Introduction Small-area coordinate measurement technology has significant engineering application value. Coordinate sensors are key components in this field. Photoelectric elements are used due to their high precision, high resolution, and large dynamic range. They utilize the phenomenon that the photocurrent on a photosensitive element changes with light intensity to achieve geometric increments, resulting in photoelectric sensors that can be widely used in static measurement, dynamic measurement, and automation control. To meet the needs of practical engineering, small-area coordinate measurement technology is gradually gaining attention; however, current literature on sensor applications lacks discussion on this aspect. This article discusses the working principle, circuit design, applications, and processing methods of sensor detection information. I. Working Principle of Photoelectric Sensors The basic conversion principle of photoelectric sensors is to convert the measured parameter into a change in light signal, and then apply this light signal to a photoelectric element to convert it into an electrical signal output. Commonly used photoelectric sensors use light-emitting diodes (LEDs) as the light source, which is focused onto a point in space by a lens. If there is an obstacle at that point, the light will not reach the photodiode, the circuit will be biased, the PN junction will be cut off, and the reverse current will be very small. When there are no obstructions, light shines on the photodiode, generating electron-hole pairs near the PN junction. Under the combined influence of external and internal electric fields, these pairs drift through the PN junction, generating a photocurrent. At this time, the photocurrent is proportional to the light intensity, and the photodiode is in a conducting state. Specifically, a light-emitting diode (LED) is used on the light source side, and a photodiode is used on the light-receiving side, integrating the signal processing circuit onto a single chip. Its advantages include small size, high reliability, wide operating voltage range, significantly reduced interface circuit complexity, and direct connection to TTL, LSTTL, and CMLS circuit chips. II. Working Principle of Photoelectric Sensor for Displacement and Direction Measurement 1. Sensor Structure Design If the rotating disk to be measured is placed between the light-emitting and light-receiving sides of the photoelectric interruptor, the disk has many slits. As the disk rotates, the light emitted by the light source is intermittently blocked by the slits, resulting in intermittent strong and weak light signals on the light-receiving side. As shown in Figure 1, if the rotating disk is not rotating and the light beam detected by the optical path is not blocked, the output voltage waveforms of the X-ray photodiode and the Y-ray photodiode in the measurement circuit are the same, with a phase difference of π/2. If the disk rotates, the output voltage waveforms of the dual-output type are shown in Figure 2 (only the timing diagram of Q1 is shown; the timing diagram of Q2 is the same). If the disk rotates to the left, the output voltage of Q2 lags behind and is shielded; conversely, if the disk rotates to the right, the output voltage of Q1 lags behind and is shielded. Therefore, the phase relationship between the two output voltages reflects the rotation direction of the disk, and the displacement of the disk can be obtained by the algebraic sum of the number of output pulses of Q1 and Q2. Figure 1 Figure 2 2 Sensor Circuit Design The X-ray photodiode and the Y-ray photodiode differ in phase by π/2, so the signals they obtain on the photoelectric element must differ by π/2. When the disk rotates in the forward direction, the X signal leads the Y signal. Because the circuit is relatively complex, the CPLD device is programmed using ispEXPXRT software from Lattice Semiconductor, Inc. The circuit diagram shown in Figure 3 is based on the CPLD design. The signal generated by OR gate C1 serves as the set input of D latch Q1, allowing only the positive pulse generated by X to pass through. Since the Y signal is still low when C1 is active, the signal of D latch Q2 is shielded, and Q2 outputs a low level. The gate circuit performs addition in the up/down counter. When the disk rotates in the opposite direction, the negative signal generated by Y leads the signal generated by X. The signal generated by OR gate C1 serves as the set input of D latch Q2, allowing only the negative pulse generated by Y to pass through. Since the X signal is still low when C1 is active, the signal of D latch Q1 is shielded, and Q1 outputs a low level. The gate circuit performs subtraction in the reversible counter. This completes the direction detection process. OUT0 is the output, OUT1 is the carry, and Z is the control input. The working principle diagram is shown in Figure 4. Figure 3 Figure 4 III. Structural Design and Coordinate Algorithm of Photoelectric Coordinate Sensor 1. Structural Design In the actual design process, the measurement accuracy and range of the sensor are designed according to the needs. The accuracy can be achieved by calculating the slit density on the disk. The shape and size of the sensor disk are determined by the measurement range. The structural block diagram of the entire sensor system is shown in Figure 5. Figure 5 2. Coordinate Algorithm When the coordinates of the measured object change, the disk 7 rotates, and the signals of the photodiodes 4 and 6 change. The changes are automatically transmitted to the computer through the interface circuit, and the computer automatically collects the data of the input channel signals. If the forward or backward angle changes, the angle of the displacement disk 9 also changes. The rotation of the drive shaft drives the direction disk to rotate, causing the light on the photodiodes to be switched on and off by the slits on the disk 9, emitting an electrical pulse signal corresponding to the forward or backward movement. This signal is automatically transmitted to the computer through the interface circuit, and the computer collects the data of the input channel signals. The collected data is stored to form a database, so that the computer can determine its own coordinate position through data calculation and output the data through the corresponding interface. The formulas for calculating relative coordinates XN and YN are as follows: XN = Z * cos YN = Z * sin To ensure measurement accuracy, the computer's sampling time should not be too long; it should be close to the response time of the photoelectric sensor, ideally synchronized or a multiple thereof. 3. Coordinate to Voltage Conversion: Based on the principle of photoelectric conversion, the output voltage changes periodically. The sensitivity of this change is related to the distance between the slits. The distance between the slits can be manufactured as needed, but is limited by process and technical capabilities. It can also be obtained through computer-based compensation. 4. Linearization of Input Signals: If the input and output quantities have a linear proportional relationship, it is called a linear relationship. However, sensors with ideal linear relationships are extremely rare. To achieve linearization, electronic circuits or computer correction functions can be used. IV. Test Results The self-designed sensor was applied to test the coordinate positions of the soccer robot's field and goal. Tests were conducted on a 1.000 m × 1.000 m planar field. The measured data is compared with the actual data as follows: The above figure shows that within a small area, the low-cost combined sensor can automatically detect coordinates, and the output is relatively stable with strong anti-interference capabilities, meeting the expected design requirements. V. Conclusion Through the design method presented in this paper, a photoelectric coordinate sensor was designed. It features integration, high sensitivity, strong anti-interference capabilities, and small size, and can be widely used in static measurement, dynamic measurement, and automation control. Therefore, the photoelectric coordinate sensor has broad applicability. Disadvantages: During measurement, attention must be paid to the continuity of the sensor's measurement; it cannot be used intermittently. Professional installation skills are required, and the light source should be matched to the application environment. Further efforts are needed in this area to improve and perfect the functionality of the coordinate sensor.