Servo system, also known as follow-up system, belongs to one of the automatic control systems. It is used to control the rotation angle (or displacement) of the controlled object so that it can automatically, continuously and accurately regulate the change law of the input command. It is usually a closed-loop control system with negative feedback. In some cases, its function can also be realized by open-loop control. In practical application, the automatic control system usually takes the mechanical position or angle as the control object, such as NC machine tool and so on. The drive motor used in the servo system is required to have the characteristics of fast response, accurate positioning and large moment of inertia. This kind of special motor is called servo motor. Its basic working principle is no different from that of ordinary AC / DC motor. The special drive unit of this kind of motor is called servo drive unit, sometimes referred to as servo, which generally includes torque (current), speed and / or position closed-loop. Its working principle is simply that on the basis of open-loop controlled AC / DC motor, the speed and position signals are fed back to the driver through rotary encoder and rotary transformer for closed-loop negative feedback PID regulation control. Coupled with the current closed-loop inside the driver, through these three closed-loop adjustments, the accuracy and time response characteristics of the motor output following the set value are greatly improved. Servo system is a dynamic follow-up system, and the steady-state balance is also dynamic balance.

The full digital servo system generally adopts the three loop structure of position control, speed control and torque control. The system hardware is roughly composed of the following parts: power supply unit; Power inverter and protection unit; Detector unit; Digital controller unit; Interface unit. Corresponding to the three closed loops of “position”, “speed” and “torque” of the servo system from outside to inside, the servo system is generally divided into three control modes. When using the position control mode, the servo completes all three closed-loop control. When using the speed control mode, the servo completes the two closed-loop control of speed and torque (current). Generally speaking, our system that needs position control can use both servo position control mode and speed control mode, but the processing of the upper computer is different. In addition, some people think that the position control mode is easy to be disturbed. The torque control mode is that the servo system only carries out the closed-loop control of torque, that is, current control. It only needs to send a target torque value to the servo unit, which is mostly used in a single torque control occasion. For example, in a small angle cutting machine, one motor uses the speed or position control mode to transmit materials forward, and the other motor is used as the torque control mode to form a constant tension.

Brief description of servo principle

The principle of servo is as follows:

1. Frequency converter is the actuator of servo;

2. Servo is an arbitrary control system in the automatic control system, which takes the position, orientation and posture of the object as the control quantity to form the tracking target;

3. The frequency converter executes the start, stop, speed regulation, braking and other commands of the servo;

4. The servo detects the actual displacement of the feedback motor and workpiece through the encoder, and the servo inputs the target control quantity through the upper computer. The target control quantity – actual displacement = servo command;

5. For example:

1) Command pulse – feedback pulse = command pulse >> 0, start and accelerate;

2) Command pulse feedback pulse = command pulse ≥ 0, slow down and stop;

3) Command pulse feedback pulse = feedback pulse, reverse addressing;

6. The start and stop commands generated by the servo “command pulse feedback pulse” are accurate, but whether the actual start and stop position of the motor controlled by the frequency converter is in the given position cannot be controlled!

7. Therefore, the servo realizes the precise command of command and detection feedback, but does not realize the precise control of the motor;

8. The characteristic of stepping motor is that it realizes the accurate control of the motor, and the further walking degree is determined before each step;

9. If the start and stop commands generated by the servo “command pulse feedback pulse” are used to directly control the stepping pulse current of the motor, the defect that the servo can not accurately control the motor will be completely changed, and the accurate control of motion will be truly realized!

Leave a Reply

Your email address will not be published. Required fields are marked *