Identify the increment of which position by the output pulse. It can generate pulse signal equivalent to displacement increment, and its function
It provides a sensing method for discretization or increment of continuous displacement and displacement change (speed), which is relative to a certain
The relative position increment of datum point can not directly detect the absolute position information of axis. Generally speaking, incremental photoelectric editing
The encoder outputs two pulse signals (so-called two sets of orthogonal output signals) with a difference of 90 degrees between A and B, thus becoming a square.
It is convenient to judge the direction of rotation. At the same time, there are Z-phase mark (indication) pulse signals used as reference zero. Whenever the code wheel rotates,
One week, only one signal is sent. The marking pulse is usually used to indicate the position of the machine or to clear the accumulated quantity.
Incremental photoelectric encoder is mainly composed of light source, code wheel, detection grating, photoelectric detection device and conversion circuit, as shown in the figure.
1- 1. The code wheel is engraved with radial light-transmitting gaps with equal spacing, and an incremental cycle is represented between two adjacent light-transmitting gaps.
Period; The detection grating is engraved with two groups of light transmission gaps A and B corresponding to the code wheel for passing or blocking the light source and photoelectric detection.
Light between devices. Their pitch is equal to the pitch on the code wheel, and the two groups of light transmission gaps are staggered by 1/4 pitch, so that,
The phase difference of the signals output by the photoelectric detection device is 90 degrees. When the encoder rotates with the detected rotating shaft, the grating is detected.
Under the condition of not moving, the light shines on the photoelectric detection device through the gap between the code wheel and the detection grating, and the photoelectric detection device outputs.
Two sets of electrical signals with a 90-degree difference are similar to sine waves. After signal processing by the conversion circuit, the electrical signal can
Obtain the rotation angle or speed information of the measured shaft. The output signal waveform of incremental photoelectric encoder is shown in figure 1-2.
The advantages of incremental photoelectric encoder are: simple principle and easy implementation; The average life of machinery can reach tens of thousands.
Hours or more; High resolution; Strong anti-interference ability, long signal transmission distance and high reliability. Its disadvantage is that it can't
Directly read the absolute position information of the rotating shaft.
Figure 1- 1 Composition of incremental photoelectric encoder
Figure 1-2 Output signal waveform of incremental photoelectric encoder
1.2.2 Basic technical specifications
In the process of using incremental photoelectric encoder, different requirements are usually put forward for its technical indicators, the most important of which is
The key is its resolution, accuracy, stability of output signal, response frequency and signal output form.
(1) resolution
The resolution of the photoelectric encoder is represented by the number of basic cycles of the output signal generated by one rotation of the encoder shaft, that is
Pulse number per revolution (PPR). The number of light-transmitting slits on the code wheel is equal to the resolution of the encoder, and the more slits engraved on the code wheel,
The higher the resolution of the encoder. In industrial electric drive, according to different application objects, the selectable resolution is usually in
The incremental photoelectric encoder with 500~6000PPR can reach tens of thousands of PPR at the highest. Communication in AC servo motor control system
Encoders with a resolution of 2500PPR are often selected. In addition, the photoelectric conversion signal can be logically processed to obtain frequency doubling.
Or quadruple pulse signal to further improve the resolution.
(2) accuracy
The precision of incremental photoelectric encoder has nothing to do with resolution, but two different concepts. Accuracy is a measure.
The ability to determine the position of one pulse relative to another within a selected resolution range. Accuracy is usually measured by angle, angular minutes or
Angular seconds. The accuracy of encoder is related to the processing quality of the light transmission gap of the code wheel and the manufacturing accuracy of the mechanical rotation of the code wheel.
Elements, but also related to the installation technology.
(3) Stability of output signal
The stability of encoder output signal refers to the ability to maintain specified accuracy under actual operating conditions. Affect encoder output
The main factors of the stability of the output signal are the drift of electronic devices caused by temperature, the deformation force exerted on the encoder by the outside world and the light source.
Changes in characteristics. Due to the influence of temperature and power supply changes, the electronic circuit of the encoder cannot maintain the specified output characteristics.
Design and use should be fully considered.
(4) Response frequency
The response frequency of encoder output depends on the response speed of photoelectric detection device and electronic processing circuit. When the encoder is at high speed
When rotating, if the resolution is high, the frequency of the signal output by the encoder will be high. If the photoelectric detection device and electricity
If the working speed of sub-circuit components can't adapt to it, the output waveform may be seriously distorted and even pulse loss may occur.
Therefore, the output signal cannot accurately reflect the position information of the shaft. Therefore, the resolution of each encoder is 1.
Under certain conditions, its maximum speed is also certain, that is, its response frequency is limited. Maximum response frequency of encoder
The relationship among speed, resolution and maximum speed is shown in the formula (1- 1).
60
N R
f
maximum
maximum
×
= ( 1- 1)
Among them, Max
F is the maximum response frequency, max R is the maximum speed and n is the resolution.
(5) Signal output form
In most cases, the signal directly obtained from the photoelectric detection device of the encoder has low level and irregular waveform.
Can not meet the requirements of control, signal processing and long-distance transmission. Therefore, in the encoder, this signal must also be amplified.
Plastic surgery. The processed output signal is usually similar to sine wave or rectangular wave. Because the rectangular wave output signal is easy to digitize
Processing, so this kind of output signal is widely used in positioning control. When sine wave output signal is used, it is basically eliminated.
It can locate the oscillation phenomenon when it stops, and it is easy to obtain higher resolution at lower cost by electronic interpolation.
The signal output forms of incremental photoelectric encoder are: open collector output and voltage output.
(voltage output), line driver, complementary output and push-pull.
Type output (totem pole).
Open collector output, an output mode, uses an NPN transistor at the output of the encoder to convert the power of the transistor.
The emitter terminal is connected to 0V, the collector is disconnected from the terminal of +Vcc, and the collector is used as the output terminal. Power supply in encoder
This type of output circuit is recommended when the voltage is inconsistent with the voltage of the signal receiving equipment. The output circuit is shown in the figure.
1-3. The main application fields are elevators, textile machinery, oiler, automation equipment, cutting machinery, printing machinery,
Packaging machinery and knitting machinery, etc.
Figure 1-3 open collector output circuit
Voltage output is an output mode, which uses an NPN transistor on the output side of the encoder to guide the emitter of the transistor.
The output terminal is 0V, and the collector terminal is +Vcc and the load resistor as the output terminal. At the encoder supply voltage
It is suggested to use this type of output circuit when the voltage of signal receiving devices is consistent. The output circuit is shown in figure 1-4.
Show. The main application fields are elevators, textile machinery, oiler, automation equipment, cutting machinery, printing machinery and packaging machinery.
Machinery and knitting machinery.
Figure 1-4 Voltage Output Circuit
The line drive output uses the line drive IC chip (26LS3 1) as the encoder output circuit, including
Because of its high-speed response and good anti-noise performance, the line-driven output is suitable for long-distance transmission. The output circuit is shown in the figure.
1-5. The main application fields are servo motors, robots, CNC machining machinery, etc.
Figure 1-5 Line Drive Output Circuit
Complementary output This output mode consists of two transistors, PNP and NPN, when one of them
When one transistor is turned on, the other transistor is turned off. This output form has high input impedance and low output impedance,
Therefore, it can provide a wide range of power supply under the condition of low impedance. Because the input and output signals have the same phase and frequency range
Wide, so it is suitable for long-distance transmission. The output circuit is shown in figure 1-6. Mainly used in elevator field or special field.
Figure 1-6 complementary output circuit
Push-pull output This output mode consists of upper and lower NPN transistors. When one of the transistors is turned on,
When the other transistor is turned off. Current flows in both directions through the two transistors at the output end, always outputting electric energy.
Flow. Therefore, the impedance is low and is less affected by noise and deformation waves. The output circuit is shown in figure 1-7. Mainly used for collar.
This field includes elevators, textile machinery, oiler, automation equipment, cutting machinery, printing machinery, packaging machinery and knitting machinery.
Figure 1-7 push-pull output circuit