For article 7 of 8 in Cruden’s Motion Series, Bastiaan Graafland assesses the performance, logistical, safety and cost considerations of chassis mockups in simulator use.
Specifying a driving simulator for the task at hand is essential if the customer is to derive maximum value from its latest investment. In our earlier article comparing motion for immersion versus motion for vehicle dynamics, we stressed that a motion system should always be fit for purpose: it should suit the simulator’s use case. The same principle applies to the structure that sits on top of the motion platform, from a simple seat and steering wheel to a full or partial chassis mockup.
Around 50% of Cruden’s simulator clients opt for a mockup, anything from a basic structure to a full car. The other 50% specify a generic, top-frame structure that has some form of dashboard and other essentials for a driver to control a car. We’ve said before that the visual system is by far the most important element of a driving simulator, so holding a steering wheel and looking out at a virtual world generates a certain level of immersion. For many users – from vehicle OEMs to universities – this is sufficient for their applications.
Chassis mockups take immersion and realism to the next level. Cruden builds these for its customers, either sourcing a car itself or working from a base car delivered by the client. Some customers are proficient in completing some of the conversion themselves, prior to Cruden undertaking work to install the necessary mechanical and electrical connections to the rest of the simulator.
Some customers are proficient in completing some of the conversion themselves, prior to Cruden undertaking work to install the necessary mechanical and electrical connections to the rest of the simulator.
Not all mockups are alike
Depending on the use case, mockups may either take the form of a generic vehicle to be used in human behavior research, for example, or may represent a specific vehicle like the latest GT racing car. Each scenario comes with a number of factors for consideration.
Universities and other research organizations typically tend towards a more generic structure. They value the flexibility of being able to mount parts and equipment in order to conduct different experiments. Cruden makes platforms with generic mounting structures, so that measurement equipment, cameras, computers or anything else needed for an experiment can be loaded on board.
That’s not always ideal for a specific car mockup, however, where the user may not want to detract from the immersion by adding equipment that’s not in the real vehicle. Instead, simulators with specific mockups often come with the need for flexibility in a different area – the ability to reconfigure the simulator to accommodate different types of car mockups, such as exchanging a GT car for a formula-style car mockup in the example of a motorsport engineering simulator.
This scenario widens the potential uses of the simulator but brings additional complexity. The simulator must be designed with the ability to temporarily break the mechanical and electrical connections between the mockup and the dynamic frame while ensuring a very stiff connection between the two.
How frequently the mockup exchanges will be required, defines how easy to make the swap. In some cases, an annual exchange for a new vehicle may be sufficient. In that case, a few days of downtime to disconnect the mockup and install its replacement may be acceptable. This option likely requires fewer additional components than a requirement to switch cars in 20 minutes, which would necessitate a different type of exchange mechanism and has a greater impact on the configuration of the simulator.
It’s important to pinpoint the exchange requirements up front because the faster the exchange needs to happen, the more complex and expensive the simulator design. There is a delta of around $50k between a generic frame and one with a fixed mockup. Moving to an exchangeable mockup might then add another $100k to the simulator price to cover the additional mockup as well as the associated interchange equipment, connectors, clamping devices and everything else that’s necessary to make the switch.
Mockup stiffness
For certain use cases that demand high motion performance, it is important to have a very low payload for the motion system. This might mean a minimalistic mockup structure with only the essential driver environment structure and parts. It should be a very stiff structure to make full use of the bandwidth that the motion system can deliver. High-frequency motions must travel through the mockup to reach the driver, so the dynamic performance will be lost if there is too much flexibility in the mockup itself or in its attachment to the motion system actuators.
This provides a challenge when converting an actual car into a mockup for simulator use. Removing the engine, drivetrain suspension and other parts invisible to the driver and passenger positions, in order to save weight, will damage the structural integrity that has very carefully designed in by the OEM. Real-life load paths from the engine and suspension are no longer in play as the chassis floor is mounted to the motion system. In cases where precise, high-frequency signals applied by the motion system are crucial to the test being performed, mockup stiffness ensures the signals arrive at the seat rail or parts of the vehicle mockup touched by the driver, without other parts vibrating unrealistically.
A stiff mockup could be crucial when using the simulator for NVH development in vehicles, too. When precision is required in the medium-frequency range, around 10-30Hz, then the stiffness of the mockup structure must match the experiment’s targets and bandwidth. By contrast, a stiff mockup might not be so important in use cases where lower-frequency excursions are the priority to create motion to immerse the driver, rather than to precisely reproduce real-world vibrations.
The bigger picture
The desire to exchange mockups raises questions for driving simulator projects that are not always addressed in the product brochures. The facilities in the simulator building and equipment around the simulator must be designed to meet the customer’s needs while keeping simulator operators and drivers safe.
There are different means of effecting the mockup exchange. A gantry crane is useful, and Cruden will specify the requirements if this forms part of a new simulator installation, but a large projection system may make overhead access difficult. Mobile lifting equipment or custom mockup exchange systems are other options that can be sourced and delivered as part of a simulator project. The simulator’s placement in the room must be carefully mapped out in the planning stage to ensure access, by whatever means, and that any exchange equipment – including mockups not installed on the motion system – can be safely stored outside of the platform’s motion envelope.
Moreover, the exchange system – as well as the driver access system – must be integrated into the simulator’s safety and control system to comply with local health and safety legislation, such as the European Union’s Machinery Directive. Cruden’s simulator control system, for example, not only drives the actuators of the motion system, but also includes a safety PLC that safeguards the access doors to the simulator area and ensures safe operation of the integrated simulator systems.
The Cruden approach
The strong focus on what meets the customer’s needs, and a flexible, modular approach to simulator specification is what distinguishes Cruden from other suppliers. The specification can be adapted to precisely match the customer’s needs, rather than being restricted to a fixed portfolio of designs.
In Cruden’s experience, customers often start the simulator procurement process with a clear idea of their chassis mockup requirements, just as they might have a mental picture of how the motion system should look. But discussions about a possible simulator project should always focus on what the customer needs, and why. That results in a specification that meets those needs and will provide greater value to the client’s engineering or research programs.
Identifying the simulator’s stakeholders, use cases, experiments and expected outcomes will help determine whether a chassis mockup is needed at all. If one is required, should it be exchangeable? If so, how often and how quickly must it be exchanged? Does the mockup need to be lightweight and very stiff, or is an immersive, car-like interior more important? Will a generic structure suffice? The answers to these questions have a major impact on the project’s costs, so decisions made upfront will manage the customer’s expectations and lead to a smoother implementation.
At Cruden, our engineers are always happy to help and are very good at answering the list of questions you’ll have about your simulator project. Please don't hesitate to consult us.
For more information, please contact Dennis Marcus via d.marcus@cruden.com or on +31 20 707 4646.
Links to subsequent articles will be added below as they are published.
View all articles in our Motion Series of articles: here.
Article 1: Driving Simulator Motion Systems 101
Article 4: Motion for immersion VS motion for vehicle dynamics and motorsport
Article 5: Good Vibrations: Using a Driving Simulator for NVH development
Article 6: Acceleration is not enough: there’s more to accurate motion cueing than meets the eye
.jpg)