GB2410311A

GB2410311A - Accelerator and method of accelerating an object

Info

Publication number: GB2410311A
Application number: GB0401177A
Authority: GB
Inventors: Peter Bignell; Leslie John Wall
Original assignee: TRL Ltd
Current assignee: TRL Ltd
Priority date: 2004-01-20
Filing date: 2004-01-20
Publication date: 2005-07-27
Anticipated expiration: 2024-01-20
Also published as: WO2005068966A1; GB0401177D0; GB2410311B

Abstract

The invention relates to an accelerator, for accelerating an object to be tested in an impact testing rig, and a method of accelerating an object. The accelerator comprises two releasably connected carriages, a front carriage 630, to which the object is mounted, and a rear carriage 632; elastic (bungee) cords 620; pulleys 660 662 664 666 668; and a motor and winch mechanism 612. As the carriages are pulled back by the winch mechanism a pin 622, mounted on the underside of the front carriage, engages with the elastic cords 620. The elastic cords are therefore extended by a combination of a) the movement of the rear carriage and b) the movement of the front carriage once the pin has engaged. The front carnage is then accelerated when a coupling 634 between the two carriages is released. This arrangement reduces the intial acceleration of the object to be fired, which as a result reduces the likelihood of disturbing the fired object before impact.

Description

Accelerator and Method of Accelerating an Obiect

Field of the Invention

The invention relates to accelerators that accelerate objects. Such devices may, for example, accelerate objects for impact testing.

Backaround

Vehicle manufacturers need to test vehicle components, in order to examine how they will behave in impacts. One important test of some components requires the simulation of forces occurring in a vehicle crash. In order to do this, a vehicle component may be accelerated up to a typical vehicle speed.

The component is then allowed to collide with another object. The impact itself can be observed, and the component can be examined after the test.

One known device for performing such tests is an 'impact rig'. This consists of a straight path, along which the component under test is accelerated. The path may be defined by a pair of parallel rails. The component under test is generally mounted on a platform, and the actual acceleration force is applied to the platform. The platform may, for example, have wheels. The wheels run along the inside surface of the parallel rails, as the platform accelerates.

Many different arrangements are known for providing the acceleration force.

One particular arrangement employs elastic ropes. These may consist of commercially available ropes, commonly termed 'bungee cords'. When these bungee cords are extended from their rest position, they provide a pull force.

The force is approximately that predicted by Hooke's law, with the force depending on the ratio of the cord's extended length to its rest length.

Although an impact rig has been described, accelerators can be used in other fields. These include devices that accelerate projectiles. An accelerator might also have applications in metal deforming, where large forces may be required. The terminology used in the prior art for accelerators varies, and some are referred to as 'sleds'.

Figure 1 represents a prior art accelerator, seen in side elevation.

A trolley 2 moves within a pair of rails 8, along axis X. Wheels 6 run on the inside of rails 8, and confine the trolley's movement in other axes than X. An object to be accelerated 4 is mounted on the trolley, on suitable means 3.

The object to be accelerated may be a crash test dummy, which may be equipped with measuring instrumentation.

Winch 12 and cable 10 combine to move trolley 2 from a forward position F to a 'pulled back' position B. As the trolley moves from forward position F to pulled back position B. an elastic means is stretched. The elastic means is not shown in figure 1, but is explained in figures 2 and 3.

Figure 2 represents the accelerator of Figure 1, seen in plan view. The rails and wheels have been omitted, for clarity of illustration.

Winch 212 and cable 210 are arranged to pull back trolley 202. An elastic means 220 is anchored at both ends, at a fixed plane 224. Elastic means 220 runs over two pulleys, 250 and 252.

Between pulleys 250 and 252, elastic means 220 crosses the path of trolley 202 as it moves along axis X. In figure 2, the dotted portion of elastic means 220 merely indicates the elastic means passing underneath trolley 202, when seen in plan view.

A pin 222 is fitted to the underside of trolley 202. Pin 222 is of sufficient length to reach down to the level of elastic means 220. Shortly after winch 212 and cable 210 begin to pull trolley 202 from forward position F to pulled back position B. pin 222 engages with elastic means 220. Further movement of trolley 202 therefore causes stretching of elastic means 220. This stretching stores energy in elastic means 220.

Figure 3 shows a plan view of the accelerator of Figure 2, at a different point in its operating cycle.

In figure 3, the trolley is part way between the forward position F and the pulled back position B. Elastic means 320 has been extended from its rest length, and is exerting a corresponding pull force on trolley 302. The force exerted by elastic means 320 is along axis X, In a direction from B to F. Winch 312 and cable 310 pull back trolley 302 to the pulled back position B. The trolley can then be released. Trolley 302, and the object to be accelerated 4, see figure 1, will be accelerated by elastic means 302.

Trolley 302 and the object to be accelerated will continue to accelerate towards the forward position F. until trolley 302 reaches the forward position F. At this point, pin 322 will disengage from elastic means 320. This situation generally resembles that shown in figure 2 once again, except that trolley 302 will be moving forward at high speed. After pin 322 has disengaged from elastic means 320, the trolley is essentially in 'free flight', with no force exerted either by cable 310 or elastic means 320.

If the accelerator is being used for an impact test, then the impact will be arranged to occur at a point in space just after pin 322 has disengaged from elastic means 320. A measurement of the trolley's speed may be performed shortly before the impact.

Figure 4 shows a side view of a known prior art variant of the arrangements of figures 1-3, including two moveable trolleys.

Winch 412, cable 410, wheels 406 and rails 408 fulfil the same functions as the corresponding elements in figure 1.

First trolley 402 has means 403 for mounting the object under test. A pin 422 is mounted on first trolley 402, for engaging the elastic means. The elastic means is not shown in figure 4, but corresponds to elastic means 220, 320 shown in figures 2 and 3.

First trolley 402 is connectable via a releasable coupling 434 to a second trolley 414. Cable 410 is connected to second trolley 414.

With the arrangement of figure 4, cable 410 can pull both trolleys 402 and 414 from the forward position F to the pulled back position B. When coupling 434 is released, first trolley 402 accelerates along axis X to the forward position F. Here an impact test may occur. Second trolley 414 remains at the pulled back position throughout the acceleration step.

Typically, an accelerator will need to perform several acceleration cycles in a working day. Therefore the prior art accelerator of Figure 4 may be provided with a mechanism for returning the second trolley 414 to the forward position F. after each acceleration cycle has been conducted.

One known arrangement to return the second trolley 414 to the forward position comprises a chain, mounted between the rails, under the trolley. A motor can drive the chain, which engages with second trolley 414 and pulls it to the forward position. Cable 410 can remain attached to second trolley 414 throughout the entire operation cycle. In this case, winch 412 will pay out cable 410 as the chain pulls second trolley 414 to the forward position F. The chain may take the form of an endless loop, driven by a motor. However, such an arrangement is not shown in Figure 4. s

In the accelerators of figs 1-3 and 4, the elastic means may comprise one or more elastic ropes. These elastic ropes may be so-called 'bungee cords'.

Bungee cords are mass produced, so are relatively cheap. One advantage of using these bungee cords is that the number of bungee cords mounted in parallel can be varied. This allows a range of acceleration forces to be applied in an accelerator that is of fixed length from the forward F to the pulled back position B. Summarv of the Invention An accelerator in accordance with the invention comprises the features of appended claim 1. Details of preferred embodiments of the accelerator of the invention are provided in claims 2-6.

A method of accelerating an object in accordance with the invention comprises the steps of appended claim 7. Details of preferred embodiments of the method of accelerating an object in accordance with the invention are provided in dependent claims 8-11.

At the start of an acceleration cycle, the invention can provide lower acceleration forces on an object than in prior art accelerators of comparable dimensions that provide a similar terminal velocity. This advantage means that an object to be accelerated is less likely to be disturbed at the start of the acceleration. Alternatively, the invention can provide a higher terminal velocity for an object than prior art accelerators of comparable dimensions, for the same initial acceleration force. A trade-off between these advantageous features is possible, in order to select the desired balance between object terminal velocity and initial acceleration force.

Drawinas Figure 1 represents a prior art accelerator, seen in side elevation.

Figure 2 represents the accelerator of figure 1, seen in plan view.

Figure 3 shows a plan view of the accelerator of figures 1 and 2, at a different point in its operating cycle.

Figure 4 shows a side view of a known prior art variant of the arrangements of figures 1-3.

Figure 5 shows the forces applied in the prior art arrangements of figures 14.

Figure 6 shows an arrangement in accordance with the invention, in plan view.

Figure 7 shows a further plan view of the arrangement of figure 6.

Figure 8 shows a further plan view of the arrangement of figure 6.

Figure 9 shows a further embodiment of the invention.

Figure 10 shows the forces applied in the arrangement of the invention.

Detailed description

The inventors have considered a known problem with the arrangements of figures 14. This concerns the force applied by the elastic means 220, 320. In particular, the magnitude of the force applied to the trolley has significant implications for the success of any test.

Figure 5 shows the force applied to trolley 202, 302, by the elastic means 220, 320 shown in figures 2 and 3. The force applied is plotted against the displacement of the bungee cords. The force has a maximum magnitude of T1, at point A. This corresponds to the pulled back position B. where the elastic means has been extended the furthest. This extension corresponds to displacement 'd' on figure 5.

Now consider the release of trolley 202, 302, for an acceleration cycle. As the trolley moves along the acceleration path, the force acting on it falls as the percentage extension of the elastic means falls. When the elastic means return to their rest length, the force applied to the trolley falls to zero. This is the point where pin 222 no longer contacts the bungee cords 220.

By Newton's laws, the acceleration of the trolley 202, 302 and object 4 are proportional to the force applied to them. The kinetic energy built up by the trolley is proportional to the integral of the force. This is represented by the area under the force curve in figure 5, area OdA.

At the start of each test, the force on the object under test changes from zero to that corresponding to T1 in figure 5. The inventors have experienced problems in the test, at this point. The object to be accelerated may be dislodged or undesirably reoriented at this point.

For example, consider the case where the object to be accelerated 4 is a crash test dummy held in a safety belt, see figure 1. In this case, the limbs or upper body of the dummy may move undesirably at the start of the test. Even if this occurs at the start of the acceleration phase, the dummy is likely to remain in this incorrect attitude through to the moment of impact. This may invalidate the test. Possible consequences include the necessity to make a judgement about whether or not the impact was acceptable, extra time for re- testing, and possibly the expense of a new dummy or safety belt for a re- test.

The inventors have devised an arrangement that can provide a lower acceleration than that indicated in figure 5 at displacement d, the pulled back position B. The arrangement of the invention provides significantly lower disturbance of object to be accelerated 4, at the start of the acceleration.

Apparatus of the invention Figure 6 shows an accelerator in accordance with the invention, in plan view.

The accelerator comprises a first moveable member 630 and a second moveable member 632. First and second moveable members 630, 632 are displaceable along a common axis X, between a forward position F and a pulled back position B. In figure 6, both the first 630 and second 632 moveable members are shown generally at the forward position F. Figure 6 has been drawn to show clearly the arrangement of the elastic means. In applications of the accelerator to impact testing, the spacing between the forward position F and pulled back position B would be considerably greater in proportion to the size of first 630 and second 632 moveable members as illustrated. Figures 7 and 8 have also been illustrated with a reduced separation F-B.

Moveable members 630, 632 may take the form of trolleys, mounted on wheels and confined within a pair of parallel rails, as generally described in connection with prior art figures 1-4. However, the moveable members may take other forms, and may be constrained to move along axis X by a variety of alternative engineering arrangements. Various bearing arrangements could, for example, replace the wheels.

Means 3 for mounting an object to be accelerated 4 are arranged on the second moveable member 632. These means correspond generally to means 3, 403 of figures 1 and 4, and have been omitted from figure 6 for clarity of illustration.

An elastic means 620 is provided. Elastic means 620 is reversibly extensible over the range of extension described in figures 6-10. Elastic means 620 is arranged, when extended, to provide a pull force along the common axis X, the pull force depending on the amount of extension of the elastic means.

Elastic means 620 is connectable 680, 682, to the first moveable member 630. The second moveable member 632 and the elastic means 620 are adapted to engage releasably. This adaptation may take the form of a pin 622, which can engage elastic means 620.

Figure 6 shows a particular spatial arrangement of the elastic means 620, although other arrangements are possible. Starting from the centre of elastic means 620 and considering the course of the elastic means towards one of its ends, the elastic means passes over a pulley 660. The elastic means then runs approximately the distance d, from the forward position F to the pulled back position B. The elastic means then rounds a second pulley 664, and runs a further distance d back to third pulley 668. After rounding third pulley 668, the end of the elastic means 620 is connected to first moveable member 630 at point 680.

Starting from the centre of elastic means 620 and considering the path to the other end of elastic means, the elastic means passes over fourth pulley 662, fifth pulley 666 and sixth pulley 670. The elastic means is then joined to second moveable member 630 at point 682.

In a preferred embodiment of the invention, a releasable coupling 634 may be arranged to couple the first 630 and second 632 moveable members releasably.

Elastic means 620 may comprise one or more bungee cords. Elastic means 620 may be pretensioned to exert a pull force on the first moveable means 630 at the forward position F. Figure 7 shows another plan view of the accelerator of figure 6. In figure 7, the accelerator is shown at a different point of operation. The numbering of elements in figure 7 corresponds to that used in figure 6, with a 7 prefix on each number replacing the 6 prefix in figure 6.

In figure 7, winch 712 and cable 710 have been used to pull first 730 and second 732 moveable members from the forward position F to the pulled back position B. Elastic means 720 has been extended relative to the rest position shown in figure 6, through two actions: (i) The connection of the ends of elastic means 720 to the first moveable means 730, at 780 and 782. This causes the elastic means to extend as the first moveable means 730 moves from the forward position F to the back position B. (ii) Pin 722 engages with the mid portion of elastic means 720, as the winch begins to pull back the second moveable portion 732. As the second moveable means 732 moves from the forward position F to the back position B. the pin causes the elastic means to extend.

In the pulled back position B. the elastic means 720 is extended by an amount depending on: (i) the distance travailed by the first moveable member 730 along the common axis X from the forward position F; and (ii) the distance travelled by the second moveable member 732 along the common axis X after engaging the elastic means 720.

Clearly, the pull force provided by the elastic means 720 in the pulled back position B depends on these distances.

If the first 730 and second 732 moveable members are coupled by releasable coupling 734, then winch 712 and cable 734 can move both moveable members back together, in one operation.

However, the invention could also be operated in a second node, whereby winch 712 pulls the first moveable member 730 to the pulled back position B in a first step, after which the first moveable member 730 would be anchored in the pulled back position B. Then, in a second step, the winch would be connected to second moveable member 732, and would pull this to the pulled back position B. Here the two moveable members could be coupled with releasable coupling 734, if fitted. Otherwise the two members could be individually anchored in the pulled back position B. prior to release of second member 732 and the acceleration phase.

Figure 8 shows a further plan view of the arrangement of figure 6. Figure 8 shows the accelerator of figures 6 and 7, following release of second moveable means 732 and the acceleration phase. In figure 8, the numbering of elements corresponds to that used in figures 6 and 7, with an 8 prefix.

In figure 8, first movable means 830 remains in the pulled back position B. Second moveable means 832 has, however, returned to the forward position F. Scrutiny of the elastic means 820 in figure 8 shows clear differences over the degree of extension of the elastic means 620, 720 in figures 6 and 7.

Because first moveable means 830 remains in pulled back position B. there is some extension of the elastic means 820. However, pin 822 of second moveable means 832 has disengaged from elastic means 820, so no longer causes any extension of elastic means 820. So the tension in elastic means 820 is less than that in elastic means 720 shown in figure 7, but greater than that in elastic means 620 in figure 6.

Comparison of the length of elastic means 720 in figure 7 with the length of elastic means 620 in figure 6 shows the maximum extension of the elastic means. Limiting the maximum extension to this range helps ensure good linearity between the force exerted by the elastic means and the amount of extension. Bungee cords are able to provide a force roughly proportional to the extension, up to the point where their length exceeds double their rest length. This force curve is also repeatable, so they are suitable to repeated extension cycles, which are needed for repeated acceleration cycles.

Figure 9 shows a further embodiment of the invention. The arrangement of figure 9 functions according to the same principle as the arrangement shown in figures 6-8. However, the arrangement of figure 9 includes only one pair of pulleys, 990 and 992.

Winch 912 and cable 910 pull the first 930 and second 932 moveable members from the forward position F to the pulled back position, B. This causes elastic means 920 to extend. However, this extension results in elastic means 920 extending over a far greater proportion of its rest length than was the case for the arrangement of figures 6-8.

Elastic means 920 in figure 9 has to be of higher quality than that required in figures 6-8 to produce a pull force that is proportional to the amount of extension. If bungee cords were used in the arrangement of figure 9, they would be extended beyond the range in which they provide their best performance.

Figure 10 shows the force versus displacement curve for the arrangement of the invention, for displacement between the positions shown in figures 7 and 8.

Point C on figure 10 corresponds to the arrangement of figure 7, with both the

_ A -

first 730 and second 732 moveable members at the pulled back position B. The force in the elastic means 720 at this point is T2.

Point D on figure 10 corresponds to the arrangement of figure 8, with only the first moveable member 830 at the pulled back position B. The second moveable member 832 has been released, and is in the forward position F. The force in the elastic means 820 at this point is T3.

The force curve and point A, from figure 5, have been shown in dotted form on figure 9. This is to facilitate comparison.

The acceleration of second moveable member 832 is proportional to the force applied to it, by Newton's second law of motion. So the energy of second moveable member 832 as it reaches forward position F is proportional to the area of trapezium CdOD.

The force is maximum at point C. As second moveable member 832 moves along the acceleration path, the force on it falls from T2 to T3 at point D. The maximum force applied to second moveable member 832 is T2 at point C. This force is considerably less than force T1, exerted by the prior art arrangement of figures 1-4 at point A. However, the area of trapezium CdOD in figure 10 is equal to the area of the triangle OdA. So second moveable member 832 reaches the same terminal velocity, immediately prior to impact, as trolleys 2, 402 in figures 1-4. This assumes that the weight of the second moveable member 832 is equal to that of trolleys 2, 402, and assumes that differences in friction and air resistance forces are not significant.

The acceleration at point C is significantly less likely to disturb an object under test than the acceleration at point A in the prior art arrangement. Each impact test is therefore more likely to be successful.

There is a further beneficial feature of the arrangement of figures 6-10. Once an acceleration run has been conducted, the elastic means 820 is still under tension T3. Tension T3 acts on the first moveable means 830. So first moveable means 830 can be returned to the forward position by allowing the winch cable to pay out, and the elastic means to return to the position shown in figure 6 or figure 9. The arrangement of the invention therefore does not need a separate chain drive, corresponding to that used in the arrangement of figure 4 to return trolley 414 to the forward position F. Method of the Invention The explanation of figures 68 above included a number of method steps.

The invention provides a method of accelerating an object, comprising: a) arranging a first moveable member 630, 730 and a second moveable member 632,732 to be displaceable along a common axis X, between a forward position F and a pulled back position B; b) mounting an object to be accelerated 4 on the second moveable member 632,732; c) connecting the first moveable member 630,730 to an elastic means 620, 720 the elastic means being reversibly extensible and, when extended, providing a pull force along the common axis X, the pull force depending on the amount of extension of the elastic means; d) pulling the first and second moveable members from the forward position F to the pulled back position B. thereby engaging 622 the second moveable member 632 and the elastic means 620 releasably; e) the elastic means 620 providing a pull force T2 in the pulled back position B depending on: (i) the distance travailed by the first moveable member 630 along the common axis X from the forward position F; and Is (ii) the distance travelled by the second moveable member 632 along the common axis X, after engaging the elastic means 620; releasing the second moveable means 732.

Clearly, in the pulled back position (B), the elastic means 620,720 is extended by an amount depending on: (i) the distance travailed by the first moveable member 730 along common axis X from the forward position; and (ii) the distance travelled by the second moveable member 732 along common axis X, after engaging the elastic means.

Pin 622 on second moveable means 632 can be used to releasably engage with the elastic means 620.

The method of the invention may further comprise releasably coupling the first 630 and second 632 moveable members, prior to the step of pulling the first and second moveable members from the forward position F to the pulled back position B. In this case, the step of releasing the second moveable member 732 may further comprise releasing the coupling 734.

When the second moveable means 832 has resumed to the forward position F. the residual pull force T3 in the elastic means 820 may be used to return the first moveable means 830 to the forward position F. In this case, the first moveable means 830 remains attached to the cable 810 during the step of returning to the forward position F. Following this, the cable 610 and a winch 612 are then once again able to provide the pull force for the step of pulling the first and second moveable members from the forward position F to the pulled back position B. A preferred embodiment of the invention may comprise the step of pre- tensioning the elastic means 620, so that the elastic means exerts a pull force on the first moveable means 630 at the forward position F. The effect of providing pretension in elastic means 620 can be seen by considering figure 10. If there were significant pretension in the elastic means, then the gradient of line DC would be lower for the same area CdOD. The difference between forces T2 and T3 would be less than that shown in figure 10, for any given terminal velocity of the second moveable means in an acceleration cycle. This would reduce the magnitude of force T2, and thereby further reduce the likelihood of disturbance to an object to be accelerated.

Claims

Claims 1. An accelerator, comprising: a first moveable member (630) and a

second moveable member (632), the first and second moveable members being displaceable along a common axis (X), between a forward position (F) and a pulled back position (B); means (3) for mounting an object to be accelerated (4) on the second moveable member (632); an elastic means (620), the elastic means being reversibly extensible and being arranged, when extended, to provide a pull force along the common axis (X), the pull force depending on the amount of extension of the elastic means; the elastic means (620) being connectable (680, 682) to the first moveable member (630); the second moveable member (632) and the elastic means (620) being adapted (622) to engage releasably; whereby, in the pulled back position (B), the elastic means (620) is extended by an amount depending on: (i) the distance travelled by the first moveable member (630) along the common axis (X) from the forward position (F); and (ii) the distance travelled by the second moveable member (632) along the common axis (X) after engaging the elastic means (620).
2. An accelerator in accordance with claim 1, wherein: a releasable coupling (634) is arranged to couple the first (630) and second (632) moveable members.
3. An accelerator in accordance with any previous claim, wherein: the elastic means (620) comprises one or more bungee cords.
4. An accelerator in accordance with any previous claim, wherein: the elastic means (620) is pretensioned, so that the elastic means exerts a pull force on the first moveable means (630) at the forward position (F).
5. An accelerator in accordance with any previous claim, wherein: the first moveable means (630) is attached to a cable (610) and a winch (612), the cable and winch being adapted to pull the first moveable member (630) and the second moveable member (632) from the forward position (F) to the pulled back position (B).
6. An accelerator in accordance with any previous claim, wherein the accelerator is an impact rig, and the object to be accelerated (4) is a component under test.
7. A method of accelerating an object, comprising: arranging a first moveable member (630) and a second moveable member (632) to be displaceable along a common axis (X), between a forward position (F) and a pulled back position (B); mounting (3) an object to be accelerated (4) on the second moveable member (632); connecting (680, 682) the first moveable member (630) to an elastic means (620), the elastic means being reversibly extensible and, when extended, providing a pull force along the common axis (X), the pull force depending on the amount of extension of the elastic means; pulling the first and second moveable members from the forward position (F) to the pulled back position (B), thereby engaging (622) the second moveable member (632) and the elastic means (620) releasably; the elastic means (620) providing a pull force (T2) in the pulled back position (B) depending on: (i) the distance travailed by the first moveable member (630) along the common axis (X) from the forward position (F); and (ii) the distance travelled by the second moveable member (632) along the common axis (X), after engaging the elastic means (620); releasing the second moveable means (632).
8. The method of claim 7, further comprising: releasably coupling the first (630) and second t632) moveable members, prior to the step of pulling the first and second moveable members from the forward position (F) to the pulled back position (B); the step of releasing the second moveable member (632) further comprising releasing the coupling between the first (630) and second (632) moveable members.
9. The method of claim 7 or claim 8, further comprising the step of: after the second moveable means has resumed to the forward position (F), allowing the residual pull force (T3) in the elastic means (620) to return the first moveable means (630) to the forward position (F).
10. The method of claim 9, whereby: the first moveable means (630) remains attached to a cable (610) during the step of returning to the forward position (F): the cable (610) and a winch (612) providing the pull force for the step of pulling the first and second moveable members from the forward position (F) to the pulled back position (B).
11. The method of any of claims 7-10, further comprising the step of: pretensioning the elastic means (620), so that the elastic means exerts a pull force on the first moveable means (630) at the forward position (F).