Current Limits

Current limits protect motors and electrical components from damage and optimize robot performance.

Stator Current Limit

Definition: Limits the current flowing through the motor windings. This current is directly proportional to torque output.

Why it matters:

• Prevents motors from overheating.

• Controls the torque applied by the motor.

• Useful for mechanisms where excessive torque causes mechanical damage (e.g., arms, elevators, shooters).

Use when:

• You want to cap torque output.

• Thermal protection is important.

• You’re tuning for consistent mechanism response.

---

Supply Current Limit

Definition: Limits the current drawn from the battery into the motor controller. This affects the total load on the power distribution system.

Why it matters:

• Helps prevent brownouts and voltage drops during high-load events.

• Manages battery performance and electrical stability.

• Useful for subsystems that run simultaneously, such as drivetrains under defense or full-speed paths.

Use when:

• You want to prevent robot-wide brownouts.

• You’re tuning drivetrain acceleration and stability.

• You’re optimizing for electrical system reliability.

---

3. How the Two Limits Interact

CTRE motor controllers act like buck converters. They step down voltage and trade current for torque. This is governed by the power balance:

$$\frac{V_{\text{supply}}}{I_{\text{supply}}} \approx \frac{V_{\text{stator}}}{I_{\text{stator}}}$$

This relationship explains why:

• Stator current is often higher than supply current when motors are operating at low duty cycles.

• Supply current limits affect overall power draw, while stator current limits affect torque.

---

Tuning

Replace battery before every test for accurate results

Drivetrain

1. Start off with 80A supply limit, 120A stator limit

2. Test Stator: Command full throttle while against a sturdy wall, if the wheels slip than stator limit is too high

3. Tune: Iterate on the limit, there is an issue if it is below 90A

4. Test Supply : You should be able to drive for 2 matches straight without mechanisms before browning out

5. Tune: Iterate on the limit, there is an issue if it is below 50A

Mechanisms/Rollers

1. Start off with 40A stator limit and a 20A Supply Limit

2. Tune Stator: Temp, Torque

3. Tune Supply: Should end the match on the verge of brownout (Max performance)