2. INTRODUCTION TO KINEMATICS OF MECHANISMS
À pConsider a pair of adjustable vice grips.
Relative/Absolute - a position, velocity, etc. is measured based on a fixed (absolute) or moving (relative) point.
A Degree Of Freedom (DOF) is an independently controllable variable. As an example, a machine that has two degrees of freedom might need two motors to control it.
Lower Pairs, - constrained position/orientation of both sides of the joints are identical
Higher pairs include, - typically other equations are needed to constrain the joints, such as gear ratios (if the joint has more than a single degree of freedom)
The definition of higher/lower pairs given in Shigley [1995] is, "the lower pairs, such as the pin joint, have surface contact between the pair elements, while higher pairs, such at the connection between a cam and its follower, have line or point contact between the surface elements." They go on to point out that the definition is not exact, which is somewhat disappointing.
A better definition of a higher pair is - A higher pair is not a lower pair, where a lower pair permits the following relative motions between links; circular, linear, helical, cylindrical, spherical, planar.
If a link has one joint, it is a unary link. A link with two joints is binary, with three it is ternary, with four it is quaternary, etc.
Planar linkages use lower order pairs, and are constrained to a single plane of motion.