Control analysis for self-driving cars

People often have many questions about the control of self-driving cars. For example, when it comes to steering, does the interaction with the vehicle involve inputting the steering angle or the force? When it comes to braking, does the IPC tell the hardware how much force to use, or is it intelligent enough to specify the braking percentage?

To implement these control instructions, it is first closely related to the chassis group components of the reference vehicle. To understand the interaction with the various components of the vehicle chassis, you must first understand these control components.

Execution by wire

Simply put, the drive-by-wire execution mainly includes the brake, steering and throttle by wire. On some high-end cars, the suspension can also be controlled by wire. Braking is the most difficult part of the drive-by-wire execution.

Throttle by Wire

The throttle-by-wire is quite simple and has been widely used, that is, the electronic throttle. All vehicles with cruise control are equipped with an electronic throttle. The electronic throttle uses a wire harness (wire) to replace the cable or rod, and installs a micro motor on the throttle side to drive the throttle opening.

Generally speaking, increasing or decreasing the throttle means changing the engine throttle opening by the accelerator pedal, thereby controlling the flow of the combustible mixture and changing the engine speed and power to meet the needs of vehicle driving. The traditional engine throttle operating mechanism works through a cable or a pull rod, one end of which is connected to the accelerator pedal and the other end is connected to the throttle linkage plate.

However, the application scope of this traditional throttle is limited and lacks precision. The main function of the electronic throttle is to convert the angle of the driver's accelerator pedal into a voltage signal proportional to it, and at the same time make various special positions of the accelerator pedal into contact switches, and convert engine operating conditions such as idling, high load, acceleration and deceleration into electrical pulse signals and transmit them to the controller ECU of the electronic engine to achieve optimized automatic control of fuel supply, injection and speed change.

The electronic throttle control system is mainly composed of an accelerator pedal, a pedal displacement sensor, an ECU (electronic control unit), a data bus, a servo motor and a throttle actuator.

Bosch Intelligent Electronic Accelerator Pedal

The displacement sensor is installed inside the accelerator pedal to monitor the position of the accelerator pedal at any time. When the height position of the accelerator pedal is detected to change, the information will be instantly sent to the ECU. The ECU will process the information and the data information sent from other systems, calculate a control signal, and send it to the servo motor relay through the line. The servo motor drives the throttle actuator. The data bus is responsible for the communication between the system ECU and other ECUs.

In adaptive cruise control, the ECU in ESP (ESC) controls the motor, which in turn controls the opening and closing range of the intake valve and ultimately controls the vehicle speed.

Steer-by-wire

Nissan's Infiniti Q50 is one of the first mass-produced vehicles to use steer-by-wire

Wire-controlled steering has also been put into practical use, such as Nissan's Infiniti Q50. In fact, the current electronic power steering (EPS) is very close to wire-controlled steering.

The main difference between EPS and steer-by-wire is that steer-by-wire eliminates the mechanical connection between the steering wheel and the wheels, uses sensors to obtain the steering wheel angle data, and then the ECU converts it into specific driving force data, using the motor to drive the steering gear to turn the wheels. EPS, on the other hand, increases the steering force based on the driver's steering angle.

The disadvantage of steer-by-wire is that it needs to simulate the force feedback of a steering wheel. Since the steering wheel is not connected to the mechanical part, the driver cannot feel the resistance transmitted from the road surface and will lose the sense of the road. However, this does not need to be considered in unmanned vehicles. The steer-by-wire device on the Q50L still retains the mechanical device to ensure that even if all electronic systems fail, it can still steer normally.

BBW

Braking by wire is the most critical and the most difficult. To understand brake by wire, you must first understand the braking principle of the car. Light vehicles usually use hydraulic brakes.

The traditional brake system is mainly composed of a vacuum booster, a master cylinder, a fluid reservoir, a wheel cylinder, a brake drum or a brake disc. When the brake pedal is pressed, the brake fluid in the fluid reservoir enters the master cylinder and then enters the wheel cylinder.

The pistons at both ends of the wheel cylinder push the brake shoes outward, causing the friction plates to rub against the brake drum, thereby generating braking force.

When the driver steps on the brake pedal, the mechanism will transfer the force of the driver's foot to the wheel through hydraulic pressure. But in fact, it takes a lot of force to stop the car, which is much greater than the power of human legs. Therefore, the braking system must be able to amplify the power of the legs. There are two ways to do this: one is leverage; the other is to use Pascal's law and use hydraulic amplification. The braking system transfers the force to the wheel, giving the wheel a friction force, and then the wheel also gives the ground a corresponding friction force.

Before we discuss how the braking system works, let's understand three principles:

Leverage;

Hydraulic action;

Friction effect.

There is no need to explain the lever effect, and everyone must have been familiar with it. A force F is applied to the left side of the lever. The length of the left side of the lever (2X) is twice that of the right side (X). Therefore, the right end of the lever can get twice the force 2F of the left end, but its stroke Y is only half of the left end stroke 2Y.

The brake pedal is a lever. Considering the inclination of the pedal, the design stroke of the pedal is generally no more than 18 cm. The hydraulic principle needs special explanation. Liquid cannot be compressed. The pressure of liquid in a closed container has a characteristic: whether it is inside the liquid or the force pressing against the container wall, it is the same everywhere. That is: if there is a ton of force on one square meter, then the force on one square meter is one ton everywhere. This is called Pascal's theorem.

Since liquid cannot be compressed, this method is very efficient in transmitting torque, with almost 100% force transmission. The biggest advantage of the hydraulic force transmission system is that it can connect two cylindrical hydraulic cylinders at any length, or bend into various shapes to bypass other components. Another advantage is that the hydraulic pipe can be branched, so that a master cylinder can be divided into multiple slave cylinders. Another function of hydraulics is to amplify torque. If the diameter of the master cylinder is 1 inch and the diameter of the wheel cylinder is 3 inches, then any force applied to the master cylinder will be amplified 9 times on the wheel cylinder. However, the piston of the master cylinder pushes 9 cm, and the piston of the wheel cylinder only pushes 1 cm, and energy is conserved. Usually, the diameter of the master cylinder of a car is 22 mm, the diameter of the front wheel cylinder is 32 mm, and the diameter of the rear wheel cylinder is 28 mm.

Different material surfaces have different serration structures; for example, rubber is harder to slide against rubber than steel. The type of material determines the coefficient of friction. So the friction force is proportional to the normal pressure on the contact surface of the object. For example, if the coefficient of friction is 0.1, one object weighs 100 pounds and the other weighs 400 pounds, then if you want to push them, you must apply a 10-pound force to the 100-pound object and a 40-pound force to the 400-pound object to overcome the friction and move forward.

ABS

Having said that, let's talk about ABS.

ESP is very similar to ABS. The biggest difference between ESP and ABS is that ESP can output brake pressure to the wheel cylinder without pressing the brake pedal, while ABS can only output pressure from the master cylinder to the wheel cylinder after pressing the brake pedal. The pressure generator is the motor and plunger pump. Compared with ABS, there are 4 more plunger pumps and 4 solenoid valves, namely VLV and USV.

Bosch's ninth-generation ESP adds two special functions, one is ACC, adaptive cruise control, ESP can partially control the electronic throttle, and the other is AEB, ESP can partially control the braking system.

Some people think that ESP can control both the accelerator and the brakes, and is a very good wire control system, but it is not. Bosch generally only opens ACC and ESP mass production interface protocols to domestic manufacturers. The maximum braking force is about 0.5g, and the standard braking force is above 0.8g. 0.5g is far from enough.

Again, at the beginning of the design, the ESP controlled braking system was only used in a few emergency situations and might be used less than twice a year. The capacity of a general pump is only 3 ml. Each time it is used, the plunger pump has to withstand high temperature and high pressure. Frequent use will cause the plunger pump to heat up severely and its precision will decrease, causing the life of ESP to drop sharply. A conventional braking system may be used several times in an hour. If ESP is used as a conventional braking system, it may be scrapped in a month.