第4次课--Chapter 3.ppt

Chapter 3 Kinematic Analysis of Mechanisms(机构的运动分析) 3.1 Tasks and Methods of Kinematic Analysis1. Tasks: to find position, velocity and acceleration(加速度) of driven links according to the input parameters(参数) of driving link(s) and kinematic dimensions of all links. Position: to determine whether all links will interfere(干涉) with each other, and to find locus(轨迹) Velocity: to calculate the stored kinetic energy(动能) or power(功率) P Acc.：To calculate the dynamic forces 2. Methods Graphical method(图解法) (a) Geometrical(几何学的) method for position (b) Instant(瞬时的) centre method for velocity (c) Vector(矢量) equation method* Analytical method (解析法) (a) Whole mechanism method*(b) Assur group method Experimental method (实验法) 3.2 Velocity Analysis by the Method of Instant Centres(瞬心法) 1. Definition of the Instant Centre(瞬心) In Theory Mechanics(理论力学）: One object move in a plane, at anytime, there is one point, the absolute(绝对的) velocities of which is zero, the motion of the object can be regarded as rotation relative to this point; This point can be called Instantaneous centre (瞬心) of relative rotation, or more briefly the instant centre, denoted(标为) as “P” , and VA?AP. Extend the above concept to two objects There is a pair of coincident(重合) points, the absolute(绝对的) velocities of which are the same, in both magnitude(大小) and direction. Classification(分类) of instant centres Absolute (绝对) instant centre: one of links is the frame. Its velocity is zero, but its acceleration may not be zero. Relative(相对) instant centre: both links are moving links, velocity of which may not be zero 2. Number N of instant centres Provided that a mechanism consists of K links, then the total instant centre should be: 3. How to find the Instant Centre Location of two Links Revolute pair If two links 1 and 2 are connected by a revolute pair, the centre of the revolute pair is obviously (明显地) the instant centre P12 or P21 (2) Sliding pair Relative