Study on switching from position to stiffness control for musculoskeletal-inspired robot arm based on direct sum decomposition

Figures & data

Figure 1. Musculoskeletal-inspired robot arm driven by three pairs of antagonistic actuators. $m_1$ and $m_2$ move the first joint, $m_3$ and $m_4$ move the second joint, and $m_5$ and $m_6$ move both joints.

Figure 2. Force generation model of the actuator. It consists of an elastic element, a viscous element, and a force generator that generates active forces.

Figure 3. Coactivation pattern of actuators represented by the new bases ${\hat{\mathit{e}}}_1,{\hat{\mathit{e}}}_2,\dots ,{\hat{\mathit{e}}}_6$. The black actuators output a unit active force in the positive direction, the white actuators in the negative direction, and the gray actuators do not output any actives. (a) ${\hat{\mathit{e}}}_1$, (b) ${\hat{\mathit{e}}}_2$, (c) ${\hat{\mathit{e}}}_3$, (d) ${\hat{\mathit{e}}}_4$, (e) ${\hat{\mathit{e}}}_5$, (f) ${\hat{\mathit{e}}}_6$.

Table 1. Physical parameters of the musculoskeletal-inspired robot arm.

	First link	Second link	Unit
Length	0.31	0.27	$\left[\mathrm{m}\right]$
Mass	1.93	1.32	$\left[\mathrm{kg}\right]$
Center of mass $(x,y)$	$(0.165,0.05)$	$(0.135,0.03)$	$\left[\mathrm{m}\right]$
Inertia moment	0.141	0.120	$[\mathrm{kg}\cdot {\mathrm{m}}^2]$
Moment arm	0.05	0.035	$\left[\mathrm{m}\right]$

Figure 4. Stiffness ellipsoid representing the tip stiffness properties.

Figure 5. Response of the tip position and velocity. We performed position control in the first five seconds, switched from position control to stiffness control at $t=5$ seconds, and applied an external force to the hand at $t=10$ seconds. (a) Tip position (x-direction), (b) Tip position (y-direction), (c) Tip velocity (x-direction), (d) Tip velocity (y-direction).

Figure 6. Active forces. We performed position control in the first five seconds, switched from position control to stiffness control at t = 5 seconds, and applied an external force to the hand at t = 10 seconds. (a) $m_1$, (b) $m_2$, (c) $m_3$, (d) $m_4$, (e) $m_5$, (f) $m_6$.