Design and Prototyping of a Collaborative Station for Machine Parts Assembly

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper presents a collaborative robotic station design and prototype that could replace manual assembly operations. While this is a very important topic, there may be a shortage of new, useful and factual knowledge that a reader could gain from the present form of the paper. How are the effectiveness, speed, cost, precision, etc. of the assembly operations justifiably influenced by the introduction of the robotic system? 

Major concerns/suggestions:

-What is the novelty in this work? Where does it stand in relation to the state-of-the-art? Are the assembly planning, gripping and performance assessment methods used comparable to the state-of-the-art?

-The presented outcomes and discussions about the robotic assembly operations hardly provide knowledge that can be used or reproduced by the journal community. It would be more useful if a generalized paradigm, that improves on present methodologies, is given for the use of cobots for assembly operations.

-The experimental analysis (which revalidates a known cobot as a cobot) was not actually significant to the specific assembly operations outlined in the paper. One will expect the experimental studies here to focus on quantitative/qualitative assessments of how well the cobot carries out the assembly operations; preferably comparing results of different assembly sequences.

 

Minor comments:

-General writing problems exist, where consecutive statements are not logically connected very effectively.

-Assembly 1 needs to be used consistently. Is it assembly 1 or component 1?

-The variation in the time taken for assembly should also be given, along with any other metric that could be used to assess the performance of the assembly operations.

 

Comments on the Quality of English Language

-some spelling/grammar issues:

in line 19 “recently” is spelled wrong,

 in line 44, use of “face off” may be wrong as it means some sort of confrontation

in line 94, “… by human operator manually” has redundant use of human and manual

 

in line 151, use of “tolerated diameter”

There could be additional minor language problems that need revision.

Author Response

First of all, the authors thank the Reviewer for his suggestions. The paper has been carefully checked and thoroughly revised according to the Reviewer's requests.

Comments 1: What is the novelty in this work? Where does it stand in relation to the state-of-the-art? Are the assembly planning, gripping and performance assessment methods used comparable to the state-of-the-art?

Response 1: The authors thank the Reviewer for his suggestions. The following parts have been added to the paper to contextualize the research activities performed by the authors in this work:

Introduction

The aim of the present paper is to investigate a collaborative and flexible assembly station, designed to handle the tolerated assembly of multiple machine components by using a single gripper and by sharing the workspace with human personnel. Here, the presence of the operator is necessary to perform the final testing of the assembled components, which is essential to ensure proper mounting in the associated machine. In other words, while the cobot handles all the heavy and repetitive tasks, the operator adds value to the process with his experience and insight.
Collaborative robots were used in previous works for assembly applications. In [21], the authors programmed a dual-arm cobot to assemble the electrical components of a PCB board by using a Time Petri Net to schedule the automated activities, whereas the human-robot interactions were handled by means of a depth camera exploited to acquire the positions of the operator’s hands over time. A similar solution was developed in [22], where a depth sensor, capable of recognising body gestures via a skeleton tracker application was integrated in the station to prevent contacts between the operator and the industrial robots. Collaborative machine parts assembly was investigated also in [23]: here, the authors used a cobot equipped with a custom gripper to screw the bolts of a car wheel. Finally, collaborative robots admittance control was exploited in [11] to obtain the collaborative assembly of a homokinetic (Rzeppa) joints. Therefore, compared to the aforementioned works, the main novelty of this paper are:

• using a collaborative robot to handle tolerated assembly of multiple machine parts without the need of a tool changing system
• developing a flexible and reconfigurable station, which can be modified or expanded to process different parts
• establish a method to validate safety measures by identifying a proper set of devices.

Conclusions

This study provides three main outcomes:

• The developed solution improved the efficiency of the manual assembly processes by significantly reducing the cycle time of component 2, while meeting the requirements imposed by the company, including the manual tests and inspections performed by the operator;
• The design methodology introduced in this work, along with the tools used to verify the compliance of the adopted collaborative strategy, could be applied to other projects to certify the collaborative nature of robotic applications, even when non-collaborative robots are involved; this is anticipated to be supported by the forthcoming new version of the regulations, which is currently in development;
• The auxiliary device developed to insert the Seeger rings can be industrialized to automate the assembly of machine components.
• The development of this application demonstrates the growing interest and research in implementing collaborative solutions in the manufacturing industry.

Comments 2: The presented outcomes and discussions about the robotic assembly operations hardly provide knowledge that can be used or reproduced by the journal community. It would be more useful if a generalized paradigm, that improves on present methodologies, is given for the use of cobots for assembly operations.

Response 2: The authors agree with the Reviewer. The following part has been added to the paper to detail the design and integration methodology followed by the authors in this work:

Furthermore, the purpose of this paper is also to present a general methodology that can be used to investigate the possibility of integrating a collaborative robot into industrial processes. The three steps of the proposed method are illustrated in the flowchart shown Fig.1 and briefly explained below 

• Cobot choice: the first step of the procedure is the verification of the payload, the reachability of all areas of interest, and the cycle times. This analysis can be conducted by consulting the user manuals and through simulation, providing the most suitable cobot manufacturer and model for the application under study as an output.
• Application analysis and implementation: the intermediate step aims to verify that the collaborative robot can efficiently perform all the operations required in the application. If the result of this investigation is negative, the integration of auxiliary support systems for the cobot must be considered.
• Risks analysis and safety measures validation: finally, in the last step, the risks arising from the application - and from the auxiliary systems in particular - are analyzed and evaluated. If the presence of the operator in the shared workspace of the robot is one of the project requirements, appropriate strategies must be adopted to mitigate any risks highlighted by the analysis in accordance with current regulations. Then, the chosen safety measures must be experimentally validated with appropriate and certified tools.

The assembly station presented in this work was designed according to the aforementioned procedure, as it will be detailed in the following sections of the paper. However, as stated before, the workflow and its tools can be generally applied to cobots integration analysis regardless of the application under study. As a matter of fact, the methods as well as the hardware and software tools used by the authors represent a workbench which can be exploited to overcome the challenges associated with using collaborative robots in industrial processes

Figure 1 has also been added to the paper to show the flux diagram associated to the presented methodology. Moreover, the following sentence has been added to the Conclusions.

• The design methodology introduced in this work, along with the tools used to verify the compliance of the adopted collaborative strategy, could be applied to other projects to certify the collaborative nature of robotic applications, even when non-collaborative robots are involved; this is anticipated to be supported by the forthcoming new version of the regulations, which is currently in development;

Comments 3: The experimental analysis (which revalidates a known cobot as a cobot) was not actually significant to the specific assembly operations outlined in the paper. One will expect the experimental studies here to focus on quantitative/qualitative assessments of how well the cobot carries out the assembly operations; preferably comparing results of different assembly sequences.

Response 3: The aurhors thank the Reviewer for his suggestion. The article has been deeply revised to clarify the significance of the tests.

The experimental analysis had the following purpose. The robot has been programmed to perform the assembly at the highest possible speed in order to reduce cycle time and increase process efficiency. Since the manual testing area is located within the robot's workspace, there is a possibility of impacts with the operator when the robot places the finished parts in the unloading zone. Therefore, it is necessary to identify the maximum speed at which the robot can move when the operator is in the shared area, to prevent any potential contact from being too forceful, in accordance with the force and pressure limits imposed by current regulations. The tests, repeated at different speeds, are thus intended to experimentally measure a safety threshold, expressed in terms of the robot's linear and joint speed, to be incorporated into the station's logic as required by the Power and Force Limitation collaborative strategy. In other words, the purpose of the test was to validate the collaborative nature of the application.

Regarding the experimental investigation of the robot's ability to perform assembly processes, the preliminary tests detailed in the second section were aimed at determining which tasks could be performed by the cobot and which would require the development of auxiliary systems. Tests for gripping, manipulating, and inserting rings were also conducted to identify the limits of performance for a collaborative robot.

Regarding the assessments of how well the cobot carries out the assembly operations, the following sentences have been added to the paper:

The cycle times for assembling the two components are as follows:

• For component 1: 14 seconds, compared to 15 seconds for the manual process.
• For component 2: 93 seconds, compared to 2 minutes for the manual process.

Moreover the following part has been added to the conclusions:

From a qualitative perspective, a rigid automation system based on dedicated machines would certainly be capable of performing the same assembly operations with very high performance. However, the components under study are characterized by medium-low and discontinuous production rates; additionally, the company has expressed the need for a flexible and reconfigurable assembly station that can be quickly adapted to assemble different components with simple modifications to auxiliary systems and gripping tools — tasks that the company, experienced in the production of precision mechanical components, can carry out independently once trained in cobot programming. These reasons underpin the choice to use a collaborative robot instead of rigid machines.

Comments 4: Assembly 1 needs to be used consistently. Is it assembly 1 or component 1?

Response 4: The authors agree with the Reviewer. The paper has been thoroughly modified so that each component is identified by a unique name that is repeated consistently throughout the text, captions, and figures. Moreover, the following sentence has been added to the paper:

In the following, the machine components which are object of this study will be referred as component 1 and component 2, whereas the term 'part' will be generally used to indicate the respective sub-components such as bearings, Seeger rings and metal cases.

Comments 5: The variation in the time taken for assembly should also be given, along with any other metric that could be used to assess the performance of the assembly operations.

Response 5: The authors thank the Reviewer for his suggestion. The following sentence has been added to the paper:

The cycle times for assembling the two components are as follows:

• For component 1: 14 seconds, compared to 15 seconds for the manual process.
• For component 2: 93 seconds, compared to 2 minutes for the manual process.

Comments 6: some spelling/grammar issues:

in line 19 “recently” is spelled wrong,

 in line 44, use of “face off” may be wrong as it means some sort of confrontation

in line 94, “… by human operator manually” has redundant use of human and manual

in line 151, use of “tolerated diameter”

Response 6: The authors thank the Reviewer for his notifications. The text has been edited according to the Reviewer's suggestions.

Reviewer 2 Report

Comments and Suggestions for Authors

The work is a practical contribution to the implementation of cobots in a work-station, and is therefore considered to be of interest in the implementation of this technology. It is recommended to take into account the following observations:

1. The meaning of SME remains to be defined

2. The images in Figure 2a are not shown.

3. Reference Figure 8 in the text.

4. Add the units to the axes in Figure 10.

Author Response

First of all, the authors thank the Reviewer for his suggestions. The paper has been carefully checked and thoroughly revised according to the Reviewer's requests.

Comments 1: The meaning of SME remains to be defined

Response 1: The authors thank the Reviewer for his suggestion. The abstract has been modified as follows:

The collaboration between humans and machines is the core of the Industry 5.0 paradigm and collaborative robotics is one the most impacting enabling technologies for Small and Medium Enterprises, SME

Comments 2: The images in Figure 2a are not shown.

Response 2: The authors thank the Reviewer for his notification. Figure 2 has been edited according to the Reviewer suggestion.

Comments 3: Reference Figure 8 in the text.

Response 3: The authors thank the Reviewer for his suggestion. Figure 8 - Figure 10 in revised version of the paper - has been referenced in the text.

Comments 4: Add the units to the axes in Figure 10.

Response 4: The authors thank the Reviewer for his notification Figure 10 - Figure 13 in revised version of the paper -  has been edited according to the Reviewer suggestion.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript proposed the development of human and robot collaboration in machine part assembly.

In the introduction, what does it mean by “Cobots can create smooth, strong virtual surfaces and other haptic effects within a shared human/cobot workspace”?

There is no literature review section in the manuscript. How do the authors contextualize this research? What are the research gaps in the existing research?

The explanation of the assembly process should be referred to the figure. In other words, same words in the figure and manuscripts should be used, so that readers can understand easily. For example. “cone” was used in manuscript, but there is no “cone” in the figure. Is it refer to conical element? If yes, use the same words.

Figure 8: Instead of showing the sensors used, please show where these sensors are installed.

Please revise the sentence in Line 384. “Thanks to these sensors, it was possible to collect data on the pressure and force developed during the simulation of an operator impact.” It is not appropriate to use “Thanks to these sensors” in a manuscript.

Please use figure to explain the two types of collaborative tests mentioned in Section 4.3. What is the purpose of conducting test? When or what circumstance during the assembly that these tests are needed? It seems just to check whether the application is collaborative. Why the collaborative assessment is necessary?

Please use the terms that are commonly used in research manuscript.

Comments on the Quality of English Language

The quality of the English language in this text is generally understandable. However, there are a few awkward sentence structures that could be improved for better readability.

Author Response

First of all, the authors thank the Reviewer for his suggestions. The paper has been carefully checked and thoroughly revised according to the Reviewer's requests.

Comments 1: In the introduction, what does it mean by “Cobots can create smooth, strong virtual surfaces and other haptic effects within a shared human/cobot workspace”?

Response 1: The authors thank the Reviewer for his question. The sentence was not of significant importance to the article and has been consequently removed.

Comments 2: There is no literature review section in the manuscript. How do the authors contextualize this research? What are the research gaps in the existing research?

Response 2: The authors thank the Reviewer for his questions. References 21-23 have been added to contextualize the research activites described in the paper. Moreover, the following paragraph has been added to the Introduction:

Collaborative robots were used in previous works for assembly applications. In [21], the authors programmed a dual-arm cobot to assemble the electrical components of a PCB board by using a Time Petri Net to schedule the automated activities, whereas the human-robot interactions were handled by means of a depth camera exploited to acquire the positions of the operator’s hands over time. A similar solution was developed in [22], where a depth sensor, capable of recognising body gestures via a skeleton tracker application was integrated in the station to prevent contacts between the operator and the industrial robots. Collaborative machine parts assembly was investigated also in [23]: here, the authors used a cobot equipped with a custom gripper to screw the bolts of a car wheel. Finally, collaborative robots admittance control was exploited in [11] to obtain the collaborative assembly of a homokinetic (Rzeppa) joints.

Comments 3: The explanation of the assembly process should be referred to the figure. In other words, same words in the figure and manuscripts should be used, so that readers can understand easily. For example. “cone” was used in manuscript, but there is no “cone” in the figure. Is it refer to conical element? If yes, use the same words.

Response 3: The authors thank the Rewiever for his suggestion. Figure 2, 3, 6, 7, 8, 9 have been deeply edited to comply with the descriptions provided in the text. Moreover, The paper has been thoroughly modified so that each part of the assembly processes is identified by a unique name that is repeated consistently throughout the text, captions, and figures. For instance, the term "cone", used in the manual assembly process description, has been replaced by "conical interface". The same term has been used in the corresponding figure 2 as well as in its caption.

Comments 4: Figure 8: Instead of showing the sensors used, please show where these sensors are installed.

Response 4: The authors agree with the Reviewer. We added this sentence to the caption of figure 11 to detail where the sensor is positioned to simulate an impact with the operator's back. 

Robotic station setup during the collision tests. The GTE Cobosafe sensor is positioned between the robot and the unloading area to simulate an impact with the operator's back. 

Comments 5: Please revise the sentence in Line 384. “Thanks to these sensors, it was possible to collect data on the pressure and force developed during the simulation of an operator impact.” It is not appropriate to use “Thanks to these sensors” in a manuscript.

Response 5: The authors thank the Reviewer. The sentence has been modified as:

By means of these sensors it was possible to collect data on the pressure and force developed during the simulation of an operator impact

Comments 6: Please use figure to explain the two types of collaborative tests mentioned in Section 4.3. What is the purpose of conducting test? When or what circumstance during the assembly that these tests are needed? It seems just to check whether the application is collaborative. Why the collaborative assessment is necessary?

Response 6: The authors thank the Reviewer for his suggestion. The tests have the following purpose. The robot has been programmed to perform the assembly at the highest possible speed in order to reduce cycle time and increase process efficiency. Since the manual testing area is located within the robot's workspace, there is a possibility of impacts with the operator when the robot places the finished parts in the unloading zone. Therefore, it is necessary to identify the maximum speed at which the robot can move when the operator is in the shared area, to prevent any potential contact from being too forceful, in accordance with the force and pressure limits imposed by current regulations. The tests, repeated at different speeds, are thus intended to experimentally measure a safety threshold, expressed in terms of the robot's linear and joint speed, to be incorporated into the station's logic as required by the Power and Force Limitation collaborative strategy.

Then, the following sentences has been added to the paper to clarify the purpose of the tests

Following the flux diagram of Fig. 1, a Risk Assessment was also performed to prevent injuries to the operator and ensure that the whole application is collaborative. Since the area dedicated to the manual inspections and final tests lies inside the cobot workspace, safety measures must be implemented to mitigate the risks due to unexpected collisions between the robot and the operator. 

In order to prevent injuries to the operator, the station logic integrates the Power and Force Limitation collaborative strategy. Particularly, when the operator moves inside Area 1, the system reduces the robot's speed below a safe threshold which will be measured in the following subsection.

By means of these sensors, it was possible to collect data on the pressure and force developed during the simulation of an operator impact. The tests were repeated for different robot speed to identify a safe threshold according to the limits specified in ISO/TS 15066:2016 in terms of maximum contact force and pressure

The results of the tests carried out are shown in the table 2. The values reported are below the limit values indicated by the standard. The joint and linear speed set during the tests of Fig 12-13 and table 2 were then set as the safe threshold in the station logic according to the Power and Force Limitation collaborative strategy. Therefore, the station's work cycle complies with the regulations on human-machine collaboration and the application can be classified as collaborative.

Comments 7: Please use the terms that are commonly used in research manuscript.

Response 7: The authors thank the Reviewer for his suggestion. The paper has been carefully checked to remove terms not suitable for a scientific publication

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors addressed significant parts of the comments and concerns raised.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have thoroughly addressed all the comments.

Comments on the Quality of English Language

The English is clear and understandable.