Development and application of laser ranging sensor
1. The development of transmission time laser distance sensor
Laser ranging is one of the earliest applications of lasers. This is due to the fact that lasers have many advantages such as strong directionality, high brightness, and good monochromatism. In 1965, the Soviet Union used lasers to measure the distance between the earth and the moon (380 ́103km), with an error of only 250m. In 1969, the Americans placed a mirror on the lunar surface after landing on the moon and also used a laser to measure the distance between the earth and the moon, with an error of only 15cm.
The basic principle of measuring distance using laser transmission time is to determine the target distance by measuring the time it takes for the laser to travel to and from the target. Namely: .
Transmission time laser ranging, although simple in principle and structure, was previously mainly used in military and scientific research, but is rare in industrial automation. Because laser ranging sensors are too expensive, they are usually in the few thousand dollars. In fact, all industrial users are looking for a sensor that can achieve precise distance detection at long distances. Because in many cases, sensors are limited by physical location and production environments, today's transmission time laser ranging sensors will solve the problem for engineers in these situations.
2. Working principle
Transmission time When the laser sensor works, the laser diode is first aimed at the target and emits laser pulses. After being reflected by the target, the laser scatters in all directions. The partially scattered light is returned to the sensor receiver, received by the optical system and imaged onto the avalanche photodiode. An avalanche photodiode is an optical sensor with an internal amplification function, so it can detect extremely weak light signals. The target distance can be determined by recording and processing the time elapsed from the time the light pulse is sent to when it is returned to be received. Transmission time laser sensors must determine transmission times with extreme accuracy because the speed of light is too fast.
For example, with a speed of light of about 3 ́108 m/s, to achieve a resolution of 1 mm, the electronic circuitry of the transmission time ranging sensor must be able to distinguish the following extremely short times:
0.001m¸(3´108m/s)=3ps
To distinguish the time of 3ps, this is an excessive requirement for electronic technology, and the cost is too high to achieve. But today's inexpensive transmit-time laser sensors cleverly avoid this obstacle, using a simple statistical principle that achieves a resolution of 1 mm and guarantees a fast response.
3. Solve problems that cannot be solved by other technologies
Transmission time laser distance sensors can be used where other technologies are not available. For example, ordinary photoelectric sensors that calculate the reflected light from the target can also perform a large number of precision position detection tasks when the target is very close. However, when the target distance is far away or the target color changes, ordinary photoelectric sensors are difficult to cope with.
While advanced background noise suppression sensors and triangulation sensors work well with color changes in the target, their performance becomes less predictable when the target angle is not fixed or the target is too bright. In addition, the general range of triangulation sensors is limited to 0.5m.
Ultrasonic sensors, although often used to detect objects at a distance, are not affected by color changes because they are not optics. However, ultrasonic sensors measure distances based on the speed of sound, so there are some inherent drawbacks that cannot be used in the following situations.
(1) When the target to be measured is not perpendicular to the transducer of the sensor. Because the target of ultrasonic detection must be within an angle of no more than 10° perpendicular azimuth to the sensor.
(2) When the beam diameter is small. Because the diameter of the ultrasonic beam is 0.76cm when it is 2m away from the sensor.
(3) Occasions where the position calibration of the visible spot is required.
(4) Windy occasions.
(5) Vacuum occasion.
(6) Occasions with large temperature gradients. Because in this case, it will cause changes in the speed of sound.
(7) Occasions that require rapid response.
The laser distance sensor can solve the detection in all of these situations.
4. Wide range of uses in the field of automation
Nowadays, in addition to ultrasonic sensors and ordinary photoelectric sensors, a new method that can solve long-distance measurement and inspection has been added to the automatic detection and control methods, the transmission time laser distance sensor. It provides application flexibility for a variety of different occasions, which can include the following:
(1) Equipment positioning.
(2) Measure the material level of the bale.
(3) Measure the distance and height of objects on the conveyor belt.
(4) Measure the diameter of the log.
(5) Protect the overhead crane from collision.
(6) Error-free inspection occasions.
5. Several application examples
1. Measure the width of the box on the conveyor belt
Two divergent transmission time laser sensors are used and mounted face to face on both sides of the conveyor belt. Because the position of the box with the change of size falling on the conveyor belt is not fixed, each sensor measures its own distance from the box, setting one distance as L1 and the other as L2. This information is sent to the PLC, which subtracts L1 and L2 from the total distance between the two sensors to calculate the width of the box W.
2. Protect the hydraulic forming die
The manipulator puts a pre-formed pipe into the lower die of the hydroforming machine, and the operator must ensure that the position is accurate each time. Before the upper die falls, a divergent sensor measures the distance from the critical section of the tube, which ensures that the die is in the correct position before closing.
3. Positioning of two-axle cranes
Mounted with two reflector-type sensors facing the reflector, which is mounted on the two moving units of the overhead crane. One unit moves forward and backward, and the other moves left and right. When the crane drives the plate frame rollers, the two sensors monitor their respective distances to the reflectors and continuously track the exact position of the crane through the PLC.
With this high-speed, stable transmission time laser ranging sensor, long-distance position detection of reflective or multi-colored targets is no problem even when detecting angle changes