Biomedical Engineering Reference
In-Depth Information
FIGURE 2-34
Photograph of an
incremental encoder
with encoder disk
exposed. (Courtesy
of GPI http://
www.gpi-encoders.
com/.)
With this technology, resolutions of 40,000 ppr are possible.
For even higher resolution, some encoders replace square wave signals with sine
wave outputs that allow for interpolation factors as high as 10 times. Through 10 times
interpolation of a sine wave, a 256 ppr encoder is capable of 2,560 ppr.
To illustrate the benefits of particular resolutions, one could use an example of an
incremental encoder in monitoring conveyor position. An encoder could be mounted to a
conveyor drive wheel with a circumference of 150 mm at a 1:1 ratio such that each revolu-
tion of the encoder is equivalent to 150 mm of overall conveyor travel. By using an output
of 40,000 ppr, the encoder is able to provide positional feedback accurate to 0.012 mm.
Unfortunately, in the event of a power failure incremental encoders will usually lose
positional information, and a reset or homing cycle must be performed to synchronize the
encoder with the control device.
Figure 2-34 shows an example of the 7700 optical encoder and disk manufactured by
Gurley Precision Instruments (GPI). It is 13 mm tall by 43 mm diameter with a resolution
of up to 5000 ppr.
2.4.6 Ranging Sensors
Noncontact distance measurement methods can be classified into three categories: (1)
interferometry; (2) triangulation; and (3) time of flight. The method used by a particular
sensor usually depends on the maximum range and the measurement accuracy required.
For example, interferometric methods can be extremely accurate but are prone to range
ambiguity, whereas time-of-flight methods operate at longer ranges with poorer accuracy;
triangulation sits somewhere in the middle.
2.4.6.1 Interferometry
Optical interferometry operates using the superposition of two monochromatic light beams
so that extremely small displacements can be measured. The operational principle is best
explained in conjunction with Figure 2-35.
