An incremental encoder's code disk consists of many grating lines. It has two (or four, we'll discuss four-eye encoders later) sensors that read signals A and B. The density of the grating lines determines the resolution of the incremental encoder, that is, the smallest angular change it can distinguish. The parameter representing the resolution of an incremental encoder is PPR, which is pulses per revolution. For example, if there are 360 grating lines per revolution, and A and B each output 360 pulses per revolution, the resolution parameter is 360 PPR. So, what is the smallest angular change this encoder can distinguish in degrees? Is it just 1 degree?
The A/B output waveform of an incremental encoder generally has two types: one is a square wave signal with a steep rising edge and a steep falling edge, and the other is a sin/cos curve waveform signal output with a slow rising and falling edge, similar to a sine curve. A and B are 90 degrees out of phase with a period of 1/4T. If A is a sinusoidal sin curve, then B is a cosine cos curve.
For a square wave signal, phases A and B are 90 degrees out of phase (1/4T). Thus, there are rising and falling edges at four phase angles: 0 degrees, 90 degrees, 180 degrees, and 270 degrees. Therefore, angle changes can be determined within 1/4T of the square wave period. This 1/4T period is the minimum measurement step size. By judging these rising and falling edges, the circuit can read angle changes four times the PPR (Pressure Proportion) value; this is the fourth harmonic of the square wave. This judgment can also be done using logic, where 0 represents low and 1 represents high. The changes in phases A and B within one period are 00, 01, 11, and 10. This judgment not only allows for the fourth harmonic but also the determination of the rotation direction.
Therefore, the minimum resolvable angle of a square wave signal is 360 degrees / (4 x PPR).
The previous question: An incremental encoder with a square wave A/B output of 360PPR has a minimum resolution angle of 0.25 degrees.
Strictly speaking, square waves can only be divided up to 4 times the frequency. Although some people use the time difference method to divide them into finer subdivisions, this is generally not recommended for incremental encoders. Higher frequency divisions require incremental pulse signals, which are sine/cosine signals like SIN/COS. Subsequent circuits can then subdivide the signal by reading the phase changes of the waveform and using an analog-to-digital converter (ADC) to subdivide it by 5, 10, 20, or even more than 100 times. After subdivision, the result is a square wave output (PPR). The frequency division factor is actually limited. First, ADC has a time response issue; the speed of ADC and the accuracy of resolution are contradictory. Infinite subdivision is impossible; excessive subdivision will cause problems with response and accuracy. Second, the original encoder's engraving precision, the consistency of the output sine/cosine signal, and the perfection of the waveform are limited. Excessive subdivision will only expose the original code disk's errors more clearly, introducing further errors. Subdivision is easy to implement, but difficult to execute well. It depends on the engraving precision and output waveform perfection of the original code disk, as well as the response speed and resolution accuracy of the subdivision circuit. For example, Heidenhain's industrial encoders recommend an optimal subdivision of 20x. Higher subdivisions are recommended for more accurate angle encoders, but the rotational speed is very low.
Our company's IDE sine wave subdivision frequency multiplier can provide sine and cosine wave subdivision frequency multiplication of 5, 10, 20, 25, and up to 100 times.
An incremental encoder that outputs A/B/Z square waves after subdivision can be multiplied by 4 again. However, please note that subdivision has requirements on the encoder's rotational speed, which is generally low. In addition, if the original code disk has low engraving precision, the waveform is imperfect, or there are limitations in the subdivision circuit itself, subdivision may cause severe waveform distortion, large or small steps, or missed steps. These factors should be considered when selecting and using the encoder.
The previous question: A sine/cosine A/B output 360PPR incremental encoder may have a minimum resolution angle of 0.01 degrees (if the frequency is divided by 25 times and the original code disk accuracy is guaranteed).
Some incremental encoders can have an initial 2048 lines (2 to the power of 11, 11 bits). By subdividing by 16 times (4 bits), a 15-bit PPR is obtained. Then, by multiplying the frequency by 4 times (2 bits), a 17-bit resolution is achieved. This is how some Japanese encoders achieve their 17-bit high-resolution output, which is typically expressed using "bit". At higher speeds, these Japanese encoders still internally use the unsubdivided low-bit signals to process the output; otherwise, the response wouldn't be fast enough. So don't be misled by the "17 bits".
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