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Published 2026-04-13
servoload testing is the process of verifying whether aservomotor and its mechanical transmission system can maintain specified position accuracy, torque output, and response speed when subjected to an external resistance torque. This is a necessary validation step before anyservosystem is deployed in actual machinery, such as robotic arms, CNC feed axes, or automated guided vehicles. Without proper load testing, a servo system may experience position deviation, stalling, oscillation, or even driver damage when encountering real-world working resistance. This guide provides a standardized, step-by-step methodology for conducting servo load testing, based on common industrial practices and accessible tools, without referencing any specific brand or company.
The primary goal of load testing is to confirm that the servo system’s actual output characteristics match its theoretical specifications under the intended operating conditions. Specifically, load testing answers three fundamental questions:
Torque margin: Can the servo provide sufficient torque to accelerate, decelerate, and hold the load under worst-case friction and inertia?
Position accuracy under load: Does the actual position error remain within the allowable tolerance (e.g., ±0.05 mm) when an external force opposes motion?
Thermal behavior: Does the servo motor’s temperature rise stay below the insulation class limit (typically ≤80°C for Class B insulation) during continuous operation at rated load?
A typical case: A robotic arm for pick-and-place operations was designed using theoretical calculations. During deployment, when picking a 2 kg workpiece, the arm’s wrist joint would overshoot its target by 3 mm. Load testing revealed that the servo’s actual continuous torque at 80% duty cycle was 22% lower than the theoretical value, leading to position error. The solution was to either increase the servo size or reduce the payload.
To perform a valid load test, you need the following equipment. None of these require specific brands; generic industrial-grade components work equally well.
Setup procedure (common for bench testing):
1. Mount the servo motor on a rigid test bench. Use a flexible coupling to connect the motor shaft to the torque sensor.
2. Connect the torque sensor output to the load generator (brake) shaft. Align all shafts within 0.1 mm runout to avoid parasitic loads.
3. Attach the encoder to the load side (after the coupling) or use the servo’s built-in encoder if it provides direct shaft feedback. For highest accuracy, use a separate load-side encoder.
4. Connect all sensors to the data acquisition system. Set sampling rate to at least 1 kHz if you need to capture acceleration transients.
5. Place the thermocouple on the motor housing’s hottest point (usually near the winding end cap). Secure it with thermal paste and heat-resistant tape.
Perform the test in three progressive phases: no-load verification, step load test, and continuous load test.
Before applying any load, verify that the servo system operates correctly under zero external torque.
Command the servo to perform a defined motion profile: e.g., 0° → 90° → 180° → 90° → 0° at 50% rated speed.
Record position error (difference between commanded and actual position). Acceptable no-load error is typically ≤±0.02° for absolute encoders or ≤±1 encoder pulse for incremental encoders.
Measure no-load current at constant speed. This value serves as the baseline for calculating load-induced current.
If no-load error exceeds the limit, check for mechanical misalignment, loose couplings, or incorrect servo parameter settings (e.g., position loop gain too low).
Apply incremental torque loads while the servo maintains a constant low speed (e.g., 10% of rated speed). This test reveals the maximum torque the servo can output without stalling or excessive error.
1. Set servo to constant speed mode at 10% rated speed (e.g., 30 RPM for a 300 RPM rated motor).
2. Starting from 0% rated torque, increase load torque in steps of 10% of rated torque. Wait 5 seconds at each step for stabilization.
3. Record at each step: actual torque (from torque sensor), actual speed (from encoder), and position error (if in position mode).
4. Continue increasing load until any of these stop conditions occur:
Position error exceeds 5° (for position mode)
Speed drops below 90% of commanded speed (for speed mode)
Motor current reaches 150% of rated current
Servo driver triggers an overload or following error alarm
Interpretation: The torque at which any stop condition occurs is the practical maximum continuous torque. For reliable operation, your actual working torque should not exceed 80% of this value.
Common case: A CNC feed axis servo was rated for 4 Nm continuous torque. Step load testing showed that at 3.2 Nm (80% of rated), the position error was already 0.12 mm (exceeding the 0.05 mm tolerance). The actual usable torque was only 2.8 Nm. The cause was insufficient position loop gain. After tuning the gain from 15 to 28 (1/s), the error at 3.2 Nm dropped to 0.04 mm.
Apply a constant torque equal to the intended maximum working torque (e.g., 80% of the value found in Phase 2) and run the servo through its actual working duty cycle for at least 60 minutes or until thermal equilibrium.
Procedure:
Set the load brake to the target torque value.
Command the servo to repeat its real-world motion profile (acceleration, constant speed, deceleration, dwell).
Record motor housing temperature every 2 minutes.
Also record current and torque every 30 seconds.
Acceptance criteria (based on insulation class):
Class B (130°C): Housing temperature ≤80°C, winding temperature ≤120°C (winding can be estimated as housing + 15°C for small motors)
Class F (155°C): Housing ≤95°C, winding ≤140°C
Class H (180°C): Housing ≤110°C, winding ≤165°C
If temperature exceeds limits, either reduce the load or improve cooling (add forced air or increase heat sink area).
A real example: A servo used in a packaging machine conveyor was tested at 2.5 Nm (rated 2.8 Nm). After 35 minutes of continuous back-and-forth motion (0.5 Hz, 90° amplitude), the housing reached 92°C, exceeding the Class B limit of 80°C. The solution was to add a 120 mm fan blowing directly on the motor fins, which reduced steady-state temperature to 74°C.
During all three phases, record the following data points. This data is essential for diagnosing issues and certifying the servo system.
How to calculate mechanical output power:
For rotary motion: P_out (W) = Torque (Nm) × Angular speed (rad/s)
Angular speed (rad/s) = RPM × (2π / 60)
How to calculate electrical input power (for three-phase servo):
P_in (W) = √3 × V_rms × I_rms × Power factor
If power factor is unknown, assume 0.85 for loaded condition.
Load testing often reveals problems that are invisible during no-load operation. Here are the most frequent issues and their fixes.
A documented case: An automated guided vehicle’s steering servo passed no-load testing but failed continuous load testing. After 12 minutes of driving with a 150 kg payload, the driver triggered an overcurrent alarm. Load testing revealed that the required torque for turning on carpet flooring was 3.1 Nm, but the servo’s actual torque at 80°C was only 2.4 Nm (due to magnet degradation at high temperature). The fix was to increase the servo size from 100 W to 200 W, providing 4.0 Nm rated torque.
Based on load test results, you must define three operational limits for the actual machine:
Maximum continuous torque (MCT): The highest torque the servo can sustain for 60 minutes without exceeding thermal limits. Set this as 90% of the torque measured at thermal equilibrium.
Maximum intermittent torque (MIT): The torque allowed for short durations (≤5 seconds). This is typically 150–200% of MCT, but verify that the driver’s current limit does not trip. From the step load test, MIT is the torque just before stall or alarm.
Maximum speed under full load: The highest speed at which the servo can deliver MCT without torque derating. If speed is too high, torque drops due to back-EMF. Typical limit is 70–80% of no-load speed.
Important: Never operate a servo continuously above its MCT. Even brief overloads (more than 10 seconds) can cause winding insulation degradation, leading to premature failure. Always include a torque limit parameter in the servo driver set to 100% of MCT.
After completing the three-phase load test and analyzing the data, take these specific actions to ensure reliable long-term operation:
1. Create a load test certificate: Document the test date, ambient temperature, measured MCT, MIT, temperature rise, and position error at working load. This certificate serves as proof of system validation.
2. Set driver protection parameters based on test results:
Current limit = 110% of MCT (for continuous protection)
Overload time limit = 5 seconds at 200% MCT
Stall protection torque = 120% of MIT
Position error limit = 2× the maximum measured error under load
3. Implement a periodic re-test schedule: For high-cycle applications (e.g., pick-and-place robots operating 24/7), re-test every 2000 operating hours or 12 months. Servo torque degrades over time due to magnet aging and bearing wear. A typical degradation rate is 5–10% over 10,000 hours.
4. Add thermal monitoring in the actual machine. If the load test showed a 50°C rise at MCT, install a thermistor (PTC type) in the motor winding and set a warning at 90% of the maximum allowable temperature (e.g., 90°C for Class B). This prevents silent overheating when ambient temperature is higher than test conditions.
5. Adjust the motion profile if test results show marginal torque margins. For example, if your working torque is 85% of MCT, reduce acceleration by 15% to lower peak torque during acceleration phases.
Core takeaway: Load testing is not a one-time checkbox. It is the only way to validate that a servo system will perform reliably under real working conditions. A servo that passes no-load tests but fails load testing will cause unexpected downtime, product damage, or safety hazards. Always perform step load and continuous load tests before integrating any servo into production machinery. Then, use the test data to set protection limits, schedule maintenance, and optimize the motion profile. This practice reduces unexpected servo failures by an estimated 70% based on industry maintenance records.
Action step for engineers: If you have not yet load-tested a servo currently in service, schedule a test within the next two weeks using the procedure above. Start with the step load test at 10% rated speed to measure actual torque margin. If the margin is less than 20% above your working torque, either reduce the load or upgrade the servo before a failure occurs.
Update Time：2026-04-13
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