TWISTS AND TURNS OF MOTION SIMULATOR RIDE REPLICATED USING NEW ACTUATOR SIMULATION TECHNOLOGY

Iwerks Entertainment, Inc., Burbank, California, is one of the world’s leading suppliers of location-based entertainment attractions, such as the TurboRide cinematic simulator shown here. This type of simulator was the subject of an engineering analysis by amusement ride expert Ed Pribonic using Mechanical Event Simulation software from ALGOR, Inc., Pittsburgh, Pennsylvania.

By: Edward M. Pribonic
President, Edward M. Pribonic P.E. Engineering and Consulting, Seal Beach, California, www.aimsintl.org/pribonic.htm

As an engineering consultant specializing in amusement rides, theme park design and water parks, my job often seems more like play than work. I have worked on countless theme park simulations -- flight, free-fall, explosions, train rides and space travel, just to name a few. While it is fun and exciting, engineering in the entertainment industry is also a very serious business. Millions of people take tens of millions of thrill rides each year in amusement parks. Assuring their safety as they roar through the loops of a roller coaster at 60 mph, or as they dive and dodge exploding asteroids in a flight simulator, is a demanding engineering responsibility.

Fortunately, rapid advances in science and computer technologies, especially simulation software, now support engineering analysis. However, for slide rule-trained engineers like myself, realizing the benefits of the latest simulation tools can require some extra time and training. My efforts paid off recently when my firm created an engineering simulation using physics-based Mechanical Event Simulation (MES) software from ALGOR, Inc., Pittsburgh, Pennsylvania, to verify the integrity of a cinematic motion simulator ride.

A family entertainment complex in Edinburgh, Scotland, purchased a new TurboRide from Iwerks Entertainment, Inc. (Iwerks), headquartered in Burbank, California. Iwerks contracted Edward M. Pribonic P.E. Engineering and Consulting (Pribonic Engineering and Consulting) to verify that the American-built simulator would meet requirements set by the British Fairground Standard for amusement ride safety. After recognizing the inherent limitations of using traditional linear static stress analysis to analyze a dynamic event, my firm researched alternative FEA and kinematic analysis solutions and concluded that MES provided the best solution for the engineering analysis needs of the project. Ultimately, the MES enabled the installation of the ride to move forward and provided my client with a virtual prototype for future product developments.

The Dilemma

The Edinburgh installation consists of 12 four-seat motion bases. The fiberglass seats are supported by a welded steel frame of rectangular tubing, known as the flying platform. Six double-acting hydraulic cylinders connect the flying platform and floor-mounted base unit. Each cylinder is fastened at one end to the base unit via a cast iron bearing. The base unit has three mounting plates, each with two bearings to accept the lower ends of the cylinders. The computer-controlled hydraulic cylinders extend or retract independently, providing the seat and occupants roll, pitch, yaw, heave, surge and sway motion with six degrees of freedom in coordination with the on-screen adventure action.

After reviewing the simulator design, Iwerks, their client and I determined that all components of the ride should be structurally analyzed with the bearings as the main focus of the engineering analysis. We needed to verify that the simulators would safely withstand the dynamic loading caused by the actuating cylinders, the weight of the entire assembly and presence of four adult passengers. Of special concern was a portion of the iron bearing housing beneath the bearing inserts. We needed to prove that there was no possibility of a catastrophic failure of this part.

Cast iron bearings, similar to this one, were the focus of the linear static stress analysis, which aimed to verify that the bearings could withstand stresses resulting from the motion of six actuating hydraulic cylinders and provide extended service life.

While the goal of the project was straightforward, the method of achieving it was not. Our first approach to the problem was to perform a linear static stress analysis. However, we discovered that the inherent limitations of static analysis made it unsuitable for studying the dynamic nature of the simulator. The problem arose in determining loads created by the accelerations of the multiple double-acting cylinders to use as input into a linear static stress analysis.

Control valves connect to the top and bottom of each cylinder. As the ride begins, computerized controls lift all six cylinders to their neutral positions at half the cylinder extension capability (maximum extension is 25.25 inches) by raising fluid levels in the lower portion and releasing fluid in the upper portion of the cylinder. Computer commands conduct unique multi-directional extension and retraction sequences for each of the cylinders. An accumulator provides for fluid surges, while a central HPU provides a constant fluid pressure of 2000 PSI.

Due to the complexity of movement, calculating the loading on the bearings from the six independently moving cylinders was not feasible. A detailed solid model of the bearing was created in SolidWorks and captured directly in ALGOR using InCAD^Plus for SolidWorks, without translation to a neutral file format. The model was fully constrained at the stainless steel inset in the center of the bearing. We decided to analyze a worst case scenario, applying an artificially high static load to the bearing.

In addition to the bearing analysis, Iwerks requested another analysis to verify that the welds of the motion base also would withstand the dynamic loads created by actuator movement and the presence of simulated passengers. A detailed solid model of the motion base was created in SolidWorks and transferred to ALGOR in the same manner as the previous model. A load that would have been distributed to three mounting plates was applied to just one mounting plate in the vertical and horizontal directions. The points where the seat is attached to the frame were fully fixed for the static analysis. The results showed only moderate stress levels in the structure despite an artificially high load case. The welds in the seat frame performed well under the applied loads.

The linear static stress analysis results showed that a bearing could withstand loading 2.5 times the maximum-recommended load for the component. Because of the number of assumptions and simplifications made in the analysis, Mr. Pribonic was not convinced that the results portrayed an accurate picture of the mechanical system behavior.

The static analyses conducted separately on the bearings and seat frame yielded low to moderate stresses as well. While the results were favorable, both my firm and our client were not convinced that the results portrayed an accurate picture of the mechanical system behavior because of the number of assumptions and simplifications made in the static analysis.

Consequently, my firm decided to go beyond traditional static FEA methods to take advantage of the high level of engineering simulation and computing power available with ALGOR. I recommended that we build a complete, fully detailed, full-motion computer model of the simulator that could be used to run the motion profile at hand as well as new motion profiles as they are developed for future films. Stresses on the equipment can change with every new motion profile so Iwerks needed an engineering model that could run each motion profile and produce results.

With the help of ALGOR’s technical support, we determined that replicating the actuating movement of the hydraulic cylinders using ALGOR’s new actuator element technology and Mechanical Event Simulation (MES) software was the best method of evaluating real dynamic loads over time. In addition, a reliable computer model based on a detailed CAD solid assembly would help all involved to better understand the dynamics of the design and apply this knowledge to future programming and simulator design decisions.

The Solution

For a simple MES, such as a manually shifted lever, the engineer simply specifies a prescribed displacement or, if it is known, the force needed to set up the MES. Then the software will compute the acceleration and resulting stresses. In the case of the simulator, applying prescribed displacements for each cylinder to get dynamic motion and stress results over time was not feasible. At best, we could determine stresses at just one instant in time if we used this approach with the available software capabilities. Such an analysis would yield a similar result as a motion load transfer analysis, in which loading determined in a kinematic analysis is applied to a static stress analysis. My client had already agreed that this would not meet the safety analysis requirements for the project.

The complex actuating movement could be simulated using a new actuator element technology invented by ALGOR. Actuator elements are engineering elements (like contact or dashpot elements) that replicate linear extension and contraction movement in three-dimensional space, typical motion for hydraulic and pneumatic cylinders and electric solenoids. An actuator element, which appears as a line, was used to represent each cylinder in the finite element model.

To proceed with the MES, we created a detailed CAD assembly of the simulator that would be captured in ALGOR for finite element modeling. Using SolidWorks once again, engineers at Pribonic Engineering and Consulting modeled more than 100 individual components based on Iwerks drawings and merged the components into five subassemblies.

The surfaces of the subassemblies were meshed separately by choosing the InCAD^Plus menu selection in SolidWorks, which activates ALGOR’s surface meshing. InCAD^Plus captures the exact CAD geometry, and group information is preserved in the finite element model.

Once the subassemblies were meshed, they were merged into one model in Superdraw III. While some detailed surface matching was needed to align welded components in the seat frame, we performed very little surface mesh enhancement. The initial finite element model contained approximately 500,000 elements -- far too many for a reasonable analysis processing time.

With the help of ALGOR’s technical support, my firm was able to reduce the overall number of finite elements in the model to approximately 141,000 elements. This was due in part to ALGOR’s solid mesh engine, which automatically creates better aspect ratios for each solid element based on the quality level chosen by the engineer. We also replaced flexible brick elements with ALGOR’s kinematic elements where possible to reduce the analysis computation time. Kinematic elements behave just like flexible finite elements, but do not produce stresses. Engineers can insert kinematic elements in areas of the assembly where dynamic effects are essential but for which stresses are of secondary importance. This saves time and the engineer can focus the analysis on the part of the mechanism being optimized -- a set of bearings in our case.

After the model size was reduced, my firm defined the analysis type, unit system, element and material properties by group. A translator program was used to read in the displacement vs. time load curves for each actuator element. The program extracted the load curves for an 11.6-second segment of the motion profile provided by Iwerks. We chose this particular segment because it contained the most extreme range of accelerations across all of the load curves. Material properties for steel and cast iron were defined using ALGOR’s Material Library Manager. Global analysis parameters included the duration of the event and capture rate. A rate of 30 captures per second was chosen to match the Iwerks motion file data points. Gravity also was applied to the model.

During processing, ALGOR’s built-in visualization capabilities and Monitor utility were activated so we could watch the event unfold as it was processed. ALGOR enables WYSIWYG visualization by showing the movement of the mechanism and stresses as they occur over time. Had we found an error at the beginning or during the run, we could have stopped the analysis and fixed the problem without waiting for the entire run to complete. ALGOR’s Monitor utility works like a virtual oscilloscope, displaying velocity, displacement, acceleration, reaction forces or maximum stresses vs. time for a specified node. Using Monitor, we viewed acceleration vs. time data for the six cylinders.

Created using ALGOR’s Monitor utility, this graph shows the acceleration vs. time curve for each of the six independently moving actuator elements used in the Mechanical Event Simulation.

As soon as the processing was finished, we viewed analysis replays of the MES in .avi format. These served as visual aids, helping my firm explain to Iwerks the dynamics of the event in terms that non-engineers could understand.

A Mechanical Event Simulation (MES) with built-in actuator element technology of the motion simulator replicated the movement of computer-controlled hydraulic cylinders. The cylinders were represented with actuator elements (shown here as blue lines). Replays of the MES show the cylinders extending and retracting independently, providing the seat and occupants with roll, pitch, yaw, heave, surge and sway motion in coordination with the on-screen adventure action. ALGOR’s actuator elements (shown here as blue lines) were invented to simulate axial extension and retraction movement in three-dimensional space, typical motion for hydraulic and pneumatic cylinders and electric solenoids.

The Test

The MES results showed that the stresses experienced by the bearings under loading from the six cylinders were within the acceptable range. A comparison of the maximum stresses found in the MES with those of the linear static stress analysis showed the static results to be very conservative. The accuracy of the MES stress results for the simulator was important to ensuring that the ride met the assigned British Fairground Standard.

ALGOR’s Mechanical Event Simulation (MES) software results showed that the stresses experienced by the bearings were within the acceptable range. A comparison of the maximum stresses found in the MES (shown left) with the linear static stress analysis results (shown prior) showed that the linear static stress analysis was quite conservative. Much lower loadings (and therefore stresses) were found in the dynamic analysis of the MES. As predicted by the MES results, the bearings performed well in physical testing.

The actuating motion produced by the MES appeared to be very realistic when we viewed the analysis replays in real time. However, to verify the accuracy of the actuator elements used to produce the motion, we compared the MES output to actual test data. Using one of the motion bases produced for the Scottish TurboRide, engineers placed accelerometer test equipment at key areas of the simulator. One such point was placed in the left rear seat of the motion base. Data acquisition software compiled the acceleration data as the motion program was run on the simulator.

Acceleration vs. time data for a node in a similar location as the accelerometer testing was acquired from the Monitor program. We found that the MES and physical acceleration testing results for the same timeframe correlated very well, giving my firm and Iwerks a high confidence in the accuracy of the results.

The ALGOR Mechanical Event Simulation acceleration vs. time data for the X, Y and Z directions (right) correlates well with the acceleration data acquired through laboratory testing using accelerometers (left). Both sets of data were captured at similar points in the motion base.

Through this engineering challenge, Pribonic Engineering and Consulting has recognized the value of developing virtual prototypes using ALGOR’s MES. We can represent product designs using detailed CAD assemblies and simulate complex, dynamic behavior. In this case, the virtual prototype verified that the ride would withstand stresses caused by the high-speed actions. Just as importantly, its true-to-life form will enable it to serve as a virtual prototype for future product developments by Iwerks.

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