# Design and Control of a Pneumatic Muscle Servo Drive Applied to a 6-DoF Parallel Manipulator
## 4 Conclusions  
This article contains the results of research aimed at developing a pneumatic servo drive composed of two pneumatic muscles. This servo drive is being developed to be used to drive a parallel manipulator with six degrees of freedom.  
The article presents a comparison of the authors' original muscle with the commercial muscle. Both solutions have unique features. Each of them has advantages and disadvantages, which means that both can be successfully used to drive manipulators. Comparing them in full is very difficult because they are completely different products. However, by analyzing the results of an unbiased comparison, it can be noticed that the authors' original muscle is characterized by greater strength and greater relative shortening. At the same time, it should be assumed that commercial muscles will be much more durable and resistant to damage. However, considering the very low cost of making the authors' original muscle, it should be assumed that replacing it more often will not be a big problem.  
This article presents a mathematical model of the authors' muscle. This is a functional model. It was determined through a series of many experiments: mostly step responses. The procedure for its development makes this task time-consuming and labor-intensive. The lack of a structural model means that this procedure must be repeated for each muscle separately. This may be considered a disadvantage of this solution. In a 6-DoF manipulator, 12 muscles need to be used. However, as will be presented in the next publication, even the authors' muscles performed in a repeatable manner. Thanks to this, the parameters of the mathematical models of individual muscles do not differ significantly. It is sufficient to perform parameterization for several muscles (for example, four) and average the results. Moreover, to further simplify and shorten the identification stage, a test stand was developed to automate the entire process. This significantly reduced the time needed for identification.  
The article also presents the synthesis of a servo drive consisting of two pneumatic muscles. The use of the developed mathematical models (in the form of inverse models) in the control process significantly eliminated the non-linearities of muscle work. This approach allowed us to get rid of the influence of the non-linearity of the static characteristics of the object. This made it possible to use a PID controller with constant coefficients. However, other non-linearities, especially those resulting from the friction of the braid and rubber bladder, still negatively affect the operation of the drive. This is especially visible at low operating speeds. This is one of the main limitations of the adopted servo drive synthesis method and will be one of the most important topics of further research.  
Based on the analysis of the presented results and the conclusions drawn, the decision was made to justify the use of the developed servo drive for the construction of a delta-type parallel manipulator with six degrees of freedom. The manipulator, mentioned several times in this article, will be presented in subsequent work.  
**Author Contributions:** Conceptualization, D.S.P. and P.A.L.; methodology, D.S.P.; software, D.S.P. and J.Z.; validation, P.A.L. and J.Z.; formal analysis, D.S.P. and P.A.L.; writing--original draft preparation, D.S.P.; writing--review and editing, P.A.L.; supervision, P.A.L. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the authors on request.  
**Acknowledgments:** The authors would like to thank Parker for donating 10 Hoerbiger Tecno Plus valves.  
**Conflicts of Interest:** The authors declare no conflicts of interest.