A case study in human–robot collaboration in the disassembly of press-fitted components

Abstract

Human–robot collaborative disassembly is an approach designed to mitigate the effects of uncertainties associated with the condition of end-of-life products returned for remanufacturing. This flexible semi-autonomous approach can also handle unpredictability in the frequency and numbers of such returns as well as variance in the remanufacturing process. This article focusses on disassembly, which is the first and arguably the most critical step in remanufacturing. The article presents a new method for disassembling press-fitted components using human–robot collaboration based on the active compliance provided by a collaborative robot. The article first introduces the concepts of human–robot collaborative disassembly and outlines the method of active compliance control. It then details a case study designed to demonstrate the proposed method. The study involved the disassembly of an automotive water pump by a collaborative industrial robot working with a human operator to take apart components that had been press-fitted together. The results show the feasibility of the proposed method.

Keywords

Remanufacturing robotic disassembly human–robot collaboration collaborative robot active compliance control

Introduction

Remanufacturing is an important element of a circular economy,¹ even more so than recycling.² Disassembly is a key operation in remanufacturing, as it must be carried out successfully to enable the other steps in the process chain to take place.^3–7 Considering all the uncertainties associated with products that have reached the end of their service, in particular, their unpredictable physical conditions, complete robotisation of disassembly is technically challenging and still economically infeasible.⁸ Flexible systems are needed to handle uncertainties in disassembly.⁹ This article advocates semi-autonomous disassembly with humans working shoulder to shoulder with robots as an approach to achieve the required flexibility.

In human–robot collaborative disassembly, robots and other ancillary equipment carry out tasks that are repetitive and heavy. This could leave people to assume a supervisory role or perform work that requires greater dexterity, flexibility, or cognitive ability than machines can provide.¹⁰ Effective human–robot collaboration (HRC) involves robots sharing the workspace with humans to perform their tasks jointly or independently.¹¹ In other words, there is no physical barrier between them. To achieve this, robot manufacturers have produced machines capable of working alongside people safely without having to be fenced in Unhelkar et al.¹² Examples of these new types of collaborative robots (or ‘cobots’) include the YuMi from ABB, the CRs range from Fanuc, the LBR iiwa range from KUKA, the TM range from Techman, the TX2 range from Staubli and the UR range from Universal Robots.

The state of the art in HRC in industrial applications was surveyed, focussing on safety and interfacing aspects.¹³ An extensive review was carried out of collaborative robot programming in industrial environments.¹⁴ There have been a number of HRC cases reported.¹⁵ A hybrid disassembly workstation with a collaborative robot performing unscrewing operations was designed for electric vehicle battery recycling.^16,17 In addition, HRC was implemented for the disassembly of EoL lithium-ion batteries from electric vehicles.¹⁸ HRC was employed in the disassembly of unknown models of electrical vehicle motors.¹⁹ Collaborative robots were used in e-waste management to classify and dismantle electronic devices.²⁰ A robotic system with two collaborative robots was validated on a mock-up of a gearbox assembly station in a car factory.²¹ A robotic disassembly assistant controlled by an intelligent software agent was proposed for collaborative disassembly to improve the ergonomics of disassembly workstations.²² A HRC system was developed for applications in areas such as food packaging, aerospace component assembly, and automotive engine assembly.¹⁰ A hybrid cell was constructed to enable HRC in bin-picking tasks during assembly operations.²³ A disassembly system using collaborative robots was proposed and validated on PC disassembly.²⁴

In HRC, it is necessary for the robot to exhibit compliance to avoid injuring humans should accidental physical contact between them occur.^25,26 Active compliance implemented via controlling the robot joints and sensing (force/ torque and vision) also increases adaptability and helps mitigate the effects of uncertainties. For example, to cope with workpiece variance, active compliance control has been adopted for industrial processes such as grinding, polishing, deburring, and finishing.²⁷ Active compliance was employed to enable a robot to perform knee surgery.²⁸ Surface alignment and surface following were demonstrated for an assembly operation using a collaborative robot with active compliance control.²⁹ Similarly, a compliant robot was successfully implemented for curve-following tasks under impedance control.³⁰ Learning-based variable compliance control was employed to perform the robotic insertion of a peg into a hole.³¹

Note that although HRC has been implemented in disassembly, using the active compliance of the robot to perform tasks such as separating press-fitted components has never been reported. Currently, in manual disassembly, the human operator usually has to hold the part as it is being pressed out of its mating component, so that he can follow its movement until it is completely separated. However, the operator risks injury when he has to simultaneously operate the press and hold the part to be separated. In HRC, a collaborative robot could replace the operator in holding the part and follow its movement during the separation process, releasing the operator from a potentially dangerous disassembly operation.

In this article, a novel HRC method for the common disassembly task of separating press-fitted components is presented. The proposed method uses active compliance to achieve complex disassembly tasks with flexibility and adaptability. The article is organised as follows. Section ‘Human–robot collaborative disassembly’ outlines the proposed HRC disassembly strategy and introduces the method of active compliance control. The new method of press-fitted component separation using HRC is described in section ‘HRC disassembly of press-fitted components’ and its validation on a case study involving HRC disassembly of an automotive water pump is reported in section ‘Implementation and case study’. Finally, the conclusions are presented in section ‘Conclusion’.

Human–robot collaborative disassembly

As previously mentioned, considering both flexibility and efficiency, HRC is implemented for disassembly to handle uncertainties associated with returned products. Active compliance control enables robots to overcome external influences and achieve complex disassembly tasks safely together with humans.

Collaborative disassembly

The advantages and disadvantages of manual and automated disassembly are shown in Table 1 adapted from Carrell et al.³² It can be seen that manual disassembly has low efficiency and high labour cost. Fully automated disassembly could deal with large volumes but is inflexible and would incur high capital costs.³³ However, due to variability in the returned EoL products and in the disassembly process, a higher degree of flexibility is needed of the disassembly system.³⁴

Table 1.

Comparison between manual and automated disassembly methods.

Method	Operator/machine	Advantages	Disadvantages
Manual disassembly	Humans	High flexibility	Low efficiency High labour cost
Automated disassembly	Automated machines or robots	High efficiency	Low flexibility High capital cost

The main factors that hinder the industrial adoption of disassembly automation are lack of flexibility and high capital cost. Human–robot collaborative disassembly can overcome these problems by combining the respective strengths of manual and automated disassembly.³⁵

As illustrated in Figure 1(a), most conventional industrial robots work in a specified workspace within a safety cage or behind a fence to prevent serious hazard to humans. This means a human operator cannot work in a robot-occupied workspace. However, the new work model, human–robot collaborative disassembly (as shown in Figure 1(b)), enables human operator and robot to work together and share the workspace without a safety cage. The flexibility and cognitive ability of the human operator, and the efficiency and accuracy of the robot, are combined in an HRC disassembly cell. The main components of such an HRC disassembly cell are robots, human operators, and ancillary equipment such as the cell controller, disassembly tools, robot end-effectors, fixtures, and sensors.

Figure 1.

Disassembly of cells: (a) using conventional robot and (b) using collaborative robot.

The complementarity between robot and human could help reduce the workload of the latter and lower requirements for dexterity, adaptability, and cognition on the former, while enabling complex disassembly tasks to be performed efficiently, flexibly, and cost-effectively. In addition, the use of collaborative robots in HRC disassembly can also improve safety and decrease production setup and reconfiguration time, therefore further reducing cost.

Active compliance

Without external sensors (such as force/torque sensors), most standard industrial robots can only work under position and velocity control, performing tasks such as materials handling, welding, painting, and coating.²⁸ However, as previously mentioned, due to uncertainties in the location, size and shape of the workpiece, and errors in the position and velocity of the robot, it is sometimes useful to control force or compliance rather than position or velocity.³⁶ Active force/compliance control enables robots to be employed in applications such as grinding, polishing, deburring, and automatic assembly/disassembly.

Active compliance is implemented to control the movement of a robot in response to force or tactile stimuli to minimise the magnitude of the stimuli.³⁷ Active compliance control can be divided into two main categories, hybrid position/force control and impedance control.³⁸ In hybrid position/force control, force and torque information is combined with positional information to satisfy simultaneous position and force trajectory constraints in a convenient task-related coordinate system.³⁹ Impedance control is generally adopted to achieve a certain desired dynamic behaviour of the robot when it interacts with the environment. The dynamic behaviour of the controlled end-effector as a generalised mechanical impedance can be expressed as⁴⁰

K_{M} ({\overset{\cdot}{υ}}_{d}^{e} - {\overset{\cdot}{υ}}_{e}^{e}) + K_{D} (υ_{d}^{e} - υ_{e}^{e}) + h_{Δ}^{e} = h_{e}^{e}

(1)

where $K_{M}$ and $K_{D}$ are symmetric and positive-definite matrices, ${\overset{\cdot}{υ}}_{d}^{e}$ and $υ_{d}^{e}$ are acceleration and velocity of a desired frame $Σ_{d}$ , ${\overset{\cdot}{υ}}_{e}^{e}$ and $υ_{e}^{e}$ are acceleration and velocity referred to the end-effector frame $Σ_{e}$ , and $h_{Δ}^{e}$ and $h_{e}^{e}$ are the elastic wrench and the external wrench. As shown in the above equation, a compliant dynamic behaviour is imposed on the end-effector in response to its interaction with the environment.⁴⁰ A force/torque sensor is needed to measure the contact force and moment for impedance control.

Impedance control enables the robot to interact with humans safely to achieve complex tasks. Being able to control its stiffness over a wide range, a collaborative robot can perform highly precise tasks with maximum stiffness (in other words, operate in the stiff mode) and safely collaborate with humans with low stiffness (in other words, work in the compliant mode). In the compliant mode, the robot can avoid hard collisions with humans operating in its workspace. This designed-in safety feature allows for greater flexibility and efficiency as both parties can work normally instead of the robot having to stop or slow down whenever a human operator enters its cage.

The robot employed in this work, a KUKA LBR iiwa, can work under impedance control using a Cartesian impedance controller, which is modelled on a virtual spring-damper system with configurable stiffness and damping.^41,42 Due to impedance control, the behaviour of the robot is compliant. An external force or a motion command results in a deviation between the set position and actual position of the tool centre point (TCP) on the robot. This induces a deflection of the virtual spring as shown in Figure 2. The resultant force could be calculated according to Hooke’s law

F = K Δ x

(2)

Figure 2.

Virtual spring of Cartesian impedance control.⁴¹

where F, K, and Δx are the resultant force, spring stiffness, and deflection, respectively. The configurable spring stiffness ranges for the translational and rotational degrees of freedom are 0–5000 N/m and 0–300 N m/rad, respectively.⁴¹

HRC disassembly of press-fitted components

Parts that are assembled by press fitting are held together by friction. Examples of press-fitted parts include bearings and housings, and bearings and shafts. To separate two press-fitted parts, the operator needs to apply a large force on one part while restraining the other.

In manual disassembly, the part to be pushed out is usually held in the hands of the operator so that he can follow its movement during the separation process (as shown in Figure 3(a)) until it is completely separated. However, the operator risks injury when he has to simultaneously operate the press and hold the part to be separated. Alternatively, a jig could be used to catch the extracted part, as shown in Figure 3(b). The problem is that it is difficult to ensure that the part drops into the component in the right position in the jig. Moreover, directly and continuously holding the separated component is more efficient than picking it up again from the jig, an operation affected by uncertainty in the location of the component and the positioning of the robot gripper. Another alternative is proposed that involves HRC to facilitate the task of the human operator and enable him to work in a safe environment. The new method uses the gripper of a compliant robot to hold the part being extracted and follow its movement during the extraction process. This is shown in Figure 3(c).

Figure 3.

Press-fitted component separation: (a) operator’s hand, (b) jig, and (c) robot gripper.

In some situations, the part to be pushed out is located completely inside another part (like a housing), as shown in Figure 4(a). The HRC removal procedure for this kind of press-fitted component is illustrated in Figure 4 and the related flow chart is shown in Figure 5. First, the robot waits at a set position for the shaft to touch the gripper (Figure 4(a)), while the operator applies a force on the component with the help of a press. When internal force sensors indicate to the robot that the part has touched the gripper (Figure 4(b)), it moves down to disengage from the contact and opens its fingers (Figure 4(c)). Next, it moves back up to grasp the part to be extracted (Figure 4(d)) and then follows its downward movement in compliant mode (Figure 4(e)). When, according to the force or position information fed back to the robot, the part has been completely removed, the compliant mode or velocity control mode is switched off, and the robot returns to its initial position.

Figure 4.

Press-fitted component separation using HRC: (a) waiting, (b) touching, (c) moving down and opening fingers, (d) clamping and turning on compliance, and (e) following movement and turning off compliance.

Figure 5.

Flow chart of HRC procedure.

Implementation and case study

To demonstrate the proposed method, a case study was conducted involving the disassembly of a small car water pump. A mentioned robot, the KUKA LBR iiwa, was adopted for the collaborative task of separating press-fitted components.

Description of disassembly cell

As shown in Figure 6, the human–robot collaborative disassembly demonstration cell consists of a KUKA LBR iiwa 14 R800 fitted with a two-fingered gripper (Robotiq 2-FINGER 140), a small manually operated press, the press operator, the water pump to be dismantled plus positioning jigs and part collection boxes located on a workbench. The robot has a built-in facility for detecting torques at its joints. It is a sensitive, flexible, and safe collaborative robot with seven axes and a payload of 14 kg. A press is used by the operator to separate the press-fitted pump components. Two of the jigs are to hold the components in the press during the dismantling operation. The third jig acts as a temporary receptacle for some subassemblies following their removal from the pump. The robot uses the gripper to handle the pump components and positioning jigs. The collection boxes keep the different dismantled pump components separated from one another.

Figure 6.

Human–robot collaborative disassembly demonstration cell.

The water pump to be disassembled (Unipart GWP187) is found in classic cars like the MG Midget (1961-1980) and Mini (1962-1993). As illustrated in Figure 7, the pump comprises a flange (A), pump casing (B), bearing (C), shaft (D), seal unit (E), and impeller (F). The material of the flange, shaft, and bearing is steel. The materials of the pump casing and the impeller are aluminium alloy and cast iron, respectively. The fit between the two parts of each pair A-D, B-C, C-D, E-D, and F-D is press fit.

Figure 7.

Exploded view of the water pump.

HRC tasks in disassembly

The main tasks in the disassembly of the water pump are shown in Table 2. There are three HRC tasks: the separation of the rotor subassembly (C-D-E-F) from A and B (Step 2), the removal of F from the rotor subassembly (Step 5), and the separation of E from the bearing-shaft subassembly (C-D), which will not be broken down into two components due to high cost (Step 8). The HRC procedure described in section ‘HRC disassembly of press-fitted components’ is followed to separate two parts that are press-fitted together. As previously explained, this involves the operator using the press to apply the required separation force, with one part remaining in the press while the robot holds the other and follows its movement as it is being pushed out. The next section gives the details of this procedure.

Table 2.

Main tasks in the disassembly of the water pump.

Step no.	Tasks	HRC
1	Pick up the water pump and place it in the press	No
2	Separate the rotor subassembly from parts A and B	Yes
3	Place the rotor subassembly on its temporary receptacleand parts A and B in their collection boxes	No
4	Place jig 1 in the press and the rotor subassembly in jig 1	No
5	Remove part F from the rotor subassembly	Yes
6	Put part F in its collection box and the rest of the rotorsubassembly (subassembly C-D-E) back on its temporary receptacle	No
7	Replace jig 1 with jig 2 and place subassembly C-D-E in jig 2	No
8	Separate part E from the bearing-shaft subassembly (C-D)	Yes
9	Put part E in its collection box	No
10	Put subassembly C-D in its collection box	No
11	Put jig 2 back on the workbench	No

HRC: human–robot collaboration.

Procedure of the HRC tasks

The procedure for separating the rotor subassembly from part A and part B is detailed in Figure 8, together with the related robot control programme implemented in KUKA Sunrise Workbench.⁴¹ First, the gripper moves to a set position inside the press to wait for the rotor subassembly (CDEF) to be pushed out of the pump casing (B) and flange (A). The ram of the press pushes CDEF downwards, separating it from A and B. When the robot detects an increase in the vertical component of the force on its gripper caused by CDEF touching it, the robot moves the gripper down slightly to be clear of CDEF and then opens the fingers. Next, the robot moves the gripper back up to grasp CDEF. Once the robot has taken hold of CDEF, it turns on the compliant mode to enable the gripper passively to follow the downward movement of CDEF. When CDEF has been removed completely, leaving A resting on B in the press, there will be a step change in the vertical force on the gripper due to the weight of CDEF which it now carries. This causes the position of the gripper to drop sharply. At that point, the robot switches off the compliance mode and the procedure ends.

Figure 8.

Procedure and programme for separating the rotor subassembly from part A and part B.

Figure 9 shows the separation procedure for part F and the related computer programme. First, the robot grips subassembly CDE and turns on the compliant mode. Then, the gripper passively follows the movement of CDE as the operator uses the press to separate it from F. When CDE has been completely extracted, leaving just F in the press, the vertical position of the gripper drops sharply due to the weight of CDE on the gripper. The robot then switches off the compliant mode.

Figure 9.

Procedure and programme for separating part F from the bearing-shaft subassembly CDE.

Results and discussion

The demonstration cell has been constructed and trials carried out on it to show that the procedures described in the previous sections work.

Figure 10 presents selected image frames captured from the process of separating the rotor subassembly (CDEF) from part A and part B. The top left-hand frame depicts the robot carrying the whole pump to the press. The next frame (Figure 10(1)) shows the gripper positioned under the pump, waiting for the rotor subassembly that the operator is extracting to contact it. Contact occurs in the third frame (Figure 10(2)). The fourth frame (Figure 10(3)) shows the gripper lowered, its fingers opened and clear of the rotor subassembly, while the fifth frame (Figure 10(4)) has the gripper moved back up to grasp the rotor assembly. In the sixth frame (Figure 10(5)), it can be observed that the gripper has tracked the downward movement of the rotor subassembly. The seventh frame (Figure 10(6)) reveals the freed rotor subassembly held in the gripper and parts A and B resting in the jig inside the press. Finally, the eighth frame (at the bottom left corner of Figure 10) sees the robot placing the rotor subassembly on its temporary receptacle on the workbench.

Figure 10.

Separation of the rotor subassembly from parts A and B: (1) the robot waits for the rotor subassembly, (2) contact with gripper occurs, (3) the robot lowers and opens the gripper, (4) the robot moves the gripper back up, (5) the robot grasps the rotor subassembly and follows its movement, and (6) the robot holds the freed rotor subassembly.

Figure 11 illustrates the process of removing part F from the rotor subassembly CDEF. The bottom left frame in the figure shows the robot moving the rotor subassembly back to the press after picking it up from its temporary receptacle. The next frame (Figure 11(1)) depicts the robot holding subassembly CDE in the press. In the third frame (Figure 11(2)), the robot gripper can be seen to have followed the movement of CDE as the operator applies force to drive it and F apart. The fourth frame (Figure 11(3)) has Part F separated from CDE which is still held in the gripper. The final frame (at the bottom right corner of Figure 11) shows subassembly CDE placed on the temporary receptacle on the workbench.

Figure 11.

Removal of part F from the rotor subassembly: (1) the robot holds subassembly CDE, (2) the robot follows the movement of CDE, and (3) the robot holds the separated subassembly CDE.

Preliminary trials show that the proposed HRC disassembly method is feasible. The cell is able to take a water pump apart in approximately 5 min and can be readily adapted to handle different pump models requiring the same basic disassembly operations. Work is being carried out to give the cell additional tools and capabilities, such as screwdrivers to remove threaded components and force/torque/vision sensors to enhance the cognitive ability of the robot, enabling it to collaborate even more effectively with the human operator. In addition, methods of disassembly task assignment between robots and human operators are being explored. Appendix 1 gives the URL of the video showing the disassembly operation.

Conclusion

Remanufacturing, in particular, its first stage which is disassembly, is an operation beset with uncertainties due to unpredictability in the condition of the returned EoL products and variance in remanufacturing processes. Human–robot collaborative disassembly is aimed at achieving the flexibility needed for cost-effectively dealing with those uncertainties. The separation of components held together by press fitting is a common disassembly task. This article has presented a new method for human–robot collaborative disassembly of press-fitted components. The robot uses its built-in active compliance facility to behave as the human operator’s left arm and assist him with securely catching the components being separated. A case study has been detailed that has demonstrated a successful application of the proposed method to the disassembly of a water pump by a collaborative industrial robot working together with a human operator to separate press-fitted components. The results indicate the feasibility of the proposed method. In addition, the case study has also shown that employing a collaborative robot enables the human operator to be near it without requiring it to be fenced in. This not only reduces installation costs but also increases efficiency as there is no need for the robot to stop or slow down when the operator enters its workspace, which would be the situation with ordinary industrial robots in a cage.

Judiciously employing the active compliance capability of the collaborative robot also lessens requirements for jigs and fixtures, as illustrated by the elimination of the jig in Figure 3(b). This lowers tooling costs and enhances the flexibility of the disassembly cell, helping it economically to deal with a wider range of EoL products than a system equipped with an ordinary industrial robot would be capable of.

Finally, the demonstration cell should be seen as only the first step towards realising a practical collaborative disassembly system, which would need additional functionalities such as the ability to undo screws and bolts and remove snap-fit and spring elements.

Footnotes

Appendix 1

The collaborative water pump disassembly operations described in section ‘Implementation and case study’ can be viewed at https://www.youtube.com/watch?v=hgEeY_gwsG0.

Acknowledgements

We thank Allan Tymen, Romain Delgres, Thibaud Thomas, Alix Le Quere, and Olivier Kabar from the Université de Brest for their contributions to the disassembly cell construction and demonstration.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was supported by the EPSRC (Grant No. EP/N018524/1) and the National Science Foundation of China (Grant No. 51775399).

ORCID iDs

Jun Huang

Yongjing Wang

Wenjun Xu

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