Position detection device as an important part of the transmission control, its role is to detect the displacement, and the feedback signal issued by the control device issued a command signal comparison, if there is deviation, the amplified control actuator to move towards the elimination of bias direction Until the deviation is equal to zero. In order to improve the machining accuracy of the mechanical device, the accuracy of the detecting element and the detecting system must be improved. Among them, rotary encoders, linear encoders (grating scale, magnetic scale), resolvers, tachometer generators and so on are more common. Among them, encoders are one of the most commonly used detection devices of various kinds of machines and are detected by an encoder as a signal Method has been widely used in CNC machine tools, textile machinery, metallurgical machinery, petroleum machinery, mining machinery, printing and packaging machinery, plastic machinery, testing machines, elevators, servo motors, aviation, instrumentation and other industrial automation. A wide range of encoders, different industry users of the encoder parameters, specifications vary.
Encoder to read out points, there are two types of contact and non-contact. Contact using the brush output, brush contact conductive area or insulation to indicate the status of the code is “1” or “0”; non-contact to accept the sensitive element is a photosensitive element or magnetic sensor, the photosensitive element Light and opaque areas to indicate whether the status of the code is “1” or “0”.
Encoder to detect the principle to points, optical, magnetic, inductive and capacitive.
Encoder to measure the way to points, a linear encoder (grating ruler, magnetic scale), rotary encoder.
Encoder signal principle (scale method and signal output form) points, there are incremental encoder, absolute encoder and hybrid three.
First, the incremental encoder (rotary type)
1, the working principle:
Optical encoder consists of a central optical encoder with a circular ring, dark engraved lines, when the disc rotates a pitch, the light-emitting components, the photosensitive element to get A, B signal is 90 degrees Phase sine wave, this group of signals amplified and shaped by the amplifier, the resulting square wave output, A compared to the B-phase lead 90 degrees, the voltage amplitude is generally 5V. Let A phase lead the B phase for the positive direction of rotation, the B phase before the A phase is the negative direction of rotation, the phase relationship between phase A and phase B can determine the encoder forward and reverse, C phase The resulting pulse is the reference pulse, also known as the zero point pulse, which is a pulse generated at a fixed position by one revolution of the shaft to obtain the zero reference of the encoder. AB-phase pulse signal frequency-voltage conversion, get the shaft speed proportional to the voltage signal, you can measure the speed and displacement.
Magnetic encoder is developed in recent years, a new type of electromagnetic sensor, it is with the development of optical encoder developed. The main advantages of optical encoders are their sensitivity to moisture and contamination but poor reliability, while magnetic encoders are less susceptible to dust and dew formation. Their simple and compact design allows high speed operation with 500 to 700 kHz response speed, The volume is smaller than the optical encoder, the cost is lower, and the multiple elements are easy to be arranged and precisely combined, and the new functional device and multi-functional device are easier to construct than the optical element and the semiconductor magnetic sensitive element. Under the requirements of high speed, high precision, miniaturization and long life, in the fierce market competition, the magnetic encoder has its unique advantages and outstanding features, which has become one of the keys to develop high-tech products.
The principle of the magnetic encoder is that a pulse train is formed by a magnetic force to generate a signal. The magnetic rubber ring is characterized in that rare earth magnetic powder is mixed with unvulcanized rubber to form a magnetic rubber billet, which is vulcanized and adhered on the reinforcing ring (1) to form a magnetic rubber ring ), And the magnetic rubber rings are circumferentially alternately magnetized on the magnetic rubber ring to generate S pole and N pole. At the same time using a new type of SMR (Hall Effect Resistor) or Hall Effect sensor as a sensitive element, the signal is stable and reliable. In addition, the use of double-layer wiring technology, but also make the magnetic encoder not only has the general encoder only incremental signals and incremental signals and exponential signal output, but also has an absolute signal output. So, although at present about 90% encoders are optical encoders, there is no doubt that in the future motion control system, the amount of magnetic encoders will gradually increase.
2, incremental encoder resolution, frequency doubling and subdivision technology
Incremental encoder code wheel is composed of a lot of grating reticle, with two (or four, discussed later four optical eye) optical eye reading A, B signal, the graduation density determines the increment Encoder resolution, that is, the minimum angle of change that can be read. The parameter that represents the resolution of the incremental encoder is the PPR, which is the number of pulses per revolution.
There are two kinds of waveforms of A / B output of incremental encoder. One is square wave signal with steep rising edge and steep falling edge. The other is rising and falling slowly. Sin / Cos with sinusoidal waveform Curve waveform signal output, A and B phase difference of 1 / 4T 90-degree phase, if A is sinusoids like sine curve, then B is cosine-like cos curve.
For the square wave signal, phase A and phase B are 90 degrees out of phase (1 / 4T). Thus, at 0 degree phase angle, 90 degree, 180 degree and 270 degree phase angle, the four positions have rising and falling edges, In this way, in fact, the square wave period of 1 / 4T can be angularly changed, so 1/4 T period is the minimum measurement step, and the judgment of these rising and falling edges by the circuit can be 4 times of the PPR read Take the change of angle, which is the fourth harmonic of square wave. This judgment can also be done by logic, 0 for low, 1 for high, A / B two-phase change in a cycle is 0 0,0 1,1 1,1 0. This judgment can not only 4 times the frequency, but also to determine the direction of rotation.
Strictly speaking, the maximum square wave can only do 4 times the frequency, although some people use the time difference method can be divided into more detailed, but that is not the basic incremental encoder recommended higher frequency to use incremental pulse signal is SIN / COS type cosine signal to do, the follow-up circuit can read the waveform phase changes, with analog-digital conversion circuit to subdivide, 5 times, 10 times, 20 times, or even 100 times more, after a good square wave Waveform output (PPR). First, the A / D conversion has time to respond to the problem. The speed of A / D conversion and resolution accuracy are a pair of contradictions, which can not be infinitely subdivided. The subdivision, response and accuracy There is a problem; Second, the original encoder scribing accuracy, the output class cosine signal itself is consistent, the waveform perfectness is limited, too sub-points, only the error of the original code plate exposed more obvious, and bring error. Subdivision easy to do, but it is difficult to do, on the one hand, depends on the original code plate engraving accuracy and output waveform perfection, on the other hand depends on the subdivision circuit response speed and resolution accuracy. For example, the German industrial encoder, the recommended best subdivision is 20 times, higher subdivision is its recommended accuracy of the angle encoder, but the rotation speed is very low.
An incremental encoder subdivides the output of A / B / Z square waves and can again quadruple the frequencies, but note that subdivision is required for the encoder’s rotational speed and is generally lower. In addition, such as the original code plate engraving accuracy is not high, the waveform is not perfect, or subdivision of the circuit itself, the subdivision may be serious waveform distortion, the size of the step, lost step, etc., should pay attention to the selection and use.
For some incremental encoders, the original graduation line may be 2048 lines (11th, 11th, 2th) of 2 lines, subdivided by 16 times (4 bits), resulting in 15-bit PPR and 4 times the second 17-bit resolution, which is some Japanese encoder 17-bit high-end encoder came, it is generally used “bit, Bit” to express the resolution. This Japanese encoder at a faster speed, the internal still use unsorted low signal to deal with the output, otherwise the response can not keep up, so do not be confused by its “17”.
3, incremental encoder features:
Incremental encoder features: non-contact, friction and wear, small size, light weight, compact structure, easy installation, simple maintenance, low driving torque, high precision, large range measurement, fast response, Digital output characteristics;
Incremental encoders are ideally suited for speed measurement, allowing infinite accumulation of measurements. However, there are zero cumulative error, poor anti-interference, receiving equipment to power off memory, boot should be zero or reference bit and other issues, such as the choice of absolute encoder can be solved.
Built-in battery technology:
Some encoders have a built-in battery to avoid loss of power signal loss, and some encoder to the lap is an absolute signal, and the number of laps is a built-in battery and circuit with incremental counting method to obtain this pseudo-absolute Encoder, which by the battery life, low temperature battery failure, by the negative impact of battery contacts and other factors, and greatly reduce the reliability.
4, incremental encoder general application:
Measuring speed, measuring the direction of rotation, measuring the movement angle, distance (relative).
Second, the absolute encoder (rotary type)
Incremental encoders output pulses as they turn, know their position through the counting device, and rely on the internal memory of the counting device to remember the position when the encoder is stationary or powered down. In this way, when the power outage, the encoder can not have any movement, when the call work, the encoder output pulse, there is no interference and loss of pulse, otherwise, the counting device memory zero will shift, and this bias The amount of migration is unknown and can only be known if the wrong production result appears.
The solution is to increase the reference point, the encoder every reference point, the reference position is amended into the memory location of the counting device. Before the reference point, the accuracy of the position can not be guaranteed. To this end, in industrial control there is every operation to find a reference point, boot and other methods.
For example, the positioning of a printer scanner uses an incremental encoder principle. Every time you turn on, we can hear a crackling sound, looking for a reference zero before it works.
This method of some industrial projects more troublesome, and even do not allow the boot change (after boot you have to know the exact location), so there is the emergence of an absolute encoder.
1, the working principle:
Absolute encoder optical codepad has a lot of road marking, each line followed by 2 lines, 4 lines, 8 lines, 16 lines …… Arrangement, so that in every position of the encoder, by reading each engraved Line pass, dark, get a group from the zero of 2 to the n-1 power of 2 unique binary code (Gray code), which is called n-bit absolute encoder. This encoder is determined by the encoder mechanical position, it is not affected by power outages, interference.
Absolute encoder is determined by the mechanical position of each location unique, it does not need power-down memory, no need to find a reference point, and do not have to count, when need to know the location, when to read its position. In this way, the encoder anti-jamming characteristics, data reliability has greatly improved.
Because absolute encoders are significantly superior to incremental encoders in positioning, they have been increasingly used in angle, length measurement and position control in a variety of industrial systems. But because of its high precision, the output digit is more, such as still using parallel output, each of its output signal must ensure that the connection is good, for more complex conditions but also isolated, connecting the cable core number, which brought a lot of Inconvenience and reduce the reliability, therefore, absolute encoder in multi-digit output type, generally choose to use serial output or bus type output.
2, from single-turn absolute encoder to multi-turn absolute encoder:
Single-turn absolute encoder to measure each groove on the optical encoder during rotation to get a unique code. When rotated more than 360 °, the code will return to the original point, so it will not meet the absolute principle of absolute coding. The coding can only be used for measuring within 360 degrees of the rotation range and is called a single-turn absolute encoder.
If you want to measure the rotation of more than 360 degrees, it is necessary to use multi-turn absolute encoder.
Encoder manufacturers use the principle of clock gear machinery, when the center code wheel is rotated, another set of code wheels (or sets of gears and sets of code wheels) is driven through gears to increase the number of turns on the basis of the single-turn code Encoding to expand the measuring range of the encoder. Such an absolute encoder is called a multiturn absolute encoder. It is also determined by the mechanical position coding. Each position code is unique and does not need to be memorized.
Another advantage of multiturn absolute encoders is that due to the large measurement range, the actual use is often more affluent, so you do not need to find a hard time to find a zero installation, the middle position as a starting point can be a substantial simplification of the installation and commissioning Difficulty.
The advantages of multi-turn absolute encoders in terms of length positioning are obvious and have been increasingly used in industrial control positioning
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