Pneumatic Automation Tools: Pneumatic Muscle
Authors: Efremova K D. , Pilgunov V. N. 
Published: August 2018

The traditional actuating motor for pneumatic drives is a differential double-acting pneumatic cylinder used to create a pushing force that is significantly larger than the pulling force. The pneumatic muscle is a single-acting operating motor to be used for creating a pulling force. The pneumatic muscle is based on a cylindrical bladder (a thin two-dimensional elastic structure) property to change its shape or size upon applying overpressure of actuating medium to it.

The paper objective is to present this new type of the actuating motor to a wide range of specialists in pneumatic automation. Using a bladder structure of the pneumatic muscle of the MAS family, company "FESTO" as an example, the paper considers a physical nature of its operation and defines a dependence of the force, developed by a pneumatic muscle in its internal cavity, on the overpressure value and the value of contraction. Describes an experimental setup to study static and dynamic characteristics of the pneumatic muscle, as well as a design of the loading and measuring device.

The experimental study allowed us to obtain static and dynamic characteristics of the pneumatic muscle MAS 10-300: dependencies "force - contraction", "force - overpressure", and “contraction -overpressure". The averaged predicted value of the braid angle of impulsion of the cord thread for three sizes of the MAS family pneumatic muscle is determined according to German FESTO Product Catalogue to be 23 ... 25.5°.

It is shown that the force curve of the pneumatic muscle is essentially nonlinear: the curve linearity is evident only when the pneumatic muscle contractions are, at most, 2% of its original length. Dynamic properties of the pneumatic muscle loaded with a constant force were evaluated through analysis of frequency characteristics: the operating frequency of the pneumatic muscle was f = 3 ... 6 Hz.

The paper presents the reproducibility data of the force characteristic of a pneumatic muscle during its cyclic constant-value over-pressurisation p = 4 bar with a frequency f = 0.5 Hz.

The researches have shown that with the cyclic over-pressurisation of the pneumatic muscle the force-value deviations from its averaged value are of systematic nature, depend on the number of loads, and so cannot be estimated by statistical characteristics. The paper considers an operating mode of the pneumatic muscle, as an extension spring, which is appropriate to the external force application to the pneumatic muscle to ensure return of the pneumatic muscle to the initial position after its contraction under over-pressurisation. An average value of the pneumatic spring stiffness is obtained from the force characteristic of the pneumatic muscle through its piecewise-linear approximation within the specified range of change in the contraction value. A comparative estimate of the forces developed by pneumatic muscles and pneumatic cylinders with equal working areas is given.  It has been found that the pneumatic muscle contraction force exceeds the pulling force of the pneumatic cylinder, on average, 12 ... 14 times, but this advantage comes out only when the contractions of a pneumatic muscle are small. The usability of a short pneumatic muscle, as a control and loading device for the gates of hydraulic and pneumatic valve-type automation devices, has been investigated.