
A linear encoder consists of a graded precision
scale, fixed, and a movable cursor connected
to the spindle that provide higher precision to
the machine.
An axis can be moved in many different ways.
Simple machine, printers, plotters etc. use
motor and belts, with problems of stiffness and
precision.
Best movements are obtained by using a motor
connected to an endless screw, that transforms the rotation of the motor in linear displacement.
Il an “open-loop” system there is no check about the fact that the motor has rotated at the angle
specified by the controller, thus the error can be very large.
In usual “closed loop” systems, the motor includes an encoder that measure the actual angle or
rotation of the motor, and corrects it if need and it is enough precise for most jobs. However, by
working many hours, the endless screw will heat up and expand due to thermal expansion, and the
displacement of the axis is not any more the same as the start of the job.
Usage of linear encoders overcome this problem. In the C6 machine, the numerical control reads
1,000 times per second the actual position of the spindle (on the 3 axes), compares with the
theoretical position and corrects if needed. See following figure.
which when assembled on each axis often reads the real position of the specific axis on a 1-second
time scale (normally 1,000), acting retroactively on the motor that controls movement so as to
minimise the positioning error (closed loop operation).
The machine is normally fitted with 4 optical scales to control movement on the 3 axes ("gantry"
system: the Y axis is actually composed of 2 axes, with 2 movements, which work in parallel)-
If the machine is fitted with the scales, this error can be continuously displayed on the machine's PC
tablet (point 10.3.7).
If the machine is fitted with scales, each machine's manual will also include the precision
measurements of each single scale, taken by the manufacturer of the scales during each of their
commissioning. This diagram normally looks like the one shown to the side.
As a result, the machine is much more precise and precision is stable
in time, independently from the temperature of the environment, heating
or cooling system, etc. and from the working time of the machine.
All linear encoder are tested one by one to be sure that they meet the
precision need, and the test of each all of them are usually included in
the machine documents for the client (see picture).