Regenerative Braking

Anoop Koganti

Goal

Design a mapped braking system that properly integrates motor braking for optimal regenerative braking while providing a validated and tuned transition from motor braking to mechanical braking.

High-Level Overview

System Flow

Brake System Encoder
        ↓
      CCM
        ↓
Motor Regen Command
        ↓
Motor Braking (0-20% Encoder Travel)
        ↓
Mechanical Braking (>20% Encoder Travel)
        ↓
Brake Pressure Generated Through Excess Slack

Concept

• The brake encoder is used as the primary input.
• The CCM reads the encoder value and determines the requested braking level.
• The first 20% of calibrated brake travel is dedicated to regenerative braking.
• Regen torque is linearly mapped from 0% to 100% over this range.
• After 20% brake travel, regen remains at its maximum value.
• Additional brake pedal travel engages the mechanical braking system through intentional slack in the brake linkage.
• Mechanical brake pressure must be tuned to account for the additional braking force produced by the motor.

Update Function

int BrakesRegen(int EncoderADC);

Function Purpose

• Takes brake encoder ADC readings as input.
• Maps encoder position from the calibrated 0-20% brake travel range.
• Outputs a regenerative braking request from 0-100%.
• Sends the corresponding negative torque request to the motor controller.

Example Mapping

Mechanical Braking Considerations

Brake Bias Tuning

The rear motor contributes braking torque during regenerative braking. Mechanical brake pressure must therefore be tuned such that total vehicle braking remains balanced.

Areas requiring validation:

• Front-to-rear brake balance
• Vehicle stability during braking
• Smooth transition from regen-only braking to regen + mechanical braking
• Driver pedal feel

Calibration Approach

• Begin with conservative regen values.
• Incrementally increase regen contribution.
• Record stopping behavior and vehicle response.
• Adjust brake pressure and slack distance as necessary.
• Validate consistency across multiple runs.

Initial Test Cases

Test 1: Encoder Range Mapping

Input

Encoder values corresponding to:

• 0%
• 5%
• 10%
• 15%
• 20%

Expected Result

Regen command increases linearly from 0% to 100%.

Verification

Confirm motor controller receives the expected negative torque command.

Test 2: Regen Saturation

Input

Encoder values greater than 20% travel.

Expected Result

Regen remains capped at 100%.

Verification

Motor torque command does not increase beyond the calibrated maximum.

Test 3: No Brake Input

Input

0% brake travel.

Expected Result

0% regenerative braking.

Verification

No negative torque command is sent to the motor.

Test 4: Mechanical Brake Transition

Input

Brake pedal moved from below 20% to above 20%.

Expected Result

• Regen reaches maximum at 20%.
• Mechanical brakes begin contributing after slack is removed.
• Braking force increases smoothly.

Verification

No sudden jump in braking force.

Test 5: Low-Speed Behavior

Input

Brake applied while vehicle speed approaches zero.

Expected Result

Regen is reduced or disabled at low speed.

Verification

Vehicle comes to a smooth stop without oscillations or jerking.

Test 6: BMS Fault Handling

Input

BMS reports a charging fault or disables charging.

Expected Result

• Regen command immediately becomes zero.
• Mechanical braking remains available.

Verification

• No negative motor torque is requested.
• Vehicle can still brake mechanically.

Test 7: Motor Controller Fault Handling

Input

Motor controller fault condition.

Expected Result

• Regen command immediately becomes zero.
• Mechanical braking remains available.

Verification

• No regenerative torque request is sent.
• Vehicle can still brake mechanically.

Test 8: Repeatability Test

Input

Repeated brake applications at identical encoder positions.

Expected Result

Same encoder value produces the same regen command each time.

Verification

Mapping remains stable and repeatable.

Future Work

• Determine optimal regen percentage range.
• Determine ideal slack distance before mechanical brake engagement.
• Tune brake bias for motor-assisted braking.
• Add battery current limiting.
• Add state-of-charge-based regen limiting.
• Add thermal derating based on motor and battery temperature.
• Collect braking data for validation and tuning.