A Stewart platform, also called a hexapod, is a parallel robot made from two rigid plates connected by six extendable linear actuators. By changing the six actuator lengths together, the top plate can move in all six degrees of freedom: three translations and three rotations. This makes the mechanism powerful for motion simulators, flight training, camera stabilization, machining, and precision positioning.
It matters because it shows how geometry, force, and control combine to create accurate three-dimensional motion.
Unlike a typical robot arm, a Stewart platform uses several legs at the same time to support and move one platform. Each actuator usually connects to the base and top plate through joints that allow angular motion while the actuator changes length. The controller solves an inverse kinematics problem, choosing the six leg lengths needed to produce a desired platform position and orientation.
Because loads are shared by multiple actuators, hexapods can be stiff, accurate, and strong for their size.
Key Facts
- A Stewart platform has 6 degrees of freedom: x, y, z, roll, pitch, and yaw.
- Each leg length can be modeled as L = |Ptop - Pbase|, where the points are measured in the same coordinate frame.
- Inverse kinematics finds actuator lengths from a desired platform pose.
- Forward kinematics finds the platform pose from six actuator lengths and is usually harder to solve.
- For small actuator changes, platform motion is often approximated by ΔL = JΔq, where J is the Jacobian matrix.
- Shared load support means each actuator carries part of the force, but the exact force depends on geometry and pose.
Vocabulary
- Stewart platform
- A parallel robotic mechanism with a moving platform connected to a fixed base by six adjustable legs.
- Hexapod
- A six-legged robotic structure, often referring to a Stewart platform with six linear actuators.
- Degree of freedom
- An independent way an object can move, such as translation along an axis or rotation about an axis.
- Linear actuator
- A device that extends or retracts in a straight line to produce controlled motion or force.
- Jacobian
- A matrix that relates small changes in actuator lengths to small changes in platform position and orientation.
Common Mistakes to Avoid
- Counting only three motions, not six, is wrong because the platform can translate along three axes and rotate about three axes.
- Assuming all six actuators always extend by the same amount is wrong because most platform motions require different length changes in different legs.
- Treating the legs as rigid fixed bars is wrong because the actuators must change length and the joints must rotate to allow motion.
- Ignoring joint limits and actuator stroke is wrong because a mathematically possible pose may be impossible for the real machine to reach.
Practice Questions
- 1 A hexapod actuator can extend from 320 mm to 470 mm. What is its total stroke length, and what is the midpoint length?
- 2 A platform must rise straight up by 25 mm without rotating. In an ideal symmetric hexapod with vertical actuators, by how much should each of the six actuators extend?
- 3 Explain why a Stewart platform is often stiffer and more precise than a single serial robot arm carrying the same top platform.