A Stewart platform is a type of parallel robot that can move in six degrees of freedom (6-DOF), meaning it can translate and rotate in all directions. Also known as a hexapod, a Stewart platform is a mechanical system composed of two rigid plates (fixed and moving plates) connected by six extendable legs (actuators). By precisely controlling the length of each leg, the platform can move in multiple directions and orientations. These movements include:
- Translation along the X (surge), Y (sway), and Z (heave) axes.
- Rotation around the X (roll), Y (pitch), and Z (yaw) axes.
This combination gives the Stewart platform its powerful motion capabilities.
Video 1 – Motion capabilities of a Stewart Platform (shown in 2x speed)
The core principle behind a Stewart platform is parallel kinematics. Unlike serial robots (like robotic arms or stacking up linear stages), where joints are arranged one after another, a Stewart platform uses multiple actuators working simultaneously to control motion.
Each of the six actuators connects the base (fixed) to the top (moving) platform via joints (often spherical or universal joints). When the actuators extend or contract, they change the position and orientation of the top platform.
Key components:
- Base platform: The fixed bottom plate fastened to the ground or table.
- Top platform: The moving upper plate where the payload is attached.
- Six actuators: Typically hydraulic, electric, or pneumatic (always electric for Symetrie hexapods).
- Joints: Allow angular flexibility between components.
Figure 1 – Components of a Stewart Platform
The system relies on inverse kinematics to compute how each actuator should move to achieve a desired position.
Advantages and limitations of a Stewart platform
Thanks to its parallel structure, a Stewart platform offers many advantages compared to serial structures:
- High precision: in a serial structure, the errors add to each other, but because all actuators of the Stewart platform contribute simultaneously, errors tend to average out, resulting in very fine positioning accuracy.
- High stiffness: the parallel structure of the Stewart platform distributes loads efficiently, making the system very rigid and stable, relative to its own mass.
- Compact design: compared to serial robots, Stewart platforms can achieve complex motion in a relatively small footprint.
- High load capacity: Stewart platform can support heavy payloads relative to their size due to their structural geometry, for instance Symetrie hexapods have payloads capabilities from 5 kg for the smallest model, up to several tons for the largest model.
- 6-degrees of freedom: this sounds trivial, but a Stewart platform always offers at least 6-DOF, sometimes at about the same price as stacking up 3 linear stages.
- No moving cables: cables are connected at the base of each actuator, next to the fixed platform, so by design, they are not moving.
- Configurable pivot point: the pivot point can be configured through the control software and can be located anywhere in space.
Despite their advantages, Stewart platforms also have some constraints, the main one being their limited travel range: unlike serial robots, moving in one direction or one orientation on a Stewart platform will limit the maximum travel range for the other degrees of freedom.
Key applications of Stewart platforms
Stewart platforms are used wherever precise, multi-axis motion is required. Their ability to deliver synchronized movement across six degrees of freedom makes them especially valuable in high-performance and safety-critical environments. Below is a non-exhaustive list of key applications for Stewart platforms:
Flight simulators
One of the most iconic applications of the Stewart platform is in flight simulation. The platform reproduces aircraft motion—roll, pitch, yaw, and translational accelerations—to create a realistic training environment for pilots. High-end simulators combine the hexapod’s motion with visual and vestibular cues to mimic turbulence, takeoff, landing, and emergency scenarios with high fidelity.
Automotive & naval industries
Similar to aircraft simulators, Stewart platforms can simulate real-world road conditions and sea swell for applications like durability testing, ride comfort evaluation, and component validation, so that engineers can test vehicle behavior without leaving the lab. They are also used in testing autonomous vehicle sensors, where precise and repeatable motion profiles are critical.
Aerospace & SATCOM
In aerospace, Stewart platforms are used for antenna alignment, satellite testing, and optical system positioning. For example, they can simulate the orientation of a satellite in orbit or precisely align instruments during integration and testing phases. Their stiffness and accuracy are key for maintaining alignment under load.
Manufacturing, metrology & precision engineering
Stewart platforms are used in precision manufacturing processes such as CNC machining, laser cutting, and semiconductor alignment. In metrology, they serve as positioning stages for measurement equipment, enabling accurate inspection of parts and assemblies. Their rigidity minimizes deformation, which is essential for maintaining measurement accuracy.
Medical & surgical systems
In the medical field, Stewart platforms are used in surgical robotics, rehabilitation platforms, and imaging systems. They enable extremely precise positioning, which is critical for minimally invasive procedures or radiotherapy systems where sub-millimeter accuracy is required.
Robotics
Stewart platforms are widely used in robotics labs and research institutions for motion control experiments, calibration systems, and validation of control algorithms. Their predictable and repeatable kinematics make them ideal for studying advanced topics such as control theory, sensor fusion, and dynamic system response.
Entertainment & virtual reality
Beyond industrial applications, hexapods are also used in motion platforms for gaming, virtual reality experiences, and theme park attractions. These systems leverage the same principles as flight simulators but are optimized for immersion and user experience rather than strict physical accuracy.
A dive into Symetrie Stewart platform solutions
Symetrie offers a wide range of high-end Stewart platforms, categorized in two families: positioning hexapods and motion hexapods. All Symetrie hexapods use electric actuators (not hydraulic nor pneumatic).
Positioning Hexapods
These Stewart platforms are designed to achieve very fine displacements, with high repeatability and accuracy. Positioning hexapods are typically slow (but not always) and are usually operated in a point-to-point fashion. They are also very stiff as many applications require holding a position, either while the hexapod is on and running or when the hexapod is off. In the latter case, the hexapod needs to be mechanically irreversible, which most of Symetrie hexapods are.
Payload capacity & resolution:
Symetrie positioning hexapods product range starts at 5 kg maximum payload capacity with the SOLANO miniature hexapod and goes all the way up to 1,500 kg maximum payload capacity with the JORAN hexapod, with sub-micron resolution for most of the catalog.
All hexapods are using absolute encoders, except the BORA and PUNA that are using incremental encoders, but an option called Virtual Homing is available to avoid having to do the homing procedure at every boot-up. The most high-end hexapods are using linear absolute encoders directly mounted on the actuator (ZONDA & JORAN hexapods).
Figure 2 – Symetrie positioning hexapods displayed to scale
Environment:
Positioning Stewart platforms became very popular tools for optics and physics labs and factories, and therefore sometimes need to be compatible with clean or harsh environments. All of Symetrie positioning hexapods are clean-room compatible with ISO 7 class in standard, and down to ISO 5 class as an option.
Some models can even be customized to be compatible with high vacuum (10⁻⁶ mbar) environments (MAUKA, BORA, ZONDA & JORAN hexapods), and with extended temperature range compatibility from -40°C up to +60°C (BORA & ZONDA), vs. 0°C to +50°C in standard.
Symetrie also has experience designing and manufacturing non-magnetic hexapods, but those are not part of the standard catalog.
Finally, some positioning Stewart platforms are being used outdoors, in dusty, and sometimes rainy and/or snowy environments (for instance when used as secondary mirror or sub-reflector positioners in telescopes). In those cases, the hexapod can be customized to be compatible with this outdoor environment, typically by adding a protective bellow between the fixed and mobile platforms.
Control software:
SYM_Positioning is the GUI software included with each hexapod that allows simple tasks like configuring the center of rotation and moving to safe (pre-validated) positions based on pre-configured payload characteristics (mass, COG position) and coordinate system, or more complex tasks like generating a sequence of positions.
Symetrie can also supply a free-of-charge API and specific SDKs, programming libraries or examples for C++, LabVIEW, Matlab, Python, EPICS, TANGO, etc …
Lastly, a hand-held control unit can be provided for manual controls, with the possibility to select the axis and increment size, and then to do some jog or continuous movements.
Video 2 – SYM_Positioning GUI software main features
To learn more about Symetrie’s positioning hexapods, visit our product pages.
Motion Hexapods
Motion Stewart platforms are typically used for simulation applications where dynamic specs like speed and acceleration matter more than resolution and accuracy. Symetrie motion hexapods still are on the high-end of the spectrum when it comes to resolution, repeatability and accuracy specs, but with typically one to two orders of magnitude difference with positioning hexapods. Symetrie hexapods are extremely popular for applications like sea-motion, aircraft motion or ground-based vehicle motion simulation or for motion compensation applications with similar motion profiles.
Payload capacity, speed & acceleration:
Symetrie motion Stewart platform’s product range starts at 50 kg maximum payload capacity, and goes up to 6 tons, with maximum rated speed typically between 1 and 2.5 m/s and acceleration rates up to 10 m/s2 or 1 g.
Figure 3- Symetrie motion hexapods displayed to scale
Environment:
Symetrie motion Stewart platforms are often used outside and can therefore be exposed to dust, rain, snow as well as to wider temperature ranges. Each one of those hexapods can be modified to be outdoor compatible, typically with anti-corrosion treatments, protective bellows or skirts to limit the intrusion of dust and water in the joints, and with the use of different grease and parts that can accommodate the wider temperature swings.
Control software:
SYM_Motion is the GUI software that allows the user to control the motion of dynamic hexapods, with the following control features: point-to-point, sinusoidal and harmonic trajectories, or more complex pre-defined or pre-recorded trajectories. All those movements require software validation before being played.
An optional API is available for customers interested in interfacing with the hexapod through their own control software.
The UPD Acquisition is another optional feature that allows the user to retrieve the exact position of the hexapod directly from the absolute encoder’s values, either in real time or in post-processing.
Lastly, the ERTT feature (External Real-Time Trajectory) is an option that allows users to control the hexapod in real-time without prior trajectory validation, for example by using a joystick, or by feeding trajectory information live.
To learn more about Symetrie’s motion hexapods, visit our product pages.
You can also read more about the applications of hexapods on this page.
This post was written by:
Vincent Renaud, Sales & Application Engineer
