What is a Hexapod?
Hexapods – also referred to as Stewart Platforms – are 6-legged robotic platforms that offer parallel kinematic motion in 6 degrees-of-freedom. In motion control applications, hexapods can either be used for high precision positioning requirements, or for generating fast and repeatable motion trajectories.
The motion of a hexapod is determined purely from the motion of six interrelated actuators on its legs. This is known as parallel kinematic motion. This differs in several ways from serial kinematic motion in which a combination of single-axis positioners are attached to each other. Most notably, a hexapod provides user-configurable pivot points and coordinate systems. As such, the versatility of hexapods makes them ideal for alignment applications both in photonics and more widely.
How does a Hexapod Work?
The six legs of a hexapod consist of motorised actuators that can all move independently of each other, but are all constrained by two plates, a heavier – and fixed – base plate, and a lighter, moving top plate that is designed to be the platform for the payload that needs to be moved or positioned. The final position of the moving platform is a summation of the positions of the actuators.
Inverse kinematic equations are required to determine this position and are calculated by a sophisticated motion controller allowing for the end user to command the positions of the top platform only, without the need to consider the individual motion requirements of each leg.
What are the advantages of using a hexapod?
Compared with a serial kinematic system, a hexapod has many advantages that can be considered for motion and positioning applications.
- For a hexapod, the applied forces of any payload are shared between all six legs. For a stacked system however, the magnitude of any payload gradually increases stage by stage, cumulatively adding up so that the largest forces are experienced by whichever stage is at the bottom of the stack, and the smallest forces are experienced by the top-most component. This lack of cumulative loading improves dynamic performance because the mechanical resonant frequency of the system is higher, and the response time is therefore quicker.
As well as the forces being distributed evenly over the legs, this design can also contribute to a superior positioning accuracy. Each individual stage in a serial kinematic system has its own positioning errors, most critically the angular pitch, yaw, and roll errors that are caused mostly due to bearing imperfections and slight orientation misalignments when bolting stages together. These angular runout errors combine cumulatively to form an Abbe error at the surface where the payload is mounted. Because an Abbe error increases the further away the measurement point is from its origin, tall stacks of stages can be known to cause large inaccuracies at the final positioning point. For a parallel kinematic hexapod, the errors are all acting on one shared platform, reducing this angular runout contribution and ultimately leading to much smaller Abbe errors for the same conditions.
Another significant disadvantage to consider for a multiple stage assembly is that each stage will have its own cable connecting it to the motion controller. These can pull on the motion stages while they move and can add an additional load to the system on each layer. Hexapods, in comparison, only have cables going to the fixed base plate, eliminating the complexity of moving cables. While cable management systems exist, they can be expensive to install and compromise the footprint of any motion system that they are attached to.
- For a hexapod, all the motion on the top-plate is mathematically derived from the various cumulative displacements of the six actuator legs. As a result, it is possible to change the shared pivot point for the rotational motion through software only. What’s more, with a capable-enough controller, this can be done on-the-fly, allowing for huge flexibility in motions and trajectories. For a serial kinematic system, the rotary stages will all have a fixed pivot point, meaning that the centre of rotation is always restricted. Of course, the orientation can be changed by adjusting the position when the system is at rest, but this is a significant limitation.
- For the same reasons as above, the user coordinate system for a hexapod system is also highly configurable. Indeed, it is possible to have multiple coordinate systems defined at once, for example a global reference workspace coordinate system and a specific coordinate system used for a tool that may be attached to the hexapod, but in its own separate orientation. Switching between the two coordinate systems can be done with a simple command, making complex alignment tasks simple to understand for the user.
- Finally, from a form-factor point-of-view, it is much easier to include a through-hole aperture on a hexapod than it is to incorporate such a design into an assembly of stages. This can be useful for allowing the transmission of light for beamline applications, or for feeding extraneous fibres/cables through for processes that require this.
What are the disadvantages of using a hexapod?
While a hexapod has many advantages, there are also a few potential drawbacks to consider.
- The permissible travel range of a serial kinematic system is very simple to understand – its constraints are just the travel range of each stage within its individual axis. This is true for motion in one axis only, or for when motion in all 6 axes is required at the same time. In comparison, the travel range of a hexapod is more complicated to define. As the struts are all confined to the same two bases, then it follows that any motion on the top plate will require a motion from all six actuators together, and conversely that motion in any one actuator will affect the position of all the others. While a hexapod can be, and often is, defined by its maximum single axis travel range, these capabilities will correspondingly shrink when motion is required in more than one axis at the same time, leading to what is known as an envelope-of-motion. This envelope-of-motion can be harder to define, especially when compound travel in multiple axes is considered, and changing the pivot point can also result in a further restricted motion space.
- For low precision applications, the cost of a hexapod system, along with a 6-axis controller that is sophisticated enough to run the inverse kinematic calculations required to determine the hexapods position, is usually higher than that of individual stages required for a serial kinematic solution. For this reason, it is important to match up the requirements of any application with the correct technological solution, so that both cost and performance can be considered.
Motion Hexapods Versus Positioning Hexapods
A hexapod can be used either as a 6 degree-of-freedom positioning system, or as a multi-axis dynamic motion system.
So called “positioning hexapods” are designed for precision alignment applications for optical systems, telescopes, and scientific experimentation, as well as handling in precision automation and manufacturing processes. “Motion hexapods” on the other hand are designed for providing dynamic motion for large payloads, predominantly required in the motion simulation and sensor characterisation fields.
Positioning Hexapod Application Examples
- A typical use of hexapods in the photonics industry is in the qualification and/or calibration of optical instruments. They can help to position lenses, mirrors, and detectors at high resolution to allow for high-class optical qualification.
- Hexapods are often used to position the secondary mirror in optical and radio telescopes around the world. These mirrors are often heavy payloads and require positional accuracy in the region of ten of µrad while being operated in an outdoor environment at high altitude.
- Precision hexapods are particularly suited for use in scientific applications, especially within synchrotrons where they are used to align samples, mirrors, and components with respect to the beamline with high precision and long term positional stability.
Motion Hexapod Application Examples
- One of the most common uses for dynamic hexapods is for motion simulation because they can perform repetitive high-speed motion, following predefined and on-the-fly trajectories. Whether this is for characterising motion sensors for automation or aerospace vessels, or simulating vibrational motion for cameras and touch screen devices, the capabilities of dynamic hexapods are ideally suited. They have even been used in the medical field for simulating human walking and running gait.
- In the naval industry, dynamic hexapods can be incorporated into multiple use cases. Large dynamic hexapods have the required angular travel range and acceleration capabilities to be used for swell simulation devices. Smaller dynamic hexapods can be used in a similar way for moving model sea vehicles within wave basins to study the fluid dynamic response and associated forces produced on the vessel.
Summary
Hexapods can be used for a variety of purposes, offering flexibility where stacked motion systems often cannot. There are many benefits to considering a parallel kinematic motion system, however these should be weighed up against the specific application requirements.
Pro-Lite is partnered with the French manufacturer Symétrie, who are hexapod specialists with over 20 years in the market. Symétrie design and manufacture over 15 standard hexapod products and have the knowledge and expertise to design customised products, with specifications tested in-house with their world-class metrology capabilities. Along with Symétrie, Pro-Lite adopts a collaborative approach to ensure that the right product is available for any requirement.
