Sigma comparator
Sigma Comparator
The Sigma Comparator is a mechanical comparator in which
the amplification is obtained by means of a compound lever.
Advantages: It has got a bold scale and larger indicating pointer.
Disadvantages: Due to motion of parts, there is wear in the moving parts.
Constructional details:
The vertical beam is
mounted on flat steel springs connected to fixed members.
• The vertical beam which in turn are screwed to a back plate
• The assembly provides a frictionless movement with restraint from the springs
• The hinged assembly carrying the forked arms incorporates a hardened fulcrums
• The metal ribbon attached to the forked arms passes around the spindle
• This spindle is to rotate in specially designed miniature ball bearings
• The indicating pointer is secured to a boss on the disc
Working principle:
The
plunger is mounted on a pair of slit diaphragms in order to have frictionless
linear movement. A knife edge is mounted on it and bears upon the face of the
moving member of a Cross strip hinge. The cross strip hinge consists of the
moving component and a fixed member. The cross strip hinge then connected by
thin flexible strips alternately at right angles to each other. Thus if an
external force is applied to the moving member it will pivot as would a hinge
about the line of intersection of the strips
To the moving member an arm `Y` shape having the
effective length `b` is attached. If the distance of the hinge from the knife
edge be `a` then the magnification of the first stage is `b/a. A
phosphor-Bronze strip is attached to the two extremities of the `Y` arm and is
passed around a radius `r` attached to the pointer spindle. If the length of
pointer be `R` then R/r is the second stage of magnification. In order to
adjust the magnification, distance `a` must be changed by slackening and
tightening the two screws attaching the knife edge to the plunger. In sigma
comparator a damping device must be employed to prevent oscillation of point
before coming to rest.