Basic waveforms and circuits of incremental photoelectric encoders
A rotary photoelectric encoder is used, with its shaft connected to the compensation knob shaft in the gravity measuring instrument. The angular displacement of the compensation knob in the gravity measuring instrument is converted into a certain electrical signal. There are two types of rotary photoelectric encoders: absolute encoders and incremental encoders .
Incremental encoders are sensors that output pulses. Their code disks are much simpler and have higher resolution than those of absolute encoders. Generally, only three tracks are needed. These tracks no longer have the same function as those in an absolute encoder; instead, they generate counting pulses. The outer and middle tracks of the code disk have an equal number of evenly distributed transparent and opaque sector areas (gratings), but the two sectors are offset by half a zone. When the code disk rotates, its output signals are A-phase and B-phase pulse signals with a 90° phase difference, plus a pulse signal generated by the third track, which has only one transparent slit (this serves as the reference position of the code disk, providing an initial zero-position signal to the counting system). The direction of rotation can be determined from the phase relationship (leading or lagging) of the A and B output signals. As shown in Figure 3(a), when the code disk rotates clockwise, the pulse waveform of track A leads track B by π/2, while in reverse rotation, the pulse of track A lags track B by π/2. Figure 3(b) shows a practical circuit where the lower edge of the A-channel shaping wave triggers a positive pulse generated by a monostable multivibrator, which is then ANDed with the B-channel shaping wave. When the code disk rotates forward, only the positive pulse is output; conversely, only the negative pulse is output. Therefore, the incremental encoder determines the rotation direction and relative angular displacement of the code disk based on the output pulse source and pulse count. Typically, if the encoder has N (code channels) output signals with a phase difference of π/N, the countable pulses are 2N times the number of gratings; in this case, N=2. A drawback of the circuit in Figure 3 is that it sometimes generates false pulses, causing errors. This occurs when one signal is at a 'high' or 'low' level, while another signal is fluctuating between 'high' and 'low'. In this case, although the code disk does not move, it generates a unidirectional output pulse. For example, this can happen when the code disk jitters or when manually aligned (as seen below in gravimeter measurements).
Figure 4 shows a quadruple frequency subdivision circuit that can both prevent false pulses and improve resolution. Here, a D-type flip-flop with memory function and a clock generation circuit are used. As shown in Figure 4, each channel has two D flip-flops connected in series. Thus, during the clock pulse interval, the two Q terminals (e.g., pins 2 and 7 of the 74LS175 corresponding to channel B) maintain the input state of the previous two clock cycles. If they are the same, it indicates no change during the clock interval; otherwise, the direction of change can be determined based on their relationship, thus generating a 'positive' or 'reverse' output pulse. When a channel fluctuates between 'high' and 'low' due to vibration, it will alternately generate 'positive' and 'reverse' pulses. This can be eliminated when the algebraic sum of the two counters is applied (this will also be relevant to the instrument readings below). Therefore, the frequency of the clock generator should be greater than the maximum possible value of the vibration frequency. Figure 4 also shows that four counting pulses are obtained within the original pulse signal period. For example, an encoder with 1000 pulses per revolution can generate 4000 pulses at 4 times the frequency, with a resolution of 0.09°. In fact, current sensor products of this type encapsulate the amplification and shaping circuits of the photosensitive element's output signal with the sensing element, so by simply adding subdivision and counting circuits, an angular displacement measurement system can be formed (74159 is a 4-to-16 decoder).
