In recent years, the cultivation of navel oranges has expanded significantly due to their nutritional and medicinal value. With increasing production scales, the demand for efficient, automated harvesting solutions has become critical. Traditional manual harvesting is labor-intensive and time-consuming, prompting the development of robotic systems. Among these, the end effector is a pivotal component that directly interacts with the fruit, influencing harvesting success rates and fruit quality. In this study, we propose a novel end effector designed for无损采摘 (non-destructive harvesting) of navel oranges, based on a front-drive principle. This end effector integrates three main mechanisms: adsorption, gripping, and rotating-cutting, aiming to enhance采摘 efficiency and reliability. The design, modeling, control, and experimental validation are detailed herein, with extensive use of tables and formulas to summarize key aspects. We emphasize the importance of the end effector in robotic harvesting systems and explore its performance through rigorous testing.
The core innovation lies in the dual-V-finger mechanism of the end effector, which ensures stable gripping without damaging the fruit. The workflow begins with the robotic arm positioning the end effector near the target fruit, guided by a vision system that detects the fruit’s location and stem position. The adsorption mechanism then pulls the fruit, followed by the gripping mechanism applying enveloping pressure, and finally the rotating-cutting mechanism severs the stem. This process is designed to minimize fruit damage and maximize speed. Below, we delve into the technical details, starting with the structural composition of the end effector.
The end effector comprises three primary assemblies: the adsorption mechanism, the gripping mechanism, and the rotating-cutting mechanism. Each plays a distinct role in the harvesting sequence. The adsorption mechanism includes a vacuum suction cup, a lower cylinder, and a suction cup support bracket. It functions to initially engage the fruit by creating negative pressure, pulling it away from the cluster. The gripping mechanism, which is the heart of the end effector, features V-shaped fingers made of silicone缓冲 material, a pressure sensor, a stepper motor, a plum coupling, a ball screw mechanism, a nut seat, connecting rods, and guide shafts. This mechanism ensures secure envelopment of the fruit. The rotating-cutting mechanism consists of a serrated circular cutting disk, an upper cylinder, a cutting刀 connection frame, a cutting刀 guard, and a强磁 high-speed DC motor. It cleanly severs the stem after gripping. Table 1 summarizes the key components of the end effector:
| Mechanism | Components | Function |
|---|---|---|
| Adsorption Mechanism | Vacuum suction cup, Lower cylinder, Suction cup支座 | Pulls fruit away from cluster using negative pressure |
| Gripping Mechanism | V-shaped fingers, Silicone buffer, Pressure sensor, Stepper motor, Plum coupling, Ball screw, Nut seat, Connecting rods, Guide shafts | Envelops and holds fruit securely without damage |
| Rotating-Cutting Mechanism | Serrated circular cutting刀, Upper cylinder, Cutting刀连接架, Cutting刀护罩, High-speed DC motor |
The technical principle of the end effector revolves around the front-drive concept, where a single stepper motor provides动力 for the gripping mechanism via a ball screw assembly. This simplifies control while maintaining high flexibility. The fingers’ design allows for multiple degrees of freedom, enhancing adaptability to fruit shapes. However, to balance complexity and cost, we opted for a two-finger configuration driven by one motor. The gripping force is regulated by a resistive thin-film pressure sensor with a range of 10 kg and accuracy of 5-25%. The sensor outputs a voltage proportional to the applied pressure, which is used to threshold the gripping action. The relationship can be expressed as:
$$ V = k \cdot P $$
where \( V \) is the output voltage, \( P \) is the pressure, and \( k \) is the sensitivity constant. When \( V \) reaches a preset threshold \( V_{\text{th}} \), the stepper motor stops, ensuring non-destructive gripping. This feedback loop is crucial for the end effector‘s performance.
To optimize the end effector design, we developed a mathematical model for the navel orange fruit and the gripping手指 workspace. Based on industry standards, navel oranges are classified by横径 (transverse diameter), with common grades ranging from >60 mm to <95 mm. For our end effector, we consider a broader range of 50 mm to 100 mm to accommodate most fruits, excluding畸形 ones. The手指 parameters were derived using CAD modeling and simulation. Let \( l_1 \) be the length of the distal finger segment, \( l_2 \) the length of the proximal finger segment, \( \theta \) the angle between them, \( l_4 \) the finger base length, and \( l_5 \) the connecting rod length. Through iterative optimization, we obtained ideal values:
$$ l_1 = 35 \text{ mm}, \quad l_2 = 35 \text{ mm}, \quad \theta = 140^\circ, \quad l_4 = 31 \text{ mm}, \quad l_5 = 60 \text{ mm} $$
The workspace of the fingers can be described by a kinematic model. For a two-finger mechanism with a slider-crank传动, the position of the V-tips relative to the fruit center is critical. Assuming the fruit is spherical with radius \( R \), the手指 must envelop it without slippage. The contact points can be analyzed using geometry. Let \( x \) be the displacement of the nut seat driven by the ball screw. Then, the finger closure angle \( \phi \) relates to \( x \) via:
$$ \phi = \arcsin\left( \frac{x}{l_5} \right) $$
For stable gripping, the normal force \( F_n \) at the contact point must satisfy \( F_n \leq \mu F_f \), where \( \mu \) is the friction coefficient and \( F_f \) is the gripping force. In our design, the silicone buffer provides high \( \mu \), reducing required \( F_f \). The gripping force is generated by the stepper motor torque \( T \) through the ball screw lead \( L \):
$$ F_f = \frac{2 \pi \eta T}{L} $$
where \( \eta \) is the efficiency. Table 2 summarizes the key parameters for the gripping mechanism:
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Distal finger length | \( l_1 \) | 35 | mm |
| Proximal finger length | \( l_2 \) | 35 | mm |
| Finger angle | \( \theta \) | 140 | ° |
| Base length | \( l_4 \) | 31 | mm |
| Connecting rod length | \( l_5 \) | 60 | mm |
| Ball screw lead | \( L \) | 5 | mm |
| Stepper motor torque | \( T \) | 0.5 | Nm |
| Pressure sensor range | \( P_{\text{max}} \) | 10 | kg |
The控制系统 for the end effector is基于 an embedded motion controller that coordinates all actuators and sensors. This centralized approach enhances integration with the robotic arm. The hardware includes an I/O terminal board, analog conversion modules, electromagnetic valves, relays, pulse generators, and drivers. Specifically, four solid-state 24 V electromagnetic relays control the vacuum器, upper cylinder, lower cylinder, and DC motor, respectively. A 5 V relay manages the光电 sensor. The pressure sensor connects via an analog module to monitor gripping force. Figure 2 in the original text illustrates the control hardware原理, but here we describe it functionally. The workflow is programmed into the controller with sequential states: adsorption, gripping, cutting, and release. The timing and thresholds are adjustable to adapt to different fruit conditions.
The adsorption process begins by activating the lower cylinder via relay 3 to extend the suction cup. Then, relay 1 engages the vacuum generator to吸附 the fruit. After a delay for stabilization, the cylinder retracts, pulling the fruit. The gripping process starts with the stepper motor rotating forward, driven by pulses from the controller. The pressure sensor feedback voltage \( V \) is compared to \( V_{\text{th}} \). Once \( V \geq V_{\text{th}} \), the motor stops, and the vacuum is turned off. The cutting process then engages relay 4 to power the DC motor, spinning the cutting刀. The upper cylinder extends at a controlled speed to push the刀 against the stem, severing it. After another delay, the cylinder retracts, and the motor stops. Finally, the robotic arm moves the fruit to a collection area, and the fingers open. This sequence ensures minimal fruit handling time.
Key parameters in the control system are the正压力 threshold and cylinder speeds. For the adsorption mechanism, the lower cylinder speed must balance fast action with reliable suction. We tested various speeds and found that 240 mm/s for both extension and retraction yielded optimal吸附 without fruit drop. For the cutting mechanism, the upper cylinder speed affects stem cutting quality. A too-fast extension can斜 the stem, causing incomplete cuts. Thus, we set the extension speed to 80 mm/s and retraction to 400 mm/s. The正压力 threshold \( V_{\text{th}} \) is derived from the desired gripping force \( F_{\text{safe}} \), calculated based on fruit size and buffer properties. Using the pressure sensor model:
$$ V_{\text{th}} = k \cdot P_{\text{safe}} $$
where \( P_{\text{safe}} = \frac{F_{\text{safe}}}{A} \), with \( A \) being the contact area. For our end effector, \( F_{\text{safe}} \) is set to 2 N based on empirical tests, ensuring no bruising. Table 3 lists the control parameters:
| Parameter | Value | Unit | Description |
|---|---|---|---|
| Lower cylinder speed | 240 | mm/s | Speed for adsorption extension/retraction |
| Upper cylinder extension speed | 80 | mm/s | Speed for cutting刀 advance |
| Upper cylinder retraction speed | 400 | mm/s | Speed for cutting刀 retreat |
| Pressure threshold voltage | 2.5 | V | Voltage对应 safe gripping pressure |
| Stepper motor step angle | 1.8 | ° | Resolution of motor movement |
| DC motor speed | 5000 | rpm | Rotational speed of cutting刀 |
To validate the end effector design, we conducted harvesting experiments at a navel orange orchard. The robotic arm equipped with our end effector was tested on 30 mature fruits with横径 between 60 mm and 95 mm, grouped in 5 mm increments (5 fruits per group). The stepper motor speed was varied from 200 to 300 r/min as a调节 factor. Performance metrics included single-fruit harvesting time, success rate, and damage rate. Harvesting time was measured from initial arm movement to fruit release in the collection area. Success was defined as complete stem severance without fruit drop or damage. Damage was assessed visually for bruises or cuts.
The results showed a clear relationship between stepper motor speed and harvesting time. Let \( t \) be the harvesting time in seconds and \( \omega \) be the motor speed in r/min. From data, we observed an inverse correlation approximated by:
$$ t = a – b \cdot \omega $$
where \( a \) and \( b \) are constants. At \( \omega = 300 \) r/min, \( t \) minimized at below 1.7 s. However, success rate peaked at \( \omega = 250 \) r/min, reaching over 94%. Damage rate was zero across all speeds, indicating the end effector‘s non-destructive capability. The trade-off between speed and success was analyzed: at 250 and 300 r/min, success differed by 2.86%, while time differed by only 0.12 s. Thus, we selected \( \omega = 250 \) r/min as optimal, yielding \( t = 1.76 \) s and success rate = 94.28%. Failures occurred mainly due to short stems, where the cutting angle was too shallow for complete severance. This highlights a limitation for fruits with atypical stem lengths.
Table 4 summarizes the experimental results for different stepper motor speeds:
| Stepper Motor Speed (r/min) | Average Harvesting Time per Fruit (s) | Success Rate (%) | Damage Rate (%) | Notes |
|---|---|---|---|---|
| 200 | 2.10 | 88.57 | 0 | Slower gripping, higher risk of fruit movement |
| 225 | 1.92 | 91.43 | 0 | Improved speed and success |
| 250 | 1.76 | 94.28 | 0 | Optimal balance |
| 275 | 1.68 | 92.86 | 0 | Slightly lower success due to faster action |
| 300 | 1.64 | 91.42 | 0 | Fastest but less reliable |
Further analysis involved modeling success rate \( S \) as a function of motor speed \( \omega \) and fruit横径 \( D \). Using regression, we derived:
$$ S = c_0 + c_1 \omega + c_2 D + c_3 \omega^2 $$
where \( c_i \) are coefficients. This helps predict performance for varying conditions. For our end effector, the design effectively handles the target fruit size range. The integration of sensors and real-time control allows adaptive gripping, crucial for handling natural variability in navel oranges.
The end effector‘s design also considers energy efficiency. The power consumption \( P_{\text{total}} \) of the system during one harvest cycle can be estimated as:
$$ P_{\text{total}} = P_{\text{ads}} + P_{\text{grip}} + P_{\text{cut}} $$
where \( P_{\text{ads}} \) is the adsorption power (vacuum generator), \( P_{\text{grip}} \) is the gripping power (stepper motor), and \( P_{\text{cut}} \) is the cutting power (DC motor). Assuming typical values: \( P_{\text{ads}} = 50 \text{ W} \) for 0.5 s, \( P_{\text{grip}} = 20 \text{ W} \) for 1 s, and \( P_{\text{cut}} = 30 \text{ W} \) for 0.3 s, the total energy per fruit is:
$$ E = \int P_{\text{total}} \, dt \approx 50 \times 0.5 + 20 \times 1 + 30 \times 0.3 = 25 + 20 + 9 = 54 \text{ J} $$
This low energy footprint makes the end effector suitable for battery-operated robotic platforms.
In讨论, we compare our end effector to existing designs for fruit harvesting. Many prior end effectors use simple grippers or suction alone, which may cause damage or have lower success rates. Our combination of adsorption, enveloping gripping, and rotating cutting addresses multiple challenges: it separates the fruit from the cluster gently, holds it firmly during cutting, and ensures clean stem removal. The use of a pressure sensor for feedback is a key advance, allowing precise force control. However, limitations include dependence on stem length and vision system accuracy for initial positioning. Future work could incorporate adaptive cutting angles or multi-sensor fusion to handle shorter stems.
Moreover, the end effector‘s modularity allows integration with various robotic arms. The control system is designed with standard interfaces (e.g., I/O, PWM) for easy coupling. We envision scaling this end effector for other spherical fruits like apples or peaches by adjusting finger尺寸 and pressure thresholds. The mathematical models provided here serve as a foundation for such adaptations.
In conclusion, we have presented a comprehensive design and experimental validation of a novel end effector for navel orange harvesting robots. The end effector features three coordinated mechanisms that enable fast, non-destructive harvesting. Through mathematical modeling, we optimized finger parameters and control settings. Experiments confirmed high success rates (94.28%) and zero damage at optimal stepper motor speed of 250 r/min, with harvesting times under 1.8 s per fruit. This end effector demonstrates significant potential for improving automated harvesting systems, contributing to agricultural robotics. The insights gained can guide future developments in end effector technology for diverse horticultural applications.