Traction Control Module

Author: Atharva Rao

Purpose

The traction control module prevents rear-wheel spin during acceleration by reducing the torque request sent to the inverter. It compares front (undriven) wheel speeds to the motor-derived rear wheel speed to detect slip, then applies a PID correction when slip exceeds a defined ratio.

Control Method

Slip Calculation

Slip is computed in VCU_GetSlip() using motor eRPM as the driven wheel speed reference, which is calculated through pole pairs and gear ratio:

motor_speed = (eRPM / POLE_PAIRS) / GEAR_RATIO
slip = (motor_speed − front_wheel_avg) / front_wheel_avg

The calculation also consists of an additional step: if the two front wheel speeds differ by more than MAX_SPEED_DIFF (35 RPM), the lower reading is used instead of the average to guard against a faulty sensor pulling the wheel speed reference high.

If the front wheel average falls below MIN_SPEED (5 RPM) — i.e., the car is nearly stationary, the function returns the target slip ratio directly, avoiding a division-by-near-zero.

PID Torque Reduction

The PID controller targets SLIP_RATIO (0.10) as its setpoint. Its output is a multiplicative reduction applied to the driver’s requested torque:

target = requested_torque − (requested_torque × PID_output)

This means that the correction scales with the driver’s pedal demand. For example, a large torque request gets a larger absolute cut when the calculated slip is high.

Note: The PID output is constrained so the torque cannot be increased beyond what the driver requested (This is implemented to abide by the rules).

Test Cases

Constants assumed: POLE_PAIRS = 10, GEAR_RATIO = 3, MIN_SPEED = 5 RPM, MAX_SPEED_DIFF = 35 RPM, SLIP_RATIO = 0.10, TC_KP = 0.5, TC_KI = 0.01, TC_KD = 0.0

Test Case 1 — No Slip

WSS1 = 100 RPM, WSS2 = 100 RPM, eRPM = 300000

requested_torque = 200 Nm

slip = (100 − 100) / 100 = 0.00

PID output = 0.5 × (0.00 − 0.10) = −0.05 → clamped to 0

torqueDemand = 200 Nm (unchanged)

Test Case 2 — Slip At Threshold

WSS1 = 100 RPM, WSS2 = 100 RPM, eRPM = 330000

requested_torque = 200 Nm

motor_speed = (330000 / 10) / 3 = 110 RPM

slip = (110 − 100) / 100 = 0.10

PID output = 0.5 × (0.10 − 0.10) = 0.00

torqueDemand = 200 Nm (unchanged)

Test Case 3 — Moderate Slip

WSS1 = 100 RPM, WSS2 = 100 RPM, eRPM = 360000

requested_torque = 200 Nm

motor_speed = (360000 / 10) / 3 = 120 RPM

slip = (120 − 100) / 100 = 0.20

PID output = 0.5 × (0.20 − 0.10) = 0.05

torqueDemand = 200 − (200 × 0.05) = 190 Nm

Test Case 4 — Heavy Slip

WSS1 = 40 RPM, WSS2 = 42 RPM, eRPM = 300000

requested_torque = 200 Nm

motor_speed = (300000 / 10) / 3 = 100 RPM, front_avg = 41 RPM

slip = (100 − 41) / 41 = 1.44

PID output = 0.5 × (1.44 − 0.10) = 0.67

torqueDemand = 200 − (200 × 0.67) = 66 Nm

Current Limitations

• PID gains (TC_KP, TC_KI, TC_KD) are placeholder values and require tuning.
• The target slip ratio (0.10) might require tweaking for effective traction control.
• Testing is yet to be done to ensure everything works well.