Basic concept of encoder

Question: Are there only two types of rotary encoders, incremental and absolute? How to distinguish between these two types of rotary encoders? What are the differences in working principles? What are the characteristics of each?

Answer: Only incremental and absolute. Incremental type only measures the angular displacement (indirect angular velocity) increment, which is the base point at the previous moment. The absolute type measures the angular displacement from the beginning of work. Incremental measurement of small angles is accurate, and large angles have accumulated errors. Absolute measurement of small angles is relatively inaccurate, but large angles have no cumulative error (this paragraph is useless). Assuming a serial absolute encoder, the output data can be transmitted using standard interfaces and standardized protocols, and the point-to-point connection has been realized in the past Serial data transmission: The use of fieldbus systems is increasing today.

 

Incremental rotary encoder, each revolution of the shaft, the incremental encoder provides a certain number of pulses. Periodic measurement or pulse count per unit time can be used to measure the speed of movement. If the number of pulses is accumulated after a reference point, the calculated value represents the parameter of rotation angle or stroke.

This paragraph is the principle of the rotary encoder. You have to figure out the principle of everything. It is not necessary to figure it out completely, but at least you have to know the working mode of the sensor of the rotary encoder! !

 

1. The principle and characteristics of rotary encoder:

Rotary encoder is a speed displacement sensor that integrates opto-electromechanical technology. When the rotary encoder shaft drives the grating disc to rotate, the light emitted by the light-emitting element is cut into intermittent light by the grating disc slit, and is received by the receiving element to generate an initial signal. After the signal is processed by the subsequent circuit, a pulse or code signal is output.

It is characterized by small size, light weight, multiple varieties, complete functions, high frequency response, high resolution ability, small torque, low energy consumption, stable performance, reliable and long service life.

 

1. Incremental encoder (program by yourself to calculate the current speed of the roller by counting pulses)

When the incremental encoder shaft rotates, there is a corresponding phase output. The determination of the direction of rotation and the increase or decrease of the number of pulses need to be realized with the help of the direction circuit and counter at the rear. The counting starting point can be set arbitrarily, and unlimited accumulation and measurement of multiple turns can be realized. It is also possible to use the Z signal for each pulse transmitted as

Refer to the mechanical zero position. When the pulse is fixed and the resolution needs to be improved, two signals with a phase difference of 90 degrees A and B can be used to multiply the original pulse number.

 

2. Absolute encoder

In the case of the absolute encoder shaft rotator, there is a code (binary, BCD code, etc.) output corresponding to the position one-to-one, and the positive and negative direction and the position of the displacement can be determined from the change of the code size, without the need for a direction circuit. It has an absolute zero code. When the power is cut off or turned off and then turned on and re-measured, the code of the power cut or shut down position can still be read accurately, and the zero code can be found accurately. Generally, the measuring range of absolute encoder is 0～360 degrees, but special models can also realize multi-turn measurement.

 

3. Sine wave encoder (useless)

Sine wave encoders are also incremental encoders. The main difference is that the output signal is a sine wave analog signal instead of a digital signal. Its appearance is mainly to meet the needs of the electrical field-as a feedback detection element for motors. On the basis of comparing with other systems, people can use this kind of encoder when people need to improve the dynamic characteristics.

 

In order to ensure good motor control performance, the feedback signal of the encoder must be able to provide a large number of pulses, especially when the speed is very low, the use of traditional incremental encoders to generate a large number of pulses, there are problems in many aspects , When the motor rotates at a high speed (6000rpm), the digital signal is transmitted and processed

Numbers are difficult. In this case, the bandwidth required to process the signal to the servo motor (for example, the encoder pulse per revolution is 10000) will easily exceed the MHz threshold; on the other hand, the use of analog signals greatly reduces the above-mentioned troubles and has the ability to simulate A large number of pulses from the encoder. This is thanks to the interpolation of sine and cosine signals (what's the matter?), which provides a calculation method for the rotation angle.

 

This method can obtain a high multiplier increase of the basic sine, for example, from 1024 sine wave encoders per revolution, more than 1000,000 pulses per revolution can be obtained. The bandwidth required to receive this signal is only slightly larger than 100KHz. The interpolation frequency multiplication needs to be completed by the secondary system.

 

2. Output signal

1. Signal sequence

In addition to the two phases A and B (the phase difference of the signal sequence of the A and B channels is 90 degrees), the general encoder output signal also outputs a zero pulse Z per revolution. . When the spindle rotates in a clockwise direction, the pulse is output as shown in the figure below, and the A channel signal is before the B channel; when the spindle rotates counterclockwise, the A channel signal is

Located behind the B channel. Therefore, it is judged whether the main shaft is rotating forward or backward.

Note: It is possible to distinguish A and B which comes first, because the phase sequence differs by 90 degrees. If the phase sequence differs by 180 degrees, it cannot be separated, because the pulse waves of A and B are just opposite and symmetrical.

 

The differential signal output by the sine output encoder is shown in the figure below: (It's useless, there are only absolute encoders and incremental encoders in the project)

 

In the above figure, the phase sequence of +A and -A is opposite, and the phase sequence of +A and +B differs by 90 degrees

2. Zero signal

The encoder sends a pulse every revolution, which is called zero pulse or identification pulse. The zero pulse is used to determine the zero position or identification position. To accurately measure the zero pulse, regardless of the direction of rotation, the zero pulse is output as the high-order combination of the two channels. Due to the phase difference between the channels, the zero pulse is only half the pulse length.

 

3. Early warning signals

Some encoders also have an alarm signal output, which can alarm for power failures and LED failures so that users can replace the encoders in time.

3. Output circuit (base, collector, emitter)

1. NPN voltage output and NPN open collector output circuit

This circuit consists of only an NPN transistor and a pull-up resistor, so when the transistor is in a static state, the output voltage is the power supply voltage, which is similar to TTL logic in the circuit, so it is compatible with it. When there is an output, the transistor is saturated, and the output turns to a low level of 0VDC, otherwise it jumps from zero to a positive voltage.

 

As the length of the cable, the transmitted pulse frequency, and the load increase, the influence of this type of line increases. Therefore, in order to achieve the desired use effect, these effects should be considered. The open collector circuit cancels the pull-up resistor. In this way, the collector of the transistor and the feedback line of the encoder power supply are

They are independent of each other, so that a current output signal different from the encoder voltage can be obtained.

 

2. PNP and PNP open collector circuit

This circuit is the same as the NPN circuit. The main difference is the transistor. It is a PNP type, and its emitter is forced to connect to a positive voltage. If there is a resistor, the resistor is a pull-down type and is connected between the output and zero volts.

 

 

3. Push-pull circuit (push-pull output is push-pull output)

This circuit is used to improve the performance of the circuit, making it higher than the aforementioned various circuits. In fact, the main limitation of NPN voltage output lines is that they use resistors, which exhibit a much higher impedance than transistors when the transistor is off. To overcome these shortcomings, an additional push-pull circuit is added. Transistor, so whether it is positive or zero direction conversion, the output is low impedance. The push-pull line improves the frequency and characteristics, which is conducive to longer line data transmission, even at high speeds. The level of signal saturation remains low, but it is sometimes higher compared to the logic described above. In any case, the push-pull circuit can also be applied to the receiver of NPN or PNP circuit.

 

4. Long-term driver circuit

When the operating environment requires electrical interference or there is a long distance between the encoder and the receiving system, a long-line driver circuit can be used. Data transmission and reception are carried out in two complementary channels, so interference is suppressed (interference is caused by cables or adjacent equipment). This interference can be regarded as "common mode interference." In addition, the transmission and reception of the bus driver are carried out in a differential manner, or the voltage difference on the complementary transmission channel. Therefore, it is not a third party to common mode interference. This transmission method can be considered compatible with RS422 when using a DC5V system; in the case of a special chip, the power supply can reach DC24V, which can be used under harsh conditions (long cable, strong interference, etc.) use.

 

5. Differential line

The differential line is used in an analog encoder with a sinusoidal long-line driver. At this time, it is required that the signal transmission is not disturbed. Like the long-line driver circuit, two signals with a phase difference of 180 degrees are generated for the digital signal. This line has a special line impedance of 120 ohms deliberately, which is in line with the receiver’s input power

The impedance is balanced, and the receiver must have equal load impedance. Usually, a 120 ohm terminal resistor is connected in parallel between complementary signals to achieve this purpose.

 

Four, commonly used terms

■Output pulse number/revolution

The number of pulses output by the rotary encoder for one revolution, for an optical rotary encoder, is usually the same as the number of slots of the grating inside the rotary encoder (the number of output pulses can also be increased to 2 times 4 times the number of slots on the circuit ).

■Resolution

The resolution indicates the maximum equal fraction of the position data read out after the main shaft of the rotary encoder rotates once. The absolute value type does not output in pulse form, but expresses the current spindle position (angle) in code form. Different from the incremental type, it is equivalent to the "output pulse/revolution" of the incremental type.

■Raster

Optical rotary encoder, its grating has two kinds of metal and glass. If it is made of metal, there is a light hole slot; if it is made of glass, a light-shielding film is coated on the surface of the glass, and there is no transparent line (slot) on it. When the number of grooves is small, the metal disc can be grooved by punching or etching. In impact resistance

Type encoders use metal gratings, which are less resistant to impact than metal gratings. Therefore, please be careful not to directly apply impact to the encoder.

■Maximum response frequency

Is the maximum number of pulses that can be responded to in 1 second

(Example: The maximum response frequency is 2KHz, that is, 2000 pulses can be responded to within 1 second)

The formula is as follows

Maximum response speed (rpm)/60×(number of pulses/revolution)=output frequency Hz

■Maximum response speed

Is the highest speed that can be responded to, and the response formula for pulses generated at this speed is as follows:

Maximum response frequency (Hz)/ (pulse number/revolution)×60=shaft speed rpm

■Output waveform

Output pulse (signal) waveform.

■Output signal phase difference

In two-phase output, the relative time difference between the two output pulse waveforms.

■Output voltage

Refers to the voltage of the output pulse. The output voltage will vary due to changes in the output current. Please refer to the output current characteristic diagram for the output voltage of each series

■Starting torque

Rotate the encoder shaft at a standstill with the necessary torque. In general, the torque during operation is smaller than the dynamic torque.

■ Shaft allowable load

Indicates the maximum load that can be added to the shaft, and there are two types of radial and axial loads. The radial load is vertical to the shaft, and the force and the eccentric angle have some

Off: The axial load is horizontal to the shaft, and the force is related to the force of pushing and pulling the shaft. The magnitude of these two forces affects the mechanical life of the shaft

■Shaft moment of inertia

This value represents the inertia of the rotating shaft and the resistance to changes in speed

■Speed

This speed indicates the mechanical load limit of the encoder. If this limit is exceeded, it will have a negative impact on the service life of the bearing, and the signal may also be interrupted.

■ Gray Code

Gray code is high-level data, because it is unit distance and cyclic code, it is very safe. Only one change per step. During data processing, Gray code must be converted into binary code.

■Working current

Refers to the load current allowed by the channel.

■Working temperature

The data and tolerances mentioned in the parameter table are guaranteed within this temperature range. If it is slightly higher or lower, the encoder will not be damaged. When the working temperature is restored, the technical specifications can be reached

■Working voltage

The supply voltage of the encoder.

Encoder is a kind of sensor. It is mainly used to detect the speed, position, angle, distance or count of mechanical movement. In addition to being used in industrial machinery, many motor controls such as servo motors and BLDC servo motors are Need to be equipped with encoder for motor controller as commutation, speed and position

Therefore, the range of application is quite wide. According to the detection principle, encoders can be divided into optical, magnetic, inductive and capacitive. According to its scale method and signal output form, it is divided into incremental encoder and absolute encoder. The photoelectric encoder uses the principle of grating diffraction to realize displacement-digital conversion.

It began to be used in machine tools and computing instruments in the 1950s. Because of its simple structure, high measurement accuracy, and long life, it has received attention and promotion at home and abroad, and has been widely used in precision positioning, speed, length, acceleration, and vibration.

 

a. Features of incremental encoder:

When the shaft of the incremental encoder rotates, there is a corresponding pulse output, and its counting starting point can be set arbitrarily, which can realize multi-turn infinite accumulation and measurement. One rotation of the encoder shaft will output a fixed pulse, and the number of pulses is determined by the number of encoder grating lines. When the resolution needs to be improved, two channels A and B with a phase difference of 90 degrees can be used

Multiply the signal or replace the high-resolution encoder.

b. Absolute encoder features

The absolute encoder has a code output corresponding to the position, usually a binary code or BCD code. From the change of the code number, the positive and negative directions and the position of the displacement can be judged. The absolute zero code can also be used for power failure position memory. The measuring range of absolute encoder is usually 0-360 degrees.

 

Incremental rotary encoder

It can be seen from these two passages that the ABZ trinomial output mentioned above only refers to an incremental encoder. The output of the two absolute value encoder is each revolution of the binary number axis, and the incremental encoder provides a certain number of pulses. Periodic measurement or pulse count per unit time can be used to measure moving

speed. If the number of pulses is accumulated after a reference point, the calculated value represents the parameter of rotation angle or stroke. The phase difference between the output pulses of the dual-channel encoder is 90º. The electronic equipment that receives the pulse can receive the rotation induction signal of the shaft, so it can be used to realize two-way positioning control; in addition, the three-channel increase

The quantitative rotary encoder generates a pulse called a zero signal every revolution.

Absolute encoders provide a unique encoded digital value for each axis position. Especially in the application of positioning control, the absolute value encoder reduces the calculation task of the electronic receiving equipment, thereby eliminating the complicated and expensive input device: Moreover, when the machine is turned on or the power fails, it is turned on again

The power supply can use the current position value without returning to the position reference point. (But if you want to change the encoder or encoder plug, you must set the reference point again)

 

The single-turn absolute encoder subdivides the axis into a specified number of measurement steps, with a maximum resolution of 13 bits, which means that a maximum of 8192 positions can be distinguished. Multi-turn absolute value coding