Servo motor speed control and torque control are both achieved using analog signals, while position control is executed through pulse signals. The specific control mode chosen depends on customer requirements and the desired motion functionalities.

Here, let's introduce the three control modes for servo motors:

Torque Control:

When there are no specific requirements for speed or position, and only a constant torque output is needed, torque mode is preferred. It sets the motor to output a constant torque, adjusting according to the analog input. This mode is suitable for applications where precise speed and position are not critical.

Position Control:

Position mode is suitable when there are precision requirements for both position and speed, but real-time torque control is not a priority. It adjusts the motor's position based on pulse inputs or communication signals. This mode is beneficial when the higher-level controller has robust closed-loop control capabilities.

Speed Control:

Speed control mode is ideal when the upper-level controller offers superior closed-loop control functionality. It provides better speed control performance compared to other modes. If real-time requirements are not high, position control mode can be used without imposing stringent demands on the upper-level controller.

In terms of servo drive response speed:

Torque mode requires the least computational effort and offers the fastest response to control signals.

Position mode demands the most computational resources, resulting in slower response to control signals.

For applications demanding high dynamic performance, real-time adjustments to the motor are necessary.

If the controller's computational speed is slow (e.g., PLC or low-end motion controllers), position control mode is preferred.

For controllers with faster computation speed, speed mode can be used to offload some tasks from the drive to the controller, thereby enhancing efficiency. Additionally, torque control mode can be employed with advanced specialized controllers, moving both speed and position loops away from the drive, although this is typically achievable only with high-end controllers.

A straightforward comparison metric for evaluating drive control performance is called the "response bandwidth." This parameter is determined by applying a square wave signal to the drive during torque or speed control mode, gradually increasing the frequency until the envelope reaches 70.7% of its maximum value. The resulting frequency indicates the control quality, with current loops capable of achieving frequencies above 1000 Hz, while speed loops typically reach only tens of Hertz.

Individual Control Modes:

Torque Control:

Torque control mode sets the output torque of the motor shaft based on external analog input or direct address assignment. For instance, if 10V corresponds to 5Nm, the motor shaft outputs 2.5Nm when the external analog input is set to 5V. This mode finds application in winding and unwinding devices where maintaining precise torque levels, adjusted according to changes in winding radius, is crucial.

Position Control:

In position control mode, the rotation speed is determined by the frequency of external pulse inputs, while the angle of rotation is determined by the number of pulses received. Some servos allow direct assignment of speed and displacement through communication. This mode is commonly used in positioning devices such as CNC machines and printing machinery.

Speed Mode:

Speed control mode regulates the rotational speed either through analog input or pulse frequency. In systems employing an outer PID control loop from an upper-level controller, speed mode can also facilitate positioning. However, it requires feedback of motor or load position signals for computation. Position mode also supports direct load outer loop position detection, enhancing system positioning accuracy by minimizing errors introduced by intermediate transmission components.

Three Control Loops:

Servo motors typically operate within three closed-loop feedback PID control systems:

Current Loop: The innermost PID loop controls the motor's current. It operates entirely within the servo drive, adjusting the output current to match the set current. This loop primarily governs torque control, resulting in minimal computational load and fast dynamic response.

Velocity Loop: The velocity loop regulates the motor's rotational speed by utilizing feedback signals from the motor encoder. Its output directly influences the set current of the current loop, effectively integrating speed and torque control. Therefore, regardless of the operating mode, the current loop remains fundamental to control, ensuring precise regulation of speed and position.

Position Loop: Positioned as the outermost loop, the position loop can be implemented either between the drive and the motor encoder or between an external controller and the motor encoder/load. The choice depends on specific system requirements. As the position control loop's output feeds into the velocity loop's setpoint, operating in position control mode necessitates computations across all three loops, resulting in the highest computational workload and slower dynamic response speed.

By understanding and selecting the appropriate control mode and loop configurations, engineers can optimize servo motor performance for various industrial applications.