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How does SMT mounter detect and sense?

There are sensors with different functions in the relevant parts of the machine to ensure the realization of machine motion coordination and function. A sensor is a device or device that can sense a specified object and convert it into a usable signal according to a certain rule. It is usually composed of sensitive components and conversion circuits. A sensor is a detection device that can feel the information of the measured object, and can transform the detected and felt information into electrical signals or other required forms of information output according to a certain rule, so as to meet the transmission, processing, and storage of information , Display, record and control requirements. It is the first link to realize automatic detection and automatic control.


One, the sensor in the placement machine
The placement machine is composed of many parts, and its operating conditions require many parts to operate in a normal state. How do you know that each component is under normal operating conditions? The sensor fulfills this mission. The sensor detects the status of each component, and then transmits the information to the control part for display, telling the operator whether it is operational, if the equipment cannot be operated, an error message clause will be displayed, and the operator will follow the prompts to troubleshoot and enable the equipment normal operation.
The placement machine is equipped with various types of sensors, mainly force and optical sensors, which play a role in collecting information in all aspects of the placement machine's work. As the automation and intelligence of placement machines increase, the types and numbers of sensors will increase. In general, the more types and numbers of sensors in the machine, the higher the automation and intelligence level of the placement machine. The following table shows the types and functions of sensors commonly used in placement machines.
Two, pressure sensor
The air pressure system of the placement machine includes various cylinder working pressures and vacuum generators, all of which have certain requirements for air pressure. When it is lower than the equipment regulations, the machine cannot operate normally. The air pressure sensor is always monitoring the pressure change, and once it is abnormal, it will alarm in time to remind the operator to deal with it.
The suction nozzle on the placement head sucks components by negative pressure. Therefore, the change of negative pressure reflects the situation of the suction nozzle sucking components (as shown in the figure below). If the feeder has no components, or the components are too large and stuck in the material bag, the suction nozzle will not suck the components; or although the suction nozzle sucks the components, but the components are sucked incorrectly, the pressure of the suction nozzle will change. , By detecting the pressure change, the placement situation can be controlled. Of course, if the negative pressure is not enough, the suction nozzle will not be able to suck the components, or although the components are sucked, they fall due to the movement force during the movement of the placement head. These abnormal conditions are all caused by the negative pressure sensor. monitor. If it is found that the components cannot be sucked or the components cannot be sucked, the system will alarm to remind the operator to replace the feeder or check whether the suction nozzle negative pressure system is blocked.
At present, the new negative pressure sensor has been miniaturized, and the negative pressure sensor is integrated with the conversion and processing circuit to form an integrated part. This component that integrates the sensor with the conversion and processing circuit is usually called a transmitter, and the transmitter outputs a standard electrical signal (0 ~ 5 V voltage or 4 ~ 20 mA current). The small negative pressure sensor can weigh less than 70 g, so it can be directly mounted on the placement head (as shown in Figure 1 below). When the multi-nozzle patch head is working, each nozzle is sucked and placed in order, so the two nozzles can also share a negative pressure sensor through the solenoid valve (as shown in Figure 2 below).
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Figure 1 The negative pressure sensor is directly mounted on the placement head
Figure 2 Two suction nozzles share a negative pressure sensor
Third, the position sensor
The transmission and positioning of the printed board includes the count of the PCB, the real-time detection of the movement of the placement head and the workbench, and the movement of the auxiliary mechanism. They have strict requirements on the position, such as the origin positioning of the placement machine, or for the safe operation of the machine. There is a photoelectric sensor in the movement area, which uses the photoelectric principle to monitor the running space to prevent damage caused by foreign objects. These are achieved through various forms of position sensors. The following takes the placement machine with the FLEXJET head for the world as an example to illustrate the two position sensors of the OTHC camera on the placement head, as shown in the figure below.
In addition, the Z-axis of the FLEXJET head also has a Home sensor, which senses that the Spindle has returned to the upper position in the Z-axis direction. The position of the detection axis is also a kind of position sensor.
• Mirror In-range/Clear: (Sliding mirror in-range/away) sensor.
• Mirror Clear: This sensor is used to ensure that the lens has safely left the road during Spindle movement, and the Mirror In-range sensor ensures that the sliding lens is in place for taking pictures. The length of the wire must be adjustable so that when the mirror clutch is activated, the lens is pulled down all the way until the in-range sensor is triggered; when the mirror clutch is not triggered, the connecting wire must be long enough to allow the lens to reach and activate the Mirror Clear sensor.
The area detection sensor is actually a kind of position sensor. For example, for various types of placement machines, the X-Y positioning system of the machine has a working range. When it is exceeded, an error will occur or even the machine will be damaged. Therefore, it is necessary to set limit switches at the ends of the X-axis and Y-axis beams. When the movement of the axis exceeds the working range, the machine will generate a limit error and stop the movement, thus acting as a zone detection function.
1. Compression on-off electrical sensor
Through mechanical contact, the sensor turns on or off the circuit to detect whether the object reaches the specified position. For example, safety cover closing sensor and component supply part safety protection device (Z-axis side safety interlock and box float sensor).
2. Photoelectric position sensor
Photoelectric position sensors are usually called photoelectric switches. According to the position of the light source and receiver, there are three common types of photoelectric position sensors:
(1) Separation of light source and receiver
The working principle of the sensor with separate emitter and receiver is that during normal operation of the device, a beam of light emitted by the emitter will be received by the receiver. When an object blocks the light and the receiver cannot receive the light, the sensor will send this information It is transmitted to the controller to display the alarm.
This kind of sensor is used to detect whether the feeder is installed in place. It is usually installed at both ends of the feeder base, as shown in the figure below.
(2) Projector and receiver are integrated
Its working principle is that when the specified part of the moving part (such as the placement head) blocks the light, the sensor receives the information of the part (the placement head) and transmits this information to the control part to tell the machine that this part is ready (as shown in the figure below) Shown).
This type of sensor is often used to detect whether the placement head is in place, whether each mechanical part returns to the zero point, or whether the mechanical position reaches the limit position, etc.
(3) The light source and the receiver are on the same plane
The working principle of the projecting light source and the light receiver on the same plane is reflective detection. As shown in the figure below, when the measured object reaches the detection position, the light emitted by the projecting light source is reflected to the light receiver, and the sensor output state changes to achieve the purpose of position detection. .
When the placement machine is working, when the substrate passes the substrate transport deceleration sensor, the substrate starts to decelerate. When the substrate reaches the detection position, the stopper advance limit sensor gives information, and the stop cylinder moves downward to stop the substrate. At the specified position, the workbench rises at this time, the stop cylinder is up, the stopper returns to the limit, the sensor gets information, and the placement starts; when the placement is completed, the workbench goes down, giving the substrate sending information to send the substrate.
3. Z-axis control sensor for placement head
With the increase in placement speed and accuracy, the "placement force" required by the placement head to place components on the PCB is getting higher and higher, which is commonly referred to as the "Z-axis soft landing functional force". The Z-axis soft landing is realized with the help of sensors installed on the placement head and the load characteristics of the servo motor. The machine software uses sensors to sense. The following takes the global FLEXJET patch head Touchdown sensor as an example to introduce the Z-axis motion control.
The new Touchdown sensor is used on the FLEXJET2/FLEXJET3 placement head to control the smooth contact of the suction nozzle with the circuit board, feeder and suction nozzle. The luminosity returned to the receiver is not as important as the consistency of the light. This is because what we are looking for is a change in luminosity, and the sensor is locked at 0.100" above the desired contact point (to avoid errors during acceleration). At this point, The optical density of the sensor is captured. During the Touchdown contact period and when the density reaches 80% of the locked value, the software considers that contact has occurred. When the small round mirror of these Touchdown sensors gets dirty, the sensor is very forgiving because the collected optical density determines When the contact occurs. The small round mirror reduces the optical density, so the small round mirror still needs to be cleaned but not so frequently. The working schematic diagram of the Touchdown sensor is shown in the figure below.
Four, laser and vision sensor
1. Laser sensor
The laser sensor has been widely used in placement machines. Its biggest advantage is its fast response speed, so it can achieve "flight centering" and increase the placement speed. The laser sensor can accurately reflect the position and direction of the component, and can distinguish the coplanarity of the device pin; the laser sensor can also identify the height of the device, which can shorten the production preparation time. At the same time, it has the ability to handle components of various shapes and sizes. However, even the most complex laser system cannot measure the lead and lead pitch.
The working principle of the laser sensor is shown in the figure below.
2. Vision sensor
At present, the commonly used image sensor is a charge coupled device (Charge Couple Device, CCD), which is divided into two types: linear array CCD and area array CCD. The former is used for the measurement of size and displacement, and the latter is used for the transmission of plane graphics and text. At present, the area array CCD has been used as a digital camera (DC), a digital video camera (DV) and various digital cameras and industrial vision systems. The camera used in the placement machine to identify PCB benchmarks and inspect components is used in industrial vision systems.
(1) The working principle of CCD
CCD is composed of many photosensitive pixels, and each pixel is a MOS (metal-oxide-semiconductor) capacitor, as shown in the figure below. A layer of SiO2 is formed by oxidation on the P-type silicon substrate, and then a metal layer (polysilicon) is vapor-deposited on the surface of the SiO2 as an electrode. The majority carriers in P-type silicon are positively charged holes, and the minority carriers are negatively charged electrons. When a positive voltage is applied to the electrode, its electric field can repel or attract these carriers through the SiO2 insulating layer. As a result, positively charged holes are repelled away from the electrode, and negatively charged electrons are attracted to the surface close to the SiO2 layer. This phenomenon forms a trap for electrons. Once an electron enters, it cannot return, so it is also called an electron potential well.
When a beam of light is projected on the MOS capacitor, the photon passes through the polysilicon electrode and the SiO2 layer and enters the P-type silicon substrate. The energy of the photon is absorbed by the semiconductor to generate electron-hole pairs. At this time, electrons are attracted and stored in In the potential well. The stronger the incident light, the more electrons collected in the potential well, thus realizing the conversion of light and electricity. The electrons in the potential well are in a stored state, even if the light is stopped, they will not be lost for a certain period of time, which realizes the memory of light.
(2) The composition of the area sensor
Area sensors are usually called image sensors. As shown in the figure below, an area sensor composed of a two-dimensional array of photodiodes. In order to extract the charge of each photodiode, a vertical CCD for charge transfer is arranged in the vertical direction of the figure. The charge from the photodiode is read by the vertical CCD through the control of the field effect tube (transfer control gate). The charge transferred to the vertical CCD is transferred to the horizontal CCD for horizontal transmission, and finally the signal is read out from the MOS field effect tube. The pulse to read out the charge of each photodiode is composed of three parts, that is, the pulse applied to the control gate, the pulse applied to each CCD pixel, and the combined control pulse applied to the gate of the output MOS field effect transistor.
In the example described above, the photodiode part (photosensitive part) and the CCD part (transfer part) are made in the same body on a semiconductor substrate. This charge transfer method is called Inter-line transfer method. . This method is conducive to high-speed image processing. However, since the non-sensitive part of each pixel occupies a larger area, the photosensitive efficiency is not high. Corresponding to the inter-line method, the method in which the photosensitive part and the transfer part are made separately is called the frame transfer method.
(3) Combination with image processing technology
The camera can only convert the two-dimensional light intensity distribution into a continuous time-varying analog electrical signal (called an image signal), but this alone is not a visual signal perceived by the human visual organs. In order to obtain the visual signal, the computer must be used for further signal processing. Usually image signals are converted into digital signals, these digital signals are easily stored in a computer's memory, and then image processing is performed as needed.
The visual inspection system of the placement machine has a dedicated image processing system, which is converted into digital signals according to the pixel distribution, brightness and color information. The image processing system performs various operations on these signals to extract the characteristics of the target, such as area, quantity, and location. And the length, and then output the result according to the preset tolerance and other conditions, including size, angle, number, pass/fail, and yes/no, etc., to realize the automatic identification function.