Previously, we discussed the resolution and accuracy of servo motor encoders . So, what is the impact of resolution and accuracy on the speed loop? And what are the factors affecting the accuracy and fluctuation of the speed loop? Today, we'll talk about the relationship between the speed loop and the encoder.
When an encoder is used as a feedback element in a closed-loop system, it mainly affects the performance of the control system in the following three ways:
I. Response speed, accuracy, and resolution
1. Response Speed: Response speed is the easiest to understand. A slow-responding encoder is like an ideal sensor connected in series with a low-pass filter. This creates phase lag in the entire closed-loop control system, affecting the stability of the entire system in the high-frequency range. In most applications today, servo motor encoders have very fast feedback speeds and can be considered ideal sensors. However, some serial communication encoders (SSI) with lower communication rates inevitably introduce some delay due to their communication method, limiting the bandwidth of the speed loop.
2. Accuracy: To understand the impact of accuracy, we first need to define the concept of speed accuracy. Accuracy is a measure of the repeatable average deviation between the actual value and the set value. As shown in Figure 1, speed accuracy represents the overall deviation between the set value and the actual value over a period of time. Therefore, speed accuracy can be understood as the absolute steady-state error between the controlled variable and the specified given value.
Figure 1 shows the speed accuracy.
3. Stability: Although many adjustment targets involve accuracy, in most cases, we prefer the "stability" of the adjustment process. This stability can be understood as the "fluctuation" of speed. Speed fluctuation is defined as : fluctuation is an undesirable characteristic of the actual value, which is superimposed on the average speed.
Figure 2. Plot of velocity ripple.
For servo control systems using Siemens' SINAMICSS120 series frequency converters and servo motors, in a laboratory environment, when the motor operates at its rated speed and rated torque, the speed accuracy can reach 0.001 % of the rated speed, as shown in the table below.
Example: Use an S120 to drive a 1FK7032-2AK71-1QA0 servo motor. This motor has a rated speed of 6000 RPM and the encoder is a 20-bit single-turn absolute value.
We operated at 60 RPM and recorded the incremental pulse count (R482) and the actual rotational speed (R61), as shown in Figure 3:
Records of Figure 3R482 and R61
We record a 3-second time interval. Ignoring error factors, the motor should add 3 * 220 = 3,145,728 incremental pulse signals. However, the motor actually rotated for 3,145,731 incremental pulses, resulting in an error of 3 incremental pulses. Converting this to a rotational speed of 60 + (3/3145728) * 60 = 60.00005722 RPM, the accuracy is : (60 RPM - 60.000057 RPM) / 6000 RPM = -0.000001 % < 0.001 %.
The accuracy value in Figure 3 is ideal, but the speed fluctuation performance is very poor.
II. Speed fluctuations are affected by the following factors
(1) The encoder has limited resolution and speed loop period (the lower the resolution, the longer the period time, and the greater the speed fluctuation).
(2) Improper encoder installation can cause low-frequency periodic fluctuations in encoder feedback.
(3) The cogging torque fluctuation of the servo motor will also cause low-frequency periodic fluctuations in the speed loop.
III. Conclusion
(1) In practical applications, the impact of speed cycle fluctuations caused by factors such as the limited resolution of the encoder, installation, and servo motor manufacturing is more important than the speed accuracy alone. Engineers need to consider the above factors during use.
(2) To reduce speed fluctuations, the following methods can be used: select a high-resolution encoder, shorten the speed loop period, improve the encoder installation accuracy, and use the cogging torque compensation function provided in our servo system to reduce the impact of speed loop fluctuations and avoid vibration and noise in mechanical equipment under high dynamic applications.
