Thermal Derating Module

Author: Atharva Rao

Purpose

Prevent the BMS and AIRs from triggering a hard reset by reducing torque and current limits before hardware-enforced thresholds are reached. Derating is applied across the battery, motor, and inverter, which takes effect on every torque command sent to the DTI.

How It Works

VCU_Derate(float temperature) returns a scalar factor in [0.2, 1.0] using piecewise linear interpolation between two temperature thresholds:

factor = 1.0 − (0.8) × (temp − TEMP_START) / (TEMP_MAX − TEMP_START)

Below TEMP_START the factor is 1.0 (no derating). Above TEMP_MAX the input is clamped and the factor floors at 0.2.

Integration

Three factors are computed independently each VCU loop iteration and the minimum is used:

float batteryFactor  = VCU_Derate(BMS_GetOrionData()->highTemp);
float motorFactor    = VCU_Derate(DTI_GetDTIData()->motorTemp);
float inverterFactor = VCU_Derate(DTI_GetDTIData()->controllerTemp);

float smallestFactor = min(batteryFactor, min(motorFactor, inverterFactor));

DTI_SetDCLimits(60.0 * smallestFactor, -2.0);
DTI_SetACLimits(150.0 * smallestFactor, -20.0);
DTI_SendAccelCommand(targetTorque * smallestFactor);

Both the CAN-sent current limits and the torque command are scaled by the same factor, so all three limit layers (BMS config, DTI config, CCM CAN override) are adjusted together.

Limitations

• A single linear ramp is used for all three thermal sources, no per-component curves.
• All temperature thresholds are placeholder (TEMP_START, TEMP_MAX, MIN_FACTOR) which require adjustments.