Servo Motor Precise Positioning And Closed Loop Characteristics

- Mar 11, 2019-

Servo motor precise positioning and closed loop characteristics

A servo mechanism is an automatic control system that enables an output controlled amount of an object to follow an arbitrary change of an input target (or a given value).

Then how to achieve precise positioning of the servo motor, how to understand its closed-loop characteristics, let's explore. First, let's look at the composition of the AC servo system, which consists of a servo driver and a servo motor. Here we mainly talk about the working principle of the servo drive, the motor is just an actuator. The schematic diagram of the driver is as follows. Similar to the main circuit of the inverter, the power supply is rectified and inverted to realize the conversion from AC→DC→AC.


Servo drive structure diagram

The input signal/command 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 the control of the ring. The torque control is the current closed-loop control, and the speed mode is the speed closed-loop control. The mode is the three closed loop control mode (torque, speed, position). Below we analyze the three closed loops of the position mode:


Three closed loop control of position mode

In the above figure, M represents the servo motor, PG represents the encoder, and the outermost blue represents the position loop. Because we finally control the position (positioning), the inner ring is the speed loop and the current loop (torque loop), respectively. The lower speed loop and current loop act as guard rings to prevent stall control and overload to ensure constant motor speed and constant motor current. We focus on how the position loop ensures that the motor can rotate exactly at a given angle.

If we give a pulse, the feedback pulse is 0, the pulse deviation Δp=1, input to the controller, at this time the drive circuit controls the IPM inverter to generate the SPWM wave to drive the servo motor to rotate, pay attention to this SPWM wave. It is different from the square wave of our plc pulse. When the motor drives the encoder to rotate, it sends a feedback pulse. At this time, △p=0, the motor stops output, and one pulse is positioned. The entire process from the pulse to the acceptance of the feedback pulse is a closed-loop process to ensure that the motor can be accurately positioned. The number of pulses determines the distance of the positioning. The frequency of the pulse determines the speed of the motor.

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