In this blog episode we'll be sharing information about Stewart Platforms and their usages in real life and various industries. We have divided this blog into the following 4 sections:
- Introduction to the Stewart Platform and Hexapods
- Tips, Tricks and Caveats of the Stewart Platform
- Why you may need a Stewart Platform?
- Glance on Acrome's Stewart Platform - An Accessible, Open Platform Solution for Education & Research
Let's start with the fundamentals first.
1.Introduction to the Stewart Platform and Hexapods
The Stewart platform is a special mechatronics system used for precision position and motion control, originally proposed in 1965 as a flight simulator [1]. Since then, a wide range of applications have benefited from the Stewart platform (more info available in Section 3 of this blog). By definition, a Stewart Platform is also a parallel robot, but with limited reach and mobility.
There are variants of the platform, but most of them have six linearly actuated legs with varying combinations of leg-platform connections. The full assembly is a parallel robotic system consisting of a rigid body top or mobile plate connected to an immobile base plate. Center of the top plateis defined by at least three coordinates, and aim is to move this center point in 3D space.
To move upper plate, each leg pair (lower and upper) must be controlled with an actuator to change the total length of the leg. By controlling each leg's length independently and with mathematical calculations called forward and inverse kinematics, the location of the top center could be changed precisely.
Devices placed on the top plate can be moved in the six degrees of freedom in which it is possible for a freely suspended body to move. These are the three linear movements x, y, z (lateral, longitudinal, and vertical), and the three rotations (pitch, roll, and yaw). Because of its motions, it is also called a or .
There are 2 critical performance criteria of every Stewart platform: Accuracy of the center location and response time/speed against a position change command.Mechanical and electrical components of the platform along with the system controller and software play important role in the performance. As being a mixture of the mentioned components, Stewart Platforms are falling under the focus of mechatronic engineering discipline.
A short definition of a hexapod would also be valuable in this context. You may see 2 different explanations when you search the term "hexapod". A hexapod robot is a mechanical vehicle that walks on six legs. But it should not be confused with the hexapod platform, which is also described as a specific type of a Stewart platform, restricted with 6-legs only. Bear in mind that there are different types of Stewart platforms having more or less than 6 legs, though you may see them rarely.
2.Tips, Tricks and Caveats of the Stewart Platform
In this section we will share some of the tips that we learnt from the hard way.
At first, the weakest link of every mechanical system is the coupling and durability of the moving parts.In Stewart Platform, these are the legs and their junction to the top and bottom plates. In our experiments we have noticed that physical characteristic and performance of the platform is highly susceptible to the movement gaps (ie. Backlash of the motors or gaps in joint bolts etc.). For the platform to move in a smooth trajectory, every joint should be precisely controlled.Joints' position uncertainty will heavily affect the final "combined" position error. To reduce the uncertainty, precision crafted joint materials with minimum tolerances should be utilized. Though with every precision manufacturing comes the burden of cost. So, price vs. performance tradeoff is also valid for Steward Platforms as with every physical system.
Another important element is weight and inertia. Equal lengths, sizes and weights are very important to minimize the effect of couple unbalances. Also placing the DUT (DUT: Device Under Test, is the device to be placed on the top plate and be tested with the Stewart platform) as close as possible to the center of the top plate is crucial in terms of reducing inertial momentum and weight (load) distribution to the actuators.
For controlling the leg movements; 2 different selections are available as for the actuators: Linear servo hydraulic systems and electrical motors. Both have some advantages and disadvantages, or we can call it caveats and describe them hereunder:
- Hydraulic systems are good for heavy loads. But they are bulky and more difficult to control, special control systems or algorithms are required for decent linear performance. Also, the response time of the hydraulic systems are limited compared to electromechanical systems.
- Electrical motors are small and easier to control. Using a linear motor / actuator makes the system compact in size. But it is difficult to find high capacity linear electrical motors. Using a rotating conventional electrical motor brings a new layer of complication to convert the rotational movement into a linear movement.
Due to cost and size constraints, we are building our Stewart Platforms using the linear electrical motors/actuators. Small size, low to mid power linear motors are easier to find and use. However, the control characteristics of every motor varies hence one needs to make calibration (input command vs. actual output) to achieve the best performance and to match the response of every joint.
Finally, we'd like to share some insights about the feedback mechanisms.
In a Stewart Platform, 2 feedback mechanisms (or sensors) are required to run a meaningful test. These are:
- Feedback for controlling the position of each leg. This comes from each actuator and used for PID or similar control loop for controlling the movement of each leg.For this task, most of the times an encoder is used.
- Feedback for sensing the actual location and orientation of the platform, which also gives idea about the location of DUT and gravitational forces applied on it. A Gyro and an accelerometer or combination of them called IMU/MPU is used for this purpose.
Let's give examples of each case based on our preferences in Acrome's Stewart Platforms (more info available at Section 4).
For leg feedback signal -as we are using linear actuators- we are using the integrated feedback sensor of the actuators. This sensor varies with the type of the actuator and manufacturer. Most of the times the actuator uses linear potentiometer as the sensor mechanism. Then an analog measurement channel is required to measure the signal. In some cases, non-contact magnetic strips are used, which also requires analog measurements. In very sophisticated applications, we have noticed the usage of digital sensors, output of these sensors also varies, but it is always in some sort of digital communication protocol (SSI or CAN are widely used selections).
For the DUT position, we prefer using a 6 DOF Gyro, or Accelerometer IMU –which combines a 3-axis gyroscope and a 3-axis accelerometer on the same device. The good thing is we have successfully used TDK's MPU-6050 in our platforms and there are quite good open source application examples showing how to use this sensor with various platforms, such as in Arduino robot kits.
3. Where you can see or need a Stewart Platform?
In this section we'll give 3 practical examples of the Stewart Platforms. From these examples (and many others that might be found through a web search) one can understand that Stewart Platforms are widely used in motion applications where precision positioning of a DUT on a fixed plane is required. So, "when we need a Stewart platform" should be an easy question to answer.
One of the major applications of Stewart Platforms is target tracking. In this application either target could be a stationary location while platform is moving (such as geostationary satellites or radar/seeker ground target) or target could be moving while platform is stationary (such as solar/stellar tracker applications). AMiBA radio telescope experiment is a good example for the latter case, where the telescope's antennas are directed towards target galaxies/stellar objects with the aid of a Steward platform [2].
Another widely seen application is the augmented reality applications seen in the gaming sector and entertainment industry. XD cinematic experience (starting from 4D-ends till 12D so far) is called for anything beyond the 3D cinema experiences. There isn't a standard in the naming but any cinematic experience where the seats are moving in synchronous with the scene uses some sort of a platform and this could also be named as a special type of Stewart Platform. Here there should also be a way to connect the platform to the scene generator using some computer communication interface. The moving "focus object" is the object that is being tracked in this type of Stewart platform where the seat (or maybe the gravity) is the stationary point.
Third, Stewart platforms are also used for experimenting terrain-based vehicle dynamics. There are many uses cases for this application area, mostly related with suspension systems. An example is the seat suspension, which could be implemented on various vehicles starting from bicycle up to space launcher. Here Stewart Platforms are used as the vibration generation platform for simulating the terrain induced vehicle vibration in laboratory. At [3] there is a very good PhD thesis giving lots of in-depth information in the design of the Stewart platform used for vehicle driver seat verification tests.
And finally, you may check our ACROME Stewart Platform's video to see a "flight simulator" where we have simulated the movement of the airplane against the pilots pitch, yaw and roll commands. There is a similar system operated by Lufthansa for training their pilots.
If you have concerns whether you'll need a Stewart platform or not, then send us a short message about your application and we'll try to help you finding out if a Stewart Platform would be suitable for your application or any other platform/solution would be better.
4. Glance on Acrome's Stewart Platform - An Accessible, Open Platform Solution for Education & Research
At Acrome we have gathered experience on motion platforms and automatic control. So, we decided to bring our know-how into a product, built for education and research. In our company's core values, we believe in the value of collaboration and sharing the information. As our motto, "Access and Accelerate", we have worked to make our Stewart Platform an open system both with the selection of the components and with the source code that runs on various controller options. As a result, the components of the platform could easily be changed according to the needs of the application or customers' demand.
As targeted for education and research purposes, all of the system (including the source code) is properly documented and these documents are available to download from the product page of the Stewart Platform. An example page out of the Stewart Platform's User Manual is given below as a reference. This document is prepared for the NI myRIO© option as for the controller of the platform. We have also Arduino™ and Raspberry Pi© options as the Stewart Platform's controller alternatives.
Acrome Stewart Platform is an optimum solution with a user-familiar, open platform made with precision crafted mechanics yet with an acceptable price tag. We offer simple but innovative experiment platforms (Check out ACROME products in action!) to practice fundamental concepts and even more with hands-on experiences. We also provide courseware prepared with both LabVIEW™ and Matlab™/Simulink™ code.
That's all from this blog. If you read until the end, then a big thank-you for your interest!
To learn more about our Stewart Platform, mechanic tips and tricks, and application codes etc. join us in an upcoming webinar of ours or feel free to send a message through info@acrome.net.
Related Readings:
[1] Stewart, D., "A platform with six degrees of freedom," Proc. Inst. Mech. Eng., Vol. 180, part I(15), 1965-1966, pp. 371-386.
[2] Koch, Patrick & Kesteven, Michael &Nishioka, Hiroaki & Jiang, Homin& Lin, Kai-Yang &Umetsu, Keiichi & Huang, Yau-De & Raffin, Philippe & Chen, Ke-Jung & Ibanez-Romano, Fabiola &Chereau, Guillaume &Locutus Huang, Chih-Wei & Wu, Ming-Hang & Ho, Paul & Pausch, Konrad &Willmeroth, Klaus & Altamirano, Pablo & Chang, Chia-Hao & Chang, Shu-Hao & Romeo, Bob. (2009). The AMiBA hexapod telescope mount. Astrophysical Journal - ASTROPHYS J. 694. 10.1088/0004-637X/694/2/1670.
Disclaimer:
Arduino is the trademark of Arduino AG and Raspberry Pi is a trademark of the Raspberry Pi Foundation. Matlab and Simulink are trademarks of The Mathworks Company. NI myRIO and LabVIEWare trademarks of National Instruments. Other product and company names listed are trademarks and trade names of their respective companies.
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