06774435

1756 IEEE/ASME TRANSACTIONS ON MECHATRONICS, VOL. 19, NO. 6, DECEMBER 2014

Multi-DOF Counterbalance Mechanismfor a Service Robot Arm

Hwi-Su Kim and Jae-Bok Song, Member, IEEE

Abstract—Low-cost but high-performance robot arms are re-quired for widespread use of service robots. Most robot arms useexpensive motors and speed reducers to provide torques sufficientto support the robot mass and payload. If the gravitational torquesdue to the robot mass, which is usually much greater than thepayload, can be compensated for by some means; the robot wouldneed much smaller torques, which can be delivered by cheap ac-tuator modules. To this end, we propose a novel counterbalancemechanism which can completely counterbalance the gravitationaltorques due to the robot mass. Since most 6-DOF robots have threepitch joints, which are subject to gravitational torques, we proposea 3-DOF counterbalance mechanism based on the double parallel-ogram mechanism, in which reference planes are provided to eachjoint for proper counterbalancing. A 5-DOF counterbalance robotarm was built to demonstrate the performance of the proposedmechanism. Simulation and experimental results showed that theproposed mechanism had effectively decreased the torque requiredto support the robot mass, thus allowing the prospective use of low-cost motors and speed reducers for high-performance robot arms.

Index Terms—Counterbalance mechanism, counterbalancerobot arm, low-cost robot, manipulator design.

I. INTRODUCTION

IN recent years, service robot arms have received much at-tention because of their increasing use in various applica-

tions [1]–[3]. However, the robot arm market is still limitedbecause of the high prices of robot arms. Therefore, much re-search has been done to develop low-cost, high-performancerobot arms [4]–[8].

Most conventional robot arms are equipped with expensivespeed reducers and high-performance motors, which are usedto produce high torques that allow the arm to withstand the loaddue to their own weight and motion. Cheap motors and speedreducers do not provide sufficient torques to support the robotarm mass and to accelerate it. Therefore, innovative strategiesare needed to reduce the required torque of robot arms in orderto achieve high performance even with cheap components. Notethat the word “low cost” in this paper means that the proposed

Manuscript received September 17, 2012; revised September 3, 2013;accepted January 14, 2014. Date of publication March 17, 2014; date of currentversion June 13, 2014. Recommended by Technical Editor Y. Li. This work wassupported in part by the Basic Science Research Program through NRF (2007-0056094), in part by the Development of Counterbalance Robots through theMOTIE, and in part by a Korea University Grant.

The authors are with the School of Mechanical Engineering, Korea University,Seoul 136-701, Korea (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMECH.2014.2308312

robot arm can achieve similar performance (e.g., payload) ofconventional robot arms more cheaply.

Several counterbalance mechanisms have been developedto decrease the capacity of motors and speed reducers ofrobot arms. In the case of industrial manipulators, a heavyweight is often attached at the opposite side of the robotarm to balance the gravitational torque due to the manipu-lator mass. However, external heavy mass increases the to-tal mass of a robot arm, which results in the increases in thetorque required for acceleration and deceleration. To cope withthis problem, a multi-DOF counterbalance mechanism usinga pseudoparallelogram mechanism based on a belt and pulleywas suggested in [9], and a passive mechanical gravity com-pensator using pulleys and tension springs was developed in[10]. These studies show that springs can be used to providea counterbalancing torque instead of a heavy external mass.A cam-type counterbalance mechanism for industrial manipu-lators was designed in [11]. This study introduced the designmethod of using a cam profile to generate a proper balancingtorque at the lower two joints of an articulated manipulator. Acounterbalance mechanism using differential bevel gears for a2-DOF robot arm operating in a hemisphere workspace was pro-posed in [12]. This study shows that the counterbalance mech-anism can be applied not only to the pitch joint, but also to theroll joint of a robot arm. Some counterbalance mechanisms havebeen widely used to design rehabilitation devices, since users(i.e., patients) do not have enough strength to operate rehabilita-tion devices. For example, a 2-DOF counterbalance mechanismusing gears, pulleys, and wires for lower limb rehabilitation wasdeveloped in [13].

However, the previous counterbalance mechanisms are lim-ited to only one or two joints of a multi-DOF robot arm. More-over, robots using these mechanisms are too heavy and bulkyto be used for service robot arms, and their operational rangesare usually restricted due to the counterbalance mechanism. Inthis study, we develop a novel counterbalance mechanism usingsprings and wires. This mechanism can effectively compensatefor the gravitational torques required at each joint to support therobot arm mass for any robot configurations. In other words, themotors and speed reducers have only to provide the torque tomove the payload and the robot without supporting all the grav-itational loads, which are automatically compensated for by thecounterbalance mechanism. Thus, low-power actuator modulesare sufficient to achieve high performance, significantly reduc-ing the cost related to the actuators.

The proposed mechanism is applicable to all joints of therobot arm because it uses a double parallelogram mechanism.A 5-DOF robot arm using the proposed mechanism was built to

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KIM AND SONG: MULTI-DOF COUNTERBALANCE MECHANISM FOR A SERVICE ROBOT ARM 1757

Fig. 1. Simulations of the torques required to operate a typical 6-DOF robotarm: (a) simulation model and condition, (b) torques required to operate joint2, and (c) joint 3 with a payload of 0–2 kg between 0 and 90%, at 90◦/s.

demonstrate the performance of the proposed mechanism. Sincethe gravitational torque due to the robot mass was effectivelycompensated for by the counterbalance mechanism, expensivemotors and speed reducers were no longer required.

The rest of this paper is organized as follows. The multi-DOF counterbalance mechanism using springs and wires, andthe double parallelogram mechanism is discussed in Section II.Section III shows the design of the counterbalance robot armusing the suggested counterbalance mechanism. In Section IV,various simulation and experimental results are presented toeffectively illustrate the performances of the designed counter-balance mechanism and the counterbalance robot arm. Finally,the conclusion is presented in Section V.

II. COUNTERBALANCE MECHANISM

In this section, a novel counterbalance mechanism usingsprings and wires is presented as well as its application to amulti-DOF robot arm.

A. Gravitational Torque Due to the Robot Mass

The highest torques required for the motion of robot armsare gravitational torques and inertial torques. The Coriolis andcentrifugal effects are not great unless the robot moves at highspeeds. The inertial torque, which accelerates or decelerates therobot, is usually small, provided that the robot moves at reason-able speeds and accelerations. A gravitational torque occurs dueto the masses of the robot and payload, but most of the gravita-tional torque is caused by the robot mass, which is much greaterthan the payload mass for most robots. This phenomenon can beseen in Fig. 1, in which the results of a simulation show the rel-atively small dependence of the required torque on the payloadmass. It is therefore clear that counterbalancing the gravitationaltorque due to the robot mass can minimize the torque requiredat each joint.

The gravitational torque Tg of a simple 1-DOF robot shownin Fig. 2(a) is given by

Tg = mglc sin θ (1)

Fig. 2. Gravitational torque of 1-DOF arm: (a) simplified model and (b) torqueas a function of θ.

Fig. 3. Gravitational torque of 2-DOF arm: (a) simplified model and (b) torqueas a function of θ1 with θ2 = 0.

where m and l are the mass and length of the link, respectively,lc is the distance from the joint axis to the link center of mass,and θ is the angular displacement of the link from the y-axis.Also, for this simple robot, the counterbalance mechanism canbe easily designed by installing a spring with a proper stiffnessk between the reference plane (base) and the link to generatea counterbalancing torque that cancels the gravitational torquedue to the link mass.

Next, let us consider a 2-DOF robot shown in Fig. 3(a). Duringthe rotation of a robot arm, the gravitational torque at each jointcan be given by

Tg1(θ1 , θ2) = m1glc1 sin θ1 + m2g {l1 sin θ1 + lc2 sin(θ1 + θ2)}

= (m1glc1 + m2gl1) sin θ1 + m2glc2 sin(θ1 + θ2)

Tg2(θ1 , θ2) = m2glc2 sin(θ1 + θ2). (2)

The gravitational torques at joints 1 and 2 are affected byboth joint angles θ1 and θ2 . For example, Tg2 is affected by θ1 ,which is the absolute angle from the base frame and θ2 , whichis the relative angle from link 1. Therefore, the counterbalancemechanism at joint 2 should be designed to generate a torquethat cancels the gravitational torque due to both the absoluteangle and relative angle.

Thus, the counterbalance mechanism for a multi-DOF robotcannot be designed by merely placing a spring at each joint.Let us consider a 2-DOF robot with a spring between each linkand the reference plane to generate the counterbalancing torque,as shown in Fig. 4. This counterbalance mechanism, however,cannot generate the proper counterbalancing torque in severalrobot configurations.

Suppose that link 2 rotates while link 1 remains stationaryas shown in Fig. 4(b). Since the reference plane for link 2 isfixed in space due to the fact that link 1 is stationary, spring 2


Fig. 4. 2-DOF counterbalance mechanism in various configurations: (a) initialconfiguration, (b) only joint 2 is rotated by θ, and (c) only joint 1 is rotated byθ.

Fig. 5. 1-DOF counterbalance mechanism: (a) concept model and (b) closeupview of triangle in general position.

can compensate for the gravitational torque at joint 2. However,the motion of link 2 induces a gravitational torque at joint 1,which cannot be compensated for by spring 1 because link 1 isstationary. Therefore, the counterbalance mechanism should bedesigned such that it can deliver a proper spring force to eachjoint in any configuration.

As another example, suppose that links 1 and 2 rotate togetherby the same angle. The reference plane for link 2, which isattached to link 1, also rotates with link 1. Therefore, spring 2cannot generate the counterbalance torque required for joint 2in Fig. 4(c) while it can do so in Fig. 4(b), although the identicalgravitational torque is applied to joint 2 in both configurations.Therefore, every link should have its own fixed reference planefor counterbalance, and this plane should be invariant to themovement of other joints.

B. Single-DOF Counterbalance Mechanism

The concept model of the proposed counterbalance mecha-nism is shown in Fig. 5. To compensate for the gravitationaltorque of the robot, a compression spring (instead of the tensionspring shown in Fig. 2) and a wire were used in the coun-terbalance mechanism. When the link is rotated, the spring iscompressed by the spring block pulled by the wire. The springforce can be given by

Fs(θ) = k {so + c(θ) − (b − a)} (3)

where k is the spring stiffness, so is the initial compressionlength of the spring, and α, a, b, and c(θ) are defined in Fig. 5.

From the equation of the closure of the mechanism shown inFig. 5(b), the counterbalancing torque Tc can be given by

Tc(θ) = bFs(θ) sin α (4)

Fig. 6. Parallelogram mechanism constructed of: (a) 4-bar linkage, and(b) pulley and wire.

Fig. 7. 3-DOF counterbalance mechanism with reference plane at each joint.

where c = (a2 + b2−2ab cos θ)1/2 and sinα = a sinθ/c. Finally,the difference torque, which is calculated as the difference be-tween the gravitational and counterbalancing torques, requiredto maintain the posture of the robot can be computed by

Td(θ) = Tg (θ) − Tc(θ). (5)

The counterbalancing torque Tc cancels the gravitationaltorque Tg in ideal conditions so that the difference torque Td

becomes 0. From (1) and (4), the relationship among the lengthlc , a, b, and the spring constant k can be described by

mglc sin θ = bk {so + c(θ) − (b − a)} a sin θ

c(θ). (6)

Therefore, if the initial compression length of the spring so isset to b − a, the spring to generate the proper counterbalancingtorque can be easily selected by the relationship k = mglc /ab.

C. Double Parallelogram Mechanism

A 4-bar linkage system is often used to construct a parallel-ogram mechanism, in which the driven link is maintained inthe same direction, as shown in Fig. 6(a). These links can bereplaced by wires and pulleys, as shown in Fig. 6(b). With theparallelogram mechanism, the reference plane, which is fixedon pulley 2, maintains a constant absolute angle (i.e., usuallyparallel to the direction of gravity). Therefore, a counterbalanc-ing torque can be generated at joint 2 with respect to referenceplane 2. Note that the wire is not used to rotate the link, but justto connect pulley 2 to the fixed pulley 1 to generate referenceplane 2.

We adopted a double parallelogram mechanism based on pul-leys and wires to provide counterbalancing torques to the mul-tiple pitch joints of a service robot arm (i.e., three pitch jointsfor most industrial robots). The reference plane with a constantabsolute angle can be generated at each joint, and a spring isinstalled between each reference plane and the correspondinglink to generate the counterbalancing torque. Fig. 7 shows that


Fig. 8. 3-DOF counterbalance mechanism for: (a) lower 2 joints, (b) wristjoint, and (c) closeup view of triangle in general position.

Fig. 9. Orientation of link 3 in various configurations.

Fig. 10. Reference planes at each joint to generate counterbalancing torques.

the proper counterbalancing torque can be generated at eachjoint when the robot arm moves from pose 1 to 4.

D. Multi-DOF Counterbalance Mechanism

Fig. 8 shows the counterbalance mechanism designed forthree continuous pitch joints: the lower two joints and the wristjoint. As each link rotates, the installed springs are compressedby the pulled wires as the distance between each fixed point andidler changes.

The counterbalance mechanism installed at the wrist jointshown in Fig. 8(b) and (c) generates a counterbalancing torquefor the gripper and grasped object. As shown in Fig. 9, theorientation of link 3 is fixed relative to pulley 1, which is fixedat the base frame. Therefore, link 3 can provide the referenceplane for the wrist joint.

The spring force Fsj and counterbalancing torque Tcj for thecounterbalance mechanism related to joint j can be given by

Fsj (θ1 , θ2 , θ3) = kj{soj + cj − (bj − aj )} (7)

Tcj (θ1 , θ2 , θ3) = bjFsj (θj ) sin αj (8)

Fig. 11. Design of the counterbalance robot arm.

where cj = (a2j + b2

j – 2aj bj cosθj )1/2 , and sinαj = aj sinθj /cj

for joints 1 and 3. In the case of the counterbalance mechanismof joint 2, c2 = {a2

2 + b22 – 2a2b2b2cos(θ1 +θ2)}1/2 since the

absolute angle is affected by the angle of joint 1.In serial robot arms, the gravitational torque of joint j (j =

1, . . . , n) is the sum of the gravitational torques of joint j tojoint n. For example, in the case of the 2-DOF arm shownin Fig. 3, the gravitational torque of joint 1 consists of thetwo terms (m1glc1 + m2gl1)sinθ1 and m2glc2sin(θ1 + θ2) in(2), which are the gravitational torques due to links 1 and 2,respectively. Note that the second term is compensated for bythe counterbalance mechanism installed at joint 2, and thus, thecounterbalance mechanism at joint 1 has only to cancel the firstterm. In conclusion, each counterbalance mechanism has onlyto cancel the gravitational torque due to its corresponding linkas shown in Fig. 10.

Therefore, the difference torque Td at each joint of the 3-DOFrobot arm can be calculated by

Td1 = Tg 1 − (Tc 1 + Tc 2 + Tc 3)

Td2 = Tg 2 − (Tc 2 + Tc 3)

Td3 = Tg 3 − Tc 3 . (9)

III. COUNTERBALANCE ROBOT ARM

In this study, we constructed a 5-DOF robot arm using theproposed counterbalance mechanisms to investigate the effec-tiveness of the mechanism. The required specifications of themotors and speed reducers can be greatly decreased since thegravitational torques due to the robot mass are compensatedfor by the counterbalance mechanisms. Therefore, comparedto conventional robot arms, the counterbalance robot arm canachieve the same performance (such as payload) using muchcheaper components. In this section, the design of the 5-DOFcounterbalance robot is presented.

A. Design of a 5-DOF Counterbalance Robot Arm

Among the three types of revolute joints (i.e., roll, pitch, andyaw joints), pitch joints are affected by gravity. Therefore, weinstalled the counterbalance mechanism at each pitch joint ofthe robot. Fig. 11 shows the prototype design of the 5-DOFcounterbalance robot. This robot arm is constructed of yaw–pitch–pitch–pitch (passive joint)–yaw–pitch joints. The passive


Fig. 12. Counterbalance mechanism for joint 2.

Fig. 13. Prototype of the counterbalance robot arm.

pitch joint has no motor and speed reducer, and it is coupled withjoints 1 and 2 through the double parallelogram mechanism tomaintain a constant absolute angle. As shown in Fig. 11, thecounterbalance mechanism is not required for joints 1(yaw) and4(yaw) since they always rotate parallel to the ground. Thus,the proposed 3-DOF counterbalance mechanism needs to beapplied to only joints 2, 3, and 5.

Joint 2 is connected with the actuator module through a pinionto provide an additional gear ratio of 1:1.9. To decrease the armmass, joint 3 is operated by the actuator module placed at thebase module of the robot via a tendon-driven mechanism with agear ratio of 1:1.9. Wires of the tendon drive and parallelogrammechanism are placed inside hollow shafts. Most parts of thecounterbalance robot arm are constructed with the aluminumalloy to achieve light weight.

The proposed counterbalance mechanism was designed sothat it can be embedded inside the robot arm. Since the counter-balance mechanisms at joints 2 and 3 should cancel the gravita-tional torques of the robot arm; the mass and the center of massof each link are optimized to obtain the best results. Severalsprings were selected to provide the proper counterbalancingtorque and sufficient compressible length. Fig. 12 shows thedesigned counterbalance mechanism for joint 2.

As shown in Fig. 12, the spring blocks are pulled by wires,which are fixed at the reference planes, through the idlers wheneach link is rotated in the CCW direction. Four bushes are in-stalled inside the spring block to minimize the friction as thespring block moves along the hollow shafts, as shown in thesection view A-A′ of the spring block. Wires for the parallelo-gram mechanism and tendon drive are passed through the hollowshafts, and connected with the driven pulleys of the next joint.

B. Prototype

The prototype of the counterbalance mechanism is presentedin Fig. 13. The low-cost actuator modules for the counterbalance

TABLE ISPECIFICATIONS OF THE ACTUATOR MODULE [14]

TABLE IISPECIFICATIONS OF THE COUNTERBALANCE ROBOT ARM

TABLE IIIDESIGN PARAMETERS FOR THE COUNTERBALANCE MECHANISM

robot arm are listed in Table I, and the specifications of therobot arm are detailed in Table II. The module contains a motor,gear reducer, controller, and communication device. Note thatthese actuator modules alone cannot provide sufficient torquesfor a 5-DOF robot arm. However, the proposed counterbalancemechanism enabled the construction of a 5-DOF arm with areach of longer than 600 mm, a payload of 2 kg and an endpointspeed greater than 2 m/s. Design parameters of the robot armand its counterbalance mechanism are presented in Fig. 13 andTable III.

IV. SIMULATIONS AND EXPERIMENTS

The simulation results that verify the performance of the pro-posed counterbalance mechanism are presented in this section.Sections IV-A and IV-B show the counterbalancing torque fromthe counterbalance mechanisms during the operations of therobot arm with and without the external load, respectively. Also,the difference torque is presented for each result. Sections IV-Cand IV-D verify the performance of the counterbalance robotarm by experiments.

A. Counterbalance Without an External Load

The simulation conditions of the counterbalance robot arm areshown in Fig. 14. As mentioned in the previous section, a totalof three counterbalance mechanisms were installed, one at each


Fig. 14. Simulation conditions of the counterbalance robot arm.

Fig. 15. Simulation results of gravitational, counterbalancing, and differencetorques without an external load at: (a) joint 2 with condition A, (b) joint 3 withcondition A, (c) joint 3 with condition B, and (d) joint 5 with condition C.

pitch joint (joint 2: shoulder joint, 3: elbow joint, and 5: wristjoint) of the robot arm. In this simulation, it was assumed thatthe robot was equipped with a 0.5-kg gripper at its end-effector.

The results in Fig. 15(a) and (b) show the gravitational, coun-terbalancing, and difference torques when joint 2 of the robotarm was rotated from 0◦ to 180◦ in the CCW direction withoutan external load. On the other hand, Fig. 15(c) and (d) shows theresulting torques for joints 3 and 5 when they were rotated from–90◦ to 90◦ CCW, respectively, while joint 2 was fixed at 90◦. Asshown in the results, the proper counterbalancing torques weregenerated when the maximum gravitational torques of 15.5, 5.2,and 0.4 N·m were applied to each joint, respectively. However,due to the counterbalance mechanism, only 1, 0.9, and 0 N·mwere required to operate joints 2, 3, and 5, respectively. Notethat the difference torques shown in Fig. 15(a)–(c) show a mi-nus value in certain configurations because the counterbalancingtorque exceeded the gravitational torque. However, the magni-tude of the difference torque is always well below the maximumallowable torque ±τmax of each joint, which are 16.0, 11.4, and2.5 N·m for joints 2, 3, and 5 with additional gear ratios of 1:1.9,1:1.9, and 1:1, respectively. Note that the maximum allowabletorque means the product of the torque of the actuator moduleand the additional gear ratio.

B. Counterbalance With an External Load

To show the performance of the proposed robot arm with anexternal load, it was assumed that the robot held a 2-kg object at

Fig. 16. Simulation results of gravitational, balancing, and difference torqueswith an external load 2 kg at: (a) joint 2 with condition A, (b) joint 3 withcondition A, (c) joint 3 with condition B, and (d) joint 5 with condition C.

Fig. 17. Simulation results of gravitational, counterbalancing, and differencetorques with a spring of 4 kN/m stiffness at joint 5.

its gripper. The simulations discussed in Fig. 14 were conductedagain and the results are presented in Fig. 16.

As shown in Fig. 16, the torques required to operate therobot arm were greatly reduced since the gravitational torqueswere compensated for by the counterbalancing torques from thecounterbalance mechanisms. Normally, gravitational torques ofup to 29.6, 13.4, and 2.0 N·m were applied to the three joints,respectively. Note that these values are above the maximumallowable torque of each joint. On the other hand, with theproposed mechanism, the required torque greatly reduced to13.8, 7.9 and 1.6 N·m, as shown in Fig. 16. They fell below themaximum allowable torque of the robot arm, which implies thatthe arm can be fully controlled with the actuator modules.

Finally, note that the counterbalance mechanism for joint 5(wrist joint) in Fig. 16 does not seem effective since the grav-itational torque due to a 2-kg payload is below the maximumallowable torque of the actuator module. Suppose that the springconstant at this joint is increased four times from 1 to 4 kN/m.The 4 kN/m spring would generate a compensating torque of–1.6 N·m, as shown in Fig. 17. This torque would obviouslyovercompensate for the gravitational torque in the case of nopayload, and the actuator would have to provide 1.2 N·m in thedirection of gravity. However, with this increased stiffness of thespring, the robot could handle even a 4-kg payload, which wouldresult in a maximum of 3.5 N·m of the gravitational torque, asshown in Fig. 17. In conclusion, a spring with higher stiffnesscould lead to a further increased payload capacity of the robotarm, which would allow joint 5 to handle more than a 2-kg load.


Fig. 18. Experiments to determine the torque required to move the robot armwith and without the counterbalance mechanism: (a) experimental setup and(b) experimental results.

Fig. 19. Repeatability test: (a) experimental setup and (b) results.

C. Experiments: Counterbalance

The torque required to operate the robot arm with and withouta counterbalance mechanism was measured by the experimen-tal setup shown in Fig. 18(a). Because it is difficult to install anexternal torque sensor without changing the robot arm design,a special sensing device was designed to indirectly measure thetorque. This sensing device consists of a force sensor and a1-DOF link. The force sensor installed at the end of the sensingdevice can measure a contact force acting in the direction or-thogonal to the link as shown in Fig. 18(a). Since the rotationaljoint of the sensing device was fixed at joint 2 (shoulder joint)of the robot arm, the force sensor recorded the force exerted onsome known point of distance l away from joint 2, while therobot arm is rotated from 0◦ to 180◦ manually. Since the momentarm l is known, the torque can be calculated by multiplying themeasured contact force by the link length l.

As shown in the experimental results in Fig. 18(b), the max-imum torque required to move joints 2 of the robot arm upwas 10 N·m without the counterbalance mechanism, but wasdecreased to 2 N·m with the counterbalance mechanism. Thismeans that most of the torque required to operate the robot armwas compensated for by the proposed counterbalance mecha-nism. A small remaining torque is caused by the friction of themechanism.

D. Experiments: Repeatability

Repeatability test for the proposed counterbalance robot armwas conducted using a digital dial gauge, as shown in Fig. 19(a)[15], [16]. To measure the repeatability, the robot arm was con-trolled to move between points A and B, and between A and Crepetitively at an endpoint speed of 1 m/s. The position of the

end-effector of the robot arm was measured by the digital dialgauge at the end of each cycle, and it was plotted in Fig. 19(b).As shown in the results, the position errors of the repeated mo-tion of the counterbalance robot arm were less than 0.25 mm.These errors were caused mainly by the backlash of the actuatormodules. This repeatability is sufficient to conduct various taskssuch as “pick and move” or “simple assembly,” which usuallydo not require a high degree of precision.

V. CONCLUSION

A novel counterbalance mechanism composed of springs anddouble parallelogram mechanisms were proposed in this paper.With this proposed mechanism, the proper counterbalancingtorque can be generated to cancel out the gravitational torquedue to the robot mass. Moreover, a counterbalance robot armwas developed using the proposed counterbalance mechanism.The following conclusions are drawn from these results.

1) The proposed counterbalance mechanism can compensatefor the gravitational torque due to the robot mass for mostrobot configurations.

2) A counterbalance robot arm which requires a muchsmaller actuator torque for its motion can be constructedat a much lower cost than current commercialized robots,which could lead to the development of robot arms suitablefor service robots.

REFERENCES

[1] DLR, Institute of Robotics and Mechatronics, Status Report 1997–2004,Part 1, pp. 35–42, 2004.

[2] H. Iwata and S. Sugano, “Design of human symbiotic robot TWENDY-ONE,” in Proc. IEEE Int. Conf. Robot. Autom., 2009, pp. 580–586.

[3] Y. Sakagami, R. Watanabe, C. Aoyama, S. Matsunaga, N. Higaki, andK. Fujimura, “The intelligent ASIMO: System overview and integra-tion,” in Proc. IEEE / RSJ Int. Conf. Intell. Robots Syst., 2002, vol. 3,pp. 2478—2483.

[4] U-X. Tan, W. T. Latt, C. Y. Shee, and W. T. Ang, “A low-cost flexure-basedhandheld mechanism for micromanipulation,” IEEE Trans. Mechatronics,vol. 16, no. 4, pp. 773–779, Aug. 2011.

[5] W. Jianhua, X. De, T. Min, and L. Yun, “Control strategy for a low costmanipulator to transport and align IC mask-plates,” IEEE Trans. ControlSyst. Technol., vol. 17, no. 5, pp. 1018–1027, Sep. 2009.

[6] E. Cambell, Z. C. Kong, W. Hered, A. J. Lynch, M. K. O’malley, andJ. McLurkin, “Design of a low-cost series elastic actuator for multi-robot manipulation,” in Proc. IEEE Int. Conf. Robot. Autom., 2011,pp. 5395–5400.

[7] M. Quigley, A. Asbeck, and A. Ng, “A low-cost compliant 7-DoFrobotic manipulator,” in Proc. IEEE Int. Conf. Robot. Autom., 2011,pp. 6051–6058.

[8] J. A. N. Cocota Jr., H. S. Fujita, and I. J. Silva, “A low-cost robot ma-nipulator for education,” in Proc. IEEE/ASME Int. Conf. Technol. Appl.Electron. Teaching, 2012, pp. 164–169.

[9] T. Morita, F. Kuribara, Y. Shiozawa, and S. Sugano, “A novel mechanismdesign for gravity compensation in three dimensional space,” in Proc.IEEE/ASME Int. Conf. Adv. Intell. Mechatronics, 2003, pp. 163–168.

[10] N. Ulrich and V. Kumar, “Passive mechanical gravity compensationfor robot manipulator,” in Proc. IEEE Int. Conf. Robot. Autom., 1991,pp. 1536–1541.

[11] K. Koser, “A cam mechanism for gravity-balancing,” Mech. Res. Com-mun., vol. 36, no. 4, pp. 523–530, 2009.

[12] C. H. Cho and S. C. Kang, “Static balancing of manipulator with hemi-spherical work space,” in Proc. IEEE/ASME Int. Conf. Adv. Intell. Mecha-tronics, 2010, pp. 1269–1274.

[13] T. Nakayama, Y. Araki, and H. Fujimoto, “A new gravity compensa-tion mechanism for lower limb rehabilitation,” in Proc. IEEE Int. Conf.Mechatronics Autom., 2009, pp. 943–948.


[14] (2013). [Online]. Available: http://robotis.com/xe/[15] J.-F. Brethe, E. Vasselin, D. Lefebvre, and B. Dakyo, “Determination of

a repeatability of a KUKA robot using stochastic ellipsoid approach,” inProc. IEEE Int. Conf. Robot. Autom., 2005, pp. 4339–4344.

[16] Y. Koren, Robotics for Engineers. New York, NY, USA: McGraw-Hill,1985.

Hwi-Su Kim received the B.S. and Ph.D. degreesin mechanical engineering from Korea University,Seoul, Korea, in 2007 and 2014, respectively.

He is currently engaged in postdoctoral work in theSchool of Mechanical Engineering, Korea University.His research interests include manipulator design andsafe robot arms.

Jae-Bok Song (M’00) received the B.S. and M.S. de-grees in mechanical engineering from Seoul NationalUniversity, Seoul, Korea, in 1983 and 1985, respec-tively, and the Ph.D. degree from Massachusetts Insti-tute of Technology, Cambridge, MA, USA, in 1992.

He is currently a Professor in the School ofMechanical Engineering, Korea University, Seoul,where he has served as the Director of the IntelligentRobotics Laboratory since 1993. His research inter-ests include safe manipulators, design and control ofrobotic systems, and indoor/outdoor navigation.

Dr. Song is a member of the American Society of Mechanical Engineers.

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