Novel multi-DOF counterbalance mechanism design based on a spring balancer for robot arms

다국어 초록 (Multilingual Abstract)

For many decades, robot arms have been traditionally used in the industrial fields for factory automation. To expand the applications of robot arms to indoor service sites, service robot arms have also been developed recently. Most of these service robots are equipped with a vertical prismatic joint between the mobile platform and the robot arm to allow the robot to perform various tasks. These robot arms and prismatic joints require significantly higher motor torques and power than those required by the wheel drive of a mobile platform. Therefore, the operating time of a battery-powered mobile service robot is short, and the payload capacity of a service robot arm is also low with limited task variation.
A counterbalance mechanism (CBM), which is a mechanical device that cancels or reduces the gravitational torque applied to a revolute linkage joint with counterweight or springs, can be an effective solution to solve this problem. The CBM can effectively support the robot arm mass and payload, thereby reducing the motor torque and power consumption required to operate the robot. In addition, a spring balancer, which is a mechanical component that applies a constant restoring force to a wire, can be easily used to counterbalance a prismatic joint. However, mounting both the prismatic CBM and revolute CBMs on the service robot arm would substantially increase the overall volume, weight, and mechanical complexity. Therefore, a CBM that can counterbalance the arm with both prismatic and revolute joints, with a simple structure, is required.
In this report, a novel multi-DOF CBM that can counterbalance a robot with prismatic–pitch–pitch joints using only one spring balancer mounted on the base is proposed. The proposed CBM consists of mechanical components such as a spring balancer, wire, and idlers. It is difficult to counterbalance the multi-DOF joints by simply connecting each joint to a single coil spring and wire, because the compensation torque of one joint can be affected by the movement of the other joints, thereby changing the spring force. Therefore, in this study, the property of the spring balancer, i.e., the restoring force is constant and not affected by a change in displacement, was exploited. The wire subjected to a constant restoring force, generated by the spring balancer initially, extends through the idlers on the link of the prismatic joint, and then forms multiple loops between the idlers, fixed to each pitch joint, and the link. Consequently, the combined force applied to each idler exerts an appropriate compensation force/ torque on each joint.
To verify the performance and practical implementation of the mechanism, a 3-DOF CBM prototype along with a 7-DOF robot arm equipped with the CBM were constructed. Various experiments were conducted, and the corresponding results were compared to confirm the performance of the proposed CBM and counterbalance robot arm. It was shown that the proposed multi-DOF CBM, based on a spring balancer, properly counterbalanced the prismatic–pitch–pitch joints and enabled the construction of the high-payload service robot arm with compact actuators and link structures.

목차 (Table of Contents)

• 1. INTRODUCTION 1
• 1.1 Research Background 1
• 1.2 Related Works 4
• 1.2.1 Various types of countebalance mechanism 4
• 1.2.2 Multi-DOF counterbalance mechanism 6
• 1.2.3 Practical use of counterbalance mechanism in robot arm 8
• 1.3 Thesis Outline 10
• 2. COUNTERBALANCE MECHANISM 11
• 2.1 Introduction 11
• 2.2 Conventional CBMs 12
• 2.2.1 Torque required for robot arm operation 12
• 2.2.2 Counterbalance for a pitch joint 13
• 2.2.3 Counterbalance for pitch-pitch joints 18
• 2.3 Multi-DOF CBM based on a Spring Balancer 23
• 2.3.1 Features of a spring balancer 23
• 2.3.2 Counterbalance for a pitch joint 26
• 2.3.3 Counterbalance for pitch-pitch joints 33
• 2.3.4 Counterbalance for a prismatic joint 37
• 2.3.5 Counterbalance for prismatic-pitch-pitch joints 39
• 2.4 Verification 41
• 2.4.1 Design of a 3-DOF CBM prototype 41
• 2.4.2 Experiments: static balancing 42
• 2.4.3 Experiments: force/torque reduction measurement 43
• 2.5 Conclusion 47
• 3. COUNTERBALANCE ROBOT ARM 48
• 3.1 Introduction 48
• 3.2 Design of a Counterbalance Robot Arm 49
• 3.2.1 Structural design of the CBM 49
• 3.2.2 Load analysis and actuator selection 61
• 3.2.3 Prototype of the counterbalance robot arm 64
• 3.3 Verification 69
• 3.3.1 Experiments: static balancing 69
• 3.3.2 Experiments: force/torque reduction measurement 70
• 3.3.3 Experiments: repeatability 75
• 3.3.4 Experiments: durability 77
• 3.4 Conclusion 79
• 4. DISCUSSION 80
• 4.1 Specifications of the Service Robot Arms 80
• 4.2 Actuator Size Reduction 82
• 4.3 CBM Comparison 84
• 4.4 Conclusion 87
• 5. CONCLUDING REMARKS 88
• REFERENCES 92
• CURRICULUM VITAE 94

참고문헌 (Reference)

1. A cam mechanism for gravity-balancing, K. Koser, vol . 36 , no . 4 , pp . 523 ? 530, , 2009

2. Effect of mass balancing on the actuator torques of a manipulator, H. Diken, vol . 30 , no . 4 , pp . 495 ? 500, , 1995

3. Gravity compensation of a 6-UPS parallel kinematics machine tool through elastically balanced constant-force generators, A. Martini, vol . 46 , no . 1 , pp . 10 ? 16, , 2018