What is a Stewart Platform, in the first place?
Six degrees of freedom (DoF) motion platforms are called Stewart Platforms, and they are utilized by many different sectors. The platform was first created for simulation in domains like aeronautical engineering, but it is being utilized in a variety of applications, including medical devices and industrial robots. Please check our blog named What Are Hexapod Robots Used For? to learn more details about popular applications with Hexapods/Stewart Platforms.
Stewart Platforms are composed of a top platform that may be moved and a permanent base that are joined by six actuators (usually hydraulic or electric). This arrangement allows the platform to move linearly in many directions—forward-backward, left-right, up-down—as well as conduct rotating movements. While designing the hexapod-legged robots, one needs to consider a large number of possibilities. Several decisions need to be considered about the design that will have an Influence on the operation and technical features. Some of the most significant design issues and constraints are as follows:
- The mechanical structure of the hexapod,
- Leg architecture,
- Actuators and drive mechanisms,
- Control architecture,
- Max size, max payload
- Power supply,
- Operational features,
- Cost, etc.
Although there are many other intricacies and details about Stewart Platforms, there is one concept called Virtual Pivot Point, which is an important and a multi-dimensional topic covering the operational performance, application capabilities and software development tasks. This blog post is aiming to give more details about this topic.
The Virtual Pivot Point (VPP) Concept
One key idea in managing a Stewart Platform's motions is the Virtual Pivot Point (VPP). VPP is a workable solution in robotics and precision movement applications because it offers notable benefits in the kinematic and dynamic control of the platform. The VPP is essentially a mathematically defined rotational center that is produced by control algorithms rather than existing physically.
Let's take a closer look at the Stewart Platform's functionality and the significance of VPP as an invention to see why it works so well to improve the platform's accuracy.
When the pivot point is defined at the center of the upper plate of the platform (this is the default pivot point) the roll motion occurs precisely around this center. However, when the pivot point is moved -let’s say 150 mm above the platform- the roll motion follows a circular trajectory with a radius of 150 mm. around this “virtual point” in space, thus the name virtual pivot point comes from.
The pivot-point concecpt is particularly useful in many systems such as satellite, imaging systems etc. where maintaining a fixed focal point is essential for precise alignment and control. ACROME's Stewart Platforms software allows the pivot point’s position to be adjusted in 2 methods:
- Via the Graphical User Interface (GUI), which is used in non-programming cases such as commissioning or simple motion tasks
- Or via the API commands (Application Programming Interface), which is used by software or robotic engineers for custom software development needs.
The next section explains these 2 alternative methods and how each can be used individually.
Software for Virtual Pivot Point
The top platform of a Stewart Platform can move in space thanks to six connecting rods. The platform's position and angular orientation can be changed by varying the length of each rod. Six axes of precision control (X, Y, Z, Roll, Pitch, Yaw) are made possible by this architecture. The intricate task of regulating a Stewart Platform is figuring just how much each connecting rod should move. To drive the upper platform in a specific direction, coordinated movement is necessary, as each of the six actuators needs to be controlled separately. There is a chance for computation errors when attempting to determine an accurate pivot point.
The VPP feature has been added to ACROME's Stewart Platforms after the software version 2.2. This feature will remain enabled in the subsequent software versions as well. The VPP setting can be changed via the default Graphical User Interface of the product or via the API commands, separately.
The API commands also can be used to change the default pivot point into a virtual pivot point, similar to the GUI controls. An example Python code is provided below to show how it is done:
message.append("enable,True")
message.append("move_platform,0,0,447,0,0,0") # Move platform to home position
message.append("SetPivot,0,0,150") # Set pivot point to (0,0,150)
message.append("move_platform_traj,0,0,447,10,0,0,5000") # Roll movement, automatically adjusted based on the pivot point
message.append("delay,5")
message.append("move_platform_traj,0,0,447,-10,0,0,5000") # Negative Roll movement, offset relative to the pivot point
message.append("delay,5")
message.append("enable,False")
The Virtual Pivot Point's Benefits
Controlling the intricate movements of the platform is made significantly more innovative with the help of the Virtual Pivot Point (VPP). This point offers more design flexibility because it doesn't physically exist and doesn't put any strain on the platform. Anywhere on the platform, or even at a virtual location off the platform, can be designated as a rotational center with VPP. Among the benefits are:
- Flexibility and Customizability: VPP facilitates platform customization for various apps. For instance, the VPP can be positioned anywhere in the cockpit of a flight simulator, increasing the realism of the pilot's motions.
- Simplifying Complex computations: Mathematical modeling and computations are made simpler by using VPP. A virtual definition of the required movement of the platform with respect to a given point allows the actuators to be synced with it. Movement accuracy and dynamic balance are enhanced as a result.
- Precision and Stability: VPP also aids in lowering system faults. There are no variations brought on by mechanical deformations in the connecting rods because there is no physical pivot point. As a result, the platform moves and positions itself more accurately.
Stewart Platform and VPP Applications
There are numerous uses for the Stewart Platform and Virtual Pivot Point in automation and robotics technologies. Here are a few instances:
- Simulations: Accurate platform movement is essential in driving and flight simulations. VPP permits the definition of a virtual rotational center, boosting the realism of the simulation.
- Surgical Robots: The robotic arms must move precisely and safely when doing surgery. Surgical robot precision is improved by VPP, leading to safer and more effective procedures.
- Industrial Robots: Precision assembly in different manufacturing lines or material handling are two uses for Stewart Platforms utilized in factories. VPP gives these robots more mobility, which facilitates quicker and more error-free production.4. Seismic Testing: To assess a structure or bridge's ability to withstand earthquakes, seismic testing is done using Stewart Platforms. By employing VPP, earthquake-like phenomena can be recreated with exact movement about a particular virtual point.
Summary
This article provides an overview of the Stewart Platform and the Virtual Pivot Point (VPP) concept, explaining the functional necessity and how it works in reality. Document emphasizes the benefits of using the VPP in various applications, and shows how to use the VPP feature in ACROME's Stewart Platforms. Examples from engineering and simulation tasks are provided.
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