BRAKE-BY-WIRE: THE HISTORY OF THE FUTURE

Two fundamental patents from 2005 describe modern brake management using brake-by-wire systems. For the first time, a brake-by-wire brake system was introduced that was able to implement braking pressure control in a fully variable manner, independent of driver influence, dynamically and with high precision. The core of the brake management setup was a powerful brushless EC motor and the complete decoupling of brake system pressure from the driver. As a result, for the first time ever, the recuperation of kinetic energy by the generator was not limited, and the highly dynamic emergency braking function (AEB) was put into action for the first time using a powerful EC motor. These two new functions have now established themselves on the market as fundamental elements of modern braking systems. Fifteen years ago it had was already been anticipated that electric and hybrid vehicles as well as autonomous driving functions would prevail in what is now the present-day.

PPC is the core concept of the high-precision braking pressure regulator (E87DE-SA, E87DE-TA1, E90DE-SA) using a brushless electric motor and a piston-cylinder unit. Braking pressure is built up through the forward movement of the piston and through the shifting of hydraulic fluid (pressure), and is reduced in a highly dynamic fashion by the return motion of the piston (pressure release). Core elements of PPC are the phase current sensor, the angular sensor detecting the rotor position of the EC motor and adaptive characteristic maps. Characteristic maps for displaying environmental factors (e.g. air bubbles, ambient temperature) are used for feed-forward control in highly dynamic operation and for diagnosing leakages for example. Using intelligent mathematical models, PPC is able to achieve impressive precision while taking into account environmental factors.

This highly dynamic and precise pressure control has been consistently pursued and continuously optimized by LSP and has ultimately led to the outstanding features of the IBSe brake-by-wire system.

In a first basic process (E90DE-TA1), pre-pressure in ABS operation is controlled through the piston  in a variable manner, i.e. the pre-pressure is flexibly adapted to suit the road situation. When operating on streets with low friction coefficients (e.g. snow, ice), pre-pressure is reduced; on asphalt the pre-pressure is increased. Variable pre-pressure can be used to reduce modulation noise, the latter of which is decisive for high-quality ABS control and thus minimizes braking distance when ABS is in use. The design for a larger cross-section of the inlet valve also results in lower fluid flow resistance and shortens pressure buildup time for the AEB (automatic emergency braking) function.

If PPC is used in pressure control mode with an Link open brake system, and e.g. pre- pressure is maintained during ABS-control, the piston must be advanced step-by-step because volume is drained into the reservoir via outlet valves in regular operation. During long braking operations (e.g. ABS on snow or ice), the result is that the volume in the working chamber for pressure supply will be used up after a certain time and needs to be replenished (G2.4). When replenishing (E107EP), the piston is moved back and volume is drawn from a reservoir, thereby briefly interrupting pressure increase in the control process.

In a modern computer, multitasking is done by processing sequential work steps so fast that they cannot be resolved by humans and are perceived as multitasking. This very concept was transferred to brake systems with 4 wheel brakes in highly demanding and dynamic ABS operation.

The revolutionary hydraulic multiplex system with power-on-demand pressure supply unit is described in (E90EP, E102EP). Pressure buildup and release is done exclusively via the piston. Conventional outlet valves were eliminated completely by using the inlet valves as bidirectional switching valves. To set different levels for wheel brakes, pressure was maintained by closing the switching valves while pressure in the other wheel brakes was increased or reduced, either sequentially or simultaneously. Only one pressure sensor and the p-V characteristic curve are required to achieve highly precise control with varying pressure gradients.
The multiplex ABS regulator achieved better braking distances and extremely low noise levels in initial runs in low-friction road conditions (snow and ice) due to the variable pressure regulation and no limitation for minimum pressure as compared with traditional ABS systems with low pressure accumulators. However, there were disadvantages when applying the system to non-homogeneous surfaces that were wet from rain, especially when simultaneous pressure release was required. The 4-channel multiplex operation was considered too risky. In addition to the limits from high friction levels, the stringent demands on the electric motor dynamics would have resulted in the development of an electric motor with a new operating concept (E88EP). Two-channel multiplex control is used in with X-Boost for various function such as electronic brake force distribution (EDB), 2-axle-blending with different recuperation of electric motors on front and rear axle and redundancy functions of 2-box solutions. Using multiplex control a special 2-channel-ABS function can be realized, which ensures both short brake distances and a high vehicle stability in case of failure of the ESC-unit. The latest function is essential for automated driving as a safe control has to be guaranteed also on non-homogenous road conditions in case of failure of the primary ABS function (function of ESC-system).

In a second basic patent (Patent no. DE112009005541B3, internal reference E112DE-TA2), the pressure in the wheel brakes is increased and reduced by moving the piston back and forth. Without effective wheel brake valves, it’s possible to install rudimentary ABS control in the form of automated cadence braking and thus implement electronic brake force distribution (EBD). This is advertised as a HAD-ABS redundancy function in some 2-box brake systems on the market.

If a pedal feel simulator is not used, the simple principle of control of differential positon of the pedal postion and booster body position combined with an elastic element (E112DE-SA) can be used to very cost-effectively and reliably implement a variable brake booster function in a follow-up brake booster.

A defined differential position can be controlled depending on the desired pedal characteristics, which in turn makes the booster characteristic curve, i.e. the boost of input force (foot force), variably adjustable using software. As a result of the booster characteristic curve, a phase current is then set in the brushless motor. Proportional to the phase current, the simple physical relationship of torque = torque constant x phase current defines the motor torque, which then, dependent on the gear box transmission ratio and efficiency of the gear, defines the booster force. That force, together with the foot force, leads to pressure in the brake master cylinder. Sensors required to operate a brushless motor are also used for the braking system as well. The motor rotor position sensor is used to control the EC motor and in addition to determine the position of the booster body, whereas the latter is essential to define the booster characteristic curve. The current sensor is used for EC motor control and in addition for open-loop control of the brake boost force  . The control method as well as the cost-effective use of the rotor position sensor of the EC motor is described in patents E87DE-TA1 and E112DE-TA1.

In this operational case, in order to also maintain the desired pedal characteristic when a generator is in place to recuperate kinetic energy, volume from the brake master cylinder will be moved to a fluid reservoir while the brake booster force is reduced. The so-called “blending control” for a follow-up-booster with ESC is described in (E117DE-SA) and in various publications from brake system manufacturers.

Last, but not least, ideas were already put into the anticipated autonomous driving (E120EP). For example, solutions were defined for redundancy functions. For example, if the electric brake booster fails and the ABS function of the ESC unit still has to be maintained, or if volume has to be released to a fluid reservoir in a closed system during emergency functions (i.e. defect of brake booster electronics and block of the gear mechanism).

In 2013, a so-called 2-box or 3-box solution made its first appearance on the market and consisted of an electrohydraulic brake booster, a smart actuator and an ESPC unit. In a first step, the brake booster was electrified with the primary purpose of dispensing with the vacuum booster and vacuum pump. When the brake pedal is actuated, the driver's desire is detected and the brake pressure induced by the driver amplified by the brake booster. This is known as a follow-up brake booster system without a pedal feel simulator.  

In 2017 a second generation of the electrohydraulic brake booster and an adapted ESC system (ESCHEV) followed. The smart actuator was omitted by integrating recuperation or so-called “blending-“control as a function in the ESChev unit. The pedal characteristics were maintained by reducing the brake boost force of the brake booster.

At the same time, X-Boost was presented and subsequently licensed to selected customers. This is also a 2-box system that differs from the previously mentioned system featuring a pedal feel simulator and a very short design. X-Boost has significant advantages in terms of recuperation, packaging and especially safety, and it targets an application for automated driving above SAE level 3 in combination with a standard ESC. Unlike follow-up brake boosters, recuperation control can be performed by the X-Boost without any need to simultaneously control the ESC-hev, which offers customers the essential advantage of increasing independence from specific manufacturer solutions such as producers of ESPhev still following the VDA360 guideline. This opens the opportunity that recuperation control can be implemented centrally and independently by the car manufacturer to the greatest degree possible, thereby reducing any reliance on specific (and often complex) solutions (ESChev). The simplified application work and the virtually unlimited recuperation of kinetic energy by an electric motor matches one of the most important needs among OEMs these days in terms of implementing domain structure.

In addition to the 2-box solutions, a first 1-box solution was introduced to the market in 2016, the MKC1 from Continental. This is an open electrohydraulic system that integrates the functions of the brake booster and the the ESC unit. It is also equipped with a pedal feel simulator. The brake booster is driven by a pressure supply unit that translates the driver desire into brake pressure. Integrating the brake booster with the ESC saves considerable installation space and weight.

LSP was also very active between 2010 and 2020, as can be seen in from the development of the X-Boost and other phases of the integrated 1-box brake systems IBS 2 and IBS 3. A main focus during these years was on package and weight as well as consistently improving the systems with regard to safety. LSP was always one step ahead here, and in many ways a trendsetter during this phase. These years of development between LSP and IPGATE brought us to our currently revolutionary approach, the so-called Link Modular System Architecture with Safety Gate (MSA).

In addition to the continuous work on our own systems, we regularly carry out comprehensive technical analyses and performance comparisons for the most important braking systems available on the market. The results not only show our developers the current state of the art, improvement potentials among the competition and the potential for our own optimization, but OEMs and suppliers can also purchase this knowledge from us and benefit from our comprehensive and detailed expertise.