Atlas Motion Platform Full-scale PrototypeDesign

Z. Copeland, B. Jung, M.J.D. Hayes, and R.G. Langlois

Department of Mechanical and Aerospace Engineering, Carleton University,Ottawa, Canadae-mail: Zack.Copeland@carleton.ca; Bradley.Jung@carleton.ca;John.Hayes@carleton.ca; Robert.Langlois@carleton.ca

Abstract. This paper presents an overview of the design of the first full-scale prototype of the At-las flight simulator motion platform for pilot training. The Atlas concept was introduced in 2005,and is unique in that orienting is decoupled from positioning, and unlimited rotations are possibleabout any axis of the mechanism. Detail design and manufacturing are complete, and assembly isin progress. The key to the design is three mecanum wheels in an equilateral arrangement, whichimpart angular displacements to a sphere that houses the cockpit, thereby providing rotationalactuation. Since the Atlas sphere rests on these mecanum wheels, there are no joints or leversconstraining its motion, allowing full 360◦ rotation about all axes, yielding an unbounded orienta-tion workspace that is singularity free. In this paper, the current state of the design and assemblyregarding actuation, the spherical S-glass shell, and modelling for motion control are discussed.

Key words: Unbounded orientation workspace; structural design; actuation; simulated dynamicresponse.

1 Introduction

The Carleton University Simulator Project (CUSP) is a fourth year capstone designproject in mechanical and aerospace engineering. Unlike capstone design projectsat most other universities, CUSP is run as a small design office with as many as30 students led by three or four faculty members. The students experience work-ing in a virtual enterprise environment. Among developing other soft skills, theyexperience a design project matrix: experience having to prioritise among multiplesupervisors and multiple tasks; communicate in a large multidisciplinary team; de-velop verbal and written communication skills; but also develop the capacity forunbiased listening. Since its inception in 2002, approximately 300 fourth-year stu-dents from mechanical, aerospace, and computer systems engineering have incre-mentally advanced the development of a novel simulator system that overcomes mo-tion limitations associated with industry-standard simulator motion platforms basedon hexapods. The full-scale design is called the Atlas simulator motion platform andis illustrated in Fig. 1. The unique design of Atlas decouples its three-dimensional

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2 Z. Copeland, B. Jung, M.J.D. Hayes, and R.G. Langlois

translational workspace from its unbounded, singularity-free orientation workspace[5]. In the current configuration, a MOOG MB-EP-6DOF Gough-Stewart platform[4, 11] is used to provide translation while Atlas provides the rotation.

Fig. 1 A 3D rendering of theAtlas Simulator.

The concept of spherical actuation is not new. Spherical dc induction motorswere introduced in 1959 [13] and developed over the next 30 years, see [2, 10], forexample. However, due to physical limitations imposed by the stator and commu-tator, angular displacements are limited. Unbounded rotational motion is achievedby the Eclipse II architecture [6]; however, there is no closed-form algebraic modelfor its kinematics and the velocity-level kinematics require estimating parametersnumerically. The Desdemona motion platform [1] uses a fully-gimballed system toenable rotation about any axis. However, because of the gimbal arrangement, theorientation workspace is not free of singularities because of the potential of gimbal-lock.

Based on the performance of several proof-of-concept small-scale demonstrators,design of the Atlas full-scale prototype began in 2011 and manufacturing beganin earnest in 2013. All of the individual components are currently in assembly. Inthis paper, the current state of the design and assembly regarding actuation, thespherical S-glass shell, and modelling for motion control are discussed. Please notethat the use of dual metric and Imperial dimensioning reflects the reality of designin Canada: the standard is metric; however, many stock components are sized inImperial units.

2 Structural Components

Structural components within the Atlas motion platform can be broken into threemain categories for consideration: external structures; the spherical cockpit; andthe internal support structures. Due to weight restrictions, a large number of the

Atlas Motion Platform Full-scale Prototype Design 3

Atlas components are machined out of 6061 T6 aluminum. Bolt-together designsare employed wherever possible.

The primary focus of the external structure of the Atlas prototype is to providesupport and stability throughout a wide range of simulated conditions. In order toaccomplish this task, it must perform several simultaneous functions: it must allowfor the spherical cockpit to be constrained under expected operational loading con-ditions and provide enough space for the actuation components to be mounted andstabilized.

Support is provided by three vertical I-beams, in an equilateral arrangement alongthe outside of the sphere, see Fig. 1. The sphere is constrained with two sets of pas-sive mecanum wheels, connected to the I-beams, located at the top and bottom of thesphere. The 12 passive mecanum wheels placed along the bottom of the sphere helpto distribute its weight and that of the internal structures, while the upper set of 12passive mecanum wheels provide downward force to ensure sufficient contact forcebetween the sphere and the three active mecanum wheels such that the sphere canbe rotated. The downward force is supplied by three pneumatic cylinders connectedbetween the I-beams and the ring of 12 upper wheels.

2.1 Sphere Structure

In order to create unbounded rotation, the cockpit of the Atlas prototype is housedin a 9.5 ft diameter fiberglass sphere, consisting of an internal support structure forincreased rigidity as well as two hatches to facilitate entry and egress. Due to me-chanical and spacial design constraints, the sphere shell has been designed to com-prise four identical quarter spheres. Using epoxy S-glass, the strength required tomaintain integrity under the loading from the active mecanum wheels was achieved,with an estimated failure load in excess of 1000 psi.

Each flange between sphere segments is bound together with a series of bolts,while an aluminum reinforcement is applied to both sides of the joining flanges inorder to provide additional stiffness to the sphere, as well as serving as a washerfor the bolts to prevent damage to the sphere flanges. In addition to these stiffeners,a series of ribs that serve as a mounting interface for the internal structures is alsoconnected at a 90 degree angle to the flange washers.

Entry points are included at two antipodal points of the sphere. They comprisetwo 30 in diameter hatches that are locked in place with a striker bolt system whichconnects to the internal support structure of the sphere. The hatches are formedfrom 3/8 in thick machined aluminum. Air circulation is provided to the pilot withfour fans placed on each hatch, which make use of 151 holes in the hatches toallow for each fan to draw and expel air from and to the surroundings. Attached tothe internal support structures are the brackets and flooring that provide a base forthe cockpit seen in Fig. 2 (a). Reconfigurability created the demand for a highly-modular internal structure, so it is bracket and bolt based, allowing for any singlecomponent to be reconfigured without requiring disassembly of the sphere. The

4 Z. Copeland, B. Jung, M.J.D. Hayes, and R.G. Langlois

flooring support structure interfaces with the stiffeners via four brackets which arebolted to them directly.

Fig. 2: (a) Floor structure and pilot support system; (b) interface platform connectedto the Moog hexapod.

3 Actuation

The Atlas motion platform possesses six degrees of freedom. What distinguishesAtlas from conventional motion platforms is that linear displacements are decoupledfrom angular displacements. Moreover, the orientation workspace is unbounded andfree from singularities [7]. This is accomplished by attaching the sphere orientationsystem to an interface platform atop a hexapod, see Fig. 2 (b). A MOOG MB-EP-6DOF Gough-Stewart platform was selected to be the hexapod motion base. Whilethis platform is capable of full six degree of freedom motion, it’s controller enablesthe use of only its translation capabilities [8], thereby providing linear combinationsof surge, sway, and heave.

The orientation degrees of freedom are supplied by the three active mecanumwheels which supply torque to the sphere, and the 12 pneumatically-controlledupper wheels ensure sufficient normal forces at the sphere-active wheel contactpatches, while the 12 lower passive mecanum wheels help support the sphere load.Initially, omniwheels were to provide this rotational actuation, as seen in Fig. 3 (b)but mecanum wheels were eventually selected in order to reduce actuation-inducedvibrations [12]. The reduction in vibration is similar to that achieved when helicalgear pairs are used in place of corresponding spur gear pairs. Fig. 3 (a) is an imageof one of the active mecanum wheels designed to apply torque to the sphere. Whilecommercial mecanum wheels exist, appropriate sizing and weights were problem-atic. Hence, the active wheels were designed in several iterations leading to the final

Atlas Motion Platform Full-scale Prototype Design 5

versions illustrated in the figures. The passive mecanum wheels are substantiallysmaller than their active counterparts, as illustrated in Fig. 4.

Fig. 3: (a) CUSP mecanum wheel design; (b) omniwheel proof of concept model.

Fig. 4: (a) Atlas active wheel hub; (b) Atlas passive wheel hub assembly.

Analysis revealed that a normal force of 1500 lbf between each active roller andthe sphere is required to move the sphere. To ensure an appropriate distribution ofthis force at each contact point without destroying the S-glass required the forces bedistributed over an area of at least 2.5 in2, while ensuring that the roller material wasstiff enough to avoid deflecting to the point that the hubs would abrade the sphereduring operation. Urethane was selected because of favourable wear characteristicsand strength limits that are adequate for the expected loading of the Atlas prototype,as well as having a large range of durometers (levels of compliance) to choose from.Testing was conducted in an MTS press, shown in Fig. 5 (a) to establish suitableurethane durometer. Contact patch size was recorded via an ink stamp test wherethe roller was compressed onto a sheet of paper and analyzed photogrammetrically.

6 Z. Copeland, B. Jung, M.J.D. Hayes, and R.G. Langlois

A sample ink blot can be seen in Fig. 5 (b). This led to the determination of theeffective pressure exerted on the sphere surface.

Fig. 5: (a) MTS press test frame with sample roller inserted; (b) Resulting ink blotafter compression testing a durometer 55A urethane to 1300 lbf.

The analysis revealed that the two durometers of urethane considered, 55A and85A, both satisfied the minimum required surface contact patch size of 2.5 in2.While the durometer 55A urethane had surface areas in excess of 5 in2, it expe-rienced deflections in excess of 0.5 in, indicating that it would likely jam and abradethe sphere during torque transmission. For this reason, the durometer 85A was se-lected for the rolling surfaces on both the passive and active mecanum wheels. Forcelimits on the sphere S-glass surface mean that the maximum surface pressure is700 psi. The durometer 85A sample was able to maintain a safety factor of at least1.5 over the operable pressure range expected.

4 Control, Dynamics, and Simulation

Motion control of the Atlas platform is currently under development. At this point,the controller is being developed using a dynamic model to simulate the sphere mo-tions given pilot flight control inputs. The following gives a high-level description ofthe feedback motion control [7], motor torque requirements [9], and the simulateddynamic response of the sphere.

4.1 Control

The overall Atlas control system comprises the vehicle simulation which determinesthe motion of the simulated vehicle in response to pilot input and environmental

Atlas Motion Platform Full-scale Prototype Design 7

disturbances, a two-part sphere orientation sensing system which fuses orientationdata from a visual orientation system (VOS) that actively tracks circular barcodesaffixed to the outer surface of the sphere and an on-board three-axis gyroscope [7];a washout algorithm that converts the vehicle state information to the desired plat-form state using classical washout theory that has been extended to accommodatelarge angular motions that are possible with the Atlas simulator; and an actuatorcontrol system that controls the six legs of the MOOG motion base to provide de-sired translational motions and the three rotation drive motors that control the sphereorientation by applying the required combination of sphere motor torques.

4.2 Torque Jacobian and Simulated Dynamic Response

The three motors that actuate each of the active mecanum wheels are positionedequilaterally around the sphere’s vertical axis and 45 degrees below the equator,indicated by θ =−45◦ for all three wheels, as illustrated in Fig. 6 (a). The mecanumwheel castor rollers are at a 45 degree angle relative to the mecanum wheel axis ofrotation, γ =−45◦ for all rollers, as indicated in Fig. 6 (b).

Fig. 6: (a) Mecanum wheel position around sphere; (b) Roller angles aroundMecanum wheel.

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The torque vector, M, generated by the ith mecanum wheel acting on the sphereis given by the cross product of the position vector of the centre of the wheel’scontact patch relative to the sphere centre with the force injected to the sphere bythe wheel, resulting in the equation,

Mi = RAi/G×FAi =τiRrCγi

[−CφiCγiSθi +SφiSγi, −SφiCγiSθi−CφiSγi, CγiCθi

]Twhere the position of the contact patch around the sphere is specified by angle, φi,and inclination angle, θi; the angle the roller axis makes with the mecanum wheelaxis is γi; the radius of the sphere is R, the effective radius of the mecanum wheelis r; the individual mecanum wheel torques are τi; and C, S, and T are the cosine,sine, and tangent trigonometric functions, respectively. Full details of the derivationof the mecanum-wheel-generated torque can be found in [9].

Combining the torque contributions for each wheel into a single matrix expres-sion results in,

M = Jττ , (1)

where M is the total moment applied to the sphere and Jτ is the torque Jacobiangiven by,

Jτ =Rr

−Cφ1Sθ1 +Sφ1T γ1 −Cφ2Sθ2 +Sφ2T γ2 −Cφ3Sθ3 +Sφ2T γ3−Cφ1Sθ1−Sφ1T γ1 −Cφ2Sθ2−Sφ2T γ2 −Cφ3Sθ3−Sφ2T γ3

Cθ1 Cθ2 Cθ3

. (2)

A sphere emulator program was designed for the purposes of dry testing spherereactions to motor torques, as well as recording the simulation run for subsequentplayback and data analysis. The emulator reads the desired mecanum wheel torquedata in real time from the custom Atlas simulation computer network or from apreviously-saved file and calculates the incremental orientation of the Atlas sphere.The program takes the sphere’s inertial matrix and torque Jacobian matrix intoaccount to provide a dynamic model of the sphere movement. The emulator pro-gram was written in MATLAB and includes a graphical user interface displayingthe sphere animation, data readouts, and user controls. Rotation calculations werecompleted using quaternion mathematics to avoid representational singularities.

Quaternion mathematics are used in the emulator because quaternion rotationdoes not have a hierarchy of rotations. Rather, quaternions represent the rotationof the object in 4-D hyperspace about an arbitrary axis. Once the desired motionis converted into quaternion notation, the sphere will rotate in a single sweepingmotion. This makes motion interpolation between large angle rotations possible.Quaternion mathematics and conversions can be found in [3].

The moment of inertia tensor I necessary for dynamic modelling was extractedfrom the Atlas composite CAD model by estimating the approximate densities ofeach component in the assembly. This is a 3 x 3 symmetric matrix that is variable toaccommodate changes to the sphere weight distribution between simulation runs.

Atlas Motion Platform Full-scale Prototype Design 9

At each simulation cycle, the initial motor torque, angular rate, and sphere at-titude (τ,ω0,θ0) are read by the emulator and then converted to quaternion formdenoted as e0. The angular momentum equation can be rearranged as

α1 = I−1 (M−ω0× Iω0) , (3)

where α is the sphere angular acceleration.The angular acceleration is numerically integrated resulting in an updated angu-

lar velocity:

ω1 = ω0 +∫

α1dt. (4)

The angular rate in Euler terms is converted to the quaternion rate using [3], yielding

e1 =12

Lω1. (5)

The quaternion rate is then numerically integrated to obtain an updated orientation:

e1 = e0 +∫

e1dt. (6)

Finally the quaternion rate is converted back into Euler terms. The calculated angu-lar rate and angle are then fed back into the system for the next iteration, e1→ θ1.This process is then repeated for the duration of the dynamic simulation.

5 Concluding Remarks

This paper has presented an overview of the current state of development of the At-las simulator motion base with emphasis on the rotational actuation system, as wellas computational tools that have been developed to support the design and opera-tion of the system. As is apparent from the design, the unusual actuation methodprovides unique motion capabilities for the simulator – most notably unbounded,singularity-free rotation. However, with that come some practical challenges relat-ing to the need for tether-free power, data transfer, and ventilation. Additionally,since the drive system is unconventional and relies on technologies for which exactclosed-form solutions do not exist in many cases, and despite the fact that the basicdesign principles are low risk, it is anticipated that much will be learned through thecommissioning and calibration phases of the full-scale prototype.

Three related areas of particular interest will be the contact forces, mecanumwheel tractive forces, and wheel slip. In total, the sphere is held in place by patchcontact with 27 mecanum wheels. The system is clearly statically indeterminate andtime-varying due to the inertial loads acting on the system as the simulator is in mo-tion and also due to the time-varying tractive forces applied by the active mecanumwheels. Design tools that were developed to approximate the worst-case contact

10 Z. Copeland, B. Jung, M.J.D. Hayes, and R.G. Langlois

forces necessarily made assumptions - such as the relative rigidity of the externalsupporting structure, the effective stiffnesses of both the small and large mecanumwheel castor assemblies, and the characteristics of urethane in this application. Themaximum tractive effort of the active mecanum wheels will depend both on the pre-vailing contact forces and the friction characteristics at the interface between theactive mecanum wheels and sphere surface. Initial testing was performed to deter-mine the required durometer of urethane and its friction properties; however, somenotable differences between test conditions and in situ operation are anticipated. Fi-nally, wheel slip due to tangential compression of the urethane material prior to itentering the active mecanum wheel contact patches is expected to result in longi-tudinal slip, similar to what occurs with pneumatic tires. The extent of this and itsimpact on the idealized kinematic equations, based upon which the system has beendesigned, will have to be assessed and integrated into the Atlas control system.

Development of the Atlas simulator has been a provider of, and continues to pro-vide excellent educational opportunities for both undergraduate and graduate stu-dents, and continues to present a series of well-bounded research questions. Theresults of this work directly benefit the CUSP project but also contribute better un-derstanding to the broader problem of spherical actuation using mecanum wheels.

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