Comparing SG90 and MG90S Micro Servo Motors for Robotic Arms

When building robotic arms—whether for industrial automation, educational projects, or hobbyist creations—the choice of servo motor can make or break your design. Among the most popular micro servos in the maker community are the SG90 and MG90S. At first glance, they appear nearly identical: both are compact, affordable, and ubiquitous in DIY projects. But beneath their similar exteriors lie critical differences that directly impact performance in robotic applications.

Understanding Micro Servo Motors: Why Size and Precision Matter

Micro servos like the SG90 and MG90S belong to a category of rotary actuators designed for precise control of angular position. Typically operating within a 180-degree range, these devices combine a DC motor, gear train, potentiometer, and control circuitry in a housing barely larger than a sugar cube. For robotic arms—where space constraints, weight distribution, and positional accuracy are paramount—micro servos offer an attractive balance of performance and miniaturization.

Core Components Breakdown

• DC Motor: Provides the raw rotational force.
• Gear Train: Transfers and modifies the motor's torque/speed.
• Potentiometer: Provides real-time feedback on output shaft position.
• Control Circuit: Compares desired position (via PWM signal) with actual position and adjusts the motor accordingly.

SG90: The Hobbyist's Workhorse

The SG90 has earned its reputation as the "go-to" servo for beginners and low-demand applications. Weighing just 9 grams and measuring 22.2 x 11.8 x 31 mm, its tiny form factor makes it ideal for applications where space is at a premium.

Performance Specifications

• Operating Voltage: 4.8V ~ 6.0V
• Stall Torque: 1.8 kg·cm (4.8V) / 2.2 kg·cm (6.0V)
• Speed: 0.1 sec/60° (4.8V) / 0.08 sec/60° (6.0V)
• Gear Material: Primarily plastic (Nylon or ABS composites)

Strengths in Robotic Arm Context

• Cost-Effectiveness: Its rock-bottom price makes it feasible to build multi-jointed arms without breaking the bank.
• Lightweight: Minimizes the overall weight of the arm structure, reducing the load on preceding servos.
• Adequate for Low-Load Applications: Perfect for finger joints, wrist movements, or small-scale educational arms that handle very light payloads.

Notable Limitations

• Plastic Gears: The most significant weakness. Under sudden loads or stalls, the gears are prone to stripping, leading to failure.
• Torque Inconsistency: The output torque can be insufficient for lifting any meaningful payload, especially when the arm is fully extended.
• Durability Concerns: Not designed for continuous, high-stress operation. The motor can overheat, and positional jitter is common over time.

MG90S: The Metal-Geared Upgrade

The MG90S is often positioned as a direct, drop-in replacement for the SG90, but with one critical enhancement: metal gears. This single change dramatically alters its performance profile and suitability for more demanding tasks.

Performance Specifications

• Operating Voltage: 4.8V ~ 6.0V
• Stall Torque: 2.2 kg·cm (4.8V) / 2.8 kg·cm (6.0V)
• Speed: 0.1 sec/60° (4.8V) / 0.08 sec/60° (6.0V)
• Gear Material: Metal (typically brass and steel)

Strengths in Robotic Arm Context

• Durability and Resilience: The metal gears can withstand sudden shocks, stalls, and continuous high-load situations without stripping. This is its single biggest advantage.
• Higher Torque Output: The slightly higher stall torque provides more "muscle" for lifting payloads and maintaining position under load.
• Reliability: Better suited for the core joints of a robotic arm, such as the shoulder and elbow, where mechanical stress is highest.

Trade-offs to Consider

• Higher Cost: The metal gearing comes at a premium, typically costing 50-100% more than the SG90.
• Increased Weight: Metal gears add a few grams of mass, which can slightly impact the design of the arm.
• Potential for Motor Burnout: While the gears are robust, the DC motor itself can still be overwhelmed if pushed beyond its limits for extended periods.

Head-to-Head Comparison: A Detailed Analysis

To make an informed decision, let's break down their performance across key metrics relevant to robotic arm design.

Torque and Lifting Capacity

While the specifications show the MG90S with only a marginal torque advantage, the real-world difference is more pronounced. The MG90S can sustain its rated torque without fear of gear damage. In a robotic arm, the torque required at a joint is a function of the payload weight and the distance from the joint (lever arm). For any joint bearing a significant load, the MG90S's reliable torque delivery is non-negotiable.

Verdict: MG90S is the clear winner for load-bearing applications.

Speed and Responsiveness

On paper, their speeds are identical. In practice, the MG90S may exhibit slightly smoother movement under load due to its more robust gearing, which has less backlash and flex than plastic gears. However, for most practical purposes, the speed is comparable.

Verdict: Tie. Both servos offer similar speed for positional changes.

Durability and Lifespan

This is the most significant differentiator. * SG90: Plastic gears wear down over time and can fail catastrophically if the servo is obstructed. Its lifespan is suitable for intermittent, low-stress use. * MG90S: Metal gears are resistant to wear and can handle impacts and stalls that would destroy an SG90. It is built for longevity and repeated use in demanding conditions.

Verdict: MG90S is vastly superior for any application requiring reliability.

Precision and Positional Accuracy

Both servos use a similar potentiometer and control circuit, offering comparable theoretical accuracy. However, the MG90S often demonstrates better holding strength. An SG90 might "sag" under a load it cannot quite handle, leading to a loss of positional accuracy, whereas an MG90S is more likely to hold its commanded position firmly.

Verdict: MG90S has a slight edge due to better performance under load.

Power Consumption and Thermal Management

Both servos draw similar current when unloaded. Under load, the SG90's plastic gears can flex and bind, sometimes leading to higher current draw and motor overheating as it struggles to reach the commanded position. The MG90S, with its more efficient power transfer through metal gears, can often run cooler under the same conditions, though its motor is still the ultimate limit.

Verdict: MG90S has a slight advantage in thermal performance under load.

Cost and Value Proposition

• SG90: Unbeatable on initial cost. The best choice for prototyping, learning, or applications where failure is low-risk.
• MG90S: Higher initial investment, but offers far better value in the long run for a finished, functional robotic arm that needs to work reliably.

Verdict: SG90 wins on price; MG90S wins on value.

Strategic Selection: Which Servo for Which Part of the Robotic Arm?

A well-designed robotic arm often uses a mix of servos to optimize performance and cost. Here’s a strategic approach:

Base and Shoulder Joints (High-Stress Areas)

These joints bear the weight of the entire arm and its payload. They require maximum torque and durability. * Recommended Servo: MG90S. The metal gears are essential to handle the constant stress and potential for shock loading.

Elbow Joint (Moderate Stress)

This joint supports the forearm, wrist, and payload. It requires a good balance of torque and reliability. * Recommended Servo: MG90S. While an SG90 might work for a very light arm, the MG90S is the safer, more reliable choice.

Wrist and Gripper Joints (Lower Stress)

These joints manage finer movements and lighter loads. Precision and speed can be more critical than raw torque. * Recommended Servo: SG90 or MG90S. This is the decision point. If the gripper is handling fragile objects or requires very fine control, the lower cost and weight of the SG90 can be sufficient. If the gripper will experience any side-loading or needs to hold firmly, the MG90S is better.

Educational or Prototype Arms

For learning the fundamentals or testing a proof-of-concept where budget is the primary constraint. * Recommended Servo: SG90. Its low cost allows for experimentation and iteration without significant financial loss from accidental damage.

Practical Tips for Integration and Control

Regardless of your choice, proper integration is key to success.

Power Supply Considerations

• Never power a servo directly from an Arduino/ Raspberry Pi's 5V pin. Servos under load can draw hundreds of milliamps, which will brown out or damage your microcontroller.
• Use a dedicated, regulated power supply capable of supplying the required current. For a multi-servo arm, a 5V/3A-5A UBEC (Universal Battery Elimination Circuit) is an excellent choice.
• Decouple the servo power from the logic power by connecting all grounds but using separate VCC lines.

PWM Signal and Control Boards

• Both servos use a standard 50Hz PWM signal (20ms period) where the pulse width (typically 1.0ms to 2.0ms) determines the angle.
• For controlling multiple servos, dedicated servo driver boards like the PCA9685 are highly recommended. They offload the timing burden from the main microcontroller and provide more stable control.

Mechanical Installation Best Practices

• Use Servo Horns and Mounts Correctly: Ensure horns are screwed on tightly and the servo is securely mounted to the robot frame to prevent movement and vibration.
• Avoid Forcing the Limits: Do not mechanically force the servo beyond its physical limits. This is a primary cause of immediate gear failure, especially in the SG90.
• Implement Software Limits: In your control code, set software limits that are slightly inside the servo's physical limits to provide a safety margin.

Gearing and Leverage

• Understand the principles of leverage. The further a weight is from the joint, the more torque is required to lift it. Design your arm links to keep heavy components as close to the joints as possible.