mph faster.
  3. If you recently disengaged the cruise control by hitting the brake pedal, hitting the resume button will command the car to accelerate back to the most recent speed setting.
  4. Holding down the coast button will cause the car to decelerate, just as if you took your foot completely off the gas. On this car, tapping the coast button once will cause the car to slow down by 1 mph.
  5. The brake pedal and clutch pedal each have a switch that disengages the cruise control as soon as the pedal is pressed, so you can shut off the cruise control with a light tap on the brake or clutch.
Controlling the Cruise Control

The brain of a cruise control system is a small computer that is normally found under the hood or behind the dashboard. It connects to the throttle control, as well as several sensors. The diagram below shows the inputs and outputs of a typical cruise control system.

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A good cruise control system accelerates aggressively to the desired speed without overshooting, and then maintains that speed with little deviation no matter how much weight is in the car, or how steep the hill you drive up. Controlling the speed of a car is a classic application of control system theory. The cruise control system controls the speed of the car by adjusting the throttle position, so it needs sensors to tell it the speed and throttle position. It also needs to monitor the controls so it can tell what the desired speed is and when to disengage.

The most important input is the speed signal; the cruise control system does a lot with this signal. First, let's start with one of the most basic control systems you could have -- a proportional control.

In a proportional control system, the cruise control adjusts the throttle proportional to the error, the error being the difference between the desired speed and the actual speed. So, if the cruise control is set at 60 mph and the car is going 50 mph, the throttle position will be open quite far. When the car is going 55 mph, the throttle position opening will be only half of what it was before. The result is that the closer the car gets to the desired speed, the slower it accelerates. Also, if you were on a steep enough hill, the car might not accelerate at all.

Most cruise control systems use a control scheme called proportional-integral-derivative control (a.k.a. PID control). Don't worry, you don't need to know any calculus to make it through this explanation -- just remember this:

• The integral of speed is distance.
• The derivative of speed is acceleration.

A PID control system uses these three factors -- proportional, integral and derivative, calculating each individually and adding them to get the throttle position.

The integral factor is based on the time integral of the vehicle speed error. Translation: the difference between the distance your car actually traveled and the distance it would have traveled if it were going at the desired speed, calculated over a set period of time. This factor helps the car deal with hills, and also helps it settle into the correct speed and stay there. Let's say your car starts to go up a hill and slows down. The proportional control increases the throttle a little, but you may still slow down. After a little while, the integral control will start to increase the throttle, opening it more and more, because the longer the car maintains a speed slower than the desired speed, the larger the distance error gets.

The derivative, Remember that the derivative of speed is acceleration. This factor helps the cruise control respond quickly to changes, such as hills. If the car starts to slow down, the cruise control can see this acceleration (slowing down and speeding up are both acceleration) before the speed can actually change much, and respond by increasing the throttle position.

Adaptive Cruise Control

Two companies are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology, called adaptive cruise control, uses forward-looking radar,installed behind the grill of a vehicle, to detect the speed and distance of the vehicle ahead of it.

Last edited by Rehaan : 12th July 2014 at 10:49. Reason: See note in post
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Old 11th July 2014, 23:23   #2
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Default re: Understanding Cruise Control

Thanks for the article.

I have a question. In a manual transmisison, do you have to engage cruise control only after shifting to the appropriate gear? For example, I am in 5th gear doing 80kmph, and have engaged cruise control (CC). But I need to slow down for some reason, to, say 50kmph, and I also downshift to 4th.

Now, to resume cruising at 80kmph, does one have to shift to 5th first? I ask because you mentioned pressing the clutch disables the CC.
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Old 11th July 2014, 23:45   #3
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Default re: Understanding Cruise Control

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Originally Posted by Karthik Gullu View Post
Adaptive Cruise Control

Two companies are developing a more advanced cruise control that can automatically adjust a car's speed to maintain a safe following distance. This new technology, called adaptive cruise control, uses forward-looking radar,installed behind the grill of a vehicle, to detect the speed and distance of the vehicle ahead of it.
I guess the HowStuffWorks article where you copy pasted this entire thing from is not really up to date. Adaptive cruise control is quite widely available in cars these days. So, it has gone from "being developed" to "being deployed"

Anyway, here is the link to the original article, which i think you forgot to put:
http://auto.howstuffworks.com/cruise-control1.htm
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Old 12th July 2014, 03:27   #4
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Originally Posted by bravo6 View Post
For example, I am in 5th gear doing 80kmph, and have engaged cruise control (CC). But I need to slow down for some reason, to, say 50kmph, and I also downshift to 4th.

Now, to resume cruising at 80kmph, does one have to shift to 5th first? I ask because you mentioned pressing the clutch disables the CC.
AFAIK, to disengage the CC, one needs to lightly tap the brake pedal and to re-engage the CC at the same point which it wad initially set (80 kmph), one needs to press cruise button or the '+' button.

I don't think pressing the clutch pedal will disengage the CC.

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Originally Posted by amitoj View Post

I guess the HowStuffWorks article where you copy pasted this entire thing from is not really up to date.

Anyway, here is the link to the original article, which i think you forgot to put
Good catch sir.

Anurag

Last edited by a4anurag : 12th July 2014 at 03:28.
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Old 12th July 2014, 07:27   #5
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Default re: Understanding Cruise Control

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Originally Posted by bravo6 View Post
Thanks for the article.

I have a question. In a manual transmisison, do you have to engage cruise control only after shifting to the appropriate gear? For example, I am in 5th gear doing 80kmph, and have engaged cruise control (CC). But I need to slow down for some reason, to, say 50kmph, and I also downshift to 4th.

Now, to resume cruising at 80kmph, does one have to shift to 5th first? I ask because you mentioned pressing the clutch disables the CC.
Since CC has no idea which gear you are in, it justs opens the throttle as said thereby accelerating the car. Hence if you are in lower gears, prepare for a screamer of an engine Anyways thats another reason behind disabling it at low speeds. The designers will ensire you will be in the upper gears as far as possible. Also since its a method of throttle control, pressimg the clutch must disable CC because once the clutch is pressed, the engine will revv to glory if CC doesnt close the throttle on time.

CC will go to 'standby' mode if you press brake or clutch. You can get back the car to the cruising speed by pressing the 'resume' button.

Last edited by audioholic : 12th July 2014 at 07:30.
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Old 12th July 2014, 07:52   #6
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Default re: Understanding Cruise Control

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Originally Posted by a4anurag View Post
AFAIK, to disengage the CC, one needs to lightly tap the brake pedal and to re-engage the CC at the same point which it wad initially set (80 kmph), one needs to press cruise button or the '+' button.
I believe you can disengage CC using the Coast or a dedicated Cancel button as well.

Quote:
Originally Posted by audioholic View Post
CC will go to 'standby' mode if you press brake or clutch. You can get back the car to the cruising speed by pressing the 'resume' button.
Hmmm makes sense to disengage CC when pressing the clutch. I've only experienced CC on an automatic and I must say that combination is sooo easy to drive all day long! As long as the speed didn't drop below ~40kmph, and with a bit of thinking ahead, I didn't have to use my leg at all. CC returned great mileage on relatively flat roads too.


OT: I've often heard people claim that lorry drivers place a brick on the A pedal to "cruise" on highways. Any truth behind it?
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Old 12th July 2014, 09:23   #7
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Default re: Understanding Cruise Control

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Originally Posted by Karthik Gullu View Post
Adaptive Cruise Control
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Originally Posted by amitoj View Post
Adaptive cruise control is quite widely available in cars these days.
A look at the adaptive cruise control system (ACCS) on the Jeep Grand Cherokee Overland, owned by BHPian rideon in Australia.

Understanding Cruise Control-adaptive-cruisectrl.jpg

The pic above shows the steering controls - the lower buttons allow for setting the cruise control in ACCS mode, with adjustment for separation from the car ahead (1-3 car lengths). Since the system responds much faster than human reaction time, a lesser gap is maintainable even at higher speeds. The car brakes in when the vehicle ahead slows down - and yes we checked - the brake lights also glow!

At the heart of the system is the radar located below the bumper (pic below).

Understanding Cruise Control-fcradar.jpg

Cruise control can also be set on non-adaptive mode.

The radar also acts as a Forward Collision Warning (FCW) with Crash Mitigation alert system even when not in ACCS mode. It automatically applies the brakes hard, and sounds an alarm with a flashing warning on the meter console, asking the driver to brake.

FCW can also be turned off by the driver via a button on the dashboard (not advisable).
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Old 12th July 2014, 09:35   #8
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Default re: Understanding Cruise Control

Quote:
Originally Posted by bravo6 View Post
Thanks for the article.

I have a question. In a manual transmisison, do you have to engage cruise control only after shifting to the appropriate gear? For example, I am in 5th gear doing 80kmph, and have engaged cruise control (CC). But I need to slow down for some reason, to, say 50kmph, and I also downshift to 4th.

Now, to resume cruising at 80kmph, does one have to shift to 5th first? I ask because you mentioned pressing the clutch disables the CC.
Yes, In a manual transmission, we have to engage cruise control only after shifting to the appropriate gear probably top gear for better performance of Cruise Control but the minimum speed has to be achieved to click"SET" button.

If you want to slow down the car, can be done in two ways
  • Keep pressing COAST button, as it reduces the speed by 1 mph.
  • once the clutch is pressed to change the gear to 4th or break is applied, CC will be off and by which vehicle can be slow down to 50kmph and if you want to resume back to previous speed(say 80kmph from 50kmph as per your example) click RESUME button irrespective of 4th or 5th gear, so CC picks the latest maximum speed which it was before disengage.

Quote:
Originally Posted by amitoj View Post
I guess the HowStuffWorks article where you copy pasted this entire thing from is not really up to date. Adaptive cruise control is quite widely available in cars these days. So, it has gone from "being developed" to "being deployed"

Anyway, here is the link to the original article, which i think you forgot to put:
http://auto.howstuffworks.com/cruise-control1.htm
Yes, I thought of mentioning source at the end but I totally forgot by the time I complete understanding of cruise control mechanism Thanks Bro.

After researching many links and website, I felt howstuffworks.com had given about Cruise control precise and easy for people to understand the concepts. So finally I ended up with this.

Regarding the Adaptive cruise control, I just mentioned gist about it as I wasn't sure on this front. May be I will go through Adaptive cruise control technology soon and come up with write up
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Old 12th July 2014, 10:16   #9
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Quote:
Originally Posted by bravo6 View Post
Thanks for the article.

I have a question. In a manual transmisison, do you have to engage cruise control only after shifting to the appropriate gear? For example, I am in 5th gear doing 80kmph, and have engaged cruise control (CC). But I need to slow down for some reason, to, say 50kmph, and I also downshift to 4th.

Now, to resume cruising at 80kmph, does one have to shift to 5th first? I ask because you mentioned pressing the clutch disables the CC.
There are cruise systems that allow you to shift gears while cruise control is active. Basically the control system ignores clutch pressed for a short duration while changing gears. But this is again not available in all vehicles.

Quote:
Originally Posted by a4anurag View Post
I don't think pressing the clutch pedal will disengage the CC.
Anurag
In my experience pressing the clutch or as a matter of fact pressing any of the pedals will disengage a normal cruise control system. But there are certain cruise control systems that allow you to change gears by ignoring short clutch presses.

Last edited by Rehaan : 12th July 2014 at 10:36. Reason: Merging consecutive posts. Please use the MULTI-QUOTE system (see the Announcements Section)
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Old 12th July 2014, 12:25   #10
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Default Re: Understanding Cruise Control

thanks for the article. I'm a bit of a Cruise Control enthusiast, if not to say fanatic. i use it all the time, even in town. I will keep it engaged even for very short durations, say 5-10 seconds.

In many modern cars cruise controls are already built in, or you can get it factory fitter as an option. A few years ago I got myself an aftermarket cruise control and fitted it to my 1982 Mercedes W123. It's a very simple system, but I thought it might be interesting to see some of the details. I did a very detailed write up for the Dutch W123 club and I still have the photographs, so here's a condensed version.

There are many aftermarkets CC available. I choose the MagicSpeed 50 for a few simple reasons. It is a very simple and cheap system. I knew of various club members who had already installed it successfully and were quite pleased with it's operation. See here for some product information: http://en.waeco.com/products/4374_452.php

The W123 had the factory option to have a CC installed. You can find all the bits in pieces second hand, but it can be a bit of a pain to get it all to work, so I decided to go the aftermarket way.

Ordered the MagicSpeed on line for about Euro 140 I believe.

Various CC have slightly different ways on how they work. The MS work has an actuator, connected to the engine vacuum. The actuator, through a bowden cable needs to be connected to the throttle cable. On the W123 there is no throttle cable, there is a linkage, so I had to modify and improvise a bit. Then there are two reed relays to be fitted to the clutch and brake pedal. The MS doesn't measure speed as such. It measure rpm of the prop shaft through another magnetic sensor. And of course there is a small control unit and the actual electronics control, so here goes;

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The whole unit comes in one box and a manual on how to install and a schematic diagram

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This is sort of upside down, you're looking at the props haft. There is a magnet attached to the prop shaft and a sensor is mounted on a bracket attached to the bottom of the car. None of standard brackets provided fitted properly so I made my own.

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Getting to the prop shaft is not that difficult. Put the W123 with its front wheels on my drive and i can slide right underneath. But of a tight fit, (and cold it was autumn in the Netherlands!)

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Not the best of photographs, but here we are looking from the footwell upwards. The shiny thing is a bracket I made. It hold one of the reed relays, in this case for the cloth. The magnet is attached to the clutch pedal. Positioning is a bit of a fiddle. As soon as you touch the clutch the reed relays need to disconnect and thus de-activate the CC. Similar for the brake, but there you can use the brake light switch.

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This is the switch to control the various CC functions. As this is a near concours state classic car I did not want to mess around with the interior. This switch I can easily remove, replace the little panel underneath and everything is completely original again.

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The vacuum actuator installed in the engine room. The bowden cable is a fix length and can't bend to much. Lucky me, the actuator fitted right on top of some existing hole in the chassis.

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Here's where it got a bit tricky. As I said the W123 does't have a throttle cable, only endless linkages. So I had to improvise a bit. So I built this little support from some aluminum flat bars and got a few bits and pieces from a bicycle shop to make a support for the Bowden cable. If you look carefully, you can just see the end of the Bowden cable resting on this support. The cable that will move the throttle linkage is sticking out underneath the Bowden cable support.

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Here's the clever bit. Another modification. You can see (vertical) throttle linkage. The inner cable front the Bowden cable is running parallel to the throttle linkage. I made, from a block of aluminum a small clamp that is attached to the throttle linkage. The inner cable runs through a hole in the aluminum support and has a stop at the end. What it does is the throttle linkage can move indecently of the cable. Once you engage the CC the stop will hit the clamp and pull put the throttle linkage. If you stomp on the accelerator you can still press the linkage. further up.

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The actuator is connected to the engine vacuum lines. The same that goes to the brake booster. So I installed a T-connector tapping the vacuum before the check valves. Meaning that any vacuum problem on the CC shouldn't affect the brake booster.

It took about 10-12 hours to install of this. I had quite some problems to get it to work properly. had to experiment quite a bit with the correct setting on the CC electronics and the number of magnets on the prop shaft.

enjoy.

Jeroen
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Old 12th July 2014, 13:28   #11
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Default Re: Understanding Cruise Control

I think with a drive by wire system, fitting cruise control will be a plug and play task. I was interested in adding it to my SX4 with steering mounted controls as in the international variant, but the job required a new contact coil for the steering which made me temporarily shed the plans I had :(
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Old 12th July 2014, 15:30   #12
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Originally Posted by audioholic View Post
I think with a drive by wire system, fitting cruise control will be a plug and play :(

Don't want to sound to skeptical but there is little plug and play around. Cruise control system are largely manufacturer and sometimes even model specific. So aftermarket system will always require modifications and such. The MagicSpeed range sells as "will fit most cars" which is just a euphemism for get your toolbox put because it's going to need modifications and or adaptions.

There is no real standard drive by wire system in the car industry so there is not going to be a plug and play CC
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Old 12th July 2014, 16:39   #13
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Default Re: Understanding Cruise Control

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Originally Posted by Jeroen View Post
Don't want to sound to skeptical but there is little plug and play around. Cruise control system are largely manufacturer and sometimes even model specific. So aftermarket system will always require modifications and such. The MagicSpeed range sells as "will fit most cars" which is just a euphemism for get your toolbox put because it's going to need modifications and or adaptions.

There is no real standard drive by wire system in the car industry so there is not going to be a plug and play CC
Plug and play meaning like how tuning boxes work. VSS signal is available. Use this signal and an embedded system to interfere with the accelerator signals. When cruise control is on, using a separate set of controls from user, this system can replicate the signals sent from the gas pedal to the ECU. Involves a lot less work than the mechanical one using a vacuum control. Can be completely overridden by cutting off power supply in case it fails at any time. When OFF, it can let the real accelarator send signals to the ecu without manipulation.
Manufacturer specific tweaks wrt to accelerator pedal signals will be enough I think in order to make it universal, or use same hardware and just vary the code and connectors.
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Old 12th July 2014, 19:15   #14
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Originally Posted by audioholic View Post
Plug and play meaning like how tuning boxes work. VSS signal is available. Use this signal and an embedded system to interfere with the accelerator signals. When cruise control is on, using a separate set of controls from user, this system can replicate the signals sent from the gas pedal to the ECU. Involves a lot less work than the mechanical one using a vacuum control. Can be completely overridden by cutting off power supply in case it fails at any time. When OFF, it can let the real accelarator send signals to the ecu without manipulation.
Manufacturer specific tweaks wrt to accelerator pedal signals will be enough I think in order to make it universal, or use same hardware and just vary the code and connectors.
I remain skeptical. Today car manufacturers don't even standardize the electrical connectors they are using. If you have a modern car today without a cruise control and you want one, you might be best off getting it from the manufacturer. Even the switch for the CC isn't standardized. Not in hardware, what it looks like, what cable, what connector what signals to which ports on the ECU or other processors.

But I would be very happy to be proven wrong! Lets what and see what hits the shelves in terms of "universal plug and play aftermarket Cruise Control". Meanwhile I suggest just to tick the option when you order your new car.

Unless, like me, you don't mind tinkering with your car.

Jeroen
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Old 14th July 2014, 21:10   #15
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@Jeroen: are you sure that the accelerator and brake connectors are not standard connectors that all pedal manufactures and OEMs use?
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