IBS-Technology

Author: Dr. Thomas Leiber (Managing Director LSP Innovative Automotive Systems GmbH)

Co-Author: Dr. Anton van Zanten (Consultant IPGATE AG)

Introduction

Since the market introduction of ABS, ASR and ESC in 1978, 1986 and 1995, pressure modulation systems for brakes based on the recirculation principle have become established. The valve block, electric motor, pump, 8 or 12 solenoid valves, accumulator chamber and, later in the case of ESC, pressure transmitter and electronic control unit (ECU) are combined in a single unit and installed in the engine compartment separately from the brake booster [1, 2]. Concepts in which brake boosting and pressure modulation are integrated were already on the market in the 1980s (e.g. MK II, ABS 3) but failed to gain acceptance for cost reasons.

The optimization of gasoline engines and the development of hybrid and electric vehicles at the beginning of the 21st century required an electric vacuum pump for the first time. The associated additional costs of classic brake systems provided the motivation for the new concept of an integrated brake system with a non-hydraulic transmission as well as an electric power-on-demand brake boosting (e-BKV) and pressure modulation (ABS/ESC function). In the years 2005 to 2009, two variants of the IBS technology in the form of Integrated Braking Systems with different travel simulators (referred to as "IBS Basic" and "IBS Premium") were developed by the companies IPGATE AG and LSP Innovative Automotive Systems GmbH and licensed to several brake system manufacturers with significant market shares. Fig. 1 provides an initial overview of the considerable progress made in terms of construction volume and installation effort compared with the vacuum brake booster with electric vacuum pump and ESC [3].

In the process, the IBS already covered all the requirements of a future braking system for partially automated driving (also referred to as "SAE Level 2") in 2009. With the introduction of a simultaneously powerful and highly dynamic electric motor, the basis was also created for new operating principles and main functions, such as the automatic emergency braking function (AEB), which is characterized by a very rapid increase in braking torque of approx. 150 ms up to the wheel lock pressure (referred to in specialist circles as "time-to-lock" or "TTL").

Fig. 1 Comparison of IBS variants with vacuum braking systems

Basics

PPC method

The core of the PPC control (PPC: Piston Pressure Control) is a permanently excited brushless electronically commutated internal rotor motor with rare earth permanent magnets (referred to as PMSM internal rotor motor or highly dynamic EC motor), which drives the pressure rod piston of a tandem master brake cylinder (TMC) via a mechanical gear (rack or spindle drive) to generate pressure in the two brake circuits BK1 and BK2. The highly dynamic EC motor has three phases, is sinusoidally commutated, has current sensor i/U as well as rotor angle encoder /U and is controlled by a three-phase rectifier (so-called B6 bridge circuit with six power semiconductors as well as a star point connection) via a microcontroller. Control via a microcontroller enables vector current control with d and q vectors (referred to as Id/Iq current control), where the q vector represents torque and the d vector represents magnetic flux density. By applying vector control, the EC motor can achieve significant performance gains in pulse power operation through field weakening (Id-current control) compared to DC motors with block commutation. Due to the well-known high resolution of the rotor angle encoder as well as the large gear ratio between the EC motor and the push rod piston, a position determination of the push rod piston of the TMC in the m-range is possible as well as a very precise speed calculation of the piston due to high sampling times of the powerful microcontroller in the ms-range. For the brake booster function, Id-current control is also possible with known efficiency of the transmission. To implement this, the relationship between pressure and the torque-forming current Iq is determined by a pressure sensor (p/U) and regularly adjusted during operation so that a map p=f(Iq) can be applied. The pressure sensor (p/U) is also used to determine a pressure volume characteristic curve or pressure piston travel characteristic curve.

By using the above-mentioned sensor technology as well as a model-based software structure, the physical relationships can be used for open-loop control and closed-loop control. The controller is cascaded with a pressure controller and/or piston position controller in the outer control loop and an Id/Iq current control of the EC motor in the innermost control loop. An actuator speed controller is also integrated between the outer and inner control loops. Adaptive maps (e.g. pressure-displacement characteristic of the push rod piston) are used either for feedforward control in the event of dynamic pressure changes or for pressure control by means of piston position. The motor sensors are used for both brake boosting and pressure modulation.

This type of control is known as the PPC process (PPC: Piston Pressure Control). The essentials of the PPC are documented in numerous patent specifications and are legally protected [5,6,7,8,9]. The most important relationships are illustrated in Fig. 2.

Fig.2 Essentials of the PPC process 

Highly dynamic EC motor

Brake systems with vacuum brake booster do not place any special requirements on the electric motor of the ABS return pump, rather it is a matter of its cost-effective manufacture. A DC motor was therefore used. The DC motor met all the requirements since the vacuum brake booster only needs to generate and statically maintain a vacuum. The brake pressure and upstream pressure for ABS control operation are generated in the master brake cylinder. The DC motor must primarily return the volume discharged into the ABS/ESC unit's accumulator chamber via outlet valves to the master cylinder. The DC motor of the ESC/ABS unit also performs certain braking force assistance functions, referred to in technical terminology as, for example, "HBA (Hydraulic Brake Assist)," "HBB (Hydraulic Brake Boost)," "HBC (Hydraulic Boost Failure Compensation," "HFC (Hydraulic Fading Compensation)," as well as driver assistance functions (referred to as "ADAS") such as AEB (Automatic Emergency Braking). These functions were specified according to the performance of the DC motor of the ESC/ABS unit and only enabled emergency braking with limited dynamics.

As an enabler of the IBS technology, a highly dynamic EC motor [4], namely a brushless internal rotor motor with rare earths [5,6,7,8,9] with a peak power of approx. 700 watts, was introduced for the first time in 2004, whereby the motor power was increased by a factor of approx. 4 compared to a DC motor of a standard ESC unit; moreover, once the maximum power had been reached, the power could be kept constant even at higher motor speeds by field-oriented vector control. The performance of a 12V on-board power supply was utilized to the maximum. In addition, the electric motor in both IBS variants had a more dynamic response by a factor of about 5. To characterize the dynamics of the EC motor, the acceleration parameter M/Θmax (ratio of the maximum torque Mmax of the EC motor to the inertial mass Θ of the moving parts) was simplified and called the time constant of the IBS technology. This simplified definition of the time constant is based on the fact that the duration of the torque build-up is of secondary importance for the dynamic pressure build-up.

Fig. 3 shows a comparison of the performance data and time constant of electric motors of a standard ESC system (ESC Standard), IBS technology with multiplex control (IBS MUX), a typical 1-box brake system (1-box brake systems are characterized by functional integration of brake boosting and pressure modulation as well as actuating device in one unit) with pressure modulation by means of inlet and outlet valves (EV/AV) as well as an electric follow-up brake booster with differential travel control between pedal travel and booster travel.

Fig. 3 Comparison of electric motors in braking systems

In contrast to standard ESC brake systems, all modern brake systems (1-box brake systems with EV/AV, electric follow-up brake booster, IBS MUX) have highly dynamic electrically commutated PMSM internal-rotor motors as the drive for the pressure supply, whereby the EC motors are controlled via a microcontroller with field-oriented vector control.

In a performance comparison, IBS-MUX is comparable to the follower brake booster and is surpassed in performance by the EC motor of a 1-box brake system. The maximum power of the 1-box motor with EV/AV is greater than the maximum power of the follower brake booster, because with the follower brake booster, the driver supports the braking effect with his foot force. In addition, more power is also required for the same highly dynamic pressure buildup (TTL) compared with the IBS-MUX due to greater hydraulic flow resistance between the pressure supply unit and the wheel brakes.

In the acceleration characteristic comparison, IBS is the most dynamic motor, primarily due to multiplex requirements for dynamic reversing of the motor. Electric follow-up brake boosters also use highly dynamic motors, greater than a factor of 3 more dynamic than motors of standard ESC systems. This is due to the fact that the operating principle of high-dynamic differential travel control has approximately comparable dynamic requirements to multiplexing.

Driver request recording

In the case of the classic vacuum brake booster, driver sensing is not mandatory. In principle, the valve function of the vacuum brake booster is realized by passive components, so that amplification is always proportional to the pedal force [10]. In the absence of vacuum, the braking pressure must be generated by the driver using foot force only, with the dimensioning of the master brake cylinder determining the pedal feel.

Newer electric brake systems are classified according to the two IBS technology variants IBS Basic and IBS Premium into electrically driven follower brake boosters and brake boosters with pedal feel simulator (also called "brake-by-wire with hydraulic fallback level"). After electric brake boosters based on force measurement with a force transducer, e.g. a piezo sensor [11,12], proved to be unsuitable primarily due to the complexity, e.g. the signal transmission of moving parts, two main operating principles of driver sensing were established, whose embodiments are shown in Fig. 4:

A: Driver request detection based on differential travel of pedal plunger and amplifier piston.

B: Driver sensing by means of sensors (pedal travel, pressure sensor and/or force-displacement sensor FDS) as well as use of a pedal feel simulator.

Fig. 4 Principles of driver request recording

Measurement principle A1 is based on the displacement difference measurement of two pistons that generate pressure and act on the primary pressure chamber of the master cylinder [13]. The measuring principle found its way into the first electrically driven series brake booster solution (designated as the e-ACT product of Hitachi Automotive Systems), but is now only produced in small quantities due to the high complexity of setting up two parallel moving pistons and higher precision requirements for leak tightness.

Measuring principle A2 is based on the differential travel control of two pistons acting on the brake master cylinder, where the first piston K1 is actuated by the brake pedal and the second piston K2 is driven by a highly dynamic EC motor, with pedal force and motor force adding up in the force effect on the pushrod piston. A highly dynamic EC motor is a basic requirement of the measuring principle, in order to ensure a good pedal feel even when the driver actuates the pedal quickly. Characteristically for the measuring principle, an idle travel Ds is also provided between piston K1 and transmission device (piston K2), whereby the EC motor acts on the transmission device via a gear. The idle travel defines a so-called jump-in phase, which corresponds to the jumper of the pneumatic brake booster, at the start of brake pedal actuation. Furthermore, in order to map a progressive spring characteristic between the brake pedal and the push rod piston, a number of F1 and F2 spring elements connected in series arrangement [14,15].

Measurement principle B1 is based on driver sensing with a pedal travel sensor and pressure sensor; a pedal feel simulator is also introduced, which consists of a first and second assembly [16]. The first simulator assembly is purely mechanical, while the second simulator is connected to the master cylinder and receives the hydraulic fluid delivered from the master cylinder. Pressure sensors measure the pressure in the pressure chambers of the master cylinder and are used for driver force sensing and monitoring of the second simulator assembly. Furthermore, the master cylinder can be hydraulically separated from the brake circuits by switchable solenoid valves, allowing pure brake-by-wire operation with a hydraulic fallback level.

Measurement principle B2 is based on a driver sensing system with redundant pedal displacement sensors and a pedal feel simulator connected to a hydraulic piston-cylinder unit. A one-piece pedal feel simulator with multiple elastic elements (spring & elastomer) is provided to represent a nonlinear force-displacement characteristic [17].

Electric follow-up brake boosters installed in electric vehicle, notably Tesla and Volkswagen, are based on measuring principle A2, while electric brake systems with pedal feel simulator in the marketplace provide measuring principle B1 and a one-piece pedal feel simulator according to measuring principle B2, which in contrast to version B2, however, is connected to a pressure chamber of the TMC. In the newer electric brake booster for automated driving X-Boost + ESC [18], a force-displacement sensor [19] is also provided for the emergency function "button brake" so that a driver request can still be detected and braking performed even if the piston is blocked.

Pedal characteristics

Fig. shows on the left-hand side different pedal characteristics of today's vehicles with vacuum brake boosters, which are generally dependent on the vehicle weight. The reason for this is that the volume required to actuate the wheel cylinders generally increases with the vehicle weight since the contact pressure of the wheel cylinder pistons also increases with the vehicle weight. With a given pedal travel, pedal ratio and volume, this results in the dimensioning of the master brake cylinder. 

Fig. 5: Main features of pedal characteristic IBS with pedal feel simulator compared to vacuum brake booster

Fig. 5 shows on the right side a typical pedal characteristic of a brake system with pedal feel simulator (IBS Premium), which can be freely varied due to the fact that the brake booster is decoupled from the brake pedal stroke in normal operation and is represented by a combination of springs and elastomers. A pedal feel simulator can thus be used to design an ideal pedal characteristic independently of the vehicle weight.

In addition to the pedal characteristic in normal operation, the pedal characteristic in case of BKV failure must also be taken into account (Fig. 6). For the dimensioning of the TMC, IBS has the great advantage of decoupling the pedal travel and the master cylinder piston travel. This decoupling was exploited in the IBS Premium in such a way that a significantly smaller TMC piston diameter (19.05 mm) was used than in standard vacuum brake boosters (23.8 mm to 26.9 mm), which is a decisive advantage in the event of a brake booster failure. This is because 50 to 60 bar brake pressure and thus over 5 m/s2 deceleration can thus be achieved for the fallback level at a pedal ratio iPed of 4 with the prescribed 500 N foot force (UN/ECE requirement: ≥2.44 m/s²), which is positively assessed by the driver and can be used for system designs of new brake systems taking into account critical individual faults, e.g. the brake circuit failure of first-generation 1-box systems with pressure-limiting design of the isolating valves. On the other hand, for an SUV with a 27 mm diameter with a pedal ratio iPed of 4 as well as 500N foot force, only about half the brake pressure can be generated in relation to IBS, which means that the UN/ECE requirement is only borderline fulfilled.

In addition, as shown in Fig. 6, the pedal characteristic is almost identical for the IBS in the initial range with the booster intact and failed, whereas for the vacuum brake booster the initial force increases significantly on failure (20 N 130 N) and is characterized by a very steep rise, i.e. the driver feels a hard pedal. This change in characteristic is relevant to safety because the driver is not prepared for the changed situation when the brake booster fails.

Fig. 6 Pedal characteristics IBS compared to conventional vacuum brake booster

In the first embodiments of brake systems with pedal feel simulator (Fig. 4, embodiment B1), brake fluid is shifted from the secondary chamber of the main brake cylinder to the pedal feel simulator. If the displacement simulator fails during braking, the amount of brake fluid that was displaced into the displacement simulator will be absent during braking in the fallback plane. The result is a partial failure of the corresponding brake circuit and reduced braking in the fallback level. This problem has been solved in the IBS Premium (Fig. 4, embodiment B2) by providing a separate first piston-cylinder unit (so-called auxiliary piston HK) specifically for the pedal feel simulator. In the event of failure, the brake pedal acts directly on the master brake cylinder by means of a mechanical through-engagement, thus no brake fluid volume is lost in the fallback plane. Alternatively or additionally, a normally closed shut-off valve to the travel simulator is provided (Fig. 4, embodiment B1).

Primary functions

Brake boosting

In the vacuum brake booster (Fig. 7, top), the brake pressure is boosted in proportion to the pedal force FPed by a boosting force Fboost. The amplification ratio is determined by the ratio of the amplifier body area to the pedal pushrod area. For this purpose, an elastic reaction disk is provided between the push rod piston and the booster body/pedal tappet. The amplification mechanism works automatically by differential travel between the pedal plunger and the amplifier body.

Using a brake system with a pedal feel simulator (Fig. 1, Fig. 4, B2), the pressure supply unit can operate independently of the pedal actuation by decoupling the effects so that any brake force boosting characteristic (sport mode, comfort mode) can be set via software. An electric motor-driven piston-cylinder unit, or "plunger pressure supply unit" for short, is used as the pressure supply. The control is based on the principles of the PPC control (Piston Pressure Control), and the most important features for brake boosting are shown again in Fig. 7 for the sake of clarity.

Fig. 7 Principles of brake boosting

Pressure modulation

In a standard ABS/ESC system with vacuum brake booster, a pre-pressure is set by the master brake cylinder and pressure modulation is implemented via solenoid control valves (inlet and outlet valves). Conventionally, one inlet valve (EV, as a linear solenoid valve, LMV) and one outlet valve (AV, as a switching solenoid valve, MV) are used per wheel brake for pressure modulation. The volume discharged during pressure reduction is absorbed by an accumulator chamber and returned to the master cylinder via a return pump, which, together with the gradual pressure buildup, results in the familiar pulsating brake pedal.

In a brake system with a pedal feel simulator and plunger pressure supply unit, the pre-pressure for ABS control is generated via the plunger pressure supply unit. This allows a further degree of freedom in pressure buildup via the inlet valves, in that the pre-pressure can be adjusted according to the coefficient of friction of the road surface. It makes sense to set a pre-pressure that is about 20% higher than the highest wheel brake pressure [20]. The pressure build-up method with variable inlet pressure has a positive effect on pressure oscillations, NVH and the accuracy of the pressure setting during pressure build-up (see Fig. 8).

Fig. 8 Conventional pre-pressure control vs. variable pre-pressure control using the PPC method

The pressure is released either by timing outlet valves into the accumulator chamber of an ESC unit (closed brake system) or is released into the reservoir (open brake systems), as introduced by Bosch EHB and Toyota working principle [16, 21]. In the Toyota working principle, the pressure is built up via two pumps, with the pumps acting at different pressure levels.

In a further development of the open system, the plunger pressure supply unit was introduced [22]. In addition to the pressure p as a controlled variable, the pressure control method also uses the controlled variables actuator speed wAktor and actuator torque MAktor in a cascade controller [23], and the position of the piston of the pressure supply is also determined and evaluated or used for control. Thus, in closed-loop operation, the piston is retracted when a certain hydraulic volume is consumed, with hydraulic fluid being drawn from the reservoir into the pressure supply during retraction. The cascade controller enables dynamic pressure control and is, therefore, advantageous because the pre-pressure can be kept constant in control mode despite the loss of volume when the pressure is reduced via outlet valves into the reservoir. A dynamic control cascade with acceleration and velocity components in return mode for component protection, referred to as the PRL function in the VDA 360 guideline [24], is also important in closed 2-box brake systems with two units (electromotive brake booster with or without pedal feel simulator [15,18], first box, in combination with an ABS/ESC unit, second box) because the closed brake system has very low hydraulic elasticities in contrast to the vacuum brake booster. The elasticities absorb pressure oscillations in the vacuum brake booster, whereas in a brake system with an electromotive spindle drive, the pressure oscillations must be reduced in terms of control technology by moving the piston.

In the IBS Premium, the pressure control method with pressure build-up and pressure reduction is introduced via bidirectionally effective switching valves (SV), whereby the pressure reduction is controlled in the simplest form of pressure modulation without switching the solenoid valves in such a way that the piston of the pressure supply is moved back by a certain distance s, whereby the EC motor is operated with a reduced current. Active retraction of the piston or alternatively reduction of the motor current results in active retraction or passive retraction caused by return springs and hydraulic pressure in the brake system, reducing the brake pressure and thus the braking force. The change in travel results in a volume change V and a physically determined target pressure change. If the pressure volume characteristic is adapted during operation, the target pressure setting can be used reliably even if the pressure volume characteristic changes, e.g. as a result of air bubbles in the brake fluid or changes to the wheel brake.

This allows various ABS pressure control methods to be implemented. The simplest form of ABS pressure control is the so-called "cadence brake", in which no wheel speed sensors are used and no control valves are actuated, and the brake pressure is modulated by driving the electric motor back and forth or alternatively by operating between two different current levels to prevent the wheels from locking permanently. Here, the proportional relationship between phase current and torque of the motor or alternatively the relationship between position and pressure is used by evaluating the pressure volume characteristic. This rudimentary ABS control is introduced as a driving stability function in electric follow-up brake boosters [25] in case of failure of the ESC unit, which is, however, limited to a deceleration of 5.8 m/s2 due to the lack of an EBD function.

For multi-channel ABS operation, wheel speed sensors are read in and the solenoid valves are actively opened or closed. In IBS, priority control, the so-called multiplex method (abbreviated as the MUX method) with PPC method and additional solenoid valve control, was introduced for this purpose. The MUX control determines whether the pressure reduction takes place sequentially [26] or (partially) simultaneously [27] in one or more wheel brakes. In the first generation of MUX control (MUX 1.0), the pressure is reduced exclusively via switching valves (SV); in the second generation, it is reduced both via switching valves SV and, in borderline cases, via outlet valves with opening of the brake circuit into the reservoir. The mixing concept with pressure reduction via pistons and outlet valves has been designated the MUX 2.0 process [28].

Fig. 9 shows a comparison of the different pressure modulation principles with pumps/pressure fluid accumulation chamber and plunger pressure supply unit.

Fig. 9 Principles of pressure modulation

Advantages/disadvantages ABS/ESC system + vacuum brake booster:

The combination of ABS/ESC systems with a pressure fluid accumulator chamber and vacuum brake booster has the great advantage of a closed hydraulic system, which has proven itself as a reliable system for more than 40 years. Therefore, in a further development, the principle was retained in 2-box solutions with electric slave brake booster + ESC unit.

The main disadvantage is a complex application, because pressure modulation operation de facto determines the pressure in the wheel brakes only with volume flow estimation models. In addition, pressure reduction by time control cannot be set as precisely as with pressure reduction using PPC methods. This means that the pressure setting is inaccurate, especially in the case of active interventions, namely ASR and ESC, if there is no feedback of measured wheel cylinder pressures.

Advantages/Disadvantages of Modulation with PPC/MUX

Pressure modulation using the PPC process has the advantage that pre-pressure can be set very precisely in the pressure buildup and can also be varied depending on the coefficient of friction of the road surface. If the pressure is reduced via outlet valves into the reservoir, higher pressure reduction gradients can be achieved compared with conventional ESC systems, especially at low pressures. A disadvantage is that the hydraulic system is opened due to volume loss during pressure reduction and replenishment, and dirt particles can enter the valves via the pressure supply during control operation [29]. The probability of brake circuit failure, therefore, tends to be higher than with closed systems. Since the introduction of the new type of plunger pressure supply unit has made new possibilities in diagnostics possible even during driving without the driver noticing, an open system could also achieve acceptable failure safety because leakages can be detected at an early stage before braking.

If the pressure is reduced according to a so-called MUX priority control via switching valves and pressure supply device, the pressure reduction is very accurate with sequential pressure reduction, but there are inaccuracies with simultaneous pressure reduction. It was shown that comparably good pressure control performance can also be achieved with 4-channel multiplexed operation. However, problems included noise in highly dynamic operation (e.g. ABS braking on asphalt) and high demands on the solenoid valve design to prevent a valve from closing due to dynamic flow effects neither during pressure build-up nor during pressure reduction. Furthermore, to optimize NVH, large pressure differences during dynamic pressure reduction had to be avoided by suitable piston position control. Unresolved noise problems with ABS control on asphalt, complex software priority control, extremely high dynamic requirements on the EC motor time constant for pressure modulation and cost-intensive solenoid valves were the biggest obstacles to series introduction in early development. An approach to improvement was found in the MUX 2.0 process, which, on the one hand, significantly reduced the pressure modulation dynamic requirements on the EC motor and opened the braking circuit for a short time only in special cases. This meant that the MUX 2.0 process de facto had the safety advantages of a closed braking system, since there was no need to feed in a control process. The MUX 2.0 process created an initial basis for further development. A robust and easy-to-apply control system with an inexpensive and at the same time tightening-resistant solenoid valve would give the MUX process a new meaning.

ABS control results with MUX 1.0 procedure

The low throttling effect of the switching valves in IBS allows a large pressure reduction gradient for braking at low-µ and low-temperature. A comparison shows the pressure reduction that is representative for braking straight ahead on ice (Fig. 10a). In the process, the pressure reduction time with IBS from 10 bar to 5 bar at 20°C is reduced from today's 60 ms by a factor of 4. using a recirculation system. In addition, IBS was significantly quieter than standard ESC systems in control mode.

Fig. 10b shows an example of IBS control on a front wheel with purely sequential control of the multiplex process during fast braking straight ahead on ice with an average locking pressure of approx. 6 bar. With IBS Premium, the higher pressure reduction gradients initially result in a differential speed between the wheel and the vehicle Δv of only 3 km/h. The following sequence shows even smaller control deviations Δv of only 1.3 km/h, corresponding to small pressure changes of 3 bar. The typical sequence for conventional ABS is shown qualitatively in dashed lines. Compared with conventional ABS, braking distance reductions of up to 20% with additionally improved driving stability were measured on snow, ice and split roads. Noise and pedal reactions are barely perceptible. The highly dynamic pressure control is very fast and accurate, which is also reflected in the small wheel accelerations. No better results are known from the EMB or the wedge brake (fully electric brake), which do not use brake fluid and, therefore, do not have the viscosity problems at low temperatures.

Fig. 10 Measurement record of ABS control mode IBS Premium on ice (mid-size vehicle) 

Fig. 10c shows the load on the vehicle’s electrical system, which is independent of the pedal force. In comparison, the conventional ABS draws increasingly higher currents with increasing pedal forces, up to more than 5 times.

Other main functions

Blending procedure for regenerative braking

Vacuum brake boosters have very limited suitability for regenerative braking due to the lack of variability in brake force support and require additional components (separating cylinders) to map the counterforce simulation [30].

In regenerative braking, the so-called blending process is carried out, i.e. the hydraulic braking torque is reduced by the generator braking torque so that the deceleration caused by the hydraulic braking system and the deceleration caused by the generator or generator-driven electric drive motor of a hybrid or electric vehicle add up to the driver's desired deceleration.

Two different methods were used for regenerative braking in IBS technology. In the follow-up brake booster principle with differential travel control (IBS Basic), a multifunctional fluid pressure accumulator chamber with piston and return spring, which can be switched on via a switching valve, was introduced in the BL connecting line between the master brake cylinder and wheel cylinder Rz or at the wheel brakes. The fluid pressure accumulator chamber is filled via a solenoid valve and emptied via a solenoid valve or passive element (throttle + non-return valve) and can also be used for brake pad clearance control. When only a generator brake torque is applied, the accumulator chamber is filled when the pedal is actuated and emptied again when the pedal is released (Fig. 11) [32]. The hydraulic volume flow is color-coded in Figure 11 as an example of the pressure build-up.

Fig. 11 Volume flow during regenerative braking with IBS Basic

Similar blending solutions can be found in today's 2-box braking systems consisting of an electric follow-up brake booster and an ESC unit, whereby a communication interface between the two physically separate units had to be introduced for the 2-box solution [33]. For the first time, the storage chamber of the ESC unit has a multifunction and is used for both ABS pressure modulation and regenerative braking. However, the application is complex because, during blending, solenoid valves connected to the accumulator chamber and the electric brake booster in two separate units must be controlled simultaneously to prevent the driver from perceiving the regenerative braking intervention in the form of a change in pedal feel. In order to ensure acceptable pedal feel, even in the case of rapid driver actuation or dynamic temporal change of the generator braking torque of powerful drive motors of e-vehicles, a highly dynamic EC motor and a highly precise pressure control using the PPC method are required.

In the IBS Premium, whose hydraulic circuit diagram is shown in Fig. 12, the brake pedal acts on an auxiliary piston HK, which in turn is operatively connected via a switching valve WA to a mechanical hydraulic pedal feel simulator (PFS) with piston, spring (8b) and rubber element (8). The hydraulic volume flow is color-coded in Figure 12 as an example of pressure buildup. In normal operation, the pressure supply and actuator unit are decoupled, and, according to the driver's request, the hydraulic braking torque is reduced by the effective generator braking torque. Blending is, therefore, very easy to apply with the PPC control method.

Fig. 12 Volume flow during regenerative braking with the IBS-Premium

Automatic Emergency Brake (AEB)

IBS technology also enabled pioneering work in developing the automated emergency brake with very fast brake pressure build-up with the introduction of a highly dynamic EC motor with initial presentation in June 2006 to Continental AG. As Fig. 13 shows, IBS technology and the PPC process enabled the motor to reach a speed of over 5000 rpm within 30 ms, and the pressure buildup began after only 10 ms due to the low throttling losses of the valves, reaching 100 bar in 134 ms (TTL=134 ms).

Fig. 13 Pressure build-up dynamics IBS up to 200 bar

This was a quantum leap in dynamics compared with conventional ESC systems, in which the pressure buildup from 1 bar to 100 bar could only be achieved in approx. 300 ms (ESC Premium) to approx. 500 ms (ESC Standard) according to the company's own measurements. At an Intelligent Brake 2011 conference, Continental presented simulation results of the first integrated 1-box brake system MK C1 [34], in which the enormous scope of the effect of rapid pressure buildup with a highly dynamic EC motor on braking distance was demonstrated. Even at a low speed of just 65 km/h at the start of emergency braking, a braking distance reduction of 5 meters was achieved. 

Procedure for reducing the friction loss of the brake

It is well known that the brake pads do not fully release from the brake disc in normal cases, e.g. without forced cornering, and generate a non-negligible residual friction torque, which results in a higher CO2 value. With the IBS, a small clearance (BLS: brake lining clearance) can be generated by appropriate piston and valve control, which reduces the residual torque appreciably. An increased clearance can also be produced by designing the brake shoes with reinforced rollback in the wheel cylinder. The disadvantage is the increased pedal travel in the fallback plane, which does not occur with IBS Premium.

As a second essential function, efforts have been made in IBS technology to significantly reduce the power dissipation of the friction brake, which in measurements in 2007 was still about 300 watts. Two variants [29,35] were defined as the BLS process and illustrated in Fig. 14:

a) Active retraction of brake pads by vacuum with PPC control method

b) Active lining clearance adjustment with PPC control method and use of calipers with strong rollback seals

The vacuum control (a) proved to be very effective but was limited at about 0.5 bar in the amount of vacuum as well as in the time period of operation with vacuum because cavitation effects could occur. Since with an electric follower brake booster, the driver feels the clearance when the pedal is applied, a brake system with a pedal feel simulator offers the possibility of effectively and easily reducing the residual friction of the brake. Thus, in a braking system with pedal feel simulator, method (b) is favored, whereby the brake pads are applied in good time before the start of a braking operation, so that no delay in the braking effect can be detected.

Fig. 14 Procedure for reducing power loss in the friction brake

Failure safety

In the design of IBS, the focus was on a fail-safe design. In all previously known brake systems, pressure modulation for pressure reduction involves opening the brake circuit with a connection to the accumulator chamber (in the case of the return principle of ABS and ESC already mentioned) or fault-critical into the reservoir (open brake system). With IBS, the pressure modulation takes place via the MC pistons in a closed brake system. This means that brake circuit failure detection can also be evaluated directly. For this purpose, the piston travel is correlated with the pressure-volume characteristic of the brake system using the pressure transmitter signal. The same applies to wheel brake failure detection. Since the wheel brake pressure is measured (also for ABS, as explained above), the piston travel can also be correlated here with the pressure-volume characteristic of each individual wheel brake. In the event of a failure due to leakage outside the hydraulic control unit HCU, with IBS technology, the switching valve to the failed wheel cylinder can be closed, and braking can continue with three wheel cylinders instead of four. This is a significant safety gain because a shorter braking distance can be achieved with three remaining wheel brakes compared with two wheel brakes, and, in addition, yaw rate control can still take place, albeit to a limited extent.

Using the existing sensor technology (pressure sensor, motor current, piston position) as well as the current-pressure correlation, complete monitoring of all components is possible even if individual sensors fail. In the event of failure of the travel simulator, the system switches over to an electric slave brake booster. Even in the event of engine failure, a high level of vehicle deceleration is still possible in the fallback plane with a design featuring a master brake cylinder diameter of 19.05 mm.

An open brake system has many critical fault sources and must rely on the fallback level in many cases. 1-box braking systems in the market benefit from the fact that a very good fallback level has been created with the small-diameter TMC (19.05 mm2), but they rely on continuous fault diagnostics due to the fault issues and are consequently still limited to SAE level 2 automated driving. The error sources were addressed at a VDI event in 2017, and new smart diagnostic approaches were proposed, e.g. different tests during driving: park stop tests (PST), braking diagnostics (BED), park stop diagnostics (PSD), and car stop diagnostics (CSD) [29,30].

 

Summary and outlook

In 2010, after about 5 years of development, the IBS technology as well as its brake management was presented to the public for the first time at a conference (Fig. 14) [3,36]. In addition to the standard functional scope of ESC systems, three new main functionalities were included:

(1) Pre-crash: highly dynamic automatic emergency braking AEB,

(2) Brake blending with powerful generators or e-drives of a vehicle.

(3) Method for realizing a low-friction or frictionless brake.

Fig. 15 Brake management with IBS

The new functionalities defined in 2010 are now considered standard, as the market penetration of hybrid and electric vehicles, as well as partially automated driving according to SAE Level 2, requires all core functions as well as new main functions.

The following comparison shows a summary of the main characteristics of both variants of IBS technology in comparison with vacuum brake boosters:

Main features IBS Basic (electromotive follower brake booster with differential travel control):

  • Usual good pedal feel in brake boost function

  • Regenerative braking possible without affecting pedal feel up to a certain deceleration (depending on the dimensioning of the accumulator chamber)

  • Driver recognizes the condition of the brake (e.g. fading) by the ratio of pedal travel/pedal force to vehicle deceleration

  • Very fast brake pressure build-up for ADAS functions, in particular, very high-performance automatic emergency braking

Main features IBS Premium (electromotive brake booster with pedal feel simulator):

  • Shorter pedal travel, significantly faster brake pressure build-up for ADAS functions, in particular very high-performance automatic emergency braking

  • Variable, adaptive pressure jump at the start of braking (optimization for the individual series)

  • Better fallback level in case of brake boost failure (almost familiar pedal characteristics and higher pressures with the same pedal forces)

  • Unchanged pedal characteristics with fading compensation by pressure increase

  • Regenerative braking in hybrid and electric vehicles is theoretically not limited by the braking system

  • No pedal breakdown in case of brake circuit failure

  • Automatic diagnosis of the venting condition

  • No irritation of the driver due to pedal pulsations during pressure modulation (ABS, ADAS)

  • Lower noise in pressure modulation mode, especially with low external noise (ABS braking on snow and ice)

In 2017 and 2019, the challenges for future braking systems arising in connection with the fail-operational requirements of automated driving were addressed in technical depth for the first time in the technical community [29,37]. As a first family member of a future 2-box brake system kit for SAE level 3-5, the X-Boost Technology© (www.x-boost-technology.com ) is being developed, which was homologated in February 2022, as a pilot application in a technically highly demanding hyper-car with approx. 2000 hp of electric drive power, the RIMAC Nevera.

In connection with further changes in the automotive industry (electric vehicles, automated driving of SAE levels 3-5, vehicle motion control), the system architecture of vehicles and requirements for braking systems will continue to change dynamically. Due to the still large, partly untapped innovation potential and the significant cost advantages of electrohydraulic brake systems compared to EMB, brakes will remain predominantly hydraulic or partially hydraulic for a long time. Nevertheless, there is still a need for action in improving fail-safety so that electromotive braking systems can achieve the very high reliability of conventional mass-produced vacuum brake boosters and ESC units.

Literature

[1]     Leiber, H.: ABS - Comparison of separate and integrated systems. VDI-ATG Conference, Wolfsburg, Dec. 1984

[2]     van Zanten, A., Erhardt, R., Pfaff, G.: FDR - The vehicle dynamics control system from Bosch. ATZ (1994) 11, pp. 674 - 689

[3]     Leiber, T., Köglsperger, C., Unterfrauner, V.: Modular braking system with integrated functions. ATZ 2011-06, p. 466 - 472

[4]     Leiber, H., Leiber, T.: "Permanenterregte Drehfeldmaschine", patent application E81 of the company IPGATE AG, patent application DE102004030063A1, filing date: 23.06.2004

[5]        Leiber, H., Leiber, T.: "Bremsssystem mit elektromotorisch angetriebenem Kolben-Zylinder-System", granted patent E87DE-SA of IPGATE AG: DE 102005018649B4, filing date: 21.04.2005

[6]        Leiber, H., Leiber, T.: "Bremsssystem mit elektromotorisch angetriebenem Kolben-Zylinder-System", granted patent E87DE-TA1 of IPGATE AG: DE 102005063659B3, filing date: 21.04.2005

[7]        Leiber, H., Leiber, T.: "Bremsssystem mit elektromotorisch angetriebenem Kolben-Zylinder-System", granted patent E87DE-TA4 of IPGATE AG: DE 102006063691B3, filing date: 21.04.2005

[8]     Leiber, H., Leiber, T.: "Bremsssystem mit elektromotorisch angetriebenem Kolben-Zylinder-System", granted patent E87DE-TA5 of IPGATE AG: DE102005063697B4, filing date: 21.04.2005

[9]     Leiber, H., Leiber, T.: "Bremsssystem mit elektromotorisch angetriebenem Kolben-Zylinder-System", granted patent E87DE-TA6 of IPGATE AG: DE102005063699B4, filing date: 21.04.2005

[10]       Wikipedia contributors. (n.d.). Bremskraftverstärker. Wikipedia. Retrieved March 21, 2025, from https://de.wikipedia.org/wiki/Bremskraftverstärker[11]    Kobayashi, K., Inoue, H.: "Electrically driven Brake Booster." granted patent of Robert Bosch GmbH US 6634724, filing date of Japanese priority application 26.04.2001

[12]    Witte, B., Barthenheimer, T. et al.: German patent application of Volkswagen AG: DE 102005025577, filing date: 25.05.2005

[13]    Yamaguchi, T., Obata, T., Oikawa, H.: "Brake apparatus", European patent application EP1964739 of Hitachi Automotive Systems, filing date of Japanese priority application: 27.02.2008

[14]    Leiber, H., Leiber, T., Unterfrauner, V.: "Brake system without travel simulator" granted patent E112 family of IPGATE AG: DE102009004636B4, DE112009005536B3, DE11 2009 005 541B3, filing date. 03.02.2009

[15]    Weiberle, R. Jahnz, T., Meyer,J. : "Brake booster and method and device for its operation", granted patent of Robert Bosch GmbH: DE 102010001939, filing date 15.02.2010

[16]    Toyota, J., Toyota, A.: "Brake system for minimizing high-load operation of a pump device for operating brake cylinders", German patent application of Toyota AG: DE 10015810, filing date of Japanese priority application. 01.04.1999

[17]    Leiber, H., Unterfrauner, V.: "Actuating device for a motor vehicle brake system", granted patent family E122 of IPGATE AG: DE112011102270B4, US9878697B2, CN103108785B,  DE112011106127B3, US10493967B2, filing date: 17.09.2010

[18]    Leiber, T., Leiber, H., van Zanten, A.: "Electromechanical brake booster for automated driving". ATZ 03/2019, pp. 50-54, Fig. 1

[19]    Leiber, H.: "Actuating device with travel simulator" granted patent family E123 of IPGATE AG: US 9541102B2, CN103282251B, KR101804337B1, filing date: of the German priority application:  03.11.2010

[20]       Leiber, H., Leiber, T. et al.: "Druckmodulatorsteuerung ", granted patent E90DE-TA1 of IPGATE AG: DE 102005063658B3, Filing date: 21.11.2005

[21]       Schmidt, G. Binder, J., Winner, H., Gottwick, U.: "Method and device for checking a brake system", granted patent of Robert Bosch GmbH: DE 198073678, filing date: 21.02.1998

[22]       Roll, G., Böhm, J., Jäger, J.: Method for controlling an electrohydraulic braking system and electrohydraulic braking system“, European patent specification of the company Continental AG: EP 2595847, filing date: 13.07.2011.

[23]    Roll, G., Böhm, J.: "Method and device for controlling an electrohydraulic braking system for motor vehicles", granted patent Continental AG: EP 25800951, filing date: 30.06.2010

[24]       Stringer, S., Spöri, T., Lu, W., Schwab, M., Hägerle, N.: "New brake system products - with VDA interface ready for the future" p. 803, Function PRL (Pressure Reduction Logic).

[25]      Bosch Mobility Solutions. (n.d.). iBooster: Elektronisch geregelte Bremskraftverstärkung. Bosch Mobility Solutions. Retrieved March 21, 2025, from https://www.bosch-mobility-solutions.com/de/loesungen/fahrsicherheit/ibooster/

[26]    Leiber, H., Leiber, T.: "Druckmodulatorsteuerung " granted patents of IPGATE AG: E87WOEP - EP1907253B1 and E87WOUS - US8,371,661B2; E90WOEP - EP 1874602 and E90WOUS - US8,540,324B2 , filing date of the priority application: 21.04.2005

[27]    Leiber, H., Leiber, T.: "Braking System with Simultaneous or Partially Simultaneous Brake Cylinder Pressure Increase or Decrease from Different Brake Cylinder Pressure Levels", granted patents E102 of IPGATE AG: DE102009008944B4, EP2396202B1, US9,211,874B2, CN102325676B, IN340432B; filing date: 13.02.2009

[28]    Knechtges, C., Knechtges, J., Passmann, A.: "Electrohydraulic Vehicle Brake System with Electromechanical Actuator and Method for Operating the Brake System", German patent application of ZF Active Safety GmbH: DE 102013014188, filing date: 28.08.2013

[29]    Leiber, T., Köglsperger, C., Hecker, C.: " Herausforderungen und Lösungsansätze für zukünftige Bremssysteme", presentation script VDI Wissensforum, Oct 25th, 2017.

[30]    Leiber, H., van Zanten, A.: "Diagnostic method for a braking system", granted patent family E142 of IPGATE AG: US10059321B2, CN107472232B, filing date: 21.04.2015.

[31]    Plewnia, H.: "Bremsaggregat für ein Landfahrzeug", patent specification Lucas Automotive GmbH: DE102006033890, filing date: 21.07.2006

[32]    Leiber, H., Leiber, T., Köglsperger, C., Unterfrauner, V.: "Brake system with storage device and multiple function", granted patent E117DE-TA1 of IPGATE AG: DE102009061809B3, filing date: 30.09.2009

[33]    Vollert, H., Strengert, S., Kunz, M., Kolarsky, J.: "Method for operating a recuperative braking system of a vehicle, control device for a recuperative braking system of a vehicle and recuperative braking system", granted patent DE 102012211278 of Robert Bosch GmbH, filing date: 29.06.2012   

[34]    Feigel, H. -J., Ross, S., Linkenbach, S.: " Brake System Concepts for Hybrid Vehicles", presentation Intelligent Braking 2011, IQPC Köln, presentation on 28.09.2011

[35]    Leiber, H., Unterfrauner, V.: "Brake system with adaptively controllable lining clearance" granted patent family E113 of IPGATE AG: DE 102008049739B4, DE 102008064942B3, CN102171080B, filing date of the German priority application: 21.12.2007.

[36]    Leiber, T., Köglsperger, C., Unterfrauner, V.: "IBS- Integriertes Bremssystem", Automobilelektronik Kongress Ludwigsburg, Presentation script, June 10th, 2010

[37]    Leiber, T., Leiber, H., van Zanten, A.: "Impact of autonomous driving on future braking systems, ATZ special edition March 2019.