There is indeed a possibility of failure. Analysis of cruise control system

Nowadays, the cruise control function is no longer an exclusive feature of high-end cars. Whether it is originally installed or installed later, more car owners have enjoyed the convenience brought by this function. Indeed, it can play a positive role in driving comfort and fuel consumption. However, a news report two days ago about a traffic accident caused by the failure of the cruise control system has questioned this once popular function. Many people have also raised objections. However, if we only look at the cruise control system, the system does have the possibility of being completely out of control. This "root of evil" was planted in 1959.

In 1959, Cadillac became the first automaker to apply the cruise control system to mass-produced cars. The cruise control system installed at that time was different in structure from the system we are exposed to now. Don’t forget that the throttle (throttle is the accelerator) was still controlled by a mechanical cable at that time, and the electronic throttle was gradually popularized in most cars in the past 10 years. Indeed, for various reasons, the cruise control system was not popularized in the era of mechanical cable throttle. However, the change of the engine throttle from mechanical cable to electronic has guided the development of cruise control to a certain extent. Not only that, with the increasing integration of vehicle electronics and electrical equipment, the coordination between the cruise control control system and other links has become more complicated than before.

The structural changes of the cruise control system have not overturned the most basic control logic that was originally formulated. Whether it is the old antiques or the latest cruise control technology, they all need to collect relevant signals and transmit them to the cruise control module for calculation during the execution process. After the command signal is determined, it is sent to the execution control module to control the throttle opening. In this way, the vehicle can cruise at the speed set by the driver, and the driver does not need to control the accelerator pedal during the whole process. With the increasing integration of system components, the current cruise control system only needs one or two components to complete the above steps.

In addition, how to stop the cruise control function is the most concerned point at this stage. Since the birth of the cruise control system, engineers have been working hard to completely solve this problem.

● Electronic injection and carburetor

The mechanical cable-type throttle belongs to both the EFI and carburetor eras. The cruise control system was first installed on carburetor engines, and then this device was extended to EFI engines. However, the cruise control systems installed on the two engines have different control concepts. It is more direct to look at the picture for this section.

After briefly understanding how the cruise control system of a carburetor engine works, I will now mainly use the electronic fuel injection engine as the discussion background to take a look at how cruise control works.

● Cruise control system based on mechanical cable throttle structure

Technically, it is not difficult to install a cruise control system on the basis of a mechanical cable throttle. We only need to use a servo motor to replace the driver's accelerator pedal to control the throttle cable, and the acquisition of vehicle speed information depends on the vehicle speed signal of the instrument. The driver controls the servo motor installed at the throttle through the relevant buttons to adjust the throttle opening. The throttle position sensor transmits the throttle opening signal to the engine control unit, and adjusts the injection pulse width, ignition, and even the gear position of the automatic transmission in combination with the engine operating condition map, so that the vehicle speed in the cruise state can be changed.

The picture shows the structure of the cruise control system installed in the first generation Lexus LS model. The throttle of this car adopts a mechanical cable type, and the throttle opening is controlled by a servo motor.

In terms of how to turn off the cruise control function, the driver can cancel the function by controlling the switch. In addition, stepping on the brake pedal will also be recognized by the system as a termination signal. For manual transmission vehicles, the cruise control function will also be canceled when the clutch pedal is stepped on. They transmit electrical signals to each other.

This type of cruise control system has a relatively simple control logic for the throttle, and the probability of the system being unable to stop due to electrical circuit failure is small. However, in fact, a cruise control system with this structure also has a certain risk of failure. We assume that in the cruising state, the servo motor used to control the throttle opening fails, causing the throttle cable to be unable to be released normally. At this time, the throttle opening cannot be effectively adjusted, and the system cannot be controlled by the driver.

● Cruise control system based on electronic throttle structure

Compared with the mechanical cable throttle, the cruise control system based on the electronic throttle has a simpler structure because the servo motor that controls the throttle opening is eliminated.

In addition to the structural changes, there are also differences in the execution concepts. The cruise control system originally based on the mechanical cable-type throttle structure relies on the motor-driven cable to adjust the throttle opening, while the electronic throttle's execution signal is sent from the engine control unit (this is also the essential difference between the two). Therefore, the cruise control system needs to negotiate with the engine computer in the control link of the throttle opening.

☆ How does the vehicle speed change?

The following section will only discuss the cruise control system installed on models with electronic fuel injection engines and electronic throttle.

-- Driver proactive adjustment

The driver uses the lever to send a request to adjust the vehicle speed to the cruise control unit, and this request is transmitted to the engine control unit, which will formulate an adjustment plan based on the target vehicle speed.

If the change in vehicle speed is not large and is within the adjustment range of the throttle opening, the engine control unit only needs to issue a command to adjust the throttle opening, and the injection amount and ignition timing will also be adjusted accordingly. That is, when the throttle opening becomes larger, the vehicle speed increases, and when the throttle opening is reduced, the vehicle speed decreases.

Another situation is that the driver's request to change the speed cannot be achieved by adjusting the throttle opening alone, so an automatic transmission is needed to cooperate, which is only for automatic transmission vehicles. The change of vehicle speed is achieved by changing the gear position. For example, the engine speed is increased by using a lower gear of the gearbox, so as to reduce the vehicle speed by using the braking effect generated by the engine. On the contrary, when the driver issues an acceleration command, if there is a higher gear available, when the engine speed climbs to a certain speed as the vehicle speed increases, the gearbox will also increase the gear. Of course, whether the driver sends an acceleration command or a deceleration command through the lever, the whole process will be very smooth.

-- The cruise control system actively adjusts to different road conditions

In order to further improve the performance of cruise control and thus enhance driving comfort, a program for dealing with special road conditions has been added to the entire system. For example, when the vehicle is driving on a non-level road, the set cruise speed may change. When driving on an uphill section, the engine load increases, and the vehicle speed decreases as a result. At this time, the cruise control system will send a speed compensation instruction to the engine computer based on the change in vehicle speed. The principle of compensation is still to adjust the throttle opening first, so that the engine can take in more air and increase the injection pulse width to compensate for the fuel. Secondly, as a transmission mechanism, the gearbox also responds very actively to changes in vehicle speed. If the change in vehicle speed is not enough to adjust through the throttle opening, the automatic transmission can also contribute to compensating the vehicle speed by switching gears, relying on the connection between the cruise control system, the engine control unit and the automatic transmission control unit.

Of course, if the vehicle speed fluctuation range is too large, the cruise control system may not be able to compensate for the vehicle speed. In this case, the driver needs to take over the vehicle speed. As for how the system responds when the vehicle speed approaches the target speed again, this depends on the working logic of the system. It may restore the original setting state, or the driver may need to activate the system function by controlling the lever.

● How to cancel cruise control?

I have to say that under normal circumstances, it is very easy to cancel the cruise control function, and sometimes you may even feel that it is a little too sensitive in this regard.

-- The driver actively cancels the cruise control system

The driver can cancel the cruise control function by pressing the brake pedal or the accelerator pedal, the cruise control lever button, or switching the automatic transmission to neutral (for manual transmission, pressing the clutch pedal). The action of pressing the accelerator pedal is set to be temporarily canceled in the cruise control system of some cars, that is, when the driver finishes the acceleration action, the vehicle speed will return to the originally set speed. It is worth noting that these signals are transmitted to the cruise control system control unit in the form of electrical signals.

-- Cruise control system automatically cancels

When the system finds a related fault, the cruise control function will be suspended. The brake pedal position sensor is the component that sends a cancellation signal to the cruise control function. When this component fails, it means that it may not be able to send a cancellation signal to the cruise control system. For the sake of driving safety, the cruise control system will automatically shut down at this time.

The signal collection and execution mentioned above in the introduction of the cruise control logic is also the basis for ensuring the normal operation of the system. If there is a fault in this link, the cruise control function will also be cancelled.

The vehicle speed is calculated after obtaining the signal from the wheel speed sensor. The vehicle stability system, speedometer display and other systems that require the vehicle speed signal will obtain the signal from the wheel speed sensor. Therefore, when the wheel speed sensor fails, these related systems will be affected, and cruise control is no exception.

In fact, in daily car use, the failure of the wheel speed sensor is not due to a problem with the sensor itself. More often, it is due to foreign matter accumulated between the sensor and the target wheel (generally fixed on the half-axle, with a toothed structure on its surface), which makes the sensor unable to monitor the rotation of the target wheel. Just clean the surface. This is worth noting when dealing with wheel speed sensor failures to avoid unnecessary expenses.

The throttle valve is the executive component that controls the vehicle speed. If it fails, it will also cause the cruise control system to be interrupted. In addition to mechanical failure of the throttle valve, if there is severe carbon deposits on the throttle valve, it will also cause an electronic throttle valve fault alarm. After removing the carbon deposits, re-match the throttle valve and clear the fault code, the alarm light will go out.

● The cruise control function may fail.

After reading the above content, we can be sure of the statement that "the cruise control system has the possibility of control failure" because, from the control principle, all cancellation signals are transmitted to the cruise control system control unit in the form of electrical signals. The control unit analyzes these data and decides whether to exit the cruise control state.

Simple circuit failures (such as brake pedal sensor failure, wheel speed sensor failure and related circuit failures, etc.) will not cause the cruise control system to lose control. It will only terminate the cruise control, resulting in the vehicle speed being controlled by the driver.

The problem of cruise control failure may occur in the data processing link of the control unit. It is like a computer crash. After power failure, the system may return to normal state after restarting, as if nothing happened. There is really no way to avoid such problems fundamentally.

Editor's summary:

When driving on the highway, the cruise control system can indeed improve the driver's comfort, but this incident also made us see the hidden dangers of this system. Even if this is only a low-probability event, considering that this function is mostly used for high-speed driving, the dangers brought by the hidden dangers of failure cannot be ignored.

While writing this article, the question of how to avoid such problems has been lurking in my mind like a background program. Is that all? Can we add multiple "backdoor programs" to this system? When the cruise system freezes, can we activate the emergency system in some feasible way to effectively intervene in the phenomenon of being unable to slow down? This is just my very superficial idea. If you have a better suggestion, please speak freely in the comments. Maybe it will be recognized by some manufacturer. But what if the car really can't stop? You can click the link below to get relevant content.