The 12th International Conference on Ubiquitous Robots and Ambient Intelligence (URAI 2015)October 28 ∼ 30, 2015 / KINTEX, Goyang city, Korea

Wall Cutting Strategy for Circular Hole Using Humanoid Robot

Beomyeong Park1, Hyunbum Cho1, Wonje Choi1 and Jaeheung Park1

1Dyros Lab, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, Republic ofKorea,

(Tel : +82-31-888-9167; E-mail: {on2lord, myugun, chwj79, park73}@snu.ac.kr)

Abstract - This paper explains the strategy of cutting acircular hole in a wall and removing the cut piece witha humanoid robot. The two difficult aspects of this taskare how the robot turns on the drill and how the wall iscut following the desired trajectory while the robot avoidsthe torque limit of its joints. This is because the robotwe used, THORMANG, especially has low payload andlarge gripper. Therefore, we proposed the following twoapproaches. First, we devised a special device on therobot gripper to turn on the drill immediately when therobot grasps the drill. Secondly, only one high-poweredshoulder joint, rather than all the arm joints, is used toavoid the torque limits on some of the arm joints and cuta circular hole efficiently. These approaches were veryeffective on our robot THORMANG but they can also beapplied to other humanoid robots. We demonstrated ourstrategy at the 2015 DARPA Robotics Challenge (DRC)Finals by succeeding to cut a hole in a wall.

Keywords - Humanoid Robot, Manipulation,Planning,Gripper, DARPA Robotics Challenge

1. IntroductionSince the humanoid robot was invented, many re-

searchers have developed hardware and control algo-rithms so that the humanoid robot could substitute therole of humans in industrial areas and disaster scenes [1]-[3]. However, when the Fukushima-Daiichi Tsunami dis-aster occurred, most of the rescue robots could not workat that scene due to the low communication bandwidthbetween the operator and the robot, uncertain terrain, andthe difficulty to perceive objects. Therefore, demand forthe humanoid robot, which is presently available at disas-ter scenes, has increased.

The Defense Advanced Research Projects Agency(DARPA) held the 2013 DRC Trials in order to encour-age researchers to develop platforms and algorithms for ahumanoid robot that can be available at a disaster scene[4]. Furthermore, in June 2015, twenty-five teams(highranking teams from the DRC trials and newly participat-ing teams) accomplished eight missions that simulated adisaster scene. The eight missions are as follows: driv-ing a car, egressing from a car, opening a door, closinga valve, cutting a wall with a tool, performing a surprisemanipulation task, passing rubble, and climbing stairs[5].

Especially, the success in cutting a wall requiredmany technical components such as perception, dexter-ity, robot-environment interaction, and decision makingskills. Accordingly, the wall mission was one of mostdifficult tasks also because the robot was required to use

Fig. 1 THORMANG of Team SNU performing cuttingwall mission at DRC Finals 2015.

a tool. Actually, the success rate of the wall mission in theDRC trials and finals was lower than 30 percent[6]-[8].

Specifically, the wall mission at the DRC finals re-quired the robot to hold a drill and cut a hole in a wall.There were two types of drills: trigger type switch andpush-button type switch. Each team chose one type ofdrill. Turning on the trigger type or button type drill wasa major key point to perform in the wall mission. Also,there was a black circle on the wall with a 20 centimeterdiameter. Each robot was required to cut a hole from thewall (including the black area) and remove the piece fromthe wall.

Therefore, the first problem is how the robot turns onthe drill. Team SNU (Seoul National University) chosethe button type drill. However, the hand of the THOR-MANG is larger than the handle of the drill. Thus, push-ing the button switch by using the other hand is impossi-ble. The second problem is how the robot pierces and cutsthe wall while holding a drill. Cutting a trajectory whileholding a drill was difficult for our robot platform be-cause of the torque limits of the robot arm motors. More-over, we needed a way to check whether the drill pene-trates the wall enough for cutting without an F/T sensor.

This paper introduces how Team SNU overcame theabove two problems. We added a passive palm to therobot hand, which helped orientate the drill in the di-rection where the robot hand could turn the drill on.Then, the protruding part on the palm pushes the buttonswitch so that the drill could be turned on simultaneouslywhen the robot hand grasps the drill. We could check onwhether the drill penetrated the wall enough with graph-ical user interface (GUI), which shows the torque valueof each joint. In addition, using only one shoulder joint,which is the most powerful joint of the arm joints, enables

Fig. 2 System diagram of THORMANG.

the robot to perform circle trajectories while avoiding thetorque limit.

In this paper, we present our robot system at the DRCFinals in Section 2. In section 3, we introduce TeamSNUs strategy and procedure for the wall mission. Sec-tion 4 presents the results of the wall mission at the DRCFinals, and compares the results with the other teams.Section 5 provides the conclusion.

2. Robot System

In this section, we introduce our robot hardware andsoftware and how we control the upper body of THOR-MANG.

2.1 Hardware and operation systemWe used the THORMANG (ROBOTIS Co., South Ko-

rea) in the 2015 DRC Finals as the robot hardware plat-form. THORMANG has a height of 140cm and a weightof 60kg. THORMANG is assembled with modular-typeactuators and has 32 DOF : eight in both arms and six inboth legs, and two for the torso and two for the head. TwoLIDARs (Light detection and Ranging) are on the chestand head, respectively, and one IRIS camera is equippedon the back of the head for driving. Three cameras areinstalled in front of the head and have a microphone func-tion. THORMANG uses two computers. One computeris for robot control and the other computer is for the vi-sion and perception tasks. Figure 2 shows the overall sys-tem of the THORMANG.

For the operating system, we use two systems. Onesystem works for sending an order to control the robot.The other system shows the posture of the robot and vi-sion data, which is obtained from the cameras and LI-DARs.

2.2 Upperbody controllerThis section presents how we control the upper body

of the THORMANG. We use two types of controllers:joint control and task control.

Fig. 3 Two separate tasks of the wall mission.

A. Joint controllerAt the joint control level, we directly control each joint

by commanding a desired angle. This control is used toexecute previously determined motions such as a certainposture in which the THORMANG is ready to reach itsarm to the drill. We obtain the trajectory from the currentangle to the target angle of each joint by using a cubicspline.

B. Task controllerTask control is used to control the position and ori-

entation of the end-effector of the robot arm. Each armis controlled and their coordinate is based on the Pelvisframe. We used Constrained Closed Loop Inverse Kine-matics as our upper-body task control algorithm, [9].

q = J∗(xd +K(xd − x)) (1)

where K is the square matrix for the gain, vector q =[q1,. . . , qn]T is the input angle to each joint of the robot arm.Vector x = [pr, wr]T represents the position and orienta-tion of the end-effector, and xd means the desired pose.

For the weighted pseudo inverse Jacobian, used inEq.(1), J∗ is obtained by

J∗ = W−1JT (JW−1JT + λ2I)−1, (2)

where W is the weighted matrix, λ is a scalar value foravoiding singularity and I is an identity matrix.

3. Strategy for Wall Mission

In this paper, we will focus on the manipulation task.The following contents will cover our approach for thewall mission. Specifically, this section will introduce adevice that helps THORMANG easily grasp and turn onthe drill, and a strategy for cutting a hole in the wall whileavoiding the torque limit problem.

3.1 Procedure for the missionWe classify wall mission into two separate tasks: task

of docking with the drill for grasping and turning on thedrill, and the task of cutting the wall (i.e., cutting the holeand removing the piece from the wall). Figure 3 showsthe two tasks.

Fig. 4 Grasping and docking with the robot gripper.

A. Task 1: Docking with the drillThe task of docking with the drill includes acquiring

the position of the drill, and grasping and turning on thedrill.• Perception : obtaining the position and orientation ofthe drill through the LIDAR sensor.• Reaching : moving the robot’s arm to the drill with theobtained position data with perception.• Grasping : holding the drill handle and turning on thedrill.

B. Task 2: Cutting wallThe task of cutting wall includes cutting a hole in the

wall and removing the piece.• Penetrating : positioning the drill toward the wall andletting the drill penetrate the wall.• Cutting : cutting a hole in the wall by following circletrajectory.• Pushing : pushing the cut piece behind the wall.

3.2 Docking with drillThe main problem in docking with the drill is how to

turn on the drill. We solved this problem by using spe-cial equipment. This section will explain the equipment,which is designed for turning on the drill.

A. Grasping and DockingSince the robot performs the task using a drill, we con-

sider the drill as a new end-effector in the wall task ofthe robot. Therefore, this process is called a docking asshown in Fig 4. The concept of docking is used in theself-reconfiguring modular robotics area and means thetask where modular robots are combined to each other[10]-[12]

For successful docking, perceiving the position of theobject accurately and deliberate manipulation is consid-ered as an ideal method. However, we have to considerthat accurate perception and manipulation might not beperformed ideally when applying the robot in a disas-ter scene. Accordingly, we designed a new device thatenables robust docking without accurate perception andprecise manipulation. This method is different from theother teams and might be an important factor to helpquickly complete the mission.

Fig. 5 Developed gripper, drill and sketch of gripperand drill.

B. Characteristics of dockingThe docking in this mission has specific characteris-

tics. Only the robot has intelligence among the robot anddrill, which is the object for docking. The robot has toconduct the overall task for docking. This is because therobot can only move and the drill remains at rest until therobot moves the drill. We planned the docking methodbased on these characteristics.

When the position of the drill is determined based onthe LIDAR data, the robot approaches the drill and pre-pares for docking by positioning the gripper to wherethe docking is possible. While the angle between thepalms is reduced to grasp the drill, the designed hard-ware equipment makes the drill oriented and translated inan intended kinematic status. The docking ends when thegripper completely grasps the drill and pushes the powerbutton.

C. Design of device for dockingWe set up the requirements for design as follows.

• The device should not change the original robots status(i.e., shape, weight, and inertial moment).• Regardless of the drill status, the device could succeeddocking.• The drill should be in the uniform position with therobot, after docking.• The drill should be turned on right after docking is fin-ished.

Based on the above requirements, we designed the de-vice as shown in Fig 5(a). In order to satisfy the firstrequirements, the device was designed lightly with a sim-ple shape but without an additional actuator. The deviceis designed in a way that changes the palm of the origi-nal gripper, which consists of sliding panels, spring hingeand tip. The sliding panels rotate and pull the battery part

Fig. 6 Docking sequence with a designed device.

Fig. 7 Experiments for docking.

of the drill. The spring hinge gives elastic force to helpthe sliding panels restore its original condition. The tip isused for pushing the drill power button.

Figure 6 shows the docking process of the designedgripper. The gripper has one actuator, which causes thegripper clench its fist. Each step of the docking is con-ducted with the gripper grasp. We assumed that the drillis located between palms and is randomly rotated. As thegripper folds, the battery of the drill is touched by thespring hinges and the drill is rotated along the motion ofthe gripper. If the sliding panels cover the entire part ofbattery, the spring hinge pulls the drill inside the gripper.Consequently, the push button is placed before the tip. Asthe drill is pulled, the tip begins to push the button. Whenthe gripper holds the drill entirely, the tip pushes the but-ton and the drill is turned on. If the docking succeeds,the drill is attached to the robot in a uniformed positionregardless of the condition prior to the docking.

D. Performance of deviceWe experimented to verify the performance of the de-

signed device. Figure 7(a) shows the status before thedocking and the drill is randomly placed. Figure 7(b)shows the status after the docking and the drill is placedin the intended position.

The experiment was conducted in a way that the drill isrotated 5◦ from 0◦ to 360◦. A success criterion is whetherthe drill is turned on. Figure 8 shows the result. Thestandard angle is set as 0◦ and docking succeeds at from140◦ to 220◦ and from 325◦ to 45◦. This result showsthat the success boundary of docking is 80◦ at the frontand back sides of the drill. If the drill is rotated in 44.4%of the possible area, the docking process could succeed

Fig. 8 Available range of docking from the experiment.

Fig. 9 The GUI of Team SNU displaying the status ofthe robot’s each joint: angle and current.

since the success bounds cover 160◦ of 360◦.

3.3 Cutting wallWe focus on two factors for cutting the wall. One fac-

tor is how we check whether the drill penetrates the wallenough for cutting the wall. The other factor is cutting acircle trajectory for removing the cut piece from the wallwhile avoiding the torque limit.

A. Penetrating the wallPrior to cutting the wall, the drill bit should go through

the wall sufficiently in order to cut the wall. However, theoperator controls the robot in a different space and with-out an F/T sensor. Assuring whether the drill bit pene-trates the wall is difficult. Therefore, a system for check-ing whether the drill bit penetrates the wall is essential.Our strategy is to check the moment when the torque ofarm joint exceeds a threshold value. If the drill contactsthe wall and the robot tries to push the drill to the wall,the torque exceeding the threshold value would be ap-plied. Figure 9 shows GUI for operation, joint angle, and

Fig. 10 Posture and trajectory for cutting the wall.

motor current. Especially, the red block means that thetorque of the joint overpasses its threshold. Accordingly,when the status of the arm joint turns to the red block, theoperator can realize that the drill penetrates the wall.

B. Posture for cutting the wallThe main goal of the wall mission was to cut and

remove the colored 20-cm-diameter circle. Creating aproper trajectory in front of the robot with the task controlwas the most effective method. However, the end-effectorcould not follow the exact circle trajectory when it holdsthe drill since the circle trajectory requires more torquethan what elbow and wrist joint motors could generate inour robot.

For this reason, we prepared a plan that utilizes thecharacteristics of the robot. We used only the shoulder-pitch joint for cutting the wall since the shoulder-pitchjoint is the most powerful motor among the arm motorsand it can rotate 360 degrees in one direction. Accord-ingly, the shoulder-pitch motor became the axis of thecircle and we drew a circle trajectory on the right side asshown in Fig. 10(a). By controlling the end-effector upand down, we were able to change the diameter of the tra-jectory up to 40 cm, which is two times larger than whatthe mission requires. Thus, using a shoulder motor as acompass to create a trajectory could make an almost per-fect circle. The wall is also vertical to the shelf and drill.Therefore, we draw a circle to the right-side of the robot.The strategy has the obvious advantage of being able todraw a circle without moving posture of THORMANG.

C. Trajectory for cutting the wallIn the wall mission at the DRC Finals, the piece of the

wall is cut and removed from the wall. If the cut part ofthe wall falls toward the robot legs, the robot could loseits balance or the locomotion of the robot is interrupted

Fig. 11 Analysis of the wall mission at the DRCFinals. (Measured time begins from docking se-quence. Seven teams, including Team DRC-HUBO,succeeded wall mission. Since the video of TeamDRC-HUBO was not provided, we could not ana-lyze the time of Team DRC-HUBO.)

from the cut wall. Thus, we consider the trajectory of theend-effector such that the cut wall falls down toward theopposite side of the robot.

We control the orientation of the end effector to makethe conic trajectory as shown in Fig. 10(b). When thewall is cut along the trajectory, the inner circle area issmaller than the outer circle area(d1 < d2). Therefore,when the wall is cut, the piece falls behind the wall be-cause the outer hole is larger than the inner hole. Also,we made a closed-figure trajectory to ensure that the startpoint and end point cross as shown in Fig.10(c). The tra-jectory rotates counter-clock wise and ends by crossingthe start line. Thus, when the motion is finished, the tra-jectory makes a closed figure and assures that the sectioncould be completely removed from the wall.

4. DRC Finals 2015 Results

The 2015 DRC Finals took place in Pomona, Califor-nia. Team SNU succeeded in four missions, includingthe wall mission. The wall mission was the most difficulttask. Only seven teams succeeded in this mission, andteam SNU achieved remarkable success in this mission.Many teams failed to turn on the drill and several teamsdid not attempt the wall mission. In addition, some robotstook a long time to cut the hole from the wall becausethey did not remove the piece entirely at once. However,Team SNU succeeded in the wall mission remarkably incomparison to the other teams as shown in Fig. 11. Al-though we turned on the drill in the second attempt, teamSNU recorded the fastest time in comparison to the otherteams. For example, Team SNU timed at 140 secondsand Team KAIST timed at 240 seconds. Furthermore, therun-time for cutting the wall was 56 seconds, which is al-most half of the run-time of Team IHMC at 115 seconds.Moreover, the cut piece from the wall was an almost per-fect circle shape and fell behind the wall right after it wascut. Consequently, the pushing sequence was not neededas shown in Fig. 12. Including the walking period, ourrun-time of the wall mission was the fastest since the ma-nipulation time was significantly shorter than any otherteam. This result proved that our strategy for cutting thewall is very effective.

Fig. 12 THORMANG at the 2015 DRC Finals fulfilling the wall mission.

5. Conclusion

This paper presented an effective strategy by TeamSNU for cutting a wall at the 2015 DRC Finals. Thecutting wall mission, which had the lowest success ratein the 2015 DRC Finals, had two major difficulties. Thefirst difficulty was turning on the drill and the second dif-ficulty was cutting the wall along the planned trajectory.Many teams failed to turn on the drill and cut the holeon the wall. However, we overcame these two problems.We added equipment to the robot gripper to turn on thedrill efficiently. Then, we used only a shoulder pitch jointwhen the robot generates a cutting trajectory. The shoul-der pitch joint was used as an axis of the conic trajectory.The results of the 2015 DRC Finals show that our ap-proach was more effective than that of other teams.

Acknowledgement

This research was supported by the MOTIE under theRobot industry core technology development project(No.10050036) supervised by the KEIT. We specially givethank to ROBOTIS Co. for providing THORMANG andtechnical support. Also, we are thankful to the MIPALlabs and SimLab for software development.

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