Some technical features of using equipment to measure bullet speed..
PETROV Sergey Ivanovich
SOME TECHNICAL FEATURES OF USING EQUIPMENT TO MEASUREMENT BULLET SPEED
When conducting ballistic examinations in forensics, development and testing of samples of small arms and artillery weapons, there is a need to measure the speed of projectiles and bullets. There are universal and special measurement methods that are based on the use of equipment with significant differences in the ability to solve various types of scientific and technical problems.
Universal measurement methods include high-speed photography and pulse radiography. The equipment used (high-speed optical cameras such as SFR, ZhLV-2, VSK, VFU-1 and X-ray pulse devices such as MIRA-5B/1, PIR-600A) allows for a wide range of scientific research and technical measurements. The use of this equipment for measuring speeds has not become widespread due to the fact that the equipment is quite expensive, and the accuracy of measurements does not always correspond to the intended purpose.
At present, the most widely used special methods of measuring speeds are those using equipment specially designed to measure speeds with specified metrological and operational characteristics (type of firing device, range of measured speeds, measurement error, preparation time for a repeat experiment, service life).
Special methods of measuring speeds use two basic measurement principles:
- measuring the frequency shift of a signal reflected from a moving body relative to the frequency of the main signal (Doppler effect);
- measuring the time interval between signals from bullet (projectile) flight sensors spaced by the value of the measurement base.
Doppler velocity meters are complex and expensive measuring systems (like the Ariel measuring system), suitable for measuring velocities in areas of internal and external ballistics. As a rule, such meters are used in research and development as part of measuring systems with computer processing of results and are currently of limited use.
Velocity meters with a fixed measuring base, in turn, are divided into two large groups by type:
contact:
- disk;
- frame (for breaking or closing the measuring circuit);
- charging;
non-contact:
- optical (LED or laser);
- inductive;
- inductive (magnetic and electromagnetic).
This article considers only special time-interval meters with a fixed measuring base, as they are the most widely used in experimental technology.
CONTACT-TYPE VELOCITY METERS
Disk meters
A disk speed meter is a system of two uniformly rotating thin-walled but fairly rigid disks located on a common shaft and spaced apart by the size of the measuring base.
The bullet, whose movement is parallel to the axis of rotation of the disks, sequentially penetrates first the first, then the second disk. During the time the bullet flies the inter-disk distance, the second disk rotates at a certain angle. Knowing the magnitude of the angular velocity and the distance between the disks, we can calculate the speed of the bullet.
The disadvantages of such a measurement system include: increased requirements for uniformity of disk rotation (a speed stabilization device is required), additional error due to the dynamic impact on the system when a bullet hits a disk and their deformation during penetration, a decrease in bullet speed during penetration of the first disk, a large number of consumables, inconvenience of operation and a significant time for preparation for repeated measurement.
This speed measurement system does not have obvious advantages and has not become widespread.
Frame meters
Frame speed meters usually consist of two flat target frames spaced apart by the measurement base. The target frames can be made of thin wire or metal foil. The main design requirement for the frames is minimal stretching at the moment of rupture (closure).
The frames are included in the corresponding electric circuits through which the electric current passes. The bullet (projectile) alternately breaks each of the circuits. Potential surges occur at the signal pickup points, which, accordingly, start and stop the counting device (frequency meter in timer mode). Such frame targets operate on breaking.
Frame targets operating on closing are usually made of two sheets of thin metal foil, electrically insulated from each other by a thin sheet of dielectric material (polyethylene, paper, etc.). The bullet alternately closes the plates of the first and second frame targets, forming, accordingly, the start and stop circuits of the timer.
The advantages of frame speed measurement systems are the simplicity of the design and low cost, high noise immunity and metrological characteristics when working on large measurement bases. At the same time, the following can be noted as disadvantages: a large number of consumables, significant preparation time for repeated measurements, additional error due to deformation of the sensor at the moment of interaction with the bullet.
Frame velocity meters have become quite widespread in testing laboratories and at proving grounds, especially when working with shells and grenades for artillery systems.
Charge meters
Charge velocity meters are a further development of frame measurement systems. Their operation is based on the following physical principle.
The timer start and stop sensors are a two-channel system consisting of a charge receiver and charge amplifiers. The charge receiver is a thin metal plate fixed on the ballistic track perpendicular to the bullet's trajectory. The charge amplifier is a device that converts a change in charge on the charge receiver into a change in potential at the timer input.
The bullet, electrifying in flight through the air, accumulates an electric charge Q, the value of which mainly depends on its geometric dimensions and shape, i.e. on its own capacitance C1 and the value of the electrostatic potential difference U1:
Q = U1 • C1
The charge receiver also has its own capacitance C2. At the moment the bullet touches the first charge receiver, a “bullet-charge receiver” system with a capacitance C is formed:
C = C1+ C2
The electric charge accumulated by the bullet is redistributed between the bullet and the charge receiver, and a potential surge dU is formed at the input of the charge amplifier:
dU = U1– U2, where U2 = Q/(C1+ C2)
The signal is amplified and fed to the timer start input. The second channel (timer stop) works identically.
While retaining all the main characteristics of frame systems, the charge velocity meter has a significantly lower measurement error due to the fact that the spatial position of the bullet at the moment of starting and stopping the frequency meter (the moment the bullet touches the charge receiver) is determined with the highest possible accuracy, regardless of the subsequent deformation of the charge receiver. And therefore it is possible to use a measuring base of much shorter length.
Charge velocity meters have not become widespread, since they have been replaced by non-contact velocity meters, which are more preferable in operation.
NON-CONTACT TYPE VELOCITY METERS
Optical meters
Optical speed meters operate on the principle of photoelectric blocking and are usually made in the form of a system consisting of two measuring optical planes spaced by the value of the measuring base.
Depending on the method of forming the optical planes, optical meters are divided into LED and laser.
In LED meters (photo 1), the optical plane is formed by an LED line (emitter) and a photodiode line (radiation receiver), installed in clips with thin translucent slits.
Photo 1. Optical speed meter
A bullet passing through an optical plane weakens the light flux coming to the photodetector, as a result of which a start (stop) signal for the timer is formed at the output of the electronic device.
In laser measuring devices, the optical plane is formed by multiple reflections of the laser emitter beam in such a way that the step of the beam grid is less than the minimum caliber of the bullet, and the beam, having been repeatedly reflected from the system of mirrors or mirror prisms, hits the photodetector. A bullet, passing through the optical plane in any part of it, completely interrupts the light flow coming to the photodetector, which activates the electronic circuit for generating the start (stop) signal of the timer.
Optical speed measuring devices are characterized by high performance, constant readiness for work, do not require consumables and operate in a wide range of speeds.
A significant disadvantage of such measuring devices is the possible significant measurement error, especially in the case of work with barrel systems with poor obturation of powder gases in the barrel bore (worn barrel or homemade barrel) or in the case of the formation of an intense ballistic shock wave propagating in front of the flying body with a speed different from it. In both cases, the interruption of the light flux is carried out not by the bullet, but by the turbulence front (shock wave), moving in space with a variable speed and having an indirect relation to the speed of the bullet itself. Moreover, in a number of cases in one experiment, it is possible to interrupt the light flux in one optical plane by the turbulence front, in another plane — by the bullet itself.
In addition, the disadvantage of these meters is the need for preventive maintenance of optical units. A characteristic feature of laser meters is their relatively high cost.
Nevertheless, LED speed meters have become quite widespread, in particular the FEB-7, as basic equipment for testing laboratories.
Laser meters are not yet mass-produced.
Inductive meters
The operating principle of inductive speed meters is based on changing the inductance of the measuring coils, which are the sensors of the bullet's flight.
When a bullet flies inside the coil, its inductance decreases due to the shielding effect (for bullets made of non-ferrous metals) or increases due to magnetic shunting (for bullets with a steel core). The measuring coil is included in the oscillatory circuit of the master oscillator, which accordingly changes the frequency of the generated oscillations. The change in the frequency of the master oscillator signal is converted by the frequency discriminator into a change in the output signal voltage. The output start (stop) pulse generator works directly on the timer.
The measuring base is equal to the distance between the geometric centers of the magnetic fields of the measuring coils.
Inductive speed meters are characterized by higher performance parameters than optical meters, but have a significant measurement error and are very sensitive to electromagnetic and mechanical interference (vibrations, impacts, etc.). They have received limited distribution.
Inductive meters
Inductive speed meters implement the same principle of measuring speed — measuring the time interval on a fixed measuring base. They differ from other non-contact type meters in the design and operating principle of the speed sensors.
Induction meters are divided into magnetic and electromagnetic. In a magnetic meter, the sensor is based on a permanent magnet, and in an electromagnetic meter, on an electromagnet (photo 2). The operating principle of both sensor designs is similar.
The sensor has its own magnetic field (in the electromagnetic version, the field is created by the magnetizing current). The bullet, flying inside the sensor coil (magnet ring), creates a change in the sensor's magnetic field pattern. If the bullet is made of a magnetic material, the main contribution to the change in the sensor's magnetic field is made by the magnetic shunting effect. If the bullet is made of a non-magnetic material (copper, lead), the change in the sensor's magnetic field occurs due to the introduction of the bullet's magnetic field into it. The bullet's magnetic field is secondary (induced by the resulting Foucault currents).
Photo 2. Induction speed meter
Changing the magnetic field of the sensor causes interference in the signal winding of the magnet or in the magnetization winding of the electromagnet. These interferences are a useful signal, carrying information about the bullet's flight through the measuring plane of the speed sensor. The signals are then amplified, compared and converted into rectangular pulses of a given amplitude and duration (timer start and stop pulses).
The main advantage of inductive velocity meters compared to inductive and optical ones is that the output signal levels of the sensors reach units and tens of volts, which allows choosing a significant comparison threshold and, as a result, reliable operation at a high level of external interference. In addition, turbulence fronts accompanying the process of shooting and bullet flight do not have a negative effect on the accuracy of measurements.
The main disadvantage of inductive velocity meters is inherent in the very principle of signal acquisition: the output signal of the velocity sensor is directly proportional to the bullet speed. This limits their use for measuring low speeds (the minimum measurable speed is several tens of meters per second) and completely excludes static measurements.
Due to the fact that in the overwhelming majority of cases it is necessary to measure bullet speeds in the range from 90 … 100 m/s and higher, inductive velocity meters are becoming the most promising direction in solving the problem of high-precision measurement of bullet speeds of 4.5 … 14.5 mm calibers.