Automated technique for high-pressure water-based window cleaning and accompanying parametric study.

The maintenance of buildings has become an important issue with the construction of many high-rise buildings in recent years. However, the cleaning of the outer walls of buildings is performed in highly hazardous environments over long periods, and many accidents occur each year. Various robots are being studied and developed to reduce these incidents and to relieve workers from hazardous tasks. Herein, we propose a method of spraying high-pressure water using a pump and nozzle, which differs from conventional methods. The cleaning performance parameters, such as water pressure, spray angle, and spray distance, were optimized using the Taguchi method. Cleaning experiments were performed on window specimens that were contaminated artificially. The cleaning performance of the proposed method was evaluated using the image-evaluation method. The optimum condition was determined based on the results of a sensitive analysis performed on the image data. In addition, the reaction force due to high pressure and impact force on the specimens were investigated. These forces were not sufficient to affect the propeller thrust or cause damage to the building's surface. We expect to perform field tests in the near future based on the output of this research.

1. Introduction

With the development of architectural technology, many high-rise buildings have been constructed worldwide in recent years. As a result, maintenance tasks such as the cleaning of outer walls present problems with regard to cost and worker safety. The working environment is exceptionally hazardous because workers are suspended either on gondolas or by ropes from very tall buildings.

Many exterior wall-cleaning robots have been studied [1-7] and developed to reduce the cost of cleaning and to relieve workers from hazardous tasks. For example, CSIC’s Tito [8], IPC Eagle’s HighRise [9, 10], and SkyPro [11, 12] are commercially available with certain product lineups, as shown in Fig 1. These robots are transported up and down buildings by using cables and winches. These commercial robots use typical equipment to clean the exterior walls of buildings, such as water nozzles, squeegees, and brushes [8-12]. Although these devices are highly effective for cleaning, their use results in uncleaned zones. In the process of traversing optimized moving paths, which prevent collision of the devices with obstacles, uncleaned zones are generated near obstacles such as façades. Another popular robot is Gecko [13-15], which was developed using a highly effective vacuum suction pad [16-18]. However, the applicability of these robots are limited owing to the shapes of the buildings and the decorative fittings on them. The motion speeds are extremely low when vacuum pads are used; therefore, long cleaning times result. In addition, there are limitations on the payload required for the cleaning device.

Fig 1

Various exterior wall cleaning robots (a) Tito [8] (b) IPC EAGLE [9, 10] (c) SKYPRO [11, 12] (d) Gecko [12–15].

Several studies on high-pressure cleaning have been conducted to enhance the cleaning performance through spray optimization. Zhang reported that the shape characteristics of the nozzle can enhance the performance of high-pressure cleaning [19]. Peng analytically investigated the effect of the spray distance of the nozzle on the cleaning performance [20], and Yang studied the effect of the installation of nozzle angle on the spray pressure in the spray system [21]. A theoretical model for evaluating the optimal and critical distances for cleaning using high-pressure water was proposed by Guha [22]. Xu and Chen pioneered the studies on spray properties and their effect on nozzles at high pressures [23, 24]. Medan optimized a cleaning equipment to measure the reaction force of the spray nozzle and to determine the main factor that affects high-pressure cleaning [25]. Zhong studied surface erosion according to nozzle type [26], and Sertore examined the state of a nozzle based on the injection force measured by a load cell [27]. However, these research studies employed very high pressure for special purposes; the findings are therefore not applicable in cleaning the outer walls of buildings. This is because high pressure has certain limitations in terms of surface damage, robot orientation control, and commercialization.

To overcome these limitations, we propose a method for cleaning outer walls of buildings using high-pressure water. Unlike the conventional robot or the previously mentioned very-high-pressure method, the proposed device uses high-pressure water. It can clean over a wider range by varying the injection angle and direction of the nozzle. The robot can clean windows continuously while overcoming obstacles because this method does not use cleaning device that needs to be in direct contact with the wall for the cleaning. In addition, it does not generate uncleaned zones because the spray direction is variable. It is also possible to clean areas that are difficult to access using a brush.

The quantitative measurement of cleaning performance is also an important issue in this study. Various measurement methods have been introduced by researchers to evaluate the cleaning performance of devices [28, 29]. Kang evaluated contamination by using infrared transmissivity [30], and Moon et al. used an experimental method to evaluate cleaning performance [31].

A cleaning device that uses high-pressure water for cleaning is introduced in this study. A method to determine the optimal design parameter conditions and ensure reliable cleaning performance is also described herein. Design parameters that influence the cleaning performance of the proposed cleaning devices were selected.

The selected performance design parameters were optimized experimentally to maximize the cleaning performance. The experimental results of cleaning by high-pressure water were evaluated according to image data obtained using a digital camera. We adopted Taguchi orthogonal array for effective experimentation [32, 33]. The Taguchi method enables the optimization condition of experiment factors to realize inexpensive and rapid product design. Further, it is possible to determine more effective conditions and verify these (and thereby, improve product performance) with an identical number of experiments. The high-pressure water that the robot uses for cleaning generates reaction and impact forces. In this study, to examine the influence of the reaction force on the propeller thrust and that of the impact force on the surface of the building's exterior wall, the magnitudes of the reaction force and the impact force were measured at the nozzle and on specimen, respectively.

The remainder of this paper is organized as follows. Section 2 introduces the overall cleaning system and working environment. Section 3 presents the configuration of the proposed cleaning device. Section 4 defines the optimization problem and experimental design, including the objective function, design parameters, and user condition. Section 5 describes the results of an experiment performed with a test bench and presents a discussion. Finally, the concluding remarks are presented in Section 6.

2. Overall description of cleaning robot and devices

A robot installed in a high-pressure cleaning module is shown in Fig 2. The robot can move over the building’s surface using two ropes that are secured at the top of the building. Additional devices are installed on both edges of the building’s top to secure the ropes’ ends. This system involves simple installation and enables the operation of robots for buildings without the use of gondolas or separate winch devices. The robot uses two winches that are embedded into it. The force for attaching on to the wall is provided by the propeller. This type of robot system is relatively insensitive to the reaction force against the spray pressure and the force balance of the nozzles’ pressure. Therefore, the operational stability is higher than that of a robot that uses a suction plate. In addition, severe issues such as the occurrence of simultaneous suction of water and air, the required payload for the devices, and damages to the adsorption plate caused by foreign matter and wear do not arise. In total layout of the robot, it is advantageous on a space efficiency because of the simple structure of the cleaning unit.

Fig 2

Configuration of winch-mounted wall-climbing robot [39, 40].

The robot largely consists of three parts: a propeller thrust unit, winch unit, and cleaning and frame unit. The cleaning unit is placed at the bottom of the robot considering water leakage. a) Overview of winch-mounted wall-climbing robot (b) Assembly of the winch unit and cleaning and frame unit.

Triangular wheel sets such as star wheel of MSRox [34] (each wheel with a diameter of 150 mm) are applied for traversing on walls. These can overcome obstacles with heights less than 100 mm, such as window frames. To prevent damage to the building’s surface by slippage, wheels made of a soft material were applied. In addition, a suspension device and air-injected wheels were adopted to minimize the transmission of impact force and to protect the surface. Unlike conventional robots, this robot sprays high-pressure water continuously while overcoming obstacles. This is an important advantage for reducing the amount of uncleaned zone compared with that for existing cleaning robots.

The cleaning device can be assembled in parts or as a single device. Here, the device was assembled in two parts considering the space efficiency of the robot. For a large weight (2 kg), the pump should be placed close to the line of gravity of the robot to enhance control efficiency and thereby, minimize the influence of the moment of inertia. In addition, the cleaning unit components were placed at the bottom of the robot because of concerns of water leakage. A guide wall was installed to prevent recontamination of the cleaned area. The cleaning range can be altered by varying the angle of the replacement nozzles, which is a significant advantage with regard to mass production and robot maintenance.

To ensure effective cleaning, the robot control algorithm optimization should consider various scenarios, e.g., spray control for continuous cleaning while overcoming obstacles, movement control against the reaction force generated by the high-pressure water spray, and orientation stability control at the beginning of spray. Therefore, the capability for cleaning and for overcoming obstacles must be evaluated simultaneously. It would be more effective to test after achieving control optimization. Therefore, the optimization of the entire system would be addressed in future studies. The optimization of the cleaning performance, which is the main focus of this study, is critical for the overall system. If the cleaning performance is not satisfactory, it would be ineffective to apply the robot at cleaning sites. Therefore, it should be designed to achieve a high level of cleaning performance. In the following sections, the specifications for the design and optimization of the cleaning performance are presented in detail.

3. Specification of proposed cleaning device

The proposed cleaning device consists of nozzles for spraying high-pressure water, a pump for generating high pressure, a pressure sensor for measuring the inlet pressure of the nozzle unit, and a wall for preventing recontamination. Four nozzles are used to create a spray zone with minimal overlap. A manifold is adopted to minimize the pressure loss in each nozzle and maintain a uniform spray pressure.

The criteria for selecting the nozzle tip were based on two aspects: 1) the impact efficiency of spray water, which represents the capability for removing contaminants, and 2) the uniformity of the shape of the water spray at the nozzle tip, which determines the uniformity of cleaning performance. According to the data sheets provided by the manufacturer, the impact efficiency of the flat-type nozzle at 50° is 10%, whereas those of the full-cone and hollow-cone-type nozzles are < 1% and 1%, respectively [35, 36]. The spray shape uniformity according to the nozzle type was verified at the designated pressure. The flat-type nozzle displays higher spray uniformity with a fan shape and is the most widely used type of nozzle for cleaning. In addition, it displays a higher impact efficiency than that of the hollow-cone- or full-cone-type nozzle. Finally, the HM_V type (Hanmi Nozzle co. ltd [35]) of flat nozzle was selected, as shown in Fig 3. A single unit nozzle made of stainless steel was selected to ensure reliability.

Fig 3

A flat-type nozzle displays good spray uniformity.

In terms of impact efficiency, it displays higher cleaning performance than those of hollow-cone- or full-cone-type of nozzle. (a)–(c) are flat nozzle tips. Their spray angles are 15°, 25°, and 40° respectively, (d) spray angle of 40° at 6 bar [35, 36].

The nozzles sprinkle a specialized solution that contains an alcohol-based material to clean organic pollutants. Pulse width modulation (PWM) flow control and a solenoid valve are used to control the amount of cleaning solution applied. The water pump can discharge a maximum pressure of 10 bar. However, to ensure working stability, the applied pressure is designed to be < 80% of the maximum discharge pressure. The discharge pressure is controlled by the rotational speed of the pump. A pressure sensor is installed at the inlet of the nozzle to monitor the pressure supplied to the nozzle. It enables the accurate control of the pressure supplied to the nozzle. The pressure sensor detects the status of the pump when the water supply is intermittent or the pump does not operate. During cleaning, the sensor monitors the pressure drop of the pump. In addition, it detects the variation from the designated injection pressure when the device is overcoming a low obstacle. The measured data from the sensor is transmitted to the controller to maintain the pre-set pressure.

A guide-wall is important to prevent recontamination of the cleaned area, although this is not directly related to the cleaning performance. It is installed between the manifold. We designed the nozzle spraying direction to be tilted at an angle of approximately 10° toward the transfer direction, to minimize the splashing of the contaminant toward the cleaned area. Although the fundamental specification ranges of the cleaning appliance have been selected, the detailed design parameters must be verified experimentally as described in the following section.

4. Planning for robust optimal design and experimental setup

4.1. Control parameters and user condition

We examined the sensitivity of the performance of the high-pressure water cleaning to the following variables: nozzle inlet pressure, nozzle spray distance, and spray angle. The variables that are to be optimized to achieve the maximum performance are presented in Table 1 and are shown schematically in Fig 4. The nozzle inlet pressure is related to the flow rate as follows [21]:

Table 1

Control parameters and user condition for optimization of the cleaning device.

	Parameter		Level 1	Level 2	Level 3
Control Parameters	Nozzle inlet pressure (bar)	A	3	6	8
	Spray distance (mm)	B	200	300	400
	Spray angle (°)	C	15	25	40
User condition	Cleaning speed (m/s)	D	3	6	-

Fig 4

Control parameters (a–c) and a user condition (d). a) Nozzle inlet pressure, b) Spray distance, c) Spray angle. d) Cleaning speed of the robot [29, 37].

Here, Q, P, and n represent the flow rate, pressure, and specific gravity, respectively, of the fluid. According to Eq (1), we control the pressure rather than the flow rate, as shown in Table 1. Considering the large impact force and the most frequently used nozzle types at cleaning sites, three spray angles of flat nozzle were selected. Considering the cleaning performance, the impact force is reduced considerably at over 40°, and satisfactory cleaning performance is unlikely. The spray distance is defined as the distance between the building surface and nozzle tip. The distance is closely related to the overall layout of the cleaning robot. The space between the outer wall and nozzle tip as well as the cleaned area are closely related to the robot’s structure. In addition, the distance is a factor that determines the maximum height of obstacles that can be overcome. It was determined to be 200–400 mm considering the performance and the robot’s layout.

The cleaning speed is a user condition that determines user operating speeds. Therefore, two levels of descending speed of the robot (3 and 6 m/min) were selected to verify the best and worst parametric combinations, respectively, as shown in Table 1 and Fig 4.

4.2. Evaluation of cleaning performance

All the parameters should be optimized to maximize the cleaning performance. Therefore, it is important to design and perform an evaluation of the cleaning performance. Although many researchers have recommended methods for measuring and evaluating the contamination on the exterior walls of buildings, there are no international or domestic standard. In addition, there is no standard definition of the cleanliness of exterior walls. The perspectives differ among individuals and industries. Therefore, an image processing method [29, 38] involving the acquisition of photographs before and after cleaning was used for a quantitative evaluation of the cleaning performance in this study.

Fig 5 shows the setup for evaluating the cleaning performance, as well as the result sample. Photographs were captured before and after cleaning by using the same camera, which was set at a constant height and with constant illumination using an aluminum frame. Thus, the post-cleaning data were obtained (as shown in Fig 5C), and the cleaning performance was expressed as the ratio of the area of contamination before cleaning to that after cleaning, after the two photographs were filtered to a certain threshold value. Freeware was used to evaluate the area on which dust remained after the test [38].

Fig 5

Device setup for the image data measurement and conversion to image data from the test results.

a) Device setup to capture photographs for the image data, b) A test result photograph after cleaning, c) Image data converted from the photographs [29, 38].

4.3. Objective function and experimental design

The selected design parameters are large-the-better characteristic for the cleaning performance. Because a higher cleaning performance would be obtained when a wider area is cleaned, the signal-to-noise ratio (SNR) was applied as shown in Eq (2):

Here, y represents the measurement data, and n represents the number of data. Taguchi orthogonal matrices [32, 33] provide a highly popular method to design and perform experiments using standardized arrays. These special orthogonal arrays define the minimum number of experiments that could yield the sensitivity of all the factors that affect the performance. We decided to use the L9(33) of the orthogonal array on the basis of the three levels of design variables. The experimental sequence and the combination of variables are presented in Table 2. Two levels of user conditions (device cleaning speeds of 3 and 6 m/min) were considered, as shown in Fig 4.

Table 2

L9(33) orthogonal array and evaluation results of image data.

Exp.#	Target function variables			Descending speed = 3 m/min			Descending speed = 6 m/min			SNR
	A	B	C	Panel 1	Panel 2	Panel 3	Panel 1	Panel 2	Panel 3	(dB)
	(Level)	(Level)	(Level)	(%)	(%)	(%)	(%)	(%)	(%)
1	1	1	1	12.89	10.53	10.11	0.13	0.26	0.14	-12.98
2	1	2	3	52.30	43.93	40.41	27.82	41.84	37.92	31.70
3	1	3	2	31.22	28.67	24.01	9.75	1.79	7.95	12.44
4	2	1	3	36.20	32.72	41.29	10.66	19.85	5.28	20.83
5	2	2	2	28.50	21.16	31.46	2.41	1.87	5.85	10.87
6	2	3	1	1.18	1.98	0.67	0.04	0.92	2.43	-19.00
7	3	1	2	72.80	74.11	72.60	23.55	45.47	43.60	32.50
8	3	2	1	0.85	2.17	2.75	0.21	2.73	1.94	-6.12
9	3	3	3	98.12	70.58	91.23	4.00	28.48	48.79	19.69

4.4. Test bench configuration and experimental setup

The test bench was designed with two parts: a water spray unit and a cleaning-performance evaluation (Fig 6). A water spray unit consists of a load cell, a pressure sensor, and a nozzle device assembly. In addition, a cleaning-performance evaluation unit has a window frame with a load cell and transfer devices with two axes. In the water spray unit, the nozzle feed pressure was the control factor. The pressure was varied by modifying the rotational speed of the pump. In addition, a pressure sensor monitored the nozzle inlet pressure. Although the nozzles used in the experiment had identical orifice shape, their spray angles were different [35]. The cleaning-performance evaluation unit was designed with a distance-adjustment device for evaluating the cleaning performance according to the distance between the nozzle and panel. In addition, the speed of motion of the panel was considered as a user condition. Two load cells were applied: One was used to measure the reaction force from the high pressure on the nozzle inlet, and the other was located on the back of the window panel and was used to identify glass damage.

Fig 6

The test bench consists of two parts: A cleaning performance evaluation unit for investigating spray characteristics and a nozzle spray unit for implementing movement of the robot for cleaning.

Each unit has a load cell to measure the reaction force and impact force. (a) Overall schematic of test bench and that of (b) the water spray unit.

One of the important factors affecting the cleaning performance is the pressure. The rotational speed of the pump was adjusted to three values (as shown in Table 2), and a pressure sensor was installed in the nozzle inlet to measure the supplied pressure. A tube with an outer diameter of 12 mm and specially manufactured nipples were used to minimize the pressure drop of the pipeline. In addition, manifolds were employed to enable convenient nozzle-tip replacement, achieve a uniform pressure, and minimize the influence of the pressure-sensor mounting, as shown in Fig 7.

Fig 7

Experimental test bench setup for evaluating the cleaning performance.

a) Test scenario with high-pressure water cleaning, b) Cleaning performance evaluation unit and transfer devices in two directions (X, Y). c) Water-spray-unit assembly. Specifications of main parts: Pump: 8905-902-290(SHURflo), Pressure sensor: GP-M025, (KENYCE), Load cell: BCA-5(CAS), Nozzle: Flat type, HM_V5 (Hanmi Nozzle.co.Ltd) [35].

5. Test results and discussion

5.1. Sensitivity analysis and results review

The orthogonal array and the experimental results are summarized in Table 2. As shown on the right part of the table, to obtain the image data result for each plate, the area cleaned by the high-pressure water was calculated by comparing the window surfaces before and after cleaning. The SNR was calculated using the experimental data via Eq (2). As indicated by the test results in Table 2, the cleaning performance achieved with a high pressure was better than that achieved with a low pressure, and the cleaning performance at a low descending speed was better than that at a high descending speed. However, the robot's descending speed was not more sensitive than nozzle inlet pressure, spray distance, and spray angle, as shown in the experimental results in Fig 9G–9L.

Fig 9

Test results for the highest and lowest cleaning performances.

The highest performance results were those of Exp.#7 (Table 2) for the user condition of 3 m/min, as shown in a)–c). d)–f) display the results for 6 m/min. The worst performance result was for Exp. #6 under both the user conditions (3 mm/s and 6 mm/s).

The result of the sensitivity analysis of the selected design variables is shown in Fig 8. This is calculated via Eq (2). The optimal combination of the design variables was determined as follows: nozzle inlet pressure = 8 bar, spray distance = 0.2 m, spray angle = 40°, and transfer speed of the specimen = 3 m/min. The highest and lowest sensitivities were to the nozzle inlet pressure and spray angle, respectively. The cleaning performance increased as the pressure increased. In addition, the spray distance exhibited an inverse relationship with the cleaning performance.

Fig 8

Sensitivity analysis results of the control parameters based on Eq (2).

It is noteworthy that the cleaning performance decreased significantly below a critical spray angle. As shown in the experimental results in Fig 9, although both ends of the spray area are clean, the results are not effective in the central part. We conjecture that this is because the flow rate was not uniform over the water-spray area when the spray angle of the nozzle was 15° or 25°. In particular, an identical pattern of cleaned areas appeared on the specimen with these small spray angles, as shown in Fig 9D–9F. These results of the cleaning pattern caused by the flow imbalance at small angles are highly helpful and provide guidance for developing devices for cleaning external walls using a high-pressure spray that display good performance.

The experimental results shown in Figs 8 and 9 reveal that a higher cleaning performance was achieved with the condition characterized by a high nozzle inlet pressure, long spray distance, and wide spray angle than with that characterized by a small nozzle inlet pressure, short spray distance, and narrow spray angle. This is because a wide spray angle can cover a wide range of cleaning area with a high nozzle inlet pressure. Within the experimental set of spray distances, the cleaning performance was more sensitive to the injection pressure than to the spray distance, as shown in Fig 8. Although the cleaning performance under the condition characterized by a high nozzle inlet pressure and narrow injection angle is normally good, it is not better in terms of the definition of cleaning performance. This is because the total area to be cleaned by the robot per unit distance moved is marginal.

5.2. Verification experiment and discussion

An experiment for verifying the optimal values of each factor was performed based on the results of the sensitivity analysis. The optimum conditions were as follows: nozzle input pressure = 8 bar, spray distance of the nozzle = 200 mm, and injection angle = 40°. The pressure could not exceed 8 bar owing to the pump’s limitation, as mentioned earlier. The injection angle and spray distance were determined according to the layout of the robot and the sensitivity analysis results. The results of the verification experiment based on the optimum conditions were better than those of Experiment #7, as shown in Fig 10.

Fig 10

The results of verification test performed under the optimal conditions.

Nozzle inlet pressure: 8 bar; spray distance: 0.2 m; injection angle: 40°. Image data results: 74.8% (average).

The reaction force at the nozzle and the impact force at the acryl specimen when the nozzle injected high-pressure water for cleaning were measured. This was performed to investigate 1) the influence of the reaction to the thrust force from the propeller that is applied for the robot to adhere to the wall and 2) the damage to the building’s outer wall by the impact force. As shown in Fig 11, the reaction force at the nozzle and the impact force on the outer wall were approximately 2.6 N and 2.7 N, respectively. The nozzle spray reaction force was approximately 4.5% of the thrust force of the propellers. This indicates that the spraying at a pressure of 8 bar did not significantly affect the absorption or damage the exterior wall of the building.

Fig 11

Reaction force on nozzle and impact force on panel under the condition of 8 bar, 0.3 m, 15°.

6. Conclusion and future works

In this study, a method of spraying high-pressure water for cleaning the outer walls of buildings was developed, and the cleaning performance was optimized. The proposed cleaning device was installed on a robot equipped with a winch. Cleaning was performed by spraying high-pressure water using nozzles, a high-pressure pump, and a guide wall to prevent recontamination. The control parameters were optimized by utilizing the Taguchi optimization method for different descending speeds. The design variables were identified based on experience to evaluate the cleaning efficiency through image processing. We carried out a sensitivity analysis to identify the design variables to which the cleaning performance is most sensitive. The following are the variables arranged in decreasing order of influence on the cleaning performance: nozzle inlet pressure, spray distance, and injection angle. The cleaning performance was not good for a slow speed of motion under the condition of a small spray angle or low spray pressure. We verified the optimized conditions through a test under the following conditions: nozzle inlet pressure = 8 bar, spray distance = 0.2 m, nozzle injection angle = 40°, and descending speed = 3 m/min. The reaction force and impact force generated by the pressure were 2.6 N and 2.7 N, respectively. These are not adequate to affect the propeller thrust force or damage the building surface.

In the near future, based on the results of the proposed method, lab tests are expected to be carried out with the upgraded ascender control algorithm for overcoming obstacles. Through this test, the cleaning quality of the high-pressure-water spray while overcoming obstacles would be verified, and the uncleaned zone generated by obstacles would be assessed. Thus, a study will be conducted to reduce the uncleaned zone and thereby achieve good cleaning quality. Subsequently, a field test will be conducted to verify the cleaning performance of the proposed cleaning method.

28 Aug 2020

PONE-D-20-12174

Parametric study on the automated high-pressure water-based window cleaning device

PLOS ONE

Dear Dr. Seo,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

Please submit your revised manuscript by Oct 12 2020 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. You should upload this as a separate file labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. You should upload this as a separate file labeled 'Manuscript'.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter. Guidelines for resubmitting your figure files are available below the reviewer comments at the end of this letter.

If applicable, we recommend that you deposit your laboratory protocols in protocols.io to enhance the reproducibility of your results. Protocols.io assigns your protocol its own identifier (DOI) so that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

We look forward to receiving your revised manuscript.

Kind regards,

Hongbing Ding, Ph.D.

Academic Editor

PLOS ONE

Journal Requirements:

When submitting your revision, we need you to address these additional requirements.

1. Please ensure that your manuscript meets PLOS ONE's style requirements, including those for file naming. The PLOS ONE style templates can be found at

https://journals.plos.org/plosone/s/file?id=wjVg/PLOSOne_formatting_sample_main_body.pdf and

https://journals.plos.org/plosone/s/file?id=ba62/PLOSOne_formatting_sample_title_authors_affiliations.pdf

2. Please provide vendor details of all equipment and materials used. Please ensure that the methods section is described in enough detail for another researcher to reproduce the findings.

3. Please clarify where the data underlying the findings of the study can be found. We note for instance that no Supporting Information has been provided.

4.We suggest you thoroughly copyedit your manuscript for language usage, spelling, and grammar. If you do not know anyone who can help you do this, you may wish to consider employing a professional scientific editing service.

Whilst you may use any professional scientific editing service of your choice, PLOS has partnered with both American Journal Experts (AJE) and Editage to provide discounted services to PLOS authors. Both organizations have experience helping authors meet PLOS guidelines and can provide language editing, translation, manuscript formatting, and figure formatting to ensure your manuscript meets our submission guidelines. To take advantage of our partnership with AJE, visit the AJE website (http://learn.aje.com/plos/) for a 15% discount off AJE services. To take advantage of our partnership with Editage, visit the Editage website (www.editage.com) and enter referral code PLOSEDIT for a 15% discount off Editage services.  If the PLOS editorial team finds any language issues in text that either AJE or Editage has edited, the service provider will re-edit the text for free.

Upon resubmission, please provide the following:

The name of the colleague or the details of the professional service that edited your manuscript

A copy of your manuscript showing your changes by either highlighting them or using track changes (uploaded as a *supporting information* file)

A clean copy of the edited manuscript (uploaded as the new *manuscript* file)

Additional Editor Comments (if provided):

Thank you for submitting your manuscript to PLOS ONE. The reviewers recommend reconsideration of your paper following major revision. I invite you to resubmit your manuscript after addressing all reviewer comments.

[Note: HTML markup is below. Please do not edit.]

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #2: Yes

**********

2. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: N/A

**********

3. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: No

Reviewer #2: No

**********

5. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Summary

The authors present a technique for window cleaning of high-rise buildings---an increasingly prominent problem, as many high-rise buildings have been constructed recently. Manual window cleaning of such structures is dangerous for various reasons. Various robots have been proposed to assist with this problem. In this paper, the authors propose a method of spraying high-pressure water by using a pump and nozzle. Unlike other approaches, utensils, e.g., brush, squeeze, manipulator, typically used for manual cleaning, are not used in the proposed plan. Omitting such tools has essential implications for maneuvering around obstacles, such as building facades. The method was evaluated by polluting windows, using the robot on the windows, and quantitatively comparing before and after images. A field study is planned for the future.

Pros

- Relevant and timely topic. Cleaning windows on high-rise buildings manually is dangerous.

- The proposed approach seems to have substantial advantages over existing methods. The benefits are particularly significant when considering that buildings may have facades and other obstacles.

- Preliminary results are encouraging.

Cons

- The quality of the writing is poor. Not even spell check was seemingly executed on this article.

- No field study. The current research may not be complete.

- The novelty of the proposed approach over existing approaches is not apparent.

Major Details

- Why do existing solutions not satisfy you? What is wrong with them, specifically? What are the disadvantages of those approaches?

- How about qualitative analysis of performance?

- What are the effects of wheel damage on the buildings that your device may cause?

- "In case of higher than 100mm of obstacles, a separate mechanism is needed." -> Isn't this the problem you are solving over existing approaches, i.e., not being hindered by obstacles without using separate mechanisms?

- "In other words, the further away the distance, the poorer the cleaning performance." -> Isn't this expected?

- Why weren't obstacles used in the evaluation?

- Why does slow speed impede performance? Isn't this counterintuitive?

Minor Details

- Title: The paper is not really about a study. Instead, an approach is proposed; the study helps evaluate the proposal. I suggest changing the title to something like: "An Automated Technique for High-pressure Water-based Window Cleaning and Accompanying Parametric Study" (the latter half about the study may be dropped).

- The motivation is that many high-rise buildings need cleaning. However, with COVID-19, many of these buildings are empty. COVID-19 may have a significant impact on commercial real estate. How does this affect the impact of the proposed approach? Are there other contexts where the device can be used in light of COVID-19?

- The abstract is too long—too many details.

- It would be better to have the figures at the top of the page (more comfortable to read).

- The conclusion presents new information not found in the introduction (recontamination). The opening and conclusion should mirror each other. If this information is essential, it should also be included in the introduction. Otherwise, it should only be in the discussion section of the experimentation.

- No future work section. Highlighting future work is crucial since you are planning future studies.

Reviewer #2: This paper involved a method to spray high pressure water in the process of cleaning exterior building walls, in addition to optimizing cleaning performance. Although the premise is clear, the outcome is significant for an effective performance of building exterior wall cleaning and maintenance, and the developed methods and data are relatively sound, the paper is extremely hard to read, given the poor English writing, improper sentence structure, multiple typos and grammatical errors, making it not only unsuitable for submission in the current state, but also ambiguous to read and understand technically, which often does not allow the reader to grasp some vital information to the understanding of the methods, procedures, and results. The paper cannot be submitted in this state of poor writing and requires thorough revision by a native English speaker. Further comments might even arise upon a re-read once some of the concepts and procedures become more clear.

Although the research problem identified and discussed in section 1 is understood, the authors are encouraged to elaborate more on the problem formulation, with clear justification of the need for their research. The authors mention some relevant studies, but do not go in length in delineating how past studies and precedents have failed to achieve the desired objective regarding exterior wall cleaning performance (more in terms of metrics and data, rather than anecdotal information). This should help ground more the research and justify the discussed methods and procedures later on. Perhaps a brief discussion about how precedent studies specifically addressed similar methods and cleaning parameters and other relevant data could help bring more justification to decisions such as the choice of the Taguchi method and the image evaluation method (which are currently described very briefly and there is no elaborate grounding of why they are selected for this specific experiment and study).

Section 2 is mostly ambiguous and it is not clear from the current version of the paper whether the discussed topics related to cleaning devices and equipment are mostly guidelines to follow, methods adopted by the authors, or work developed by the authors. This needs to be classified and demonstrated clearly. Are the cleaning devices mentioned used by the authors? Or are they just referred to as examples of previous work? This is not clear, perhaps more due to the ambiguous language more than anything else. There are also some strong assumptions in the third paragraph in page 3 that require some clarification. If some of the cleaning devices and robots are to be adopted, there needs to be some more explanation regarding specific relations to the proposed methods described later on and how they fit together. This is missing.

Section 3 lays out well some of the specifications for the proposed device, but requires some more explanation, especially in the second paragraph discussing the selection criteria. There appear to be some gaps in the description of the selection criteria. This requires further elaboration.

Most of the figures, especially 4, 5, 9, and 10 are unreadable and difficult to read. These are crucial illustrations that should be much more developed and more clear, perhaps enlarged in size, to demonstrate the suggested differences in cleaning performance, and so they require much more clear images and image resolution. Some figures are misplaced, such as figure 6, which needs to belong under section 4.4. Table captions need to be above the tables not below.

The first two paragraphs in page 5 are relatively confusing in relation to the tables and figure that follow. Please describe very clearly the content of each table (in a more elaborate discussion) so that the flow of information reads well. There is no adequate discussion regarding the mentioned levels (1, 2 and 3) and how they relate each of the parameters. Table 2 and 3 are quite short and their inclusion is questioned. Perhaps another tabulation format can be included, otherwise they could be eliminated and included as text description, but with further explanation.

Section 4.2 lacks both adequate description of how previous research fall short in terms of method or standards (this requires specific citations and identification of gaps, and not just general talk), and a sufficient description of process that is conducted by the authors (this is quite truncated and not described clearly). The authors do not mention also the "certain threshold value" for cleaning in the first paragraph in page 6, adding to the ambiguity in this section. Section 4.3 is quite hard to read and understand. The only useful piece of information is the equation which is utilized effectively later on in the results. Other than that, the text needs substantial work to be understood in the first place. The conclusion of the paragraph following the equation needs to be clear. The experiment setup in section 4.4 lacks organization and clarity, making it also hard to follow. Please organize the description of the setup into clear chunks of information that are easy to follow. Table 4 is primarily related to the evaluation results, so it seems more appropriate to locate this table later on in section 5, rather than under 4.4 which is more related to the methods and procedures. You come back to it later and recall it in the results section in a confusing manner, so it suits that location in the text better. This should organize the flow of ideas better.

The sensitivity analysis described in section 5 is interesting and revealing of your findings. However, there seem to be some anecdotal observations and assumptions in the third paragraph in page 8. These should be avoided; please stick to the data and pure results. The interpretation of the results should be addressed far more scientifically than what is mentioned here. The results also need more elaboration in the second paragraph in page 8; seems quite short and truncated. In other words, how you arrive at the optimum results (0.2m spraying distance, 40 degrees angle, and 3m/min transfer speed) is the core of your findings. This needs to be analyzed, interpreted and discussed widely. The confirmation section (5.2) reads well but needs more elaboration. Figures need to be more visually clear. There needs to be a discussion here or in the earlier section about comparisons to base cases.

The paper ends relatively in an anti-climactic manner. There needs to be an elaborate discussion section that goes thoroughly through the interpretation of these results and how they relate to the robotic cleaning device and the consequences of the optimized values. There also needs to be a macro discussion that relates to buildings, and the generalizability of such findings. Are these results applicable for example and true for all locations, conditions, heights? Many other factors need to be considered, such as the climate, dust, etc. It is understood that this is not the scope of the paper, but there needs to be a clear description of the assumptions of the paper, in terms of weather conditions, building orientation, wind speed, etc. that all might critically affect some of the results/parameters such as spray angle and spraying distance. What is deduced here in this paper might apply for example in a southern facade but not in a western facade, and at certain elevation levels not others, and in specific climatic zones and not others. Grounding the findings of your research should specify in detail these assumptions or at least the conditions under which you conducted your experiments. If this is in a controlled environment, what must be accounted for in field testing? There also needs to be a discussion and justification regarding the effect of the suggested 2.6-2.7N impact force and how the assessment of minimal damage to the building surface is justified; this is not clear. There needs to be more elaboration regarding how to carry your findings from the experimental setup into field tests; at least in terms of the assumptions in each case and what additional experiments need to be conducted to transfer to real conditions and what to account for.

**********

6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

27 Sep 2020

Thank you for the valuable comments on this article. We carefully revised the paper according to the comments. The main corrections are as follows:

The title, abstract and introduction parts were revised to clearly point out our novelty and contributions of the proposed method.

All Figures in the text were revised image the clarity to increase the readability for the paper.

Tables containing variables are combined into one table to help easy understand variables and pictures.

To identify the cleaning performance, a simple additional test was performed, and the contents were added in the answer sheet.

To improve the readability, sentences were revised grammatically and unnecessary words with no meaning were deleted for easy reading. (The editing is done by a professional editor (https://www.editage.co.kr/_

Please refer the attached revision summary and revised draft for details.

Submitted filename: 200928_PlosOne_Revision summary.pptx

Click here for additional data file.

3 Nov 2020

Automated technique for high-pressure water-based window cleaning and accompanying parametric study

PONE-D-20-12174R1

Dear Dr. Seo,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Hongbing Ding, Ph.D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

The authors have done a good job in revising the manuscript. Now it can be accepted for publication in PLOS ONE.

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #2: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #2: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #2: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #2: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Thank you for addressing my concerns. I have one small comment, I prefer "Future Work" to "Future Works." Using the former makes it seem like there are many more possibilities, while the latter makes it seem like there is a smaller number of things that can be done in the future.

Reviewer #2: The authors have addressed all comments adequately. They have also added more figures and explanations to further clarify some of the raised comments. They have significantly improved the fashion of writing and revised the use of the English language.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: No

12 Nov 2020

PONE-D-20-12174R1

Automated technique for high-pressure water-based window cleaning and accompanying parametric study

Dear Dr. Seo:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Professor Hongbing Ding

Academic Editor

PLOS ONE