Figure 6.5-5 - Two-Channel Optical Encoder Scale And Read Head Assembly - Newport ESP6000 User Manual

The detector signal is similar to a sine wave. Converting it to a digital
waveform, we get the desired encoder signal. But this is only one phase,
only half of the signal needed to get position information. The second
channel is obtained the same way but from a mask that is placed 90% out of
phase relative to the first one (Figure 6.5-5).
Figure 6.5-5 — Two-Channel Optical Encoder Scale and Read Head Assembly
There are two basic types of encoders, linear and rotary. The linear encod-
ers, also called linear scales, are used to measure linear motion directly.
This means that the physical resolution of the scale will be the actual
positioning resolution. This is their main drawback since technological
limitations prevent them from having better resolutions than a few mi-
crons. To get higher resolutions in linear scales, a special circuitry must be
added, called a scale interpolator. Other technologies like interferometry or
holography can be used but they are significantly more expensive and need
more space.
The most popular encoders are rotary. Using gear reduction between the
encoder and the load, significant resolution increases can be obtained at
low cost. But the price paid for this added resolution is higher backlash.
In some cases, rotary encoders offer high resolution without the backlash
penalty. For instance, a linear translation stage with a rotary encoder on
the lead screw can easily achieve 1µm resolution with negligible backlash.
NOTE
For rotary stages, a rotary encoder measures the output angle directly.
In this case, the encoder placed on the rotating platform has the same
advantages and disadvantages of the linear scales.
Section 6 — Motion Control Tutorial 6 - 2 1
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