Method and system for stabilising a vehicle during cornering on extremely low-friction surfaces

Abstract

The invention extends an existing vehicle dynamics control system to stabilise cornering on extremely low friction surfaces with an estimated friction coefficient mu in the range of approx. 0.02 - 0.05. A controller superimposes sharp-edged steering impulses around a dynamic neutral steering position δ0(t) with an asymmetric bias towards the curve direction. The impulses are synchronised with short brake pulses (preferably at the inner-rear wheel) and a drive-torque limiter so that the combined tyre force remains inside the friction ellipse sqrt(Fx2 + Fy2) ≤ mu (estimated) -TV. A state machine (ARMING, ACTIVE, STABILISATION and ABORT) activates automatically when mu (estimated) ≤ 0.05 and vehicle speed v≤ 20 km / h, and deactivates on stability restoration or driver override. The result is maintained lateral guidance and convergence of yaw / trajectory towards a desired course r_des under μ-split or polished-ice conditions. Disclosed are a method, a system, a computer program and a computer-readable medium.

Description

00269178-0016 04.12.2025 PCT / DE2025 / 000121TACTOS-5.0 - PCT Description Date: 27 Nov 2025DescriptionMethod and system for stabilising a vehicle during cornering on extremely low-friction surfaces (TACTOS-5.0 Cornering, ZZBL principle)Field of the Invention

[0001] The invention relates to vehicle dynamics control systems and methods for improving stability and lateral guidance during cornering on extremely low-friction surfaces.It particularly concerns an extension of the TACTOS-4.0 architecture to a control concept referred to as TACTOS-5.0 Cornering, based on the ZZBL (zig-zag steering motion) principle.The invention enables active stabilisation of vehicles under friction conditions of mu (estimated) ~ 0.02 - 0.05, such as mirror-smooth ice or polished road surfaces, without requiring additional sensors beyond those commonly available in ABS / ESP systems.Background of the Invention

[0002] Conventional stability systems such as ABS, TCS and ESP are primarily designed to prevent wheel lock-up or excessive yaw deviation by modulating brake pressure and torque distribution.Under extremely low friction conditions (for example, icy roads), these systems reach their functional limits because the available tyre forces are too small to maintain the desired yaw rate or lateral guidance during cornering.

[0003] Known control strategies attempt to compensate by early brake pressure build-up, torque reduction or steering interventions, but none of them generate an active lateral force modulation suitable for maintaining stable cornering at mu <0.05.In particular, under mirror-ice conditions the driver’s steering input becomes largely ineffective due to loss of tyre-road adhesion, and the vehicle tends to slide tangentially out of the curve.

[0004] Previous developments, such as the TACTOS-4.0 system, introduced a control structure capable of temporary torque decoupling and steering impulse modulation for straight-line or split-mu conditions.However, those systems did not yet provide an automatic coordination of steering, braking and torque limitation specifically for low-friction cornering situations.Page 1 to 600269178-0017 04.12.2025 PCT / DE2025 / 000121.0 “22-2025-00168178~001'7TACTOS-5.0 - PCT Description Date: 27 Nov 2025

[0005] Therefore, there is a need for a control method and system that automatically detect the onset of understeer or loss of lateral grip at extremely low friction levels, and that apply targeted steering and torque impulses to stabilise the vehicle trajectory without driver intervention.Summary of the Invention

[0006] The invention provides a control method and system that actively stabilise a vehicle during cornering on extremely low-friction surfaces.It extends the previously known TACTOS-4.0 architecture by introducing a synchronised coordination of steering, braking and drive-torque modulation.The method generates a defined sequence of steering impulses with asymmetric bias and synchronised torque interaction control between steering, braking and torque limitation.

[0007] When the estimated friction coefficient mu (estimated) drops below approximately0.05 and the vehicle speed v< 20 km / h, the control system enters an ARMING state.Upon detection of understeer or loss of lateral grip, it transitions to an ACTIVE state in which alternating steering impulses <5 mp(t) are applied around a dynamic neutral steering position <y0(t).Simultaneously, short brake-pressure pulses act on at least one inner-rear wheel, and the drive-torque is regulated so that the total tyre-force vector remains inside the friction ellipse (see Eq. 1)Mdrive=M(j • (7 — ' 3y / liy,max) (1)

[0008] The control logic monitors the yaw-rate deviation (r - r des) and the vehicle’s lateral acceleration to determine when stability is restored.If stability is achieved, or if the driver applies significant corrective steering torque, the program product switches to the STABILISATION or ABORT state and suspends the impulse control.

[0009] By cyclically applying small steering and torque impulses, the method maintains tyre adhesion and lateral guidance even under mu (estimated)-split or mirror-ice conditions.This behaviour can be compared to the directed paddle strokes of a canoe, each impulse producing a brief, corrective yaw moment that realigns the vehicle with the desired trajectory.Page 2 to 600269178-0019 04.12.2025 PCT / DE2025 / 00012104-12-2025-00269178-0019TACTOS-5.0 - PCT Description Date: 27 Nov 2025

[0015] FIG. 4 depicts the state machine of the TACTOS-5.0 control algorithm with the states OFF, ARMING, ACTIVE, STABILISATION and ABORT.Activation occurs automatically when mu (estimated) < 0.05 and v< 20 km / h; abort is triggered by rear-axle instability or driver override.

[0016] FIG. 5 shows an example of a cornering manoeuvre on a mirror-smooth road surface (mu (estimated) ~ 0.03).The dashed line indicates the actual trajectory r, the solid line the desired trajectory r des. After activation of the ZZBL control, the actual trajectory converges towards the desired path, and the vehicle maintains lateral stability even under extremely low-friction conditions.Detailed Description of the Preferred Embodiments

[0017] FIG. 1 illustrates the general structure of the TACTOS-5.0 vehicle dynamics control system.The control unit (12) receives signals from the sensor set (20), including wheel-speed sensors, steering-angle sensor, yaw-rate sensor and acceleration sensors.From these input quantities the friction estimator (22) determines the estimated friction coefficient mu (estimated).The value of mu serves as a key parameter for activating and parameterising the impulsebased stabilisation according to the ZZBL principle.

[0018] The ZZBL (zig-zag steering motion) principle relies on short, alternating steering-angle impulses around a dynamic neutral steering position <5o(t).The impulses are sharp-edged and of small amplitude, typically in the range of ± 1-2°, and are superimposed on the driver’s steering input.An asymmetric bias of approximately 60 % towards the curve direction is applied so that the generated yaw moment supports lateral guidance.

[0019] In the ACTIVE state, steering impulses Δδ_imp(t), brake-pressure pulses and drive-torque limitation are synchronised in time.The brake-pressure pulse acts preferably on the inner-rear wheel, producing a stabilisation yaw moment.The drive torque M_drive is temporarily reduced according toM drive — MQ • (7 - k(mu) • dy / Hymax) (1)where ayis the lateral acceleration and ay,maxis the maximum lateral acceleration determined from vehicle geometry and tyre data.Page 4 to 600269178-0020 04.12.2025 PCT / DE2025 / 00012104-12-2025-00269178-0020TACTOS-5.0 - PCT Description Date: 27 Nov 2025

[0020] Within the friction ellipse defined bysqrt(Fx2+ Fy2) ≤ mu (estimated) · Nthe resultant tyre-force vector remains below the adhesion limit.This ensures that longitudinal and lateral forces can coexist without exceeding the available friction potential.The algorithm continuously monitors the actual yaw rate r and compares it with the desired yaw rate r des calculated from steering angle, velocity and curvature K

[0021] When the yaw-rate deviation (r - r des) falls below a defined threshold and the lateral acceleration signal aystabilises, the program product switches from ACTIVEto STABILISATION.If the driver applies a steering input that contradicts the system intervention, or if the guardrail logic (30) detects unstable rear-axle behaviour, the system enters the ABORT state and returns control to the driver.

[0022] FIG. 2a shows the typical time sequence of the three key variables <5(t), p(t) and drive(t).The synchronisation of steering and braking impulses produces periodic yaw moments that compensate lateral-force loss.FIG. 2b demonstrates the relation between (t) and < X>(t); within the neutral range |<y- <5o| < £ (£~ 1°) no impulses are generated.When the deviation exceeds, a new impulse is triggered in the appropriate direction.

[0023] FIG. 3 illustrates the force distribution within the friction ellipse.The control acts so that the vector of combined longitudinal and lateral tyre forces remains inside the stable area of adhesion.The use of small, frequent corrections prevents the tyre from leaving the stable region and thus maintains controllability on icy or highly polished surfaces.

[0024] FIG. 4 shows the state machine that manages the transitions between OFF, ARMING, ACTIVE, STABILISATION and ABORT.The thresholds for activation and deactivation are stored in calibration tables depending on mu (estimated) and vehicle speed v.This finite-state logic guarantees reliable operation without oscillation or unwanted re¬ activation.Page 5 to 600269178-0021 04.12.2025 PCT / DE2025 / 000121J04- 1 ^2025"-00269178-0021TACTOS-5.0 - PCT Description Date: 27 Nov 2025

[0025] FIG. 5 presents a vehicle trajectory example.The dashed line represents the actual path r, the solid line the desired path r des.Each steering impulse corresponds to a brief corrective yaw moment similar to a paddle stroke of a canoe, progressively aligning the vehicle with its intended course.Through these cyclic corrections the system maintains both lateral stability and driver confidence even at extremely low friction levels.

[0026] The invention may be realised purely as a software extension of an existing stability control unit.No additional sensors are required, and the algorithm can be integrated into current ESP or ABS program product by software update.A computer program product containing the corresponding code sections is therefore also part of the invention.Page 6 to 600269178-0022PCT / DE2025 / 000121Reference Signs List (TACTOS-5.0 - PCT Application)Ref. No. Description10 Vehicle (overall system)12 Control unit (TACTOS-5.0 controller)14 Steering actuator / EPAS system16 Brake actuator / ESC or IBC system18 Drive-train actuator (motor / inverter / eAWD)20 Sensor system (wheel-speed, steering-angle, yaw-rate, acceleration sensors)22 Friction estimator (determining / / (estimated))24 ZZBL impulse generator (steering impulse module)26 Brake-pulse unit (inner-rear wheel single-wheel actuation)28 Drive-torque limiter (torque-modulation module)30 Guardrail logic (safety and abort conditions)32 HMI module (driver information “Mirror-ice detected - ZZBL active”)34 Yaw rate r (actual value)36 State: OFF38 State: ARMING39 State: ACTIVE40 State: STABILISATION41 State: ABORT42 Indicator “ZZBL active - stabilisation”6 Steering angle) Dynamic neutral steering position8 imp(t) Steering-impulse deviationNeutral steering tolerance band^(0 Brake-pressure profile■ irive(t) Drive torqueFxLongitudinal tyre forceFy Lateral tyre forceN Normal forcer Actual yaw rate00269178-0023 04.12.2025 PCT / DE2025 / 000121-22- G2 ">002g 17S-0023rdesDesired yaw ratev Vehicle speed / / (estimated) Estimated friction coefficientK Path curvatureay, ay,max Lateral acceleration / maximum lateral acceleration A, AoSteering impulse amplitude / base amplitude f, f0Steering-impulse frequency / base frequency ( / / ) Friction-dependent correction factort Time1 Steering-impulse waveform (FIG.2a)2 Brake-pressure waveform (FIG. 2a)3 Drive-torque waveform (FIG. 2a)4 Curve-direction bias indicator5 Time axis6 Brake-pulse envelope (FIG. 2a)7 Torque-reduction phase (FIG. 2a)8 Diagram reference for FIG. 2b9 Actual steeringcurve10 Curve direction (FIG. 2b)11 Impulse sequence around <f0(t)12 Tyre-force vector (FIG. 3)13 Longitudinal-force axis Fx14 Lateral-force axis Fy15 Rear-axle instability signal16 Transition signal to ARMING17 Threshold condition ( / z and v) for activation 18 Road course / lane boundary (FIG. 5)19 Driving direction arrow (FIG. 5)20 Actual path r(FIG. 5)21 Desired path r des (FIG. 5)22 Estimated friction zone (FIG. 5)

Claims

00269178-0025 04.12.2025 PCT / DE2025 / 000121TACTOS-5.0 - PCT Claims Date: 27 Nov 2025Claim 1 (Independent claim - method)A method for stabilising a road vehicle when cornering on very low friction surfaces, in particular as a module within the TACTOS vehicle-dynamics system,characterised in that- when understeering is detected and the estimated friction coefficient ^(estimated) is < 0.05 and the vehicle speed pis < 20 km / h,- frequent, amplitude-limited steering impulses are generated via the steering actuators, the impulses having a frequency of 3 - 5 Hz, a road-wheel amplitude of 1.5 - 3.5°, and a time bias of > 60% in the curve direction,- and simultaneously the drive torque is controlled in accordance with equation (1) in the description such that the friction ellipse is not exceeded and lateral guidance is prioritised, wherein the target rotational speeds of the individual wheels are determined from the geometric parameters of the curve and from the steering angle.Claim 2The method according to claim 1,characterised in thatthe steering impulses are generated as sharp-edged step impulses with a slew rateof 60 - 1807s, wherein the time share in the curve direction amounts to 60 - 65%, and wherein the steering impulses are generated relative to a dynamic neutral steering position A(t), which is derived from a driver steering angle and / or an external lane reference, and wherein, after each impulse, the steering is returned to a neutral range 16- <5o| < £ without passing through the opposite steering direction.Claim 3The method according to any of the preceding claims,characterised in thatsynchronised brake impulses are generated at an inner-rear wheel with a duration of80 - 120 ms and with a low pressure level in accordance with a predefined pressure-time characteristic.Claim 4The method according to any of the preceding claims,characterised in thatdrive torque and recuperation torque are treated identically, and in all-wheel-drive vehicles are distributed such that additional torque is provided at the outer rear wheel as long as the friction ellipse is respected.00269178-0026 04.12.2025 PCT / DE2025 / 00012104-12-2025-00269178-0026TACTOS-5.0 - PCT Claims Date: 27 Nov 2025Claim 5The method according to any of the preceding claims,characterised in thatthe method is activated only when understeering, friction-threshold and speed-threshold conditions are simultaneously present, activation taking place in time windows of 0.5 - 1.5 s with subsequent re-evaluation.Claim 6The method according to any of the preceding claims,characterised in thatwhen rear-axle instability is detected, the steering impulses are immediately deactivated and the drive torque is reduced to a drag-torque level.Claim 7 (System claim)A system,characterised in thata vehicle-dynamics control system comprises sensors for yaw rate, lateral and longitudinal acceleration, wheel rotational speeds and steering angle, a friction estimator, an EPAS actuator, a brake actuator, a drive-train actuator, and a control unit configured to carry out the method according to any of claims 1-6.Claim 8 (Computer program)A computer program,characterised in thatthe computer program comprises program-code means which, when executed on a computing unit of the vehicle, perform the steps of the method according to any of claims 1-6.Claim 9 (Computer-readable medium)A data carrier,characterised in thata non-transitory, machine-readable data carrier contains the computer program according to claim 8.

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