MOTION SIMULATOR MODULE CYBERBALL

ABSTRACT

Introduction to a new concept for motion simulation module CYBERBALL. A simulator system with three rotational degrees of freedom for unbounded bi-directional angular displacement is described, in which the scenery as well as the instrument panel and also the whole cockpit are simulated by digital video graphics applying the novel VR technology by means of a VR head mounted display and VR glove for virtual command. It follows a description of selected topics of the applied system solutions, such as axis drives, motion control, system kinematics and dynamics.

Keywords: Flight Simulator, Virtual Reality, Automatic Control, Electrical Drives, Interaction.

1 INTRODUCTION

A motion simulation module Cyberball is an equipment capable of submitting its user to mechanical sensations similar to the ones felt when utilising the vehicle being simulated.

Flight simulators are, no doubt, the most utilised ones and they allow the physical reproduction of some sensations experimented during the aircraft’s motion. Such simulators reached a high degree of sophistication, reproducing accurately the cockpit’s environment of a given aircraft aiming at training, specially for taking off and landing operations, and to make the crew to become acquainted to the on-board equipment in commercial aircrafts.

A graphics processor generates the images of the flight scenery, which are projected onto panels, in perfect synchronism with the mechanical events of the interactive simulation process.

This concept has been leading to the development of entirely specific simulators for each model or version of aircraft, produced as prototypes or in a small batch, requiring the development of aeronautical and avionics equipment specially adapted for simulation purposes, resulting in an extremely high cost equipment.

Consequently, crews of small-sized aircraft usually do not have access to this kind of training, which directly affects their performance and above all, the flight safety.

Besides its high cost, such simulators are also very limited at reproducing the motions. This makes them inadequate for training acrobatic, combat/fight and other manoeuvres, as well as for the class of smaller-sized/higher dynamics aircrafts, such as helicopters.

Static simulators, with very accessible costs, but limited to audio-visual effects only, can even be used in introductory training phase, but are not adequate in terms of hi-fi reproduction of flight sensations.

2 CONCEPTION OF A MOTION SIMULATOR SYSTEM

The integration of new technological developments, specially in the field of informatics and computers graphics and of electromechanical high dynamic drives, helped the conception of an entirely new motion simulator, whose patent has already been requested.

In this conception, the rotating motions of the vehicle (aircraft, car, boat, etc.. ) about the three mutually orthogonal Cartesian co-ordinate axes are reproduced mechanically, as shown in FIG.1.

FIG.1 Conception of the simulator system.

The mechanical conception of the simulator Cyberball consists of 4 basic structural elements, starting with a stationary support base for the module, and of 3 movable elements, one per co-ordinate axis, which are a fork that rotates about A axis, an outer ring that rotates about B axis articulated on the fork, and a cockpit (bubble) mounted onto an inner ring, that rotates about C axis relative to the external ring. Utilising these three axes of the simulator’s plant, it can be reproduced the rotating motions about the three axes that define the attitude of a vehicle: Y (yaw), P (pitch) and R (roll). However, it is possible to show through kinematics analysis that the dependency relation between the axes of the simulator’s plant with the axes of the simulated vehicle do not remain constant.

Instead of making physically available all the panel instruments, these are also visually simulated through a graphics image processor in real time, together with the operations scenery, and presented to the user in the form of a special display mounted on a helmet-goggle for virtual reality (FIG.2), which, whenever necessary, is integrated to the piloting helmet utilised in the operation.

FIG.2 A display helmet for virtual reality.

With two small colour LCD screens of very high resolution, located in the helmet, very near the user’s eyes, this display permits the perfect illusion of 3-D stereoscopic images. Multiple orientation (spacial angular position) sensors, installed in the helmet itself, detect the motions of the user’s head, and inform the graphics processor which portion of the scenery should be shown on the screen at that instant, and in this way the user get a panoramic virtual view of 360 degrees horizontally and vertically, which is identical to the real view. Stereo sound is available through integrated phones.

Only the main devices for vehicle command are physically present, which are, in the case of an aircraft, the control lever, the pedals, the power command, and eventually the devices for special operations command, e.g. for weapons. The buttons, keys and the other command devices are also graphically simulated and actuated through instrumented gloves for virtual reality, applied to the user’s hands, whose positions in the 3-D space are continually informed to the simulation processor, in such a way that a sufficient (virtual) approximation to a certain command device will trigger its execution. If the hands with gloves penetrate the instantaneous field of the user vision, these will also be reproduced on the screens. There is also inside the simulation cockpit a seat and safety devices for one (or more) user(s), who are also exact replicas of the ones in the vehicle being simulated.

With these devices it is assured the full interaction between the user and the system, which is a vital characteristic of virtual reality systems, and distinguish them from other systems such as movies, video, standard television, etc., that have a passive nature.

Thus, the majority of the functions, which previously were carried out by hardware, have been transferred to software solutions, that can be replaced much more easily, quickly reconfiguring the simulator. Therefore, in the same basic simulator module, with few alterations to the command device (hardware) and by changing the simulation program (the kinematics, dynamic models and their parameters), it is effectively possible to simulate a variety of vehicles and environments, either of similar nature or different.

It can be noticed that this conception of a simulator, being preponderantly generic, favours a series production with a very significant cost reduction, which inclusively makes it viable its application as an equipment for training users of land vehicles, both military and civil, and also opening a perspective of utilisation as leisure equipment, in a high level range, allowing to foresee a very significant market for it.

3 THE SIMULATOR MODULE

Besides the cost, also the weight of the movable parts of the simulator are very much lower than the traditional equipment’s, which gives it much higher dynamic characteristics, i.e. much greater realism in the execution of the motions, and therefore a perfect illusion of reality for the user, enabling also the simulation of radical motions, such as acrobatics. The effects of the rectilinear accelerations and displacements, which are difficult to simulate mechanically in an exact way, are embedded in the graphic processing of the scenery images, imposed to the user in synchronism with the rotating motions.

The rotating motions of the simulator module (FIG.1) result in inertia variation on the three axes, which is a function of their relative positions, superposed to varying gravity effects due to the asymmetry of the movable part’s weight, and also related to Coriolis and centrifugal effects.

The specific dynamics of the vehicle being simulated needs to be imposed, in real time and in a sufficiently precise way, to the own dynamics of the simulator module.

In FIG.3 it is shown a block scheme of the main functions that are part of the command and control system.

FIG.3 Scheme of the simulator command / control system.

The calculation of the vehicle’s dynamic model is done from the command variables, which are the signals originating from the command elements available to the user (and eventually to an instructor) and from the whole simulation program source (e.g. winds, amount of fuel, road relief, interacting mobile objets, etc.). The result from the dynamic model evaluation of the vehicle, which is applied to the command of the simulator module, are the angular accelerations, rotations and orientation of the vehicle along the three rotating axes from their reference.

However, since the simulated vehicle’s kinematics usually differs from the simulator module kinematics, it is necessary to calculate an inverse kinematics transformation, in order to transform the variables (e.g. acceleration) of the vehicle into commands of each of the movable parts of the plant, i.e. fork, external ring and cabine, along the three axes of its reference.

In order to synchronise the mechanical motion with the respective visual graphical representations, the calculation of the dynamic model of the vehicle requires information on the angular orientations where the vehicle is forced by the imposed accelerations. Therefore, it is necessary that, through another kinematics transformation, direct transformation, the current orientations of the simulator module are transformed back into those of the vehicle’s cabine.

The synchronisation of the motions with the visual graphical part of the simulation, requires that every information applied to the dynamic model is also available to the graphics software, that manages the co-ordinate system of the movable and geodesic elements.

4 DRIVE SYSTEM

It is intended to generate tangible mechanical motions in the form of accelerations and displacements capable of creating tactile sensations similar to the ones that the user would experience if he utilised the real vehicle. Consequently the reproduced motion must correspond to the commands fed by the module’s command devices and other variables generated in the simulation session program.

In order to achieve that, the simulator module has a drive system with three electromechanical servo-drives of high dynamics, one for each axis of rotating movement. These are commanded co-ordinately by the motion management processor (FIG.3), to which the calculated acceleration references are imposed through the first kinematics transformation.

The motion manager performs many functions, as can be seen in FIG.3: one of these is the calculation of the dynamic model of the mechanical plant of the simulator module, in which, from the acceleration, rotation and position references of the three axes (generalised co-ordinates), the torque references for each of the servo-drives are calculated.

It is observed that the parameters of the dynamic model of the plant vary with the orientation (attitude) and with the three orthogonal components of rotation (speed) of the cabine: The necessary torques for the acceleration are proportional to the inertia, which varies with the attitude of the cabine; the required torques for overcoming the forces due to the mutual influences of the motions of the three movable parts (centrifugal and Coriolis effects) are functions of both, attitude and rotations of the cabine; in turn, the torques requested to overcome gravity are function of the cabine’s attitude only.

Since there is no direct relation between the three reference axes of the vehicle’s cockpit, X, Y, Z (that correspond to those of the simulator’s cabine), and the three driven physical axes of the plant, q_i, the execution e.g. of a pure yaw of  degrees requires co-ordinate ( simultaneous ) motions on the three physical axes of the plant (FIG.4).

FIG.4 References of the simulator module. where: rotation about: Z corresponds to yaw, Y to pitch; and X to roll of the cockpit and/or vehicle; qi: rotations about the driven physical axes of the plant.

In order to the cabine to follow the pilot’s, instructor’s and/or scenery’s information determined motions, with the specified rotational speeds and accelerations, the motion manager must calculate the torque to be applied to each of the movable parts of the plant by the respective drives. To calculate these torque, in each situation, the manager receives as inputs the generalised co-ordinates, i.e. the angles and their derivatives (rotational speeds and accelerations) of each movable element, given relative to the fixed reference base.

It is a function of the inverse kinematics to execute the transformation of the yaw, pitch and roll angles, which describe the vehicle’s attitude, their rotations and accelerations, on the generalised co-ordinates, i.e. on angles, rotations and accelerations of each of the movable elements of the plant.

Once the torque references are calculated by the simulator’s dynamic model, these are then applied to three decentralised drive systems, with independent control, one by rotating axis of the plant.

Other functions of the motion manager correspond to full supervision of the drives and the gathering of information on rotational speed based on the position information of the sensors of the three axes.

5 IMPLEMENTATION

Every transformations and calculations of the dynamic models are processed in real time, with a calculation cycle that must be lower than 1ms.

The dynamic model of the simulator is processed in real time by a high performance DSP, hosted on a PC-like hardware with which it communicates through a double-port RAM. ASICs for the interfacing of the (angular) orientation measuring systems (encoders) of incremental type, operate in parallel with the DSP. A DSP also calculates the direct and inverse kinematics transformations.

The calculation of the vehicle’s dynamic model is carried out by a simulation manager processor that also controls the graphics accelerator processor.

The graphic processor (FIG.3) for real time image generation is based on a graphic accelerator , and it can be utilised the digitalisation of real scenery previously recorded, which allows for instance the simulation of a flight procedure over a given ground, during the day, night, under various meteorological conditions, etc.

Each one of the three drive systems (FIG.5), is characterised as a servo-system with an AC servo-motor (2) and a servo-converter with vectorial control, commanded on torque, coupled with the special reduction (3) with high reduction factor with negligible play and coupled also to the angular position sensor (1) of the motor axis. The digital control of each drive is done in closed loop, with a DSP carrying out simultaneously the calculation of the control algorithms of the three axes.

FIG.5 Simulator’s drive system.

For energising as well as information transfer between the various movable parts of the module, it is utilised slip ring and brush devices. The information is transmitted via local network of high transmission rate.

6 CONCLUSION

In order to increase even more the realism of the simulations, the command elements (control lever, etc.) may present tactile, acoustic, etc. reactions corresponding to the real instantaneous dynamic state of the motion. The cabine can be pressurised.

Simulators of this kind can be interconnected via a network aiming at performing simulation procedures with multiple participants interacting simultaneously in real time, each of them with his/her own vehicle, for e.g. race simulation, etc.

The new conception also favours the future integration of new visualisation technologies, drives, etc. and their application to still unexplored activities.

A down-sized instrumented model dynamic mock-up was built in order to carry out advanced studies of the system’s kinematics (FIG.6). The first prototype in natural size is being manufactured.

The significant cost reduction achieved relative to state-of-the-art simulators really will enable its application also as an equipment for leisure, at least on the high end range, permitting to foresee a very significant market for it.

FIG.6 Aspect of simulator’s small-sized driven mock-up.