Photoelectric sensors come in various types, primarily categorized based on their detection modes. These include:
1. **Reflective Photoelectric Sensors** – These sensors combine the light source and the photodetector in one unit. The emitted light is reflected by the object back to the sensor, enabling it to detect the presence of an object. This type is particularly useful for detecting objects with different reflectivity, such as black and white surfaces.
2. **Transmissive Photoelectric Sensors** – In this configuration, the light source and the detector are placed opposite each other. When an object passes between them, it blocks the light beam, triggering a signal. This method is often used in applications where precise object detection is required, such as in conveyor systems or automated sorting.
3. **Focusing Photoelectric Sensors** – These sensors are designed to focus the light at a specific distance. The detector only receives the light when the object is at that exact focal point, making them ideal for applications requiring high precision, like in industrial automation.
A reflective photoelectric sensor can be used to distinguish between black and white objects. Since black and white materials have different reflection coefficients, the sensor can be adjusted so that it only responds to the reflected light from the white surface. When a black object is detected, the weak reflection is not enough to activate the sensor, resulting in a low signal output. Conversely, a white object reflects more light, causing the sensor to trigger a high signal. This principle is widely used in robotics and automation for line-following applications.
The circuit works as follows: when a black object is present, the infrared LED emits light that is poorly reflected, so the phototransistor remains off. This causes point A to be high, which is then inverted by a 7414 inverter, resulting in a low signal being sent to the microcontroller. When a white object is detected, the light is strongly reflected, turning on the phototransistor, which lowers point A and results in a high signal being received by the microcontroller. This allows the system to determine whether the object is black or white.
Integrated photoelectric sensors come in several forms, including reflective, transmissive, and focusing types. They also vary in terms of light sources, such as visible red (650 nm), green (510 nm), and infrared (800–940 nm). Each light source has its own advantages; for example, infrared is suitable for long-range detection, while red or green light is better for color contrast. The shape of the sensor can also vary, with common options including cylindrical, square, and groove types.
In practical applications, such as detecting a black line on a white surface, the sensor uses the difference in reflection coefficients to determine the presence of the line. One simple approach involves using LEDs and photoresistors, but this setup is vulnerable to external light interference. A more advanced solution uses pulse-modulated infrared signals, which significantly reduce noise and improve reliability. This method is commonly used in commercial products due to its simplicity, stability, and effectiveness.
When selecting a motor for a small robot or trolley, two main options are available: DC motors and stepper motors. Stepper motors offer precise control, quick start/stop capabilities, and high sensitivity, but they are generally more expensive. DC motors, on the other hand, provide smooth speed control, strong overload resistance, and are easier to implement in circuits. Due to their cost-effectiveness and ease of use, DC motors are often preferred in many applications.
For motor drive schemes, several methods exist. Using a resistor network or digital potentiometer to adjust voltage is possible, but it may lead to inefficiency and complexity. Relay-based control is simple but suffers from slow response times and mechanical wear. The most efficient and reliable method is the H-bridge circuit, which uses four high-power transistors to control the motor's direction and speed. This design is widely used in modern electronics due to its high efficiency, fast switching, and stability.
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