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In order to understand how motion can be cued, a brief overview of human physiology is required.
The way we perceive our body and our surroundings is a function of the way our brain interprets signals from our various sensory systems, such as sight, touch and hearing. Special sensory pick-up units (or sensory "pads") called receptors, translate stimuli into sensory signals. External receptors (exteroceptors) respond to stimuli that arise outside the body, such as the light that stimulates the eyes, sound pressure that stimulates the ear, pressure and temperature that stimulates the skin and chemical substances that stimulate the nose and mouth. Internal receptors (enteroceptors) respond to stimuli that arise from within blood vessels.
Postural stability is maintained through the vestibular reflexes acting on the neck and limbs. These reflexes, which are key to successful motion cueing, are under the control of three classes of sensory inputs:-
Proprioceptors are receptors located in your muscles, tendons, joints and the inner ear, which send signals to the brain regarding the body's position. An example of a "popular" proprioceptor often mentioned by pilots, is the "seat of the pants".
Proprioceptors respond to stimuli generated by muscle movement and muscle tension. Signals generated by exteroceptors and proprioceptors are carried by sensory neurons or nerves - electrochemical signals. When a neuron receives a signal, it sends it on to an adjacent neuron through a bridge called a synapse. A synapse "sparks" the impulse between neurons through electrical and chemical means. These sensory signals are processed by the brain and spinal cord, which then respond with motor signals that travel along motor nerves. Motor neurons with their special fibres carry these signals to muscles, which are instructed to either contract or relax.
In other words, these sensors present a picture to your brain as to where you are in space and external acceleration forces on your body. For example, picture yourself sitting at the traffic lights in your car. The light changes to green and your foot presses the accelerator. As you accelerate away from the lights you will "feel" yourself being pushed back in to the seat. That experience is transmitted to your brain via the proprioceptors, in particular, through the sensors in your backside and back. The brain interprets this information as an acceleration in the forward sense. If you now stand on the brakes to stop suddenly, you will find different proprioceptors at work. The deceleration will be felt through your hands and feet and your backside will now be trying to slide forward in the seat. This information is again presented to your brain and thus it interprets the deceleration taking place. In turn, the brain now signals the muscles in your arms and legs to contract and stop you from sliding forward in the seat. A similar sensation will take place when you turn a corner. If you turn left, your body will slide across the seat toward the right and vice versa for a turn to the right.
The downfall with our
internal motion sensors is that once a constant speed or velocity is reached,
these sensors stop reacting, so your brain has to now rely on visual cues to
determine what's taking place until either a further acceleration or
alternatively a deceleration takes place. In flight simulation development
we can use this to our advantage. This is further explained below in "Washout".
Vestibular System
The Vestibular System is the balancing system of the body that includes the vestibular organs, ocular system, and muscular system. The vestibular system is contained in the inner ear. It consists of three semicircular canals or tubes arranged at right angles to one another. In space, there are three planes that we can move through, forward/backward or longitudinally, left/right or laterally and up/down, or vertically, thus there is one canal assigned to detect movement in each plane. Each canal is partially filled with fluid and has a series of hair follicles which stand vertically inside each tube. The fluid indicates rotation in the yaw, pitch, and roll axes. When acceleration takes place in a particular direction, the fluid in the appropriate canal is displaced which in turn causes the hair follicles to move. The movement of the hairs is interpreted by the brain as an acceleration.
 Three semicircular canals are orthogonal and deal with angular turns. The saccule with vertical linear movement, the utricle with horizontal movement. |
There is however a short coming to this clever piece of biological engineering. If sustained acceleration (10 - 20 seconds) takes place in one direction, the fluid in the appropriate canal also remains continually displaced and after a brief moment the hair follicles will return to the vertical position, therefore the brain will perceive that the acceleration has stopped. This can be a serious problem for pilots, as the pilot can be in a turn and not know it. In addition, there is a fixed acceleration threshold, below which the semicircular canals cannot sense any rotation at all. This threshold is approximately 2 degrees per second; if the rotation is gradual enough, the pilot won't sense any change and will develop "the leans" - otherwise referred to as spatial disorientation. The leans can occur in the same or opposite direction of the motion. Both will occur when movement is below the threshold of sensitivity for the semicircular canal. This biological shortcoming can be a huge advantage to flight simulator developers. This is also explained further in "Washout" below.
Eyes are the most important source of information. In relation to flying, they send pictures to the brain about the aircraft's position, velocity, and attitude relative to the ground. As a result, it is equally important that the motion works in cue with what's happening on the screen in front of you.
Motion Based Platform (Hydraulics)
A motion platform can cue motions through the computer algorithms which command the relative extension or retraction of the "legs" (hydraulic actuators) of the motion base.
Motion-based platforms add the sensation of movement to the pilot’s simulated flight. The cockpit and visual display system are mounted on the platform, which will have a number of hydraulic actuators or "legs" which do the lifting. The number of hydraulic powered legs dictates the number of degrees-of-freedom motion that can be achieved. These legs allow the platform to follow the pilot’s motions during flight. For example, if a pilot were taking off, the front of the platform would pitch so the pilot feels as if he is climbing.
Further information on the Types of Motion Platforms is detailed below.
The "washout" or cueing system makes pilots think they are making a continuous movement when actually the motion is restricted. For instance, when the aircraft is turning around. Since the simulator is on a platform, there are some movements which it cannot physically complete. As the cockpit has one fixed direction, it must provide the illusion to the pilots that they are actually turning around. The system does this by completing the first part of the turn, for example a left turn. Then the system slides the cockpit back into the main position using a tilt angle, so that the pilots do not know they are being returned to the neutral platform position. The old position data is thus "washed out". Hence when the pilots turn again, they are still able to get the sensation of a new turn. In other words, "washout" is where the simulator actually returns to a central or reference position in anticipation of your next movement, without you actually realising that it's happened. This is an important aspect in simulators as the flight sensations must be as close to real as possible.
As indicated above, the vestibular system is unable to interpret continued or sustained acceleration, so as long as we move the simulator at a speed below the threshold at which the human body can sense motion, you will be totally unaware that this movement has taken place. Likewise, if we apply full power in our aircraft and pull the nose up to initiate a climb, once the aircraft has settled into the climb and is maintaining a constant speed, the proprioceptors no longer supply information to your brain about the climb. Instead, we are using the visual cues, i.e instrument indications to interpret our situation. At this point, we can again return the simulator to its reference position and as long as the movement speed is kept below our detection threshold, you won't even be aware that it's happened.
G-Seat
Some organisations (such as the Air Force) take this all a step further and enhance the realism of the flight experience, through the use of a G-seat. G-seats are typically capable of reproducing vertical load, roll, pitch, forward acceleration or deceleration (e.g. by operation of airbrakes), and vibrations such as by buffeting, gun-firing and braking.
As with all products, not all G-seats operate in the same way. Features may include:
Some Aerospace groups use a G-seat with a helmet loading device, which complements their 6-DoF motion system. The helmet loader generates a load of the pilot's head. The combination of all three, is that it enables both low frequency sustained cues and high frequency vibrations to be simulated realistically.
There are various types of motion platforms. These range from state-of-the-art Six-Degrees-of-Freedom (DoF) motion platforms, to those which provide the more basic motions of pitch, roll and heave. Not surprisingly, the more DoF that the simulator can perform, the more complex (and expensive) the motion platform.
Six-Degrees-of-Freedom
 Six-degrees-of-freedom motion base. |
The most sophisticated of motion based flight simulators provide six DoF. This means that the simulator can be moved in the six ways an aircraft moves. Specifically:-
Vertical (also know as "Elevation" or "Heave" in flight sim terminology)
Longitudinal (Forward)
These motions can be divided into two categories:-
Three Translational Degrees of Freedom
|
Vertical (Heave) |
 Lateral |
 Longitudinal (Forward) |
Three Rotational Degrees of Freedom
|
Pitch |
Roll |
Yaw |
Typically, motion plaforms with 4 DoF provide all three rotational DoF, that is, pitch, roll and yaw and one of the translational DoF - heave.
 Four-degrees-of-freedom motion base. |
Our prototype - the GS-1, is a Three-Degrees-of-Freedom flight simulator, providing pitch, roll and heave movements. Each pod moves through:-
It does this through using three hydraulic rams driven by a 3 phase 3000 PSI hydraulic power unit.
 Pitch |
 Roll |
 Heave |
1. Response of Airline Pilots to Variations in Flight Simulator Motion Algorithms by Lloyd D. Reid and Meyer Nahon, AIAA Journal of Aircraft, Vol. 25, No. 7, pp. 639-646, 1988.
