On November 15, 2021, Russia blew one of its own retired spy satellites from the sky in a test of its “Star Warrior” anti-satellite (ASAT) system. The Kremlin’s message was clear: Push too far in Ukraine and risk losing the 32 satellites NATO relies on for its mission-critical GPS services, including ground troop and missile navigation. With such an obvious, looming systemic hazard, how can a world that now depends on those 32 GPS satellites mitigate its risk?

The University of Birmingham, with help from Warwickshire, England-based Computer Controlled Solutions Ltd. (CCS), may be closing in on an answer, and a key part of its solution involves ultra-precise motion control from Delta Motion to facilitate terrestrial gravity mapping.

The Need for Extreme Stability

Working from a background in physics, computing, and physical electronics, Directing Manager Paul Riley founded CCS in 1994 to make test machines that focused on “control, logging data, and breaking things.” Over the years, CCS has plied its expertise across a host of industries, from landfill gas management to brakes and clutches for racing car parts manufacturer AP Racing. Most recently, CCS has found a niche in aerospace parts testing rockets and determining if structures like flaps and wing tips can withstand up to 90 kilonewtons of air pressure.

CCS Stewart platform – 6 DOF. (Image: Delta Motion)

Along the way, CCS cultivated a relationship with the University of Birmingham, which has a specialty in rail research. That academic concentration led to the school’s ingenious “blue sky” research idea: Could there be a way to measure the microgravity of a specific point and, from those measurements, construct a topographical map of the world that could rival and potentially replace the GPS system, if needed? Furthermore, could those measurements be taken from a moving train as it crossed the land?

With such a grand scope, the line between “blue sky” and “pie in the sky” might be invisible. And that was before contemplating the equipment involved.

Paul Riley explained, “Researchers used to measure gravity by dropping a ball, then another ball, and then, if you could measure the acceleration between the two, you could get an idea of gravity. They could measure to an accuracy of maybe times 10-4 to 10-5. But now they have a new gravity system where they take cold atoms in superposition, position them with magnetic fields, and do essentially the same thing. They’re dropping atoms and measuring how far they’ve fallen with lasers. The results are so precise that they can get a gravitational ‘fingerprint’ of an area.”

The inverted Stewart platform devised for the terrestrial gravity mapping DOF 2 application. (Image: Delta Motion)

However, imagine trying to take such atomic-level readings with apparatus on a moving train (or an aircraft, which is also in planning). Keeping the equipment level would be like balancing a brimming glass of water on a tray while hurtling over rough terrain, only orders of magnitude worse. The person holding the tray would require superhuman reflexes and precision.

Riley had worked with Delta Motion on prior projects and expected they had the expertise that it would take to successfully tackle the train problem. His initial concern was that many of the applications for Delta’s products revolved around hydraulic motion control and Riley expected that hydraulics would be too large for the confined space of the rail car or aircraft fuselage. After a little more research, Riley was happy to find that Delta’s RMC motion controllers were capable of precision control of the electromechanical actuators that he wished to use. His final concern was that the magnetic fields of the electric motors would interfere with the sensors that measure and map the gravitational field. After testing it was determined that this would not pose an issue.

Delta Motion RMC200 Motion Controller. (Image: Delta Motion)

Any solution was likely to require active stability to maintain a level system during travel. Riley was very well acquainted with Stewart platforms, which often provide the six degrees of freedom needed for platform-based movement — much like our hypothetical water tray.

Riley knew from previous experience working with Delta Motion that Delta’s expertise extended well beyond simple point-to-point motion. For example, the company had worked with controlling Stewart platforms in applications such as automotive testing and even in filmmaking, where actors would jump onto a platform that mimicked the action of a moving vehicle. Riley suspected that the principles of a Stewart platform might work for his application… with some creative modification.

Get It Small, Keep It Steady

It didn’t take much study to realize that the weight and dimensions of their test apparatus combined with a conventional Stewart platform would exceed the size limitations of a train car. So, Riley and his team, in a sense, thought outside the box by designing inside it.

“We inverted the Stewart construction,” he said. “We have a ring of stainless steel that anchors the six arms. Then the arms go down, rather than up, and connect to a lower, six-sided platform. That means we can put these two containers of a hundred kilograms or so on this suspended platform, and it all fits in a train carriage.”

Even with the size issue solved, that still left motion control to conquer. Riley understood that control loops would have to be extremely tight to facilitate instantaneous reaction to train conditions. He had designed such systems for PID control with a field programmable gate array (FPGA), but the mathematics behind such solutions can be challenging, and what he finally wrote as one-off software ended up being so uniquely tied to himself and his own thinking that it was practically impossible for anyone else to support.

“That’s why I knew we needed to bring in Delta Motion,” he said. “Delta started with getting the math right for motion control, then built their algorithms around the theory. They looked beyond first-order positioning to speed, acceleration, and much more. Their software allows a standard engineer and anyone else to understand it and learn their processes because it’s a formal software implementation method. And they’ve sold so many units, they’ve got the control unit beautifully optimized for extremely fast control.”

The control software for the terrestrial gravity mapping system. (Image: Delta Motion)

Years before, CCS had integrated Delta motion controllers and hydraulic actuators into a military aircraft testing system simulating high G load on wings. That solution had required high response control, but now CCS could push even further with electric Bosch Rexroth drives and motors, with Delta’s motion control layered in for superior tuning.

To measure x/y/z position and roll, pitch, and yaw, CCS used a high-precision inertial measurement unit (IMU) much like those used on drones. Beyond that, the system had to account for things like bumps in the rail, slight accelerations and decelerations, and curves in the rail line. All of this had to be countered by the system in real-time to keep the platform stable. Such superhuman response times practically required looking into the future, so that’s exactly what CCS did.

“We implemented look-ahead analysis by placing another IMU in the carriage ahead. If that carriage saw a bump, we would pick up the speed through GPS and say, okay, we should see that bump in about 30 milliseconds. We could account for the coming change before it happens and synchronize any oscillations. It’s still a work in progress, but it’s quite a clever way of tying it together.”

Success and Support

The CCS team — Paul Riley is first on the left. (Image: Delta Motion)

Work on Birmingham’s gravity mapping project continues, and CCS keeps on refining its methods. For example, Riley’s team found IMUs were very good at determining dynamic positions, but they’re less accurate with DC levels and prone to drifting. So, CCS is now integrating inclinometers into its solution and pumping that data into the motion system to offset inaccuracies.

Ultimately, though, the project’s measure of success was whether the solution could provide the desired gravimetric measurements, despite the many challenges involved. On that front, the answer has been a resounding “yes.”

A key factor in this success was the time savings that CCS realized by using the Delta motion controllers. Rather than lean on a team of engineers to work out the math and code necessary for the solution, much of it came readymade off the shelf within Delta’s software. In turn, those engineers could focus more on solving larger design challenges and testing, allowing the team to stay within schedule and budget. Riley noted how regular meetings with Delta’s UK team were instrumental even at the earliest stages, just to determine if and how the group’s solution might be feasible. And whenever implementation questions arose, especially around math, Delta’s support was always just a call away.

Riley hopes to leverage the accomplishments of this gravimetry project into other fields, starting with electric vehicles. Currently, automotive manufacturers have a sizable headache with electric car batteries and the threat of them catching fire, especially in enclosed spaces like parking structures. Being able to test those batteries, and perhaps the entire vehicle, on a Stewart platform to simulate extreme use cases, could be extremely beneficial for millions of drivers.

As it turns out, motion control can not only help map the world — it might even make it a safer place.

This article was contributed by Delta Motion (Battle Ground, WA). For more information visit here .