A series elastic actuator is a robotic actuator that places a spring between the motor and the load. This simple mechanical change makes the actuator less rigid, which helps robots interact safely with people and uncertain environments. Instead of only commanding position, the robot can estimate and control the force it applies.
SEAs are used in legged robots, exoskeletons, robotic arms, and rehabilitation devices where impact tolerance and gentle contact matter.
The key idea is that the spring deflects when force passes through it, and that deflection can be measured with sensors. If the spring stiffness is known, the actuator force is found from Hooke's law, F = kx. A controller compares the desired force with the measured force and adjusts the motor through the gearbox to reduce the error.
This gives accurate force control, shock absorption, and safer physical interaction, but it also adds compliance that can limit bandwidth and position precision.
Understanding Robotics: Series Elastic Actuator
The spring creates two moving sides inside the actuator. One side is driven by the motor and gearbox. The other side pushes the robot limb, tool, or joint.
Their difference in motion is useful information, but it is not perfect information by itself. Friction in gears, sensor noise, loose parts, and temperature changes can affect a reading.
Designers choose spring material and shape carefully so its bending or twisting stays predictable. They must keep the spring below its fatigue limit, because repeated loading can eventually cause cracks or a permanent change in shape.
A compliant actuator has its own natural vibration. When the motor pushes, the spring can stretch and rebound while the load moves. This is similar to a mass on a spring.
If the controller reacts too aggressively, it can add energy at the wrong time and make the joint shake. Engineers therefore tune the controller for stability, not only for fast response. They may filter noisy sensor signals and limit how quickly the requested force changes.
The gearbox ratio, load mass, spring stiffness, and control update rate all influence this behavior. A system that feels stable on a test bench can behave differently when attached to a moving person or a walking robot.
The spring can briefly store mechanical energy. In a running or hopping robot, this can soften a landing and return some energy during the next push. Human tendons do something similar in the body.
This does not mean the actuator creates free energy. The motor still supplies energy, and losses occur through electrical resistance, friction, and damping. The stored energy mainly changes when and how the energy is delivered.
This can reduce sharp impact forces and make motion feel less jerky. It is especially valuable when a robot foot meets uneven ground, since the contact point may move before the robot has time to calculate a correction.
Real devices need safety limits beyond the spring. An exoskeleton can limit its maximum joint force so it cannot push too hard if software fails. A rehabilitation machine can detect an unexpected resistance and slow down.
A robot arm can use gentle contact to guide a part into place without damaging it. When studying these actuators, pay attention to the tradeoff between softness and control. A very soft spring gives clear force measurements, yet it allows more lag and bounce.
A very stiff spring gives tighter positioning, yet small sensor errors become more important. Good designs match the spring and controller to the job rather than treating compliance as automatically better.
Key Facts
- A series elastic actuator places an elastic element in series between the motor or gearbox and the output load.
- Spring force is estimated using Hooke's law: F = kx, where k is spring stiffness and x is spring deflection.
- For rotational SEAs, torque is often estimated as τ = kθ, where k is torsional stiffness and θ is angular deflection.
- Force control uses feedback: error = desired force - measured force.
- Lower spring stiffness improves shock tolerance and force sensitivity, but can reduce position accuracy and response speed.
- SEA output motion depends on both motor motion and spring compression: load position can lag behind motor position during force changes.
Vocabulary
- Series Elastic Actuator
- A robotic actuator that includes a compliant spring element in series with the motor and load to enable force sensing and safer interaction.
- Compliance
- The tendency of a component or system to deform when a force is applied.
- Spring Deflection
- The change in length or angle of a spring caused by an applied force or torque.
- Force Control
- A control method that commands an actuator to apply a desired force rather than only reaching a desired position.
- Feedback Loop
- A control process that measures the system output, compares it with a target value, and adjusts the input to reduce the error.
Common Mistakes to Avoid
- Treating the spring as a weakness, which is wrong because the spring is the sensing and safety element that allows force estimation and impact absorption.
- Using motor current as the only force measurement, which is wrong because gearbox friction and motor dynamics can make current a poor estimate of output force.
- Forgetting units in F = kx, which is wrong because stiffness in N/m must be multiplied by deflection in meters to get force in newtons.
- Assuming a softer spring always improves performance, which is wrong because too much compliance can make the actuator slow, oscillatory, or inaccurate in position tasks.
Practice Questions
- 1 A linear SEA has a spring stiffness of 800 N/m. If the spring compresses by 0.015 m, what output force is being applied?
- 2 A rotational SEA has a torsional spring stiffness of 120 N·m/rad. If the angular deflection is 0.05 rad, what torque is measured at the output?
- 3 Explain why placing a spring between the gearbox and the load can make a robot safer during contact with a human or the ground.