Embedded vs. External Torque Sensors in Robotic Joints Module: Engineering Trade-offs and Future Trends

In recent years, with the rapid growth of humanoid robots and collaborative robots, high-precision force control has become a core capability for achieving compliant and safe interaction. At the heart of this capability lies a critical component inside the robotic joint module: the torque sensor, responsible for perceiving force.

However, the installation method of torque sensors directly determines joint performance, reliability, and cost. Today, two main technical approaches dominate the market: embedded (integrated) and external (add-on) torque sensors.

This distinction goes beyond simply “built-in” versus “attached.” The fundamental difference lies in whether the torque sensing structure is an intrinsic part of the mechanical transmission chain and integrated within the joint itself.

1. Embedded Torque Sensors: The “Native Core” of Integrated Joints

Embedded torque sensors are deeply integrated into the joint structure, becoming the fourth core component alongside the motor, reducer, and encoder. This enables a fully integrated system combining drive, transmission, sensing, and control.

Common Implementation Methods

Strain Gauge-Based Sensing

Strain gauges are applied to components such as the output flange, hollow shaft, or a dedicated elastic element. Torque is calculated by measuring minute torsional deformation. This is the most classic and widely used method.

Magnetic / Magnetoelastic Sensing

Torque is detected by measuring angular differences via magnetic rings or changes in material magnetic properties. This non-contact approach offers excellent sealing and resistance to oil and contaminants, making it a rapidly growing trend.

Structural Reuse Design

Existing elastic components—such as the flexspline in harmonic drives or output flanges—are reused as sensing elements. This approach requires advanced mechanical decoupling to avoid interference from axial forces and bending moments.

Advantages

Compactness & Lightweight Design

No additional axial length is introduced, enabling ultra-compact joint modules and improving payload-to-weight ratio.

High Reliability & Protection

Internal wiring allows full sealing (high IP rating), protecting against dust, water, and mechanical wear.

Superior Dynamic Performance

High structural stiffness and low inertia support high-bandwidth, responsive force control.

High Product Consistency

Pre-calibrated at the factory, offering plug-and-play integration and reducing system complexity.

Challenges

High Design Complexity

Requires solving strength, stiffness, overload protection, temperature drift compensation, and multi-axis decoupling within a confined space.

Limited Maintainability

Sensor failure often requires returning the entire joint module for repair, leading to longer downtime.

Hidden Overload Risks

External shocks may cause irreversible micro-damage or zero drift that is not visually detectable.

Typical Applications

Humanoid robots

Collaborative robots (cobots)

Quadruped robots

Exoskeletons

Dexterous robotic hands

These applications prioritize compactness, lightweight design, and high reliability in mass production.

2. External Torque Sensors: Flexible and Independent Measurement Modules

External torque sensors are standalone standardized components installed between the joint output and the load via flanges or couplings.

Common Implementation Methods

Flange-Type Torque Sensors

Installed between the joint output flange and the load, acting as a measurement ring. This is the most common approach.

Shaft-Type / Hollow Shaft Sensors

Mounted directly on the transmission shaft, suitable for specific spatial configurations.

Motor Current Estimation (Virtual Sensing)

Although not a physical sensor, torque is estimated from motor current in low-cost applications. However, accuracy is affected by friction, temperature drift, and gearbox efficiency, making it unsuitable for high-precision force control. This method is widely used in cost-sensitive robotic applications.

Advantages

High Flexibility & Maintainability

Sensors can be quickly replaced, minimizing downtime. Ideal for R&D and iterative development.

“Gold Standard” for Calibration

High-precision, certified sensors can be used to validate and calibrate embedded solutions.

Wide Selection & Mature Technology

A large variety of products are available with different ranges, accuracies, and interfaces (e.g., CAN, EtherCAT).

Challenges

Increased Size & Reduced Aesthetics

Adds axial length and bulk, making joints less compact.

Reduced System Stiffness

Additional interfaces and elastic elements may limit control bandwidth and introduce alignment errors.

Protection & Cabling Issues

Exposed wiring requires additional protection against dust, liquids, and mechanical stress.

Typical Applications

R&D and testing platforms

High-load or impact-prone environments (using sacrificial sensors)

Retrofit or upgrade projects

3. Key Comparison: Embedded vs. External Torque Sensors

4. Selection Guide: How to Choose the Right Solution

Primary Decision

If your goal is productization and mass production, prioritizing compactness, lightweight design, reliability, and clean integration →

Choose embedded torque sensors.

If you are in R&D, prototyping, testing, or require high flexibility and easy maintenance →

Choose external torque sensors.

Key Validation Criteria (for Both Approaches)

Cross-Axis Decoupling Performance

Verify specifications for resistance to bending moments and axial loads (e.g., <2% FS).

Overload Protection & Zero Recovery

Ensure the sensor maintains accuracy after 2× or 5× overload conditions.

Mounting Rigidity (Critical for External Sensors)

Proper flatness, stiffness, and alignment are essential—poor installation can degrade even the best sensors.

Embedded torque sensors represent the future of highly integrated robotic joints, enabling scalable, high-performance, and cost-effective mass production. Meanwhile, external sensors retain enduring value in flexibility, validation, and adaptability.

There is no absolute “better” solution—only the one that best fits your application. Understanding these engineering trade-offs allows you to equip your robot with the most suitable “sense of touch,” achieving the optimal balance between precision, reliability, and cost.