A vision sensor is an instrument that uses optical elements and imaging devices to acquire image information of the external environment. The vision sensor is the direct source of information for the entire machine vision system, mainly composed of one or two image sensors, sometimes accompanied by a light projector and other auxiliary equipment. The primary function of a vision sensor is to acquire sufficient raw images for the machine vision system to process. Image sensors can use laser scanners, linear and area array CCD cameras, TV cameras, or the more recently developed digital cameras, etc.
The working principle of vision sensor technology is essentially image processing technology. It calculates the characteristic quantities (area, center of gravity, length, position, etc.) of an object by processing the image captured by the camera, and outputs data and judgment results.
A vision sensor has thousands of pixels that capture light from an entire image. The sharpness and detail of an image are typically measured by resolution, expressed in pixels. After capturing an image, the vision sensor compares it to a reference image stored in memory to make an analysis. For example, if a vision sensor is programmed to identify a machine part with eight bolts correctly inserted, the sensor knows it should reject a part with only seven bolts, or a part with misaligned bolts. Furthermore, the vision sensor can make this judgment regardless of the machine part's location within the field of view, and regardless of whether the part is rotated within a 360-degree range.
Industrial applications utilizing vision sensors include inspection, metrology, measurement, orientation, defect detection, and sorting. Here are some application examples:
At a car assembly plant, the inspection team checks whether the adhesive beads applied by robots to the door frame are continuous and of the correct width.
At the bottling plant, checks are made to ensure the bottle caps are properly sealed, the liquid level is correct, and no foreign objects have fallen into the bottle before sealing.
On the packaging production line, ensure that the correct packaging labels are affixed in the correct positions.
On the pharmaceutical packaging production line, the blister packs of aspirin tablets are inspected for any damaged or missing tablets.
At a metal stamping company, stamped parts are inspected at a rate of over 150 pieces per minute, which is more than 13 times faster than manual inspection.
The image acquisition unit of a classification vision sensor mainly consists of a CCD/CMOS camera, an optical system, an illumination system, and an image acquisition card. It converts optical images into digital images and transmits them to the image processing unit. The two most commonly used image sensor devices are CCD image sensors and CMOS image sensors.
1. CCD image sensor
CCD, or Charged Coupled Device, is a light-sensing system similar to the film in a traditional camera. It's a circuit device that senses light, which can be imagined as tiny sensing particles spread behind the lens. When light and an image pass through the lens and are projected onto the CCD surface, the CCD generates an electric current, converting the sensed content into digital data stored in the camera's internal flash memory or built-in hard drive. The more pixels a CCD has, and the larger the size of each pixel, the clearer the captured image.
A CCD image sensor consists of three layers: a microlens, a color filter, and a photosensitive element. Each photosensitive element of a CCD image sensor comprises a photodiode and a storage unit that controls the charge of adjacent cells. The photodiode captures photons, converting them into electrons. The stronger the light collected, the more electrons are generated. A stronger electron signal is easier to record and less likely to be lost, resulting in richer image details. A CCD sensor is a special semiconductor material composed of a large number of independent photodiodes, typically arranged in a matrix, much like the film in a traditional camera.
There are two types of CCD sensors. The first type is specialized CCD sensors, such as infrared CCD chips (infrared focal plane array devices), high-sensitivity back-illuminated and electron-impact CCDs, EBCCDs, etc. There are also large-area visible light CCD sensors such as 2048*2048 and 4096*4096, and wide-spectrum (ultraviolet—visible—near-infrared—3-5µm mid-infrared—8-14µm far-infrared) focal plane array sensors. Commercial products are already available and widely used in various fields. The second type is general-purpose or consumer-grade CCD sensors, which have made significant progress in many aspects, with the overall direction being to improve the comprehensive performance of CCD cameras.
CCD sensors offer several advantages, including high resolution, low noise, high sensitivity, wide dynamic range, excellent linearity, large-area photosensitive capability, low image distortion, small size, light weight, low power consumption, immunity to magnetic fields, high charge transfer efficiency, mass production capability, stable quality, robustness, resistance to aging, ease of use, and simple maintenance. However, as the application scope of CCDs expands, their disadvantages have become increasingly apparent: First, CCD chip technology is complex and incompatible with standard processes. Second, CCD chips require high voltage and power consumption, making them expensive and inconvenient to use.
2. CMOS image sensor
Due to the complexity and other drawbacks of CCD manufacturing, CMOS image sensors have gradually emerged. Their working principle is as follows: First, external light illuminates the pixel array, causing a photoelectric effect that generates corresponding charges within the pixel units. The row selection logic unit selects the appropriate row of pixel units as needed. The image signal within each row of pixel units is transmitted through its respective column's signal bus to the corresponding analog signal processing unit and A/D converter, where it is converted into a digital image signal for output. The row selection logic unit can scan the pixel array row by row or interlaced. The row selection logic unit, in conjunction with the column selection logic unit, can achieve image window extraction. The main function of the analog signal processing unit is to amplify the signal and improve the signal-to-noise ratio. Furthermore, to obtain a usable camera of acceptable quality, the chip must include various control circuits, such as exposure time control and automatic gain control. To ensure that the various circuits within the chip operate at a predetermined rhythm, multiple timing control signals must be used. For ease of camera application, the chip is also required to output timing signals, such as synchronization signals, row start signals, and field start signals.
Complementary metal-oxide-semiconductor (CMOS) image sensors utilize CMOS technology to integrate image acquisition and signal processing units onto a single chip. Due to these characteristics, they are suitable for mass production and applications requiring small size, low cost, and less demanding image quality, such as security cameras, mobile phones, computer network video conferencing systems, wireless handheld video conferencing systems, barcode scanners, fax machines, toys, biological microscope counting, and certain automotive camera systems—a wide range of commercial applications.
The selection guide outlines the choice of vision sensors based on accuracy, output, sensitivity, the cost of the machine vision system, and a thorough understanding of the application requirements.
The camera is the eye of a machine vision system, and the heart of the camera is the image sensor. CCD image sensors offer excellent image quality, noise resistance, and flexibility in camera design. Although the addition of external circuitry increases system size and complexity, it also allows for greater flexibility in circuit design, enabling the optimization of specific performance characteristics that are of particular interest to CCD cameras. CCDs are better suited for applications with very high camera performance requirements but less stringent cost controls, such as astronomy, high-resolution medical X-ray imaging, and other scientific applications requiring long exposures and stringent image noise control.
CMOS image sensors are image sensors manufactured using modern large-scale semiconductor integrated circuit production processes, featuring high yield, high integration density, low power consumption, and low price. CMOS technology is the technology that many image sensor semiconductor R&D companies worldwide have attempted to use to replace CCDs. After years of effort, CMOS, as an image sensor, has overcome many of its early shortcomings and has reached a level where it can rival CCD technology in image quality. The advancements in CMOS technology make it more suitable for applications requiring small space, small size, and low power consumption, where image noise and quality requirements are not particularly high. These include most industrial inspection applications with auxiliary lighting, security applications, and most consumer commercial digital camera applications.
In a given application, three key factors determine the choice of sensor: dynamic range, speed, and responsivity. Dynamic range determines the quality of the images the system can capture, also known as its ability to render details. Sensor speed refers to the number of images the sensor can generate per second and the output amount the system can receive. Responsivity refers to the efficiency with which the sensor converts photons into electrons, determining the brightness level of the useful image the system needs to capture. The sensor's technology and design together determine these characteristics; therefore, system developers must have their own criteria for selecting sensors, and a detailed study of these characteristics will help in making the right judgment.
Troubleshooting and handling camera malfunctions caused by visual sensor failure
Cause of the malfunction:
(1) The camera video power conversion circuit fails due to static electricity, hot-swapped video cables, etc.
(2) The controller video communication chip is damaged due to electromagnetic interference, noise interference, etc.
The following anti-interference measures were taken on site:
① Shielded cables are used for communication lines and signal lines, and they are reliably connected.
② Communication lines and signal lines should be laid separately from high-current power cables.
③ The product should be installed in isolation from high-power or high-frequency equipment.
④ Ensure that the input power supply does not share a ground with the power supply of high-power equipment.
