From Fig. 2a, based on tilted angle (Ang) and length of ESPs the compliance centre is calculated and located at 71mm from the bottom of the middle plate. A force exerted at that point will cause pure translation of the peg and a moment about that point will make it rotate without translating.
Thus, for the given dimensions and geometry, the proposed device behaves as an RCC mechanism with a remote centre of compliance positioned 71mm below the middle plate. The compliance matrix of the device expressed in a coordinate system located at the compliance centre is a diagonal matrix given by Eq. 1, in which Kx=Ky=\(420N/mm\), Kz=\(2.62N/mm\), Kα=Kβ=\(420Nmm/rad\),K𝛾=\(48.95Nmm/rad\).
[C] = \(\left[\begin{array}{c}\begin{array}{cc}Kx& 0.0\\ 0.0& Ky\end{array} \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array} \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array}\\ \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array} \begin{array}{cc}Kz& 0.0\\ 0.0& K\alpha \end{array} \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array}\\ \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array} \begin{array}{cc}0.0& 0.0\\ 0.0& 0.0\end{array} \begin{array}{cc}K\beta & 0.0\\ 0.0& K\gamma \end{array}\end{array}\right]\) (1)
As shown in Fig. 4a and Fig. 4b, when the RCC device was not used, it is obvious that the peg could not be pulled out of the hole as the relative position between the peg and the hole did not change. Moreover, there was a huge equivalent stress change on the peg due to jamming as the direction of the withdrawal force was equal to the jamming angle.
The FEM software gave a warning that the contact status had experienced an abrupt change during the simulation process (see the blue line in Fig. 5), and was unable to converge on a solution for this problem. As shown in Fig. 5c, the maximum equivalent stress (represented by the green line) kept rising with time.
As shown in Fig. 6a, with the RCC device fitted, the peg could be completely pulled out of the hole. The stress distribution in the peg during the separation process can be seen in Fig. 6b. The stresses in the ESPs, especially at the connections between them and the middle plate were significantly greater than the stresses in other components, causing them to deform, the middle plate to move and the peg to accommodate itself to the hole thus facilitating the withdrawal process.
The stresses on the peg (see Fig. 6c) were much reduced when the RCC device was used, never approaching the material yield stress during the disengagement process. This shows the effectiveness of the device at preventing damage to components, enabling them to be reused following disassembly. It was also noted in the simulation that the contact state between the peg and the hole changed to one-point contact from two-point contact, which eliminated the possibility of wedging between the peg and the hole.
The colour of the stress map for the body of the RCC device (the integrated top and bottom plates) stayed blue during the whole separation process, which indicates that, throughout, there were negligible stresses in the body of the device. Therefore, it could be made of a suitably light aluminium alloy to keep the weight of the device down.
Figure 7 shows the evolution of deformations in the system. The red line represents the minimum deformation which remained close to zero during the disengagement of the pin from the hole. This indicates that there were parts of the system, notably, the body of the RCC device, where the deformation could be neglected, as previously assumed. The green line in Fig. 7 represents the maximum deformation in the entire geometry. This occurred at the ESPs where, according to Fig. 6b, stresses were also the highest, which shows the ability of the RCC device to absorb the load transferred to it during the disengagement of the peg from the hole. As the peg moved to conform to the position of the hole, the maximum deformation kept increasing until the peg became fully disengaged when all deformations became virtually null. Note that the initial deformations in the system were non-zero because it started from a stressed state with the peg and hole being misaligned relative to each other. The final deformations were also not exactly null as the weight of the peg caused the ESPs to remain slightly stretched or compressed even after full disengagement.
Figure 8 plots the stresses in the system. The red line in Fig. 8 represents the evolution in the minimum stress over the entire geometry, which confirms the previous observation that the minimum stress remained close to zero throughout the peg removal operation. The green line represents the maximum stress in the system. As mentioned above, stresses were highest in the ESPs and at the joints between them and the middle plate. The maximum stresses were well below the shear strength of the neoprene rubber in the shear pads as previously observed.
Again, it can be noted that the green line in Fig. 8 does not start from zero due to the initial stressed state of the RCC device. The line does not drop fully to zero when the peg was completely removed from the hole. This is because there are still small residual stresses due to the weight of the peg transmitted to the ESPs via the middle plate of the RCC device.
The results obtained from this finite-element modelling work have shown that the proposed device has a remote compliance centre and the effect of fitting the device to the wrist of a robot is to enable it to remove a peg from a hole without jamming or wedging even when the peg-hole clearance is very small.