Force and Torque Sensors
Force and torque sensors act as the central nervous system for modern AGVs and mobile robots, providing the tactile feedback necessary for precise interaction. By measuring external forces in real-time, these sensors enable safe human-robot collaboration, adaptive navigation, and complex manipulation tasks that vision systems alone cannot handle.
Core Concepts
6-Axis Measurement
The ability to simultaneously measure forces (Fx, Fy, Fz) and torques (Tx, Ty, Tz) across all three spatial axes gives robots complete tactile awareness of their physical interactions.
Active Compliance
Unlike rigid programming, compliance allows the robot to "feel" resistance and adjust its trajectory instantly, preventing damage to the payload or the environment during assembly.
Collision Detection
High-sensitivity sensors detect unexpected contact faster than motor current monitoring, allowing AGVs to trigger emergency stops immediately upon touching an obstacle.
Payload Verification
Sensors on the end-effector or lifting mechanism verify that an object has been gripped correctly and determine its weight, ensuring process quality control.
Center of Gravity
By analyzing torque data, mobile manipulators can calculate the Center of Gravity (CoG) of a carried load, adjusting their balance to prevent tipping during movement.
Lead-Through Teaching
Sensors enable "Zero-G" modes where operators can manually guide the robot arm to teach pathing points, as the robot actively compensates for its own weight.
How It Works
Force and torque sensors typically utilize strain gauges, capacitive elements, or optical technologies embedded within a rigid structure. When an external load is applied, the structure undergoes microscopic deformation. This deformation is measured and converted into electrical signals, which correspond to force vectors along the X, Y, and Z axes, as well as rotational torque around these axes.
The raw analog data passes through an onboard Digital Signal Processor (DSP), which filters out noise and temperature drift. The DSP then calculates the precise force and torque values using a calibration matrix unique to that sensor.
Finally, this data is transmitted to the AGV or robot controller via fieldbus protocols (like EtherCAT or PROFINET). The robot's control loop uses this feedback at high frequencies (often 1kHz+) to adjust motor torque, enabling responsive and "feeling" behaviors that mimic human touch.
Real-World Applications
Precision Assembly
In manufacturing, robots use force sensors for "peg-in-hole" insertion tasks. The sensor detects misalignment by feeling resistance and micro-adjusts the position, preventing jamming or part damage.
Surface Finishing
Tasks like sanding, polishing, or deburring require constant contact pressure. Force sensors allow the robot to maintain exact pressure against curved or uneven surfaces, ensuring a uniform finish.
Warehouse Logistics
AGVs equipped with force-sensing forks or grippers can confirm that a pallet is fully seated and detect if a load is unstable or shifting before transport begins.
Medical Robotics
In robotic surgery or rehabilitation, force sensors provide haptic feedback to the surgeon, allowing them to feel tissue resistance remotely, enhancing safety and precision.
Frequently Asked Questions
What is the difference between a 1-axis and a 6-axis force sensor?
A 1-axis sensor measures force in a single direction (usually compression or tension), acting essentially as a sophisticated scale. A 6-axis sensor measures force in three directions (X, Y, Z) and rotation (torque) around those three axes simultaneously. 6-axis sensors are required for complex manipulation where forces can come from any angle.
How does force sensing improve AGV safety compared to LiDAR?
LiDAR handles non-contact navigation and obstacle avoidance at a distance. Force sensing handles physical contact safety. If an AGV accidentally touches a person or object that was in a blind spot, force sensors detect the impact immediately and stop the robot, minimizing injury or damage.
Do force sensors require frequent calibration?
Most modern industrial force sensors are factory calibrated and robust against drift. However, "zeroing" (tare) is performed frequently via software before a task begins to account for the weight of the end-effector. Full recalibration is typically only needed after a severe crash or overload event.
What is "Cross-talk" in force sensors?
Cross-talk occurs when a force applied on one axis creates a false reading on another axis due to mechanical coupling within the sensor. High-quality sensors use decoupling matrices in their DSP to mathematically correct this, ensuring that pure Z-axis force doesn't register as X-axis force.
Can I retrofit a force sensor onto an existing robot arm?
Yes, wrist-mounted force/torque sensors are designed for easy retrofitting. They are installed between the robot flange and the end-effector. You will need to ensure the robot controller supports the communication protocol (e.g., EtherCAT, Modbus) and update the tool center point (TCP) settings to account for the sensor's height.
What is the typical overload capacity?
Sensors usually have a safe overload capacity of 200% to 500% of their rated range. Exceeding this can permanently deform the sensing elements (plastic deformation), requiring replacement. It is crucial to size the sensor based on peak dynamic forces, not just static loads.
How does temperature affect force sensor readings?
Temperature fluctuations can cause materials to expand or contract, altering strain gauge readings (thermal drift). Industrial sensors include built-in temperature compensation algorithms that actively adjust the output signal to maintain accuracy across operating temperature ranges.
What is the difference between Joint Torque Sensors and Wrist Sensors?
Wrist sensors measure forces at the tool tip (interaction with the part). Joint torque sensors are integrated into each motor of the robot arm. Joint sensors are better for whole-arm collision detection and compliance, while wrist sensors provide higher precision for delicate assembly tasks.
Are optical force sensors better than strain gauge sensors?
Optical sensors measure deformation using light and photodiodes rather than electrical resistance. They are generally more resistant to electrical noise (EMI) and can offer higher shock tolerance. Strain gauges are more traditional, widely available, and typically less expensive, but may require better shielding.
How important is data latency in force control?
Extremely important. For stable force control loops, the sensor data must reach the controller quickly (low latency) and frequently (high refresh rate, e.g., 1000Hz). High latency can cause the robot to oscillate or react too slowly to contact, potentially damaging the part.
What IP rating is required for these sensors?
This depends on the environment. For clean assembly, IP40 or IP54 is sufficient. For CNC machine tending (coolant spray) or food processing (washdown), IP65 or IP67 ratings are necessary to prevent liquid ingress from damaging sensitive electronics.