*2.1. Design of the Gripper Inspired by Human Hand Grasping*

This work compared the grasping experiments of different size spherical fruits by hand. When picking larger fruit, such as apples and oranges, each finger joint rotated to make the palm close to the fruit, realizing the enveloping grasping. In contrast, when picking small fruit, such as cherry tomatoes and plums, only the ends of thumb, index finger, and middle finger contact with the fruit. The rotation angle of the metacarpophalangeal joint was the largest. The distal and proximal phalangeal joints turned slightly. Fingers formed a more fixed arc shape in the grasping process. The comparison of fruit grasping in different volumes shows in Figure 1.

**Figure 1.** Comparison of fruit grasping in different sizes: (**a**) grasping of larger fruit; (**b**) grasping of small fruit.

Therefore, inspired by the dynamic harvesting of small fruits by human hands, the gripper was designed, which has three features, including sinusoidal contour, single-joint rotation, and finger-end grasping. The equation of the sinusoidal curve refers to the outer shape, and its shape is shown in Figure 2a. The equation of the sinusoidal curve of the single finger is Equation (1):

> +6 √

3, *x* ∈ [0, 45*π*] (1)

<sup>30</sup> (*<sup>x</sup>* <sup>−</sup> <sup>20</sup>*π*)

*<sup>y</sup>* <sup>=</sup> 12sin <sup>1</sup>

**Figure 2.** The design of gripper: (**a**) opening and closing of gripper; (**b**) the constraint part.

The design of gripper is shown in Figure 2. Three fingers were in each of the three holes of the constraint part. The fingers were fixed to the chassis by the "L-shaped" steel parts. The steel parts can produce elastic deformation. The fingers close when the constraint part advances. When the constraint part retreats, the fingers gradually recover to their initial positions. Due to the sine curve is smooth, continuous, and differentiable, the constraint part moves smoothly in contact with the fingers. On the one hand, it is intended to transform the finger opening and closing into the constraint behavior, which realizes the stable movement and precise control of the fingers. On the other hand, the problems of grasping caused by rotational speed, torque, and precision of joint motor are avoided, to a certain extent.
