Photoelectric encoder is a kind of sensor that converts the mechanical geometric displacement on the output shaft into pulse or digital quantity through photoelectric conversion. This is currently the most widely used sensor, photoelectric encoder is composed of grating disk and photoelectric detection device. Grating plate is in a certain diameter of the circular plate evenly open a number of rectangular holes. The photoelectric code plate is coaxial with the motor. When the motor rotates, the grating plate and the motor rotate at the same speed, and several pulse signals are detected and output by a detection device composed of led and other electronic components. Its schematic diagram is shown in Figure 1. The current motor speed can be reflected by calculating the number of output pulses per second of the photoelectric encoder. In addition, in order to determine the rotation direction, the code plate can also provide phase difference of 90 dry pasture hinge meahuan lotus root
According to the principle of detection, encoders can be divided into optical, magnetic, inductive and capacitive types. According to its calibration method and signal output form, it can be divided into incremental, absolute and mixed.
1.1 Incremental encoder
The incremental encoder outputs three square wave pulse phase A, B and Z directly by photoelectric conversion principle. The pulse phase difference of group A and Group B is 90 sea commission. The phase is one pulse per rotation for reference point positioning. Its advantages are simple principle structure, average mechanical life can be more than tens of thousands of hours, strong anti-interference ability, high reliability, suitable for long distance transmission. Its disadvantage is that it cannot output the absolute position information of shaft rotation.
1.2 Absolute encoder
Absolute encoder is direct digital output of the sensor, on its round encoder, several concentric yards along the radial direction, each consisting of pervious to light and opaque and sector, relationship between adjacent code number of sectors is double, the code number on the encoder is the binary digital digits, in the side of the encoder is a light source, the other side corresponding to every yard a photosensitive element; When the code disk is in different position, each photosensitive element will convert corresponding level signal according to the illumination or not, forming binary number. The characteristic of this kind of encoder is that it does not need a counter and can read out a fixed digital code corresponding to the position at any position of the rotating shaft. Obviously, the more code channels, the higher the resolution. For an encoder with n-bit binary resolution, its code disk must have N-bar code channels. At present, there are 16 bit absolute encoder products in China.
The absolute encoder uses the natural binary or cyclic binary (Gray code) mode for photoelectric conversion. The difference between the absolute encoder and the incremental encoder lies in the lines and graphs on the disk that transmit and transmit light. The absolute encoder may have several codes and detect the absolute position according to the codes on the readout code disk. The design of coding can adopt binary code, cyclic code, binary complement code and so on. Its features are:
1.2.1 The absolute value of the Angle coordinates can be read directly;
1.2.2 No cumulative error;
1.2.3 The position information will not be lost after the power supply is removed. But the resolution is determined by the number of bits in binary, that is, the accuracy depends on the number of bits, there are currently 10 bits, 14 bits and so on.
1.3 Hybrid absolute value encoder
Hybrid absolute value encoder outputs two sets of information: one set of information is used to detect the position of magnetic pole, with the function of absolute information; The other set is exactly the same as the output information of the incremental encoder.
Photoelectric encoder is an Angle (angular velocity) detection device, it will input to the axis of the Angle, using the photoelectric conversion principle into the corresponding electrical pulse or digital quantity, with small volume, high precision, reliable work, digital interface and other advantages. It is widely used in CNC machine tools, rotary table, servo drive, robot, radar, military target determination and other angular detection devices and equipment.
2. Application circuit of photoelectric encoder
2.1 Application of EPC-755A photoelectric encoder
Epc-755a photoelectric encoder has good performance, strong anti-interference ability in Angle measurement and displacement measurement, and has stable and reliable output pulse signal, and the pulse signal can be measured digital signal after counting. Therefore, we in the development of vehicle driving simulator, the steering wheel rotation Angle measurement to choose the EPC - 755 - a photoelectric encoder as the sensor, the output circuit type, choose open collector output resolution 360 pulses/circle, considering the car steering wheel rotation is a two-way street, as well as clockwise rotation, can also be counterclockwise, need to the output of the encoder signal phase to count. FIG. 2 shows the phase identification and bidirectional counting circuit actually used by the photoelectric encoder. The phase identification circuit is composed of 1 D flip-flop and 2 nand gates, and the counting circuit is composed of 3 pieces 74LS193.
When the photoelectric encoder rotates clockwise, the output waveform of channel A is 90° ahead of that of channel B, the output waveform of D flip-flop Q(waveform W1) is high level, and Q(waveform W2) is low level. The upper and non-gate are opened, and the counting pulse passes through (waveform W3) and is sent to the pulse input terminal CU of two-way counter 74LS193 for addition counting. At this point, the following and non-gate are closed and its output is high (waveform W4). When the photoelectric encoder rotates counterclockwise, the output waveform of channel A is delayed by 90° compared with that of channel B; the output Q(waveform W1) of D flip-flop is low level; Q(waveform W2) is high level; the upper and non-gate are closed, and the output is high level (waveform W3). At this point, the following and non-gate open, count pulse through (waveform W4), sent to the bidirectional counter 74LS193 minus pulse input CD, subtraction counting.
When the steering wheel rotates clockwise and counterclockwise, its maximum rotation Angle is two and a half turns. An encoder with a resolution of 360 pulses/turns is selected, and its maximum output pulse number is 900. The actual counting circuit is composed of 3 pieces of 74LS193. When the system is initialized with power, it is reset (CLR signal) first, and then its initial value is set to 800H, namely 2048(LD signal). Thus, when the steering wheel rotates clockwise, the output range of the counting circuit is 2048 ~ 2948; when the steering wheel rotates counterclockwise, the output range of the counting circuit is 2048 ~ 1148; when the wheel rotates counterclockwise, the output range of the counting circuit is 2048 ~ 1148. The data output D0 ~ D11 of the counting circuit is sent to the data processing circuit.
In practice, the steering wheel frequently rotates clockwise and counterclockwise. Due to the quantization error, after a long period of work, the output of the counting circuit of the steering wheel may not be 2048, but a few words of deviation. To solve this problem, we added a steering wheel in the back detection circuit, the system work, data processing circuit in the simulator is in a state of not operation, the system detection detection circuit in the back, if the steering wheel in the back in the state, and counting circuit data output is not 2048, can be reset to the counting circuit, and to set the initial value.
2.2 Application of photoelectric encoder in gravimeter
A rotary photoelectric encoder is used to connect its rotating shaft with the compensation knob shaft in the gravimeter. The angular displacement of the compensation knob in the gravimeter is converted into some kind of electrical signal. There are two rotary optical encoders, absolute encoders and incremental encoders.

Incremental encoders are sensors that are output in the form of pulses. Their code disks are much simpler and have higher resolution than those of absolute encoders. Generally, only three bar code channels are needed. In fact, the code channels here no longer have the meaning of absolute encoder code channels, but generate counting pulses. The outer channel and the middle channel of its code disk have the same number of transparent and opaque sector (grating) evenly distributed, but the two sectors stagger half of each other. As the dial rotates, its output signals are phase A and Phase B pulses with A phase difference of 90° and pulse signals generated by A third channel with only one transparent slit (which serves as the reference position of the dial and provides an initial zero signal to the counting system). The direction of rotation can be judged from the phase relation (lead or lag) of the two output signals A and B. As can be seen from FIG. 3 (a), the a-channel pulse waveform is more /2 advanced than the B-channel pulse when the code disk is in positive rotation, while the A-channel pulse is more /2 delayed than the B-channel pulse when it is reversed. FIG. 3 (b) is an actual circuit, in which the positive pulse generated by the monostable pulse triggered by the lower edge of the A-channel shaping wave is' matched 'with the B-channel shaping wave. When the code disk is turning in A positive direction, only the forward port pulse is output, and conversely, only the reverse port pulse is output. Therefore, the incremental encoder determines the rotation direction and relative angular displacement of the code disk according to the output pulse source and pulse count. In general, if the encoder has N (code channels) output signals, the phase difference is/N, and the countable pulse is 2N times the number of gratings, now N=2. Figure 3 circuit fault is sometimes mistake pulse generated error, this situation occurs when a signal in a 'high' or 'low' level, and the other a signal is in the midst of the "high" and "low" change state, back and forth between the encoder while not produce displacement, but the single direction of the output pulse is produced.