Principle and application scenarios of photoelectric encoder

1. The principle of photoelectric encoder

2. A photoelectric encoder is a photoelectric conversion sensor that determines how much displacement of the machine on the output shaft is converted into pulses or digital quantities

The device.

3. This is currently the most commonly used sensor, and the photoelectric encoder consists of a grating disk and a photoelectric detection device.

4. Grid disk refers to the number of rectangular holes partially opened on a circular plate of a certain diameter.

5. Due to the coaxial nature of the photoelectric encoder disk and the motor, when the motor rotates, the grating disk rotates at the same speed as the motor, and the detection is carried out

How many pulse signals are output by the detection device composed of electronic components such as light-emitting diodes;

6. The number of output pulses per second of the photoelectric encoder reflects the current motor speed.

According to the detection principle, encoders are divided into optical, magnetic, sensory, and capacitive types. According to its scale essentials and signal output

The methods can be divided into three types: incremental, proportional, and turbid.

1.1 Incremental Encoder

The incremental encoder directly utilizes the principle of photoelectric conversion to output three sets of square wave pulses in phases a, b, and z; A. Two sets of pulse phase errors with a surprise of 90 tetrasar awards, using milbri to locate the reference point for each pulse rotation. Its advantages include a rough principle configuration, a machine lifespan of tens of thousands of hours or more, strong anti-interference ability, high reliability, and suitability for long transmission cuts. The disadvantage is that it cannot output paired position information for shaft rotation.

1.2 Incremental Encoder The encoder is a sensor that directly outputs digital signals. There are several radially concentric code tracks on the circular code wheel, each consisting of transparent and opaque sectors. The number of adjacent code tracks is doubled. When the code wheel is in a different position, each light receiving element transforms the corresponding level signal according to whether it receives light, forming a binary number. The characteristic of this encoder is that it does not require a counter and can read digital codes that correspond exactly to any position of the rotating axis. It is obvious that the more code lines there are, the higher the discrimination rate. In an encoder with n-bit binary discrimination rate, there must be n code channels in the code wheel.

At present, there are 16 bit encoder products in China.

The reciprocal encoder uses natural binary or cyclic binary (Gray code) method for photoelectric conversion. The difference between absolute encoders and incremental encoders lies in the transparent and opaque line patterns on the disk, which detect the correct position by reading the code and reading the code. The plan can be implemented using binary code, loop code, binary completion code, etc. Its characteristics are as follows

1.2.1 Can directly read the absolute value of angle coordinates;

1.2.2 No cumulative deviation

1.2.3 Even if the power is turned off, the location information will not be lost. However, the recognition rate is determined by the number of binary bits, which means the accuracy depends on the number of bits. There are now various types such as 10 bits, 14 bits, etc.

1.3 Thick turbidity absolute value encoder

A robust parental encoder that outputs two sets of information. A set of information is used to detect the position of magnetic poles and serves as a parent message; The other group has exactly the same output information as the incremental encoder.

A photoelectric encoder is an angle (angular velocity) detection device that uses the principle of photoelectric conversion to convert the angle of the feed axis into corresponding electrical pulses or digital quantities. It has the advantages of small size, high accuracy, reliable labor, and digital interface. Widely used in angle detection devices and configurations required for CNC machine tools, flip display tables, servo drives, robots, radar, military target measurement, etc.

2. Application circuit of photoelectric encoder

Application of EPC-755A Optical Encoder

EPC-755A photoelectric encoder has excellent utilization performance, outputting anti-interference, stable and reliable pulse signals when measuring angles and displacements. By counting the pulse signals, the measured digital signals can be obtained.

Therefore, when developing a car driving simulator, we used the EPC-755A photoelectric encoder as a sensor to measure the rotation angle in another direction. The collector open circuit was used in its output circuit, and the output resolution was 360 pulses/turn. The bidirectional and clockwise rotation of the car steering wheel is shown in Figure 2, which shows the actual phase synchronization detection circuit used by the photoelectric encoder. The phase synchronization detection circuit includes a d-flip-flop and two NAND gates, and the counting circuit includes three 74LS193.

When the photoelectric encoder rotates clockwise, the output waveform of channel a is 90 degrees ahead of the output waveform of channel b. The output of flip-flop d, q (waveform W1), is at a high level and q (waveform W2) is at a low level. The upper NAND gate is open, and the pulse determines (waveform W1). At this time, the lower NAND gate is closed, and its output becomes high (waveform W4). When the photoelectric encoder rotates counterclockwise, the output waveform of channel a is delayed by 90 compared to the output waveform of channel b. The d-flip-flop outputs q (waveform W1) as a low level and q (waveform W2) as a high level. The upper surface NAND gate is closed, and its output is a high level. At this time, the lower NAND gate is opened to count the pulse width resolution (waveform W4) and send it to the downlink pulse input terminal CD of the bidirectional counter 74LS193 for subtraction counting. When the car steering wheel rotates clockwise and counterclockwise, its maximum rotation angle is 2 and a half turns. Choosing an encoder with a discrimination rate of 360 pulses per revolution, its maximum output pulse count is 900; The actual counting circuit used consists of three 74LS193s. In the case of electrical initialization on the system, the first step is to reset (CLR signal) and set its initial value to the 2048 LD signal of 800H; Yunyun, when the handle is rotated clockwise, the output range of the counting circuit is 2048~2948, and when the handle is rotated counterclockwise, the output range of the counting circuit is 2048~1148. The data outputs D0~D11 of the counting circuit are sent to the data processing circuit.

In practical use, due to frequent clockwise and counterclockwise rotation of the steering wheel, there is a quantization deviation. After long-term operation, the output of the counting circuit when the steering wheel returns is probably not 2048. In order to deal with this title with a few word errors, when the system is started during the steering wheel rotation process and the simulator is in a non dominant state, the data processing circuit can detect the circuit. If the handle is in the circuit and the data output of the counting circuit is not 2048, the data processing circuit can reset the counting circuit and reconfigure the initial value.

2.2 Application of photoelectric encoder in gravity measuring instrument

Using a rotary photoelectric encoder, connect its rotating shaft to the compensating knob shaft in the gravity length gauge. Supplement to the gravity measuring instrument

Convert the angular displacement of the compensation knob into a certain electrical signal quantity; There are two types of rotary photoelectric encoders, namely encoder and incremental encoder

The device.

Incremental encoders are sensors that output pulse patterns, and their code disks are much larger and have higher discrimination rates than those of traditional encoders. Normally, only three barcode tracks are needed, but these tracks no longer have the same meaning as encoder tracks and instead generate memory pulses. The outer and central tracks of its encoder have a similar and evenly distributed number of transparent and opaque sector areas (gratings), but the two sectors are offset by half a zone from each other. When the code wheel rotates, its output signals are A-phase and B-phase pulse signals with a phase difference of 90 °, as well as the pulse signal generated by the third code track with only one transparent slit (which serves as the reference position of the code wheel and provides an initial zero position signal to the counting system). The direction of rotation can be determined by the phase relationship (leading or lagging) between the output signals A and B. As shown in Figure 3 (a), when the code wheel rotates forward, the pulse waveform of track A leads by π/2 compared to track B, while when it rotates backward, the pulse waveform of track A lags by π/2 compared to track B. Figure 3 (b) shows an actual circuit in which the positive pulse generated by the monostable triggered by the lower edge of the A-channel shaped wave is' matched 'with the B-channel shaped wave. When the code wheel rotates forward, only the forward port pulse is output, and vice versa, only the reverse port pulse is output. Therefore, incremental encoders determine the direction of rotation and relative angular displacement of the encoder based on the output pulse source and pulse count. Usually, if an encoder has N output signals with a phase difference of π/N, the countable pulses are 2N times the number of gratings, and now N=2.

The disadvantage of the circuit in Figure 3 is that it may occasionally generate erroneous pulses causing deviation. This environment occurs when one signal is in a 'high' or 'low' state, while another signal is in a back and forth state between 'high' and 'low'. At this time, although the encoder does not produce a displacement, it will produce unidirectional output pulses. For example, when the code wheel shakes or manually aligns its position (as can be seen below, this environment occurs when measuring with a gravimeter). A fourth harmonic subdivision circuit that can prevent false pulses and improve discrimination. Here, impressive D-type flip flops and clock generation circuits are employed. As shown in Figure 4, each track has two D flip flops connected in series. Therefore, in the clock pulse interval, the two Q terminals (such as pins 2 and 7 of 74LS175 corresponding to track B) retain the input state of the previous two clock periods. If they are identical, it indicates that there is no change in the clock interval; Otherwise, its direction of change can be identified based on the relationship between the two, resulting in a 'forward' or 'reverse' output pulse. When a certain path oscillates back and forth between 'high' and 'low' due to vibration, the melon will produce 'forward' and 'reverse' pulses, which can eliminate their effects by replacing the sum of the two counters (as shown below)

The readings of the instrument will also involve this point. From this, it can be seen that the frequency of the clock generator should be greater than the approximate maximum value of the vibration frequency.

From Figure 4, it can also be seen that four counting pulses were obtained within the cycle of the original pulse signal. For example, the original number of pulses per cycle

The encoder with a capacity of 1000 can generate 4000 pulses at 4 times the frequency, with a discrimination rate of 0.09 °. Actually, nowadays this

Class sensor products package the amplification and shaping circuits of the output signal of the photosensitive element together with the sensing and detection element, so as to

Adding subdivision and counting circuits can form an angular displacement measurement system (74159 is a 4-16 decoder).

Fundamental skill specifications

In the utilization process of incremental photoelectric encoders, different skill specifications are usually required to deal with them, and the most critical one is

It is its discrimination rate, accuracy, stability of output signal, corresponding frequency, and signal output pattern.

(1) Discrimination rate

The discrimination rate of the photoelectric encoder is represented by the fundamental number of cycles of the output signal generated by one rotation of the encoder shaft, that is, the number of pulses per revolution (PPR). The number of light transmission defects on the code wheel is the recognition rate of the encoder. The more defects engraved on the code wheel, the higher the recognition rate of the encoder. In property electrical transmission, incremental photoelectric encoders with a discrimination rate typically ranging from 500 to 6000 PPR can be selected based on the different applications, with a maximum of tens of thousands of PPR. Exchange servo

(2) Accuracy

The accuracy and discrimination of incremental photoelectric encoders are completely independent, which are two different insights. Accuracy is a measure of the ability to determine the position of any pulse relative to another pulse within a selected discrimination range. Accuracy is usually expressed in terms of angles, minutes, or seconds. The accuracy of the encoder is related to the processing quality of the light transmission drawbacks of the encoder, the manufacturing accuracy factors of the machine rotation environment of the encoder, and also to the placement skills.

(3) Stability of output signal

The stability of the encoder output signal refers to the ability to preserve the accuracy of the rule under actual operating conditions. The important factors affecting the stability of the encoder output signal are the drift caused by temperature on electronic devices, the deformation force applied to the encoder by the outside world, and the changes in the characteristics of the light source. Due to the influence of temperature and power supply changes, the electronic circuit of the encoder cannot retain the output characteristics of the rule, and should be given ample consideration in both planning and utilization.

(4) Corresponding frequency

The corresponding frequency output by the encoder depends on the corresponding speed of the photoelectric detection device and electronic processing circuit. When the encoder rotates at high speed, if its discrimination rate is high, the frequency of the signal output by the encoder will be very high. If the labor rate of photoelectric detection devices and electronic circuit components is not suitable, it may cause distortion of the output waveform and even result in the appearance of lost pulses. Such output signals cannot accurately reflect the position information of the axis. Therefore, in an environment where the discrimination rate of each encoder is certain, its maximum speed is also certain, that is, its corresponding frequency is limited. Please refer to other sources for the relationship between the maximum corresponding frequency, discrimination rate, and maximum speed of the encoder.

(5) Signal output style

In most environments, the signal level obtained directly from the photoelectric detection device of the encoder is low, the waveform is irregular, and it is not suitable for control, signal processing, and requirements for remote transmission. Therefore, it is necessary to amplify and shape this signal within the encoder. The output signal processed by the resolution is usually similar to a sine wave or rectangular wave. Due to the ease of digital processing of rectangular wave output signals, they have been widely used in positioning control. When using a sine wave output signal, the oscillation phenomenon during positioning interruption is completely eliminated, and it is easy to determine the electronic interpolation method, achieving high discrimination with lower capital. The signal output styles of incremental photoelectric encoders include: OpenCollector, VoltageOutput, LineDriver, ComplementariOutput, and TotemPole. The open collector output method decides to use an NPN transistor on the output side of the encoder. Connect the emitter terminal of the transistor to 0V, disconnect the collector from the+Vcc terminal, and use the collector as the output terminal. Initiate the use of this type of output circuit in an environment where the power supply voltage of the encoder and the voltage of the signal receiving device are not uniform. The output circuit is shown in Figure 1-3. Important application areas include elevators, textile machines, oiling machines, automated configurations, cutting machines, and printing

Machines, packaging machines, knitting machines, etc.

The voltage output method is decided to use an NPN transistor on the output side of the encoder, and the emitter of the transistor is led out of the terminal

Connected to 0V, the collector terminal is connected to+Vcc and the load resistor, and serves as the output terminal. In the encoder power supply voltage and signal

Initiate the use of this type of output circuit in an environment where the voltage of the receiving device is uniform. Important application areas include elevators and textiles

Machines, oiling machines, automated configurations, cutting machines, printing machines, packaging machines, knitting machines, etc.

The line driven output method uses the line driven dedicated IC chip (26LS31) for the encoder output circuit. Due to its high-speed response and excellent noise resistance, the line driven output is suitable for long interval transmission. Important application areas include servo motors, robots, CNC machining machines, etc. The complementary output method consists of two transistors, PNP and NPN, respectively. When one transistor is turned on, the other transistor is turned off. This output style has high input impedance and low output impedance, so it can also provide a wide range of power in low impedance environments. Due to the similarity in phase and wide frequency range of input and output signals, it is necessary to use long interval transmission. Important applications in the elevator or specialized field. Push pull output is an output method consisting of two NPN transistors, one of which is turned on while the other is turned off. The current flows through the two transistors on the output side in two directions and always outputs current. Therefore, it has low impedance and is less affected by noise and deformation waves. Important application areas include elevators, textile machines, oiling machines, automated configurations, cutting machines, printing machines, packaging machines, and knitting machines