Understanding Stall in Motors — Lessons for Robotics and Arduino Projects

 Introduction

In robotics and embedded systems, motors are the muscles that bring mechanical designs to life. Whether you’re building a robotic arm, a mobile rover, or a servo-controlled pan-tilt camera, motors are everywhere. But motors are not invincible. One of the most important—and often misunderstood—concepts in motor control is stall.

Stall occurs when a motor is powered but unable to rotate due to excessive load or mechanical blockage. For hobbyists working with Arduino Uno and small motors like the SG90 servo or DC gear motors, understanding stall is essential to prevent burnt drivers, overheated coils, or broken gearboxes.

This blog explores stall in detail: what it means, why it matters, how to detect it, and how to design robotics projects that avoid or manage stall conditions.

What is Stall?

• Definition: Stall is the condition when a motor’s shaft speed drops to zero while voltage is applied.
• Stall Torque: The maximum torque a motor can produce at zero speed.
• Stall Current: The maximum current drawn when stalled, often several times higher than normal operating current.

In simple terms: imagine trying to push a heavy door with a weak motor. If the door doesn’t move, the motor is stalled.

Why Stall Matters in Robotics

Robotics projects often involve motors working against loads: lifting arms, driving wheels, or rotating sensors. If the load is too heavy or the mechanism jams, stall occurs.

Consequences of stall in robotics:

• Overheating: Continuous stall current generates heat in motor windings.
• Electronics damage: Motor drivers or Arduino boards may reset or burn out.
• Mechanical wear: Gears in servos can strip under stall torque.
• Robot failure: A stalled motor means the robot cannot complete its task.

For example, in a robotic arm powered by SG90 servos, if the arm tries to lift an object heavier than its torque rating, the servo stalls. The Arduino continues sending signals, but the servo buzzes without moving.

Stall in Different Motor Types

1. DC Motors

• At stall, current is maximum because back EMF = 0.
• Stall torque is the highest torque the motor can produce.
• Prolonged stall damages windings and drivers.

2. Stepper Motors

• Stall occurs when load torque exceeds available torque per step.
• Symptoms: missed steps, loss of synchronization.
• In Arduino projects, stall detection can be implemented using current sensors or encoders.

3. Servo Motors (SG90)

• Stall occurs when the shaft is blocked from reaching the commanded angle.
• The servo draws high current, gears strain, and buzzing noise appears.
• Arduino users often encounter stall when forcing servos against mechanical stops.

Detecting Stall in Robotics

Methods to detect stall:

• Current sensing: Stall current spikes can be measured with sensors.
• Encoder feedback: If commanded steps don’t match actual rotation, stall is detected.
• Software logic: Arduino can monitor time taken for movement; if no change occurs, assume stall.

For example, Arduino forums discuss detecting stall in stepper motors using driver shields and current monitoring.

Arduino and Stall Management

Arduino Uno is often used to control motors via drivers or libraries. But Arduino itself cannot handle stall currents directly—it must rely on external hardware.

Best practices:

• Use motor driver ICs (L298N, TB6612, TMC series) with current limiting.
• Add fuses or polyfuses to protect against stall current.
• Implement software stall detection: stop sending signals if no movement is detected.
• Use external power supplies for motors; don’t power them directly from Arduino’s 5 V pin.

Case Study: SG90 Servo Stall

The SG90 micro servo is popular in Arduino robotics. It has a torque rating of ~1.8 kg·cm at 5 V.

• If you attach a long lever arm and try to lift a heavy object, torque demand exceeds 1.8 kg·cm.
• The servo stalls, buzzes, and draws high current.
• Arduino may reset if powered from USB due to voltage sag.

Solution:

• Keep loads within torque rating.
• Use external 5 V supply with sufficient current.
• Avoid forcing servo beyond 0–180° mechanical limits.

Stall Detection in Robotics Competitions

In robotics competitions (like FLL or DIY Arduino challenges), stall detection is critical. EV3 lessons highlight how stall detection helps robots recover from failures. Instead of waiting indefinitely, robots can skip tasks or retry when stall is detected.

Arduino projects can adopt similar logic:

• Monitor motor current.
• If stall detected, stop motor and alert user.
• Retry or move to next task.

Preventing Stall in Arduino Robotics

Design strategies:

• Choose correct motor size: Match torque rating to load.
• Gear reduction: Use gearboxes to increase torque at the cost of speed.
• Limit mechanical load: Avoid overloading arms or wheels.
• Add sensors: Use encoders or current sensors for feedback.
• Software safeguards: Implement timeouts and stall recovery routines.

Advanced Stall Management

• Predictive control: Use PID controllers with feedback to avoid stall.
• Thermal monitoring: Add temperature sensors to detect overheating.
• Adaptive algorithms: Adjust motor current dynamically based on load.

Arduino can integrate these techniques with libraries and shields, making robotics safer and more reliable.

Conclusion

Stall is not just a technical term—it’s a real-world challenge in robotics and Arduino projects. Understanding stall torque, stall current, and detection methods can save your motors, electronics, and hours of frustration.

By designing with stall in mind—choosing the right motors, adding sensors, and writing smart Arduino code—you can build robots that are resilient, efficient, and long-lasting.