As a kind of signal conversion device, encoder is widely used in our life around.
In the intelligent car race, it is necessary to use the encoder to detect the instantaneous speed of the car model and realize the closed-loop feedback control of the car model speed, so that the control board of the car can execute the instructions given by the software according to the changes of the track and road conditions, such as acceleration, deceleration, turning, etc.
In celestial exploration, scientists use large astronomical telescopes to track stars, astronomical telescopes to achieve a certain speed control accuracy, it is necessary to select the appropriate encoder. However, the requirements for the encoder are very high at this time. For example, when the star speed is 0.004%, the resolution of the encoder is 26 bits to meet the requirements of the speed measurement.
In addition, there are elevator encoders, machine tool encoders, servo motor encoders and so on, it can be said that encoders are everywhere.
From stepper motor to intelligent system, how to choose the encoder?
So what exactly is an encoder?
By definition, an encoder is a device that compiles signals (such as bitstreams) or data into the form of signals that can be communicated, transmitted, and stored.
The simple understanding is to convert signals that humans cannot directly understand into signals that we humans can directly understand, so that we can control equipment or devices.
Encoder according to the scale method and signal output form, can be divided into incremental, absolute value, mixed type.
Incremental and absolute are more common, but the difference between the two has become a difficult problem for the majority of users.
Therefore, only incremental and absolute are compared here, so that users can make a better choice in the future.
First, the two work differently:
1. Working principle of incremental encoder:
The incremental encoder converts the displacement into a periodic electrical signal, and then converts the electrical signal into a counting pulse, using the number of pulses to represent the size of the displacement.
Take pouring water to describe, incremental encoder is like, find a cup of unknown size and pour water into it, when filled once, empty the cup once, then pour water, and finally calculate the distance according to the number of times the cup is filled.
In terms of structure, incremental encoder is composed of connecting shaft, code plate, light source and output circuit. In fact, the encoder is basically this composition, the following will not repeat.
The incremental encoder obtains four groups of sine wave signals from phototransmitting devices and receiving devices, which are combined into A, B, C and D respectively. Each sine wave has a phase difference of 90 degrees, and the four groups have a phase difference of 360 degrees (that is, one cycle). The C and D signals are reversed and superimposed on the A and B phases, so as to enhance the stable signal; In addition, a Z-phase pulse is output at each turn to represent the zero reference bit.
Since the difference between phase A and phase B is 90 degrees, the forward and reverse of the encoder can be determined by comparing which phase A and phase B come before.
The zero reference bit of the encoder can be obtained by the zero pulse. Parameters such as distance and Angle are calculated through zero reference bits and the number of pulses.
2. Working principle of absolute encoder
There are many lines on the code plate of the absolute encoder to arrange each position on the encoder. Since each position is different, you only need to know the starting position and ending position to know the displacement, rather than counting all the time like an incremental encoder.
As an example of pouring water, an absolute encoder would take a higher cup with a scale, pour water into it, and calculate the distance based on the start and end scales.
In terms of structure, there are many optical channel lines on the optical code plate of the absolute encoder, and each line is divided into 2, 4, 8, 16 lines...... Arranged so that, at any position of the encoder, a unique set of binary codes (Gray codes) from 2 to the power of zero to 2 to the power of n-1 can be obtained by reading the open and dark of each engraved line, which is the n-bit absolute encoder.
Such an encoder is determined by the mechanical position (starting and stopping position) of the optical code plate, so it will not be affected by power failure and external interference, which is also one of the excellent characteristics of the absolute encoder.
Because of this feature, the absolute encoder does not need memory, no change reference point, and does not have to count all the time. Therefore, the anti-interference characteristics of the encoder and the reliability of data are greatly improved.
Based on the construction of the absolute encoder, it must face a problem: counting to the maximum value.
In order to solve this problem, the multi-turn absolute encoder appears.
For multi-turn absolute encoder, there are the following three common design schemes:
The first, inside the encoder, uses mechanical gears to couple multiple shafts to calculate the total number of turns.
Take pouring water, the graduated cup mentioned earlier. When this cup is full, find a larger graduated cup, pour the water from the smaller cup into the larger cup, and finally add the two cups to calculate the distance.
The second is to use an electronic counter and a capacitor to count the total number of turns.
From stepper motor to intelligent system, how to choose the encoder?
Again, take the example of pouring water, this time when the graduated cup is full, pour the water out, and use a counter to measure the number of times it is full, and finally calculate the distance by adding the counter and the cup.
The third, in some magnetic encoders, uses the Weigen line and uses the Weigen effect to count.
The above three methods all need to pay a certain price, for example, the first one, because of the use of mechanical gear, so it will bring wear to the encoder, making the accuracy reduced.
As for the scheme of multi-circle absolute value encoder, it is not described here, and interested friends can go to check the relevant information.
Because of the different working principle and mechanical composition, the two have two very big differences:
1, on and off memory is different
The incremental encoder has no memory, and the power outage restart must go back to the reference zero position to find the required position, and the power outage must be restarted each time.
The most common incremental encoder is the positioning of the printer scanner. Every time the printer is turned on, we can hear a crackle, which is actually the printer looking for the reference zero point after which it can work.
The absolute encoder has memory, power outage restart without going back to zero, you can know the location of the target. This makes the absolute encoder will not be disturbed in the process, and its anti-interference characteristics and data reliability are greatly improved.
2, the code plate is different
Because the counting method of the two is not the same, so the code plate of the two is also very different.
The difference of code plate is one of the biggest differences between absolute encoder and incremental encoder.
In addition to the above differences, there are many small differences between absolute encoders and incremental encoders:
3, the output signal is different
An incremental encoder outputs a pulse signal, while an absolute encoder outputs a set of binary values.
4, the number of different restrictions
The number of revolutions of the incremental encoder is not limited, while the absolute encoder cannot exceed the range of revolutions.
5, the application field is not exactly the same
With or without breakpoint memory, incremental encoder and absolute encoder are very different in the application field. Incremental encoder is more suitable for determining speed, distance or direction of motion, while absolute encoder is more and more widely used in the field of industrial control positioning due to its characteristics.
6. The price is different
Because of the excellent characteristics of the absolute encoder, the price is higher than the incremental encoder.
With the difference between the two, let's take a look at what elements need to be paid attention to when choosing an encoder:
Whether power outage is required to maintain
Absolute encoders must be used if continuous inspection is required.
The required measurement accuracy
Relatively speaking, the accuracy of absolute encoder is higher than that of incremental encoder.
Resolution of resolution
The resolution of the encoder, that is, the number of pulses output by the encoder when the motor rotor shaft rotates once. The resolution is one of the most critical factors affecting the speed measurement effect.
Maximum speed required
The speed measurement methods of encoders are divided into three categories: T method, N method and M/T method.
Generally speaking, T method has the best effect in low speed area, and M method is better than T method in high speed area. Although M/T method is much higher than M and T method, in most cases, its speed measurement accuracy is also better than the other two methods.
Required code plate material
Encoder code plate materials are glass, metal, plastic.
From stepper motor to intelligent system, how to choose the encoder?
Glass code plate is deposited on the glass very thin engraved line, its thermal stability is good, high precision.
Metal code plate is directly through and through the engraving line, not easy to break, but because the metal has a certain thickness, the accuracy may be affected, its thermal stability is much worse than that of glass.
Plastic code plate is economical, its cost is low, but the accuracy, thermal stability, life are worse.
Encoder selection in addition to the above listed factors, but also many other factors, specifically based on the use of the occasion and environment to make a choice.
The best option is to communicate directly with the manufacturer, communicate your needs and concerns to them, and they will give you good advice. At that point, you can consider their suggestions based on your own understanding.
