A new approach to rotary 2D metrology using an IME (incremental motion encoder)
Machines, such as gantry robots, CNC or milling machines, must be able to perform
consistently over time, and any degradation in performance due to changes in speed, age or
mechanical failure must be detected or predicted as early as possible. Ultimately these
changes lead to positioning errors and vibrations within the machine. To maintain accuracy
the machines may checked 'on-site' or they may be 'mapped' in terms of desired and actual
tool-tip position. These maps can then be integrated into the controlling software or hardware
to improve performance. Monitoring the machines can form part of a feedback control loop.
In order to carry out machine monitoring a number of devices are available. These include
interferometers, ballbars, grid encoders and accelerometers for measuring vibration levels.
None of these techniques forms a complete metrology solution. Some have limited coverage
of the machine bed, others are too fragile to use on a working machine, and some do not
provide the level of detail required to carry out a proper assessment of the machine. Initial
investigations showed that vibrations are a key parameter to be monitored when making an
accurate measurement of machine performance. Vibrations can reveal wear in components
indicating imminent breakdown and they can themselves cause damage to the machine.
Vibrations are commonly caused by rotating mechanical components. Patented in 1992, and
developed within the Intelligent Machines Group, the Incremental Motion Encoder (IME) is a
novel means of monitoring the motion of rotating shafts.
The IME is a development of the angular encoder which uses 3 or more read heads. One read
head allows the measurement of angular change. The use of multiple read heads allows it to
detect lateral, as well as angular, changes in position. It is capable of measuring the submicrometre
vibrations in rotating shafts over a wide bandwidth. By attaching an IME to the
end effector of a machine it is possible to monitor the complete range of vibrations that occur
at the end effector.
A novel metrology technique, based on the attachment of the IME to the machine head, has
been developed. This technique complements the measurement of vibrations with the remote
monitoring of machine position. Laser triangulation is used to constantly track the position of
the IME as it moves with the machine. The angles required to perform the triangulation are
measured by using a laser that rotates with the IME. The laser beam strikes photosites at the
periphery of the machine's range of motion allowing their angular position to be recorded by
the rotating IME.
The new technique has been developed and a test rig scanning unit has been constructed for
evaluation. The test rig's construction allows for the measurement of angles against
background torsion al vibrations. Lateral vibrations were prevented from occurring. Results
from the test rig scanning unit indicate that the IME can be used for the measurement of angles
of the order of 0.006 radians, which can then used for the triangulation of the scanner's
position. Small changes in the position of the scanner (O.lmm) can also be detected making
the technique suitable for the tracking of crude machine position. Further, investigations have
involved the addition of an external torsional vibration source to the scanning unit and it has
been shown that they distort the angular measurements. Algorithms based on filtering have
been developed to use the IME data to compensate for such distortions. The algorithms have
been evaluated using known angular changes and show a considerable reduction in the angular
errors due to vibrations (a five-fold decrease in standard deviation from just under 0.0006
radians to 0.0001 radians).