With the development of science and technology and the improvement of people’s living standards, robots have begun to enter people’s lives. With the advent of this era, various new types of robots have emerged, such as sweeping robots and security robots. One of the most important parts of mobile robots is the navigation system, which has attracted more attention in the field of robotics. Robot navigation systems are critical for many robotic applications such as: smart warehouses, supermarket shopping guides, home robots, automated libraries, and smart hospitals. More common is a line-finding robot. The author participated in this type of robot competition and design several times during the graduate school. The actual design found that: Because of the constraints of the fixed line, this system can not achieve true full-scale autonomous operation. In addition, this method is greatly affected by light and cannot be practically applied in life. Lines on the ground also affect the appearance of the ground. After a thorough literature search and reflection, a new robot navigation system was proposed, integrating RFID, geomagnetic induction, and DSP technologies. The actual system hardware and software design and stability testing were performed.

The hardware system we developed consists mainly of: DSP core board, RFID board, system board, electronic compass, and driver board.

2.1 DSP core board

Our self-designed DSP system uses the TMS320F2812 as the core, and the 2812 is a high-performance, multi-functional, cost-effective 32-bit fixed-point DSP chip used by TI. The core uses a high-speed processor, mainly to consider the subsequent development of the system. And convenient algorithm migration. The instruction system can work at a frequency of up to 150 MHz with an on-chip SRAM of 128k and a 128k on-chip access time of 36ns FLASH. Its on-chip peripherals mainly include ADC, dual SCI, SPI, McBSP, eCAN, etc. With event management module (EVA, EVB), including six PWM/CMP, two QEP, three CAP, two 16-bit timers. One of the two SCI can be connected to the sensor all the way to the PC to output debugging information in a timely manner. 16-bit PWM can achieve fine speed regulation. The CAP facilitates the interface with the sensor. The 16-channel, 12-bit input with a maximum input of 3V and an ADC with a conversion time of 80ns can simultaneously connect to 16-channel distance sensors. The 2812 has a 16-bit 16-bit double-multiplier accumulator that provides sufficient processing speed for RF and geomagnetic direction signals. Because of the particularity of 2812 core voltage (1.8V) and starting frequency, this core board uses passive crystal.

The core board adopts TPS767D318 dual-output low-dropout LDO. It provides dual power supply and ensures a stable supply of 1.8V core voltage. The TPS767D318 has a high-speed transient response and is dedicated to the DSP, providing up to 1A of current. Time-sharing reset of IO and core can be achieved. Power-on reset function, low-voltage protection function, its reset delay time is 200ms. There is overheat protection function. The maximum power calculation can be calculated using Equation 1.

Among them TJMAX is the maximum allowable temperature, according to the experience TPS767D318 is generally about 125 degrees. TA is the ambient temperature. RθJA is the connection impedance. For the 28 pin is usually 27.9 °C / W. The actual power consumption can be calculated by Equation 2, where VI and VO are the input and output voltages, IO is the output current.

In the peripheral voltage filtering, multi-stage tank capacitors (104 and 220uF) are used in parallel. This design can produce low ESR effects. According to experience, in the frequency domain, 104 can greatly filter high-frequency interference, and 220uF can best reduce the low-frequency interference of the voltage.

2.2 System Board and Sensor Module

With regard to the power supply, the system uses a 12V lithium battery and uses a three-terminal positive voltage regulator to regulate the voltage. Internal integrated power protection. The output current can reach 1A. The input voltage can reach 30V. The charging interface is provided, and the switch is used for control. The circuit board has an interface to charge the lithium battery.

The positioning device uses radio frequency technology (RFID). RFID has become a hot technology. Recently, Wal-Mart has adopted a "requirement for its top 100 suppliers to use RFID technology when sending pallets and boxes to their distribution centers by January 2005, and gradually use this technology in single-item products after 2006." resolution. From the basic principle of information transmission, radio frequency identification technology is based on the transformer coupling model (energy transfer and signal transfer between primary and secondary) in the low frequency range, and is based on the spatial coupling model of radar detection target at high frequency (the radar emits electromagnetic signals. After reaching the target, it carries the target information and returns to the radar receiver). The most basic RFID system consists of three parts: tags, readers, and antennas. According to the action distance can be divided into close-coupled card (acting distance less than 1 cm), near-coupled card (acting distance less than 15 cm), loose coupling card (working distance of about 1 meter) and long distance card (working distance from 1 meter to 10 Meters, even further). The system uses a near-coupled card. The RF module communicates with the 2812 SCIA port. The data stream is decoded to determine the robot's position.

Followed by infrared obstacle avoidance module. In general, obstacle avoidance of robots can use the infrared reflection method, which is more common in robot games. The GPIO controls the infrared emission, and if the obstacle is reflected back, the receiving tube will cause the resistance change after receiving the light, and the resistance change can be detected to determine whether there is an obstacle. However, this method is easily interfered by light noise. Therefore, the distance is relatively short, generally only up to 2-3cm. After many games, after investigation data and research, a circuit using the infrared signal of standard high-frequency signal 38KHZ to detect obstacles was proposed. Because of the use of high-frequency signals and high-frequency op amps, it has a certain degree of anti-jamming capability, while the maximum detection distance is increased to 8 cm. First, infrared radiation is transmitted through the 555. Then, after passing through the infrared receiving tube, the signal passes through the DC blocking capacitor and is sent to the high-frequency op amp LM318N, as shown in Figure 4. Then, it is amplified 50 times. As shown in Figure 5 and Figure 6.

Figure 1 555 transmitter circuit

Figure 2 frequency identification circuit

Figure 3 detection amplifier circuit

LM567 is a phase-locked loop circuit, 8-pin DIP package. Pin 5 and pin 6 external resistors and capacitors determine the center frequency of the internal VCO. Pins 1 and 2 are usually grounded via a capacitor, generating an output filter network and a loop-level low-pass filtering network. The capacitor connected to the second pin determines the capture bandwidth of the phase-locked loop: the greater the capacitance, the narrower the loop bandwidth. The center frequency and filter bandwidth of the VCO can be determined by Equation 3 and Equation 4.

(where Vi is the input voltage)

Then there is the electronic compass module. The CMPS03 electronic compass is a planar angle sensor. By detecting the direct angle between the current sensor and the Earth's magnetic field, the electronic compass can obtain an absolute rotation angle with a resolution of 0.1 degrees. This electronic compass module is specially made for robots with the purpose of providing the robot with proper direction navigation signals. A unique code can be generated for any direction. The sensor uses PHILIPS KMZ51 geomagnetic sensor chip, its accuracy is very high. There are two output methods, the first one: I2C mode, output by Pin2 (SCL) and Pin3 (SDA). Pin7 and Pin5 must be left open. Pin6 is used for correction. These pins are all connected to the motherboard. Since the module is powered by 5V and the DSP is 3.3V, it is still necessary to use 74LVC245 for level conversion. The system is calibrated by pin 6 through 2812 GPIOB. The calibration only needs to be done once because the data is stored in the EEPROM of the PIC microcontroller in the electronic compass. Pin 6 has a pull-up resistor. Correcting only requires a negative-phase pulse to Pin6 via GPIO, and since there is a pull-up resistor, this pin can be disconnected from the system.

The last is the design of the system's driver module. Using L298 chip. The L298 is a product of SGS. The more common is the L298N in a 15-pin MulTIwatt package. The internal part also contains a 4-channel logic drive circuit. The power output device included in the L298 is designed and fabricated on a quartz substrate. Due to the same manufacturing process, it has unmatched performance parameters of discrete component combination circuits and is stable in operation. Pin 15 is the output current feedback pin, the other is the same as L293. In normal use, these two pins can also be directly grounded. It is a high voltage, high current dual full bridge driver chip. Standard TTL logic levels can be directly accepted. Can drive a variety of loads such as motors, relays and so on. There are two enable inputs that control the effectiveness of the PWM wave. L298 integrates two energy output blocks A, B. In addition, we design the board with a freewheeling diode.

2.3 Driver Design

By writing the InitSysCtrl() function, the watchdog control register WDCR is set, and the WDFLAG bit is the watchdog reset status bit. If this bit indicates a watchdog reset. Writing a 1 to the WDDIS bit will invalidate the watchdog module. Write 0 to the watchdog enable. The WDPS bit mainly determines the clock rate of the watchdog counter. Since the program contains many loops, special attention should be paid to the setting of the watchdog. Then set the system PLL by setting the PLLCR. At this time, it should be noted that the program needs to be added, 5000 cycles to wait for the lock to stabilize. This is not necessary in 2407, but pay attention to this point for the 2812 system. High-speed and low-speed peripherals are then matched by HISPCP and LOSPCP. Since interrupts are used in the program, the peripheral interrupt extension PIE needs to be set. The direction sensor connects to the DSP core through the capture unit. The capture of one pulse requires two interrupts, and the pulse encoding of the direction information is taken out by controlling and reading the FIFO register. In addition, the infrared obstacle avoidance module through 74LV245 module, after changing the level, and interrupt pin connection. So you want to program the interrupt function. Communication with RFID sensors is via the 2812's two-wire asynchronous serial port. The SCI module supports digital communications between the CPU and asynchronous peripherals using a non-return-to-zero (NRZ) standard format. The 2812 SCI receiver has a 16-level deep FIFO, which can reduce short service. The program determines if there is an RFID interrupt by determining the TxRDY bit. In this way, it can be timely discovered whether the robot has reached a new position. Then read SCIRXBUF. Finally, set the PWM pulse through the EV unit to control the direction of the robot.

The actual operation of the robot in the supermarket

Finally, debug the entire system near the supermarket. After repeated debugging and modification of the program, navigation of the RF path point was realized. The final system operation is shown above.

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