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ORIGINAL ARTICLE Study on mobile mechanism of a climbing robot for stair cleaning a translational locomotion mechanism and turning motion Takahisa Kakudou Keigo Watanabe Isaku Nagai Received 31 March 2012 Accepted 23 October 2012 Published online 16 November 2012 ISAROB 2012 AbstractInhumanlivingenvironments itisoftenthecase that the cleaning area is three dimensional space such as a high risebuilding Anautonomouscleaningrobotisproposed soastomoveonallfl oorsincludingstairsinabuilding When a robot cleans in three dimensional space it needs to turn for directioninadditiontoclimbdownstairs Theproposedrobot selectsmovementusinglegsorwheelsdependingonstairsor fl at surfaces In thispaper a mobilemechanism and a control method are described for translational locomotion The translational mechanism is based on using two wheel drive type omni directional mobile mechanism To recognize a stair using the position sensitive detector the robot shifts from translational locomotion to climbing down motion or edge following motion It is shown that the proposed robot turns to face a stair with the accuracy of 5 KeywordsCleaning robot Climbing robot Stair 1 Introduction Recently various robots have been developed to support and execute human s work in various fi elds One of such robots is the autonomous cleaning robot 1 The automa tion of cleaning by robots reduces labors and saves energy for a cleaning task so that there is an increasing need for it in large areas such as stations and airports In human living environments it is often the case that the cleaning area is a three dimensional space such as a high rise building Considering cleaning in a three dimensional space the cleaning area includes the steps of stairs which lead from one level of a building to another However many of cleaning robots are not considered to move on places between fl oors Tajima et al 2 3 have developed a robotic system in which the cleaning robot cooperated with the elevator to clean fl oors in a high rise building However this system did not consider the cleaning of stairs As a result a cleaning robot itself needs the ability to move on stairs for cleaning in a three dimensional space Also the several types of climbing robots using crawlers wheels and legs were proposed to move on stairs 4 6 Those robots were developed to move on stairs only for improving transfer performance of them in uneven surfaces and transporting people or objects Therefore a climbing robot for cleaning in a three dimensional space needs to be able to turn itself and keep posture level on the tread board of stairs as well as moving on stairs The objective of this study is to develop a climbing robot that can move on all fl oors including stairs in a building for autonomous cleaning in a three dimensional space A climbing robot has been already proposed where its structure was divided into two mechanisms for climbing down stairs and translational movement 7 8 In this paper a mobile mechanism and a control method are described for translational locomotion The operational check of the translational mechanism was conducted by facing the robot to the edge of stairs using the position sensitive detector PSD This work was presented in part at the 17th International Symposium on Artifi cial Life and Robotics Oita Japan January 19 21 2012 T Kakudou ypsd and xpsd ypsd in an x y coordinate respectively On the other hand the encoders are used to control the velocity of the robot or measure the angle of the circular plate 3 2 Controller design Cleaning robots keep the quality of cleaning constant by controlling the velocity of the robot The velocity of the Fig 1 Outline of climbing robot for cleaning stairsFig 2 Overview of translational mechanism Artif Life Robotics 2013 17 400 404401 123 robot is controlled using a PD controller with encoders to measure the rotation of wheels Figure 4 shows the block diagram of the velocity control The motors for the wheel can take states such as normal rotation reverse rotation stop and braking by sending the signal from the micro computer SH7125 through a motor driver 4 Locomotion on stairs The locomotion of the robot on stairs is assumed to consist of two motions i e the cleaning motion and the shifting motion 4 1 Cleaning motion The locomotion of commonly marketed cleaning robots is classifi ed into four basic motions such as parallel spiral wall refl ection and wall following motions The parallel motion is one of the more suitable motions for cleaning stairs because the shape of step is a rectangular form that makes a path planning easy to make for cleaning In the cleaning motion based on such parallel motion the robot repeats straight going and 90 turn keeping the posture as shown in Fig 5a 4 2 Shifting motion When the robot shifts from translational locomotion to climbingdownmotionoredge followingmotionthatmoves tocleanalongthe edge ofstairs it might falldown stairsdue to its posture Some ball casters of the robot fall down stair by design when the posture of the robot is inclined at more than 10 over the edge of stairs Also it is diffi cult for the robot to stop when the robot once starts falling in descent stair Therefore the robot turns to face the edge of stair whenever stairs are recognized as shown in Fig 5b 4 3 Kinematic model motion The kinematic model of the mechanism for translational locomotion is given by x y h 2 4 3 5 r 2sinhc r 2sinhc r 2coshc r 2coshc r 2d r 2d 2 4 3 5 hR hL 1 where x and y are the position coordinates of the robot hc is the rotated angle of a circular plate hRand hLare the a b PSD 1 Body plateCircular plate PSD 2 y x Oc Body plateCircular plate Rotational joint Connected part Encoder Fig 3 Design of a variety of sensors a top view b lateral view Motor driverSH 7125Motor Target value Output value Encoder PWMVoltage Fig 4 Block diagram of velocity control b a Fig 5 Translational locomotion on stairs a parallel motion to clean stairs b turning motion to face stairs 402Artif Life Robotics 2013 17 400 404 123 rotational angles for the right and left wheels r is the wheel radius and 2d is the distance between wheels 5 Stair recognition using PSD The robot recognizes stairs by discriminating a difference between the ground clearance and the rise of stairs using a PSD The PSD outputs the value that converts a distance to an object into DC voltage When a PSD value is less than or equal to a threshold for discriminating stair and fl oor the robot recognizes it as a stair whereas the robot recognizes it as a fl oor respectively The robot turns to face the edge of stairs using two PSDs If anyone of PSDs recognizes a stair the circular plate of the robot is rotated so that a reacted PSD is located on the wheel axis Next the robot turns in direction so the other PSD recognizes a stair Finally the robot stops if both PSDs recognize stairs Also the robot turns as shown in Fig 6 if the angular velocities of inner and outer wheels have a relationship in their turning radii such as vin vout Rin Rout 2 where vinand voutare the rotational velocities of the inner and outer wheels and Rinand Routare the turning radii of the inner and outer wheels 5 1 Operational check We had conducted an operational check to verify the accuracy of recognizing stairs using PSDs The evaluation item is the posture of the robot when it was turned to face stairs at a constant velocity of 150 mm s from start angles where the posture angle exceeded from the edge of stairs was assumed to be positive whereas one under the edge of stairs was assumed to be negative Experimental conditions are as follows start angles were assumed to be 15 30 45 60 and 75 as shown in Fig 7 A rise is defi ned as 180 mm in this study A threshold for recognizing a stair was decided as 80 mm which is the mid distance between the rise of step and the ground clearance of the robot Table 1 shows the posture angles of the robot in which the robot faced the edge of stairs every start angle Figure 8 shows the average and standard deviation SD of posture angles every start angle inR outR psdO inv outv v Fig 6 Kinematic model of turning motion Start angle Exceeded angle Shortened angle Edge Fig 7 Experimental condition Table 1 Posture angles Start angle Posture angle TrialsAverageSD 12345 150 4 0 30 50 3 0 20 140 33 300 70 70 51 21 50 920 37 453 03 53 24 03 03 340 38 604 22 04 01 31 72 641 21 754 13 82 52 02 02 880 90 1 0 1 2 3 4 1530456075 Postur angle for step deg Start angle deg Fig 8 Average and SD of posture angles Artif Life Robotics 2013 17 400 404403 123 5 2 Consideration If the posture of the robot is inclined at more than 10 to the edge of stair anyone of ball casters in front side falls down stair The margin of error in the posture angle is defi ned as 5 to shift from translational locomotion to climbing down motion As a result the error in the posture angle was within 5 and the largest value of SD was 1 2 after turning motion to face a stair A relationship between the angular velocity and the turning radius of the wheel was vin ffi 0 42vout This performed with an enough accuracy because a relationship of them gave a close agreement with vin 0 43vout defi ned by Eq 2 in ideal condition 6 Conclusion A cleaning robot to climb down stairs has been developed for cleaning a three dimensional space In particular mobile mechanisms and a locomotion control method were proposed for the robot to clean and climb down stairs The operational check of the robot was conducted for recog nizing stairs In the result it was confi rmed that the pro posed robot was able to recognize a stair with an enough accuracy to shift from translational locomotion to climbing down motion As future work a function is improved in the level of recognizing a surrounding environment such as walls or balusters in stairs References 1 Tribelhoron B Dodds Z 2007 Evaluating the roomba a low cost ubiquitous platform for robotics research and educations In Proceedings of the IEEE international conference on robotics and automation pp 1393 1399 2 Tajima S Aoyama H Seki T et al 2004 Development of robotic fl oor cleaning system for high rise buildings in Japanese J Robotics Soc Jpn 22 5 595 602 3 Aoyama H Ishimura S Nishihara I et al 2009 The development of the cleaning device of the offi ce building cleaning robot in Japanese In Proceedings of the JSME conference on robotics and mechanics 1P1 B14 pp 1 2 4 Yuan J Hirose S 2