The working principle of the encoder

First, the photoelectric encoder works

An optical encoder is a Sensor that converts mechanical geometric displacements on an output shaft into pulses or digital quantities by photoelectric conversion. This is the head

Before the most widely used sensor, the photoelectric encoder was composed of a grating disk and a photoelectric detection device. The grating disk is equally divided on a circular plate of a certain diameter

Open a number of rectangular holes. Since the photoelectric encoder and the motor are coaxial, when the motor rotates, the grating disk rotates at the same speed as the motor and transmits

A detection device composed of an electronic diode such as a photodiode detects and outputs a plurality of pulse signals, and a schematic diagram thereof is shown in FIG. 1; by calculating light per second

The number of pulses output by the electric encoder can reflect the current motor speed. In addition, to determine the direction of rotation, the code disc can also provide phase difference

90o two-way pulse signal.

According to the detection principle, encoders can be classified into optical, magnetic, inductive and capacitive. According to its calibration method and signal output form, it can be divided

For the incremental, absolute and mixed three.

(I) Incremental Encoder

The incremental encoder directly outputs three groups of square-wave pulses A, B, and Z phases using the principle of photoelectric conversion; A, B two groups of pulses have a phase difference of 90o, which can be

The direction of rotation is conveniently judged, and the Z phase is one pulse per revolution for reference point positioning. Its advantage is that the principle is simple and mechanically average

Its lifetime can be more than tens of thousands of hours. It has strong anti-interference ability and high reliability and is suitable for long-distance transmission. The disadvantage is that it cannot output shaft rotation

For location information.

(B) Absolute Encoder

The absolute encoder is a sensor that directly outputs a digital quantity. There are several concentric code paths in the radial direction on its circular code disk, each path is made of light and

The opaque sectors are formed in phases. The number of sectors in adjacent code channels is a double relationship. The number of code channels on the code disc is the bit of its binary digits.

The number is a light source on one side of the encoder and the other side has a photosensor for each code channel; when the encoder is in a different position, the photosensors are

According to the light or not, the corresponding level signal is converted to form a binary number. The characteristic of this encoder is not to counter, any on the spindle

The position can read a fixed digital code corresponding to the position. Obviously, the more code channels, the higher the resolution, for one with N bits

Binary-resolution encoders must have N code channels on their encoders. At present, there are 16 absolute encoder products in China.

Absolute encoders use natural binary or circular binary (Geray code) methods for photoelectric conversion. Absolute encoders and incremental

The encoder differs in that the disk is translucent and opaque, and the absolute encoder can have several codes, according to the code on the readout code

Code, detect absolute position. The coding design can use binary code, cyclic code, two's complement and so on. It is characterized by:

1. The absolute value of the angular coordinates can be read directly;

2. No cumulative error;

3. Position information will not be lost after the power is cut off. However, the resolution is determined by the number of binary bits, which means that the accuracy depends on the number of bits.

There are currently 10, 14 and so on.

(III) Hybrid Absolute Encoders

Hybrid Absolute Encoder, which outputs two sets of information: one set of information used to detect magnetic pole position with absolute information function; the other set is finished

Same as incremental encoder output information.

The photoelectric encoder is an angle (angular velocity) detection device, which will input the amount of the angle to the shaft and convert it into a corresponding one by using the photoelectric conversion principle.

Electrical pulses or digital quantities have the advantages of small size, high accuracy, reliable operation, and digital interface. It is widely used in CNC machine tools, rotary table

, servo drives, robots, radars, military targets, and other devices and devices that require angle detection.

Second, the application circuit of photoelectric encoder

(I) Application of EPC-755A Optical Encoder

EPC-755A photoelectric encoder has good performance, and has strong anti-interference ability in angle measurement and displacement measurement, and it is stable and reliable.

The output pulse signal and the pulse signal can be counted to obtain the measured digital signal. Therefore, we are developing a car driving simulator

When measuring the rotation angle of the steering wheel, EPC-755A photoelectric encoder is used as the sensor, and the output circuit of the sensor is open collector type.

The resolution is 360 pulses/turn. Considering that the steering wheel of the car is bi-directional, it can be rotated clockwise or counterclockwise.

It is necessary to phase-count the output signal of the encoder before counting. Figure 2 shows the phase and bidirectional counting circuit actually used by the photoelectric encoder.

The circuit consists of a D flip-flop and two NAND gates. The counting circuit is composed of three 74LS193s.

When the photoelectric encoder rotates clockwise, the channel A output waveform leads the channel B output waveform by 90°, and the D flip-flop output Q (waveform W1) is high

Flat, Q (waveform W2) is low, the upper NAND gate is open, the count pulse passes (waveform W3), and is sent to the two-way counter 74LS193.

The pulse input terminal CU counts up; at this time, the following NAND gate is closed and its output is high (waveform W4). When the photoelectric encoder is inverse

When the hour hand rotates, the channel A output waveform is delayed by 90° from the channel B output waveform, and the D flip-flop output Q (waveform W1) is low, Q (waveform W2

) is high, the above NAND gate is closed, its output is high (waveform W3); at this time, the following NAND gate is opened, and the counting pulse passes (

The waveform W4) is sent to the minus pulse input terminal CD of the bidirectional counter 74LS193 to perform subtraction counting.

When the steering wheel rotates clockwise and counterclockwise, the maximum rotation angle is two laps and half. The encoder with a resolution of 360 pulses/turn is selected.

The maximum number of output pulses is 900; the counting circuit actually used is composed of 3 pieces of 74LS193. When the system is powered on, it is first

Line reset (CLR signal), and then set its initial value to 800H, ie 2048 (LD signal); thus, when the steering wheel rotates clockwise, the count

The output range of the road is 2048 to 2948. When the steering wheel rotates counterclockwise, the output range of the counting circuit is 2048 to 1148; the counting circuit

Data outputs D0 to D11 are sent to the data processing circuit.

In actual use, the steering wheel frequently rotates clockwise and counterclockwise. Due to the existence of quantization error, after a long period of work, the direction

The output of the counting circuit may not be 2048, but there is a deviation of a few words. In order to solve this problem, we added a steering wheel.

Back to the detection circuit, after the system works, the data processing circuit detects the circuit in the middle of the simulator when the simulator is in the non-operating state.

The disc is in the back state, and the data output from the counter circuit is not 2048. The counter circuit can be reset and the initial value can be reset.

(B) The application of photoelectric encoder in gravity measuring instrument

A rotary optical encoder is used, and its rotation axis is connected with the compensation knob axis in the gravimeter. The angular displacement of the compensation knob in the gravimeter

The quantity is converted into some kind of electrical signal quantity; the rotary type photoelectric encoder is divided into two kinds, absolute encoder and incremental encoder.

Incremental encoders are sensors that output pulses. The encoder is much simpler and has a higher resolution than an absolute encoder. Usually only

For three code channels, the code channels here do not actually have the meaning of an absolute encoder code channel, but instead generate count pulses. Its yard outside

There are a number of light-transmitting and opaque sectors (gratings) that are equally distributed in the middle track, but the two sectors are staggered by a half area. When the code

When the disk rotates, its output signals are the A-phase and B-phase pulse signals with a phase difference of 90° and the third code track with only one light-transmitting slit.

The pulse signal (which serves as the reference position for the encoder provides the counting system with an initial zero signal). From A, B two output signals

The phase relationship (lead or lag) determines the direction of rotation. When the code wheel rotates forward, the pulse waveform of track A advances by π/2 than that of track B, and when it reverses,

The A track pulse lags behind the B track by π/2. It is an actual circuit that triggers the monostable generated positive pulse with the track B shaping wave using the lower edge of the shaping A shaping wave

And ', when the code disk is forward only the positive port pulse output, on the contrary, only the reverse port pulse output. Therefore, the incremental encoder is based on the output

The pulse source and pulse count determine the rotational direction and relative angular displacement of the code wheel. Usually, if the encoder has N (code channel) output signals,

The phase difference is π/N, the countable pulse is 2N times the number of gratings, and now N=2. The disadvantage of the circuit is that sometimes mistaken pulses cause errors.

The situation occurs when a signal is in the "high" or "low" state, and the other signal is between "high" and "low."

In the round-trip changing state, although the code wheel does not have a displacement, a unidirectional output pulse is generated. For example, jitter or manual code

When you align the position (as you can see below, this will happen when measuring with a gravimeter).

It is a quad-frequency subdivision circuit that can prevent false pulses and increase the resolution. Here, D-type flip-flops with memory function are used

The clock occurs in the circuit. Each channel has two D flip-flops connected in series so that in the interval of the clock pulses, two Q terminals (such as 74LS175 corresponding to B channels)

The 2nd and 7th pins) hold the input state for the previous two clock periods. If the two are the same, it means there is no change in the clock interval; otherwise, it can be rooted.

According to the relationship between the two to determine its direction of change, resulting in 'forward' or 'reverse' output pulse. When a path is 'high' due to vibration,

When 'low' fluctuates, it will alternately generate 'forward' and 'reverse' pulses, which can be eliminated by replacing the sum of two counters.

Their influence (the instrument readings below will also cover this). It can be seen that the frequency of the clock generator should be greater than the maximum possible vibration frequency

value. It can also be seen from FIG. 4 that four count pulses are obtained within the period of the original one pulse signal. For example, the original number of pulses per revolution is 1000

The encoder can produce 4 times the number of pulses is 4000, its resolution is 0.09 °. In fact, current sensor products of this type will have photosensitive elements.

The circuit for the amplification and shaping of the output signal and the sensing and detecting element are packaged together. Therefore, as long as subdivision and counting circuits are added, they can be combined.

Angle displacement measurement system (74159 is a 4-16 decoder).
Third, the application of the problem analysis and improvement measures

(I) Analysis of problems in application

The transmitting and receiving devices of the photoelectric detection device are installed at the production site, and many defects are exposed during use. The internal and external factors are also external.

Factors, mainly in the following areas: 1. Displacement or displacement of the transmitting device or receiving device due to mechanical vibration or other causes

The receiving device cannot reliably receive the optical signal and cannot generate the electrical signal. For example, photoelectric encoders are used in rolling mill speed control systems.

The photoelectric encoder is directly bolted to the housing of the motor. The shaft of the photoelectric encoder is connected to the shaft of the motor through the harder spring plate.

Then, because the load on the motor is an impact load, the vibration of the motor shaft and the housing can be caused when the rolling mill passes steel. After the determination; when steel

The vibration speed of the photoelectric encoder is 2.6mm/s. Such a vibration speed will damage the internal functions of the photoelectric encoder. Causes a false pulse and leads to

Causes the control system to be unstable or malfunctions, resulting in an accident.

2. Because the photoelectric detection device is installed at the production site, the photoelectric detection device cannot be reliably operated due to environmental factors at the production site. Ruan

The temperature of the mounting site is high and the humidity is high, resulting in changes or damage to the characteristics of the electronic components inside the photoelectric detection device. For example, in the caster to send the spindle tracking

In the system, since the position of the photoelectric detection device is close to the casting blank, the ambient temperature is high, causing the photoelectric detection device to send a signal or damage by mistake.

Causes production or personal accidents.

3. The various electromagnetic interference sources at the production site interfere with the photoelectric detection device, resulting in a distortion distortion of the output waveform of the photoelectric detection device.

, Make the system misoperation or cause production accidents. For example; photoelectric detection device installed in the body of the production equipment, the signal transmitted to the control system via the cable

The distance of the system is generally 20m to 100m. Although multicore shielded cables are generally used for transmission cables, the wire resistance and line capacitance

The effect of this, together with the laying of other cables together, is highly susceptible to various electromagnetic disturbances, thus causing distortion of the waveform and thus the

The deviation of the signal fed to the governor system from the actual value leads to a drop in the accuracy of the system.

(B) Improvement measures

1. Change the installation of the photoelectric encoder. The photoelectric encoder is not mounted on the motor casing, but is made on the basis of the motor.

Fix the bracket to mount the photoelectric encoder independently. The axis of the photoelectric encoder and the center of the motor shaft must be at the same level. The two axes are made of soft rubber.

Or the nylon hose is connected to reduce the mechanical impact of the motor impact load on the photoelectric encoder. In this way, it is measured by a vibrometer, which

The vibration speed drops to 1.2mm/s.

2. Reasonably choose the photoelectric detection device output signal transmission medium, replace shielded cable with twisted-pair shielded cable. Twisted shielded cable has

Two important technical characteristics, one is that the electromagnetic interference received by the cable has strong protection capabilities, because the space electromagnetic field generated on the line

Interference currents can cancel each other out. Another technical feature of the twisted-pair shielded cable is that the distance between the two wires after the twisted pair is small, and the distance between the two wire pair disturbing lines

Basically equal, the distributed capacitance of the two-pair shield network is also basically the same, which is more effective in suppressing common-mode interference.

3. Use PLC software to monitor or interfere. The process of feeding the ingots in the continuous casting production requires the photoelectric detection device to generate a time-sequential electrical signal.

This signal corresponds to different stages of the entire process. As shown in Figure 5.

(1) The photoelectric signal 1 is "1" before the start of the ingot-initiating process. (2) After the start of the start of the ingot process, in the A stage, the roller table starts, on the bar

give away. When the dummy bar blocks the infrared light emitted from the photoelectric device, the photoelectric signal is [0"; when the infrared light passes through the two small round holes in the middle of the dummy bar,

At this point, the optoelectronic device emits signals 2 and 3, both of which are "1". (3) Initiate the ingot process in the B stage, the photoelectric signal is "0", the roller stop, cited

The spindle bar is suspended and sent, and the sector-shaped segment is pressed down. The tension leveler and [synchronization 1" are started, and the dummy bar continues to be sent. (4) Initiate the ingot process in stage C,

The dummy bar is sent and no longer blocks the infrared light. The photoelectric signal 4 is "1", and the "synchronization 2" is started, the "synchronization 1" is stopped, and the dummy bar continues

give away. At this point, the operation of the photovoltaic device is completed. According to the working process of the light detection electrical device, as long as the on-site measurement of each photoelectric

The time when the signal occurs, combined with the relationship between the process of sending the ingot spindle and the photoelectric signal, using the relevant data in the PLC application program to compile the requirements

PLC program, the PLC program output signal input to the PLC input module, replace the original photoelectric signal input signal. Its block diagram is shown in Figure 6.

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