Stewart platform….

Hmm… this time… lets cover something which is a little bit more scientific :p heheheheh… I made an article about this stewart platform for one of my subject in NTU , Precision Mechanism Design. Well… the lecturer asked the student to make some kind of review, and in addition to that, analyse the design and made an improvisation based on the analysis results. Hmm… I know.. I know… its a pain in the ass. But.. its an assignment… so no choice here😦 I chose this stewart platform for my assignment since its really interesting, simple yet complicated😀 A lot of factors need to be considered for the design … and the applications are enormous. Thats why I expect that I can get a really good mark for this one, and I did!!😀 Well… lets see if its really interesting….

image002

Fig. 1 Schematic of a Stewart platform
Source : Van Silfhout, R. G., “High-precision hydraulic stewart platform”, Review of Scientific Instruments, v. 7, no. 8 (1999) pp. 3488-3494.

Hmmm…. lets talk about this topic in general. Thus, I will not put in the analysis or improvisation made for this platform. Well… since the science and technology of robotics is originated by the objectives of developing mechanical systems which would carry out various tasks normally ascribed to the human beings, it is quite natural that the main thrust in the development was towards using open-loop serial chains as robot manipulators. Such kind of robot manipulators will have the advantage of sweeping large workspaces and also good manoeuvre ability like the human arm. Examples of this kind of robot manipulator are factory robot arms, diffractometers, wafer steppers, milling machines, etc. The disadvantages of these systems are that each axis has to support the weight of the other entire axis which is stacked on top of it in addition to that of the object weight. Additionally, the load carrying capacity is poor due to the cantilever structure. The stress load for the first axis is very large and the structure tends to bend under heavy load. The other problem is that the structure will vibrate at high operation speed. Therefore, the precision positioning capability of this manipulator is poor.

Robot manipulators for applications with high load carrying capacity, good dynamic performance and precise positioning are very important and it is desirable to make an alternative from this conventional serial manipulator. In general, it can be expected that mechanism with the end effectors connected to the ground trough several parallel link will not affected by the aforementioned disadvantages of conventional robot manipulator. In this case, each axis will acts directly on the object where the forces experienced and produced by the axis are completely compressive or expansive along the direction of its orientation. There will be no bending moments which usually tend to be a source of the mechanical play within the apparatus. The construction of this parallel mechanism is light but very stiff with an even distribution of the weight of the supported object over all axes.

The Stewart Platform is one example of a parallel robot manipulator. This mechanism offers very high stability and stiffness. It was originally developed by Gough for tire testing and then implemented as an aircraft motion simulator by Stewart. Hunt adopted this mechanism for a robotic arm and addressed some of its advantages, disadvantages and alternative mechanical incarnations. The illustration of the Stewart platform can be seen from Fig. 1.

This platform has a fixed platform as a base, a moving platform, and 6 parallel-actuated extendible legs between the two platforms. Each leg is connected to the moving platform trough spherical joints and to the base with universal joints. The position and orientation of the moving platform are controlled by the length of the six legs actuated by the struts. In this parallel mechanism, changing the length of the single strut will causes a realignment of the entire struts, causing the platform to move in all six degrees of freedom. Therefore, even the simplest motion of this platform requires a change in all the struts that must be calculated for each strut separately.

The precision positioning capability of the Stewart platform is superior compared to the conventional robot manipulator. The high accuracy of better than 10 um is achieved by using high-precision linear actuator and high-precision joints. One commercially Stewart platform from Physik Instrumente (1996) can move within a working envelope of approximately 100x100x 50 mm and 15 degress rotation in the central working area. The absolute positioning accuracy is better than 20 um, with repeatability better than 2 um or better than 2 arc-seconds rotational and a motion resolution of 1 um even under high loads of up to 500 N. Therefore, this mechanism is suitable for the precision positioning application which requires high repeatability, high resolution and high accuracy.

Application of Stewart Platform in Precision Engineering

Sensor application

The static force transformation and singularities of a Stewart platform have direct relevance to the use of this mechanism as a force-torque sensor. As the structure has good stiffness and the reconstruction of the wrench applied at the platform from measured legs forces is quite straightforward, a Stewart platform with instrumented elastic legs can be used as a wrist force sensor. Fig. 2 illustrates the Stewart platform based force-torque sensor.

image007

Fig. 2 Stewart platform based force-torque sensor
Source : Dwarakanath, T.A., Dasgupta, B. and Mruthyunjaya, T. S., “Design and development of a Stewart platform based force-torque sensor”, Mechatronics, v. 11 (2001) pp. 793-809.

Optical alignment in telescope application

The application of Stewart platform for astronomy has been implemented by European Space Observatory (ESO) in large Cassagrain telescope such as VLT telescope. A Stewart platform maintains the proper optical alignment of the secondary mirror with respect to the main mirror by correcting small deviations which are due to flexure and thermal distortions of the telescope. The photograph of the platform is illustrated in Fig. 3.

image009

Fig. 3 Photograph of the hydraulic Stewart platform
Source : Van Silfhout, R. G., “High-precision hydraulic stewart platform”, Review of Scientific Instruments, v. 7, no. 8 (1999) pp. 3488-3494.

Surgery

Applications of Stewart platform in medical world have been developed in recent years for neurosurgery, microsurgery, orthopaedics, ophthalmology, etc. Fig. 4 shows the Stewart platform with the endoscope and the glass phantom used in Wapler experiments [5]. The endoscope is mounted on a linear axis, which can be adjusted to optimize the working volume and to quickly remove the endoscope during operation.

image011

Fig. 4 Operating robot with phantom
Source : Wapler, M., Urban, V., Weisener, T., Stallkamp, J., Durr, M., and Hiller, A., “A Stewart platform for precision surgery”, Transactions of the Institute of Measurement and Control 25, 4 (2003) pp. 329-334.

Motion simulator
Stewart platform has been used widely in motion simulator. One of the example is the high frequency motion simulator shown in Fig. 5. The primary function of this mechanism is to apply high frequency, small amplitude disturbance to its payload. The capability of the system allows effective simulation of the disturbances experienced by a typical missile guidance system in terminal flight.

image013

Fig. 5 High frequency motion simulator final design
Source : Peterson, R., Novokov, M., Hsu, J., Gass, H., and Benson M., “6 DOF High-Frequency Motion Simulator Phase II”, Proceedings of SPIE – The International Society for Optical Engineering, v 4717 (2002) pp 56-67.

Acceleration compensation

Compensation for accelerations of an object on top of a moving robot was used to reduce the effect of inertia force. The objective of this method is to decrease the movement of a liquid surface for the transport of sensible fluid. The schematic diagram of the system is shown in Fig. 6.

image015

Fig. 6 Schematic diagram of a Stewart platform for acceleration compensation
Source : Graf, R. and Dillmann, R., “Acceleration compensation using a stewart platform on a mobile robot”, Proceeding of Third European Workshop on Advanced Mobile Robots (Eurobot ’99), 1999, pp. 17-24.

9 Responses

  1. can u explain it again?

    otak gak nyampe nih

  2. okeh..let me explain to you nang..
    it’s a project that very very usefull for a winner jomblo cup like you to get a get =)
    mas wiku, am i right?
    hehehhee

  3. i’m just the second winner, as far as i remember, u’re the 1st winner benx, right mas wiku?

    hahaha

  4. Danang: Nope. can not lah (singlish mode :p)

    Benx: Hahahhahaha…. now i am lost ….

    Danang: Still lost… sighhh…..

  5. “Now, explain it to me like I’m a four-year-old”
    –Joe Miller, Philadelphia (1993)

  6. waduh…………..

  7. Okie okie… I understand. I’m editing the article right now so that it will be easier to digest🙂

  8. so how you can control its movement?

  9. Kuda: You can control it by changing the length of the leg. Each desired movement can be achieved by adjusting the length of different legs at the same time. For example. If you want to move it upward (vertical), you extent all the legs at the same time. Well.. this is the simplest movement that you can make. More complicated movement like tilting and translation motion at the same time can be made.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: