## 3 Operational Requirements
### Fruit Identification and Selection
#### 3.1.3 Proximity and Touch Sensors  
Proximity sensors detect the presence and distance of nearby objects without the need for physical contact. There are several types of proximity sensors, but the most common in agricultural robotics are ultrasonic sensors, infrared (IR) sensors, and induction sensors:  
1. **Ultrasonic sensors:** They emit high-frequency sound waves and measure the time it takes for the echo to return after bouncing off an object. The distance is calculated based on the flight time of the sound [27].
2. **Infrared (IR) sensors:** They emit infrared light and detect the amount of light reflected by a nearby object. The distance is determined by the intensity of the reflected light [28].
3. **Induction sensors:** They generate a magnetic field and detect changes in the field when a metal object approaches. They are less common in agriculture, but useful in industrial settings.  
In harvesting robots, proximity sensors are used to:  
1. **Fruit detection:** Identify the presence of fruits in the robot's field of action.
2. **Navigation and Obstacle Avoidance:** Helping the robot move through the field without colliding with plants, branches, or other objects.
3. **Precise positioning:** Facilitate the precise approach of the robotic arm to the fruits to ensure damage-free harvesting [29].  
Touch sensors, also known as force and pressure sensors, detect physical contact and applied force. These sensors can be piezoelectric, resistive, or based on capacitive technology:  
1. **Piezoelectric sensors:** They generate an electrical charge when pressure is applied. The amount of charge generated is proportional to the force applied [30].
2. **Resistive sensors:** They change their electrical resistance in response to pressure. The variation in resistance is used to measure the force applied [31].
3. **Capacitive sensors:** They detect changes in capacitance when pressure is applied to a sensitive surface. They are sensitive and accurate, suitable for detecting light forces [32].  
In harvesting robots, touch sensors are used to:  
* **Firmness assessment:** To determine fruit ripeness based on its firmness, which is a key indicator of ripeness in some fruits.
* **Gentle handling:** Allowing the robot to apply the correct amount of force when grasping and picking fruit, preventing damage and ensuring gentle picking [33].
* **Real-time feedback:** Provide real-time information on contact and force applied, allowing immediate adjustments during picking to improve accuracy and reduce damage to fruits [34].  
The use of pressure and touch sensors in harvesting robots presents several challenges that must be addressed to ensure effective and safe operation in agricultural environments.  
Pressure and touch sensors need to be sufficiently sensitive to detect subtle variations in the pressure applied to fruits without exerting excessive force that could cause damage. Accuracy is critical for assessing fruit ripeness and firmness, as these factors can vary widely among different fruit types and environmental conditions.  
Fruits come in various shapes, sizes, and textures, which complicates the design of sensors that are effective across all varieties. Some fruits may be more delicate and require gentle handling, while others may be more robust and tolerate greater pressure without damage.  
Agricultural environments are also unpredictable with regard to factors such as temperature, humidity, and lighting. These conditions can impact the accuracy and reliability of pressure and touch sensors, making it challenging to maintain consistent and precise measurements across different environments.