Assume for the moment that you are the engineering manager for a company that will design and build positioners for products weighing thousands of pounds. Moreover, the positioning will require rapidly changing angles with some lateral movement. Such operation is similar to that used in motion platforms, Figure 1. These units are purchased by entertainment firms dedicated to building small action theaters. These may simulate a run-away mine car traveling underground at fantastic speeds, or a trip through space avoiding unfriendly space cadets, or other thrill rides. Some move the audience up, down, and tilt. More complex units move up, down, sideways, and tilt. All the moves must be coordinated with an image projected on a large screen. If it isn’t, viewers get upset real fast. Many theaters will have multiple modules with a single screen.
Similar motion platforms move interactive simulators for training truck drivers. Each simulator contains a truck cab mounted on a motion platform. The driver views typical scenes he would encounter on the road while feeling the associated motion. Again, the motions he sees and feels are coordinated.
As might be expected, the motions in the entertainment simulators are more severe than for training applications, which must relate to reality, rather than provide thrills.
Interestingly, many of these platforms for entertainment are sold to the companies in the Pacific Rim, because there is little usable real estate. Rather than build new motion theaters, they convert old movie theaters to ones that move the audience while they watch visual and hear audio stimuli.
Platform designs
Typically two basic designs are used for these applications. One has four axes of motion — called four degrees of freedom (DOF), Figure 2. This can move each side independently up and down, thus tilting as well as moving the whole platform up and down. The other design has six axes, Figure 3. This design offers the four axes plus the ability to move the platform in yaw and sway directions, Figure 4.
Quantifying requirements
As with any engineering assignment, factors must be quantified. The factors involved in these motion platforms include load weight, centers of gravity, motion profiles, and enclosure dimensions. Load weight establishes the minimum and maximum weights the motion platform will move, both empty and fully loaded and weights in between.
Centers of gravity (CG) include all possible loads. For example, the CG must be established for one person in the module, two in each possible seating configuration, etc.
Motion profile is often difficult to establish, especially if this type of device is new to the ride manufacturer. Frequently, the directions are vague, such as, “Let’s make the audience feel as if they are in an aerobatic aircraft during an air show.” To establish the acceleration and deceleration rates for all directions, engineers often put sensors on something that approximates the desired thrill. Then, using the values obtained empirically, they write a motion profile that is synchronized with the visual effects and emulates the measured quantities. (Programming is discussed later.)
Dimensions of the load and the enclosure must be established. It may be necessary to restrict the maximum movements to prevent the top of the moving unit from poking holes in the surrounding enclosure.
Safety and reliability
Without doubt, safety and reliability, including long-term maintenance costs, are among the most important engineering considerations. People must not be endangered even if a component should fail or power is lost, or both at the same time. Should an unsafe situation occur, or should an operator believe a dangerous situation might arise, the system must enable all personnel to leave the unit in an orderly manner.
In addition to the safety restrictions, reliability takes on added values in the entertainment industry than even in many competitive industrial installations. From an economic standpoint, if a ride or other attraction is out of commission, it has lost irreplaceable revenue for as long as it is inoperable. No amount of overtime will make up for the lost revenue. Moreover, typical thrill rides operate continuously over 12 hours per day, 7 days per week.
Equipment selection
Based on equipment capabilities and customer preferences, two types of linear actuators have emerged for these motion platforms. For payloads over 10,000 lb, hydraulic actuators are often the best choice. These payloads are typically encountered in flight simulators for large aircraft such as the B-747s. Large simulators usually include a full cockpit, computer for controlling the entire system, audio-visual units, and room for three people.
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For smaller loads, electromechanical devices are usually the choice. These include servomotors powering either ball or roller screws. For lighter loads with shorter useful lives (say 2 to 5 years), the ball screws may be the most economical selection. However, if the planned useful life is expected to last 10 years and downtime is to be avoided, the higher priced roller screw is selected. It is impractical to set specific rules of thumb on the cutoff points, so each case must be analyzed and the tradeoffs established.
One important factor must be in the calculations: The cost to replace an actuator. Because these support loads, often weighing a few tons, the cost to bring in a crane or jack to raise the load while the actuator is changed must be included.
Regardless of the type of screw selected, lubrication is important. Most of the operational life is spent in the vertical or near vertical position, so special cups or other devices are added to the traveling nut. Such cups carry the lubricant to the top of the screw and let it flow down. Otherwise the lubricant will stay in the bottom and leave the top portions dry.
To achieve fast response and accurate positioning requires selecting servomotors, typically brushless dc (BLDC). These are controlled by amplifiers rated to handle the continuous and peak loads demanded by the duty cycle.
If maintaining a low platform height in the egress position (lowest position so people can leave a module) is critical, it is often advantageous to use two motors mounted parallel to each actuator body. With such an arrangement, each motor has a spur gear that mates with a common gear, which is connected to the screw. The lead motor is position regulated and the second motor serves as a torque-regulated slave motor, thus assuring load sharing.
If a single motor powers each actuator, the motor is connected to the screw with a synchronous belt or is directly coupled in-line to the screw.
Although hydraulic actuators were the first type of actuators used, electric actuators are now becoming the choice for three main reasons: reduced noise, absence of environmental problems, and elimination of the need for a separate pump room and the related plumbing.
Calculating the forces requires a series of complex tasks, because the operating parameters — load magnitude, position, motion profile, acceleration rates, etc. — are so variable, it is common for engineers to invest significant time to develop extensive computer programs. These are used for selecting individual components. After a platform is built, sensors are placed on the platform and data are taken to verify the design.
Control schemes
For entertainment rides, a PC typically contains the stored motion profile, which commands a PLC and the motor drives. The PLC also receives signals from endof- travel limit switches, other safety inputs, and operator’s controls. Each actuator and motor controller closes an outer position loop and an inner velocity loop for stability.
Two feedback configurations are commonly used. An encoder mounted on the motor produces a speed feedback signal to the drive, and a linear position transducer mounted on each linear actuator produces a position signal that closes the position loop through the motion controller.
A second configuration uses each encoder to supply both the speed and position feedbacks. In this system, the unit resets itself to a zero count after each cycle when the actuators return to home (egress) position.
Units for driver training are interactive so it responds to the “driver’s” actions. Although the software is more complex, the basic platform is similar to entertainment units.
Programming
Considered by many to be somewhat of an art, writing computer programs for entertainment typically involves creating a program that contains the equivalent of 70,000 lines of code. To do this, the programmer typically uses a joy stick operating in a teach mode to create a ride program with one or two degrees of freedom in each run. Then the operator combines the axes and fine tunes the program to produce the desired “feel.”
During start-up, the program is frequently modified via a telephone hook-up although the programmer and the unit may be in different countries.
Acknowledgments
The following engineers at Moog Inc., East Aurora, N.Y., contributed information to this article: Ron Benczkowski, manager, product engineering; William A. Egger, product manager; Rick Emerling, senior software engineer; Chris Layer, senior design engineer; William J. Lyons, senior project engineer; Paul J. Morrell, sales and marketing engineer. Their contributions are appreciated.
Mr. Bartel is the manager of application engineering for electromechanical actuator products at Moog Inc., East Aurora (Buffalo), N.Y.