How does one precisely control a motion base platform? (or how make a non-linear system linear)

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How does one precisely control a motion base so that the user can know where the motion base is in space and can modify where the centroid of the moving platform coordinate system is in space? Servos and Simulation’s engineers were recently presented with that exact problem. To compound the problem, Servos & Simulation’s motion base platform designs are non-linear in nature which makes modeling the motion base platform's math model very complex.

Servos and Simulation’s engineers have designed, programmed, and tested precision software to control motion bases.  Since there is no closed form solution to a non-linear Steward  Platform, a specially engineered math model was developed to take the commanded roll, pitch, yaw, X (longitude), Y (lateral), and Z (vertical) commands from the host computer and sends control signals to each of the six non-linear actuators on the motion base to instruct the motion base to accurately follow the commands.  The software takes care of all the non-linearities of the cranks and pushrods.  In addition, the software allows the center of the moving platform coordinate system to be moved to any location in space.  For example, the center of the moving platform coordinate system can be moved 20 inches above the center of the moving platform, and roll and pitch will be centered about this new coordinate center. Applications for aircraft pilot feel and realism or off the nose in missile testing scenarios require this ability.  With customization, the software can determine the location of a point(s) on a Unit Under Test in free space such as in antenna testing.

As part of the software package, Servos and Simulation’s engineers developed a Dynamic Software Evaluator package that reads the feedback potentiometers from each of the six actuators and calculates the roll, pitch, yaw, X, Y, and Z position of the motion base.  The roll, pitch, yaw, X, Y, and Z positions can be sent to a Host computer (or server) so the Host computer knows exactly where the moving platform of the motion base is at any time. The Control Software has an iteration rate of 1KHz, but the Host Software package can send data anywhere from 200Hz up to the 1KHz. Typical bandwidth of the Control Software is around 3 to 5Hz.

A separate Server Module handles the exchange of data to and from the Host computer to the Precision Control Software. Through a special software command set (SDK), motion can be started or halted, communications turned off or on, moved to specific locations etc. via the Server Module. The Server Module is also responsible for reporting position data from the Dynamic Evaluator back to the Host.

These two software programs (Precision Controller and Dynamic Evaluator) are independent of each other, but are memory mapped to work seamlessly between the Server Module, Precision Controller and Dynamic Evaluator.  Either one can be used (with the Server Module) by itself or used together.  As the Server Module is responsible for all communication, both software programs (Precision Controller and Dynamic Evaluator) are completely unaware of user inputs and rely solely on inputs derived from Host communications.

The video attached to this article shows the power of the software. The motion base is clearly a non-linear system. Notice that in each axis, the platform only moves in that axis and that none of the other axes interfere or become part of the axis in motion. At 55 seconds, the video's perspective is changed so that the observer can see how the actual center of rotation for the axes has been moved. In this case, it is about 44" from the center of the top platform.

As a possible customer of one of our systems, please be aware that we know how to remove the non-linearity of the motion base if that is an issue. For more information on how the software works, please call engineering.

For more information on our entire product and this software package, please check out our web site at or email us at

Specially designed motion base platform

Motion Base running with the Precision Software Package

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