*4.1. Control Method of Constant-Pressure Feedback*

The required servo output torque *Md* can be obtained by identifying the diameter of the target fruit. To ensure that *fc* is less than *Fm* at this time, the output torque must be adjusted further. When the *fc* obtained at this time is greater than *Fm*, it should be ensured that the maximum torque can be output while the fruit is safely held. From Equation (11), let *fc* equal *Fm* at this point to obtain the critical torque *Mm* of safe clamping, which is set as the servo's output torque. The gripper control method of the constant-pressure feedback is shown in Figure 12.

**Figure 12.** Gripper control method of constant-pressure feedback.

During the no-load closing motion of the gripper, the servo output torque is stable at an initial torque. When the feedback torque of the servo is greater than the initial torque, the finger and the fruit seem to be in contact. To avoid fruit damage due to the impact of the dynamic load, close the gripper quickly to reach the contact position before touching the fruit, and slowly close the gripper after the finger is in contact with the fruit. When the feedback torque reaches the preset value, it is assumed that the fruit has been grasped, and the servo stops rotating.

In contrast to the sensor system embedded in the finger, the servo with feedback information is used as the driver to ensure constant-pressure contact between the finger and the fruit, simplifying the structure of the soft harvesting gripper and facilitating the fruit harvesting in complex growth environments.

#### *4.2. Control Method of Slip Detection*

Fin Ray soft fingers have great advantages in dealing with the problem of fruit unilaterally damaged by extrusion. The cross beams act as rigid support rods to ensure the stiffness of the fingers while also allowing the fingers to adaptively wrap the entire fruit, preventing fruit damage due to the stress concentration.

However, it is difficult to ensure that relative slippage between the fingers and the fruit does not occur during the fruit detachment process. Because of the rough silicone pads attached to the surface of the fingers, the sliding friction force between the fingers and the fruit is relatively great when there is relative slippage between them, and it is easy to cause bruises and scratches on the fruit pericarp. As a result, effectively avoiding relative slippage is essential to ensure that the fruit is not damaged. The condition of the relative slippage, which causes the fruit damage, is complicated and will not be discussed in this paper.

A slip detection method is proposed for the designed soft gripper, which obtains the fruit position in real-time through the distance sensor. One believes that when the relative slip distance between the fruit and the fingers Δ*L* reaches *Ls*, the fruit tends to slip off, as shown in Figure 13. At this time, the output torque can be increased on the premise of ensuring that the maximum gripping force *Fm* is not exceeded, and the fruit can be clamped to prevent further sliding; if the relative slip distance Δ*L* can still reach *Ls* after increasing the output torque, clamping and pulling the fruit will increase the risk of damage, such as bruises and scratches. It means that the fruit is difficult to harvest at this point, and it is considered a harvesting failure, and the soft gripper is released. Controlling the gripper to perform the aforementioned operations *n* times, if harvesting fails all *n* times, give up picking this fruit. The slip detection control method is shown in Figure 14.

**Figure 13.** Slipping trend of fruit.

**Figure 14.** Gripper control method of slip detection.

Although the risk of harvesting failure is increased by the method proposed above, it does not cause damage to the fruit, and the fruit after harvesting failure can still be harvested manually without affecting its economic value or reducing economic losses.

#### **5. Test and Analysis**

#### *5.1. Test Analysis of the Mechanical Properties of Apple*

The Model E43 of MTS Exceed® Electromechanical Test Systems was used to conduct the relevant tests to obtain the relevant mechanical properties of the apples as the basis for the design of the gripper in this study. The range is 100 N, and it has a force and displacement sensor. Yantai Fushi apples were chosen as the test samples during the experiments.

In our study, a silicone pad is attached to the surface of the finger to improve the grasping performance by increasing the friction of the fruit's surface. To measure the maximum static friction coefficient *μ* between the silicone pad and the fruit, the pressure *Fn* was applied to the fruit through Model E43, and a silicone pad was pasted on the upper indenter and lower support, respectively. The tensile force of horizontally pulling the fruit was measured with a tension meter, as shown in Figure 15, and the horizontal pulling force *Fp* was measured from the beginning of the fruit slippage.

**Figure 15.** Diagram of mechanical properties test device.

Ignoring the apple's weight, it can be obtained from the static balance of the apple,

$$F\_p = 2\mu \times F\_n.\tag{17}$$

The test results and fitting function are shown in Figure 16, R2 = 0.92. Therefore, *μ* = 0.8 can be obtained.

**Figure 16.** Test data and fitting curve.

To obtain the detachment force *Fd* required for fruit detachment, the apple was fixed on the support and kept still; then one end of the branch was fixed with the collection of the Model E43 and pulled axially. When the fruit branch was broken through the force sensor, the maximum pulling force was recorded. The experimental results are shown in Figure 17. The experiment used twenty apple samples with diameters ranging from 65 mm to 95 mm.

**Figure 17.** The influence of apple diameter on detachment force.

The results show that *Fd* is distributed between 8.88 N and 39.6 N. *Fd* generally increased as the apple diameter increased, but a small portion showed an irregular distribution. This could be because fruits with larger stem diameters have more connection force between branches and apples, necessitating more detachment force. At the same time, in the report of Bu [53] et al., the detachment force is much greater when the natural growth angle of the fruit is obtuse than when it is acute, as shown in Figure 18. In this experiment, we did not pay too much attention to the relation of detachment force to stem diameter and fruit growth angle. The test results were consistent with those of Bu [53] et al.

**Figure 18.** The natural growth angle of apple.

To avoid damaging the apple pericarp due to excessive gripping force, the maximum pressure *Fm* that the fruit pericarp can withstand must be known. We make a rectangular apple sample block of 10 mm × 10 mm × 20 mm near the apple's pericarp, place it on the middle of the support of the Model E43, and apply a load to the apple sample until it is destroyed. The force–displacement relationship during the apple-sample compression experiment was recorded, and the results are shown in Figure 19.

**Figure 19.** Force–displacement curve of the apple samples.

It can be seen that, once the force reaches 15.35 N, it remains almost unchanged with a one-stage displacement increase. This demonstrates that, when the force reaches 15.35 N, the apple begins to tend to plastically deform. According to the energy principle of the apple damage proposed by Schoorl [54], the damage volume of the apple is proportional to the energy it absorbs. To reduce the amount of energy transmitted to the apples during harvesting, set the maximum pressure *Fm* that apples can withstand to 15.35 N. The test results were consistent with those of Grotte [55] et al.

#### *5.2. Gripping Force Verification Experiment*

The rated torque that the servo can provide in this soft gripper is 12 kg·cm (1.2 N·m), assuming that the maximum torque that a single finger can provide is 0.4 N·m. *LFc* is 28 mm; *LR* is 12 mm; *h* is 65 mm; *α* is 80◦, and *μ* is 0.8. According to the structural design of the gripper, *θFF* is between −12◦~15◦, and *θFR* is between −30◦~53◦. Given that the fruit radius *r* varies, *σ* is customarily between 0◦ and 40◦, and *δ* is traditionally between 0◦ and

25◦. In the test, the diameter of the apple sample is about 90 mm. At initial contact, the finger and the fruit can be regarded as point contact. *θFF* is generally around 10◦, and *θFR* is generally around 30◦, as shown in Figure 20a. According to Equation (11), the maximum initial gripping force *fc* of a single finger is approximately 15.34 N. The output torque *Md* is little as the first contact, so the contact force between the finger and the fruit is far less than the maximum initial gripping force. When the gripper continues to close, *σ* and *δ* become larger, *γ* becomes smaller, and *β* becomes larger, so the finger gripping force *fc* becomes larger, as does the pulling force *F*. At full contact, *θFF* is typically around −12◦, and *θFR* is typically around −30◦, as shown in Figure 20b. Therefore, the maximum gripping force *fc* of a single finger is about 16.21 N. On the basis of Equation (16), the maximum pulling force *F* of a single finger is about 14.18 N, resulting in the maximum pulling force of the entire gripper being approximately 42.55 N.

**Figure 20.** Contact model of soft gripper: (**a**) initial contact; (**b**) full contact.

According to the above test results, the detachment force when pulling to harvest the fruit is about 8.88 N–39.6 N, indicating that the designed gripper's maximum pulling force meets the detachment requirement.

The gripping force resulting from the adaptive bending deformation of the soft fingers in contact with the fruit surface, which was measured by a thin-film pressure sensor (RP-L TDS REV C.) mounted between each finger and the silicone pad, as shown in Figure 21a. The RP-L type soft thin-film pressure sensor was composed of polyester film, high conductive material, and pressure-sensitive material. It converts the pressure acting on the thin-film area of the sensor into a change in resistance.

The test started when the finger made contact with the apple, and the output torque of the servo increased by 0.2 kg·cm (0.02 N·m) each time until it reached the rated torque of 12 kg·cm (1.2 N·m). To compare the difference in the gripping force of the finger on the surface of the fruit when the diameter of the fruit changes, apples with diameters of 70 mm, 80 mm, and 90 mm were chosen for the test, as shown in Figure 21b. In each test, the pressure output by the sensor and the servo torque was recorded, as shown in Figure 22.

As can be seen from the figure, there is an approximate positive relationship between the gripping force of the soft finger and the servo torque, and the image fits the theoretical curve well. Furthermore, it can be found that the effect on the gripping force is not very significant when the diameter of the fruit changes. Therefore, the finger output force during picking can be controlled by adjusting the servo output torque.

Nevertheless, the single-finger gripping force at a torque up to 1.2 N·m for the fruit diameter of 90 mm does not reach the theoretically calculated maximum value, which is probably due to the lack of accuracy from the thin-film pressure sensor.

**Figure 21.** Experimental structure diagram: (**a**) finger with a RP-L sensor; (**b**) experimental platform.

**Figure 22.** Relationship between torque and gripping force.

*5.3. Test Analysis on the Harvesting Performance of the Soft Gripper*

During the grasping and harvesting tests, the soft gripper was fixed on Franka, a seven-axis robotic arm with a high-sensitivity force-control performance, as shown in Figure 23. The tests were carried out in an orchard located in Changping Distrct, Beijing.

**Figure 23.** Experimental scene in the orchard.

5.3.1. Feasibility Test Analysis of Constant-Pressure Feedback System

To ensure that the finger gripping force is less than 15.35 N, the servo output torque is set to control the maximum gripping force *fc*. Assuming that the detachment force required for fruit detachment is 40 N, it can be obtained from Equation (16) that the required output torque is 10.25 kg·cm (1.025 N·m). A single finger's grasping force *fc* is 13.89 N, which is not harmful.

Therefore, a grasping comparison test was performed to verify the improvement of the soft gripper's safe grasping performance by the force feedback system. In this experiment, a total of 20 apple samples with no damage on the fruit skin were selected and divided into two groups of ten apples each. In the first set of experiments, the force feedback system was turned on, and the clamping test was performed on each apple. The clamping process followed the logic of the flowchart in Figure 12, and the clamping posture is shown in Figure 24a. After the gripper has completely and stably grasped the apple, hold it still for 5 s before releasing the fruit. In the second set of experiments, all experimental conditions were the same except that the force feedback system was turned off. As there is no output torque control, the clamping will stop until the servo reaches the locked rotor torque, and the clamping posture is shown in Figure 24b. The contact area on the fruit was marked after each release, and the fruit was then stored at the same constant temperature for 7 days. After taking them out, make a note of the damage on the apple surface's contact area. The radius of the damaged area was less than 10 mm for slight damage and greater than 10 mm for serious damage.

**Figure 24.** Gripper attitude with force feedback on and off: (**a**) force feedback on; (**b**) force feedback off.

Observing the apple surface, the contact area of the apples clamped by the gripper with an open force feedback system was not damaged, so the damage rate was 0%; however, the apples were clamped by the gripper with a closed force feedback system. On the other hand, the slight damage rate was 10%, and the severe damage rate was 10%; the specific pericarp damage is shown in Figure 25. The experimental results show that activating the constant-pressure feedback system improves the soft gripper's safe grasping performance and effectively ensures non-destructive fruit grasping.

**Figure 25.** Specific damage to apple pericarp: (**a**) slightly damaged; (**b**) severely damaged.

5.3.2. Test Analysis of Harvesting Success Rate and Apple Damage Rate

We carried out picking experiments to verify the stability and safety of the soft harvesting gripper designed in this paper. The harvesting process followed the logic of the flowcharts in Figures 12 and 14 with the force feedback system on. The soft finger length is 120 mm, while the effective gripping length is 100 mm. In the tests, *Ls* was set to 10 mm. To grab and separate the fruit, the soft picking gripper was controlled by Franka's arm with a pulling speed of 2 mm/s.

First, we analyzed various situations that occurred in the fruit harvesting process with the fruit slip detection turned on. The process began with the gripper approaching the fruit and ended with the fruit being harvested. The condition of the fruit slip and the change in the servo output torque for the three situations of no obvious slip, first slip, and second slip was recorded afterwards, as shown in Figure 26.

The figure shows that, even if the fruit did not slip for the first time, there would be a slight relative movement to the finger during harvesting, which might be due to the fingertip not being completely in contact with the fruit. After the fruit slipped slightly, the fingertip and the fruit made complete contact, providing adequate support for the apple. It was also conceivable that the measurement distance was floating within the accuracy range due to a lack of sensor accuracy. When the fruit slipped for the first time, the occurrence time was approximately 10 s, implying that the gripper pulled the fruit 2 cm. At this point, the fruit branch was completely straightened, and sufficient force was required to detach it from the branch; if the fruit slipped for the second time, it proved that it was not enough to harvest the fruit under the premise of safe harvesting; in addition, further harvesting might damage the fruit.

It can be ascertained that, during fruit harvesting, the stable servo output torque can ensure that the fruit does not break free due to the gripper loosening.

To further verify the effectiveness of the gripper harvesting, the tests for the three cases of rigid fingers and soft fingers with or without slip detection under the gripper structure of this study were carried out, as shown in Figure 27. For each group of the experiments, 25 apples with completely undamaged pericarps were selected. The picking situation and fruit harvesting damage were observed and recorded. The experimental results are shown in Tables 2–4.

**Figure 26.** The slip condition of the fruit and the change of the output torque with time.

**Figure 27.** Three sets of outdoor picking experiments: (**a**) rigid fingers; (**b**) soft fingers with or without slip detection.


#### **Table 2.** The harvesting situation of rigid fingers.

<sup>1</sup> When the slip detection is turned off, we define the visible slippage as the fruit that is about to slide to the fingertips of the gripper or that has already broken from the gripper (the same as below). <sup>2</sup> Because the fruit damage due to slippage in the gripper and due to grasping are quite different in character, we can distinguish them more easily (the same as below).

**Table 3.** The harvesting situation of soft fingers without slip detection.


#### **Table 4.** The harvesting situation of soft fingers with slip detection.


<sup>1</sup> With slip detection on, the second slippage of the fruit means that the picking has failed. At this point, each fruit was picked twice; it implies that the fruit has failed, and the next fruit would be chosen if both pickings failed.

Comparing Tables 2 and 3, the picking success rate for the rigid fingers is 100%, with a damage rate of 16%, while the success rate for the soft fingers is 96%, and the damage rate is 8%, both of these have the silicone gasket applied to the surface. This shows that the optimized Fin-Ray soft fingers in this paper are able to reduce the fruit damage better. At the same time, we can see that visible slippage of the fruit was common in both cases and that most of the damage occurred during the fruit slippage in the gripper. In the rigid fingers experiment, three fruits were damaged by slippage and one by grasping, which

also shows that the rigid support structure is prone to fruit damage despite the flexible silicone gasket applied to the surface. In the soft fingers experiment, both damaged fruits were caused by slippage. Therefore, the effective control of the fruit slip in the gripper is essential to reducing the risk of fruit damage.

Comparing Tables 3 and 4, although the picking success rate dropped to 80% with slip detection on, there was no fruit damage. It turns out that the soft gripper with slip detection can effectively reduce fruit damage. Despite the fact that the harvesting success rate will decrease, the fruit will not be harmed, and its economic value will not be impacted after manual harvesting. In addition, we can see from Table 4 that 13 fruits made the first slippage, and in 7 of them, the second slippage occurred, further demonstrating the prevalence of fruit sliding during picking. Although five fruits failed in the second picking, no fruit were damaged, which indicates that the proposed control method for slip detection is effective in preventing damage to the fruits.

According to the above experimental results, the proposed Fin-Ray soft harvesting gripper with force feedback and fruit slip detection enables stable and non-destructive fruit picking. Notably, to improve the harvesting lossless rate, it is necessary to sacrifice some harvesting success rates by detecting slippage between the fruit and the fingers.

**Remark 2.** *It should be noted that the experimental results of the outdoor harvesting could be regarded as the effect of combining both force feedback and slip detection on the basis of the optimized harvesting gripper.*
