Engineering the Impossible: A Custom Six-Axis Motion Base with Extreme Offset CG

Over the past several decades, Servos & Simulation has delivered hundreds of motion base platforms—most of them custom-built, and many still in operation decades later. Recently, we were challenged with one of our most demanding builds yet: a custom six-axis motion base platform with a highly specific and complex set of requirements.

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The Challenge

The client needed a motion base that could:

Operate with a 40-inch center-of-gravity (CG) offset from the platform center
Support a 500-pound payload at that offset
Return to its starting position with extremely tight tolerances
Fit through a standard doorway
Eliminate non-linearity in motion while preserving angularity

Some of the system specifications are under NDA, but we can share that this project pushed the boundaries of motion platform design. It was, quite literally, a moonshot.

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The Solution

Our engineering team designed a system that not only met but exceeded all performance criteria outlined in the Acceptance Test Procedure (ATP). Below are highlights from the servo system frequency response testing:

Servo System Architecture

Inner Rate Loop: Uses motor encoder feedback, achieving a bandwidth > 40 Hz
Outer Position Loop: Initially limited to 4 Hz due to amplifier gain constraints
Enhanced Performance: By integrating custom electronics, we increased the gain by a factor of 4, boosting the position loop bandwidth to 16 Hz

Mechanical Design

The motion base structure and motor/gearbox assemblies were engineered for high stiffness, ensuring that structural flex did not limit bandwidth
Backlash was virtually eliminated from the gearboxes—critical for maintaining precision at high frequencies
Our engineers reworked stock gearmotors to achieve near-zero backlash, enabling accurate micro-movements even at high speeds

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Performance Metrics

With the upgraded 16 Hz position loop, the system demonstrated significantly improved high-frequency response. Key performance limits included:

Roll Velocity: 16.47°/sec

Roll Acceleration: 230°/sec²

Pitch & Yaw: Similar limits

Longitudinal Velocity: 15 in/sec

Longitudinal Acceleration: 188 in/sec²

Lateral & Vertical: Comparable performance

These limits can be further increased by adjusting motor specs and gear ratios, offering scalability for future applications.

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Conclusion

This project is a testament to what’s possible when engineering precision meets innovation. Despite the extreme CG offset, tight tolerances, and compact footprint, the system not only met but surpassed all expectations.

At Servos & Simulation, we don’t just build motion platforms—we engineer solutions that redefine what’s possible.