In the study of incremental encoders , we know that the encoder contains two glass plates: one is a large circular disk (called the code disk), which has equidistant radial engravings; the other is a fan-shaped plate (called the aperture plate), which has two sets of stripes A and B engraved on it, staggered by 1/4 pitch (referring to the distance between every two stripes on the code disk). As shown in Figure 1.
Why are light barriers needed? What would happen if we did it this way, as shown in Figure 2?
Without the aperture plate, there is only one type of waveform output (A). The distance traveled can also be determined by counting the A pulse sequence.
However, since rotary encoders are typically used to measure the displacement of linear shafts within their travel range, while pulse sequence counting detects incremental distance, Figure 2 is just a unidirectional pulse counter and cannot determine the direction of encoder rotation, i.e., the direction of shaft movement.
Therefore, we considered using an aperture diaphragm to output two sequential pulses. Obviously, these two pulses must be different.
What are the differences? Frequency? Amplitude? Phase?
Since the frequencies cannot be different, processing pulses of different amplitudes would increase costs, so we use phase to represent their differences.
What is the phase difference between the two pulses?
One cycle is 360°, 1/2 cycle? 1/4 cycle? When the encoder changes its rotation direction, the two pulses with a phase difference of 1/4 cycle (i.e., 90°) show a difference, as shown in Figure 3.
The distance between the stripes in groups A and B on the aperture plate is not equal to the distance between the stripes on the code disk; they are not perfectly aligned and are offset by 1/4 pitch, as shown in Figure 4. This results in a 90° phase difference between the two pulses.
Therefore, the aperture plate exists to determine the direction of rotation of the encoder. If you only need unidirectional counting, then an encoder without an aperture plate is also a simple one.
