Both speed control and torque control for servo motors are implemented using analog signals, whereas position control is achieved by generating pulses. The specific control mode to be adopted is selected based on the client's requirements and the specific motion functions that need to be fulfilled.
the three control modes for servo motors
- If you have no specific requirements regarding the motor's speed or position-and simply need to output a constant torque-the Torque Mode is the obvious choice.
- If you require a certain level of precision regarding position and speed, but are not particularly concerned with real-time torque values, using either Speed Mode or Position Mode is generally preferable.
- If your host controller possesses robust closed-loop control capabilities, utilizing Speed Control will typically yield superior results. Conversely, if your requirements are not particularly stringent-or if there are virtually no demands for real-time responsiveness-using Position Control places fewer demands on the host controller.
In terms of servo drive response speed: Torque Mode involves the lowest computational load, resulting in the fastest response from the drive to control signals; conversely, Position Mode involves the highest computational load, resulting in the slowest response from the drive to control signals.
When high dynamic performance is required during motion, real-time adjustments to the motor become essential.
- If the controller itself has a relatively slow processing speed (e.g., a PLC or a low-end motion controller), Position Control should be used.
- If the controller possesses a relatively fast processing speed, Speed Control can be utilized; this involves shifting the position loop from the drive to the controller, thereby reducing the drive's workload and enhancing overall efficiency.
- If an even more advanced host controller is available, Torque Control can be employed; this involves shifting the speed loop-in addition to the position loop-away from the drive. This approach is typically feasible only with high-end, specialized controllers.
Generally, a highly intuitive metric for evaluating the quality of drive control is known as "response bandwidth."
When operating in Torque Control or Speed Control modes, a pulse generator is used to input a square-wave signal, causing the motor to continuously alternate between forward and reverse rotation. As the signal frequency is progressively increased, an oscilloscope displays a swept-frequency signal. When the peak of the signal's envelope drops to 70.7% of its maximum value, it indicates that the system has lost synchronization (i.e., "stepped out"). The frequency at which this occurs serves as a direct indicator of the control system's quality; typically, the current loop can achieve bandwidths exceeding 1000 Hz, whereas the speed loop is generally limited to a few tens of Hertz.
Torque Control
Torque control is a method used to set the magnitude of the motor shaft's external output torque, achieved either through an external analog input signal or by directly assigning a value to a specific address. Operationally, this means-for example-that if a 10V input corresponds to 5 Nm of torque, then setting the external analog input to 5V will result in a motor shaft output of 2.5 Nm. Under these conditions: if the load on the motor shaft is less than 2.5 Nm, the motor rotates in the forward direction; if the external load equals 2.5 Nm, the motor remains stationary; and if the load exceeds 2.5 Nm, the motor rotates in the reverse direction (a scenario typically encountered in applications involving gravitational loads). The set torque value can be adjusted in real-time either by altering the analog input signal or by modifying the numerical value of the corresponding address via a communication interface.
This control mode is primarily utilized in winding and unwinding systems where strict requirements exist regarding the tension applied to the material-such as in wire-winding machinery or fiber-optic drawing equipment. In such applications, the torque setting must be dynamically adjusted in real-time to compensate for changes in the winding radius, thereby ensuring that the tension exerted on the material remains constant regardless of variations in the winding diameter.
Position Control
In Position Control mode, rotational speed is typically determined by the frequency of externally input pulses, while the rotational angle is determined by the pulse count. Additionally, some servo systems allow for the direct assignment of speed and displacement values via communication interfaces. Since Position mode enables rigorous control over both speed and position, it is commonly employed in positioning mechanisms.
Application fields include CNC machine tools, printing machinery, and similar equipment.
Speed Control Mode
Rotational speed can be controlled either through analog inputs or by adjusting pulse frequency. When integrated into an outer-loop PID control system managed by a higher-level controller, Speed mode can also be utilized for positioning tasks; however, it is essential to feed back the motor's position signal-or the position signal directly from the load-to the higher-level controller for computational processing. Position mode also supports the detection of position signals directly from the load's outer loop; in this configuration, the encoder mounted on the motor shaft monitors only the motor's rotational speed, while the actual position signal is provided by a detection device located at the final load end. The primary advantage of this approach is that it minimizes errors introduced during intermediate transmission stages, thereby enhancing the overall positioning accuracy of the entire system.
Three-Loop Control
Servo motors typically employ a three-loop control structure-specifically, a system comprising three closed-loop negative-feedback PID control systems. The innermost PID loop is the "current loop". This loop operates entirely within the servo drive itself; it utilizes Hall-effect sensors to detect the output current flowing from the drive to each phase of the motor, and then applies negative feedback to the current setpoint to perform PID regulation. The objective is to ensure that the actual output current closely matches the desired setpoint. Fundamentally, the current loop controls the motor's torque; consequently, when the drive is operating in torque mode, the computational load is minimal, resulting in the fastest dynamic response.
The second loop is the "velocity loop". It performs negative-feedback PID regulation based on signals detected by the motor's encoder. The PID output generated within this loop serves directly as the setpoint for the current loop. Therefore, when operating in velocity control mode, the system effectively engages both the velocity loop and the current loop. In other words, the current loop is indispensable in any operating mode; it constitutes the fundamental basis of control. Even during velocity and position control operations, the system is simultaneously executing current (torque) control to achieve the precise regulation of speed and position.
The third loop is the "position loop". As the outermost loop, it can be configured either between the drive and the motor encoder, or between an external controller and the motor encoder (or the final load), depending on the specific application requirements. Since the internal output of the position control loop serves as the setpoint for the velocity loop, operating in position control mode necessitates the simultaneous execution of all three loops. Consequently, this mode places the highest computational burden on the system and results in the slowest dynamic response speed.