TARGETED SHIPPING TRAYS

To ease the automation of the packing task, a returnable tray and a single-layer tray were selected as the targeted shipping trays. Dimensional data for the shipping trays are shown in table 1. The returnable tray employs cushioning materials with holes, into which fruits are placed vertically. The single-layer tray also has hollows, in which fruits are placed horizontally at an angle with respect to the long side of the tray.

THE SUPPLY UNIT

Components

The supply unit picks up the fruit in random postures from the harvesting container (L 26 cm

×W 54 cm×H 8 cm), and then puts it on the fruit conveyer. A schematic diagram of the unit is shown in figure 2. It consists of the machine vision system, a suction hand, a manipulator with three degrees of freedom (3DOF), and a fruit conveyer for supplying the fruit to the packing unit. A photo sensor is installed at the height of 15 mm above the conveyer around the placement point of a fruit, and the supply unit stops the operation when the photo sensor detects a fruit. When the supply unit puts the fruit on the conveyer, the photo sensor detects it, and the manipulator decelerates and stops. The suction hand always releases the fruit at the same height of less than 10 mm above the conveyer, regardless of its size or posture. It appears that the photo

sensor could not detect the fruits if they were tilted, so the photo sensor’s detection radius was

increased to 40 mm (FU-E40, Keyence, Japan). Furthermore, 5 mm-thick cushioning material was attached to the belt of fruit conveyer to protect the fruits from damage.

Figure 1. Schematic diagram of automatic packing system for strawberries.

Suction Hand

Table 1. Dimensional data for shipping trays.

Figure 2. Schematic diagram of supply unit.

Suction Hand

The suction hand of the supply unit is shown in figure 3.It consists of an ejector, a suction tube, a photo sensor, and a pressure sensor. It also has a pipe carrying purge air to release the sucked fruit smoothly. Approximately 10-mm thick cushioning material is attached to the top of the suction tube.

Tillett et al. (1995), who developed the vision-guided robot manipulator for packing horticultural produce,reported that a few tomatoes could not be picked up during the autonomous packing operation because the suction cup failed to make a seal with fruit due to the indentation around the stem. For strawberries, it appeared that making a seal with the fruit would be more difficult than for tomatoes because of the indentation caused by the calyx. At first we tried to apply a general ejector to the suction hand,but it generated a negative pressure of

less than -20 kPa and damaged the pericarp when sufficient air was supplied.Conversely, it was unable to make a seal with the fruit if it had an insufficient air supply. A blower, which also generates negative pressure, needs a thick pipe or a large manipulator, making it unsuitable for a suction hand.Finally, we decided to apply a special ejector for handling liquid crystal sheets in the manufacturing field: a cyclone pad (XT661-6A-R , SMC Corporation, Japan), which is a type of ejector that generates relatively gentle negative pressures ranging from -7 to -5 kPa with sufficient vacuum airflow to make a seal with the fruits, and which allowed miniaturization of the unit. Discharge air flow from the ejector comes out through the interstices of about 1 mm between the ejector and the frame of the suction hand. This air flow draws in air in the suction tube, then generates airflow from the top of the suction tube to the bottom of the suction hand. The suction hand approaches the fruit in the harvesting container from above. If the approach height is constant, however, the suction hand may be pushed by the manipulator onto the surface of the fruit and damage it. To prevent this type of damage, we allowed the suction tube to move freely up and down and installed a photo sensor which detected the motion of the tube. The motion of the suction hand is as follows: first, it approaches the target fruit, then the bottom of the suction tube touches the surface of the fruit. Next, the suction tube is raised. When the photo sensor detects the lifting motion of the suction tube, the z-axis actuator decelerates and stops. As a result,the load placed on the fruit is only the weight of the suction tube (approximately 30 g) during the approaching motion.