University of Birmingham A strategy for human-robot collaboration in taking products apart for remanufacture

Using humans and robots working together to disassemble end-of-life (EoL) products so that they can be remanufactured enables catering for the effects of uncertainties in the condition of those products and unpredictability in remanufacturing operations. Collaborative robots are one of the key elements of Industry 4.0. This paper presents a strategy for human-robot collaborative disassembly that exploits the active compliance control capability of collaborative robots. The paper outlines the proposed human-robot collaboration strategy and active compliance control. It then presents a case study of robotic disassembly designed to show a practical implementation of the proposed strategy to take products apart. The study involved the dismantling of an automotive water pump by a collaborative industrial robot working with a human operator and by two collaborative robots and a human operator. Active compliance control was implemented to separate press-fitted components from the water pump. The results demonstrate the feasibility of the strategy and its effectiveness at increasing flexibility and adaptability by reducing setup efforts.


INTRODUCTION
Remanufacturing is increasingly important to delivering economic and environmental benefits in a circular economy [1][2][3][4].Autonomous remanufacturing reduces costs and makes remanufacturing viable for a wider sector of industry.Disassembly automation is critical to enabling autonomous remanufacturing [5,6].The main technical barrier to autonomous disassembly comes from uncertainties in the condition of returned EoL products and the associated unpredictability in remanufacturing operations [7][8][9].Semi-autonomous disassembly using human-robot collaboration (HRC) could provide the flexibility needed to handle such uncertainties and unpredictability [10].In human-robot collaborative disassembly, robots and human operators work side by side to perform joint or independent disassembly tasks without traditional safety guarding.
There has not been much reported research into using HRC in disassembly.A systematic framework for the implementation of human-robot collaborative disa-ssembly was presented for sustainable manufacturing [11].A workstation employing a collaborative robot as an assistant for a human operator was presented for the disassembly of electric vehicle batteries [12,13].In the workstation, the human operator performed complex tasks, and the robot handled simple and repetitive tasks.Collaborative robots were applied to classify and dismantle e-waste [14].In a common workspace, humans identified complex materials and components, and robots carried out heavy and dangerous tasks.An informed software agent was developed for HRC to increase efficiency and improve ergonomics in disassembly processes [15].A method to assess manufacturing capability was investigated for HRC disassembly [16].A flexible multi-sensorial system was proposed for robotic disassembly using collaborative robots [17].The system was validated on the disassembly of CD drivers, battery covers and electronic circuits.In addition, human robot collaboration was implemented in the assembly of aircraft structures to optimise aircraft production [18].
Active compliance control can enable collaborative robots to avoid colliding with and injuring humans.It gives robots the flexibility and adaptability needed to achieve complex tasks [19].For instance, to handle workpiece variances, active compliance control was implemented in industrial processes such as grinding, polishing and deburring.Active compliance control was also employed for industrial finishing operations using HRC with a collaborative robot [20].To handle uncertainties in the environment and workpieces, active compliance control was used for surface-surface alignment tasks [21].Active compliance control was applied to enable a robot to assist surgeons in the execution of knee surgery resections [22].
This paper presents a strategy suitable for HRC disassembly in remanufacturing.The proposed strategy uses the active compliance control facility of the robot to achieve complex disassembly tasks.The paper is organised as follows.Section 2 introduces the state of the art in HRC disassembly.An outline of active compliance control follows in Section 3. Section 4 describes a case study showing an implementation of the strategy.Section 5 concludes the paper.

HRC DISASSEMBLY
Currently, most disassembly operations in remanufacturing are performed manually, which results in low efficiency and high labour cost [10].Although fully automated disassembly could deal with large volumes, it lacks flexibility and requires high capital investment.Fully automated disassembly is unable to handle the uncertainties and variances associated with EoL products and disassembly processes [23,24].HRC disassembly, a semi-autonomous disassembly method, has greater potential to afford both efficiency and flexibility.In HRC disassembly, robots and other ancillary equipment carry out tasks that are repetitive, heavy and dangerous, leaving people to assume a supervisory role or perform work that requires greater dexterity, flexibility or cognitive ability than robots or machines can provide.In other words, the strength, precision and repeatability of robots and human know-how, manual flexibility and dexterity could be combined.In that way, human operators can handle the uncertainties and variances associated with EoL products and disassembly operations with lower equipment investment compared with fully automated disassembly.Figure 1 illustrates an example of a HRC disassembly cell.Without physical barriers such as safety cages, the cell enables human operator and robots to work together and interact safety in a shared workspace to perform their tasks jointly or independently.
To achieve HRC, robot manufacturers have produced collaborative robots ("cobots") capable of working alongside people without having to be fenced in safety cages.Table 1 lists five representative industrial collaborative robots on the market and their specifications.In addition, AURA from COMAU, as one of the largest cobots, has a payload of 170 kg and a reach of 2.8 m [26].The International Federation of Robotics reported that cobots were an important element of Industry 4.0 and were expected to account for 34% of all robot sales on the market by 2025 with an annual demand growth of 60% [27].Cobots provide manufacturers with more flexible automation solutions which can easily and quickly be adapted to meet shifting demands with an economic investment cost.The combination of humans' experience and cognitive abilities and robots' strength, dependability, reach and endurance will significantly facilitate applications of human-robot collaboration [28].

ACTIVE COMPLIANCE CONTROL
Compliance helps robots to mitigate the effects of position and velocity errors and handle uncertainties in the location, size and shape of the workpiece [29].Robot compliance can be active or passive [30].Table 2 lists the key features of active and passive compliance [31,32].Active compliance is software-based.Therefore, it is more easily adapted to different situations and was adopted for this application.
Active compliance control enables robots to interact with human operators safely and achieve complex tasks such as assembly and disassembly [33].For example, the end effector of a compliant robot can move to different positions and exert different forces on an object depending on external conditions.
Hybrid position/force control and impedance control are two approaches to achieving active compliance control [21].Hybrid position/force control combines force and torque information with positional information to satisfy simultaneous position and force trajectory constraints in a convenient task-related coordinate system [34].Impedance control is adopted to produce a certain desired dynamic behaviour of the robot when it interacts with the environment.Stiffness and damping coefficients are important parameters in active compliance control.Having a controllable stiffness enables a collaborative robot to perform precise tasks with high stiffness (in the stiff mode) and safely collaborate with humans with low stiffness (in the compliant mode).In the compliant mode, the robot can avoid hard collisions with humans by reducing impact forces during physical interaction [35].As shown in Figure 2, parts or components (such as bearing-housing, and bearing-shaft) are often assembled with an interference fit by a press force.To disassemble the press-fitted components, a press can be employed used to apply a force on one of the components.With active compliance control, collaborative robots can be used to hold the component to be pressed out during the disassembly process and follow its movement until it is separated completely, releasing human operators for other disassembly tasks.

CASE STUDY
To demonstrate the proposed strategy and method for human-robot collaborative disassembly, a case study was conducted in the authors' Autonomous Remanufacturing Laboratory.The case study involved the robotic disassembly of an automotive water pump.The press-fitted components in the water pump were separated by HRC using collaborative robots with active compliance control.

HRC disassembly cells
As shown in Figure 3, the first HRC disassembly cell consists of a collaborative robot (KUKA LBR iiwa 14 R800) fitted with a two-fingered gripper (Robotiq 2-FINGER 140), a small manual press, the press operator, the water pump to be disassembled plus positioning jigs and part collection boxes located on a workbench.The robot can work under impedance control using a Cartesian impedance controller with configurable stiffness and damping.The configurable spring stiffness ranges for the translational and rotational degrees of freedom are 0 -5000 N/m and 0 -300 Nm/rad, respectively [36].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.The water pump was a Unipart GWP187.As illustrated in Figure 3, the pump comprises a flange, a pump casing and a rotor subassembly, which contains a bearing, a shaft, a seal unit, and an impeller.The material of the flange, shaft and bearing is steel.The material of the pump casing is aluminium alloy.The fit between the flange and rotor and between the pump casing and rotor subassembly is press fit.
Figure 4 shows another HRC disassembly cell with two industrial collaborative robots (KUKA LBR iiwa 14 R800).Robot-1 is equipped with a two-fingered gripper (Robotiq 2-FINGER 140).Robot-2 has a three-fingered gripper (Robotiq 3-FINGER adaptive robot gripper).The disassembly tasks were assigned among Robot-1, Robot-2 and a human operator.The proposed strategy and method of taking press-fitted components apart was implemented and tested in the above two robotic disassembly cells.

Cell HRC disassembly tasks
The separation of the rotor subassembly from the flange and the pump casing is one of the collaborative tasks in the disassembly of the water pump.As illustrated in Figure 3 and Figure 5, the collaborative disassembly task involved 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 details of this separation procedure are illustrated in Figure 5 and Figure 6. Figure 5(a) shows the robot gripper at a set position under the water pump as the robot waits for the rotor subassembly to be pushed out of the flange and the pump casing.When the robot detects an increase in the vertical component of the force on its gripper caused by the rotor subassembly touching it (as shown in Figure 5(b)), the robot moves the gripper down slightly to be clear of the rotor subassembly and then opens the fingers (as shown in Figure 5(c)).Figure 5(d) depicts the gripper moved back up to grasp the rotor subassembly.In Figure 5(e), the robot has taken hold of the rotor subassembly and turned on the compliant mode to enable the gripper passively to follow the downward movement of the rotor subassembly.When the rotor subassembly has been separated completely as shown in Figure 5(f), leaving the flange resting on the casing in the press, there is a step change in the vertical force on the gripper due to the weight of rotor subassembly, 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.
The related robot control program is shown in Table 3 for separating the rotor subassembly from the flange and the pump casing.The program is written in Java using the KUKA Sunrise Workbench [36].Class "HRCAB" (line 1) describes the separation procedure and the contact force is set in class "waitingforce" (line 6).Class "newstiffness" (line 11) is designed to turn the robot compliance on and off according to the position feedback information during the disassembly process.Table 4 lists the values of translational stiffness and rotational stiffness of the robot during the disassembly process shown in Figure 5.

Results and discussion
The procedure described in the previous section was carried out in the demonstration cell shown in Figure 3. Figure 7 presents selected image frames captured from the process of separating the rotor subassembly from the flange and the pump casing.
The robot lifts the water pump and places it in the press fixture (Figure 7(a)).Once the pump is located and released, the robot positions the gripper below the fixture (Figure 7(b)) and waits for the press operator to extract the rotor subassembly from the flange and pump casing.
As the rotor subassembly is pressed out, it eventually contacts the gripper (Figure 7(c)), signalling to the robot to continue the disassembly operation.The robot lowers the gripper, taking it away from the rotor subassembly (Figure 7(d)), and then opens its fingers (Figure 7(e)).Next, the robot moves upwards before closing the gripper fingers to grasp the rotor subassembly (Figure 7(f)).As the press continues to force the rotor subassembly downwards, the robot holds on to it and compliantly tracks its motion.When the rotor subassembly is fully released, the robot takes control (Figure 7(g)), moving it away from the press.The robot finishes the disassembly operation by placing the subassembly on the workbench fixture (Figure 7(h)).
Figure 8 shows the HRC disassembly process for the water pump using the two collaborative robots in the second demonstrating cell (Figure 4).Preliminary trials show that the proposed HRC disassembly method is feasible.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.

CONCLUSIONS
In remanufacturing, there are uncertainties due to unpredictability in the condition of the returned EoL products and variance in remanufacturing processes, which are main barriers to achieving autonomous disassembly.HRC disassembly as a semi-autonomous solution is aimed at achieving the flexibility and adaptability needed for cost-effectively dealing with those uncertainties.This paper has presented a strategy for HRC disassembly using active compliance control of collaborative robots.The collaborative robot uses its compliance facility to assist the human operator with complex disassembly tasks.
A case study has been carried out to demonstrate the successful applications of the proposed strategy and method in two HRC disassembly demonstration cells.The case study involved the disassembly of press-fitted components (the rotor subassembly, the flange and the pump casing) from an automotive water pump by a collaborative industrial robot working together with a human operator and by two collaborative robots and a human operator without requiring special safety measures.

Figure 3 .
Figure 3. First HRC disassembly demonstration cell with a collaborative robot.

Figure 4 .
Figure 4. Second HRC disassembly demonstration cell with two collaborative robots.

Figure 5 .
Figure 5. Procedure of separating rotor subassembly from flange and pump casing.

Figure 7 .
Figure 7. Separation of the rotor subassembly from the flange and pump casing.

Figure 8 .
Figure 8. HRC disassembly of the water pump using two collaborative robots.

Table 3 . Robot control program. Input: external
force by contact on robot manipulator Output: finishes wait state, grasp and follow the movement of the rotor subassembly