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Page 98 Li et al. Intell Robot 2022;2(1):89–104 I http://dx.doi.org/10.20517/ir.2022.02

Figure 5. Comparison of the actual and expected joint displacements: (A-D) Joints 1–4.

Figure 6. The trajectory tracking under D-ILC and PD-ILC.

It is observed that both ILC laws can realize trajectory tracking control, and the proposed law is superior to
the D-ILC law.

Figure 7 shows the varying tracking errors of each joint. The maximum and mean tracking errors of the two
controllers are given in Table 2. As shown in Figure 7, the two controllers have similar error trends. The errors
of Joints 1 and 3 increase rapidly from the beginning of the rotational motion and reach the maximum values
after the complete rotation, of which the maximum values are 0.94 and 0.81 for D-ILC and 0.71 and 0.61 ◦
◦
◦
◦
for PD-ILC, respectively. The other two joints can achieve good performances after iterative learning, with the
maximum errors approximating to zero, as shown in Table 2.