Working against a looming deadline, the production company for the movie "Master and Commander: The Far Side of the World" was informed that the special-effects coordinator needed a new control solution—fast. Commander Productions' Dan Sudick wanted to make sure that the hydraulic motion controllers that powered the positioning gimbal underneath his main prop—a replica of the H.M.S. Surprise—were truly synchronized and the ship was made safe for the crew who would soon be walking the decks.

Sudick was concerned that an earlier control system was not capable of the functions required to properly operate the motion platform which, combined with the ship, weighed approximately 500,000 lbs. To avoid impacting the movie's shooting schedule, the special-effects crew had only a week to upgrade the design.

This time constraint required a motion control system that would be quick to install, program, and tune, plus a design team that could pull it off. A brief search led to the selection of Delta Computer Systems' RMC100 motion controller and Concept Systems Inc. of Albany, OR, a system integrator with hydraulic controls experience. Three days later, Concept Systems' co-owner and principal engineer Michael Gurney was on a plane to the Fox Studios production site in Baja, Mexico, with RMC100 hardware in hand.

The gimbal platform that supported the full-size ship replica was capable of three axes of motion: pitch (pivoting along the transverse axis), roll (pivoting around its longitudinal axis), and heave (vertical displacement). Heave control was actually implemented with four independent hydraulic axes, two at each end of the motion platform (see "Heave, pitch, and roll control" graphic).

Hydraulics was a natural choice for this application because hydraulic power excels at moving and holding heavy loads. Delta Computer Systems' controller was chosen because one unit can simultaneously coordinate the motion of up to eight hydraulic cylinders. However, controlling hydraulics smoothly enough to make the ship's motion look realistic while ensuring safety of the actors and crew required a hydraulic motion control program capable of producing a high degree of synchronization and precision.

In particular, avoiding damage to the gimbal structure required the positioning accuracy of each of four heave rams to be maintained to ±1 inch of each other, even as the 115-ton ship was tossed in a simulation of heavy seas. See "Hydraulic system components" for some specs of this huge hydraulic system.

Beyond meeting functional requirements of the system, Concept Systems' solution also had to provide a PC-based interface that would allow the special-effects operator to define motion profiles for the six axes. Programmed in rough form before hardware installation, the system was then ready for safety mechanisms to be tested and axes to be tuned.

"Early on, my only task was to stay ahead of the special-effects coordinator, director, and producer," Gurney added. So, I would make sure we could heave the ship all day long, if required. Once we got through all the basic moves and synchronization, proving the system would do what was required, I was put in a support role where I would move the ship to certain positions when required. It was during this time that I developed the main portions of the code that would make the ship sail."

Working around the clock, Gurney supervised the installation and had the system wired and powered up just as the crew was finishing mechanical modifications of the gimbal platform—three days after his arrival on site. The system was configured and tested and ready for operation the day shooting was scheduled to begin. Special-effects coordinator Sudick suggests that a different approach could have led to the loss of two or three weeks of expensive shooting time.

"As it turned out, I had very little time to test my code," said Gurney. "The centerpiece of my work was also the focus of everyone else's work. Painters, actors, lighting, everyone needed access to the ship. So, I found myself fighting for time with her, which I usually got right before they shut down the studio for the night."

To ensure strict adherence to safety procedures and that the ship was safe to work on and around, programming of more mundane items, such as fault recovery routines and operator interface screens, was also done and tested after hours.

Part of Gurney's work required the development of motion profiles that could be executed by the motion control system to cause the ship to move in specific ways. "These motion profiles could be stored, creating a library of different virtual ocean conditions—rough seas, calm seas, port winds, etc.," said Gurney. To gain inspiration for developing these profiles, Gurney watched videos of real ships sailing at sea and breaking through waves. Actual development of the corresponding preset motion profiles occurred during the after-hours testing.

Implementing motion profiles required tuning the motion control system much like any multi-axis hydraulic system. The first step was determining the dynamics of each axis and factoring those into the control system's operations. It is important to know how the system is going to react when factors such as command signals, load, friction, hydraulic flow characteristics, and distance from the valve to the cylinder come into play.

The procedure started with commanding very small, low-speed moves, analyzing the response of the axis, and making necessary adjustments. Once Gurney was sure that the system was under control, he started increasing the speed and size of the moves to final operating levels, making adjustments as necessary. Next, the motion of multiple axes working together was tuned up, again starting with small moves and graduating to realistic full-scale motion profiles.

The completed motion profiles allowed the system operator to change the ship's motion with the click of a mouse, making it possible to keep up with the rigorous production schedule.

"When the director would ask for high seas, we would drop that file into the motion controller and start running it," Gurney says. "The director typically would want to refine things—more delay here, more dramatic pitch there—which we could change on the fly while the ship was in motion. Profiles were developed such that they could run continuously, repeating on a regular basis by having the same start/end point. Once we found something the director liked, we would save it to file for use later."

This work proved to be an interesting departure from most of Concept Systems' projects. Industrial system integrators are usually called in to develop high-speed, highly accurate control systems, whereas the Fox Studios project was neither. During Gurney's first night of tuning, he was really fighting the system. Due in part to more than 250 feet of flexible hose between the hydraulic valves and cylinders, Gurney couldn't get cylinder positioning accuracies to within an inch of target values. For someone accustomed to tuning a system to the nearest few thousandths of an inch, this was disconcerting.

Gurney fought with the problem until someone came over and asked him how he was doing. Gurney's explanation of the problem was met with a laugh and the admonition: "You're done, put it away." Except for keeping the heave axes in lock step to prevent gimbal damage, positioning a 230,000 lb, 127-ft long ship to within ±4 inches was fully adequate to yield excellent visual results. In true engineering form, Gurney added, "I could have gotten every axis positioned to plus or minus an inch!"