Servo Motor

Servo Motor

Three Control Modes of Servo Motors:

• Speed Control Mode
• Torque Control Mode
• Position Control Mode

Regarding the response speed of servo drives, torque mode involves the least computational load, enabling the fastest response to control signals. Position mode requires the most computation, resulting in the slowest response to control signals.

When demanding high dynamic performance during motion, real-time motor adjustments are required. If the controller itself has slow processing speed (e.g., PLC or low-end motion controllers), use position control mode. If the controller has fast processing speed, speed mode can be used. This shifts the position loop from the drive to the controller, reducing the drive's workload and improving efficiency (e.g., most mid-to-high-end motion controllers). If a superior higher-level controller is available, torque mode can also be employed, moving the speed loop away from the drive as well. This is typically only feasible with high-end specialized controllers, and in such cases, servo motors are not required at all.

Torque Control Mode

Torque control mode sets the motor shaft's output torque magnitude via external analog input or direct address assignment. For example, if 10V corresponds to 5Nm, setting the external analog input to 5V results in a motor shaft output of 2.5Nm: - If the motor shaft load is less than 2.5Nm, the motor rotates forward. - When the external load equals 2.5Nm, the motor stops rotating. - If the load exceeds 2.5Nm, the motor rotates backward (Typically occurs under gravity-loaded conditions). Torque magnitude can be adjusted by instantly modifying the analog input setting or by altering the corresponding address value via communication protocols.

This application is primarily used in winding and unwinding devices with strict material force requirements, such as wire winding equipment or fiber optic drawing machines. Torque settings must be dynamically adjusted based on changes in winding radius to ensure material force remains constant regardless of radius variation.

Position Control Mode

Position control typically determines rotational speed via the frequency of externally input pulses and calculates rotational angle based on pulse count. Some servos also support direct speed and displacement assignment via communication protocols. Due to its precise control over both speed and position, this mode is commonly used in positioning systems.

Applications include CNC machine tools, printing machinery, etc.

Speed Control Mode

Rotational speed can be controlled via analog input or pulse frequency. When integrated with an upper-level controller's outer-loop PID control, speed mode can also perform positioning. However, this requires feeding back the motor's position signal or the direct load's position signal to the upper-level controller for calculation. Position mode also supports direct load-end position signal detection. In this case, the encoder at the motor shaft end only monitors motor speed, while the position signal is provided by a detection device directly at the final load end. This approach reduces errors introduced by intermediate transmission mechanisms, enhancing the overall positioning accuracy of the system.

Working Principle of Linear Motors

A linear motor is a drive device that directly converts electrical energy into linear mechanical motion by unfolding a closed magnetic field into an open one, eliminating the need for any intermediate conversion mechanisms.

Structure

The structure of a linear motor can be visualized by radially sectioning a rotary motor [see Fig. 3] and unfolding its circumference into a straight line. The stator corresponds to the primary winding of the linear motor, while the rotor corresponds to the secondary winding. When current flows through the primary winding, a traveling wave magnetic field is generated in the air gap between the primary and secondary windings. This traveling wave magnetic field interacts with the permanent magnets in the secondary winding to produce driving force, thereby achieving linear motion of the moving components. In recent years, several developed countries have begun applying linear motor technology to the linear motion drive systems of CNC machine tools, replacing the traditional servo motor + ball screw drive system, achieving tremendous success.

Fig. 3 Radial cross-section of a rotating motor

Comparison Between Linear Motors and Traditional Rotary Motor + Ball Screw Motion Systems

In machine tool feed systems, the primary distinction between direct drive using linear motors and conventional rotary motor transmission lies in eliminating mechanical transmission components between the motor and the worktable (saddle). This reduces the length of the machine tool feed transmission chain to zero, hence this transmission method is also termed "zero transmission." This "zero transmission" approach delivers performance metrics and advantages unattainable with conventional rotary motor drives.

1. High-Speed Response

By eliminating mechanical transmission components with significant response time constants (such as ball screws), the overall dynamic response performance of the closed-loop control system is greatly enhanced, enabling exceptionally swift and sensitive reactions.

2. Precision

The linear drive system eliminates transmission backlash and errors caused by mechanical components like lead screws, reducing tracking errors during interpolation motion that result from transmission system lag. Through linear position detection feedback control, the positioning accuracy of machine tools can be significantly enhanced.

3. High Dynamic Stiffness

The "direct drive" approach eliminates motion lag caused by elastic deformation, friction wear, and backlash in intermediate transmission links during startup, speed changes, and directional reversals. This also enhances transmission stiffness.

4. High Speed and Short Acceleration/Deceleration Cycles

Originally developed for magnetic levitation trains (reaching speeds up to 500 km/h), linear motors effortlessly meet the ultra-high feed rates (60–100 m/min or higher) demanded by high-speed machining. Their "zero transmission" design delivers exceptional high-speed responsiveness, drastically shortening acceleration and deceleration cycles. This enables instantaneous high-speed startup and precise, instantaneous stopping during high-speed operation. High acceleration/deceleration rates are achievable, typically reaching 2–10g (g=9.8m/s²), whereas roller screw drives generally only achieve maximum acceleration of 0.1–0.5g.

5. Unlimited stroke length

By connecting linear motors in series along the guide rail, the stroke length can be extended indefinitely.

6. Quiet Operation with Low Noise

By eliminating mechanical friction from components like transmission screws and utilizing rolling guides or magnetic levitation guides (no mechanical contact), operational noise is significantly reduced.

7. High efficiency

The absence of intermediate transmission elements eliminates energy loss from mechanical friction, substantially improving transmission efficiency.

A comparison between linear motors and traditional rotary motors is shown in Table 1-1:

Comparison of Linear Motors and Conventional Rotary Motors