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Actuator Parameters

Internal Control Loop of Motor Driver

motor_circuit

Current Limit and Current Limit Margin

You can understand current limit and current limit margin as:

"Target current limit" + "protective redundancy margin"

Together, they define the safe operating range of the current controller.

📌 Basic Concepts

Current Limit

👉 The maximum current that the controller is allowed to output under normal operation

Current Limit Margin

👉 A reserved "buffer space" for the controller to handle transient fluctuations / control errors

Actual Hard Limit (Absolute Upper Bound)

current_lim + current_lim_margin

👉 Exceeding this limit will trigger protection (e.g., error / current limit violation)

Block Diagram (Logical Structure)

current_lim

🧩 Detailed Explanation

1️⃣ Role of current_lim (Soft Limit)

In the control loop (FOC):

Iq_cmd = clamp(Iq_target, -current_lim, +current_lim)

👉 The Iq setpoint is constrained within ±current_lim

2️⃣ Role of current_lim_margin

Used to detect whether the "actual measured current" is abnormal

Possible sources of error: - ADC error - PWM delay - Back EMF - Dynamic disturbances

👉 Therefore, short-term exceeding of current_lim is allowed

3️⃣ Protection Trigger Condition

|Iq_measured| > current_lim + current_lim_margin

👉 This will trigger:

ERROR_CURRENT_LIMIT_VIOLATION

📊 Example

Assume:

  • current_lim = 40 A
  • current_lim_margin = 8 A
Range Meaning
0 ~ 40A Normal operation
40 ~ 48A Transient allowed (margin zone)
>48A ❌ Protection triggered

⚠️ Engineering Significance

current_lim determines

👉 Maximum motor output torque

Torque ∝ Iq

current_lim_margin determines

👉 System stability / robustness

  • Too small: prone to false protection triggers
  • Too large: may cause overcurrent damage to MOSFETs or motor

🧠 Summary

  • current_lim → "desired maximum current"
  • current_lim_margin → "allowed transient overshoot range"

🚀 Engineering Recommendations

Applicable for:

  • High dynamic motion (jumping / locomotion)
  • Strong nonlinear control (RL control)

Recommended configuration:

  • Set margin to 20% ~ 30%
  • Combine with:
    • Bus current limit
    • Temperature protection
    • Torque limiter