MD5算法的研究与实现数据存储加密.doc

ecause of the complex nature of cutting operations it is difficult to establish relationships that quantitatively define the machinability of a material In manufacturing plants tool life and surface roughness are generally considered to be the most important factors in machinability Although not used much any more approximate machinability ratings are available in the example below 20 9 1 Machinability Of Steels Because steels are among the most important engineering materials as noted in Chapter 5 their machinability has been studied extensively The machinability of steels has been mainly improved by adding lead and sulfur to obtain so called free machining steels Resulfurized and Rephosphorized steels Sulfur in steels forms manganese sulfide inclusions second phase particles which act as stress raisers in the primary shear zone As a result the chips produced break up easily and are small this improves machinability The size shape distribution and concentration of these inclusions significantly influence machinability Elements such as tellurium and selenium which are both chemically similar to sulfur act as inclusion modifiers in resulfurized steels Phosphorus in steels has two major effects It strengthens the ferrite causing increased hardness Harder steels result in better chip formation and surface finish Note that soft steels can be difficult to machine with built up edge formation and poor surface finish The second effect is that increased hardness causes the formation of short chips instead of continuous stringy ones thereby improving machinability Leaded Steels A high percentage of lead in steels solidifies at the tip of manganese sulfide inclusions In non resulfurized grades of steel lead takes the form of dispersed fine particles Lead is insoluble in iron copper and aluminum and their alloys Because of its low shear strength therefore lead acts as a solid lubricant Section 32 11 and is smeared over the tool chip interface during cutting This behavior has been verified by the presence of high concentrations of lead on the tool side face of chips when machining leaded steels When the temperature is sufficiently high for instance at high cutting speeds and feeds Section 20 6 the lead melts directly in front of the tool acting as a liquid lubricant In addition to this effect lead lowers the shear stress in the primary shear zone reducing cutting forces and power consumption Lead can be used in every grade of steel such as 10 xx 11xx 12xx 41xx etc Leaded steels are identified by the letter L between the second and third numerals for example 10L45 Note that in stainless steels similar use of the letter L means low carbon a condition that improves their corrosion resistance However because lead is a well known toxin and a pollutant there are serious environmental concerns about its use in steels estimated at 4500 tons of lead consumption every year in the production of steels Consequently there is a continuing trend toward eliminating the use of lead in steels lead free steels Bismuth and tin are now being investigated as possible substitutes for lead in steels Calcium Deoxidized Steels An important development is calcium deoxidized steels in which oxide flakes of calcium silicates CaSo are formed These flakes in turn reduce the strength of the secondary shear zone decreasing tool chip interface and wear Temperature is correspondingly reduced Consequently these steels produce less crater wear especially at high cutting speeds Stainless Steels Austenitic 300 series steels are generally difficult to machine Chatter can be s problem necessitating machine tools with high stiffness However ferritic stainless steels also 300 series have good machinability Martensitic 400 series steels are abrasive tend to form a built up edge and require tool materials with high hot hardness and crater wear resistance Precipitation hardening stainless steels are strong and abrasive requiring hard and abrasion resistant tool materials The Effects of Other Elements in Steels on Machinability The presence of aluminum and silicon in steels is always harmful because these elements combine with oxygen to form aluminum oxide and silicates which are hard and abrasive These compounds increase tool wear and reduce machinability It is essential to produce and use clean steels Carbon and manganese have various effects on the machinability of steels depending on their composition Plain low carbon steels less than 0 15 C can produce poor surface finish by forming a built up edge Cast steels are more abrasive although their machinability is similar to that of wrought steels Tool and die steels are very difficult to machine and usually require annealing prior to machining Machinability of most steels is improved by cold working which hardens the material and reduces the tendency for built up edge formation Other alloying elements such as nickel chromium molybdenum and vanadium which improve the properties of steels generally reduce machinability The effect of boron is negligible Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel Oxygen has been shown to have a strong effect on the aspect ratio of the manganese sulfide inclusions the higher the oxygen content the lower the aspect ratio and the higher the machinability In selecting various elements to improve machinability we should consider the possible detrimental effects of these elements on the properties and strength of the machined part in service At elevated temperatures for example lead causes embrittlement of steels liquid metal embrittlement hot shortness see Section 1 4 3 although at room temperature it has no effect on mechanical properties Sulfur can severely reduce the hot workability of steels because of the formation of iron sulfide unless sufficient manganese is present to prevent such formation At room temperature the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese sulfide inclusions anisotropy Rephosphorized steels are significantly less ductile and are produced solely to improve machinability 20 9 2 Machinability of Various Other Metals Aluminum is generally very easy to machine although the softer grades tend to form a built up edge resulting in poor surface finish High cutting speeds high rake angles and high relief angles are recommended Wrought aluminum alloys with high silicon content and cast aluminum alloys may be abrasive they require harder tool materials Dimensional tolerance control may be a problem in machining aluminum since it has a high thermal coefficient of expansion and a relatively low elastic modulus Beryllium is similar to cast irons Because it is more abrasive and toxic though it requires machining in a controlled environment Cast gray irons are generally machinable but are Free carbides in castings reduce their machinability and cause tool chipping or fracture necessitating tools with high toughness Nodular and malleable irons are machinable with hard tool materials Cobalt based alloys are abrasive and highly work hardening They require sharp abrasion resistant tool materials and low feeds and speeds Wrought copper can be difficult to machine because of built up edge formation although cast copper alloys are easy to machine Brasses are easy to machine especially with the addition pf lead leaded free machining brass Bronzes are more difficult to machine than brass Magnesium is very easy to machine with good surface finish and prolonged tool life However care should be exercised because of its high rate of oxidation and the danger of fire the element is pyrophoric Molybdenum is ductile and work hardening so it can produce poor surface finish Sharp tools are necessary Nickel based alloys are work hardening abrasive and strong at high temperatures Their machinability is similar to that of stainless steels Tantalum is very work hardening ductile and soft It produces a poor surface finish tool wear is high Titanium and its alloys have poor thermal conductivity indeed the lowest of all metals causing significant temperature rise and built up edge they can be difficult to machine Tungsten is brittle strong and very abrasive so its machinability is low although it greatly improves at elevated temperatures Zirconium has good machinability It requires a coolant type cutting fluid however because of the explosion and fire 20 9 3 Machinability of Various Materials Graphite is abrasive it requires hard abrasion resistant sharp tools Thermoplastics generally have low thermal conductivity low elastic modulus and low softening temperature Consequently machining them requires tools with positive rake angles to reduce cutting forces large relief angles small depths of cut and feed relatively high speeds and proper support of the workpiece Tools should be sharp External cooling of the cutting zone may be necessary to keep the chips from becoming gummy and sticking to the tools Cooling can usually be achieved with a jet of air vapor mist or water soluble oils Residual stresses may develop during machining To relieve these stresses machined parts can be annealed for a period of time at temperatures rangiand then cooled slowly and uniformly to room temperature Thermosetting plastics are brittle and sensitive to thermal gradients during cutting Their machinability is generally similar to that of thermoplastics Because of the fibers present reinforced plastics are very abrasive and are difficult to machine Fiber tearing pulling and edge delamination are significant problems they can lead to severe reduction in the load carrying capacity of the component Furthermore machining of these materials requires careful removal of machining debris to avoid contact with and inhaling of the fibers The machinability of ceramics has improved steadily with the development of nanoceramics Section 8 2 5 and with the selection of appropriate processing parameters such as ductile regime cutting Section 22 4 2 Metal matrix and ceramic matrix composites can be difficult to machine depending on the properties of the individual components i e reinforcing or whiskers as well as the matrix material 20 9 4 Thermally Assisted Machining Metals and alloys that are difficult to machine at room temperature can be machined more easily at elevated temperatures In thermally assisted machining hot machining the source of heat a torch induction coil high energy beam such as laser or electron beam or plasma arc is forces b increased tool life c use of inexpensive cutting tool materials d higher material removal rates and e reduced tendency for vibration and chatter It may be difficult to heat and maintain a uniform temperature distribution within the workpiece Also the original microstructure of the workpiece may be adversely affected by elevated temperatures Most applications of hot machining are in the turning of high strength metals and alloys although experiments are in progress to machine ceramics such as silicon nitride SUMMARY Machinability is usually defined in terms of surface finish tool life force and power requirements and chip control Machinability of materials depends not only on their intrinsic properties and microstructure but also on proper selection and control of process variables Abstract A bipedal architecture is highly suitable for a robot built to work in human environments since such a robot will find avoiding obstacles a relatively easy task However the complex dynamics involved in the walking mechanism make the control of such a robot a challenging task The zero moment point ZMP trajectory in the robot s foot is a signifi cant criterion for the robot s stability during walking If the ZMP could be measured on line then it becomes possible to create stable walking conditions for the robot and here also stably control the robot by using the measured ZMP values ZMP data is measured in real time situations using a biped walking robot and this ZMP data is then modelled using an adaptive neuro fuzzy system ANFS Natural walking motions on fl at level surfaces and up and down a 10 slope are measured The modelling performance of the ANFS is optimized by changing the membership functions and the consequent part of the fuzzy rules The excellent performance demonstrated by the ANFS means that it can not only be used to model robot movements but also to control actual robots 1 Introduction The bipedal structure is one of the most versatile setups for a walking robot A biped robot has almost the same movement mechanisms as a human and it able to operate in environments containing stairs obstacles etc However the dynamics involved are highly nonlinear complex and unstable Thus it is diffi cult to generate a human like walking motion The realisation of human like walking robots is an area of considerable activity 1 4 In contrast to industrial robot manipulators the interaction between a walking robot and the ground is complex The concept of a zero moment point ZMP 2 has been shown to be useful in the control of this interaction The trajectory of the ZMP beneath the robot foot during a walk is after taken to be an indication of the stability of the walk 1 6 Using the ZMP we can synthesise the walking patterns of biped robots and demonstrate a walking motion with actual robots Thus the ZMP criterion dictates the dynamic stability of a biped robot The ZMP represents the point at which the ground reaction force is taken to occur The location of the ZMP can be calculated using a model of the robot However it is possible that there can be a large error between the actual ZMP value and the calculated value due to deviations in the physical parameters between the mathematical model and the real machine Thus the actual ZMP should be measured especially if it is to be used in a to parameters a control method for stable walking In this work actual ZMP data taken throughout the whole walking cycle are obtained from a practical biped waling robot The robot will be tested both on a fl at fl oor and also on 10 slopes An adaptive neuro fuzzy system ANFS will be used to model the ZMP trajectory data thereby allowing its use to control a complex real biped walking robot 2 Biped walking robot 2 1 Design of the biped walking robot We have designed and implemented the biped walking robot shown in Fig 1 The robot has 19 joints The key dimensions of the robot are also shown in Fig 1 The height and the total weight are about 380mm and 1700 g including batteries respectively The weight of the robot is minimised by using aluminium in its construction Each joint is driven by a RC servomotor that consists of a DC motor gears and a simple controller Each of the RC servomotors is mounted in a linked structure This structure ensures that the robot is stable i e will not fall down easily and gives the robot a human like appearance A block diagram of our robot system is shown in Fig 2 Out robot is able to walk at a rate of one step 48mm every 1 4 s on a fl at fl oor or an shallow slopes The specifi cations of the robot are listed in Table 1 The walkingmotions of the robot are shown in Figs 3 6 Figures 3 and 4 are show front and side views of the robot respectively when the robot is on a fl at surface Figure 5 is a snapshot of the robot walking down a slope whereas Fig 6 is a snapshot of the robot walking up a slope The locations of the joints during motion are shown in Fig 7 The measured ZMP trajectory is obtained from ten degree of freedom DOF data as shown in Fig 7 Two degrees of freedom are assigned to the hips and ankles and one DOF to each knee Using these joint angles a cyclic walking pattern has been realised Our robot is able to walk continuously without falling down The joint angles in the four step motion of our robot are summarised in the Appendix 2 2 ZMP measurement system The ZMP trajectory in a robot foot is a signifi cant criterion for the stability of the walk In many studies ZMP coordinates are computed using a model of the robot and information from the encoders on the joints However we employed a more direct approach which is to use data measured using sensors mounted on the robot s feet The distribution of the ground is reaction force beneath the robot s foot is complicated However at any point P on the sole of the foot to the reaction can be represented by a force N and moment M as shown in Fig 8 The ZMP is simply the centre of the pressure of the foot on the ground and the moment applied by the ground about this point is zero In other words the point P on the ground is the point at which the net moment of the inertial and gravity forces has no component along the axes parallel to the ground 1 7 Figure 9 illustrates the used sensors and their placement on the sole of the robot s foot The type of force sensor used in our experiments is a FlexiForce A201 sensor 8 They are attached to the four corners of the plate that constitutes the sole of the foot Sensor signals are digitised by an ADC board with a sampling time of 10ms Measurements are carried out in real time The foot pressure is obtained by summing the force signals Using the sensor data it is easy to calculate the actual ZMP values The ZMPs in the local foot coordinate frame are computed using 毕毕 业业 设设 计计 论论 文文 MD5MD5 算法的研究与实现算法的研究与实现 数据存储加密数据存储加密 论论文作者姓名 文作者姓名 申申请请学位学位专业专业 申申请请学位学位类别类别 指指导导教教师师姓姓名名 职职称称 论论文提交日期 文提交日期 MD5MD5 算法的研究与实现算法的研究与实现 数据存储加密数据存储加密 摘摘 要要 随着网络技术的广泛应用 网络信息安全越来越引起人们的重视 针对数 据在存储的时候存在大量的安全问题 目前通常将需要存储的数据进行加密然 后再存储 应用 MD5 算法是一个不错的选择 MD5 算法的全称是 Message Digest algorithm 5 是一种用于产生数字签名的单项散列算法 它的作用是 让大容量信息在用数字签名软件签署私人密钥前被 压缩 成一种保密的格式 即将一个任意长度的 字节串 通过一个不可逆的字符串变换算法变换成一个 128bit 的串 该毕业设计是运用 microsoft visual c 6 0 软件而开发的 主要是通过 算法实现数据的加密存储 文章分成五部分 第一 二部分描述了 MD5 的目前 现状和相关理论知识 也让我们了解 MD5 的定义 重点是 MD5 的流程实现和封 装 DLL 在 MD5 算法的 DLL 封装这章 主要是描述我们为什么要选用封装 DLL 的原因 以及封装的好处 设计流程这一部分里包含读取 修改 插入 删除 这几个功能的实现情况 并用流程图的方式来分别描述了这四大功能模块的实 现过程 最后一部分显示了系统测试的内容和系统主要功能运行界面图 关键词 关键词 信息安全 MD5 加密 封装 The Research an