*2.2. Design and Working Principle of End-Effector*

The end-effector designed in this paper was used to harvest small spherical fruits. The picking strategy of "Holding-Rotation" was used to realize picking. It was mounted on a picking robot based on binocular stereo vision. The picking robot guided the end-effector to aim at the fruit employing binocular vision. The gripper closed, grabbed the fruit, then rotated with the fruit to separate the fruit from the stalk (Figure 3).

**Figure 3.** The design of end-effector for small spherical fruit picking. Note: 1, Chassis; 2, Finger; 3, Pressure sensors; 4, Constraint Part; 5, Gear; 6, DC Motor; 7, Screw; 8, Slide track; 9, Slider; 10, Coupling; 11, DC motor drivers; 12, Stepper motor driver; 13, Stepper motor; 14, Arduino control panel; 15, Power; 16, Pole; 17, Linear bearings; 18, Change the channel.

During the holding stage, the gripper opens and closes to achieve fruit gripping. Its essence is to restrain the movement of fingers by linear movement of constraint part. Therefore, the end-effector rotates the screw through a stepper motor, driving the pole and the slider in reciprocating linear motion along the Slide track. Rotating stage. The DC motor drives the gear to turn after grasping the fruit. Then, the gripper and the fruit are rotated synchronously, separating the fruit from the stalk. The pressure sensors are FSR-402 sensors, which can obtain the gripping force in real time, and the measuring range is 0.1~100 N. The stepper motor is two-phase four-wire 42-stepper motor (42BYGH24). The torque is 0.13 N, and the step angle is 1.8◦. The stepper motor drive model is DM542C. The DC motor is the planetary DC decelerating motor with the working voltage of 24 V and a rotating speed of 300 r/min.

The picking time was taken as a technical index to realize the quick picking in this work. It was set to 1.5 s, which means from the beginning of the clamping to the separation of the stalks, including 1 s for the clamping phase and 0.5 s for the rotation phase.
