The below figure shows that the position of the link AB goes on changing, therefore the centre about which the motion is assumed to take place and also goes on changing, that is called Instantaneous Centre of a moving body.
It is defined as that centre which goes on changing from one instant to another. The locus of all such Instantaneous Centre is known as centrode. A line drawn through an Instantaneous Centre and perpendicular to the plane of motion is called Instantaneous Axis. The locus of this axis is known as axode.
Properties of Instantaneous Centre:
the following properties of the instantaneous centre are important from the subject point of view:
1. A rigid link rotates instantaneously relative to another link at the instantaneous centre for the configuration of the mechanism considered.
2. The two rigid links have no linear velocity relative to each other at the instantaneous centre. At this point, the two rigid links have the same linear velocity relative to the third rigid link.
In other words, the velocity of the instantaneous centre relatives to any third rigid link will be same whether the instantaneous centre is regarded as a point on the first rigid link or on the second rigid link.