What Is a Series Elastic Actuator? How Springs Improve Robot Force Control.

What Is a Series Elastic Actuator?

A series elastic actuator, or SEA, is an actuator designed with an elastic element placed in series between the motor or transmission and the load.

The elastic element, often a spring, deforms when force or torque passes through the actuator.

By measuring that deformation, the system can estimate the force or torque being transmitted.

Series elastic actuators are used in robotics research, wearable devices, legged robots, and other applications involving controlled physical interaction.

Key Takeaways

• Series elastic actuators deliberately place an elastic component in the force-transmission path.
• Measuring elastic deformation can provide information used for force or torque control.
• Compliance can absorb certain impacts and support controlled interaction, but it introduces trade-offs in stiffness, response, and control.

How Does a Series Elastic Actuator Work?

Imagine a robotic joint containing a motor, gearbox, and spring.

The motor drives one side of the spring.

The other side connects to the joint's load.

When the motor applies torque, the spring twists or deforms slightly.

The amount of deformation depends on its stiffness.

Sensors measure the relative movement across the elastic element.

The controller can use this information to estimate the torque transmitted to the load.

For a simplified torsional spring:

Torque = Spring Stiffness × Angular Deflection

Suppose a spring has a stiffness of 100 newton-meters per radian.

If it twists by 0.05 radians, the ideal transmitted torque is:

100 × 0.05 = 5 newton-meters

Real systems may require calibration and compensation for other mechanical effects.

Why Put a Spring Inside a Robot Actuator?

A conventional stiff actuator can transmit sudden contact forces directly through its mechanical structure.

A series elastic actuator allows some controlled deformation.

For example, when a robot leg unexpectedly contacts the ground, the elastic element can temporarily store energy.

The controller can measure the resulting deformation and adjust actuator commands.

This can be useful for contact-rich movement and force regulation.

However, elasticity also introduces oscillatory behavior that must be considered in control design.

Series Elastic Actuator vs. Quasi-Direct Drive

A series elastic actuator deliberately includes an elastic component in the mechanical power path.

Quasi-direct drive generally combines a relatively high-torque motor with low gear reduction.

Both approaches can support responsive physical interaction, but they achieve their characteristics differently.

For example, an SEA may use measured spring deformation to estimate output torque.

A quasi-direct-drive actuator may rely on low transmission resistance and suitable motor-current or torque sensing.

Some actuator designs combine elements of both approaches.

What Are the Limitations?

The spring introduces an additional mechanical dynamic into the system.

This can limit how quickly force changes can be transmitted or controlled.

The actuator also needs suitable sensors and control algorithms.

Spring stiffness must be chosen for the task.

A very soft spring may allow too much movement.

A very stiff spring may provide less useful measurable deformation.

Frequently Asked Questions

Does an SEA always use a metal spring?

No. Different elastic materials or mechanisms can provide the required compliance.

Does an SEA automatically make a robot safe around people?

No. Safety depends on the complete robot, its movements, tooling, loads, and protective functions.

The Bottom Line

Series elastic actuators use deliberate mechanical flexibility to support force measurement and controlled physical interaction. They are useful where responsive contact matters, although the added elasticity requires appropriate engineering.