This paper presents recent work in magnetic suspension wind tunnel development in National Cheng Kung University. In this phase of research, a control-based study is emphasized to implement a robust control system into the experimental system under study. A ten-coil 10 cm x 10 cm magnetic suspension wind tunnel is built using a set of quadrant detectors for six degree of freedom control. To achieve the attitude control of suspended model with different attitudes, a spacial electromagnetic field simulation using OPERA 3D is studied. A successful test for six degree of freedom control is demonstrated in this paper

Jou, Hui-Long

Lin, Chin E.

Sheu, Yih-Ran

English

NASA Technical Reports Server

MAGNETIC LEVIATION SYSTEMDESIGN AND IMPLEMENTATION
FORWIND TUNNEL APPLICATION
// !
Chin E. Lin
Institute of Aeronautics and Astronautics
National Cheng Kung University
Tainan, Taiwan, R.O.C.
Yih-Ran Sheu
Department of Electrical Engineering
Nan-Tai College, Tainan, Taiwan, R.O.C.
Hui-Long Jou
Institute of Aeronautics and Astronautics
National Cheng Kung University
Tainan, Taiwan, R.O.C.
ABSTRACT
This paper presents recent work in magnetic suspension wind tunnel development in
National Cheng Kung University. In this phase of research, a control-based study is emphasized
to implement a robust control system into the experimental system under study. A ten-coil
10 cm x 10 cm magnetic suspension wind tunnel is built using a set of quadrant detectors for six
degree of freedom control. To achieve the attitude control of suspended model with different
attitudes, a spacial electromagnetic field simulation using OPERA 3D is studied. A successful
test for six degree of freedom control is demonstrated in this paper.
I. INTRODUCTION
Magnetic Suspension and Balance Systems (MSBS) are being developed for wind tunnels to
support aircraft models operating free from flow interference caused by the conventional
mechanical supports. There are several MSBS's operated for wind tunnel tests described in the
literature. A typical MSBS consists of an array of electromagnets around the wind tunnel test
section, generating electromagnetic forces and moments on the suspended model. The suspended
model is designed with several permanent magnets embedded inside the non-metallic envelope.
MSBS's of this type are inherently open-loop unstable, i.e., the position and attitude of the
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suspendedmodel should be continuously monitored with adjustment and control of the magnetic
field within the wind tunnel test section via a feedback control system [ 1,2]. From the literature,
the MSBS for wind tunnel application requires a dedicated electromagnetic loop design and a
feedback control system. These two major parts are dominant to the success of the MSBS in
wind tunnel operation. To accomplish such a development, a control-based study for developing
a modem position and attitude control technology should be emphasized. In preparation for this
research development, a comprehensive force model has been developed to look into more
correlated problems in the large-gap MSBS [3,4]. From basic control approaches, we understand
that many conventional control technologies can be adopted in the MSBS for wind tunnel
applications. The control methodology can become useful knowledge in our laboratory from
previous studies [5]. This paper presents an MSBS for wind tunnel applications from the view
of the control-based study and implementation. A test system, consisting of an array of ten
electromagnets for 10 cm x 10 cm wind tunnel test section, has been built and tested. The
configuration of typical MSBS consists of power supplies, position sensing system, dynamic
model, and feedback controller. The power supply system consists of ten PWM switching
power units. Each power unit uses the high gain current feedback method to shift the poles of
the electromagnet coil far from the imaginary axis. The electromagnet dynamics is neglected [2].
The position sensing system is composed of a laser optical circuit, quadrant detectors, and lateral
offset detectors. The system can measure the motion of the suspended model with six degrees of
freedom in wide range operation. Its accuracy is proved by experimental testing. The operating
rate of the system is above 1 KHz such that the stiffness in model control is high. The dynamic
model in our case indicates the relationship between the electromagnetic coil currents and the
motion of suspended model. The electromagnetic field is simulated by the OPERA 3D magnetic
field simulation software to look into the electromagnetic field problem, and the magnetic forces
and moments in relation with magnetic field are calculated by the theoretical electromagnetic
formulas. After the equations of motion are introduced [3], the linearized system dynamic model
at a normal operation point is obtained. From the Bode plot of the linearized system dynamic
model, we can consider the system to be decentralized since each motion of degree of freedom is
dominated by its corresponding electromagnets. Then, the PID controllers are used in the
decentralized system for stable regulation control. This controller is simple and easy to
implement. Combining the decentralized PID controller with the optical position sensing
system, the proposed system does a good job to achieve control requirements in the MSBS.
II. SYSTEM DYNAMIC MODELING
The sketch of the electromagnet circuit and its system coordinates are shown in Fig. 1. It is
composed of ten electromagnet coils and a suspended model. Two air cored coils, No. 1 and
No. 2, and the other eight iron cored coils, from No. 3 to No. 10, generate the electromagnetic
forces and moments acting on the suspended model against the aerodynamic drag force, moment
and the gravity force. The size of test section is 10 cm x 10 cm. The symmetric arrangement of
the ten electromagnet coils is arranged to have a uniform magnetic field distribution in the test
section. The system dynamic model gives the relationship of coil currents and the positions and
attitudes of the controlled suspended model. The ten coils of the MSBS are divided into six
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groups. Theinput coil currentsare11,I2, I3, I4, I5, and I6, individually. The system outputs are
the positions of x, y, z, and the attitude of pitch, yaw and roll. The coil currents will produce
magnetic field and exert the magnetic forces and moments on the suspended model such that
motion in six degrees of freedom can be generated. As the equations of motion are introduced [6]
and are generally linearized for use, the system dynamic model at its normal operation point is
obtained. The magnetic field in our experiments is simulated by the OPERA 3D magnetic field
simulation software to understand the magnetic field problem, and the electromagnetic forces and
moments in relation with magnetic field are calculated by the improved theoretical electro-
magnetic formulas. The formulas are simple and their accuracies are proved by experimental
measurements [4]. This process is valuable to understand the relationship of coil currents to each
desired model position and attitude. Using different test conditions as inputs, the simulation
results quickly show the correct direction for test preparations and control gain setting.
III. POSITION SENSING SYSTEM
The position sensing system is composed of a laser optical circuit, quadrant detectors, and
lateral offset detectors. The configuration of the system is shown in Fig. 2. The laser beam from
the laser tube passes through a spatial filter and a convex lens, and then extends to parallel beams.
The light beam is marked by a covered sheet to form a slender rectangular beam. It goes through a
splitter and then divides into two light beams. One beam goes in y-axis direction and the other in
z-axis direction as shown in Fig. 5, each will be detected by a quadrant detector located at the
center of the sensing position. The quadrant detector is a cell which is composed of four photo-
diodes for each different direction. As the light intensity on the cell is balanced, the output of
sensor voltage is zero. For the suspended model in the test section, part of the light beam will be
blocked by the model and the residual light will still be received by the quadrant detector. If the
model is located at the center position in z-axis direction, the top part of the residual light is the
same as the bottom part and the output voltage of the detector is zero. If the model moves down
in z-axis position, the top part is larger than the bottom part and the output voltage is negative,
or vice versa.
This is the example for the z-axis displacement measurement. Similarly, from the residual
light of the left part and the right part, the y-axis displacement can be measured. In pitch angle
and yaw angle measurement, the angle variation is proportional to the sum of the unmarked light
intensity by a sinusoidal function. We can pick up the data of the sum of the unmarked light to
obtain the model attitude. The x-axis displacement and roll motion is measured using the lateral
offset detector. The lateral offset detector is first applied to the model position sensing in
MSBS. It is like the CCD position sensing system to measure the position of the marked light
points on the suspended model [7], but its outputs are analog voltages with relative to the x
position of the reflected light point on the detectors. The resolution and speed are much higher
than CCD, and its operation is simpler.
The sensing system is calibrated by several experiments. The experimental instruments are
built-up by two 0.01 mm accuracy height gauges, a 0.01 mm accuracy meter gauge, and a 10000
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pulsespercyclephoto-encoder.Theheightgaugesareusedto measurethey, z, pitch,andyaw
displacementsof themodel,themetergaugeisusedto measurethex-axisdisplacement,andthe
photo-encoderis usedto measuretheroll displacement.As thepositions,andattitudesare
changed,thedatafrom thedetectorsignalamplifieris pickedup. Thecalibrationresults,which is
therelationshipof amplifieroutputsignalandattitudechange,andits erroranalysis,which is
obtainedfrom thedifferencesbetweenthepicked-updataandthefitted curves,areshownin
Fig. 3. Fig. 3(a)is theresultof x-axisdisplacement,Fig. 3(b)is thez-axisdisplacement,Fig. 3(c)
is thepitchangle,andFig. 3(d) is theroll angle.They-axisdisplacementandyawanglerotation
calibrationresultaresimilar to thez-axisdisplacementandpitchangle.We canseethatthe
accuracyofx displacementis limited in 0.04mm, z displacementis limited in 0.01mm,andpitch
angleis limited in 0.2degree.Theoperationrangesof thesensingsystemfollows: thepitch
angleis+/= 25°, theyaw angleis +/= 25°, andtheroll angleis+/= 20°.
Concerningthepositionsensingsystem,thereareseveralproblemsto bediscussed.Oneis
thecouplingproblembetweenthemeasurementsin eachdegreeof freedom,for example,the
movementof z-axiswill effectthemeasurementof roll displacement.But in thereal application
of thewind tunnel,themasscenterof themodelis fixed suchthatthecouplingproblemis not
consideredhere.Anotherproblemis aboutthesensitivityof theanalogdevices.Theyare
effectedby thelight intensityvariationandnoise,mainly producedby thecomputer. In practice,
theoutputsignalof theamplifier is normalizedby thelight intensityto havethesignalrelatively
insensitiveto intensitychangesandtheaccuracyisacceptableconsideringthenoisyenvironment.
To summarize,theadvantagesof thepositionandattitudesensingsystemarehigh bandwidth,
accuracy,robustness,easyrealization,andwiderangein operation.
IV. SYSTEM IMPLEMENTATION
Basedon theoreticalanalysis,sinceeachelectromagneticoil contributesadifferenteffectto
positionandattitudecontrolof thesuspendedmodel,adecentralizedcontrolcanbe implemented
ona PC-basedcontrolsystem.ThedecentralizedcontrolcommandscoilsNo. 1andNo. 2 acting
on thex displacement,coilsNo. 3, No. 4,No. 5 andNo. 6 actingon thez displacementandpitch
motion control,andthencoilsNo. 7,No. 8,No. 9andNo. 10actingon they displacementand
yawandroll anglecontrol. Thesuspendedmodelhasembeddeda cylindricalpermanentmagnet
insideits aerodynamicenvelopefor magneticfield vectorcontrol. Theenvelopeof ourexperi-
mentmodelis arocketof 90mm lengthwith totalmassof 100g,embeddedwith fivehigh flux
densitypermanentmagnetsof 10mm diameter.Themeasuredvalueof themagnetizationis
equalto 839250A/m.
By anintegrationof thedecentralizedPID controllerwith loopclosedfrom theposition
sensingsystem,thesuspendedmodelin thetestsectionis stablein regulationcontrol. The
photographof configurationof theMSBSis shownin Fig. 4 andits controlresultsis shownin
Fig. 5. SixPID controllersfor six degreesof freedomof motioncontrolarerealizedusingan
Intel 486-33personalcomputer.Thehighestsystemsamplingrateis 2 KHz, determinedby
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stiffness of the suspended model. The higher sampling rate can provide higher operation
stiffness, and the lower sampling rate is more difficult to design the controller to meet system
stability requirement.
V. CONCLUSION
In this paper, the implementation of a magnetic suspension wind tunnel using effective
measurement and control is demonstrated. The objective of the proposed idea accomplishes the
operation requirements of a model position and attitude measurement and control for six degrees-
of-freedom. By calibration procedures, the accuracy of the proposed measurement system using
an integration of quadrant detector and lateral offset detector has achieved an acceptable level [8].
Other advantages of such an implementation are beneficial operation range, bandwidth, stiffness,
and stability, in addition to its high accuracy with low cost. This magnetic suspension wind
tunnel is used for low speed model tests under continuous attitude changes as a simulation of real
flight conditions. Also, the measurement system is established for other related research projects
to test their developing control methods for such nonlinear and unstable characteristics of
magnetic suspension system.
REFERENCES
[1] C. P. Britcher; and M. J. Goodyer: "Digital Control of Wind Tunnel Magnetic Suspension
and Balance System," ICIASF '87 Record, Williamsburg, VA, June 22-23, 1990.
[2] C. E. Lin; and L. A. Lee: "Dynamic Control of Magnetic Suspension System," IEEE
Advanced Motion Control Conference, Tokyo, Japan, March 30, 1990.
[3] N. J. Groom: "Analytical Model of a Five Degree-of-Freedom Magnetic Suspension and
Positioning System," NASA TM- 100671, March 1989.
[4] C. E. Lin; and H. L. Jou: "A Dynamic Method for Magnetic Torque Measurement of
Permanent Magnet," IEEE Instrumentation and Measurement Technology Conference IMTC
'94, Hamamatsu, Japan, May 9-12, 1994.
[5] C. E. Lin; and Y. R. Sheu: "A Hybrid Control Approach for Large-Gap Magnetic
Suspension System," CAC Journal of Control Systems and Technology, Vol. 2, No. 1, Jan.
1994, pp. 1-9.
[6] Nelson J. Groom: "Analytical Model of a Five Degree of Freedom Magnetic Suspension and
Positioning System," NASA TM- 100671, March 1989.
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[7] C. E. Lin; and A. S. Hou, "A Real Time Position and Attitude Sensing Using CCD in
Magnetic Suspension System Application," IEEE Trans. on Instrumentation and
Measurement," Vol. 44, No. 1, pp. 8-14, February 1995.
[8] C. E. Lin; J. H. Jou; and Y. R. Sheu: "System Implementation of Measurement and Control
for a Magnetic Suspension Wind Tunnel," IEEE Instrumentation and Measurement
Technology Conference 95, Boston, April 23-26, 1995.
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Fig. 1 Sketch of the MSBS and its system coordinate.
Fig. 2 Sketch of the position sensing system.
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491
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Fig. 4 Photograph of the configuration of the MSBS.
Fig. 5 Photograph of the control results of the MSBS.
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Magnetic Leviation System Design and Implementation for Wind Tunnel Application

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