Design of electromagnetic hitting system for soccer robot

　Soccer robots (also known as intelligent soccer robots) are a high-tech competitive activity that has been rapidly developed internationally in recent years. They involve cutting-edge research in the fields of artificial intelligence, mechanics, automation, computers, signal processing, wireless communications, sensors, precision machinery, and other fields [1]. Soccer robots use full vision and wireless communications in the game. The soccer robots controlled by the main control machines of both sides compete in a fully autonomous mode in a specific venue. It is not only a typical intelligent soccer robot system, but also a vivid research model of cooperation and confrontation between multiple robots. The theoretical technology involved can be applied to the fields of national defense, industrial production, rescue, education, etc., thereby effectively promoting the development of national science and technology, economy, and security.
　The biggest similarity between robot kicking and human kicking is that the players can overcome the opponent's obstacles and successfully kick the ball into the opponent's goal. In order to make the robot kick and dribble as flexibly as a star player, a fully functional kicking (striking) system is essential. Therefore, the design of a hitting system is of great significance for the shooting and passing of soccer robots.
1 Design of electromagnetic striking mechanism and striking electromagnet
1.1 Working principle of electromagnetic striking mechanism
　The electromagnetic striking mechanism is the most common mechanism in the current soccer robot striking mechanism. Its advantages are strong force, high action frequency, fast reaction speed, and relatively simple and reliable mechanical structure [2]. Here, the electromagnetic force is used as the power for the soccer robot to shoot the ball horizontally and lob the ball, which can achieve the purpose of passing the ball and shooting the ball. The design of the electromagnetic striking mechanism of the soccer robot is shown in Figure 1, which mainly includes a reset spring, an iron core, a horizontal shooting coil, a bracket, a horizontal shooting rod, a lob shooting coil, and a lob shooter. When there is no obstacle in front of the soccer robot, the horizontal shooting method is used to pass the ball or shoot the ball. When shooting the ball horizontally, the iron core drives the horizontal shooting rod to hit the ball forward violently. After the shooting is completed, the iron core is reset under the action of the reset spring; when there is an obstacle in front of the soccer robot, the lob shooting method is used. The lob shooting coil works and drives the lob shooting rod to move quickly to the back of the robot, so that the lob shooter rotates around the axis to complete the lob shooting process. After the ball is shot, it is reset by the reset spring, and the ball is shot over the front football robot, achieving the goal of dribbling past the person (football robot).


　As can be seen from formula (1), increasing the number of coil turns and current can increase the attraction of the solenoid electromagnet; the use of an armature made of soft magnetic material with high magnetic permeability and low hysteresis loss is also an important factor in improving the attraction. The
　same electromagnet is used for the ball-shooing electromagnet and the flat-shooting electromagnet. The ball-shooing electromagnet controls the conduction time of the electromagnetic coil to adjust the hitting force [3], and drives the ball-shooing club and the flat-shooting club respectively to complete the action of the ball-shooing and flat-shooting.
2. Design and implementation of boost module
　The battery is the energy source for the football robot to move, and it is powered by a 12 V, 1 800 mAh lithium battery. Directly using the battery to power the ball-shooing electromagnet, the low-voltage power supply cannot directly enable the ball to obtain enough kinetic energy in a short time, and a higher voltage is required to power the DC electromagnet. This paper adopts the DC-DC Boost high-voltage conversion principle [4] to boost the power supply voltage. The boost circuit is shown in Figure 2.

　This paper uses an integrated control chip UC3843 that can generate adjustable pulse bandwidth to realize the MOS tube shutdown control and voltage boost of the control circuit. As shown in Figure 2: (1) Adjusting R10 can change the PWM duty cycle of pin 6 output, thereby adjusting the charging speed. The larger the duty cycle, the faster the charging; (2) The PWM pulse frequency f is determined by the resistor R10 and the capacitor C6 (f=1.72/RC). By adjusting the ratio of R10 and C6, PWM waves of different frequencies can be obtained; (3) Pin 2 is the voltage feedback terminal. When the feedback voltage exceeds 2.58 V, the chip stops working. Therefore, the resistance value of resistor R5 can be adjusted to control the working area of ​​UC3843 to indirectly adjust the charging voltage of the energy storage capacitor C5.
　At the beginning of the charging process, the PWM generator starts working by enabling the pin KEY. The continuous pulses generated by the PWM generator drive the high-power MOS tube, producing a regular switching characteristic. When the high-power MOS tube is turned on, the current of the inductor L increases exponentially until the rated value; when the high-power MOS tube is turned off, the current quickly drops to 0, and the inductor generates a high-amplitude induced voltage pulse. The voltage pulse charges the energy storage capacitor C5 through the diode D0. After multiple reciprocating charges, the amplitude on the energy storage capacitor can be close to the induced pulse voltage of the inductor. In order to make the energy storage capacitor reach the required voltage of the design, the resistance value of the potentiometer R5 can be adjusted to feed back the voltage of the energy storage capacitor to the PWM generator. If the feedback voltage is greater than or equal to 2.58 V, the PWM stops working; otherwise, the PWM generator continues to work and the voltage of the energy storage capacitor increases. The charging voltage time is related to the inductance coefficient characteristics and is also closely related to the output duty cycle of the PWM generator.
3 Design and control of the driving circuit of the electromagnet
　for hitting the ball As shown in Figure 3, it is the schematic diagram of the driving circuit of the electromagnet for hitting the ball. When the football robot receives the command of flat shooting or lob shooting sent by the host computer, it should quickly connect the Q1 and Q2 of the high-power MOS tube through the MOS tube driver chip MC34152, and add the voltage across the energy storage capacitor to the two ends of the flat shooting electromagnet coil L1 or the lob shooting electromagnet coil L2, and complete the hitting action through the electromagnetic effect of the flat shooting electromagnet or the lob shooting electromagnet. When the hitting is completed, the connection between the energy storage capacitor and the electromagnet coil is immediately disconnected, and the hitting rod returns to its original position under the action of the reset spring. In order to prevent the induction current generated by the electromagnet coil at the moment of stopping the hitting from destroying the high-power MOS tube, the diodes D1 and D2 are connected in parallel with the electromagnet coils L1 and L2 to form a loop.

　This article uses the high-speed dual MOSFET driver MC34152 as the driver chip. This driver is simple to use and does not require complex peripheral circuits. It only needs a 12 V power supply to work. It has two independent high-current output channels and is compatible with COMS and LSTTL logic circuits. In addition, MC34152 can quickly charge and discharge large capacitive loads. For a 1,000 pF load, the output rise and fall time is only 15 ns, and the transmission delay (rising edge and falling edge) from the logic input to the driver output is only 55 ns. Therefore, this high-speed driver chip can respond well to the shooting command to ensure the accuracy and timeliness of the shooting effect.
4 Experiments and results analysis

　In order to test the design effect of the hitting mechanism, the soccer robot hitting system was tested for lob shooting and flat shooting, and the real-time tracking and measurement system (RTTS) of the moving target was used to track and measure the actual video of the test, and the motion trajectory and range of the "soccer" were measured.
By controlling the PWM generator to make the voltage of the energy storage capacitor reach 160 V, and controlling the driving of the hitting electromagnet to make the electromagnet conduction time in the range of 0.8 ms to 5.6 ms, the flat shooting and lob shooting were tested respectively. The results of the motion trajectory test are shown in Figure 4. Figure (a) is the test of the soccer robot's lob shooting effect, and Figure (b) is the test of the soccer robot's flat shooting effect. The "soccer" moves horizontally after leaving the 515 cm high flat shooting table. As shown in Figure 4, the "soccer" trajectories of lob shooting and flat shooting are different under different electromagnet power-on times. The hitting experiment data are shown in Figure 5.
It can be seen from the experiment that:

　(1) The boost circuit under the control of the PWM generator chip of UC3843 can effectively generate a large DC voltage, and can automatically stop charging after the charging reaches the set value, avoiding the damage of components caused by excessive charging voltage.
　(2) The hitting force of the football robot can be controlled by the power-on time of the hitting electromagnet. Within a certain range, the longer the power-on time, the greater the hitting force.
　(3) The flat shooting club, the lob shooting club and the lob shooter all have a great influence on the speed of the "football". Aluminum alloy has a certain rigidity while ensuring light weight, which is better than the iron hitting club.
　This paper designs an effective and flexible football robot hitting system with two ways of hitting, shooting and passing. The experimental results show that the designed boost module circuit can boost the DC voltage of 12 V to about 160 V; the designed drive circuit can realize lob shooting and flat shooting, and obtain a lob shooting stroke of 180 cm and a flat shooting initial velocity of 7 m/s. The football robot hitting system has a satisfactory hitting effect.
References
[1] Chen Tianhua, Guo Peiyuan. The significance of the development of soccer robots to industry and research [J]. Robotics and Applications, 2008 (5): 18-21.
[2] Yu Kaiping, Li Yongxin, Zhang Jie, et al. Design and simulation analysis of electromagnetic batting for soccer robots [J]. Mechanical Research and Applications, 2007 (1): 77-79.
[3] Li Shangrong, Liu Jinglou. Design of small DC electromagnet boost and drive circuit based on SG3525 [J]. Journal of Jiangsu Polytechnic Normal University, 2009 (2): 28-32.
[4] Chen Li, Zou Qinghua. Implementation of a low voltage DC-DC boost circuit [J]. Electromechanical Product Development and Innovation, 2010, 23 (3): 158-160.