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9 detection modes of photoelectric sensor

baijiahao | 30/06/2021

The detection modes of photoelectric sensors are divided into the following categories: through-beam type, reflector type, polarized reflector type, direct-reflection type, wide beam type, focus type, fixed area type and adjustable area type. Among them, the direct-reverse type, wide beam type, focus type, fixed area type and adjustable area type are sometimes classified as "photoelectric proximity detection mode" (note: not to be confused with capacitive or inductive proximity switches). For fiber optic sensors, if you use a through-beam fiber, it is a through-beam detection mode; if you use a straight-and-reflex fiber, it is a proximity detection mode. Ultrasonic sensors have two detection modes: on-beam type and proximity type.

1. Opposite shooting

The transmitter and receiver of the through-beam detection mode are installed opposite each other, and the light of the transmitter is directly aligned with the receiver. When the measured object blocks the beam, the sensor output changes to indicate that the measured object is detected. Through-beam type is the earliest used photoelectric detection mode. Before the advent of modulated light, the alignment of the transmitter and receiver was a big problem. Today, for photoelectric sensors that use high-energy modulated light, it is very easy to align the transmitter and receiver.

2. Reflective plate type

In the reflective detection mode, a sensor itself has both a transmitter and a receiver. The transmitter emits light onto the reflector, and the reflected light returns to the receiver. When an object blocks the light beam, the measured object is detected. The detection distance of the reflector sensor is the distance from the sensor to the reflector. The effective beam is usually cone-shaped, from the edge of the lens to the edge of the reflector. This is different in special cases, such as: when the sensor is too close to the reflector, the beam cannot cover the entire reflector; or when the emitted light is a laser beam. In these cases, the size of the effective beam does not extend to the entire area of ​​the reflector.

3. Proximity detection mode

The photoelectric and ultrasonic sensors in the proximity detection mode determine whether there is an object under test by detecting the energy reflected from the object under test. For example, when the ultrasonic sensor receives the sound waves reflected by the measured object, the measured object is detected. The transmitter and receiver of this sensor are assembled together and on the same side of the sensor. In this detection mode, when the measured object appears, it reflects a certain amount of light back to the sensor instead of blocking the light as in the through-beam detection mode. The photoelectric proximity detection mode is divided into the following detection methods: direct-reflection, wide-beam, focus, fixed-area, and adjustable-area.

4. Straight and trans

Among the photoelectric sensors, the direct-reverse sensor is a commonly used detection mode. In this way, the light emitted by the transmitter hits the surface of the measured object at various angles, and the surface of the measured object also reflects the incident light at multiple angles, of which only a small part is reflected back to the receiver.

The direct-reverse detection mode has a relatively low utilization rate of light energy, because its receiver can only receive a small part of the reflected light. Like other proximity detection modes, the direct and trans-type are also affected by the reflectivity of the object's surface. For the object to be measured with a bright white surface, the detection distance of the sensor will be farther than that of an object with a dark surface.

5. Focused

Most focus sensors add a lens to the transmitter to focus the emitted light at a certain point in front of the lens, while the focal point of the receiver lens is also here. In this way, a small energy-intensive detection area is formed at a fixed distance.

The focus sensor has a high utilization rate of reflected light. It can reliably detect small objects and objects with very low reflectance that cannot be detected by general direct-reflection or wide-beam direct-reflection sensors.

6, fixed area

The fixed area sensor is a photoelectric proximity sensor, which has a very clear detection range, and can ignore objects outside this range without being affected by the reflectivity of the surface of the object.

The fixed area sensor judges whether the measured object is present by comparing the amount of reflected light falling on the two receivers. If the reflected light falling on the receiver R2 is equal to or more than the reflected light falling on R1, the sensor detects the object.

7. Adjustable regional type

Just like the fixed area type, the adjustable area type sensor can distinguish objects located at different distances. In this case, the detection distance is adjustable. The receiver of the adjustable area sensor can generate two currents I1 and I2, and the ratio of these two currents will vary depending on where the reflected light falls on the receiver. The position of the sensor's cut-off point is directly related to this ratio. The position of the cut-off point can be adjusted by a potentiometer. Even if the object outside the cut-off point has a strong reflectivity, the sensor will still ignore it.

8. Optical fiber type

In fact, the optical fiber type is not a specific detection mode, and different detection modes can be formed by using different optical fibers. Separate optical fibers can be used to form a through-beam detection mode, and integrated optical fibers can be used to form a reflective plate or proximity detection mode. The optical fiber uses a special structure in the detection head to form a focused detection lens.

9. Ultrasonic approach mode

After the AC voltage is supplied, the ultrasonic generator will vibrate. This kind of vibration alternately compresses and strikes air molecules to continuously transmit ultrasonic waves, and the ultrasonic generator can also receive ultrasonic echoes.

Ultrasonic sensors are divided into electrostatic and piezoelectric types according to their generators. Electrostatic sensors are used for long-distance detection, usually up to 6-7 meters. This long-distance sensor is often used to detect the liquid level of a large container, etc. Piezoelectric sensors usually have a short detection distance, generally around 1 meter, but they are well sealed and can be used in harsh environments.

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