As a PLC engineer, do you feel a bit lost when facing various servo motors in the factory workshop? Don’t worry! To truly master servo motors, the key is to first understand their control methods.
Today, let’s talk about the three common control methods for servo motors—torque control, speed control, and position control. Once you’ve learned these, not only will you become more adept at PLC programming, but you’ll also avoid the awkward situation of "using the wrong mode and getting half the result with twice the effort!"
Torque Control: Powerful and Precise
In simple terms, torque control is about controlling how much force the motor outputs. Through external analog input (such as providing a voltage signal via a PLC), you can set the torque of the motor shaft. For example, if 10V corresponds to 5Nm of torque, when an external signal gives a 5V voltage, the motor outputs 2.5Nm of force. If the load matches 2.5Nm, the motor stays still; if the load is less than this, the motor rotates forward; if it’s more, the motor rotates backward. This control method is mainly used in scenarios where force needs to be precisely regulated, such as in winding devices or fiber-drawing equipment. These devices experience changes in winding radius, so the torque must be adjusted in real time to ensure constant tension on the material.
This mode requires the least computation and offers the fastest response. If you’re the type who demands real-time motor reactions, torque control is definitely your go-to.
Position Control: Go Exactly Where You Want
Compared to torque control, position control is much more complex. It determines the motor's speed and position using external pulse signals. The frequency of the pulses determines the speed, while the number of pulses determines the angle of rotation. Simply put, how far and how fast the motor moves depends entirely on the pulse count and frequency. Position control is commonly used in equipment requiring high precision, such as CNC machines and printing machinery.
Some servo drives can even directly set speed and position through communication, further improving control accuracy. If you need precise positioning and motion control, position control is your best bet.
However, position control has the highest computational load of the three methods, meaning its dynamic response is the slowest. So, if you need the motor to respond “lightning fast,” you might want to reconsider.
Speed Control: The Wisdom of Balance
Speed control strikes a balance between the other two. By using analog input or pulse frequency, you can control the motor’s speed. Keep in mind, speed control usually requires a higher-level controller (like a PLC) with PID control to achieve closed-loop control, enabling precise speed adjustments. What’s even better is that speed control can achieve positioning by detecting the load’s position, reducing errors from intermediate transmissions, thereby improving system accuracy.
If your controller has a fast enough processing speed, speed control works even better. You could even offload the position loop control from the servo drive to the controller, reducing the drive’s burden. Overall, speed control is a flexible option suitable for scenarios that require both high speed and position accuracy.
The "Three Loops" of Servo Motor Control
When talking about servo motors, we must mention their "three-loop control" system: current loop, speed loop, and position loop. The three loops form a progressively nested closed-loop control system that achieves precise control.
Current loop: The innermost loop, used to control the motor’s output torque. It has the least computational load and the fastest dynamic response.
Speed loop: Adjusted by feedback signals from the motor’s encoder, the speed loop includes the current loop within its control.
Position loop: The outermost loop, containing the calculations of the previous two loops, is responsible for controlling the motor’s position. It has the highest computational load, making its dynamic response the slowest.
If you’re an engineer aiming for ultimate control, understanding how these three loops work together will help you navigate complex industrial automation with ease.
Conclusion
Each of the three control methods for servo motors has its advantages and disadvantages, and the choice depends on your application needs. If real-time torque output is critical, torque control is your top choice. If you need high positional accuracy, position control will be your best companion. If you’re looking for a balance between speed and position, speed control offers a more flexible solution.
No matter what type of PLC engineer you are, mastering these control methods will take you beyond simple on-off control and allow you to excel in more complex industrial control scenarios.
