Industrial Camera & Machine Vision

Writer: SHD-TECH        Date: 2023-04-14 12:01:38        Read: 1793

Industrial Camera &  Machine Vision

Machine vision is one of the key technologies in manufacturing because of increasing demands on the documentation of quality and the traceability of products. It is concerned with engineering system, such as machines or production lines, that can perform quality inspection in order to remove defective products from production or that control machines in other ways, e.g., by guiding a robot during the assembly of a product.

Some of the common tasks that must be solved in machine vision system are as follows:

1.     Object identification is used to discern different kinds of objects, e.g., to control the flow of material or to decide which inspections to perform. This can be based on special identification symbols, e.g., character strings or bar codes, or on specific characteristics of the objects themselves, such as their shapes.

2.     Position detection is used, for example, to control a robot that assembles a product by mounting the components of the product at the correct positions, such as in a pick-and-place machine that places electronic components onto a printed circuit board (PCB). Position detection can be performed in two or three dimensions, depending on the requirements of the application.

3.     Completeness checking is typically performed after a certain stage of the assembly of a product has been completed, e.g., after the components have been placed onto a PCB, to ensure that the product has been assembled correctly, e.g., that the right components are in the right place.

4.     Shape and dimensional inspection is used to check the geometric parameters of a product to ensure that they lie within the required tolerances. This can be used during the production process but also after a product has been in use for some time to ensure that the product still meets the requirements despite wear and tear.

5.     Surface inspection is used to check the surface of a finished product for imperfections such as scratches, indentations, protrusions, etc.

Among the machine vision system, camera is the key unit. The camera's purpose is to create an image from the light focused in the image plane by the lens. The most important component of the camera is a digital sensor, CCD and CMOS. They differ primarily in their readout architecture, e.g., in the manner in which the image is read out from the chip.

 

CCD Sensors

 

--Line Sensors

CCD sensor consists of a line of light-sensitive photodetectors, typically they are photogates or photodiodes. A line sensor would create an image that is one pixel high, which would not be very useful in practice. Therefore, typically many lines are assembled into a 2D image. Of course, for this image to contain any useful content, the sensor must move with respect to the object that is to be imaged. To acquire images with a line sensor, the line sensor itself must be parallel to the plane in which the objects lie and must be perpendicular to the movement direction to ensure that rectangular pixels are obtained. The line readout rates of line sensors vary between 10 and 200 kHz. This obviously limits the exposure time of each line. Consequently, line scan applications require very bright illumination.

 

--Full Frame Array Sensors

Compared with line sensor, the feature of full frame array sensor is, the light is converted into charge in the photodetectors and is shifted row by row into the serial readout registers, from where it is converted into a video signal in the same manner as in the line sensor. Because the upper pixels are shifted through all the lower pixels, they accumulate information from the entire scene, and consequently appear smeared. To avoid the smear, a mechanical shutter or a strobe light must be used.

 

--Frame Transfer Sensors

This kind of sensor is designed to reduce the smearing problem in the full frame sensor, the frame transfer sensor uses an additional sensor that is covered by a metal light shield, and can thus be used as storage area. In this sensor type, the image is created in the light-sensitive sensor and then transferred into the shielded storage array, from which it can be read out at leisure. Since the transfer between two sensors is quick, usually less than 500μs, smear is significantly reduced. The biggest advantage of the frame transfer sensor is that it can have fill factors of 100%. Furthermore, no mechanical shutter or strobe light needs to be used.

 

--Interline Transfer Sensor

There is no smear in the image taken by interline transfer sensor, consequently no mechanical shutter or strobe light needs to be used. The biggest disadvantage of this king of sensor is that the transfer registers take up space on the sensor, causing fill factors that may be as low as 20%, consequently, aliasing effects may increase.

 

CMOS Sensors

 

CMOS sensors, typically use photodiodes for photo detection. In contrast to CCD sensors, the charge of the photodiodes is not transported sequentially to a readout register. Instead, each row of CMOS sensor can be selected directly for readout through the row and column select circuits. In this respect, a CMOS sensor acts like a random access memory.

Like interline transfer CCD sensors, CMOS sensors have low fill factors and therefore normally use micro lenses to increase the fill factor and to reduce aliasing.

Another big advantage is the parallel analog-to-digital conversion that is possible in CMOS sensors. This can give CMOS sensor s speed advantage even if AOIs are not used. It is even possible to integrate the ADCs into each pixel to further increase the readout speed.