Brake-by-wire
Encyclopedia
Drive-by-wire technology in automotive industry replaces the traditional mechanical and hydraulic control system
s with electronic control systems using electromechanical actuators and human-machine interface
s such as pedal and steering feel emulators. Brake-by-wire represents the replacement of traditional components such as the pumps, hoses, fluids, belts and vacuum servo
s and master cylinders with electronic sensors and actuators.
Some x-by-wire technologies have been already installed on commercial vehicles such as steer-by-wire, and throttle-by-wire. Brake-by-wire technology is still under development by some automobile and automotive parts manufacturers industry worldwide and has not been widely commercialized yet. This is mainly due to the safety-critical nature of brake products. So far, Mercedes-Benz (Sensotronic) and Toyota (Electronically Controlled Brake
) already use almost fully brake-by-wire systems, on the Mercedes-Benz E-class and SL models and on Toyota's Estima.
Once the driver inputs a brake
command to the system via a human-machine interface
- HMI (e.g. the brake pedal
), four independent brake commands are generated by the ECU based on high level brake functions such as anti-lock braking system
(ABS) or vehicle stability control (VSC). These command signals are sent to the four electric caliper
s (e-calipers) via a communication network. As this network might not be able to properly communicate with the e-calipers due to network faults, HMI sensory data are also directly transmitted to each e-caliper via a separate data bus.
In each e-caliper a controller
uses the brake command (received from ECU) as a reference input. The controller provides drive control commands for a power control module. This module controls three phase drive currents for the brake actuator which is a permanent magnet DC motor, energised by 42V sources. In addition to tracking its reference brake command, the caliper controller also controls the position and speed of the brake actuator. Thus, two sensor
s are vitally required to measure the position and speed of the actuator in each e-caliper. Because of the safety critical nature of the application, even missing a limited number of samples of these sensory data should be compensated for.
s) and be handled by more than the bare necessity hardware. Three main types of redundancy
usually exist in a brake-by-wire system:
In order to utilize the existing redundancy, voting algorithms need to be evaluated, modified and adopted to meet the stringent requirements of a brake-by-wire system. Reliability
, fault tolerance and accuracy are the main targeted outcomes of the voting techniques that should be developed especially for redundancy resolution inside a brake-by-wire system.
Example of a solution for this problem: A fuzzy
voter developed to fuse the information provided by three sensors devised in a brake pedal design.
sensors and the wheel speed sensor
s. The electronic control unit
must always be informed of the driver’s intentions to brake or to stop the vehicle. Therefore, missing the pedal sensor data is a serious problem for functionality of the vehicle control system. Wheel speed data are also vital in a brake-by-wire system to avoid skidding. The design of a by-wire car should provide safeguards against missing some of the data samples provided by the safety-critical sensor
s. Popular solutions are to provide redundant sensors and to apply a fail-safe
mechanism. In addition to a complete sensor loss, the electronic control unit
may also suffer an intermittent (temporary) data loss. For example, sensor data can sometimes fail to reach the electronic control unit
. This may happen due to a temporary problem with the sensor itself or with the data transmission path. It may also result from an instantaneous short circuit
or disconnection, a communication network fault, or a sudden increase in noise. In such cases, for a safe operation, the system has to be compensated for missing data samples.
Example of a solution for this problem: Missing data compensation by a predictive filter.
Recent designs for brake-by-wire systems use resolver
s to provide accurate and continuous measurements for both absolute position and speed of the rotor of the actuators. Incremental encoders are relative position sensors and their additive error needs to be calibrated or compensated for by different methods. Unlike the encoders, resolvers provide two output signals that always allow the detection of absolute angular position. In addition, they suppress common mode noise and are especially useful in a noisy environment. Because of these reasons, resolvers are usually applied for the purpose of position and speed measurement in brake-by-wire systems. However, nonlinear and robust observers are required to extract accurate position and speed estimates from the sinusoidal signals provided by resolvers.
Example of a solution for this problem: A hybrid resolver-to-digital conversion scheme with guaranteed robust stability and automatic calibration of the resolvers used in an EMB system.
is a relatively expensive component in an EMB caliper. The cost is derived from its high unit value from a supplier, as well as marked production expenses because of its inclusion. The later emanates from the complex assembly procedures dealing with small tolerances, as well as on-line calibration for performance variability from one clamp force sensor to another. The successful use of a clamp force sensor in an EMB system poses a challenging engineering task. If a clamp force
sensor is placed close to a brake pad, then it will be subjected to severe temperature conditions reaching up to 800 Celsius that will challenge its mechanical integrity. Also temperature drifts must be compensated for. This situation can be avoided by embedding a clamp force sensor deep within the caliper
. However, embedding this sensor leads to hysteresis
that is influenced by friction
between the clamp force sensor and the point of contact of an inner pad with the rotor. This hysteresis prevents a true clamp force to be measured. Due to the cost issues and engineering challenges involved with including the clamp force sensor, it might be desirable to eliminate this component from the EMB system. A potential opportunity to achieve this presents itself in accurate estimation of the clamp force based on alternative EMB system sensory measurements leading to the omission of a clamp force sensor.
Example of a solution for this problem: Clamp force estimation from actuator position and current measurements using sensor data fusion.
Control system
A control system is a device, or set of devices to manage, command, direct or regulate the behavior of other devices or system.There are two common classes of control systems, with many variations and combinations: logic or sequential controls, and feedback or linear controls...
s with electronic control systems using electromechanical actuators and human-machine interface
Human-machine interface
Human-machine interface is the part of the machine that handles the Human-machine interaction- Overview :In complex systems, the human-machine interface is typically computerized. The term Human-computer interface refers to this kind of systems....
s such as pedal and steering feel emulators. Brake-by-wire represents the replacement of traditional components such as the pumps, hoses, fluids, belts and vacuum servo
Vacuum servo
A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the USA it is commonly called a brake booster.-Background:...
s and master cylinders with electronic sensors and actuators.
Some x-by-wire technologies have been already installed on commercial vehicles such as steer-by-wire, and throttle-by-wire. Brake-by-wire technology is still under development by some automobile and automotive parts manufacturers industry worldwide and has not been widely commercialized yet. This is mainly due to the safety-critical nature of brake products. So far, Mercedes-Benz (Sensotronic) and Toyota (Electronically Controlled Brake
Electronically Controlled Brake
Electronically Controlled Brake developed by Toyota Motor Corporation initially for its hybrid and Lexus models, is the world's first production brake-by-wire braking system. The ECB went on sale in Japan in June 2001, first appearing on the Toyota Estima hybrid , and making its North American...
) already use almost fully brake-by-wire systems, on the Mercedes-Benz E-class and SL models and on Toyota's Estima.
Architecture of an Electromechanical Braking System
General architecture of an electromechanical braking (EMB) system in a drive-by-wire car is shown in Fig. 1. The system mainly comprises five types of elements:- ProcessorMicroprocessorA microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
s including an Electronic Control UnitElectronic control unitIn automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
(ECU) and other local processors - MemoryMemoryIn psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
(mainly integrated into the ECU) - SensorSensorA sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s - ActuatorActuatorAn actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s - Communication network(s).
Once the driver inputs a brake
Brake
A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes....
command to the system via a human-machine interface
Human-machine interface
Human-machine interface is the part of the machine that handles the Human-machine interaction- Overview :In complex systems, the human-machine interface is typically computerized. The term Human-computer interface refers to this kind of systems....
- HMI (e.g. the brake pedal
Pedal
The word pedal comes from the Latin and relates to the foot.A pedal is a lever activated by one's foot. The term "foot pedal" is redundant, and should be avoided...
), four independent brake commands are generated by the ECU based on high level brake functions such as anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...
(ABS) or vehicle stability control (VSC). These command signals are sent to the four electric caliper
Caliper
A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...
s (e-calipers) via a communication network. As this network might not be able to properly communicate with the e-calipers due to network faults, HMI sensory data are also directly transmitted to each e-caliper via a separate data bus.
In each e-caliper a controller
Controller (computing)
In computing and especially in computer hardware, controller is a chip, an expansion card, or a stand-alone device that interfaces with a peripheral device...
uses the brake command (received from ECU) as a reference input. The controller provides drive control commands for a power control module. This module controls three phase drive currents for the brake actuator which is a permanent magnet DC motor, energised by 42V sources. In addition to tracking its reference brake command, the caliper controller also controls the position and speed of the brake actuator. Thus, two sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s are vitally required to measure the position and speed of the actuator in each e-caliper. Because of the safety critical nature of the application, even missing a limited number of samples of these sensory data should be compensated for.
Voting
A brake-by-wire system, by nature, is a safety critical system and therefore fault tolerance is a vitally important characteristic of this system. As a result, a brake-by-wire system is designed in such way that many of its essential information would be derived from a variety of sources (sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s) and be handled by more than the bare necessity hardware. Three main types of redundancy
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
usually exist in a brake-by-wire system:
- Redundant sensors in safety critical components such as the brake pedalPedalThe word pedal comes from the Latin and relates to the foot.A pedal is a lever activated by one's foot. The term "foot pedal" is redundant, and should be avoided...
. - Redundant copies of some signals that are of particular safety importance such as displacement and force measurements of the brake pedal copied by multiple processors in the pedal interface unit.
- Redundant hardware to perform important processing tasks such as multiple processors for the electronic control unitElectronic control unitIn automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
(ECU) in Fig. 1.
In order to utilize the existing redundancy, voting algorithms need to be evaluated, modified and adopted to meet the stringent requirements of a brake-by-wire system. Reliability
Reliability engineering
Reliability engineering is an engineering field, that deals with the study, evaluation, and life-cycle management of reliability: the ability of a system or component to perform its required functions under stated conditions for a specified period of time. It is often measured as a probability of...
, fault tolerance and accuracy are the main targeted outcomes of the voting techniques that should be developed especially for redundancy resolution inside a brake-by-wire system.
Example of a solution for this problem: A fuzzy
Fuzzy control system
A fuzzy control system is a control system based on fuzzy logic—a mathematical system that analyzes analog input values in terms of logical variables that take on continuous values between 0 and 1, in contrast to classical or digital logic, which operates on discrete values of either 1 or 0 .-...
voter developed to fuse the information provided by three sensors devised in a brake pedal design.
Missing data compensation
In a by-wire car, some sensors are safety-critical components, and their failure will disrupt the vehicle function and endanger human lives. Two examples are the brake pedalPedal
The word pedal comes from the Latin and relates to the foot.A pedal is a lever activated by one's foot. The term "foot pedal" is redundant, and should be avoided...
sensors and the wheel speed sensor
Wheel speed sensor
A wheel speed sensor or vehicle speed sensor is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation...
s. The electronic control unit
Electronic control unit
In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
must always be informed of the driver’s intentions to brake or to stop the vehicle. Therefore, missing the pedal sensor data is a serious problem for functionality of the vehicle control system. Wheel speed data are also vital in a brake-by-wire system to avoid skidding. The design of a by-wire car should provide safeguards against missing some of the data samples provided by the safety-critical sensor
Sensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
s. Popular solutions are to provide redundant sensors and to apply a fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
mechanism. In addition to a complete sensor loss, the electronic control unit
Electronic control unit
In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
may also suffer an intermittent (temporary) data loss. For example, sensor data can sometimes fail to reach the electronic control unit
Electronic control unit
In automotive electronics, electronic control unit is a generic term for any embedded system that controls one or more of the electrical systems or subsystems in a motor vehicle....
. This may happen due to a temporary problem with the sensor itself or with the data transmission path. It may also result from an instantaneous short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
or disconnection, a communication network fault, or a sudden increase in noise. In such cases, for a safe operation, the system has to be compensated for missing data samples.
Example of a solution for this problem: Missing data compensation by a predictive filter.
Accurate estimation of position and speed of brake actuators in the e-calipers
The caliper controller controls the position and speed of the brake actuator (besides its main task which is tracking of its reference brake command). Thus, position and speed sensors are vitally required in each e-caliper and an efficient design of a measurement mechanism to sense the position and speed of the actuator is required.Recent designs for brake-by-wire systems use resolver
Resolver
Resolver may refer to:* Resolver , a type of rotary electrical transformer used for measuring degrees of rotation* Resolver , an album by the band Veruca Salt...
s to provide accurate and continuous measurements for both absolute position and speed of the rotor of the actuators. Incremental encoders are relative position sensors and their additive error needs to be calibrated or compensated for by different methods. Unlike the encoders, resolvers provide two output signals that always allow the detection of absolute angular position. In addition, they suppress common mode noise and are especially useful in a noisy environment. Because of these reasons, resolvers are usually applied for the purpose of position and speed measurement in brake-by-wire systems. However, nonlinear and robust observers are required to extract accurate position and speed estimates from the sinusoidal signals provided by resolvers.
Example of a solution for this problem: A hybrid resolver-to-digital conversion scheme with guaranteed robust stability and automatic calibration of the resolvers used in an EMB system.
Measurement and/or estimation of clamp force in the electromechanical calipers
A clamp force sensorSensor
A sensor is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated...
is a relatively expensive component in an EMB caliper. The cost is derived from its high unit value from a supplier, as well as marked production expenses because of its inclusion. The later emanates from the complex assembly procedures dealing with small tolerances, as well as on-line calibration for performance variability from one clamp force sensor to another. The successful use of a clamp force sensor in an EMB system poses a challenging engineering task. If a clamp force
sensor is placed close to a brake pad, then it will be subjected to severe temperature conditions reaching up to 800 Celsius that will challenge its mechanical integrity. Also temperature drifts must be compensated for. This situation can be avoided by embedding a clamp force sensor deep within the caliper
Caliper
A caliper is a device used to measure the distance between two opposing sides of an object. A caliper can be as simple as a compass with inward or outward-facing points...
. However, embedding this sensor leads to hysteresis
Hysteresis
Hysteresis is the dependence of a system not just on its current environment but also on its past. This dependence arises because the system can be in more than one internal state. To predict its future evolution, either its internal state or its history must be known. If a given input alternately...
that is influenced by friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
between the clamp force sensor and the point of contact of an inner pad with the rotor. This hysteresis prevents a true clamp force to be measured. Due to the cost issues and engineering challenges involved with including the clamp force sensor, it might be desirable to eliminate this component from the EMB system. A potential opportunity to achieve this presents itself in accurate estimation of the clamp force based on alternative EMB system sensory measurements leading to the omission of a clamp force sensor.
Example of a solution for this problem: Clamp force estimation from actuator position and current measurements using sensor data fusion.
Further reading
- Hoseinnezhad, R., Bab-Hadiashar, A., Missing data compensation for safety-critical components in a drive-by wire system (2005), IEEE Transactions on Vehicular Technology, Volume 54, Issue 4, pp. 1304–1311.
- Hoseinnezhad, R., Signal Processing Methods and Apparatus (Missing Data Handling by A Multi-Step Ahead Predictive Filter), International Patent number PCT/AU2005/000888.
- Hoseinnezhad, R., Bab-Hadiashar, A., Fusion of redundant information in brake-by-wire systems, using a fuzzy Voter (2006), Journal of Advances in Information Fusion (JAIF), Volume 1, Issue 1, pp. 35–45.
- Hoseinnezhad, R., Position sensing in by-wire brake callipers using resolvers (2006), IEEE Transactions on Vehicular Technology, Volume 55, Issue 3, pp. 924–932.
- Hoseinnezhad, R., Harding, P., Signal Processing and Position Determining Apparatus and Methods, International patent application number PCT/AU2006/000282.
- Hoseinnezhad, R., Bab-Hadiashar, A., Automatic calibration of resolver sensors in electro-mechanical braking systems: A modified recursive weighted least squares approach (2007), IEEE Transactions on Industrial Electronics, Volume 54, Issue 2, pp. 1052–1060.
- Anwar, S., Zheng, B., An antilock-braking algorithm for an eddy-current-based brake-by-wire system (2007) IEEE Transactions on Vehicular Technology, 56 (3), pp. 1100–1107.
- Anwar, S., Anti-lock braking control of a hybrid brake-by-wire system (2006) Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 220 (8), pp. 1101–1117.
- Lee, Y., Lee, W.S., Hardware-in-the-loop simulation for electro-mechanical brake (2006) 2006 SICE-ICASE International Joint Conference, art. no. 4109220, pp. 1513–1516.
- Canuto, F., Turco, P., Colombo, D., Control development process of the brake-by-wire system (2006) Proceedings of 8th Biennial ASME Conference on Engineering Systems Design and Analysis, ESDA2006, 2006,
- Lang, H., Roberts, R., Jung, A., Fiedler, J., Mayer, A., The road to 12V brake-by-wire technology (2006) VDI Berichte, (1931), pp. 55–71.
- Emereole, O.C., Good, M.C., Comparison of the braking performance of electromechanical and hydraulic abs systems (2005) American Society of Mechanical Engineers, Dynamic Systems and Control Division (Publication) DSC, 74 DSC (1 PART A), pp. 319–328.
- Murphey, Y.L., Masrur, A., Chen, Z., Zhang, B., A fuzzy system for fault diagnostics in power electronics based brake-by-wire system (2005) Annual Conference of the North American Fuzzy Information Processing Society - NAFIPS, 2005, art. no. 1548556, pp. 326–331.
- Masrur, A., Zhang, B., Wu, H., Mi, C., Chen, Z., Murphey, Y.L., Fault diagnostics in power electronics based brake-by-wire system (2005) 2005 IEEE Vehicle Power and Propulsion Conference, VPPC, 2005, art. no. 1554615, pp. 560–566.
- Anwar, S., A torque based sliding mode control of an eddy current braking system for automotive applications (2005) American Society of Mechanical Engineers, Dynamic Systems and Control Division (Publication) DSC, 74 DSC (1 PART A), pp. 297–302.
- Anwar, S., Anti-lock braking control of an electromagnetic brake-by-wire system (2005) American Society of Mechanical Engineers, Dynamic Systems and Control Division (Publication) DSC, 74 DSC (1 PART A), pp. 303–311.