Design of electromagnetic trigger controller for firearms based on CPLD technology

O Introduction

The firing of firearms is an important link in the field of shooting range testing. Traditional firearms are fired manually, that is, the shooter pulls the trigger of the firearm manually after hearing the shooting command. This method has two problems: First, safety cannot be guaranteed. When operating, the personnel may make mistakes due to fatigue or mishearing the instructions, which may cause safety accidents to personnel in the target range. In some applications, such as bulletproof helmet armor penetration experiments, shooters must face the bulletproof helmet to shoot, and the bullet may rebound and injure the shooter. The above two safety accidents have occurred in domestic shooting ranges. Second, with the continuous development of shooting range testing technology, the types of equipment used in shooting range testing are increasing, and the accuracy is getting higher and higher. Therefore, how to maintain a certain degree of synchronization between different test equipment is becoming more and more important. Obviously, it is difficult to achieve synchronization requirements by manually firing firearms.

Based on the above considerations, some people have proposed a control method based on electromagnetic effect, which consists of an iron core, a coil, an armature, a reed, etc. When it is necessary to control the firing of a firearm, a certain voltage is applied to both ends of the coil, and a current will flow through the coil, thereby generating an electromagnetic effect. Under the action of the electromagnetic force, the armature overcomes the tension of the spring and absorbs the iron core tightly to drive the trigger to move. When the trigger needs to be released, the voltage at both ends of the coil is disconnected, the electromagnetic force disappears, and the armature returns to its original position under the action of the spring tension to release the trigger. After many tests, it was confirmed that this method is feasible, but there are certain disadvantages. For example, the recovery of the armature position depends on the tension of the spring, and the spring will produce fatigue after long-term use. On the other hand, when the coil works, there will be a large impact current, which will cause interference to the power grid and other equipment, and even cause key equipment to be triggered incorrectly.

Aiming at the above problems, a firearm electromagnetic trigger controller based on CPLD technology was designed.

1 Overall structure of electromagnetic trigger controller

In order to ensure the electromagnetic trigger controller works safely and reliably, a certain logic interlocking mechanism must be designed. CPLD is used to realize the circuit logic function, and the stepper motor is used as the execution unit.

The overall structure of the electromagnetic trigger controller is shown in Figure 1.

In Figure 1, the electromagnetic trigger controller is composed of a CPLD logic controller, an RS 232 communication module, a motor driver, and a stepper motor. Among them, the CPLD completes the logic control and serial communication functions, and the motor driver receives the logic instructions to drive the stepper motor to work. Applying the CPLD online programmable technology and serial communication technology, the designed controller has high field programmable and networking functions, and can achieve synchronization, automation and networking of the overall test system with other test equipment. Since the electromagnetic trigger controller is used in the shooting range environment, the safety of its use is a key indicator. The newly designed controller overcomes many problems of the old instrument, fully considers factors such as electromagnetic compatibility, field operability and test safety, and guarantees the safety of use to the greatest extent from the design.

The schematic diagram of the control panel of the firearm electromagnetic trigger controller is shown in Figure 2. [page]

From the perspective of eliminating interference and ensuring stability, first of all, when designing the CPLD control board, a large number of filter capacitors are added between the power supply and the ground, optoelectronic isolation is added to the data channel, and the long-line output of the control signal adopts twisted pair output and the interface adopts military aviation plugs to avoid introducing interference in the transmission path; in the stepper motor execution unit, an electromagnetic shielding box is added to eliminate the electromagnetic interference caused by the motor movement.

From the perspective of test safety, an unlocking switch is added to the control panel to ensure the overall control of the system; at the same time, an interlocking function is added to the logic of the "trigger" and "reset" buttons to ensure the correctness of the operation, thereby eliminating misoperation.

2 Mechanical design

Most of the existing trigger controllers are designed with electromagnetic principles, and the main disadvantages are: the spring is prone to fatigue after long-term use; the armature is prone to generate large impact current during the attraction process, which affects the stability of the power grid and the normal operation of other test instruments. In response to these problems, the designer uses a motor driver to drive a stepper motor to replace the original mechanism, which can accurately control the trigger operation.

The mechanical structure diagram is shown in Figure 3.

The mechanical part of the electromagnetic trigger controller is mainly composed of a stepper motor, a base, a rotary mechanism, a trigger link, a wiring box and an electromagnetic trigger shielding shell. Its working process is: the stepper motor receives the driver's command to rotate, driving the rotary mechanism to rotate, the stepper motor rotates one circle, the rotary mechanism drives the trigger to achieve a stroke, and completes the firing action.

Figure 4 is a schematic diagram of the electromagnetic trigger stroke direction and stroke length adjustment mechanism. The rotary mechanism is designed with trigger stroke adjustment holes (①, ②, ③, ④ in the figure, the four adjustment holes are gradually reduced in radius from the center of the rotary mechanism R1>R2>R3>R4), and the length of the trigger connecting rod can also be adjusted. By adjusting these two mechanisms, the trigger stroke distance can be adjusted.

The stepper motor is driven by a motor driver connected to high voltage, and the CPLD controller generates a logic control signal to control the trigger. The controller and the motor driver are connected by a long twisted wire. The tester is far away from interference sources such as the motor, and the tester can control the firing of the firearm from a distance, which not only ensures that the control system is not subject to electromagnetic interference, but also ensures the safety of the tester. [page]

3. Control part design

3.1 CPLD logic control part

The electromagnetic trigger controller uses a stepper motor as the execution unit and a CPLD as the main controller to realize the logic control and communication functions. The logic control part realizes the key reading, latching, triggering and resetting interlocking and the control function of the stepper motor driver.

The controller uses the "unlock switch" as the main switch of the instrument function. When locked, all the buttons on the instrument panel do not work; after unlocking, the controller works normally. The motor action is coordinated by the "reset" and "trigger" buttons. When firing is allowed, press the "trigger" button, the firearm fires and latches the trigger key. If you continue to press the trigger key, the instrument will not trigger; when you need to fire again, you must press the "reset" button to release the trigger latch, and then press the trigger button to trigger the instrument. The safe triggering of the controller is ensured by the mutual latching of the "reset" and "trigger" buttons. Figure 5 is a timing diagram of the CPLD circuit logic function simulation.

The Name column on the left in Figure 5 defines the pins: clk is the CPLD input clock of 1 MHz; feng is the divided clock when the system is working; green and red represent the "trigger" and "reset" buttons respectively; out is the logic output terminal; key is the unlock switch.

The logic function of the controller is shown in Figure 5. When the system is locked (key=0), the system does not work when the key is pressed (1 in Figure 5); after the system is unlocked (key=1), press the "trigger" key, the system outputs a control signal; continue to press the "trigger" key, the system is in an interlock protection state, and the system has no trigger output (3 in Figure 5); after pressing the "reset" key, you can continue to trigger, and the system can output normally (4 in Figure 5); repeat the wrong operation, continue to trigger, and there is no output (5 in Figure 5).

In Figure 5, green_lignt and red_light correspond to the trigger light and reset light respectively. When the system is started and not unlocked, the trigger light is on, the reset light is off, and the unlock light is off (2 in Figure 5); after the system is unlocked and triggered, the trigger light is off and the reset light is on, indicating that the system has been triggered and needs to be reset to release the protection before it can be triggered again. After reset, the trigger light is on and the reset light is off, indicating that the system can be triggered.

3.2 Communication interface part

Since the current shooting range test system is composed of many test instruments, and during the test process, data collection and processing require high real-time performance, the electromagnetic trigger controller is required to work through software triggering. At present, most of the test instruments in the domestic shooting range test field have serial RS 232 interfaces. The designer added a serial communication module on the basis of the control circuit to realize system networking. The entire test process can be controlled by software from the firing of the gun to the data collection and processing of the test system, and the real-time performance has been greatly improved.

RS 232 uses a negative logic level standard, with logic "1" being -3 to -15 V and logic "0" being +3 to +15 V. It has a large tolerance, fewer data lines, and strong anti-interference capabilities, and can achieve remote data transmission.

The RS 232 communication interface design based on CPLD uses MAX232 to transform the level and logic relationship. Since the CPLD and the interface are transmitted in parallel, and the interface and the peripherals are in serial mode, it is necessary to add a serial-to-parallel conversion module to the serial interface. A typical serial interface module is shown in Figure 6.

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During the data input process, serial data enters the module's "receive shift register" bit by bit. After receiving a complete character, the data is sent from the "receive shift register" to the "data input register" and then taken out in parallel through the parallel bus DATA[7:O]. The data output process is just the opposite. The data transmission speed is determined by the receive/send clock.

The RS 232 interface circuit in the electromagnetic trigger controller is shown in Figure 7.

Figure 7 shows a serial communication circuit using the MAX232 chip. The chip can adapt to a +5 V single power supply environment, and the hardware interface is simple and easy to implement. The MAX232 contains two receivers and drivers, and has a voltage doubler and a voltage inverter inside, which can convert the input +5 V power supply voltage into the RS 232 output level ±10 V. The four capacitors in Figure 7 can be replaced by 0.1 μF non-polar ceramic capacitors instead of 1μF/16 V electrolytic capacitors, and they should be as close to the chip as possible to improve anti-interference ability.

4 Conclusion

In the design of the electromagnetic trigger controller for firearms based on CPLD technology, we fully understood the problems existing in the original controller, such as poor safety, inability to accurately control, and inability to achieve networking testing. We carried out design practice based on electromagnetic compatibility, system stability, and safety, and achieved good results. The designed controller has been tested at the shooting range and can adapt to the complex and changeable electromagnetic environment in the shooting range environment, and can perform testing work safely and reliably.