Research of Insertion Mechanism of Flexible Search and Rescue Robot

Due to various factors, the global scope disaster events occur inevitably every year. Terrorism, natural disasters or destructive earthquakes tend to cause a large number of people buried in the ruins of buildings. The research of search and rescue instruments research is mainly focused on snake-like robot and life detector both at home and abroad. The endoscopic search and rescue robot in this paper combines the characteristics of these two kinds of search and rescue equipment. This thesis mainly studies the insertion mechanism of endoscopic flexible search and rescue robot. Based on the mechanics characteristics of the flexible robot body, this paper analyzes several common failure modes during the insertion into the ruins. This article puts forward the scheme of segmented gradual pulling-pushing, which plays an important role in promoting search and rescue work.


INTRODUCTION
Due to various factors, the global disaster events occur inevitably every year.Terrorism, natural disasters or destructive earthquakes tend to cause a large number of people buried in the ruins of buildings.Using scientific methods and advanced technical equipment to rescue the buried people and make the loss to a minimum becomes a hot topic of current research on search and rescue equipment [1,2] .There are already many research findings about gap search and rescue occurred at home and abroad.Snake-like robot and handheld life detector are both representatives.ACM-R3 invented by Japanese professor Hirose is one of the typical snake-like robots, which is driven by a series of modular joints in a flexible way [3] .H S1-S5 is currently the world's leading snake-like robot, and the RoboSnake designed by National University of Defense Technology is the earliest reported snake-like robot at home.Handheld life detector, represented by American "Snake Eyes" RTK616 and ZISTOS "Hawkeye", has a small size and is convenient to carry [4] .Handheld life detector can intuitively find the trapped person by using the optical probe and cables into the ruins.The CCD sensors in handheld life detector would send the video messages to the display terminal.The snake-like robot is too complicated and it is hard to get into the gaps and narrow spaces.Handheld life detectors, such as "Snake Eyes", still on the stage of manual sending which makes the operating personnel in a risk during the process of search and rescue.
This paper provides an automatic insertion way based on endoscopic gap search and rescue robot.The method uses the segmented gradual pulling-pushing method to send the endoscope.This paper analyzes the automatic insertion mechanism of endoscope at first.Based on the flexible structure characteristic, the pa-per analyzed the mechanics characteristics of the flexible endoscope.Insertion failure may happen due to the mechanical properties of flexible endoscope in the process of sending.For this kind of situation, this paper also analyzes some kinds of failure situations, and summarized the mechanical properties of insertion process, proposed segmented gradual pulling-pushing method.All of these give a theoretical basis for the insertion mechanism of endoscope.duce the friction.Structure of endoscope is shown in Figure 1.The coil spring structure makes the body have a good softness in the axial direction, with large stiffness in the radial direction.This structure features make it possible to send the endoscope by pulling-pushing method.Because of the structure characteristics, the endoscope can be sent into the gap of ruins by insertion mechanism.But considering the mechanics characteristics of helical spring structure, the body may suffer compression bending when the axial propulsive force reaches a certain level.

Compressive mechanical properties
As shown in Figure 2, assuming that pressure and the axis are coincident.When pressure gradually increases, and no more than a certain limit, the endoscope would keep the shape of straight line.Even though a tiny lateral disturbance force is used to make the body slightly bend, it can still restore line shape after disturbance force is removed.But when pressure increases to a certain limit, the balance becomes unstable, and will not restore after the force is removed.Instead, the body would keep in a curve shape balance.This pressure limit is called critical load, denoted as ij P .If there is no bending, the propulsion can be along the endoscope, and overcome frictional resistance to send the endoscope.If bending happens, the propulsion can only increase the bending.This compression bending phenomenon happens because the spring suffers compressed load, as shown in Figure 2. When pressure value exceeds the critical load, instability phenomenon appears in spiral spring, and the spring causes buckling.
As shown in Figure 2, circle angle of n-turn spring is ɔ.The radius is R with circular cross section of the spring.A pair of pressure P is applied across the center line in opposite direction.The critical load can be calculated by balance equation [5] : 1 4 ( sin sin ) 2 In the formula, Q is Poisson's ratio and EJ is bending strength.By the above formula, the greater the screw pitch is, the larger sinɔ, and the smaller the critical load, the more number of turns, and the smaller the critical load is.As shown in Figure 3, in the process of insertion, P is the front resistance, ‫ܨ‬ ௦ௗ is the main propulsion, and ‫ܨ‬ ௦ௗ P. To ensure that the flexible endoscope will not bend, then: By the above formula, the greater the screw pitch is, the larger sinɔ, and the smaller the critical load, the more number of turns, and the smaller the critical load is.The critical load need to be as large as possible, so dense coil spring is chose to make the spring angle as small as possible.In order to avoid too many turns of spring, compression length should not be too long to exceed the critical length.
Putting it into formula (3), we can obtain: (2 Rtg ) 8 ( sin sin ) 2 In the above formula, EJ is associated with the bar section of dense coil spring.Spring coil angle M is determined by dense coil spring.Therefore the critical load only relate to the compression critical length of endoscope.

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In actual situation, the radius of spring tube is 5.5 mm.The screw pitch is Relationship between number of turns of dense coil spring and the endoscope compression critical length is shown as follows:  Putting formula (10) into formula (9) we obtain the relationship between critical load of endoscope and compression critical length as the diagram below:

Mechanical properties of pushing bend
In the above assumptions of calculation, the force the endoscope bears is completely straight, and only along the axis.So the critical load calculated is very big.In actual cases, the endoscope has bending, bearing not only axial force and bending moment exists.Therefore, the endoscope moment load will be discussed in following paragraph.
Considering the couple 0 M applied on both ends of the spring, spring bends by pressure.There are internal forces on the spring rod whose central angle is D , such as bending moments M and torque a T .Each two of the three forces are vertical [6] , as shown in Figure 5.
The deformation energy of the spring is: Bending Angle is: According to the calculation of bending beam, (EI) eq is used here as a bending rigidity [7] .The relationship between bending critical load and compression critical length is shown below:

FAILURE ANALYSIS OF SEGMENTED GRADUAL PULLING-PUSHING METHOD
As the mechanical properties mentioned in previous section, there exist the following two kinds of failure modes if we merely push or pull the endoscope of the search and rescue robot into the narrow gap.

Pushing failure
If merely push the endoscope, due to the Euler critical load mentioned in the previous section (as shown in Figure 7), when the compression length of the flexible endoscope is more than the compression critical length, or the propulsive force applied at the back-end of the endoscope is greater than the critical force, the bending instability will occur and the propulsion will fail partly or even all.In Figure 7, due to bending instability, compression buckling deformation of the endoscope happens.The actual situation can be simplified to mechanical model shown in Figure 8.
Because ‫ܨ‬ ௨௦ > ‫ܨ‬ , the endoscope bends by pressure.Because propulsive force is always along the direction of the endoscope, bend of the endoscope results in deviation from the specified direction.The diagram assumes that the deviation Angle is Ʌ.Then ‫ܨ‬ ௨௦ can be resolved into ‫ܨ‬ ଵ and ‫ܨ‬ ଶ , and the relationships of ‫ܨ‬ ଵ , ‫ܨ‬ ଶ , ‫ܨ‬ ௦ , ‫ܨ‬ ௨௦ are as follows: When ‫ܨ‬ ଵ > ‫ܨ‬ ௦ , endoscope can be sent into gap smoothly.Based on the above formula, ‫ܨ‬ ଵ decreases with the increase of Ʌ, while ‫ܨ‬ ଶ increases with the increase of Ʌ. ‫ܨ‬ ଵ in the opposite direction of resistance can make the endoscope overcome resistance to move forward.However, ‫ܨ‬ ଶ causes the bending situation to become more and more serious, leading to the increase of deviation angle.The increase of Ʌ will cause ‫ܨ‬ ଶ to increase and cause ‫ܨ‬ ଵ to decrease.Simultaneously, the effect of ‫ܨ‬ ଶ causes the pressure between the endoscope and the barrier surface of ruins to increase.Because ‫ܨ‬ ௦ have relations to surface materials of the endoscope and the pressure of the endoscope to the surface of ruins, the increase of pressure of endoscope surface causes ‫ܨ‬ ௦ to increase.Therefore, the continuous decrease of ‫ܨ‬ ଵ and the continuous increase of ‫ܨ‬ ௦ will make it unable to overcome the resistance.

Pulling failure
If merely pull the endoscope at the front-end, the friction will increase with the increase of pulling length and the obstacles of ruins make the endoscope to bend.As shown in figure 9, due to the pulling distance is too long, the back of the endoscope bend.Pulling force will be difficult to overcome the friction.At this time the pulling force will fail, as mechanical model shown in Figure 10.When ‫ܨ‬ ଵ > ‫ܨ‬ ௦ , endoscope can be sent into gap smoothly.Based on the above formula, ‫ܨ‬ ଵ decreases with the increase of Ʌ, while ‫ܨ‬ ଶ increases with the increase of Ʌ. ‫ܨ‬ ଵ in the opposite direction of resistance can make the endoscope to overcome resistance to move forward.However, ‫ܨ‬ ଵ causes the pressure of the endoscope to the surface to become bigger.Because ‫ܨ‬ ௦ have relations to surface materials of the endoscope and the pressure the endoscope to the surface of ruins, the increase of pressure of endoscope surface cause ‫ܨ‬ ௦ to increase.Therefore, when the bending angle Ʌ is big enough, ‫ܨ‬ ଵ will not be able to overcome ‫ܨ‬ ௦ , the endoscope will not be able to be sent into gap.
Due to the motor torque limitation, we assume that propulsion ‫ܨ‬ ௨௦ = 5ܰ or pulling force ‫ܨ‬ ௨ = 5ܰ.
According to reference, the coefficient of friction between the endoscope and rock is 0.6 ~ 1.5, so we use the smallest value to calculate ݂ 0.6.The relationship among ‫ܨ‬ ଵ , resistance and the deflection angle Ʌ is shown in the figure below: When bending deflection angle Ʌ is greater than 58°, ‫ܨ‬ ଵ which is along the direction to overcome resistance cannot overcome resistance, leading to pushing failure.When in the state of instability, propulsion will make the endoscope bend further.
During the process of pulling, if pulling distance is too long and the bending of the endoscope caused by the environment of ruins is more than 58 °, the pulling force will not be able to overcome the friction, and there will be a pulling failure.

Segmented gradual pulling-pushing automatically insertion method
As the mechanical properties mentioned in previous section, there exist the following two kinds of failure modes if we merely push or pull the endoscope of the search and rescue robot into the narrow gap.By above knowable, there will be pushing failure or pulling failure if we merely push or pull the endoscope of the search and rescue robot into the narrow gap.So this paper has proposed a segmented gradual pulling-pushing automatically insertion method, which can effectively avoid the bad phenomena mentioned above. of pushing and pulling, known as "pulling-pushing mechanism".Therefore endoscope can be gradually sent into narrow gap, as shown in Figure 12.
Two arbitrary adjacent pulling-pushing mechanisms' installation distance should not be greater than the compression critical length of the endoscope, so that the pushing failure will not happen.Simultaneously, the combination of multistage relative motion mechanism and pulling-pushing also can effectively avoid too long pulling distance and pulling failure.Motors drive the friction wheel rotation through the bevel gear transmission, which can push the flexible endoscope to move forward.Simultaneously, the sun wheel of planetary gear train is sheathed on the friction wheel shaft.The motor drives the sun wheel and the sun wheel drives the planet wheel rotation, until the planet wheel rotate to limit position.Then the sun wheel idling and the stretch stents of the insertion mechanism have expanded to the limit position.
The experimental environment is simulation environment of ruins built by bricks.The experiment mainly simulates the turning and climbing situation in ruins.

Mechanical property testing of endoscope
The length of the endoscope in the experiment is 10m, the outer diameter is 13mm, and the weight is 2.3 KG.Pulling experiments are conducted on concrete surface.The total sliding friction is 15N.
In fact, in order to avoid the occurrence of bending failure, propulsion must be smaller than the bending critical load.Propulsion must be greater than the friction simultaneously, to overcome the resistance.
If endoscope is sent into gap by a single organization, the pressure the endoscope bears has exceeded its bending critical load and the propulsion has failed inevitably, because the endoscope is too long.While the segmented gradual sending method is adopted to send the endoscopic flexible body into narrow gap, frictional resistance is the only force to overcome.In order to ensure that compression length does not exceed the critical compression length, and bending force does not exceed the critical load.
Endoscope can be divided into 15 segments.Friction resistance of each segment is ݂ = 1ܰ.Compression length is 670 mm.Critical bending load is ܲ = 2.9ܰ.Assuming that propulsive force is F, safety coefficient is ܰ = 2, then if F meets the following formula, the endoscope can be sent into narrow gap successfully.

nf F P d d (21)
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Mechanical property testing of pulling-pushing method
The endoscope is sent into the gap by the segmented gradual pulling-pushing method.The following experiment can test the bending force of different length of the endoscope.In the experiment, one end of endoscope is fixed, the other end of the endoscope is applied a normal force perpendicular to the endoscope.See Figure 16 below.The experimental results are shown in table 1.
Table 1.Relation between the compression length of endoscope and the maximum bending stress

Pulling-pushing sending experiment
In this paper, experiment was carried out in view of the situations the endoscope of search and rescue robot may encounter in the rubble.The segmented gradual pulling-pushing method was adopted to send the robot, which simulates the turning situation of ruins.After experimental verification, segmented gradual pulling-pushing mechanism basically can send the endoscope into ruins, but when turning angle is less than 90°, the result is unstable and needs further improvement.

Figure 2 .
Figure 2. Load and deformation of endoscope

Figure 3 .
Figure 3. Compression model of the endoscope Putting Formula (8) into formula (3), we can obtain : ratio of spring steel 0.26 0.32 Q (0.30).Tensile modulus of elasticity of spring steel 200900 203950 E MPa(202000MPa).Spring section is 2 mm diameter circle.Moment of inertia of spring is as follows:

Figure 4 .
Figure 4. Relationship between critical load of endoscope and compression critical length

Figure 5 .
Figure 5. Analysis of internal forces on the spring rod only has relationship to spring itself, so: (EI) 0.0318 eq .So it can be calculated according to Euler's formu-

Figure 6 .
Figure 6.Relationship between bending critical load and compression critical length

1
Automatically insertion mechanism structure and design of experimental environmentAt the final period of design, prototype experiments are carried out.The automatically insertion mechanism structure is shown in Figure13.The real object is shown in Figure14.The insertion mechanism is composed of two symmetrical parts, installed outside the endoscope, preloaded by applying pre-tightening force.The implementation of the segmented gradual pulling-pushing automatically insertion mechanism is friction wheel gradual pushing.Friction wheel transmission diagram is shown in Figure15.The expanding of stretch stents and tailgates are controlled by the planetary gear train.

Figure 17 .
Figure 17.Pulling-Pushing Sending Experiment Research of Insertion Mechanism of Flexible Search and Rescue RobotLinyong Shen, Qian Zhu, Ya Qiu, Yuanfei Cao & Hang Ding College of Mechatronic Engineering and Automation, Shanghai University, Shanghai, China ABSTRACT: Due to various factors, the global scope disaster events occur inevitably every year.Terrorism, natural disasters or destructive earthquakes tend to cause a large number of people buried in the ruins of buildings.The research of search and rescue instruments research is mainly focused on snake-like robot and life detector both at home and abroad.The endoscopic search and rescue robot in this paper combines the characteristics of these two kinds of search and rescue equipment.This thesis mainly studies the insertion mechanism of endoscopic flexible search and rescue robot.Based on the mechanics characteristics of the flexible robot body, this paper analyzes several common failure modes during the insertion into the ruins.This article puts forward the scheme of segmented gradual pulling-pushing, which plays an important role in promoting search and rescue work.