An introduction to motion systems

by SIMRIG

At SIMRIG we build motion platforms for simulator games. This means that we build rigs that move with the simulated car to convey forces that act on the vehicle. These forces help you as a driver make better informed decisions. It may help you drive faster, safer, and to react more quickly before things get out of hand. We also think it makes the act of driving a lot more fun and engaging.

In this article we'll explore some of the common words and phrases used when discussing motion platforms. This is an introduction. We'll try to cover the basics of what you need to know. Not with the hope of covering everything there is to know, but with the aim to inform you what too look for when conducting further research.

Degrees of freedom

Words like 2DOF and 3DOF are common occurrences when discussing motion platforms. DOF stands for degrees of freedom. This is what Wikipedia has to say on the matter:

In many scientific fields, the degrees of freedom of a system is the number of parameters of the system that may vary independently.

A 2DOF platform thus contains two independently variable parameters. For a motion platform this usually means pitch and roll. A 2DOF motion platform can tilt left-right and backwards-forwards. The left-right motion is called roll. The backwards-forwards motion is called pitch.

A 3DOF platform contains an additional independently variable parameter such as heave or sway. Heave is up-down motion caused by hitting a curb or jumping over a crest. Sway is rotation caused by turning or drifting.

A 6DOF platform can apply rotation along all three axes (roll, pitch, yaw) and translation along all three axes (surge, sway, heave.) The name of these axes are taken from ship motions.

We believe all motion platforms require at-least these three degrees of freedom: pitch, roll, and heave.

Pitch gives you a sense of acceleration. It tilts the chair backwards when the car accelerates forwards. The chair shakes as the car struggles for grip on the start line. It tilts the chair forwards when hitting the brake -- in important indicator that the car is slowing down.

Roll gives you a sense of turning. It tilts the chair left when turning to the right, and right when turning to the left. Roll makes you feel the weight of the car as it shifts from left to right when approaching a corner. With practice it allows you to estimate how fast to take each corner based on the angle of the chair.

Heave makes you feel the suspension working. It allows you to appreciate the details that goes into each track and car. It makes you feel curbs and potholes in the road surface. Without movement up and down you are missing out on half the details and half the fun.

Telemetry

Telemetry is another word that pop-up often when reading about motion platforms. Let us try to explain telemetry and its role in regards to motion platforms.

Motion platforms are meant to reenact forces applied to simulated vehicles. To accomplish this we need to feed the motion platform with information about the forces that act on the vehicle. These forces are known by the game or simulator causing them. Telemetry refers to the act of sharing these forces with the motion platform and its control software.

Telemetry data usually consists of sensor values available in real vehicles such as: speed, battery voltage, boost pressure, tire temperature, etc. Sometimes it is measured by a virtual GPS, and sometimes it is taken directly from the game's physics engine.

Motion platforms are designed to react to events in many different games and simulators. These games and simulators are developed by many different studios and companies. Each game is made differently and its telemetry output is also different. Unfortunately, there is no standard to regulate how telemetry is communicated and structured. As such, each developer implements its own special version of telemetry. Motion platform vendors therefor expend considerable effort to implement and support the many different ways in which telemetry is made available.

If you are a game developer, feel free to contact us. We'd love to discuss telemetry implementations!

Control software

The control software is responsible for receiving telemetry data and feeding it to the motion platform. It must therefor run in the background while continuously receiving telemetry, converting it into motion commands, and sending those commands to the motion platform. Writing good control software is tricky due to the real-time constraints enforced by this continuous conversion.

This is very different from how a force-feedback wheel operate. A force-feedback wheel is directly connected to the game. The game reads its current angle (for steering) when feeding the forces applied to the steering shaft. The game comes with built-in support for force-feedback wheels. The motion platform however must add support for each game in its control software.

The control software is where you'll find all settings related to your motion platform. Since the control software is responsible for converting telemetry data into motion commands it is the natural place to have the settings controlling this conversion.

The control software's capabilities can vary from game to game since each game is implemented differently. Some games output more detailed telemetry data while others have none or limited support for telemetry. An accurate physics simulation combined with detailed telemetry data will usually result in the best motion.

Peripherals

Common peripherals include force-feedback wheels, pedals, handbrakes, and button boxes. But there are also lesser known peripherals. Let's take a look at some of these.

Bass shakers are powerful sub-woofers meant to convert low-frequency sound waves into vibrations. They are commonly used by rig builders to add motion queues when shifting gears and hitting curbs. They are also suitable for road noise and motor vibrations. Bass shakers are great complements for motion systems since motor vibrations and road noise are usually too fast for motion platforms. Vibrations are high frequency signals when compared to the motion usually suitable for a motion platform.

Direct drive wheels are a special type of force-feedback wheel. In a common force-feedback wheel the rim is connected to the motor shaft with a belt. In a direct drive wheel the rim is attached directly to the motor shaft. There is no belt and no gear. And most importantly, the motor is usually very beefy. Some direct drive wheels use an industrial servo motor. They can potential damage your hands and arms.

Virtual reality

Virtual reality (VR) is a broad term. We will limit the scope to only include virtual reality headsets; the display technology.

Games are commonly displayed on monitors. Monitors display the game world i two dimensions on a flat surface. VR-headsets on the other hand display the world on two different monitors, one for each eye, creating a 3D representation of the game world.

A VR-headset can trick your brain into thinking that you're sitting in the car. This is possible through a combination of depth perception, head tracking, and the simple fact that the headset covers most of your field-of-view. Depth perception makes it easier to estimate distances and speeds. Head tracking allows you to look around simply by moving your head. This is great for cornering and not crashing into your opponents.

There are downsides with VR-headsets. Firstly, the display resolution isn't great (as of early 2019.) You'll notice chromatic aberration and you'll see the individuals pixels that make up the display. Secondly, a lot of people get motion sickness. This is alleviate by motion platforms but will still require excruciating practice to overcome (it took my about two weeks of driving and feeling sick all the time.) Thirdly, the hardware requirements are high. A consistently high frame-rate is paramount for a good experience which requires a fast and modern computer. Fourthly, it is difficult to see and hear what's happening around you in the real world.

Triple screen setups are commonly compared to VR-headsets. In a triple screen setup, three screens are arranged in a half circle. One screen in-front of the driver, and one screen to the left, and one to the right. It is possible to achieve a near 180 degree field-of-view without the need for a VR-headset. Hardware requirements are still high but motion sickness is less likely to cause problems.