We know what you’re thinking… how could they possibly find more topics to discuss on safety!? Well, strap in because we haven’t even scratched the surface of the different safety considerations and mechanisms for amusement park attractions.
With the advancement of technology in video, audio, special effects, and interactivity, motion simulators have become increasingly common throughout the amusement industry. Some of the most high-profile attractions in recent years are based around compelling, safe motion simulation, such as Disney’s Millenium Falcon: Smuggler’s Run ™, Universal’s Harry Potter and the Escape from Gringotts ™, and CAVU’s The Twilight Saga: Midnight Ride ™. A motion simulator will whisk you to far away places through media and motion. You could be soaring down the slopes of Mount Fuji or rushing through the city with your favourite superheroes.
In this post, we look at how these attractions are designed, operated, and maintained safely. We will tackle the movement of people in small spaces, by looking at clearance envelopes, degrees of freedom, and redundant motion limits.
First, let’s start with the basics.
Motion simulators, as the name suggests, create the feeling of motion in a simulated environment. This movement is usually paired with synchronized media to transport guests into a storyline. Typically, this motion is achieved using a motion base, which is an electromechanical system composed of linear or rotary actuators that push and pull a platform in various directions.
Motion bases are defined by their degrees of freedom, or DOF. There are three rotational degrees of freedom (pitch, roll, yaw) and three translational or linear degrees of freedom (heave, surge, sway). Clearance envelopes refer to the maximum operating range that a ride system needs to safely move around, and redundant motion limits ensure that the range of motion for the vehicle and base fit to the space requirements.
Let’s break down those three elements of the motion base, starting with degrees of freedom (DOF).
Degrees of Freedom
In almost any industry that involves the movement of a vehicle, the term Degrees of Freedom is used. There are many mathematical and technical uses of this term, but we will focus on the 6 degrees of freedom that exist for motion-based vehicles such as dark rides or flying theatres.
Figure 1. Degrees of Freedom 
Motion-based attractions are some of the most common in the amusement industry. These consist of a set of actuators, rotary or linear, that move a platform in the directions you want. Typically, the number of actuators is equal to the degrees of freedom of the motion base. With this, designers and engineers can determine how the equipment can move. The less degrees of freedom (# of actuators) you have, the easier it is to create clearance envelopes (we will break these down next) and contain the vehicles.
The number of degrees of freedom is notated using #DOF. For instance, a 3-degrees-of-freedom system would be denoted 3DOF. For 6DOF systems, like CAVU’s Beautiful Hunan Flying Theatre and CAVU’s Mockingjay Flight experiences, the motion system can move guests in any direction. These motion bases are called Stewart platforms or hexapods. For other systems, the motion is limited to only certain directions of motion.
Figure 2. Stewart Platform (Hexapod) 
Motion simulations can create compelling experiences without being a 6DOF system. Many attractions build this into the storyline. Take CAVU’s Midnight Ride for example. Guests feel as though they are on a high-speed motorbike racing through the hilly terrain of the Pacific Northwest, which is simulated using only three degrees of freedom: Pitch, Roll, and Heave. The remaining three degrees of freedom are not needed to create a believable and fully immersive experience.
Why is all this important? First, it is crucial for understanding how motion simulators work. Second, and more applicable for this blog, the number of degrees of freedom a system has directly affects how engineers evaluate the safety clearance envelope of the attraction.
As we mentioned briefly above, clearance envelopes are the maximum operating range that a ride system needs to safely move around; the more DOF, the larger the envelope. Clearance envelopes are based on a combination of different motion envelopes, including the combined range of movement of the vehicle and cabin, and guest reach capabilities.
Say you are on a motion base attraction like CAVU’s Midnight Ride, you wouldn’t want to be able to touch the person beside you during the experience, would you? No, you might hurt yourself. What if the bike was too close to the wall and when you stretched your arms out you could touch the wall? Again, you might hurt yourself. This is where clearance envelopes come into use: engineers use these calculations and predictive tools to ensure that guests and objects do not interfere during the experience.
To create these envelopes, engineers first calculate the maximum physical travel of the vehicle and cabin to determine how much space the system needs to move around. Next, they calculate how far guests can reach outside the vehicle. Finally, they combine these two calculations to create the overall clearance envelope. As long as all objects and people are outside of that clearance envelope, it shouldn’t be possible for guests or the vehicles to interfere.
Of course, we cannot build attractions without considering any error or unexpected event. Therefore ASTM requires additional clearance past the maximum calculated clearance envelope. Many manufacturers and operators, including CAVU, include ADDITIONAL clearance, even past the ASTM requirement, to further enhance guest safety.
Clearance envelopes greatly affect the size of buildings, theming design, and anything that could get close to the guests. Think about those “headchopper” elements on roller coasters: even though they seem extremely close, they are designed to be outside of the clearance envelope. To fit exciting experiences in small spaces, many times engineers are tasked with optimizing the clearance envelope. Engineers and designers have to use many strategies to reduce clearance envelope requirements:
- Building cabin walls that prevent guests’ reach
- Increasing the restraint class (it’s easier to reach things when you just have a seatbelt, but much harder when you have an over-the-shoulder restraint)
- Limit the motion capabilities (more on that next)
- Orient the vehicles to maximize efficient space (Midnight Ride‘s staggered bikes)
Redundant Motion Limits
Many times, the range of motion of the vehicle and the motion base is more than what the designers require. Engineers must then safely reduce the range of motion of the system, without an expensive and time-consuming redesign. Otherwise, the motion will exceed the space allocated in the buildings or at the attraction site.
This is where Redundant Motion Limits are used. Engineers and programmers implement these to safely limit the travel distance of the motion system. Using redundant sensors and programs, they prevent the mechanical system from going beyond the programmed value. Engineers often supplement these electrical limits with mechanical limits to ensure the motion base operates safely.
For CAVU’s Mockingjay Flight attraction, the powerful 6DOF motion bases are built to extend great distances. However, this doesn’t work for a revolutionary indoor theme park such as Lionsgate Entertainment World. CAVU’s team implemented redundant motion limits to ensure the motion base stays under control. This ensures that the cabin moves exactly the way it is supposed to, within the clearance envelope of the building.
The next time you are on a motion simulator, take a look at the building size, the space between you and the other riders or vehicles, and the movement you are experiencing, and think about all the engineering that goes into making sure your ride is safe.
What are the SIX Degrees of Freedom? https://www.simcraft.com/6-degrees-of-freedom-full-motion-roll-pitch-yaw-surge-sway-heave/
Stewart Platform https://en.wikipedia.org/wiki/Stewart_platform