AIAA 2007-6472
AIAA Modeling and Simulation Technologies Conference and Exhibit 20 - 23 August 2007, Hilton Head, South Carolina
Performance Testing of the Desdemona Motion System Manfred Roza1, Mark Wentink2 and Philippus Feenstra3 TNO Human Factors, Soesterberg, The Netherlands TP
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In the spring of 2007 TNO Human Factors together with AMST Systemtechnik GmbH have completed the development of their newest research simulator, the Desdemona, in The Netherlands. The Desdemona research simulator features a unique motion system not seen elsewhere in the world. Its serial design and geometrical dimensions give the motion system a large cylindrical motion space and a broad range of dynamic performance capabilities, which go beyond those of a classical Stewart platform. Like any other motion-base simulator the Desdemona motion system is driven by motion filters that transform the various simulation model outputs into safe and optimal motion cues. For the development of these motion filters it is necessary to exactly determine the dynamic performance characteristics of Desdemona and check whether these characteristics meet the specified motion system requirements. This paper describes the test protocol to measure, specify and verify the dynamic performance characteristics of the Desdemona motion system. The performance test protocol builds upon and extends the classical synergistic motion system test approaches, like the AGARD standard, to suite the specific Desdemona motion system capabilities.
Nomenclature M&S DOF IMU MCC PLC
ψ centr R H
= = = = = =
modeling and simulation degree of freedom inertial measurement unit motion control computer programmable logical controller central yaw axis
φcab
= radial axis = heave axis = cabin roll axis
ψ cab
= cabin yaw axis
θ cab
= cabin pitch axis
I. Introduction TNO Defense, Safety & Security in the Netherlands has a long tradition in research into modeling and simulation (M&S) technology and applications. The M&S effort of TNO Human Factors is centered in the area of flight, driving and ship simulators for human performance, training and behavior research. Over the years TNO Human Factors has specialized in human perception research ranging from visual & vestibular research to motion sickness and fidelity, to 3D-audio and haptic interfacing experiments1,4,5,6,7,8,9. To better facilitate this kind of research and the spatial disorientation training for the Royal Netherlands Air Force, TNO Human Factors initiated the development of a new research simulator, the Desdemona11,12,13. P
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In co-operation with AMST Systemtechnik GmbH, TNO Human Factors completed the development of the Desdemona simulator, in the late spring of 2007. The Desdemona research simulator features a unique and special designed non-synergistic motion system with six degrees of freedom (DOF). Its serial design and geometrical 1
Research Scientist, TNO Defence, Safety & Security, Human Factors Department,
[email protected], AIAA Member. Research Scientist, TNO Defence, Safety & Security, Human Factors Department,
[email protected], AIAA Member. 3 Research Scientist, TNO Defence, Safety & Security, Human Factors Department,
[email protected]. TP
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1 American Institute of Aeronautics and Astronautics Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
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dimensions give the motion system a large cylindrical motion space and a broad range of dynamic performance capabilities. One of the unique motion capabilities is the ability to combine onset cueing like a classical Stewart or hexapod platform with sustained acceleration cueing as found in dynamic flight simulators. Furthermore, the rotating gimbal system gives the Desdemona motion system the possibility to replicate unusual attitudes and large attitude changes one-to-one. The motion system houses a cabin, which is equipped with a 120 degree visual system. The modular hard and software design of the cabin enables fast reconfiguration of the interior and the installation of human machine interfaces. This makes the Desdemona simulator potentially suitable for a wide range of research applications including unusual aircraft and rotorcraft maneuvers, suburban, urban and terrain driving simulation, motion perception and cueing, spatial disorientation, motion sickness, and human-performance in artificial gravity conditions.
Figure 1. The Desdemona Research Simulator Exterior Like all other motion-base simulators the Desdemona motion system is driven by motion filters that transform the various simulation model outputs into safe and optimal motion cues 13, 14. For the development of these motion filters it is necessary to exactly determine the Desdemona dynamic performance characteristics. In addition, the measured performance is used to verify whether it meets the specified motion system requirements. This paper describes the test protocol to measure, specify and verify the dynamic performance characteristics of the Desdemona motion system. The paper starts in Section II with a presentation of the Desdemona motion system configuration and properties. Next the rationale and top-level design of the dynamic motion test protocol are discussed (Section III). This protocol builds upon and extends the classical synergistic motion system test approaches described in literature such as AGARD and ASC/MIL standards to suite the specific Desdemona motion system capabilities 15,16. In two subsequent sections the various tests and associated performance metrics of the protocol are discussed. These tests are divided in two categories. The first category is the single axis tests (Section IV). These tests comprise timedomain tests for motion system position, velocity and acceleration limits and signal tracking accuracy, and frequency domain system identification based tests. Multi-axis performance tests form the second category of tests (Section V). These tests are typical for serial motion systems, like the Desdemona, where independent axes have to be moved in conjunction to reproduce the desired motion cues during simulation. Lessons-learned and experiences from the first execution of the Desdemona motion performance test protocol are presented in Section VI. The paper ends in Section VII with conclusions and future work to motion system performance testing in relation to research into motion system fidelity, perception and requirements.
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II. Desdemona Motion System Description A. Motion System Configuration and Characteristics Unlike conventional motion systems such as the Stewart platform, the Desdemona motion system is not a synergistic or parallel robotic system. Instead the Desdemona has a non-synergistic or a serial motion system with six axes that can be moved independently (Figure 2). These six axes are respectively called: central yaw (ψ centr ), radius (R), heave (H), cabin roll ( φcab ), cabin yaw (ψ cab ) and cabin pitch ( θ cab ). The Desdemona cabin is suspended in a fully gimbaled 3DoF system ( φcab ,
ψ cab and θ cab ), which allows unlimited cabin rotation around any arbitrary axis in space. This gimbaled system is mounted in a heave system (H) that translates the gimbal system and the cabin in the vertical plane. The heave/gimbal system can be moved horizontally over a sledge; the radius (R). The sledge itself can be rotated unlimited in the middle around a vertical axis (ψ centr ); the central yaw axis. This central yaw axis in combination with the radius gives the Desdemona motion system the capability of sustained g-load generation up to 3g.
Figure 2. The Desdemona Motion System DOF
All the axes are driven by electric servo-systems. The required performance characteristics for each of these Desdemona axes in terms of the maximum attainable position, velocity and acceleration are given in the table below. Central Yaw Max. position Max. Velocity Max. Acceleration
0
>360 P
0
155 /s P
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2
45 /s P
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Radius
±4m P
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3.2 m/s 4.9 m/s2
Heave
±1m
Cabin Roll 0
>360 P
Cabin Yaw 0
>360 P
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0
Cabin Pitch
>3600 P
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0
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0
2.0 m/s
180 /s
180 /s
180 /s
4.9 m/s2
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90 /s P
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90 /s P
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90 /s2 P
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Table 1 Maximum Position, Velocity and Acceleration Characteristics of the Desdemona Motion System B. Motion Control, Measurement and Integrated Test Systems The Desdemona motion platform is controlled by a motion control computer (MCC), which runs on a standard PC with a real-time operating system. The MCC hosts all necessary control logic, safety and communication I/O software to safely operate the Desdemona motion system. An Ethernet network with a dedicated protocol is used for the communication between the MCC and several PLC’s. These PLC’s form the interface between the MCC and the peripheral motion system hardware such as the engine drives, measurement and safety systems, and various analogue and digital I/O. A CanOpen field bus is used for communication and data transport between the drive PLC’s and the engine drives. The MCC software architecture allows for both running the , e.g. vehicle model and motion filter on the MCC itself or remotely on different PC’s in a distributed simulator architecture design. This gives the Desdemona simulator additional flexibility in developing and off-line testing of vehicle model and motion filter configurations. The MCC operates and generates motion control reference signals at a rate of 200Hz. The Desdemona motion system is equipped with three types of measurement systems, which could be used for dynamic performance testing. The first measurement system comprises the various position encoders mounted on each axis, which are used by the electrical drives to control each axis position. The second measurement system comprises three solid state accelerometers mounted on the cabin chair at the position of the subjects head. These 3 American Institute of Aeronautics and Astronautics
three sensors are intended for safety purposes to measure the local specific force vector exerted on the subject7. The last measurement system is an Inertial Measurement Unit (IMU), which comprises a fiber-optics and temperature compensated 3-axis gyro in combination with a solid-state 3-axis accelerometer. These high quality sensors have specifically been selected for motion performance measurement purposes. The gyro is rigidly attached to the inside of the cabin structure and the accelerometer is also attached to the cabin but with a flexible mechanical connection. This provides digital output signals of the three cabin (mechanical low-pass filtered) accelerations and the angular rates. All measurement systems are connected to the Desdemona integrated test system. This test system is capable of injecting test-signals, logging and visualizing all the actual sensor data at a rate of 200Hz. C. Post processing of measurement data: filtering and differentiating The motion performance measurement and analysis of Desdemona comprises the orientation or positions of each degree of freedom and their first and second order derivatives. However, not all derivatives are directly measured by the aforementioned Desdemona measurement systems and are therefore not directly available for analysis. Therefore, some of the required derivatives have to be obtained numerically in the Desdemona test system software. In the literature there exist various ways to approximate these derivatives. Three commonly used implementations are the forward Euler approximation, the backward Euler approximation and Tustin’s approximation. For the Desdemona motion performance analysis the backward Euler approximation is used. A derivative operation amplifies the high frequency components of a measured signal. These high frequency components are due to measurement and sampling noise. Therefore, a low-pass filter is needed to filter out these noise components without affecting the real signal too much. A second order anti-causal (reverse digital filtering) low-pass filter has been utilized for this purpose, where the filter cut-off frequencies were found by trial and error. The cut-off frequencies are in the range of 8 to 12 Hz. Moreover, an anti-causal filter prevents a phase lag between the filtered and unfiltered signal.
III. Desdemona Motion Performance Test Protocol Design A. Existing Motion Performance Test Methodologies The AGARD Advisory Report 144 is probably the first and most extensive publication on flight simulator motion system performance15. The AGARD report stems from the seventies. Another often cited publication, from the same era as the AGARD standard, is the Department of Defense MIL-STD-1558 standard. This standard has been revised and is currently included in the superseding U.S. air-force guide specification for flight simulators16. Compared to the AGARD report the air-force guide is less extensive in its metrics but provides generic requirements, based upon lessons learned, for each of its described metrics. Considering the simulation-technology advances made over the past decades one can question whether the techniques and metrics described in both reports have to be adjusted or extended to meet the today’s requirements and needs. More recent publications confirm this notion and have proposed several modifications and extensions to both original standards 17,18,19,20,21,23,24. The basis for the Desdemona motion performance test protocol consists therefore of a mixture of both classical standards and several of these proposed improvements and lessons-learned. B. The Basic Desdemona Motion Performance Concept In the context of the Desdemona simulator the major limit of the existing literature on motion system performance is that it mainly focuses on the classic Stewart platforms, i.e. a synergistic or parallel system, around a single operation point of the workspace. The Desdemona motion system, however, has a non-synergistic or serial motion system instead. Due to this the Desdemona motions system has a broader range of motion capabilities and no specific single operation point. Moreover, each axis, i.e. electric servo system, can be moved and controlled independently of the other axes. This makes it possible to execute dynamic performance tests for each single axis separately. Such tests are not possible with a Stewart platform. The advantage is that the performance of each axis, i.e. driving servo system, is directly measured unlike the classical 6-DOF (surge, sway, heave, pitch, roll and yaw) of the cabin of a Stewart platform. Therefore, the single axis performance tests can be used for both the optimization of the servo system control laws and the motion filters that build-upon it. There are, however, some limitations to single-axis tests in assessing motion performance capabilities of serial motion systems. With only single axis performance tests it is hard, if not impossible, to directly compare the performances with other motion systems. These comparisons have to be made based-upon the 6-DOF of the cabin in which the human subject experiences the simulated motion. Obviously, from the user perspective this is also the area of interest. For most simulation applications two or more axes of the Desdemona motion system have to be moved 4 American Institute of Aeronautics and Astronautics
in conjunction to replicate these 6-DOF movements to be experienced by the subject13, 14. This can be realized by means of various, often not unique, combinations of axis movements. More importantly, under these different kinds of multi-axis operations structural and other mechanical (cross) coupling effects, like dynamic changes in the moment of inertia, centre of gravity, vibrations, Coriolis and centrifugal effects, could occur22. This may require changes to or additional compensation schemes in the Desdemona motion control system to obtain acceptable overall motion performance for multi axis operation. Furthermore, multi-axis tests provide additional information for optimization of motion-cueing algorithms for specific simulation applications. P
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Such multi-axis operation effects and knowledge cannot be identified through single axis tests. Therefore, the Desdemona motion performance test protocol combines both single axis (Chapter IV) and multi-axis performance tests (Chapter V). The multi-axis tests are based-upon the Desdemona mechanical configuration and its currently foreseen operational modes, motion filter types and research applications.
IV. Desdemona Single Axis Motion Performance Test Protocol A. System Limits Tests System limits define the upper and lower bounds for the position, velocity and acceleration of each degree of freedom provided by the motion system15. These system limits are usually expressed in the frequency domain in terms of the maximum allowed acceleration per frequency. A double log scale is used to get a convenient plot (Figure 3). System limits show the dynamic motion envelope of each axis and are used in the design of the dynamic performance tests in the remainder of this paper. P
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Figure 3. A typical example of a motion system limit plot
The system limits for each Desdemona axis have been specified by TNO together with AMST as a trade-off between what TNO requires for the intended research applications and what is physically possible given the current state-of-the-art in motion system hardware (structure, controls, servo-systems, etc.). Table 1 shows the required Desdemona motion system performance limits. These motion system requirements are tested using sinusoidal reference signals that are preceded and followed by a cosine profile to ensure a smooth signal fade-in and fade-out. For each axis two of these reference signals are created with angular rates (ω) that meet the next relationships:
ω=
v amax and ω = max pmax vmax
(1) T
T
Here amax, vmax and pmax are respectively the (absolute) maximum axis acceleration, velocity and position as specified in Table 1. The above relationships (1) directly follow from differentiating a sinusoidal signal. B
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B. Frequency Domain Analysis Tests Commonly applied frequency domain analysis techniques are a powerful manner to identify, analyze and specify the dynamic behavior of both linear and non-linear systems23, 24. The basic concept behind these techniques is to excite the motion system with a reference acceleration signal of known frequency contents and analyze it against the frequency contents of the system response. There are two types of reference signals that can be used23. Deterministic sinusoidal signals or broadband sinusoidal signals, like the Schroeder multi-sine, and broad-band random input signals, like white or colored noise, or a pulse width modulated signals. Performing tests with broad-band signals is far less time-consuming than a series of separate sinusoidal signals covering the same frequency spectrum. However, for the Desdemona dynamic motion performance testing the single sinusoidal signal approach is chosen. P
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The most important rationale for this is a safety concern. The uniqueness and complexity of the Desdemona mechanical, drive and control system design requires certain care to avoid unforeseen hazardous situations and structural damage. Therefore, a rectangular grid of measurement points is chosen inside the system limits of each axis and the measurements are executed from low-power to high-power sinusoidal signals. This approach is visualized in Figure 3 by the blue arrow. For all these grid points, the next two classes of frequency domain performance measures are determined 15, 16, 17, 18, 19. P
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1. Describing Functions Describing functions presented in the form of a series of Bode plots are a common manner to analyze and specify the dynamic behavior of a non-linear system in terms off gain and phase-lag. Describing functions are a more general version of linear system’s frequency response functions23. The difference is that describing functions not only vary the frequency but also the amplitude to identify amplitude dependent non-linearity. The AGARD standard assumes linearity of motion systems and only uses acceleration amplitudes of 10% of the system limits. For the Desdemona motion system this assumption is not trivial, therefore the following acceleration amplitudes have been chosen to test the correctness of this assumption; 2%, 5%, 10%, 25%, 50% and 75% of the system limits. The frequency grid points are chosen in the interval of 0.2 Hz to twice the expected design bandwidth for each axis. On each measurement grid point (ωk) the describing function gain and phase-lag ( G(j ωk) ) is calculate by dividing the cross and power spectral density estimates of the input and output acceleration signals as follows: P
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SYU ( jωk )
G ( jωk ) =
(2)
SUU ( jωk ) T
Expression (2) gives the describing function of the driven axis. However due to mechanical cross-coupling the driven axis will also excite the other axes. This means that in-total for each axis six describing functions can be constructed, one primary and five cross-talk describing functions. A cross-talk gain of maximum 2% in any nondriven axis is commonly acceptable16. To smoothen these estimates i.e. reduce the variance and leakage, the Welch’s method of periodogram averaging estimates for the spectral densities are applied. A quantification for the accuracy of these estimates is given by the coherence (γ) function: P
γ
2
( ωk ) =
P
SYU ( jωk )
2
(3)
SUU ( jωk ) SYY ( jωk )
The value of the coherence ranges always between zero and one. A value closer to one indicates a more accurate estimate. A coherence value larger or equal to 0.6 is considered to be adequate for an accurate estimate 24. P
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There exist two classical performance metrics that can be derived from the describing functions23,24,26. The first metric is the system bandwidth, which is defined as the frequency (F-3dB) at which the system amplitude gain sinks below the -3db. The second metric is defined as the frequency at which the system response exhibits a 90-degree phase lag (F-90deg). P
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2. Noise Level Characteristics The objective of noise level measurement is to quantify the output noise characteristics of the Desdemona motion system for a single axis driven by a sinusoidal reference signal with a discrete frequency and acceleration amplitude. The basis for the noise level characterization is the variance of the measured noise, which can be estimated through calculating the average power over a frequency interval (N1
