What Is Torque Control? How Robots Regulate Joint Force.

What Is Torque Control?

Torque control is a control method that regulates the torque produced by a motor or robotic joint.

Instead of commanding only a position or rotational speed, a torque-controlled system attempts to produce a specified turning effect.

Torque control is useful in applications involving physical contact, dynamic movement, and compliant interaction.

For example, a robot may use torque control to regulate how strongly its arm pushes against a surface.

Key Takeaways

• Torque control regulates motor or joint torque rather than only position or speed.
• Electric motor torque is often related to current, allowing suitable drives to regulate torque through current control.
• Effective torque control depends on sensing, actuator models, mechanical transmissions, and operating limits.

How Does Torque Control Work?

Imagine a robot joint that needs to exert 5 newton-meters of torque.

The controller receives the requested torque.

A suitable motor-drive system uses electrical measurements and a motor model to determine how to produce the required output.

For many permanent-magnet motors, electromagnetic torque is related to the torque-producing current component.

A simplified relationship is:

Motor Torque = Torque Constant × Torque-Producing Current

The controller adjusts electrical commands to achieve the desired torque within the system's limits.

Feedback helps maintain the requested behavior.

However, output torque at a geared joint may differ from motor torque because of transmission efficiency, friction, and other effects.

Torque Control vs. Position Control

Position control regulates where a mechanism moves.

Torque control regulates its turning effort.

Consider a robotic arm interacting with a table.

A position controller may attempt to hold the commanded position despite changes in contact force.

A torque controller can regulate the effort applied through the joint.

This distinction is valuable in applications involving uncertain surfaces or deliberate contact.

However, torque control alone does not guarantee a particular tool force or movement.

Robot geometry and dynamics still determine the resulting behavior.

Torque Control vs. Force Control

Torque is rotational.

Force is a linear mechanical interaction.

A robot's joint torques contribute to forces at its tool.

Mathematical relationships involving the robot's Jacobian can connect joint torques with end-effector forces under suitable assumptions.

For example, a robot polishing a surface may use joint torque control as part of a higher-level force-control system.

The controller must account for joint configuration and other forces acting on the robot.

What Is Current-Based Torque Estimation?

Some motors estimate torque from electrical current.

For example, a motor controller may measure the torque-producing current and calculate an approximate electromagnetic torque.

However, external joint torque can differ from this estimate because of gearbox friction, motor dynamics, and other mechanical effects.

High-precision contact applications may therefore use additional sensors, such as joint torque sensors or wrist-mounted force-torque sensors.

Why Does Torque Control Matter for Legged Robots?

Legged robots experience changing ground-contact forces.

For example, a quadruped must support its body while coordinating foot placement.

Torque-controlled joints can help regulate mechanical interaction and implement certain compliant or dynamic control strategies.

However, successful locomotion also requires balance estimation, planning, and suitable control.

Torque control is one capability within a larger system.

What Are the Limitations?

Torque commands are constrained by available motor current, voltage, speed, temperature, and transmission characteristics.

Excessive or inappropriate torque can produce hazardous movement or equipment damage.

Controllers must also account for sensor noise and timing.

Torque control does not replace protective safety functions.

Frequently Asked Questions

Does torque control require a dedicated torque sensor?

Not always. Some systems use motor-current measurements and models, although direct sensing can improve selected measurements.

Can a robot use position control and torque control together?

Yes. Some control architectures combine or switch between these modes, subject to hardware capabilities.

The Bottom Line

Torque control lets robots regulate the mechanical effort generated by their joints. It is particularly useful when physical contact and dynamic movement require more than fixed positioning.