Frequency Response of a Servos & Simulation Motion Base Platform

Six axis motion base built for Raytheon

Over the past decades, Servos & Simulation has produced hundreds of motion bases. Most of them custom... most of them last decades. Recently, we were asked to build a custom motion base with a very specific set of criteria. It needed to be a six axis motion base platform, the CG was offset 40" from the center of the motion base top, the weight was 500 pounds at the 40" CG offset, it needed to always return back to the starting point within a very tight tolerance (so of the specifications for this system are under NDA, thank you for the consideration for the non-specifics), it needed to fit through a standard door and the non-linearity in the motion base had to be removed while keeping the angularity... nothing like shooting for the moon! For the final review, there was an Acceptance Test Procedure (ATP) written of which we surpassed for all specifications. Here is some of the test data from that ATP for the frequency response of the servo system.

The plots for the performance show the maximum position as a function of frequency, the maximum velocities as a function of frequency, and the maximum accelerations as a function of frequency.  The servo has an inner rate loop using the encoder on the motor for feedback allowing the servo to have a bandwidth of more than 40 Hz.  The outer loop is a position loop with a bandwidth of 4 Hz.  The position loop gain is limited by the gain available in the drive amplifier (power amplifier that drives the motor).  Servos’ engineers add electronics to the servos and increase the gain by at least a factor of 4 which increases the position loop bandwidth to 16 Hz.

The structure of the motion base is stiff enough that it does not limit the bandwidth of the position loop. This includes the motor/gearbox. Since the structure is stiff, the bandwidth does not need to be limited to less than 16 Hz.  The backlash in the gearboxes has been reduced to almost zero.  This is necessary since at high frequency the motor movement is quite small. If the gearboxes had any significant backlash, the gearbox output shaft would not move.  Servos’ engineers reworked the stock gearmotors to successfully remove the backlash from the gearmotors.

Another set of plots shows the motion base performance with a 16 Hz position loop bandwidth.  As can be seen from the plots, the high frequency response is much better with a 16 Hz bandwidth than with a 4 Hz bandwidth.

For both cases, the roll velocity is limited to 16.47 deg/sec, and the roll acceleration is limited to 230 deg/sec^2.  Pitch and yaw are similarly limited.  The Longitudinal velocity is limited to 15 inches/second and the acceleration is limited to 188 inches/second^2.  Lateral and vertical are similarly limited.

The velocity and acceleration limits can be increased within limits by changing the motors and gear ratios in the gear boxes.

If you would like more information on our entire product line, please contact us at info@servos.com or check out our web site at www.servos.com.

4Hz displacements - Frequency Response of S&SI Motion Base Servo

4Hz displacements - Frequency Response of S&SI Motion Base Servo

4Hz Velocities - Frequency Response of S&SI Motion Base Servo

4Hz Velocities - Frequency Response of S&SI Motion Base Servo

16Hz Displacement - Frequency Response of S&SI Motion Base Servo

16Hz Displacement - Frequency Response of S&SI Motion Base Servo

16Hz Velocities - Frequency Response of S&SI Motion Base Servo

16Hz Velocities - Frequency Response of S&SI Motion Base Servo

16Hz Acceleration - Frequency Response of S&SI Motion Base Servo

16Hz Acceleration - Frequency Response of S&SI Motion Base Servo

Summary
Frequency Response of a Servos & Simulation Motion Base Platform
Article Name
Frequency Response of a Servos & Simulation Motion Base Platform
Description
The Engineering Team at Servos & Simulation, Inc., has analyzed the performance of one of our motion base designs.
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Publisher Name
Servos and Simulation
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