The most important function of the servo system in CNC machines is to ensure that the output speed and position accurately follow the input commands. To achieve this, the servo system typically consists of three control loops: current control, speed control, and position control. The current loop ensures optimal performance during dynamic operations, while the speed and position loops guarantee precise tracking of the required speed and position at all times. Evaluating a servo system involves analyzing both its static and dynamic characteristics. This article will assess the performance of the servo system based on specific indicators.
**Requirements for Output Characteristics**
These refer to the static behavior of the motor and driver under control. It determines whether the system can provide sufficient torque within the required speed range to drive the load and whether it has enough overload capacity to start the mechanical load. Motor torque is the primary parameter used for evaluation. Continuous torque must not exceed the continuous operating area, and intermittent torque must not go beyond the braking and acceleration/deceleration areas. For reverse operation and braking, the system must support four-quadrant operation.
**Analysis of System Dynamic Characteristics**
Dynamic characteristics describe how the system's output changes over time in response to inputs. Both digital and analog control systems can be analyzed using discrete or continuous mathematical methods. In engineering, the sampling frequency (f₀) is chosen such that it satisfies Shannon’s theorem: f₀ ≥ f_max, where f_max is the highest frequency in the signal spectrum. This allows the system to be treated as a continuous system using Laplace transfer functions. The sampling period T₀ is the reciprocal of f₀. The current loop, composed of a current regulator, power PWM amplifier, motor winding current circuit, and current feedback, has an electromagnetic time constant in the range of tens to hundreds of microseconds. Sampling periods and corresponding power modules are shown in Table 1.
**Two Common Control Methods in the Speed Loop**
To analyze the speed loop, the current loop is often approximated as a unity gain. However, the presence of load affects the system’s dynamic characteristics. PID control is widely used in engineering to improve performance. PI and IP control methods are commonly applied. Although both are proportional-integral controllers, they differ in processing order. PI emphasizes proportion before integration, making it suitable for large machinery with low rigidity and high gaps. IP emphasizes integration before proportion, providing better stability during startup, ideal for rigid and fast-response systems.
**Load Inertia and Its Impact on Dynamic Characteristics**
Load inertia on the motor shaft includes both motor and load inertia. High inertia degrades the amplitude and frequency characteristics of the speed loop. Generally, the load inertia should not exceed 3–5 times the motor inertia. Load inertia significantly affects the system's dynamic performance.
**Position Control Dynamics**
After analyzing the speed loop, the position control loop is simplified as an integral link. The closed-loop system behaves like a first-order inertial link, with the time constant being the reciprocal of the position gain (Kp). Higher Kp values lead to faster responses. For large machine tools, Kp ranges from 20–40/s, while for smaller ones, it is 30–60/s. In high-speed, high-precision systems, Kp can exceed 100/s after improving the current loop and addressing mechanical resonances.
**Relationship Between Maximum Speed and Position Resolution**
In digital position control systems, higher resolution requires more system resources. With large gains in current and speed loops, the system can be simplified as shown in Figure 5. Kp represents the position gain, which relates the system’s speed to the position error. The higher the maximum speed (Vmax), the more bits are needed in the error register to maintain accuracy.
**Electronic Gear Ratio**
The electronic gear ratio adjusts the relationship between input and output units. By changing the coefficients C and D, the system can produce different detection unit sizes for the same input increment. This allows flexibility in matching the system to specific applications.
**Servo System Improvements**
Modern digital servo systems offer enhanced functionality through software algorithms rather than hardware. Techniques such as feedforward control reduce steady-state errors, while improved inner-loop structures and vibration damping enhance stability. Observers estimate motor states to eliminate oscillations, and digital filters remove mechanical resonances. Nonlinear compensation techniques further refine performance.
**Conclusion**
Modern CNC systems use AC digital servo technology, allowing software-based improvements in performance and functionality. However, understanding the fundamental aspects—static and dynamic characteristics, speed-resolution relationships, and electronic gear ratios—is essential for effective system design.
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