Brake failure "Rashomon"

Tesla's "brake failure" rights protection incident can be said to be the most sensational event at the Shanghai Auto Show. As the incident fermented, interpretations were overwhelming. This finally aroused the public's vigilance and even re-evaluation of the safety performance of Tesla vehicles.

Ultimately, the core of this matter is whether Tesla's brakes failed or not, or whether it was a driver's mistake.

Both sides of this matter have their own reasons. There is a lot of information hidden behind the scenes, and there is no clear answer at present. As reporters, we are more curious to explore what the technical conclusion is. However, it is still a mystery.

Dr. Belt's Investigation

What exactly is the reason for brake failure? After all, Tesla and most electric vehicles currently use the iBooster wire-controlled (that is, electronically controlled) brake system developed by Bosch, so does "brake failure" have anything to do with Bosch?

Here is an explanation. The wire-controlled brake system of pure electric vehicles now detects the force of the driver's stepping on the brake pedal, converts it into a corresponding electrical signal to the system, and then the wire-controlled system drives the hydraulic system to complete the braking. In other words, the braking action that was previously completed by the mechanical brake structure has become electronically controlled.

The wire control system not only has fast response and small space occupation, but also can adapt well to the needs of energy recovery and autonomous driving. In other words, the motor of the wire control system is turned into a generator. The energy of the car's forward braking will drive the motor's rotor to cut the magnetic flux lines to generate current. The current passes through components similar to capacitors and then flows back into the battery after stabilization, thus completing the energy recovery process.

As a pioneer in the field of electric vehicles and autonomous driving, Tesla has naturally used this system for a long time. It is officially called "Automatic Emergency Braking System" and this function has been integrated into the AutoPilot system. However, there is a fatal problem with the wire brake system. If there is an error, delay or interruption in the transmission of the electrical signal, it is very likely to cause dangers such as brake failure and sudden acceleration. Although the probability of this is very low, it is not impossible.

Next, last June, American doctor Ronald A. Belt conducted an independent investigation into a sudden acceleration incident of Tesla and published a 66-page investigation report. Prior to this, he participated in the well-known investigation into the sudden acceleration problem of Toyota cars in 2010.

After inspecting the stalled Tesla Model 3, his analysis was that the vehicle's sudden acceleration was caused by the braking system and its interaction with the energy regeneration system. In other words, the systems were in conflict.

Because the EDR (Event Data Recorder) data of the Tesla Model 3 sudden acceleration incident revealed inconsistencies between the EDR data, driver testimony and Tesla's own accident log data analysis.

The EDR data examined by Belt showed that the vehicle was not braking during the accident, but the longitudinal accelerometer data and Tesla log data both confirmed that the driver was braking. Similarly, the EDR data showed that the ABS system was not activated, while the accelerometer data and log data both verified that the ABS system was indeed activated.

This is very strange. It is said that all Tesla drivers like to drive in single-pedal mode, which is very cool. Because in this mode, the torque can be continuously adjusted between the maximum positive torque and the negative braking torque. In other words, the speed of the car is adjusted by the accelerator pedal. The only time the driver needs to step on the brake pedal is when a complete stop or emergency braking is required.

Under normal conditions, when the driver steps on the accelerator pedal to the bottom, the driver will get 100% acceleration. When the accelerator pedal is released, the driver's acceleration will decrease, and the vehicle will switch from acceleration to slight deceleration to charge the battery. This deceleration charging is called energy regeneration, or energy recovery. When the accelerator pedal is fully released, the driver will get the maximum deceleration and the maximum regenerative energy (in the old version of the software, the maximum deceleration is 0.2g, and after the software is updated, it is 0.3g, just like a fuel car in first gear).

According to common sense, in an emergency, the driver will subconsciously press the brake pedal with all his strength, regardless of whether he likes single-pedal driving. However, when the brake pedal is pressed, it turns into a sudden acceleration, or as many car owners have reported, "the brakes become hard and cannot be pressed", then who is to blame?

Cause Analysis

Let's look at the Model 3's braking mechanism. This is the braking system used on all Tesla vehicles. It consists of three main components: the brake booster and its associated electronic control module, the brake modulator unit and its associated electronic control module, the brakes and brake pads for the disc brakes on all four wheels, and the wheel speed sensors.

The brake booster used by Tesla is Bosch's iBooster. It uses an electric motor to provide brake assistance. This system replaces the traditional vacuum booster with an electric transmission mechanism. In short, the electric system replaces the vacuum booster to assist the driver in pushing the brake master cylinder, so that the brake caliper holds the brake disc to provide braking force.

It also uses an exterior brake light/STOP switch to sense the application of the brake pedal to activate the brake lights and control related vehicle functions. For safety reasons, iBooster is designed to allow the driver to manually apply the brakes if power assistance is lost for any reason.

In addition, the brake modulator unit used in Tesla's braking system is Bosch's ESP hev II module, which receives commands from the iBooster via a high-speed CAN bus. The response time of the command is 1ms to achieve rapid braking operation in an emergency. The iBooster and ESP hev II brake modulator work together to achieve braking operation.

At present, Bosch's iBooster brake-by-wire system has developed to the second generation. At the same time, iBooster adopts a dual insurance mode. If the vehicle power supply cannot work at full load, iBooster will adopt the corresponding energy-saving mode. If the iBooster fails, the vehicle's ESP will directly take over the vehicle and provide braking assistance. Unlike ABS braking, ESP can provide braking force even without input signals from the brake pedal.

There is also a more extreme situation. For electric vehicles like Tesla, even if the high and low voltage power supplies of the entire vehicle are all cut off, the Bosch iBooster system will automatically open the internal hydraulic valve. Without the assistance of a servo motor, it can also provide braking function entirely by relying on the internal hydraulic braking system. This is the same principle as the failure of traditional vacuum brake boosters.

Industry insiders have analyzed that the Bosch iBooster system with a "double insurance" mode is absolutely fine. However, Dr. Belt's analysis concluded that "the failure of the brake light switch caused the EDC/MSR function in the ESP hev II module to make an incorrect decision about the source of the negative acceleration it encountered, causing the EDC/MSR function to request the drive motor to have a large positive torque."

In other words, even if the driver insists that he did press the brake pedal, and even if the log data confirms this, the EDR data will show that the brake pedal was not pressed. So, is this the problem?

Referees and athletes

According to the data and text description of "one minute before the accident" released by Tesla, the explanation we get is:

"When the driver stepped on the brake pedal for the last time, the data showed that the vehicle speed was 118.5 kilometers per hour. Within 2.7 seconds after the driver stepped on the brake pedal, the maximum brake master cylinder pressure was only 45.9 bar. After that, the driver stepped on the brake pedal more and the brake master cylinder pressure reached 92.7 bar. Then the forward collision warning and automatic emergency braking functions were activated (the maximum brake master cylinder pressure reached 140.7 bar) and played a role, reducing the magnitude of the collision. 1.8 seconds after the ABS took effect, the system recorded the occurrence of the collision. After the driver stepped on the brake pedal, the speed of the vehicle continued to decrease. Before the collision, the speed dropped to 48.5 kilometers per hour."

The focus of the debate is the speed of 118.5 kilometers per hour. But if you go to the scene and check, you will know how it is possible to drive at this speed under the traffic light conditions where you need to slow down? Moreover, it is understood that the time of the accident happened at 6 pm during the evening rush hour, and the speed limit on the section of the road where the accident occurred was 80 kilometers per hour. Judging from common sense, it is possible that the "pedal became hard" mentioned by the owner's brother led to brake failure.

In addition, the data provided by Tesla itself lacks a lot of key information, such as accelerator pedal opening, motor signal, brake pedal position signal and iBooster/ESP signal, etc. However, these data are recorded truthfully by Tesla's background data and EDR. And the background data can indeed be modified.

In this regard, Dr. Yang Jun from the School of Automotive and Transportation Engineering of Jiangsu University of Technology made an analysis in Science and Technology Daily. There are three possibilities for this accident. One is that there is a problem with the interaction between the brake pedal and the wire-controlled brake system, which makes it impossible to brake.