What are the characteristics of a servo encoder? How to connect it?

EnDat

EnDat (Encoder Data) refers to the frequency converter using the clock signal to achieve synchronous transmission serial communication, which can not only transmit position values, but also parameter values. There is a special storage area inside the encoder to store encoder parameters, and the manufacturer can use this storage area to store encoder parameters. The host computer can send control words to the encoder. EnDat encoder has the following characteristics:

①The actual position can be read;

② Realize zero point compensation during motor communication;

③ Output incremental signals in parallel, which can reduce the delay time of systems with high requirements on dynamic characteristics;

④High data transmission reliability;

⑤Store encoder parameters. The controller can automatically read encoder parameters. The following figure shows the settings in the Starter software.

⑥Support monitoring and diagnosis functions;

⑦Short transmission time

Example of automatic code recognition when configuring a motor in Starter

SSI

When a multi-turn encoder detects 4096 turns (12 bits) and each turn has 8092 data (13 bits), 25 code channels are required according to normal logic. The transmission of so many code channels requires 25 cables. In order to overcome the disadvantages of multi-cable transmission, SSI (Synchronous-Serial Interface EIARS-422A or RS-485) is required to achieve communication through the serial port, and only 4 cables are needed to transmit all data. Fewer cables extend the transmission distance of the encoder signal. The data transmission format is either binary or Gray code, and the trigger is controlled by the clock of the SSI controller. It can not only output absolute position, but also incremental signal data (the maximum number of pulses can reach 512).

If driving the SSI encoder, the host computer is required to calculate the received signal, as shown in the figure below.

SSI Calculation Software

The control unit of Sinamics S120 also has SSI receiving programming software, as shown in the figure below.

Encoder receiving function diagram in S120

In addition to encoders with SSI interfaces, Siemens and other companies also provide encoders with Profibus-DP interfaces, which can transmit data to the controller via DP. The data transmission rate and the encoder address file should refer to the profile file of the corresponding encoder.

Sin/Cos Encoder

Like the pulse encoder, the sine encoder usually has two signal output channels, A and B. Its characteristic is that it can further refine each sine/cosine waveform. The refinement is calculated based on the sine voltage value, which can enhance the resolution of the encoder, as shown in the figure below.

A, B: 0.6～1.2V (peak to peak value)

~1V (peak-to-peak): refers to the amplitude of the sinusoidal incremental encoder signal is 1V, and the voltage value from the signal peak to the reference point is about 0.5V, as shown in the figure below.

Sine encoder segmentation

Conversion code channel: Usually when selecting an encoder, you will find characters marked with C/D after A/BR. These are the two auxiliary sine/cosine code channels of the sine encoder, which are used to provide the absolute position within 1 revolution and provide the driver with the magnetic pole position of the motor for more precise control. This is the hybrid encoder mentioned earlier, as shown in the figure below.

C/D channel of sine encoder

In addition, the analog channel of the sine/cosine encoder can also coexist with the serial port channel, as shown in the figure below.

Sin/Cos encoder with data channel

It can be said that the sine/cosine encoder is a combination of incremental and absolute encoders. The absolute position is transmitted by serial communication, while the sine/cosine signal is still transmitted by analog signal, because the frequency bandwidth required for analog signal transmission is relatively narrow, easy to implement, and can ensure long-distance transmission at high speed.

Phase adjustment of encoder

In the process of closed-loop control of the motor, the magnetic pole position of the synchronous motor must be known, including incremental encoders, absolute encoders, sine/cosine encoders, resolvers, etc. In many cases, the encoder is integrated into the motor, so when installing the encoder, attention should be paid to the adjustment of the encoder phase, which is actually to ensure that the encoder's zero pulse is consistent with the direction of the magnetic pole.

For incremental encoders with square wave output signals, in addition to the two-phase orthogonal square wave pulse output signals A and B and the zero position signal Z, they also have electronic commutation signals U, V, and W with a difference of 120°. The number of cycles per revolution of U, V, and W is consistent with the number of magnetic pole pairs of the motor rotor. The method for aligning the phase of the U, V, and W signals of the incremental encoder with commutation signals with the phase of the rotor magnetic poles is as follows:

① Use a DC power supply to pass a DC current less than the rated current through the UV winding of the motor, U in, V out, and orient the motor shaft to a balanced position;

② Use an oscilloscope to observe the U signal and Z signal of the encoder;

③Adjust the relative position of the encoder shaft and the motor shaft;

④ While adjusting, observe the encoder U signal jump edge and Z signal until the Z signal stabilizes at a high level (the default state of the Z signal is a low level), and lock the relative position relationship between the encoder and the motor;

⑤ Twist the motor shaft back and forth. After your hand is removed, if the Z signal can stabilize at a high level each time the motor shaft freely returns to the equilibrium position, the alignment is effective.

For encoders with C and D channels, in addition to the above-mentioned orthogonal sine and cosine signals, they also have mutually orthogonal 1V (peak-to-peak) sinusoidal C and D signals that appear only once per turn. If the C signal is a sine wave, the D signal is a cosine wave. In addition, after subdivision, the C and D signals of the sine/cosine encoder with C and D signals can also provide higher absolute position information per revolution, such as 2048 absolute positions per revolution. Therefore, the sine/cosine encoder with C and D signals can be regarded as an analog single-turn absolute encoder. The pole alignment of this encoder is as follows:

① Use a DC power supply to pass a DC current less than the rated current through the UV winding of the motor, U in, V out, and orient the motor shaft to a balanced position;

② Use an oscilloscope to observe the C signal waveform of the sine/cosine encoder;

③Adjust the relative position of the encoder shaft and the motor shaft;

④ While adjusting, observe the C signal waveform until the zero-crossing point from low to high appears accurately at the directional balance position of the motor shaft, locking the relative position relationship between the encoder and the motor;

⑤ Twist the motor shaft back and forth. After removing your hand, if the zero point can be accurately reproduced each time the motor shaft freely returns to the equilibrium position, the alignment is effective.

For encoders without C and D channels, the actual position value fed back by the encoder at the magnetic pole position can be read and then written to the encoder's internal memory, such as an absolute encoder with EnDat mode.

For rotary encoders, there are two sets of coils (90° apart), sine and cosine. When the rotor senses the high-frequency signal, it rotates with the motor and senses sine and cosine signals on the stator. The α angle can be calculated based on the sine and cosine waveforms to determine the position of the rotor, as shown in the figure below.

Resolver Pole Position Alignment

Adjustment method: Connect one channel of the oscilloscope to the stator voltage signal of the resolver, and the other channel to the encoder position signal. Use other devices to drive the motor to rotate, so that the motor is in the power generation state, and adjust the encoder position to make it at the zero point of the stator voltage. In addition, you can align it by verifying that the zero point of the resolver's sinusoidal signal envelope coincides with the zero point of the motor's UV line back electromotive force waveform from low to high.

Encoder frequency multiplication

The number of encoder pulses determines the accuracy of position control and speed control. For an encoder with 2048 pulses, without frequency doubling, the accuracy of each pulse is 0.175°. Therefore, the position accuracy of this encoder feedback must be greater than 0.175°, and it is difficult to achieve precise positioning. For precision instrument servo systems that require high control accuracy and fast response speed, the encoder resolution is required to be higher.

Regardless of whether the encoder signal is a sine signal or a pulse signal, the signal can be further subdivided, that is, frequency doubling. Frequency doubling is to further subdivide each pulse of the encoder through the encoder signal receiving device. Through the high-multiplier subdivision technology of sine and cosine signals, the sine/cosine encoder can obtain a nominal detection resolution that is finer than the original signal period. For example, a 2048-line sine/cosine encoder can achieve a nominal detection resolution of more than 4 million lines per revolution after 2048 subdivisions. Currently, many European and American servo manufacturers provide this type of high-resolution servo system, but domestic manufacturers are still rare.

There are also errors in subdivision. The subdivision error not only affects the positioning accuracy of the motor, but also brings high-frequency noise signals to the motor. For example, in the article "Rotary Encoders for High Dynamic Performance of Servo Drives" written by Johannes Heidenhain, the influence of subdivision error is further analyzed.