Classifications

• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D1/00—Control of position, course, altitude or attitude of land, water, air or space vehicles, e.g. using automatic pilots
  • G05D1/0011—Control of position, course, altitude or attitude of land, water, air or space vehicles, e.g. using automatic pilots associated with a remote control arrangement
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64U—UNMANNED AERIAL VEHICLES [UAV]; EQUIPMENT THEREFOR
  • B64U10/00—Type of UAV
  • B64U10/10—Rotorcrafts
  • B64U10/13—Flying platforms
  • B64U10/14—Flying platforms with four distinct rotor axes, e.g. quadcopters
• • G—PHYSICS
  • G05—CONTROLLING; REGULATING
  • G05D—SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES
  • G05D1/00—Control of position, course, altitude or attitude of land, water, air or space vehicles, e.g. using automatic pilots
  • G05D1/02—Control of position or course in two dimensions
  • G05D1/0202—Control of position or course in two dimensions specially adapted to aircraft
  • G05D1/0204—Control of position or course in two dimensions specially adapted to aircraft to counteract a sudden perturbation, e.g. cross-wind, gust
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64U—UNMANNED AERIAL VEHICLES [UAV]; EQUIPMENT THEREFOR
  • B64U2201/00—UAVs characterised by their flight controls
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64U—UNMANNED AERIAL VEHICLES [UAV]; EQUIPMENT THEREFOR
  • B64U2201/00—UAVs characterised by their flight controls
  • B64U2201/10—UAVs characterised by their flight controls autonomous, i.e. by navigating independently from ground or air stations, e.g. by using inertial navigation systems [INS]
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B64—AIRCRAFT; AVIATION; COSMONAUTICS
  • B64U—UNMANNED AERIAL VEHICLES [UAV]; EQUIPMENT THEREFOR
  • B64U2201/00—UAVs characterised by their flight controls
  • B64U2201/20—Remote controls

Definitions

• the invention relates to rotary wing drones such as quadricopters and the like.
• These drones are provided with multiple rotors driven by respective engines controllable in a differentiated manner to control the drone attitude and speed.
• a typical example of such a drone is the AR.Drone of Parrot SA, Paris, France, which is a quadricopter equipped with a series of sensors (accelerometers, three-axis gyrometers, altimeter), a front camera capturing an image of the scene to which the drone is directed, and a vertical aiming camera capturing an image of the terrain overflown.
• sensors accelerometers, three-axis gyrometers, altimeter
• a front camera capturing an image of the scene to which the drone is directed
• a vertical aiming camera capturing an image of the terrain overflown.
• WO 2010/061099 A2 and EP 2 364 757 A1 describe such a drone and its driving principle via a phone or multimedia player with touch screen and built-in accelerometer, for example a cell phone of type iPhone or a player or a multimedia tablet type iPod Touch or iPad (trademarks of Apple Inc., USA).
• the drone is piloted by the user by means of signals transmitted by the inclination detector of the apparatus, inclinations which will be replicated by the drone: for example, to advance the drone the user tilts his device according to its pitch axis, and to move the drone right or left it tilts the same device relative to its roll axis.
• the drone is commanded to tilt or "dive" downward (tilt at a pitch angle)
• it will progress forward with a speed that is higher as the tilt important; conversely if it is controlled so as to "pitch up” in the opposite direction, its speed will gradually slow down and then reverse by going backwards.
• the drone is also provided with a fixed-point control command: in this operating mode that will be called “autopilot mode” afterwards, when the user releases all the controls of his remote control device, the drone immobilizes in a fixed point and stabilizes there fully automatically.
• the general problem of the invention is the steering of such a drone in the wind.
• wind a movement of air parallel to the ground, constant in space and in time (gusts of wind will not be considered). Such a “wind” is then completely determined by its components of horizontal velocity in a given terrestrial reference, or by its norm and its direction in such a reference.
• the wind represents a disturbance which, by its characteristics, affects the drone when it is in flight.
• These disturbances are particularly sensitive in the case of very light "microdrones", such as the aforementioned AR.Drone, whose mass is only a few hundred grams.
• the wind gain of such a drone is therefore particularly sensitive, even in moderate winds.
• the drone relies to move on a moving air mass as a function of the wind, while it measures its horizontal speed components with respect to the ground (for example by means of a camera pointing downwards and giving an image of the ground), angles (by accelerometers and embedded gyroscopes) and external forces applied to it (by embedded accelerometers).
• the accelerometers have by construction a bias (which will be assumed constant in the short term) that must be estimated and compensated to correctly use the acceleration measurement in the reconstruction of angles.
• the accelerometers being used inter alia in inclinometers, an error in estimating the accelerometer biases causes a bias on the angle measurement, which is detrimental to the holding of the fixed point. In the same way this sensor bias influences the measurement of the wind forces achieved by the accelerometer.
• the drone In the presence of wind, the drone must bow to compensate the wind and maintain the fixed point (enslaved on a stable image of the vertical camera): its attitude will not be zero and the evaluation of the bias will be modified. This does not prevent to maintain the drone in fixed point (enslavement by the camera), but will have a particularly significant incidence in two cases:
• One of the aims of the invention is to propose a wind estimation method making it possible to distinguish between accelerometer construction bias and wind-induced measurement bias.
• Another object of the invention is to propose a wind compensation method making it possible to overcome the two phenomena that have just been described, that is to say i) in autopilot flight, to allow the drone to maintain its point. wind in a yaw motion and ii) in controlled mode, to provide a control of the movement which is identical in all directions, whether in the direction of the wind or in the direction opposite to the wind, of transparent way for the driver.
• the FR 2 926 637 A1 describes a system for estimating the speed of a drone with respect to the air, by applying a model expressing the mechanical relations between the forces acting on the drone and its speeds and accelerations in translation. and in rotation.
• the speed with respect to the air is notably estimated from the aerodynamic drag force the movement of the drone, which is indirectly measured and used to obtain information on the speed relative to the air.
• this document proposes to use a GPS receiver embedded measuring the speed of the drone relative to the ground.
• This way of proceeding requires the presence of such a receiver in the drone, and requires a GPS signal is available and that its accuracy is sufficient (visibility of a sufficient number of satellites).
• the precision obtained does not provide a wind speed estimate that is sufficiently fine to correctly evaluate the windward part of the acceleration sensor measurement bias, which is necessary to compensate for the windward steering disadvantages described above. .
• the evaluation of the angle of the drone can be strongly affected by this construction bias of the accelerometers.
• the invention proposes a technique for estimating the bias of the accelerometer sensor independently of that introduced by the wind.
• the invention also proposes a compensation technique making it possible to use the results of the bias estimation to correct the piloting faults in the wind exposed above, both in the autopilot mode (fixed point) and in the controlled mode (controls pilot applied by a user).
• the invention proposes a method for controlling a multi-rotor rotary wing drone driven by respective controllable engines to control the drone in attitude and speed, this method comprising, in a manner known per se, of after the above-mentioned Waslander article:
• the establishment of at least one dynamic model of the drone describing components of the horizontal velocity of the drone as a function of the drag coefficients and the mass of the drone, Euler angles characterizing the attitude of the drone with respect to a absolute landmark, as well as the speed of rotation of the drone around a vertical yaw axis; the measurement of the aerodynamic drag force of the drone, derived from a measurement of acceleration of the drone;
• the Kalman predictive filter is a six-state filter, these states comprising:
• the establishment of the dynamic model of the drone comprises the establishment of two distinct models, with: i) a model dy- drone in flight, and ii) a dynamic model of the drone on the ground, these models being used selectively according to the state of the drone.
• the dynamic model of the drone in flight can use a measurement of acceleration of the drone as a measure proportional to the speed of the air in the mark of the drone.
• the dynamic model of the drone in flight can notably be of the type:
• ⁇ , ⁇ and / being the Euler angles (respectively of roll, pitch and yaw) characterizing the attitude of the drone with respect to the NED mark, where 3 ⁇ 4 is the rate of rotation about the axis w (movement of turn in yaw), and
• R being the rotation matrix associated with the angle ⁇ , in dimension 2.
• the dynamic model of the drone on the ground can in particular be of the type:
• ⁇ being a temporal parameter of progressive decay of the estimate of the wind speed.
• the invention also proposes a step of compensating the effect of the lateral wind on the positioning and displacements of the drone, by generating corrective setpoints, a function of the estimated horizontal components of lateral wind speed, and a combination of these corrective instructions. the pitch and roll angles applied to the control loop of the drone's engines.
• the corrective instructions may especially be of the type:
• V wind being the modulus of the estimated wind speed
• Cx being the drag coefficient of the drone
• y / wind is the heading angle of the wind direction
• y / drone being the heading angle of the direction of the drone .
• the compensation step may notably include the definition for the drone of an open loop reference attitude.
• the corrective instructions are combined with the piloting instructions applied by the user.
• these corrective instructions are combined with fixed point stabilization instructions produced in response to a measurement of the horizontal velocity of the drone relative to the ground.
• Figure 1 is an overview showing the drone and associated remote control device for remote control.
• Figure 2 is a block diagram of the various control organs, servo and assisted steering of the drone.
• Figures 3 and 4 are comparative records, respectively of the module of the wind speed and the direction of the wind, values simultaneously estimated and measured under real conditions, showing the relevance of the estimation method according to the invention.
• Figure 5 is a functional schematic representation illustrating the wind estimation and compensation loops, for a flight configuration of the autopilot fixed-point drone.
• Figure 6 is a functional schematic representation illustrating the wind estimation loop, for a flight configuration of the drone in controlled flight.
• reference numeral 10 generally denotes a drone, for example a quadricopter such as the AR. Drone of Parrot SA, Paris, France, described in the aforementioned WO 2010/061099 A2 and EP 2,364,757 A1, as well as in FR 2 915 569 A1 (which notably describes the control system with gyrometers and accelerometers used by the drone ).
• the drone 10 comprises four coplanar rotors 12 whose engines are controlled independently by an integrated navigation system and attitude control. It is provided with a first camera 14 with frontal aiming to obtain an image of the scene towards which the drone is oriented, for example a wide angle camera with CMOS sensor.
• the drone also includes a second camera with a vertical aim (not visible in Figure 1) pointing downwards, able to capture successive images of the terrain overflown and used in particular to assess the speed of the drone relative to the ground.
• a second camera with a vertical aim (not visible in Figure 1) pointing downwards, able to capture successive images of the terrain overflown and used in particular to assess the speed of the drone relative to the ground.
• Inertial sensors make it possible to measure with a certain accuracy the angular velocities and attitude angles of the drone, that is to say the angles of Euler describing the inclination of the drone.
• inclination means the inclination of the drone with respect to a horizontal plane of a fixed terrestrial reference, being that the two longitudinal and transverse components of the horizontal velocity are intimately related to the inclination along the two respective axes of pitch and roll.
• a local coordinate system ⁇ u, v, w ⁇ linked to the body of the drone the drone, although strongly symmetrical by construction, comprises a front and a rear, and the position of the camera will be considered as pointing towards the front , thus defining the axis u. ; the axis v is perpendicular to u in the mean plane of the drone, and the axis w is the vertical axis directed towards the ground;
• NED North East Down
• Y N direction ED is the direction parallel to the ground plane perpendicular to the geographic North (ie the geographical East)
• ZNED direction is perpendicular to the ground plane and pointing downwards.
• An ultrasonic rangefinder disposed under the drone and an onboard barometric sensor also provide measurements which, combined, give an estimate of the altitude of the drone relative to the ground.
• the device 16 is provided with radio link means with the drone for the bidirectional exchange of data from the drone 10 to the device 16, in particular for the transmission of data.
• This connection may be for example type Wi-Fi local network (IEEE 802.1 1) or Bluetooth (registered trademarks).
• the apparatus 16 is also provided with inclination sensors to control the attitude of the drone by printing the device corresponding inclinations.
• the remote control device 16 is advantageously constituted by a multimedia touch screen phone or player with an integrated accelerometer, for example an iPhone-type cell phone, an iPod Touch-type music player or an iPad-type multimedia tablet, which are devices incorporating the various control devices necessary for the display and detection of control commands, the display of the image captured by the front camera, and the bidirectional exchange of data with the drone by link Wi-Fi or Bluetooth.
• a multimedia touch screen phone or player with an integrated accelerometer for example an iPhone-type cell phone, an iPod Touch-type music player or an iPad-type multimedia tablet, which are devices incorporating the various control devices necessary for the display and detection of control commands, the display of the image captured by the front camera, and the bidirectional exchange of data with the drone by link Wi-Fi or Bluetooth.
• the piloting of the drone 10 consists in making it evolve by controlling the engines in a differentiated manner to generate, as the case may be, movements of:
• the drone also has an automatic and autonomous hover stabilization system ("fixed point" autopilot configuration), activated especially when the user removes his finger from the touch screen of the device, or automatically at the end of the take-off phase, or again if the radio link between the aircraft and the drone is interrupted. The drone then moves to a state of levitation where it will be immobilized and stabilized automatically, without any intervention of the user.
• an automatic and autonomous hover stabilization system (“fixed point” autopilot configuration)
• control system involves several nested loops for the control of the horizon- tal speed, the angular velocity and the attitude of the drone, in addition to the variations of altitude.
• the most central loop which is the angular velocity control loop 100, uses, on the one hand, the signals supplied by gyrometers 102 and, on the other hand, a reference constituted by angular velocity instructions 104. information is applied at the input of a stage 106 for correcting the angular velocity, which itself drives a control stage 108 of the motors 1 10 in order to control separately the speed of the various motors to correct the angular velocity of the drone by the combined action of the rotors driven by these engines.
• the angular velocity control loop 100 is embedded in an attitude control loop 12, which operates from the indications provided by the gyrometers 102, by accelerometers 114 and by a magnetometer 1 16 giving the absolute orientation of the drone in a terrestrial geomagnetic landmark.
• the data from these different sensors are applied to a stage 1 18 of attitude estimation type PI (Proportional-Integrator).
• the stage 1 18 produces an estimate of the real attitude of the drone applied to an attitude correction stage 120 which compares the actual attitude with angle commands generated by a circuit 122 from commands directly.
• the possibly corrected instructions applied to the circuit 120 and compared to the actual attitude of the drone are transmitted by the circuit 120 to the circuit 104 to control the motors appropriately.
• the loop 1 12 of the attitude control calculates an angular speed setpoint using the corrector PI of the circuit 120.
• the angular velocity control loop 100 then calculates the difference between the previous angular velocity reference and the angular velocity effectively measured by the gyrometers 102.
• the loop calculates from this information the various instructions of rotational speed (and therefore of ascending force), which are sent to the engines 1 10 to perform the maneuver requested by the user and / or planned by the pilot automatic drone.
• the angle setting calculating circuit 122 also receives corrections delivered by a wind estimation and compensation circuit 128, which will be described below in detail.
• the horizontal speed control loop 130 uses a vertical video camera 132 and altitude sensors 134 (ultrasonic rangefinder combined with a barometric sensor).
• a circuit 136 processes the images produced by the vertical camera 132, in combination with the signals of the accelerometer 1 14 and the attitude estimation circuit 1 18, to estimate by means of a circuit 138 the components of the horizontal speed along the two axes of pitch and roll of the drone.
• the estimated horizontal speeds are corrected by the vertical velocity estimate given by a circuit 140 and by an estimate of the altitude given by the circuit 142 from the information of the sensors 134.
• the user 124 applies commands to a circuit 144 for calculating altitude setpoints, which instructions are applied to a calculation circuit 146 for ascending speed reference value V z via the altitude correction circuit 148 receiving the estimate of altitude given by the circuit 142.
• the calculated upward velocity V z is applied to a circuit 150 which compares this reference speed with the corresponding speed estimated by the circuit 140 and consequently modifies the control data of the motors (circuit 108). in such a way as to increase or reduce the speed of rotation simultaneously on all the motors so as to minimize the difference between the ascending velocity measured rate of climb.
• ⁇ and ⁇ being the two angles defining the inclination of the drone with respect to the horizontal (angles of Euler: if ⁇ corresponds to a rotation around the axis Z NE D of the absolute reference NED, ⁇ corresponds to a rotation around of the YNED axis rotated by ⁇ , and ⁇ corresponds to a rotation around the axis u), C x and C y being the resistance coefficients to the advancement (reflecting the friction forces experienced by the drone) in the two horizontal axes, a being a coefficient linking the thrust and the climbing speed at the speed of rotation ⁇ , and
• m being the mass of the drone.
• I x , I y and I z being representative parameters of the inertia coefficient of the drone in the three axes, and 1 being the distance separating the engine from its center of gravity.
• the first term of the left-hand member corresponds to the dynamic moment of the system
• the second term represents the contribution to the dynamic moment of the Coriolis forces
• the right-hand member corresponds to the moments exerted by the ascensional forces Fi and the couples ⁇ created by the propellers of each of the rotors.
• the invention proposes a wind estimation technique based on the dynamic equations of the drone in flight.
• This dynamic model whose equations will be given below, will be used with a "Kalman filter” state estimator, which is an infinite impulse response filter that estimates the states of a dynamic system (the drone in the case) from a series of input measures.
• Kalman filter state estimator
• the general principles of this technique can be found for example in R. E. Kalman, A New Approach to Linear Filtering and Prediction Problems, Transactions of the ASME - Journal of Basic Engineering, Vol. 82 (1960).
• the Kalman filter will use and estimate six states, namely:
• the evolution of the six states of the Kalman filter estimator can be predicted using the dynamic model describing the behavior of the drone.
• the measured values of the ground speed and the acceleration of the drone will serve to recalibrate the predictions given by this estimator, thus allowing i) to improve the accuracy of estimation of the directly measured quantities (filtering function of the estimator), such as the V D f S and V DS velocities of the drone relative to the ground, and ii) estimate the quantities inaccessible to a direct measurement (state estimator function), such as speed and wind direction corresponding to the components ⁇ k / s ⁇ and VA / s ⁇ .
• ⁇ , ⁇ and p being the Euler angles (respectively of roll, pitch and yaw) characterizing the attitude of the drone with respect to the NED mark, ⁇ ⁇ being the rate of rotation about the w axis (yaw rotation movement), and
• R v being the rotation matrix associated with the angle ⁇ , in dimension 2.
• the null terms of the two sub-matrices come from the fact that, by hypothesis, the wind and the accelerometer biases are considered constant on the short term. It should be noted that this does not prevent the estimation of these states because i) the wind speed is dynamically coupled to the speed of the drone relative to the ground, itself measured and ii) the accelerometer bias is measured at through the acceleration measurement.
• the two matrices of the above relation are not constant (which corresponds to a nonstationary Kalman filter), being a function of the attitude of the drone and its speed of rotation around the yaw axis; but these parameters are assumed to be known to the estimator, insofar as they are in practice measured or estimated as part of the attitude control of the drone.
• Equation 10 and 11 are applied to the accelerometric measurement system, the latter measures only the aerodynamic friction forces; the accelerometer therefore measures a magnitude proportional to the wind speed of a factor C x / m or C m along the axis considered (with construction bias).
• this one is measured in the reference ⁇ u, v, w ⁇ related to the drone thanks to the camera with vertical aim giving an image of the ground.
• This image is analyzed by algorithms such as those described in the aforementioned EP 2 400 460 A1, which estimates the displacement of the scene captured by the camera of a following image and applies to this estimated displacement a scale factor depending on the altitude, itself estimated by merging the data produced by an ultrasonic range finder and a barometric sensor.
• Figures 3 and 4 are comparative readings, respectively of the wind speed and wind direction modulus, of simultaneously estimated and measured values under real conditions.
• the use of a specific dynamic model for the ground UAV makes it possible to force the decrease of the wind speed towards zero, thus avoiding any destabilization of the accelerometer bias estimators.
• Wind estimation has several uses:
• This wind compensation method is advantageously implemented from the estimate obtained in the manner described above, but in a nonlimiting manner, insofar as it can be implemented with other methods. wind estimation techniques.
• Wind compensation consists in determining for the drone a reference attitude around which it will be enslaved, either in controlled flight mode or in autopilot fixed point mode:
• the reference in open loop will be added to the instructions of the pilot, so as to support the drone facing the wind and conversely to brake it in the direction of the wind, and
• the stabilization of the drone can be reached more quickly, the compensation being as responsive as the estimate.
• FIG. 5 is a functional schematic representation illustrating the wind estimation and compensation loops along the axis u, for a flight configuration of the autopilot fixed-point drone.
• the servocontrol is identical along the axis v, the terms in x or in ⁇ being substituted by terms in y or in ⁇ and the cosine function being replaced by the sinus function, respectively.
• Figure 6 is the counterpart of Figure 5, for a flight configuration of the drone in controlled mode.
• fixed point 126 comprises a gain integrator stage 154 156 and a proportional stage 158.
• the output setpoint of the adder stage 152 is applied via a stage 160 limiting the excursion of the setpoint actually applied to the control loop of the drone engines.
• the compensation term applied to the adder 152 for each of the u and v axes is:
• V wind being the modulus of the estimated wind speed
• Cx being the drag coefficient of the drone
• y wind being the heading angle of the wind direction
• Wdrom being the heading angle of the direction of the drone.

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• Engineering & Computer Science (AREA)
• Aviation & Aerospace Engineering (AREA)
• Remote Sensing (AREA)
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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Automation & Control Theory (AREA)
• Mechanical Engineering (AREA)
• Control Of Position, Course, Altitude, Or Attitude Of Moving Bodies (AREA)
• Toys (AREA)

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• 2012
  • 2012-03-30 FR FR1252895A patent/FR2988868B1/fr not_active Expired - Fee Related
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  • 2013-03-27 WO PCT/FR2013/050663 patent/WO2013144508A1/fr active Application Filing
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  • 2013-03-27 EP EP13715385.4A patent/EP2831685A1/de not_active Withdrawn
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