Talk about motor encoder again

With the frequency converter, the voltage and frequency of the motor power supply can be adjusted, so that the speed can be adjusted. The control modes of the inverter include V/f, VC, SLVC, DTC, etc. Among them, V/f and SLVC control do not require an encoder and are open loop. Open-loop control cannot guarantee the stability and accuracy of the speed, so it is only suitable for occasions where the speed regulation accuracy is not high, such as fans, pumps, conveyor belts, etc. VC and DTC are closed-loop control methods, and both speed and torque can reach high accuracy.

To carry out closed-loop control, a speed measuring device must be used. There used to be a speed measuring machine, and now there is an encoder. The encoder is installed on the motor shaft. When the motor rotates, the encoder outputs a signal that changes with the speed and transmits it to the control unit of the inverter. The accuracy of the encoder directly affects the accuracy of the control. For high-precision positioning occasions, the use of low-resolution encoders cannot meet the control requirements.

Encoders generally use 5VDC or 24VDC power supply to output pulse, sine and cosine, or data signals.

1. Pulse signal

Pulse signal is a commonly used type in speed control occasions. It is divided into HTL level and TTL level. A key parameter is the number of pulses per revolution, for example, 2048 pulses are output per revolution. The pulse signal is flat-topped and has a small dead zone. It is precisely because of the existence of this dead zone that the accuracy of the pulse signal encoder is limited. This kind of pulse signal encoder generally has two pulse channels. When rotating in the forward direction, the signal of A channel leads the B channel by 90 degrees, and when it is reversed, A lags behind B. In this way, the rotation direction of the motor can be known through the signal of the encoder. In addition, some encoders have zero pulses, which are used for signal calibration. The motor will output a narrow pulse every time it rotates. It may also be in other pulse forms. The encoder signal is a high-speed pulse signal. If the signal receiving device has a deviation in counting, for example, a circle of encoder sends out 2048 pulses, and the signal receiving device only collects 2047 pulses, and there will be accumulated errors over time. With the zero pulse, the count value will be corrected every revolution. Of course, if the receiving device has collected 3000 pulses but still has not received the zero pulse, there should be a corresponding fault message reported. In short, after the zero pulse, the monitoring function of the encoder signal will increase a lot, which improves the system availability.

When the motor rotates at 3000rpm, the encoder with a resolution of 2048 will output 3000*2048=6144000 pulses per minute. This pulse frequency is very high, so the encoder signal needs to have a special high-speed counting channel to read and output outward For speed value or position value, it is impossible to directly read the pulse signal, and it is also very difficult to capture the zero pulse of the encoder. This pulse appears too fast, and this signal is generally not introduced into the upstream control system.

In practical applications, it is more appropriate to use HTL/TTL encoders for adjusting speed, tension, and torque. This is very common in production lines using three-phase asynchronous squirrel cage motors.

2. Sin and cosine signals

The encoder that outputs sine and cosine signals is similar to that of pulse signals. For example, 2048 sine waves are output per revolution of the motor. Phase A is a sine signal, phase B is a cosine signal, and zero pulse is a pulse signal. The HTL/TTL pulse signal in each cycle has only two levels, either high or low, which is the dead zone mentioned above. The sine wave signal is different throughout the cycle. The sine wave signal is processed by the receiving device. Subdivision can get higher accuracy. The peak-to-peak value of the sine wave is generally between 1V and 1.2V. For example, there will be a 1Vpp mark on the encoder. For example, a sine-cosine encoder with 2048 pulses, each sine wave is subdivided into 2048 parts, then the motor can have 2048*2048 = 4194304 pulses per revolution, which is much higher than the accuracy of the HTL/TTL encoder, so Often used in high-precision positioning occasions.

When it comes to positioning, that is, position control, servo control, and motion control, the most commonly used motor type is a three-phase permanent magnet synchronous motor. Unlike asynchronous motors, synchronous motors need to know the rotor pole position before running, and it is best The magnetic pole position of the rotor can be directly obtained through the encoder, otherwise a Hall element must be added to identify the magnetic pole position. The identification of the magnetic pole position is another topic.

When the sine-cosine encoder is used in conjunction with a synchronous motor, another signal is needed to record the magnetic pole position. In Siemens products, there is a sine-cosine encoder with C/D Track. This C/D Track outputs a sine wave signal every revolution. When the magnetic pole position of the motor is recognized, the system will record the C/D Track. The phase difference with the magnetic pole position is permanently stored, so that the magnetic pole position of the motor rotor can be obtained according to the voltage of the C/D Track.

Some of the sine and cosine signals also have a zero pulse, which is similar to the zero pulse function of the HTL/TTL pulse encoder.

3. Resolver

Resolver is also a commonly used encoder, referred to as resolver. The resolver outputs one or several sine and cosine signals per revolution, depending on its number of pole pairs. For example, a resolver with 1 pair of pole pairs outputs a sine-cosine signal per revolution, which is the same as the C/D Track of a sine-cosine encoder. It can be seen that the accuracy of the Resolver itself is relatively low, but resolver has an advantage, that is, it can be used in harsh environments, such as large mechanical vibrations.

4. Data signal

The encoders mentioned above all use physical signals to reflect the speed and are incremental encoders. If you want to record the position of the motor rotor even when the power is off, you need to use an absolute encoder. Absolute encoders send out data signals. Commonly used interfaces include SSI, EnDAT, HyperFace (not supported by Siemens), etc., and the transmitted signals are data with practical significance. Since the absolute encoder can still record the motor position when the power is off, it is often used in positioning occasions. When it is used with a permanent magnet synchronous motor, C/D Track is not needed.

5. Encoder interface

For Siemens drives SINAMICS S120 series, there are different interface modules SMC for different encoders. SMC is the abbreviation of Sensor Module Cabinet, which means the sensor module installed in the cabinet, and its protection level is IP20. In addition, there is SME, which is Sensor Module. The abbreviation for External, which means an externally installed sensor module, and its protection level is IP65:

SMx10: resolver

SMx2x: Sine and Cosine, SSI, Endat

SMx30: HTL/TTL, SSI