iros2019国际学术会议论文集pdf1391

1、2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) Macau, China, November 4-8, 20192D Contour Following with an Unmanned Aerial Manipulator: Towards Tactile-Based Aerial NavigationSalua Hamaza, Ioannis Georgilas, Thomas RichardsonAbstract In this paper a force controller

2、 via energy tanks is implemented for novel applications in aerial contour follow. This control approach allows the aerial vehicle to trace out a boundary whilst in continuous contact with a surface by means of an actively compliant manipulator. This represents the first step towards tactile-based ae

3、rial navigation, which can be used to complement more traditional mapping approaches such as visual SLAM. Key results show that the energy-based approach can be used to apply a continuous shear force through the manipulator while the vehicle remains in contact with the surface of interest. Results a

4、lso show the robustness and repeatability of this approach for prolonged aerial interaction, and the potential for future use in more complex, un-modeled environments.Fig. 1: A sequence of frames captured during flight experiments shows the aerial system exerting a shear force along a 1.25 metres su

5、rface by means of a 1-DoF manipulator compliantly controlled. The last frame shows a front view of the same experiment.response of the system as a whole was improved, different output forces were generated at the end-effector, and fine- tuning of the overall compliant behaviour was achieved.Force ex

6、change conducted at the side of an aerial vehicle is a challenging task, and it has only been addressed in more recent years in the state of the art. In 9 contact forces up to 3 N are exerted on a vertical wall with the use of a compact Delta robot mounted on the side of the aircraft for gentle cont

7、act and NDT purposes. In 10 the exertion of large forces for object manipulation is achieved through the use of a direct thrust-vectoring actuation on a tilt-rotor, with resulting forces over 20 N. In 11 an octo-rotor UAV with one degree of freedom (DoF) manipulator is able to conduct hammering test

8、 on a bridge wall. The force generated at the end-effector is proportional to the pitch angle of the UAV when in contact with the bridge pier. In 12 a 1- DoF manipulator mounted on a quadcopter is used to install and retrieve sensors on vertical and cylindrical surfaces (tree trunks). The force exch

9、ange is implemented thanks to the combined action of the force-controlled manipulator and the UAVs pitch motion.In this paper we propose a passivity-based force controller via energy tanks tailored for prolonged aerial force exchange. This approach will allow for the exertion of a shear force over a

10、 surface while maintaining stable and un-interrupted contact with it. The main motivation behind this work is to provide the UAV with tactile feedback over the surrounding environment by implementing contour follow capabilities that could aid the navigation. Often UAVs are deployed for search and re

11、scue scenarios, for example inside wrecked buildings after an earthquake or other natural calamities. In most cases the aircraft only relies on visual sensing and SLAM for navigation due to occluded GPS signal. Tactile feedback provides the aerial platform with an additional way to safely navigate i

12、n poorly lit environments and map such enclosed areas. The contribution of this work lies in theI. INTRODUCTIONRobotic manipulation has been at the centre of the state of the art of robotics research for over forty years. However, it wasnt until 7 years ago that the research community had raised an

13、interest in the development of manipulation capabilities for unmanned aerial vehicles (UAVs). The term aerial manipulation has been coined to describe this class of robots that are able to carry out manipulation tasks airborne by means of manipulators mounted on top of the aircraft. Several challeng

14、es are faced when interacting with the environment airborne, as the exchange of forces and torques between the end-effector/gripper and the target object affects both the vehicle stability and its dynamic response, often leading to a poor performance or even potential failure. It was found that comp

15、liance is an essential feature for aerial manipulators as it aids stability and improves the overall reliability of the system. Compliance can be intro- duced in the system as part of the mechanical structure of the manipulator with the use of spring-like elements 1, 2, or via software through imped

16、ance type of controllers. Compliant control strategies have been developed addressed to the aerial platform only in the form of impedance or adaptive control. These control approaches demonstrate force estimation and motion feedback at the end-effector, improv- ing stable contact during aerial inter

17、action 36. In our previous work 7, 8, variable compliance was introduced in the aerial system in the form of a variable-gain PID controller for the manipulator, while keeping a standalone flight controller on the aerial platform. Results demonstrated that by adapting the manipulator control gains, t

18、he dynamicThis work was supported by the EPSRC Centre for Doctoral Training in Future Autonomous and Robotic Systems (FARSCOPE).S. Hamaza (s.hamazabristol.ac.uk) and T. Richardson are with the Faculty of Engineering, University of Bristol, UK. I. Georgilas is with the Department of Mechanical Engine

19、ering, University of Bath, UK.978-1-7281-4003-2/19/$31.00 2019 IEEE3664novel approach to 2D contour following by an aerial vehicle through the use of energy-tank based force control. The control laws have been tailored for a 1-DoF manipulator and integrate the aircraft states resulting in closed-loo

20、p control within the manipulator itself. The manipulator also incorpo- rates smart sensing and a high-performance motor controller within a compact design, optimised for force-driven aerialpoint, i.e. it is assumed that the end-effectors position is fixed in time and no motion is generated in any di

21、rection.In addition to the works cited above, this paper proposes a force control architecture that includes the concept of energy tanks 19, 20 for stable and prolonged force-tracking and aerial contour following over 2D surfaces. This approach was successfully tested on a fixed manipulator for 3D c

22、ontour following 21, and it is here proposed for its use on a floating base. This represents the first step towards tactile-based aerial navigation and provides the base ground for further investigation on navigation in un-modeled 3D environments using tactile feedback on an unmanned aerial system.a

23、pplications. Multiple aerial experiments demonstrate reliability and robustness of this approach, and offer potential for more complex contour following tasks.the theThe outline of this paper introduces at first the related workin the field of passivity-based controllers implemented forfixed robots.

24、 Then, the modeling of the proposed control laws is presented, followed by the manipulation system design, working principle and sensing. In the experiment section, the flight results are presented in conjunction with the analysis of both the manipulator and aircraft performance. In the last section

25、, a summary of the lessons learned are presented, together with the conclusions and ideas for further work.II. RELATED WORKLet us first consider relevant work applied in the field of fixed robots, i.e. industrial robotic arms for assembling purposes. In the works presented in 13, 14 a constrained- b

26、ased approach that allows to selectively control force, impedance and position has been proposed. The former combines the estimation of geometric uncertainty into the instantaneous task specification and allows for compensation of time-varying coordinates on the end-effector. The latter extends on t

27、he to task specification geometry requirements to a guarantee a smoother and more robust indirect force con- trol. Hybrid position/force type of control gained popularity since its formulation three decades ago as it allow to work in force and motion sub-spaces that are complimentary to each other i

28、n conjunction with the task specification. Despite the versatility of the hybrid approach, the major drawbacks are associated with the need for an accurate modeling of the contact properties a priori to achieve a good performance, and the lack of robustness during contact-loss 15. In 16 a unified me

29、thod combining force and impedance control via energy-tank is presented with a particular focus on contactIII. CONTROLA. Force Control DesignLet us start by introducing force-tracking control designed for a generic n-DoF manipulator using a Proportional-Integral approach. The motor torquemis directl

30、y propor-tional to the Jacobian matrix of the system as follows:Zhitm = JT (q) kp F (t) Fd(t) + kiF (t) Fd(t) dt0(1)JTwhereis the transpose of the Jacobian matrix which only depends on the manipulators configuration, i.e. thevector of generalised coordinates q Rn. Terms F (t) andFd(t) are the time-v

31、arying force and desired force valuesrespectively, and kp and kd the proportional and derivative gains respectively.Now, let us re-arrange the previous equation for a n- DoF manipulator actuated by brushless DC motors, which present a linear relationship between the input current and output torque.

32、These types of motors are the selected for the proposed manipulation system (described in the next section) as they provide high output torques and the ability to generate higher forces compared to servo or stepper motors of similar size and weight. Therefore, to achieve direct force-tracking on the

33、 end-effector the current-to-torque linear relationship is used as follow:Zithm = JT (q) KT kp c(t)cd(t) +kic(t)cd(t) dtloss compensation and allows tracking over 3D surfaces.forsafecontactandforce0(2)where the parameter KT is the motors torque-current con- stant and it is provided by the motors man

34、ufacturer, c(t) and cd(t) are the input current and desired current respectively.Moving to force-tracking control approaches designed foraerial manipulators, in 17 a variable-impedance control applied to an aerial platform is proposed, capable of adjusting the impedance of the multi-rotor and regula

35、te the response to time-varying interaction forces. This approach specificallyB. Energy Tank DesignEnergy tank-based methods have frequently been used for tasks concerning tele-operated manipulation 2224, but also as an addition to impedance control with variable stiffness 19. The role of the energy

36、 tank is to act as a virtual storage element and minimise the energy dissipation of the controlled system. Such energy represents the passivity threshold used by the force controller, and the tank being its reservoir. In essence the tank allows to act upon the impedance of the system by monitoring t

37、he amount of energyfocuses on safe and robust compensation of disturbances exerted by the environment, for example in case a human operator interferes with the manipulator by exerting an external force on it. In 18 forces up to 16 N are applied by an aerial vehicle equipped with a 1-DoF manipulator

38、pitching at high angles against a flat vertical surface. The strategy used addresses simultaneous control of both pitch and yaw angles to guarantee stable contact for prolonged periods of time. However, this approach has the limitation of a static contact3665dissipated during the task and amending t

39、he output force accordingly. The tank energy is:IV. MANIPULATION SYSTEMA. Design ConsiderationsA recurrent approach found in the state-of-the-art of aerial manipulation is to make use of multiple-DoFs manipulators and serial arms to carry out low-dexterity manipulation tasks airborne. Despite the ad

40、ded benefit of having additional n DoFs on the aerial system, several drawbacks come with it. First to mention is the increased weight that n additional actuators add the to the aerial platform, posing a tight constraint to battery life and manoeuvrability 25. Another disadvantage typical of high-Do

41、Fs manipulators is the higher kinematics and control complexity, which often require higher processing power and longer computation time to be solved. In general, several aerial applications may require limited manipulation capabilities, such as force-driven tasks for non-destructive-testing, contac

42、t-based inspection or sim- ply tactile sensing for motion planning purposes. These types of operations can be achieved with a simple probe oriented towards the contact surface providing the aerial system with a minimal, weight-efficient solution to the problem.1T (xt ) =x2(3)t2where the variable xt(

43、t) R is the state associated with the tank, with the condition of xt(0) 0. Now, the dynamicsare given by: xt =xT Ddx+ uTxt(4)x(t) = x(t) xd(t)TuT = w(t)xtwhere x t is the time derivative of the tank state, x represents the error between desired and actual states. The term w(t) represents the tank co

44、ntrol input, and lastly is defined as:( Tupper10if T =(5)otherwise is a design parameter that enables the storage of dissipated energy as long as the total tank energy is below its upper bound Tupper. Differently, if the tank energy is greater thanB. Manipulator DesignThe proposed design consists of

45、 a single-DoF manipula- tor embodied by a prismatic joint. The actuator on-board converts rotational motion into linear by means of a rack and pinion transmission, allowing the end-effector to slide inside the joint casing. The pinion is attached on the motor shaft directly, without any gear reducti

46、on in place. The motor chosen for this design is a DC brushless, selected for its high torque to weight ratio, increased efficiency and reliability when compared to other types of motors. The rack and pinion components are manufactured in aluminium to ensure an accurate transmission and reduce the m

47、ass. Two linear bearings guarantee adherence of the racks teeth in the linear coupling. Similarly, two ball bearings hold the pinion in place perpendicularly to the rack, and release the motor from any radial tension that might be generated during interaction. The manipulators design is displayed in

48、 Fig. 3.Tupper , becomes zero and the tank is disabled. Thiscondition allows to prevent excessive storage. The product (xT Dd x) represents the power dissipated. The tank control input w(t) is defined as:Zt xkp(Fext Fd) kiFext(t) Fd(t)(6)w(F, t) =extt0Therefore we are now able to write the extended

49、motor dynamics as:Z hxtit 0(Fext Fdes)kp(Fext Fdes)+ ki= JT (q)m0(7)where is defined as:(10if T Totherwiselower =(8)where Tlower 0 represents the lower bound below which the energy cannot be extracted by the tank, leading to = 0 and preventing singularities to occur. The proposed approach is illustr

50、ated in the block diagram of Fig. 2.Fig. 3: Computer-Aided-Drawing of the manipulators transmission mechanism en- closed in a cross-shaped casing. For clarity reasons, the distance sensor and other electronic components are not displayed in this figure.Fig. 2: Block diagram of the proposed force con

51、troller via energy-tanks.3666The end-effector is chosen to be a metal ball caster, mounted at the tip of the rack (see Fig. 3). The ball caster is selected as it minimises friction and allows for smooth contouring even on indentations that might be present on the target surface. The ball caster is a

52、lso ideal as it reduces the contact surface to a single point, therefore zeroing the moments of the external wrench ext leading to pure force exchange Fext.Lastly, the manipulator design incorporates 2 sensors: an inductive encoder on the brushless motor which measures the relative position of the e

53、nd-effector; and a distance sensor mounted at the front of the aircraft to measure the relative position of the vehicle with respect to the target ahead.B. ResultsThe outline of the experiments is as follow: the UAV approaches the target surface and once the vehicles angular states and relative posi

54、tion from the target are within a certain threshold, the manipulator autonomously initiates the task. The UAV states measured by the flight controller are communicated to the manipulators on-board computer via a MAVLink/MAVros bridge, whilst the on-board distance sensor provides the range informatio

55、n. After the end-effector is extended and contact is established with the surface, the UAV flies sideways while facing the target and the manipulator exerts a continuous force over the surface. As the obstacle becomes out of range, the manipulation task terminates and the end-effector slowly retract

56、s.The main challenge faced in these airborne tests is the ability to keep continuous contact with the target for a prolonged period of time whilst the vehicle is moving. To do so, the manipulators controller has to overcome any drifting that might be present in the vehicle, caused by for example tur

57、bulence in the proximity of the obstacle, or an error in the UAVs positioning estimate. Figure 5(a) shows the UAVV. EXPERIMENTSIn this section, the flight experiments of the proposed system are presented and discussed. The objective is to demonstrate the ability of the aerial system to apply a shear force fo