A servo system is an automatic control system that enables the output controlled variables, such as the position, orientation, and state of an object, to follow any changes in the input target (or given value).
So how does a servo motor achieve precise positioning, and how do we understand its closed-loop characteristics? Today, we'll discuss this. First, let's look at the composition of an AC servo system, which consists of a servo driver and a servo motor. Here, we'll mainly discuss the working principle of the servo driver; the motor is merely an actuator. The simplified diagram of the driver is shown below. Similar to the main circuit of a frequency converter, the power supply undergoes rectification and inversion to achieve AC→DC→AC conversion.
Servo driver structure diagram
Input signals/commands can be control signals such as position, speed, and torque, corresponding to the three control modes of the servo motor. Each control mode corresponds to a closed-loop control: torque control is a current closed-loop control, speed mode is a speed closed-loop control, and position mode is a three-loop control mode (torque, speed, and position). Below, we analyze the three-loop control of the position mode:
Three-loop control in position mode
In the diagram above, M represents the servo motor, PG represents the encoder , and the outermost blue loop represents the position loop, because we ultimately control the position (positioning). The inner loops are the speed loop and the current loop (torque loop). In position mode, the speed loop and current loop act as protection loops to prevent stall control and overload, ensuring constant motor speed and constant motor current. We will focus on how the position loop ensures that the motor can accurately rotate a given angle.
If we provide one pulse, the feedback pulse is 0, and the pulse deviation Δp = 1, which is input to the controller. At this point, the drive circuit controls the IPM inverter to generate an SPWM wave to drive the servo motor to rotate. Note that this SPWM wave is different from the square wave pulse sent by the PLC . The motor drives the encoder to rotate and generate a feedback pulse. At this point, Δp = 0, the motor stops outputting, and positioning is completed with one pulse. The entire process from sending a pulse to receiving a feedback pulse is a closed loop process, thus ensuring accurate motor positioning. The number of pulses determines the positioning distance, and the pulse frequency determines the motor speed.
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