GB2543752A

GB2543752A - Chronoscope calibration

Info

Publication number: GB2543752A
Application number: GB1518640.6A
Authority: GB
Inventors: Olden Paul
Original assignee: Key Forensic Services Ltd
Current assignee: Key Forensic Services Ltd
Priority date: 2015-10-21
Filing date: 2015-10-21
Publication date: 2017-05-03
Also published as: GB201518640D0

Abstract

A chronoscope is an instrument for measuring the time of flight of a projectile between two beams of light. The distance between the beams must be known accurately. An embodiment of the invention provides a calibration gauge 100 for a chronoscope, the gauge comprising a base portion 201 and a moveable portion 202 slidably mounted to the base portion 201 for translation along a linear translation path 203. The moveable portion 202 comprises first and second reference points 204a, 204b along the path 203, and a gap 205 between the first and second reference points 204a, 204b. The distance 206 between the first and second reference points 204a, 204b is adjustable by movement of the second reference point 204b relative to the first reference point 240a along the path 203. In use, the gauge is pushed into the chronoscope, interrupting the light beams. The distance between the reference points is adjusted. As this distance approaches the actual distance between the beams, the apparent speed measured by the chronoscope increases asymptotically. When the apparent speed exceeds a threshold value, the gauge is withdrawn and the distance between the reference points is accurately measured and used as a calibration constant.

Description

CHRONOSCOPE CALIBRATION Field of the Invention

The invention relates to calibration of chronoscopes, in particular for use in providing accurate and repeatable velocity measurements of projectiles.

Background A chronoscope is an instrument that allows for measurement of the velocity of a projectile, such as an air rifle pellet. An example of a chronoscope is the Skan Mark 9 chronoscope, available from Skan AR (skanar.co.uk). A chronoscope of this type consists essentially of an open-ended enclosure containing two separated pairs of sensors for detecting the passage of a projectile. The sensors are connected to a computer system that records the time between the pairs of sensors being triggered, being the time taken for a projectile to pass between the pairs of sensors. The system calculates a velocity v=d/t based on the measured time t and the known distance d between the pairs of sensors. A measure of the kinetic energy E=mv1t2, where m is the mass of the projectile, can then be determined. A simplified schematic diagram of a chronoscope 100 is illustrated in figure 1. The chronoscope 100 comprises first and second pairs of sensors, consisting of light emitters 101a, 101b and light detectors 102a, 102b. The pairs of sensors are connected to a computer system 103, which detects when light beams 104a, 104b between the pairs of detectors are broken. A projectile 105, for example an air rifle pellet fired from the barrel 106 of an air rifle, passes between the first and second pairs of sensors, breaking the first light beam 104a, followed by the second light beam 104b. The time taken for the light beams 104a, 104b to be broken allows the speed of the projectile 105 to be measured. An accurate measure of the speed requires an accurate measure of the distance d 107 between the light beams 104a, 104b to be measured. Any inaccuracy in the distance 107 will have a direct effect on the accuracy of the measured speed. While the time between the light beams 104a, 104b being broken can be measured to a high degree of accuracy (an accuracy of one microsecond or better is typically possible), and the mass of the projectile can also be accurately measured, the distance 107 is dependent on the exact spatial points at which each light beam 104a, 104b is broken by the projectile. These points are not easily determined, and cannot be reliably or accurately measured by simply measuring the distance between the light detectors 102a, 102b. The sensor beams 104a, 104b may, for example, not be perfectly vertical or parallel, and the sensor trigger points may not correspond exactly to the beam position. Additional sources of inaccuracy include the angle between the path of travel 108 of the projectile 105 and the light beams (which is ideally 90 degrees) and the path of travel being at a consistent position between the pairs of sensors. These additional sources of inaccuracy can be, however, readily controlled through careful control of the position and orientation of the rifle barrel 106. The distance 107 is therefore the main source of inaccuracy.

One way of determining the distance between the light beams is to use a calibrated rifle, i.e. a rifle that is sufficiently repeatable in terms of the kinetic energy imparted in each shot. A problem with this approach, however, is that even the most repeatable air rifles cannot provide a sufficiently uniform speed for calibration of a chronoscope. Air conditions, such as variations in temperature or humidity, can result in variations in speed out of the control of the user. In addition, the conditions of the rifle can vary depending on how many shots have been taken over a period of time, which can have a significant effect on the kinetic energy imparted to a pellet.

Summary of the Invention

In accordance with a first aspect there is provided a calibration gauge for a chronoscope, comprising: a base portion; and a moveable portion slidably mounted to the base portion for translation along a linear translation path, the moveable portion comprising first and second reference points along the translation path and a gap between the first and second reference points, wherein a distance between the first and second reference points is adjustable by movement of the second reference point relative to the first reference point along the translation path. A calibration gauge according to the invention can be used to accurately determine a distance between the light beams of a chronoscope by translating the reference points such that they pass the light beams in succession and adjusting the distance between the reference points until the apparent speed is sufficiently high that the distance between the reference points is close to the actual distance between the light beams.

The first and second reference points may be laterally offset from a rod slidably mounted to the base portion. The reference points are preferably rigidly mounted relative to each other, which may be achieved by offsetting them from a rigidly mounted slidable rod.

The distance between the first and second reference points may be adjustable by rotation of the second reference point about a threaded portion of the moveable portion.

The base portion may comprise a plate with first and second opposing ends, the moveable portion being slidably mounted to the first and second opposing ends.

The moveable portion may be resiliently biased in a forward direction along the translation path, for example by means of a spring biasing the moveable portion in the forward direction.

In accordance with a second aspect there is provided a method of calibrating a chronoscope, the chronoscope having first and second spaced apart transmitter detector pairs and being configured to detect passage of a projectile passing in a forward direction between the transmitter detector pairs and to measure a time interval between passage of the projectile between the first and second transmitter detector pairs, a calibration gauge according to any one of claims 1 to 6 being positioned such that the first and second reference points are translatable along the translation path to pass between the first and second transmitter and detector pairs respectively, the method comprising: i) translating the moveable portion in the forward direction; ii) measuring a time interval between the first reference point passing between the first transmitter detector pair and the second reference point passing between the second transmitter detector pair; iii) determining an apparent speed based on a distance between the first and second transmitter detector pairs and the measured time interval; iv) adjusting the distance between the first and second reference points and repeating steps i) to iii) until the apparent speed exceeds a threshold value; v) measuring a distance between the first and second reference points; and vi) calibrating the distance between the first and second transmitter detector pairs with the measured distance.

Prior to first translating the moveable portion the distance between the first and second reference points may be adjusted such that the distance is less than a prerecorded distance between the first and second transmitter detector pairs.

Step iv) may comprise increasing the distance between the first and second reference points.

Step vi) may comprise fitting an asymptotic function to a series of measurements of apparent speed as a function of distance between the first and second reference points, the distance between the first and second transmitted points being determined from a root of the asymptotic function.

In accordance with a third aspect there may be provided a computer program for instructing a computer to carry out the method according to the second aspect. The computer program may be provided on a non-transitory medium such as a disc or on computer memory.

Detailed Description

The invention is described in further detail below by way of example and with reference to the accompanying drawings, in which: figure 1 is a schematic diagram of a chronoscope; figure 2 is a view of an example embodiment of a calibration device; figure 3 is an alternative view of the example embodiment of figure 2; figure 4 is a further alternative view of the example embodiment of figure 2; figure 5 is a schematic diagram of a calibration device in position within a chronoscope; figure 6 is a flow chart illustrating an example method of calibrating a chronoscope; and figure 7 is an example chart plotting apparent velocity as a function of separation distance.

In order to obtain a more accurate measure of the velocity of a projectile from a chronoscope, the actual distance between the two light beams 104a, 104b needs to be measured as accurately as possible. This can be done by using a calibration gauge such as the device 200 shown in figure 2. The device 200 comprises a base portion 201 and a moveable portion 202. The moveable portion 202 is slidably mounted to the base portion 201 for translation along a linear translation path 203. The moveable portion 202 comprises first and second reference points 204a, 204b along the translation path, with a gap 205 between the first and second reference points 204a, 204b. A distance 206 along the translation path 203 between the first and second reference points 204a, 204b is adjustable by movement of the second reference point 204b relative to the first reference point 204a along the translation path 203. In the illustrated example, the reference points 204a, 204b are defined by air rifle pellets of the type to be used in the chronoscope.

The first and second reference points 204a, 204b are laterally offset from a first rod 207 that is slidably mounted to the base portion 201. This allows a gap 205 to be provided between the reference points 204a, 204b, while maintaining the distance 206 constant. For additional stability and rigidity, the reference points 204a, 204b may also each be mounted on the ends of second and third rods 208a, 208b that are slidably mounted to the base portion 201 and rigidly connected to the first rod 207. A third rod 208 may also be provided for additional stability, which is laterally offset from the first rod 207. The first rod 207 is rigidly connected to a slidable connection 209 mounted on the third rod 208. The effect of this arrangement is to ensure that the first and second reference points 204a, 204b are maintained rigidly relative to each other along the translation path 203 and prevented from moving in a direction orthogonal to the translation path 203, while allowing the reference points to be easily translated along the path 203.

The base portion 201 comprises a plate 211 with first and second opposing ends 212, 213, the moveable portion 202 being slidably mounted to the first and second opposing ends 212, 213. The ends 212, 213 in the illustrated embodiment are in the form of plates that are rigidly mounted to opposing ends of the plate 211.

An opening 214 is provided in the plate 211 to allow for passage of light through the plate and past the first reference point 204a.

Movement of the first rod 207 is effected by movement of a plunger 215 attached to an end of the first rod 207, causing translation of the reference points 204a, 204b along the translation path 203. This movement may be readily achieved by hand, or may alternatively be done using a motor or other type of actuator, for example a pneumatic actuator. The first rod 207 may optionally be biased in a forward direction 211 along the translation path 203, for example by means of a spring (not shown), which may be positioned against the first end 212 to bias the first rod in a forward direction. The first rod may then be actuated by pulling back on the plunger and letting go, in a similar action to that of a pinball machine plunger.

Regions of the calibration gauge 200 around the first and second reference points 204a, 204b are shown in more detail in figures 3 and 4. Figure 3 shows the gap 214 in the plate 211 aligned with the first reference point 204a. Figure 4 shows in closer detail the second reference point 204b, which in the illustrated embodiment is moveable to adjust the distance between the first and second reference points 204a, 204b. The second reference point 204b is adjustable by rotation about a threaded portion. One or more locking nuts 216 may be provided to fix the reference point 204b in position once an adjustment has been made.

Figure 5 illustrates schematically how the calibration gauge 200 is operated to calibrate a chronoscope of the type described above. The gauge 200 is positioned within the chronoscope 100, with the base 201 positioned between the first light sensor and detector pair 101a, 101b. The gap 205 is positioned to allow the first light path 104a to pass through the base 201. The distance 206 between the first and second reference points 204a, 204b is set to be less than the actual distance 107 between the first and second light beams 104a, 104b. The plunger 215 is then pushed and the reference points 204a, 204b moved forward along the translation path 203. Since the distance 206 is less than the actual distance 107, the first reference point 204a will break the first light beam 104a before the second reference point 204b breaks the second light beam 104b. The chronoscope 100 will measure an apparent speed from the time taken between the first and second light beams 104a, 104b being broken. The distance 206 can then be extended by adjusting the separation between the first and second reference points 204a, 204b and the measurement process repeated. As the distance 206 approaches the actual distance 107 between the light beams 104a, 104b, the apparent speed measured by the chronoscope will increase asymptotically. If the distance 206 is extended to exactly match or exceed the actual distance 107, the chronoscope will not be able to register an apparent speed, because the light beams will be triggered simultaneously or in reverse. The calibration process can therefore be stopped once the chronoscope measures an apparent speed that is above a predetermined threshold, which may for example be set to be a speed sufficiently far above the range of speeds to be measured in practice. A threshold may, for example, be set at 100000 m/s, which is well above the speed that may be expected from any conventional air rifle. This can result in a measurement uncertainty of around 0.5 mm based on a nominal distance of 203 mm.

Once the threshold speed has been exceeded, the calibration gauge 200 can be removed from the chronoscope 100 and the distance 206 between the first and second reference points 104a, 104b accurately measured, for example using a pair of calipers. This measurement can then be used to correct the distance used by the chronoscope, with the result that the chronoscope is calibrated to a higher degree of accuracy than before.

The calibration process may be carried out as a single operation for a given chronoscope, which can then be certified as being calibrated to a given accuracy.

The calibration process described above is illustrated in the flow chart in figure 6. The process starts (step 601) with positioning the calibration gauge within the chronoscope such that the first and second reference points are translatable along the translation path to pass between the first and second transmitter and detector pairs respectively. At step 602, the moveable portion is translated in the forward direction, such that the first and second reference points translate along the translation path and break the light beams between the first and second transmitted and detector pairs. At step 603, a time interval is measured by the chronoscope between the first reference point passing between the first transmitter detector pair and the second reference point passing between the second transmitted detector pair. At step 604, an apparent speed is determined by the chronoscope based on a distance (i.e. the previously estimated distance) between the first and second transmitted detector pairs and the measured time interval. At step 605 a decision is made depending on whether the measured speed exceeds a threshold value. If the threshold value is not exceeded, an adjustment is made to the distance between the reference points (step 606) and the process of measuring is repeated (steps 602 to 605). Once the threshold is exceeded, the distance between the reference points is measured and recorded (step 607), and the process ends (step 608).

Once the calibration gauge is positioned in the chronoscope, the above process may be carried out manually or may be automated, for example by adjusting the position of one of the reference points using a motor and translating the moveable portion using an actuator, both of which may be controlled by a computer system suitably configured and programmed to carry out the above process. The actual distance between the reference points as a function of the position of the motor configured to translate the reference point may be stored in the computer prior to the calibration process being performed.

Figure 7 shows an example of a modelled set of results, in which an apparent velocity is plotted as a function of separation distance between the reference points. In this example, the actual distance between the light beams is 203 mm. The apparent velocity increases rapidly as the separation distance gets closer to the actual distance, reaching an asymptote at the actual distance, corresponding to an infinite velocity. Since an infinite velocity cannot be measured, one way of more accurately determining the actual distance between the light beams is to fit the type of curve shown in figure 7 to an asymptotic function (i.e. a function of the form y=x/(k-x), where k is the root of the function). The function can be readily solved to determine its root, which determines the actual separation distance. This method would require a reasonably uniform plunger velocity, which can be achieved using a motorised stage, but the actual velocity of the plunger does not need to be accurate since this will not significantly affect the result, provided a sufficient number of measurements is made before fitting an asymptotic function to the results.

Other embodiments are intentionally within the scope of the invention as defined by the appended claims.

Claims

1. A calibration gauge for a chronoscope, comprising: a base portion; and a moveable portion slidably mounted to the base portion for translation along a linear translation path, the moveable portion comprising first and second reference points along the translation path and a gap between the first and second reference points, wherein a distance between the first and second reference points is adjustable by movement of the second reference point relative to the first reference point along the translation path.

2. The calibration gauge of claim 1 wherein the first and second reference points are laterally offset from a rod slidably mounted to the base portion.

3. The calibration gauge of claim 1 or claim 2 wherein the distance between the first and second reference point is adjustable by rotation of the second reference point about a threaded portion of the moveable portion.

4. The calibration gauge of any preceding claim wherein the base portion comprises a plate with first and second opposing ends, the moveable portion being slidably mounted to the first and second opposing ends.

5. The calibration gauge of any preceding claim wherein the moveable portion is resiliently biased in a forward direction along the translation path.

6. The calibration gauge of claim 5 wherein the moveable portion comprises a spring biasing the moveable portion in the forward direction.

7. A method of calibrating a chronoscope, the chronoscope having first and second spaced apart transmitter detector pairs and being configured to detect passage of a projectile passing in a forward direction between the transmitter detector pairs and to measure a time interval between passage of the projectile between the first and second transmitter detector pairs, a calibration gauge according to any one of claims 1 to 6 being positioned such that the first and second reference points are translatable along the translation path to pass between the first and second transmitter and detector pairs respectively, the method comprising: i) translating the moveable portion in the forward direction; ii) measuring a time interval between the first reference point passing between the first transmitter detector pair and the second reference point passing between the second transmitter detector pair; iii) determining an apparent speed based on a distance between the first and second transmitter detector pairs and the measured time interval; iv) adjusting the distance between the first and second reference points and repeating steps i) to iii) until the apparent speed exceeds a threshold value; v) measuring a distance between the first and second reference points; and vi) calibrating the distance between the first and second transmitter detector pairs with the measured distance.

8. The method of claim 7 wherein prior to first translating the moveable portion the distance between the first and second reference points is adjusted such that the distance is less than a prerecorded distance between the first and second transmitter detector pairs.

9. The method of claim 8 wherein step iv) comprises increasing the distance between the first and second reference points.

10. The method of any one of claims 7 to 9 wherein step vi) comprises fitting an asymptotic function to a series of measurements of apparent speed as a function of distance between the first and second reference points, the distance between the first and second transmitted points being determined from a root of the asymptotic function.

11. A computer program for instructing a computer to perform the method according to any one of claims 7 to 10.

12. A calibration gauge for a chronoscope substantially as described herein, with reference to the accompanying drawings in figures 2 to 6.

13. A method of calibrating a chronoscope substantially as described herein, with reference to the accompanying drawings in figures 2 to 6.