What Is a Force-Torque Sensor? How Robots Measure Physical Contact.

What Is a Force-Torque Sensor?

A force-torque sensor is a device that measures forces and twisting moments applied to a mechanical component.

In robotics, force-torque sensors help machines understand how they are interacting physically with objects and their environment.

For example, a robotic arm inserting a component into an opening can use force measurements to detect resistance and adjust its movement.

These sensors are commonly used in assembly, polishing, grinding, testing, and other applications where controlling contact matters.

Key Takeaways

• Force-torque sensors measure mechanical forces and torques acting on a component.
• Six-axis sensors can measure three force components and three torque components.
• The measurements support force control, contact detection, and the analysis of physical interaction.

How Does a Force-Torque Sensor Work?

Imagine a robotic arm equipped with a sensor between its wrist and gripper.

When the gripper pushes against an object, mechanical forces act on the sensor.

The sensor contains elements designed to respond to these loads.

Many force-torque sensors use strain gauges that detect extremely small mechanical deformations.

Electronics convert the resulting signals into measurements of force and torque.

The robot's controller can use these measurements to understand how strongly the tool is interacting with the object.

What Does a Six-Axis Force-Torque Sensor Measure?

A six-axis force-torque sensor measures six components.

Three represent linear forces:

• Fx: Force along the X-axis
• Fy: Force along the Y-axis
• Fz: Force along the Z-axis

Three represent moments or torques:

• Tx: Torque around the X-axis
• Ty: Torque around the Y-axis
• Tz: Torque around the Z-axis

Together, these measurements describe the force and moment acting on the sensor within its reference frame.

The exact values depend on where the sensor is mounted and how its coordinate system is defined.

Why Are Force-Torque Sensors Useful in Assembly?

Consider a robot inserting a metal peg into a closely fitting hole.

The robot approaches the intended position using its motion-control system.

If the peg makes contact with the edge of the opening, a force-torque sensor can detect the resulting interaction.

Control software may then adjust the robot's movement to reduce the contact force and improve alignment.

This can help with applications where small positioning differences would make rigid movement unreliable.

Force-Torque Sensor vs. Motor Current Sensing

Motor current can sometimes be used to estimate actuator torque.

However, it does not provide the same information as a dedicated multi-axis force-torque sensor.

Motor-current-based estimates can be influenced by friction, gearbox characteristics, acceleration, and other effects.

A force-torque sensor mounted near the tool can provide more direct information about the loads applied at that location.

The appropriate method depends on the accuracy and control requirements of the application.

Frequently Asked Questions

Does every robot need a force-torque sensor?

No. Many robots perform fixed operations using position control without dedicated external force sensing.

Can a force-torque sensor detect collisions?

It can help identify unexpected forces, but collision detection and safety functions depend on the complete system and its design.

The Bottom Line

Force-torque sensors give robots information about physical contact. They are particularly valuable for tasks requiring precise force regulation, delicate handling, or interaction with uncertain objects.