Abstract: This paper elucidates the basic working principle of photoelectric detection devices, analyzes the problems in their application, and discusses specific measures to improve the reliability of photoelectric detection devices.
Keywords: working principle, problem analysis, improvement measures
1. Working principle of photoelectric detection device
Photoelectric detection devices can be classified into two types: active and passive. Active devices consist of both transmitting and receiving circuits; passive devices have no transmitting component and consist only of a receiving circuit. The active type operates as follows: when the receiving component receives the light signal emitted by the transmitting component, the circuit remains inactive; when an obstacle blocks the light signal, preventing the receiving component from receiving the emitted light, the circuit activates. The passive type operates similarly to the active type, except that it relies on the light signal emitted by the object being detected itself. Compared to the active type, the passive type has a simpler circuit structure, is more flexible and convenient to use, and has a wider range of applications because it does not require a transmitting circuit.
(1) Photoelectric encoder
An optical encoder is a sensor that converts the mechanical geometric displacement of an output shaft into pulses or digital signals through photoelectric conversion. An optical encoder consists of a grating disk and a photoelectric detection device. The grating disk has several rectangular holes evenly spaced on a circular plate of a certain diameter. Since the optical encoder disk is coaxial with the motor, when the motor rotates, the grating disk rotates at the same speed as the motor. The detection device, composed of light-emitting diodes and other electronic components, detects and outputs several pulse signals, as shown in Figure 1(a). By calculating the number of pulses output by the optical encoder per second, the current speed of the motor can be reflected. This is currently the most widely used sensor. The working principle of the optical encoder is shown in the figure. The disk has regularly etched lines that transmit light and block light. Light-emitting elements and photosensitive elements are placed on both sides of the disk. When the disk rotates, the light flux received by the photosensitive element changes synchronously with the light-transmitting lines. The output waveform of the photosensitive element is shaped into pulses, and the code disk has corresponding markings. One pulse is output for each revolution. In addition, to determine the direction of rotation, the code disk can also provide two pulse signals with a 90º phase difference, as shown in Figure 1(b). Based on their detection principles, encoders can be classified into optical, magnetic, inductive, and capacitive types. Based on their calibration methods and signal output formats, they can be classified into incremental, absolute, and hybrid types.
1) Incremental encoders directly utilize the photoelectric conversion principle to output three sets of square wave pulses: A, B, and Z phases. The A and B pulses have a 90° phase difference, allowing for easy determination of the rotation direction. The Z phase consists of one pulse per revolution, used for reference point positioning. Its advantages include simple principle and construction, an average mechanical lifespan exceeding tens of thousands of hours, strong anti-interference capability, high reliability, and suitability for long-distance transmission. Its disadvantage is that it cannot output the absolute position information of the shaft rotation.
2) Absolute encoders utilize natural binary or cyclic binary (Gray code) methods for photoelectric conversion. The difference between absolute and incremental encoders lies in the translucent and opaque lines on the code disk. Absolute encoders can have multiple codes, and the absolute position is detected by reading the codes on the code disk. The code design can employ binary code, cyclic code, binary complement code, etc. Its characteristics are:
① The absolute value of the angle coordinate can be read directly;
②No cumulative error;
③ The location information is not lost after the power supply is cut off. However, the resolution is determined by the number of bits in the binary representation, meaning the precision depends on the number of bits, currently including 10-bit, 14-bit, and other types.
3) Hybrid absolute encoder, which outputs two sets of information: one set is used to detect the magnetic pole position and has absolute information function; the other set is exactly the same as the output information of incremental encoder.
An optical encoder is an angle (angular velocity) detection device that converts the angle of an input shaft into corresponding electrical pulses or digital quantities using the photoelectric conversion principle. It features small size, high precision, reliable operation, and a digital interface. It is widely used in CNC machine tools, rotary tables, servo drives, robots, radar, military target measurement, and other devices and equipment requiring angle detection.
(2) Photoelectric proximity switch
The key function of a photoelectric switch is its ability to handle changes in light intensity: using optical elements to alter the beam of light in the middle of the propagation medium; using the beam to reflect objects; and causing the emitted beam to return instantaneously after traveling a long distance. A photoelectric switch consists of three parts: a transmitter, a receiver, and a detection circuit. The transmitter aims at the target and emits a beam of light, typically from a light-emitting diode (LED) or a laser diode. The controller comprises several parts, including a photoelectric receiving circuit, an anti-interference circuit, an amplification circuit, a power output circuit, a working indicator, and a protection circuit. Its working principle is to convert reflected light changes into electrical signals. The emitter (light-emitting head) is composed of an oscillator made of integrated circuits, and the oscillation frequency is modulated into infrared light. The phototransistor in the receiver head receives the infrared pulse modulation signal. When the light path is blocked or unblocked, the phototransistor is in either cutoff or conduction state. An inductor L and a capacitor C are used as a parallel resonant circuit as the load of the receiver tube. Since L and C present low impedance to stable DC and high impedance to the modulation pulse, and after isolation by the capacitor, the desired modulation pulse signal can be selected. Through integrated AC amplification, its selection gate blocks spurious and noise interference. The selected signal is then output through a power amplifier to realize on/off control. Photoelectric proximity switches are widely used in automatic counting, safety protection, automatic alarm, and limit control. The light beam is emitted continuously or the pulse width is changed. The radiation intensity of the pulse-modulated light beam is selected multiple times during the emission, and it runs continuously toward the target. The receiver is composed of a photodiode or phototransistor. Optical elements such as lenses and apertures are installed in front of the receiver. Following this is the detection circuit, which filters out the valid signal and applies it. Photoelectric switches can be categorized into through-beam, diffuse reflection, and specular reflection types.
1) A through-beam photoelectric switch consists of a transmitter and a receiver, structurally separated. When the beam is interrupted, a switching signal change is generated. Typically, photoelectric switches located on the same axis can be separated by up to 50 meters. Features:
① Identify opaque, reflective objects;
②The effective range is large because the light beam crosses the sensing distance only once;
③ It is not easily disturbed and can be reliably and appropriately used in the field or in dusty environments;
④ The device consumes a lot of power, and cables must be laid in both units.
2) Diffuse reflection type photoelectric switches emit a light beam, which is diffusely reflected from the target. The transmitter and receiver constitute a single standard component. When enough combined light returns to the receiver, the switch state changes. The typical effective distance can reach 3 meters. Its characteristics include:
①The effective range is determined by the reflectivity of the target, as well as the surface properties and color of the target;
② Lower assembly costs; when the switch consists of a single component, coarse positioning is usually achievable.
③ Adjust the measurement distance using the background suppression function;
④ It is sensitive to dust on the target and to changes in the target's reflectivity.
3) A standard configuration for a mirror-reflective photoelectric switch consists of a transmitter and a receiver. The light beam emitted from the transmitter is reflected by a mirror on the opposite side, returning to the receiver. When the beam is interrupted, a change in the switching signal is generated. The light travel time is twice the signal duration, and the effective operating distance ranges from 0.1 meters to 20 meters. Its characteristics include:
① Distinguish opaque objects;
② By utilizing the reflective mirror component, a high effective distance range can be formed;
③ It is not easily disturbed and can be reliably and appropriately used in the field or in dusty environments.
4) The main applications of photoelectric switches are classified as shown in Table 1:
Table 1. Classification of Main Applications of Photoelectric Switches
5) General technical parameters of photoelectric switches are shown in Table 2:
Table 2 General Technical Parameters of Photoelectric Switches
(3) Infrared photoelectric detection device
Infrared radiation is invisible to the naked eye. All objects above absolute zero radiate infrared radiation, but the wavelengths differ depending on the temperature of the object. The two main characteristics of infrared radiation are its thermal effect and its stealth capability. Infrared sensors are created using this thermal effect, and further, they are used to construct various photoelectric control application circuits. A general block diagram of a passive infrared circuit is shown in Figure 2. The infrared sensor converts the received infrared light into an electrical signal. The amplification and processing circuit first amplifies the weak signal and then processes it in a certain way. The execution circuit indicates, records, displays, or performs a specific action based on the received signal.
2. Problem Analysis in Application
The transmitting and receiving devices of the photoelectric detection equipment are installed at the production site, and many defects have been exposed during use. These defects are caused by both internal and external factors, and are mainly manifested in the following aspects:
1) Displacement or offset of the transmitting or receiving device due to mechanical vibration or other reasons can prevent the receiving device from reliably receiving optical signals and thus from generating electrical signals. For example, in a steel rolling speed control system, the photoelectric encoder is directly bolted to the motor housing. The encoder shaft is connected to the motor shaft via a stiff spring. Since the motor's load is an impact load, the movement of the rolling mill through the steel mill causes vibration in the motor shaft and housing. Measurements show that the photoelectric encoder vibrates at a speed of 2.6 mm/s during this process. Such vibration can damage the encoder's internal functions, causing false pulses and leading to instability or malfunctions in the control system, potentially resulting in an accident.
2) Because the photoelectric detection device is installed at the production site, it may malfunction due to environmental factors. For example, high temperature and humidity at the installation location can alter or damage the characteristics of the internal electronic components. In the continuous casting machine's ingot tracking system, the photoelectric detection device may erroneously send signals or malfunction due to its proximity to the billet and high ambient temperature, potentially leading to production accidents or personal injury.
3) Various electromagnetic interference sources in the production site can interfere with the photoelectric detection device, causing distortion of the output waveform and leading to system malfunction or production accidents. For example, the photoelectric detection device is installed on the production equipment itself, and its signal is transmitted to the control system via cable over a distance of 20-100 meters. Although multi-core shielded cables are generally used, the resistance of the cable conductors and the capacitance between the conductors, coupled with the fact that they are laid together with other cables, make them highly susceptible to various electromagnetic interferences. This causes waveform distortion, resulting in a deviation between the signal fed back to the speed control system and the actual value, thus reducing the system's accuracy.
3. Improvement Measures
1) Change the installation method of the photoelectric encoder. Instead of mounting the photoelectric encoder on the motor housing, a fixed bracket is made on the motor base to independently mount the photoelectric encoder. The center of the photoelectric encoder shaft and the center of the motor shaft must be at the same horizontal level. The two shafts are connected with soft rubber or nylon hoses to reduce the mechanical impact of the motor's impact load on the photoelectric encoder. After adopting this method, the vibration velocity was reduced to 1.2 mm/s according to the vibration meter test.
2) Select a suitable signal transmission medium for the photoelectric detection device, and replace ordinary shielded cables with twisted-pair shielded cables. Twisted-pair shielded cables have two important technical characteristics. First, they offer strong protection against electromagnetic interference because the interference currents generated by the spatial electromagnetic field on the line can cancel each other out. The other technical feature is that the spacing between the two twisted wires is very small, the distance between the two wires and the interfering line is basically equal, and the distributed capacitance between the two wires and the shielding mesh is also basically the same. This makes the suppression of common-mode interference more effective.
3) Monitoring or intervention using PLC software. In the continuous casting process, the ingot feeding process requires photoelectric detection devices to generate time-sequential electrical signals, which correspond to different stages of the entire process. See Figure 3 for the setup.
Before the ingot feeding process starts, photoelectric signal 1 is "1". After the ingot feeding process starts, in stage A, the roller conveyor starts and the ingot rod is fed upward. When the ingot rod blocks the infrared light emitted by the photoelectric device, the photoelectric signal is "0"; when the infrared light passes through the two small round holes in the middle of the ingot rod, the photoelectric device emits signals 2 and 3, both of which are "1".
During stage B of the ingot feeding process, the photoelectric signal is "0", the roller conveyor stops, the ingot feeding rod pauses, the 10-segment fan is pressed down, the straightening machine and "synchronization 1" are started, and the ingot feeding rod continues to feed upward.
During stage C of the ingot feeding process, the ingot guide rod is fed upwards and no longer blocks the infrared light. Photoelectric signal 4 becomes "1", activating "Synchronization 2" and stopping "Synchronization 1". The ingot guide rod continues to feed upwards. At this point, the photoelectric device's operation process is complete.
Based on the working process of the photoelectric detection device, as long as the time of each photoelectric signal during the ingot feeding process is measured on-site, and the relationship between the ingot feeding process and the photoelectric signals is combined, a PLC program that meets the requirements can be compiled using relevant data in the PLC application. The output signal of the PLC program is then input to the PLC's input module to replace the original photoelectric signal input signal. Its program flowchart is shown in Figure 4.
4. Conclusion
Photoelectric detection devices are composed of electronic components and have certain technical requirements for their installation environment. Especially in harsh environments, appropriate protective measures must be taken to ensure the device operates under its required technical conditions and performs its intended function. Otherwise, the lifespan and reliability of the photoelectric detection device will be affected to varying degrees. Based on practical applications of photoelectric detection devices in production process control, it is not advisable to use the signals from photoelectric detection devices as critical control signals in control system design. This is to avoid accidents to other equipment caused by sudden damage or instability of the photoelectric device (due to high temperature, high humidity, mechanical vibration, external impact, etc.). Applying PLC programs to monitor or intervene in process control within the control system is an effective way to overcome the various defects inherent in the use of photoelectric devices and improve system reliability.
