The most important function of the servo system in CNC machine tools 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 current during dynamic operation, while the speed and position loops guarantee precise tracking of the required speed and position at all times. Evaluating a servo system usually involves analyzing both its static and dynamic characteristics. This article focuses on specific performance indicators used to assess the system.
**Requirements for Output Characteristics**
These refer to the static behavior of the servo motor and driver. Based on these characteristics, it is determined whether the motor can provide sufficient torque within the required speed range to drive the load and whether it has enough overload capacity to start the mechanical system. Motor torque is the primary parameter considered. The continuous working torque must not exceed the continuous operating area, and the intermittent area should not be exceeded during braking or acceleration. For reverse operation and braking, the servo 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 an input. Both digital and analog control systems can be analyzed using discrete or continuous mathematical methods. In engineering practice, according to Shannon’s theorem, the sampling frequency (f₀) of a digital system is chosen so that it is greater than or equal to the highest frequency (f_max) in the signal spectrum. This allows the system to be analyzed as a continuous system using Laplace transfer functions.
F₀ ≥ f_max (1)
The sampling period (Tâ‚€) is the reciprocal of the sampling frequency. Thus, Tâ‚€ < 1/fâ‚€. Determining the highest frequency in the signal spectrum is crucial. For example, in the current loop, which includes a current regulator, a power PWM amplifier, a motor winding current circuit, and current feedback, the electromagnetic time constant of the winding is typically in the range of tens to hundreds of microseconds. The sampling period and corresponding power modules are summarized in Table 1.
**Two Common Control Methods in the Speed Loop**
To analyze the speed system, the current loop is often approximated as a unit gain. However, the dynamic characteristics of the servo system are affected by damping and inertial load due to the attached load on the motor shaft. To improve system performance, PID algorithms are frequently used in engineering. PI and IP control methods are commonly applied in the speed loop. While both are essentially proportional-integral controllers, their implementation order differs. PI emphasizes proportion before integration, making it suitable for systems with low mechanical rigidity, large gaps, and poor response. IP emphasizes integration before proportion, leading to more stable startup and is often used in rigid, fast-response machines.
**Load Inertia and Its Impact on Dynamic Performance**
Load inertia affects the dynamic characteristics of the servo system. It includes both the motor's and the load's inertia. The presence of load inertia degrades the amplitude and frequency characteristics of the speed loop. Generally, the higher the inertia, the worse the dynamic performance. The ratio of load inertia to motor inertia is typically kept below 3–5 times to maintain stability.
**Position Control Dynamics**
After analyzing the speed loop, the position loop is examined. When the speed gain is high, the speed loop is approximately a unit gain, and the position loop behaves like an integral link. The closed-loop system becomes a first-order inertial link, with the time constant being the reciprocal of the position gain (Kp). Larger Kp values result in faster responses. Typical values for large machine tools are 20–40/s, while medium and small machines use 30–60/s. In high-speed, high-precision systems, Kp can exceed 100/s after optimizing current loop characteristics and eliminating mechanical resonances.
**Relationship Between Maximum Speed and Position Resolution**
CNC servo systems are digital position control systems. Higher position resolution increases system requirements when maximum speed remains constant. Assuming the current and speed loop gains are sufficiently large, the system can be simplified as shown in Figure 5. Kp represents the position gain, indicating how much speed the system produces per unit of position error. If the position controller has an N-bit error register, the maximum error is 2^N - 1. To achieve the maximum speed Vmax, the condition Kp(2^N - 1) ≥ Vmax must be satisfied. For example, a system requiring 1 nm resolution and 1 m/min speed needs at least 18 bits if the position gain is 100/s.
**Electronic Gear Ratio**
The electronic gear ratio accounts for differences between input and output units. It is defined as C/D = R1×M1/L, where C and D are the ratios of command to detection units. Adjusting C and D allows the system to produce different detection units from the same input increment. For instance, with a minimum input increment of 1 μm and a detection unit of 0.1 μm, C=10 and D=5.
**Servo System Improvements**
Digital servo systems offer greater flexibility through software algorithms. Techniques such as feedforward control reduce steady-state errors, while inner loop improvements enhance stability. Dual-position loop control prevents vibrations in systems with large clearances, and vibration damping control reduces mechanical oscillations. Observers estimate motor status, and digital filters eliminate mechanical resonances. Nonlinear compensation techniques like pitch and reverse compensation further improve performance.
**Conclusion**
Modern CNC systems rely on AC digital servo technology, enabling software-based enhancements to system performance and functionality. However, understanding the basic characteristics—static and dynamic behavior, speed-resolution relationships, and electronic gear ratios—is essential for effective system design.
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