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.
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.