Electronic Stability Systems (TCS and YSC)¶

SIMON models two electronic stability systems as part of each vehicle's drivetrain: Traction Control (TCS) and Yaw Stability Control (YSC), collectively referred to as the vehicle's ESS (Electronic Stability Systems). Each system is enabled individually per vehicle and configured with its own activation thresholds and rates in the Vehicle Editor. The Anti-Lock Brake System (ABS) is a separate model, described with the brake system; where both are present, YSC acts on the brake pressures first and ABS then modulates the result.

The ESS model is structured the way a production controller is: a sensor layer synthesizes the signals an ECU would measure — a speedometer signal, a yaw-rate signal, and per-wheel speed signals — and two control laws act on the resulting errors. TCS intervenes by redistributing drive torque; YSC intervenes by redistributing drive torque and modulating individual wheel brake pressures.

The description below reflects the behavior of the current version of HVE.

Sensor models¶

The sensor signals are recomputed on every pass through the wheel-torque calculation:

Speedometer. The vehicle speed signal \(V\) is the average of the wheel-speed sensors on the drive wheels (spin velocity × tire static loaded radius).
Yaw-velocity error. The expected yaw velocity follows from the driver's steering input and the vehicle geometry,

\[ \dot\psi_{expected} = \frac{V \tan\delta}{L} \]

where \(\delta\) is the front-wheel steer angle (steering-wheel angle ÷ column ratio) and \(L\) is the wheelbase computed from the axle positions (for a 3-axle vehicle, the distance to the midpoint of the rear tandem). The yaw velocity error is the sensed yaw velocity minus this expectation:

\[ e_{\dot\psi} = \dot\psi_{sensor} - \dot\psi_{expected} \]

A positive error means the vehicle is rotating faster than the driver's steering asks for.

Per-wheel spin comparisons. For each wheel, the expected spin velocity accounts for the wheel's lateral position \(y\) during a turn:

\[ \omega_{expected} = \frac{V - \dot\psi\, y}{R_{SLR}} \]

where \(R_{SLR}\) is the tire static loaded radius. From the wheel-speed sensors the model then forms, per wheel: the spin-velocity error (\(\omega_{sensor} - \omega_{expected}\)), the difference from the other side of the same axle, the difference from the axle average, the difference between axle averages (axle-to-axle), and the difference from the whole-vehicle average.

Traction Control (TCS)¶

TCS runs during the drive-torque split calculation and is considered only when the total drive torque (engine torque × transmission ratio × final drive ratio) is at or above the Minimum Drive Torque threshold. Starting from an even split among the drive axles, it makes two corrections:

Axle-to-axle. If the average wheel speed of a drive axle differs from another drive axle's average by at least the Axle Difference threshold, that axle's torque fraction is reduced in proportion to the difference:

\[ S_{axle} \mathrel{-}= R_{Tq}\,\Delta\omega_{axle} \]

where \(R_{Tq}\) is the user-set Drive Torque Rate. The fraction is clamped between 0 and 1, so torque migrates away from a spinning axle toward the axle(s) with traction.

Side-to-side. If the left and right wheel speeds on a drive axle differ by at least the Side Difference threshold, the axle's torque is re-split between the sides. The right-side share is adjusted by \(R_{Tq}\,S_{axle}\,\Delta\omega_{side}\) — proportional to both the axle's current torque fraction \(S_{axle}\) and the right-minus-left wheel speed difference — and the left side receives the remainder, clamped so neither side goes below zero or above the axle's total. This mimics a brake- or clutch-based limited-slip intervention.

When either correction fires, the TCS active flag is set — the equivalent of the TCS indicator lamp — and reported in the Variable Output.

Yaw Stability Control (YSC)¶

YSC runs per wheel inside the brake-pressure calculation, after the pedal force, proportioning, push-out and rise-time effects and before ABS. It is gated twice:

The vehicle speed must be at or above the Minimum Velocity threshold.
Either the yaw-velocity error magnitude must reach the Yaw Velocity Error threshold, or the wheel's spin-velocity error magnitude must reach twice the Spin Velocity Error threshold.

Once active, YSC makes two interventions:

Drive-torque side shift. If drive torque is at or above the Minimum Drive Torque threshold and the axle's side-to-side wheel speed difference reaches the Side Difference threshold, the wheel's share of the axle torque is adjusted by \(R_{Tq}\,S_{axle}\,\Delta\omega_{side}\) (proportional to the axle's torque fraction, as in TCS), clamped to the axle total. Each side is adjusted using its own signed side difference, so the two sides' corrections mirror one another: the faster side sheds torque share and the slower side gains it, each clamped independently.
Brake modulation. If the driver's brake system pressure is at or above the Minimum Brake Pressure threshold and the wheel's spin-velocity error magnitude reaches the Spin Velocity Error threshold, the wheel's brake pressure is adjusted in proportion to the error:

\[ P \mathrel{+}= R_{P}\, e_{\omega} \]

where \(R_P\) is the Brake Pressure Rate. A wheel spinning faster than expected receives more brake pressure; a wheel turning slower than expected (approaching lock) receives less. Pressure is floored at zero.

When either intervention fires, the YSC active flag is set — the YSC indicator lamp — and reported in the Variable Output.

User parameters¶

Both systems are configured in the Vehicle Editor's drivetrain ESS dialog. The thresholds gate when the systems may act; the rates set how aggressively they act per unit of error.

Parameter	Used by	Meaning
Minimum Velocity	YSC	Vehicle speed below which YSC never intervenes
Yaw Velocity Error	YSC	Yaw-rate error that activates YSC
Spin Velocity Error	YSC	Wheel spin error for brake modulation (2× this value also activates YSC)
Side Difference	TCS, YSC	Left/right wheel-speed difference that triggers a side torque shift
Axle Difference	TCS	Axle-to-axle average speed difference that triggers an axle torque shift
Minimum Drive Torque	TCS, YSC	Drive torque below which no torque redistribution occurs
Minimum Brake Pressure	YSC	System brake pressure below which no brake modulation occurs
Drive Torque Rate	TCS, YSC	Change in torque split per unit wheel-speed error
Brake Pressure Rate	YSC	Change in brake pressure per unit wheel-speed error

Outputs¶

The Variable Output drivetrain group includes the TCS and YSC active flags, so the timing of each intervention can be plotted against the vehicle response — the simulation equivalent of watching the dash lamps.
The Program Data report lists the vehicle's ESS configuration: which systems are fitted, and the threshold and rate values in effect for the run.

Notes and limitations¶

The vehicle data structure also carries an ESP (Electronic Stability Program) flag, but no combined ESP control law is implemented in the current version of HVE — TCS and YSC are the two active systems. In particular there is no engine-torque cut and no yaw-moment-targeted braking of a specific corner based on the yaw error alone.
TCS is purely wheel-speed based; the yaw-velocity error influences only YSC, and there only as an activation gate — the magnitude of every correction is driven by wheel-speed errors.
The sensor models are idealized: the yaw-rate signal is the true simulated yaw velocity, with no noise, bias, or latency.
The corrections are proportional with clamping. There is no integral or derivative action, and interventions persist only as long as the triggering error persists.

← Previous: Chapter 8 — Index | Index