Buschbaum, Andreas

Plassmeier, Volker

Institute of Transport Research:Publications

IOP PUBLISHING SMART MATERIALS AND STRUCTURESSmart Mater. Struct. 16 (2007) 1120–1124 doi:10.1088/0964-1726/16/4/021Angle measurement with a Hall effectsensorAndreas Buschbaum and Volker PlassmeierGerman Aerospace Center (DLR), Institute of Flight Systems, 38108 Braunschweig, GermanyReceived 8 March 2007Published 25 June 2007Online at stacks.iop.org/SMS/16/1120AbstractOne field of work of the Institute of Flight Systems of the German AerospaceCenter (DLR) is the investigation of flight characteristics of helicopters. Forthis task DLR operates the research helicopter FHS (Flying HelicopterSimulator/Type EC135) and a model helicopter wind tunnel test stand.The tail rotor of a helicopter has a significant impact on flightcharacteristics like vibration and noise. Sensors for the measurement ofthrust, rotor speed, and pressure are already available. This paper presents asensor for the acquisition of the flapping motion of the tail rotor. After anintroduction and an explanation of the challenges of the measurement task,the mechanical and electrical integration of a smart and intelligent Hallsensor into the model test bed is described. Results from a wind tunnel testare discussed.(Some figures in this article are in colour only in the electronic version)1. IntroductionHelicopters are complex and highly dynamic flight vehicles.Several aerodynamic characteristics still need to be investi-gated, such as for example the interaction between the mainand the tail rotor. This interaction is one reason for unwantednoise and vibration. In cooperation with national and inter-national partners from industry (e.g. Eurocopter, Agusta) andresearch centres (e.g. ONERA, NLR) the German AerospaceCenter develops technologies to increase the performance ofhelicopters. For this and related fields of research DLR oper-ates several test facilities. One is the research helicopter FHS(Flying Helicopter Simulator, figure 1) [1], developed by Eu-rocopter Germany and DLR.In addition to the full scale helicopter the Institute ofFlight Systems [2] has operated a helicopter model test bedfor more than 20 years [3]. During this time the requirementsconcerning the sensor equipment has increased constantly, notonly in terms of the number of sensors, but also in terms of themechanical dimension, power consumption, performance andcost reduction for each individual sensor.Over the years more and more sensors have beenintegrated into this test bed. The most important sensors arestrain gauges, pressure sensors, accelerometers and tempera-ture sensors. The latest development has been the integrationof a Hall sensor for the measurement of the flap angle.This paper presents the use of a small Hall sensor mountedon the tail rotor of a model helicopter to measure the tail rotorFigure 1. Flying Helicopter Simulator FHS.flap angle (figure 2). Additionally the integration of this typeof sensor into the main rotor helicopter blades to measurethe angle of active integrated flaps is discussed as a futureproject.In both applications, the dimension of the sensor has to bevery small, since the mounting space of the sensor is limiteddue to the mechanical construction of the tail rotor and themain rotor blades.0964-1726/07/041120+05$30.00 © 2007 IOP Publishing Ltd Printed in the UK 1120Angle measurement with a Hall effect sensor  Figure 2. Helicopter tail rotor model in the German–Dutch windtunnel (DNW).Figure 3. Flap angle on a tail rotor.2. Flap angle of the tail rotorThe tail rotor of the BO105 helicopter consists of two blades.The blades being mounted on a hinge are variable in pitch(figure 3). The flap angle is the result of different airstreamvelocities of the forward (flight direction) and backward(against flight direction) running blade of the rotor. Thisaerodynamic condition results in different lift moments, whichagain lead to a sine oscillation of the blades. The effectincreases with higher flight velocities.Figure 4. Hall sensor from Micronas.3. Hall sensorFor the measurement of the flap angle the HAL800 sensor fromMicronas was selected (figure 4). Its mechanism is based onthe principles of the Hall effect. The sensor measures theflux density of the magnetic field and delivers a proportionalelectrical output voltage [4].The sensor consists of an integrated A/D converter, aD/A converter, a low pass filter and an EEPROM. A greatadvantage of this sensor is its programmability. This allowssetting and storing specific parameters, such as for example thespecification of the magnetic material, the output voltage andthe filter ranges (i.e. selecting a cutoff frequency of 0.5 kHz or2 kHz).In order to be able to program the sensor an adapter boardis connected between the sensor and the serial interface ofa PC. The adapter board requires a power supply of 15 VDC, which is passed onto the sensor. The adapter boardcommunicates with the sensor via a modulation of this supplyvoltage. The normal operational voltage of the sensor is5 V DC. The operation with the nominal voltage disables themodification of the parameters.4. Integration of the sensor in the modelDue to the mechanical construction of the tail rotor and thesmall size of the measurement equipment, both the sensorFigure 5. Schematic view of the tail rotor model system.1121A Buschbaum and V PlassmeierFigure 6. Calibration of the flap angle.Figure 7. Flap angle at hover.and the magnet were installed with two-component glue. Thesensor was installed on the fixed side of the hinge and themagnet was installed on its moveable side (figure 5).After the integration of the flap angle sensor the systemwas calibrated. As a calibration curve the following third-orderpolynomial was chosen (figure 6):β(U ) = m3 · U 3 + m2 · U 2 + m1 · U + m0(flap angle β , sensor output voltage U and numerical constantsm0 to m3).The reason for the non-symmetrical curve is the non-symmetrical movement of the magnet out of its centre position.5. ResultsAs an example two measurements are shown. The selectedflight conditions are hover (figure 7) and level flight at a flightvelocity of v = 18 m s−1 and a heading of 18◦ (figure 8).The latter means that the x axis of the helicopter is turnedby 18◦: a heading of 90◦ would mean the flight direction issidewards. Each figure shows the flap angle as a function ofthe rotor position. An angle of 0◦ means that the referenceblade shows downwards. 32 tail rotor rotations were acquiredwith 128 samples/rotation.In the hover condition (figure 7), only the airflow of themain rotor and the construction of the fin have an influenceon the flap angle. Thus, on both blades the aerodynamicconditions are almost the same, resulting in small flap angles(note the scaling of the y axis).In the level flight condition (figure 8) the flap angle isclearly a function of the cosine of the rotor angle. Due tointerference with vortices caused by the main rotor duringforward flight no constant aerodynamic conditions exist at thetail rotor blades and thus the flap angle varies a little bit withevery revolution.For this condition a Fourier transformation of the flapangle measurement results in a peak at 92 Hz (figure 9).1122Angle measurement with a Hall effect sensorFigure 8. Flap angle at level flight (v = 18 m s−1).Figure 9. Fourier transformation of the flap angle measurement from figure 8.Figure 10. Active nose flap (developed by EADS) with Hall sensor.This value is identical to the rotation speed of the tail rotor(5520 rpm) which was measured separately.6. Integration of the sensor on the active main rotorbladesIn order to avoid stall effects in the outer area of the retreatingmain rotor blade a new blade is currently under developmentby EADS, ECD and DLR [5, 6]. This blade is equipped witha small movable flap. The hall sensor is used to measure thedeflection of this flap. The sensor is installed at the hinge of theblade and the magnet is glued on the movable nose flap. Thetests with the new blade will be conducted in one of the windtunnels of the DNW at Go¨ttingen, Germany. Figure 10 showsthe arrangement of the flap inside the blade, including sensorand actuator (the top part of the airfoil is not installed).7. ConclusionsThe usage of a Hall sensor for the measurement of the flapangle of a rotor blade has turned out to be a very effective,space-and cost-saving solution. In spite of extremely high1123A Buschbaum and V Plassmeiervibrations and the high rotation speed of the rotor, the systemshows a very robust mechanical and electrical performance.With the experience from the wind tunnel tests with themodel, good results can be expected for the active main rotorblades.References[1] Gelhaar B, Oertel H, Alvermann K, Bodenstein M, Gandert R,Graeber S and Schwaneck H-P (German Aerospace Center,Institute of Flight Research, Braunschweig) 2003FHS—experimental system for Flying Helicopter Simulatorput into operation American Helicopter Society 59th Forum(Phoenix, AZ, May 2003)[2] http://www.dlr.de/ft[3] Stephan M, Klo¨ppel V and Langer H-J 1988 A new wind tunneltest rig for helicopter testing 14th European Rotorcraft Forum(Sept. 1988) Paper No. 66[4] http://www.micronas.com/products/documentation/sensors/hal805/index.php[5] Klo¨ppel V (ECD), Kube R (DLR) and Martin W (Daimler BenzAG) 1998 AROSYS—ein forschungs-undentwicklungs-projekt zu adaptiven rotorsystemenDGLR-Jahrbuch 1998 DGLR-Jahrbu¨cher I, 1., DGLR[6] Klo¨ppel V and Kube R 2001 On the role of prediction tools foradaptive rotor system developments Smart Mater. 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Angle measurement with a Hall effect sensor

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