Feetech STS3250 Smart Actuator: Evaluation of Accuracy, Torque and Backlash

1. Introduction

The proliferation of low-cost, high-performance serial bus servos has enabled rapid development of robotic systems, from hobbyist projects to industrial automation. The Feetech STS3250 represents a notable entry in this market segment, offering 50 kg·cm rated stall torque, 12-bit magnetic encoder feedback, and TTL serial communication in a compact metal housing.

Despite manufacturer specifications, real-world performance validation remains essential for engineering applications where reliability and precision are critical. This study presents quantitative measurements across five key performance domains:

1. No-load rotational velocity
2. Dynamic load response
3. Thermal behavior under sustained load
4. Positional repeatability
5. Mechanical backlash

1.1 Servo Specifications (Manufacturer Data)

Table 1 summarizes the manufacturer specifications for the STS3250 servo motor, as documented in the official Feetech product specification sheet (Document No. 6384135881578380868917773, Edition A/0, dated 2024-01-16).

Table 1: Feetech STS3250 manufacturer specifications (Source: Official datasheet)

2. Experimental Methods

2.1 Test Equipment

• Feetech STS3250 servo unit (production sample)
• Regulated 12 V DC power supply
• Custom lever arms (86 mm, 95 mm, 100 mm)
• Calibrated weights (0.6 kg, 2.0 kg)
• Digital dial indicator (0.01 mm resolution)
• USB-TTL interface for data acquisition
• Custom logging software (sampling rate: ~50 Hz)

2.2 Data Acquisition

Servo telemetry was captured via the native serial protocol, recording:

• Position — encoder counts (0–4095)
• Target position — commanded value
• Speed — encoder counts/s
• Load — internal load estimate
• Current — milliamps (mA)
• Temperature — °C, internal sensor
• Voltage — 0.1 V resolution

All tests were conducted at ambient temperature (25 ± 2°C) with 12 V nominal supply voltage, consistent with the manufacturer’s standard test environment (25°C ± 5°C, 65% ± 10% humidity).

3. Results and Analysis

3.1 No-Load Velocity Verification

Objective: Validate rated no-load speed against manufacturer specification.

Method: Continuous rotation in velocity mode with encoder feedback sampling.

Manufacturer Specification:

• Rated speed: 0.133 s per 60°
• Calculated: (0.133 × 6) = 0.798 s per revolution
• Therefore: 1/0.798 = 1.253 rev/s = 75.2 RPM

Measured Results:

• Encoder output: 5,300 counts/second
• Resolution: 4,096 counts/revolution
• Calculated velocity: 5,300 ÷ 4,096 = 1.294 rev/s = 77.6 RPM

The measured velocity exceeds the specification by 2.4 RPM (3.2%). This deviation falls within acceptable manufacturing tolerance and may be attributed to motor winding variations, reduced internal friction in the tested unit, or measurement timing precision (±1%).

Table 2: Velocity comparison — specified vs measured

3.2 Load Test: Dynamic Response Under 2 kg Load

Objective: Characterize servo behavior under significant gravitational load.

Configuration:

• Lever arm: 100 mm
• Applied mass: 2.0 kg
• Gravitational torque: 2.0 kg × 100 mm = 20 kg·cm (40% of rated stall torque)
• Motion profile: Oscillatory positioning between encoder positions 881 and 1347

3.2.1 Position Tracking

During upward motion (lifting against gravity):

• Peak speed: ~1000–1050 counts/s
• Load reading: 450–490 (internal units, indicating significant resistance)
• Current draw: 150–200 mA sustained, peaks to 213 mA during acceleration

During downward motion (gravity-assisted):

• Speed: 800–1050 counts/s
• Load reading: Near zero or slightly positive
• Current draw: Minimal (<10 mA) during free descent

3.2.2 Steady-State Holding

When maintaining position against the 2 kg load:

• Position error: 19–22 encoder counts (target 1347, actual 1325–1328)
• Steady-state current: 80–120 mA
• Load compensation: Active PID maintaining position within 0.5°

Table 3: Telemetry data during single lift cycle

Analysis: The servo successfully tracks the commanded trajectory under 40% stall load, with position lag of approximately 20–30 counts during motion and 19–22 counts at steady state. The PID controller demonstrates appropriate gain scheduling, with higher current draw during acceleration phases.

3.3 Thermal Characterization

Objective: Quantify thermal rise under sustained load cycling.

Test Protocol:

• Continuous oscillatory motion under 2 kg × 100 mm load
• Duration: 8 minutes
• Initial temperature: 40°C (pre-warmed from previous tests)

Table 4: Temperature rise during sustained load test

Thermal Rate: Approximately 3.75°C/minute under sustained 40% load cycling.

Protection Activation: According to the official datasheet, thermal protection activates at 70°C, disabling torque output (“大于70℃关闭扭矩输出”). This threshold was reached at the 8-minute mark in our test, confirming the protection mechanism operates as specified.

3.4 Stall Torque and Protection Behavior

Objective: Verify stall torque capability and characterize protection mechanisms.

Method: Incremental loading to mechanical stall with torque measurement.

Table 5: Stall torque measurements

h4>Protection Mechanisms Observed:

The official datasheet specifies the following electronic protections, all of which were confirmed in our testing:

1. Over-current protection: Activates at >4.85 A sustained for >2 s (datasheet: “运行中电流大于4.85并持续2s后”)
2. Over-load protection: Activates at >80% stall for >2.5 s (configurable)
3. Over-voltage protection: Activates at >14 V or <4 V
4. Thermal protection: Torque disabled above 70°C

Analysis: While the datasheet lists a stall torque of 50 kg·cm at 12 V, our real-world measurements showed 25 kg·cm sustained torque after protection activation, and up to 48 kg·cm peak torque for a split second. Although the built-in protection limits continuous stall torque, the servo demonstrated excellent stability and control precision.

3.5 Positional Repeatability

Objective: Quantify positioning precision under repeated motion cycles.

Configuration:

• Lever arm: 95 mm
• Motion: Repeated positioning to fixed target
• Measurement: Dial indicator at lever tip

Table 6: Repeatability test results

Analysis: The 12-bit magnetic encoder (0.088°/count resolution, as specified in the datasheet: “电子分辨率 0.088° (360°/4096)”) combined with the PID control loop achieves sub-count effective resolution through dithering. The measured repeatability of ±0.02 mm at 95 mm radius corresponds to ±0.012° angular precision—approximately 7× better than the encoder’s native resolution.

3.6 Mechanical Backlash Measurement

Objective: Quantify gear train backlash independent of encoder feedback.

Method:

1. Position servo to reference angle
2. Apply external torque in positive direction until motion detected
3. Reverse torque direction until motion detected
4. Measure total angular displacement at lever tip

Configuration:

• Lever arm: 86 mm
• Applied torque: Manual, sub-holding-torque magnitude

Table 7: Backlash measurement results

Encoder Counts Equivalent: 0.43° × (4096/360) = 4.9 counts

4. Static Load Holding Analysis

Objective: Characterize position stability under constant gravitational load.

Configuration:

• Target position: 3010 (encoder counts)
• Applied load: 2 kg at 100 mm (20 kg·cm)
• Duration: Extended observation (>35,000 samples)

Table 8: Static load holding telemetry

Position Deflection Under Load:

• Deflection: 14 encoder counts = 14 × 0.088° = 1.23°
• At 100 mm radius: 1.23° × (π/180) × 100 mm = 2.15 mm tip deflection

Compliance Calculation:

• Applied torque: 20 kg·cm = 1.96 N·m
• Angular deflection: 1.23° = 0.0215 rad
• Torsional stiffness: 1.96 / 0.0215 = 91.2 N·m/rad

6. Comparison with Official Specifications

This section provides a comprehensive comparison between our measured results and the official Feetech STS3250 product specification (Document dated 2024-01-16, Edition A/0).

Table 9: Complete comparison — Official specifications vs measured values

6.1 Specifications Confirmed

• Speed: Measured 77.6 RPM vs specified 75.2 RPM — servo exceeds specification
• Peak Torque: Measured 48 kg·cm vs specified 50 kg·cm — within 5% tolerance
• Backlash: Measured 0.43° vs specified ≤0.5° — within specification with margin
• Thermal Protection: Confirmed activation at 70°C as specified
• Electronic Protections: Over-current, over-load, and over-voltage protections all function as documented

6.2 Important Findings Not in Datasheet

• Sustained Torque: After protection activation, continuous torque is limited to ~25 kg·cm (50% of peak)
• Thermal Rise Rate: ~3.75°C/minute under 40% load — reaches protection threshold in 8 minutes from 40°C
• Repeatability: Excellent ±0.02 mm at 95 mm radius — suitable for precision applications
• Torsional Stiffness: ~91.2 N·m/rad under load

6.3 Reliability Specifications (from Datasheet)

The official datasheet includes the following reliability specifications:

• Life Test: >100,000 cycles (60° rotation, 0.25s move, 0.5s pause, at 1/5 stall torque)
• Motor Noise: 45 ± 5 dB (at 30 cm)
• Servo Noise: 65 ± 5 dB (at 30 cm, 1/3 no-load speed)
• Waterproof: No

7. Discussion

7.1 Application Recommendations

Well-Suited Applications:

• Robotic arm joints (intermittent duty)
• Pan-tilt mechanisms
• Animatronics
• Educational robotics platforms
• Prototype development

Applications Requiring Caution:

• Continuous high-load operation (>30% stall torque)
• Bidirectional force control (backlash limitation)
• High-temperature environments (reduced thermal headroom)

7.2 Design Guidelines

1. Torque Budget: Design for ≤25 kg·cm continuous, ≤40 kg·cm intermittent
2. Thermal Management: Allow 30-second rest per minute at high load, or implement active cooling
3. Backlash Compensation: Implement unidirectional approach for precision positioning
4. Position Accuracy: Account for 1–2° compliance under load in kinematic calculations

8. Conclusion

The Feetech STS3250 demonstrates performance consistent with manufacturer specifications across all tested parameters. The servo achieves 77.6 RPM no-load speed (3.2% above specification), mechanical backlash of 0.43° (within 0.5° limit), and exceptional positional repeatability of ±0.02 mm at 95 mm radius.

Key findings for system designers:

1. Sustained torque capability is approximately 50% of peak rating due to thermal and current protection mechanisms
2. Thermal rise of ~3.75°C/minute under 40% load cycling necessitates duty cycle management for continuous operation
3. Position repeatability significantly exceeds encoder resolution due to effective PID implementation
4. Backlash of 0.43° is inherent and must be accommodated in precision applications

The STS3250 represents a capable and cost-effective solution for robotic applications where intermittent high-torque operation, good positional accuracy, and serial bus communication are required. Understanding the thermal and mechanical limitations documented in this study enables appropriate system-level design decisions.