Pressure Observer Based Adaptive Dynamic Surface Control of Pneumatic Actuator with Long Transmission Lines

Round 1

Reviewer 1 Report

The abstract is not well written to indicate the content of the article. I think that it must be as the last paragraph of the Introduction section: "In this paper, the needle insertion motion control of the MRI compatible robot developed in our lab is considered.... etc..."

CT appear the first time as abbreviation line 33, please fix it.

Author Response

Dear Professor: 

Thank you for your kind comments for our manuscript submitted to Applied Sciences. We appreciate your valuable comments and suggestions to improve it. Your suggestions were a great source of inspiration for this paper. The suggestions let us know how to improve the contents and expression of this paper. We have studied comments carefully and have made correction which we hope meet with your approval. All these changes based on your and other Reviewers’ comments are highlighted by using the "Track Changes" function in Microsoft Word. 

With regard to your comments and suggestions, we wish to reply as follows: 

1. The abstract is not well written to indicate the content of the article. I think that it must be as the last paragraph of the Introduction section: "In this paper, the needle insertion motion control of the MRI compatible robot developed in our lab is considered.... etc..." 

Response: Thanks very much for the reviewer’s careful supervision. As the reviewer's recommendation, we rewrote the abstract as follows:“ In this paper, the needle insertion motion control of a MRI compatible robot, which is actuated by pneumatic cylinder with long transmission lines, is considered and a pressure observer based adaptive dynamic surface controller is proposed. The long transmission line is assumed to be an intermediate chamber connected between the control valve and the actuator in series, and a nonlinear first order system model is constructed to characterize the pressure losses and time delay brought by it. Due to the fact that MRI-compatible pressure sensors are not commercially available, a globally stable pressure observer is employed to estimate the chamber pressure. Based on the model of the long transmission line and the pressure observer, an adaptive dynamic surface controller is further designed by using the dynamic surface control technique. Compared to traditional backstepping design method, the proposed controller can avoid the problem of “explosion of complexity” since the repeated differentiation of virtual controls is no longer required. The stability of the closed-loop system is analytically proven by employing the Lyapunov theory. Extensive experimental results are presented to demonstrate the effectiveness and the performance robustness of the proposed controller.” 

2. CT appear the first time as abbreviation line 33, please fix it. 

Response: We have made correction according to the Reviewer’s comment.

Author Response File: Author Response.pdf

Reviewer 2 Report

I have a pleasure to read an article, it is interested area and implementing of needle control, but here are some drawbacks and remarks:

No dynamic model of the analyzed system provided even it is claimed so; therefore dynamic equation looks quite strange;

No mention about equation building method (and possible mistake with sign of active forces);

I hardly can notice resisting force behavior for all system; skin penetration is highly nonlinear, but here is not taken into account. If such efforts will be not taken, then it should be provided in the discussion or conclusions;

In figure axis denoting should be separated from units by comma rather then slash;

Figures: in figures 5, 8 agenda different line types, in the graph - different colors used for series;

Typing and formatting errors, fonts sizes differs in the conclusion section.

I would suggest - to improve introduction, especially part with behavior of needle - penetrating matter (otherwise it looks like operates in the air) and really improve conclusions - there no real discussion on results achieved, which are obvious from resulting graphs.

Author Response

Dear Professor: 

Thank you for your kind comments for our manuscript submitted to Applied Sciences. We appreciate your valuable comments and suggestions to improve it. Your suggestions were a great source of inspiration for this paper. The suggestions let us know how to improve the contents and expression of this paper. We have studied comments carefully and have made correction which we hope meet with your approval. All these changes based on your and other Reviewers’ comments are highlighted by using the "Track Changes" function in Microsoft Word. 

With regard to your comments and suggestions, we wish to reply as follows: 1.I have a pleasure to read an article, it is interested area and implementing of needle control 

Response: Thanks very much for the reviewer's confirmation and encouragement. 

2. No dynamic model of the analyzed system provided even it is claimed so; therefore dynamic equation looks quite strange; No mention about equation building method (and possible mistake with sign of active forces). 

Response: Thanks very much for the reviewer’s careful supervision. We are sorry for such mistake. The dynamics of the pneumatic servo system is not included in this manuscript since this issue was extensively studied in literature and was also carefully considered in our previous work (Meng D., Tao G., Chen J., Liu H (2011) Modeling of a pneumatic system for high-accuracy position control. In: Proc. of the International Conference on Fluid Power and Mechatronics, Beijing, China, 2011. pp505-510). In our previous work, a detailed model was developed for a pneumatic cylinder controlled by a proportional directional control valve. The dynamic of the valve spool was investigated and the equation which describes the mass flow through the valve’s variable orifice was introduced. The thermodynamics in cylinder chambers was carefully considered and the friction characteristics of the pneumatic cylinder was investigated. Based on our previous work, the complete model for the system we considered in this paper was given directly. It consists of the equation for the motion of piston-rod-needle assembly (1), two equations for the chamber pressure time derivatives as (2) and (4), and the equation for the mass flow through the valve’s variable orifice (5). Due to limited space, the development process and analysis of the model was omitted. Considering the Reviewer’s suggestion, we include references in Section 2. 

3. I hardly can notice resisting force behavior for all system; skin penetration is highly nonlinear, but here is not taken into account. If such efforts will be not taken, then it should be provided in the discussion or conclusions.  

Response: Thanks very much for the reviewer’s careful supervision. It is really true as the Reviewer pointed out that the force of the needle insertion into soft tissue is not discussed in the manuscript. Since the main purpose of this paper is to find a way to deal with the issue of long transmission line and realize high accuracy control of pneumatic actuator with pressure observer, the needle-tissue interactive mechanism is not taken into account. In our experiments, needle insertions were performed in a gel phantom, whose resisting force behavior is much simpler than real skin penetration. The force differs in the different phases of the needle insertion into soft tissue. Stiffness force occurs before the penetration of the tissue capsule, while friction and cutting forces occur after the penetration. When the needle completely penetrates the soft tissue, friction force achieves the dominant position. Thus, as pointed out by the Reviewer, the skin penetration is highly nonlinear. We definitely know that soft tissue deformation, multi-layer tissue relative sliding and needle deflection can decrease the needle insertion precision in practical. It is an important issue to model the interaction of needle into soft tissue, and incorporate a more accurate needle insertion force model into the controller design. Although the current research focuses on refinement of the robot and the control algorithm for MRI-compatible pneumatic servo system with long transmission lines, the next phase of this research will definitely focus on investigating the needle-tissue interactive mechanism and improving the robot- assisted needle insertion system accuracy.

Thus, as the reviewer's recommendation, we rewrote the related paragraphs in the discussion and conclusions as follows:

“ ……However, it should be noted that these needle insertion experiments were performed in a gel phantom, whose resisting force behavior is simpler than real soft tissue. The interaction between needle and soft tissue is very complex and may decrease the needle insertion precision in practical. Since the current research focuses on addressing the issue of long transmission line and realizing high accuracy control of pneumatic actuator with pressure observer, modeling of needle-tissue interaction and further improving needle insertion precision will be the subject of the next phase of this research. ……”

“ ……The current research focuses on development of the high performance control algorithm for MRI-compatible pneumatic servo system with long transmission lines. However, precise motion control of pneumatic actuator does not necessarily lead to precise position control needle tip. Modeling of the interaction between needle and soft tissue, and incorporating a more accurate needle insertion force model in the controller design is an essential requirement for practical robot-assisted needle insertion. Thus, these issues will be explored further in the next phase of this research. ……”

4. In figure axis denoting should be separated from units by comma rather than slash. 

Response: Thanks very much for the reviewer’s careful supervision. Since the figures were prepared by following the format given by the publisher, we opine that there is no need to make this correction. 

5. Figures: in figures 5, 8 agenda different line types, in the graph - different colors used for series; 

Response: We have made correction according to the Reviewer’s comment. 

6. Typing and formatting errors, fonts sizes differs in the conclusion section. 

Response: We have made correction according to the Reviewer’s comment. 

7. I would suggest - to improve introduction, especially part with behavior of needle - penetrating matter (otherwise it looks like operates in the air) and really improve conclusions - there no real discussion on results achieved, which are obvious from resulting graphs. 

Response: Thanks very much for the reviewer’s careful supervision. In order to describe the purpose of the paper more clearly, we rewrote the first paragraph and the last paragraph in the Introduction as follows:

“ Magnetic resonance imaging (MRI) technique is widely used in clinical diagnosis due to its ability to image without the use of ionizing x-rays and superior soft tissue contrast as compared to computed tomography (CT) scanning. Recently, pneumatically actuated MRI-compatible robots, which enable real-time Magnetic resonance (MR) image-guided needle placement, are designed for brachytherapy and biopsy by several researchers [1-15]. To fulfill the requirements for MR compatibility of the robotic systems, pneumatic valves are commonly placed outside the scanner room in the aforementioned works. Therefore, long transmission lines between the actuators and valves are used. Since long transmission lines have a significant influence on the pressure dynamics of the pneumatic system and MRI-compatible pressure sensors are not commercially available, precise position control is one of the main technical challenges in the robot development.”

“In this study, the needle insertion motion control of the MRI compatible robot developed in our lab is considered. The robot is actuated by pneumatic cylinder with long transmission lines. The focus of this paper is dealing with the issue of long transmission line and realizing high accuracy control of pneumatic actuator with pressure observer. Therefore,……”

In order to describe the DSC method more clearly, a new paragraph is added in the Introduction as follows:

“Recently, the backstepping design method has been proven to be a very effective way to develop nonlinear robust controllers for pneumatic servo systems. However, this method has the problem of “explosion of complexity” since the requirement of repeated differentiation of virtual controls. Thus, a practical implementation is difficult. To solve this problem, Swaroop et al. [28] proposed the dynamic surface control (DSC) method, in which the calculation of the virtual control variable’s derivative was prevented by introducing a filter at each design step. Since then, DSC method has been the topic of significant research efforts and a number of excellent theoretical contributions have been made. The applications of DSC can be found in many engineering fields, for example, hydraulic servo systems [29], underwater/autonomous surface vehicles [30], mobile wheeled inverted pendulum [31], pneumatic artificial muscle [32], and servo motor [33].”

As the editor's recommendation, we made some discussions about the experimental results in Section 4 as follows:

“……However, it should be noted that these needle insertion experiments were performed in a gel phantom, whose resisting force behavior is simpler than real soft tissue. The interaction between needle and soft tissue is very complex and may decrease the needle insertion precision in practical. Since the current research focuses on addressing the issue of long transmission line and realizing high accuracy control of pneumatic actuator with pressure observer, modeling of needle-tissue interaction and further improving needle insertion precision will be the subject of the next phase of this research.”

“…… However, the estimation error during the charging process is a slightly bigger than the one during the discharging process. This may be due to the fact that charging process is close to adiabatic and discharging process is close to isothermal. ”

As the editor's recommendation, we rewrote the Conclusion as follows:

“In this paper, the precise motion control of a MRI compatible 1-DOF pneumatic servo system is considered. The long transmission line is characterized as an intermediate chamber connected between the valve and the cylinder in series, and a nonlinear first order system is used to approximate its dynamics. Simultaneously, a globally stable pressure observer is developed to estimate the chamber pressure. Based on the model of the long transmission line and the pressure observer, a pressure observer based adaptive dynamic surface controller is developed and the stability of the closed-loop system is proved via the Lyapunov method. In contrast to most of the existing nonlinear controllers synthesized with the backstepping method, by employing the dynamic surface control technique, the proposed controller can cope with the problem of “explosion of complexity”, since the repeated differentiation of virtual controls is no longer required. The experimental results confirm that the proposed controller is effective and has good performance robustness to sudden disturbances, thus enabling future application in pneumatically actuated MRI- compatible robots. However, precise motion control of pneumatic actuator does not necessarily lead to precise position control needle tip. Modeling of the interaction between needle and soft tissue, and incorporating a more accurate needle insertion force model in the controller design is an essential requirement for practical robot-assisted needle insertion. Thus, these issues will be explored further in the next phase of this research.”

Reviewer 3 Report

The paper presents an interesting topic. The quality of the current version of the paper is generally quite good, thus I can recommend the publication. Following, a few brief comments to help the authors to improve the paper quality.

The description of the adaptive dynamic surface controller is not very comprehensive.

The implementation of the pressure observer is not very clear. Further details are needed.

(Not compulsory: for future work, maybe could be useful a brief comparison -in the form of a table- of advantages/disadvantages of this dynamic surface control technique vs other nonlinear controllers - Not compulsory).

Author Response

Dear Professor: 

Thank you for your kind comments for our manuscript submitted to Applied Sciences. We appreciate your valuable comments and suggestions to improve it. Your suggestions were a great source of inspiration for this paper. The suggestions let us know how to improve the contents and expression of this paper. We have studied comments carefully and have made correction which we hope meet with your approval. All these changes based on your and other Reviewers’ comments are highlighted by using the "Track Changes" function in Microsoft Word. 

With regard to your comments and suggestions, we wish to reply as follows: 

1.The paper presents an interesting topic. The quality of the current version of the paper is generally quite good, thus I can recommend the publication. 

Response: Thanks very much for the reviewer's confirmation and encouragement. 

2. The description of the adaptive dynamic surface controller is not very comprehensive. 

Response: Thanks very much for the reviewer’s careful supervision. It is really true as the Reviewer pointed out that the description of adaptive dynamic surface controller is not clear. In order to describe the adaptive dynamic surface controller more clearly, a new paragraph is added in the Introduction as follows:

“Recently, the backstepping design method has been proven to be a very effective way to develop nonlinear robust controllers for pneumatic servo systems. However, this method has the problem of “explosion of complexity” since the requirement of repeated differentiation of virtual controls. Thus, a practical implementation is difficult. To solve this problem, Swaroop et al. [28] proposed the dynamic surface control (DSC) method, in which the calculation of the virtual control variable’s derivative was prevented by introducing a filter at each design step. Since then, DSC method has been the topic of significant research efforts and a number of excellent theoretical contributions have been made. The applications of DSC can be found in many engineering fields, for example, hydraulic servo systems [29], underwater/autonomous surface vehicles [30], mobile wheeled inverted pendulum [31], pneumatic artificial muscle [32], and servo motor [33].”

Also, the description of the proposed adaptive dynamic surface controller is rewrote in Section 3.1 and Section 3.2. For details please refer to the revised manuscript. 

3. The implementation of the pressure observer is not very clear. Further details are needed. 

Response: Thanks very much for the reviewer’s careful supervision. It is really true as the Reviewer pointed out that the implementation of the pressure observer is not very clear. As the editor 's recommendation, we rewrote the Section 3.1 which can be found in the revised manuscript. 

4. Not compulsory: for future work, maybe could be useful a brief comparison -in the form of a table- of advantages/disadvantages of this dynamic surface control technique vs other nonlinear controllers - Not compulsory 

Response: Thanks very much for the reviewer’s valuable suggestions and kind understanding, and we promise that these issues will be explored further in the next phase of this research.

Author Response File: Author Response.pdf