Design and Characterization of a Mckibben Pneumatic Muscle Prototype with an Embedded Capacitive Length Transducer

Abstract

The McKibben muscle types are pneumatic actuators known to be intrinsically safe for their high power-to-weight ratio. For these reasons, they are suitable for robotic, biomechanical, and medical applications. In these application fields and, above all, in collaborative robotics, where safety must be ensured for human–robot interactions, the values of pressure, force, and length are necessary and must be continuously monitored and controlled. Force and pressure transducers are commercially available to be integrated into a McKibben muscle type. On the contrary, no commercial-length transducers can be adopted. This work presents a novel McKibben muscle prototype with an embedded capacitive-length transducer. The latter is a cylindrical capacitor made of a telescopic system composed of two tubes: one of its ends is connected to the muscle. A change in the length of the muscle causes a proportional change in the transducer capacitance. The paper reports in detail on the working principle of McKibben’s muscle, its fabrication, characterization, and validation of four prototype capacitive transducers. The results achieved from the experimental activities demonstrate that it is possible to control the variations of the muscle length relative to its elongation and compression for values less than 1 mm. This is the consequence of the ability to measure the transducer capacitance with a typical statistical relative indetermination better than 0.25%, which is a figure of merit for the reliability and mechanical and electrical stability of the proposed McKibben muscle prototype. Moreover, it has been demonstrated that the transducer capacitance as a function of the muscle length is linear, with maximum deviations from linearity equal to 2.44% and 5.22% during the muscle elongation and compression, respectively.

A Current-Mode Analog Front-End for Capacitive Length Transducers in Pneumatic Muscle Actuators

“…For the electrical characterization of the circuit, the values of the input capacitance C IN have been considered to vary from 50 pF to 90 pF. These values of C IN are consistent with those achievable from the fabricated C-MKM (providing an elongation ranging about from 13 mm to 20 mm [ 22 ]) used in the input of the CV-AFE circuit for its characterization in the real application discussed below. Referring to Figure 1 , the implemented circuit employs as CCII the AD844 by Analog Devices and the following values of the other components: R 1 = 8.2 kΩ, R 2 = 98.5 kΩ, R 3 = 100 kΩ, R 4 = 100 Ω, R 5 = 10 kΩ, R 6 = 1 kΩ, R 7 = 2.2 kΩ, R 8 = 330 Ω, C 2 = 2.2 μF, and C 1 = C 3 = C L = 2.2 nF.…”

“…At the beginning of each one of the measurements, the output voltage V OUT has been zeroed by acting on the value of the offset voltage V OFF2 (see Figure 1 ). Moreover, as previously discussed, the value of the offset voltage V OFF1 has been properly regulated to change the pulse width of the square wave signal generated by the CPW CONVERTER block, thus maximizing the CV-AFE circuit output voltage variations corresponding to the considered changes of the value of the input capacitance C IN (i.e., in the present case the maximum value of the input capacitance is C MAX = 84 pF) [ 22 ].…”

“…As a real case of application, the CV-AFE circuit implemented with COTS discrete components, as shown in Figure 3 , has been employed to measure the change of the length of a fabricated C-MKM by using the value of the actual capacitance of the cylindrical capacitor embedded in the muscle as circuit input [ 22 ]. The C-MKM muscle is composed of an inner tube made of hyper-elastic rubber, an external braided gauze made of Polyamide 66, and the two ends.…”

“…This is particularly true when inductive and Hall-based sensors are employed [ 16 , 20 , 21 ]. Recently, a new version of MKM has been designed and fabricated with a structure embedding a cylindrical capacitor whose instantaneous capacitance has been experimentally demonstrated to be directly correlated to the actual muscle length [ 22 , 23 ]. The structure of this Capacitive-MKM (C-MKM) overcomes the drawbacks intrinsic of the abovementioned solutions since it: (i) directly measures the distance between the two ends of the muscle without the need of mechanical sensors; (ii) does not suffer from external electrical and magnetic disturbances; (iii) is easy to be mounted between the two muscle end caps; (iv) respects the compliance of MKM muscles.…”

Discrete-Time Backstepping Sliding Mode Control for a 2-DOF PAM-Based Exoskeleton

“…Therefore, researchers are increasingly studying PAMs and their applications in rehabilitation robots, medical devices for motor function recovery, and control programs to enhance human safety while working with robots. The cylindrical braided muscle [3], known as McKibben's in the 1950s, is currently the most popular type of artificial pneumatic muscle. Besides the mentioned advantage [4] PAMs have several limitations, including high nonlinearity, uncertain parameters, and high impact delay.…”