2.23 NEW CONCEPT IN ESC

Electronic stability control (ESC) systems aim to control the yaw and sideslip angle of a moving vehicle through individual wheel braking and engine torque reduction such that the desired path of a vehicle determined through the driver’s inputs (e.g., steering input) can be maintained. That is, ESC systems help the vehicle to follow the driver’s intent such that the driver maintains good control of the vehicle regardless of the variation of road conditions.

This paper presents the Roll Stability Control (RSC) system developed at Ford Motor Company. It is an active safety system for passenger vehicles. It uses a roll rate sensor together with the information from the conventional electronic stability control hardware to detect a vehicle's roll condition associated with a potential rollover and executes proper brake control and engine torque reduction in response to the detected roll condition so as to mitigate a vehicular rollover.

ROLL STABILITY CONTROL (RSC)

The RSC system adds a roll rate sensor and necessary control algorithms to an existing ESC system. The roll rate sensor, together with the information from the ESC system, help to effectively identify the critical roll conditions which could lead to a potential vehicular rollover. Such critical roll conditions need to be discriminated from those due to road bank variations and to be characterized with respect to vehicle loading variations. RSC then applies pressure to the brake(s) on the wheel(s) of the outside of the turn. This reduces lateral force and helps keep the inside wheels firmly on the ground, thus reducing the likelihood of a rollover event.

2.14 ESC DESIGN

ESC uses a hydraulic modulator to assure that each wheel receives the correct brake force. A similar modulator is used in ABS. ABS needs to reduce pressure during braking, only. ESC additionally needs to increase pressure in certain situations and an active vacuum brake booster unit may be utilized in addition to the hydraulic pump to meet these demanding pressure gradients. The important component in ESC system is the Electronic Control Unit (ECU). The various control techniques are embedded in it. Often, the same ECU is used for diverse systems at the same time (ABS, Traction control system, climate control, etc.). The input signals are sent through the input-circuit to the digital controller. The desired vehicle state is determined based upon the steering wheel angle, its gradient and the wheel speed. Simultaneously, the yaw sensor measures the actual state. The controller computes the needed brake or acceleration force for each wheel and directs via the driver circuits the valves of the hydraulic modulator. Via a CAN interface the ECU is connected with other systems (ABS, etc.) in order to avoid giving contradictory commands.

2.15  INCOPORATES ESC COMPONENTS

Traction control system

A traction control system (TCS), also known as Anti-Slip Regulation (ASR), is typically (but not necessarily) an electro-hydraulic system on production vehicles designed to prevent loss of traction of the driven road wheels, and therefore maintain the control of the vehicle when excessive throttle is applied by the driver and the condition of the road surface (due to varying factors) is unable to cope with the torque applied. Although similar to electronic stability control (ESC) systems, traction control systems do not have the same goal.

The intervention can consist of one or more of the following:

• Retard or suppress the spark to one or more cylinders
• Reduce fuel supply to one or more cylinders
• Brake one or more wheels
• Close the throttle, if the vehicle is fitted with drive by wire throttle
• In turbo-charged vehicles, the boost control solenoid can be actuated to reduce boost and therefore engine power.

Anti-Lock Braking System

An anti-lock braking system, or ABS is a safety system which prevents the wheels on a motor vehicle from locking up (or ceasing to rotate) while braking. A rotating road wheel allows the driver to maintain steering control under heavy braking by preventing a skid and allowing the wheel to continue interacting tractively with the road surface as directed by driver steering inputs. ABS offers improved vehicle control and decreases stopping distances on dry and especially slippery surfaces. However, on loose surfaces like gravel and snow-on-pavement, it can slightly increase braking distance while still improving vehicle control. On others, it may not improve control at all.

2.21 PROBLEM

CAR HANDLING WITH ESC AND WITHOUT ESC

UNDERSTEER

Fig. 6. ESC minimize the plowing when cornering

Understeer is a term for a car handling condition in which the application of cornering force (lateral force) also applies a rotational torque (or moment) to the car in the opposite direction of the turn. The effect is opposite to that of oversteer. In simpler words, understeer is the condition in which the vehicle does not follow the trajectory the driver is trying to impose while taking the corner, instead following a less curved trajectory. Understeer is a dynamically stable condition; in other words, if control is lost, the vehicle continues to point and travel in the direction that it is already pointing and traveling in.

Understeer is also often referred to as pushing, plowing, or refusing to turn in. The car is referred to as being "tight" because it is stable and far from wanting to spin.As with oversteer, understeer has a variety of causes such as mechanical traction, aerodynamics and suspension. Classically, understeer happens when the front tires have less traction during a cornering situation than the rear tires, thus causing the front end of the vehicle to have less mechanical grip and become unable to follow the trajectory in the corner.

OVERSTEER

                     Fig. 6. back of the vehicle will tend to slide outward can be minimize

Oversteer is a characteristic of an automobile while attempting to corner or while already cornering. The car is said to oversteer when the application of cornering force (lateral force) also applies a rotational torque (or moment) to the car in the direction of the turn. The effect is opposite to that of understeer. An oversteering car is referred to as "loose" or "free." Oversteer is a dynamically unstable condition; in other words, if control is lost, the vehicle will spin.

2. 22 ADVANTAGES

Improved starting and acceleration capability Improved stability when:

·         Braking

·         Accelerating

·         Coasting

·         Cornering

·         Shortens stopping distances in corners or on slippery surfaces

·         Informs driver of slippery conditions

·         ABS, ASR, EBR and ESC functions are combined in one control unit

·         ABS, ASR, EBR and ESC basic components are combined in one hydraulic unit.

OTHER FUNCTION RELATED ESC

•   PML (SPS) – Speed Sensitive Steering
•  CAN communication
•  Electronic Accelerator (EA) intervention
•  Electronic Transmission Control (ETC)
•  intervention
•  Sensotronic Braking Circuit
•  Distance (DTR)
•  Airmatic, ABC, SAS suspension

MAJOR COMPONENT AND FUNCTION

                                   

                   Fig. 5. Components and other programs in ESC

Hydraulic modulator unit with attached ECU

The hydraulic modulator has input and output solenoid valves for controlling the pressure in the individual wheel brakes. The integrated ECU assumes all electrical and electronic tasks as well as the control functions of the system. The unit is located in the engine compartment between the brake master cylinder and the wheel brake cylinders; so that the hydraulic lines to the brake master cylinder and the wheel brake cylinders can be kept short.

Wheel-speed sensor

The ECU processes the signals from the wheel-speed sensors to compute the speeds of the wheels. Two different operating principles are used: passive (inductive) and active (Hall) speed sensors. Active sensors are becoming more and more widespread. They use a magnetic field for the contactless detection of wheel speed and are capable of recognizing the direction of rotation as well as standstill.

           Yaw-rate and lateral acceleration sensor

A yaw-rate sensor records all yawing movements of the vehicle around its vertical axis. Together with the information from an integrated lateral acceleration sensor, the status of the vehicle ("actual state") can be determined and compared with the driver’s wishes.

Steering-angle sensor

The task of the steering-angle sensor is to measure the position of the steering wheel by determining the steering angle. This figure, together with the vehicle speed and desired braking pressure or position of the accelerator, is used to calculate the driving maneuver that the driver wishes to perform (“desired state”).

Communication with engine management

A data bus enables the ESC control unit to communicate with the engine control unit. In this way, the engine torque can be reduced if the driver accelerates too hard in a particular driving situation. Similarly, it can.compensate for excessive slip of the driven wheels provoked by the engine drag torque