Control methods of servo control motors commonly used in robot servo systems

Industrial robots have four major components, namely the body, servo, reducer and controller. Among them, the general structure of the industrial robot electric servo system is three closed-loop control, namely the current loop, speed loop and position loop. Generally speaking, for AC servo drives, multiple functions such as position control, speed control, torque control, etc. can be achieved by manually setting its internal function parameters.

The servo system (servomechanism), also known as the follow-up system, is a feedback control system used to accurately follow or reproduce a process. The servo system is an automatic control system that enables the output controlled quantity such as the position, orientation, and state of an object to follow any changes in the input target (or given value).

The servo system is a product developed based on frequency conversion technology. It is an automatic control system that uses mechanical position or angle as the control object. In addition to speed and torque control, the servo system can also perform precise, fast, and stable position control. In a broad sense, a servo system is a control system that accurately tracks or reproduces a given process, and can also be called a follow-up system. In a narrow sense, a servo system is also called a position follow-up system. Its controlled quantity (output quantity) is the linear displacement or angular displacement of the load mechanical spatial position. When the position given quantity (input quantity) changes arbitrarily, the main task of the system is to make the output quantity quickly and accurately reproduce the change of the given quantity.

Servo system structure

The structures and types of mechatronic servo control systems are diverse, but from the perspective of automatic control theory, servo control systems generally include five parts: controller, controlled object, execution link, detection link, and comparison link.

Servo system composition block diagram

1. Comparison

The comparison link is a link that compares the input command signal with the system's feedback signal to obtain the deviation signal between the output and the input. It is usually implemented by a special circuit or computer.

2. Controller

The controller is usually a computer or a PID (proportional, integral and differential) control circuit. Its main task is to transform the deviation signal output by the comparison element to control the actuator to act as required.

3. Execution

The function of the execution link is to convert various forms of input energy into mechanical energy according to the requirements of the control signal to drive the controlled object to work. The actuators in the mechatronics system generally refer to various motors or hydraulic and pneumatic servo mechanisms.

4. Accused party

The controlled object refers to the object being controlled, such as a robotic arm or a mechanical work platform.

5. Testing

The detection link refers to a device that can measure the output and convert it into the dimension required by the comparison link, generally including sensors and conversion circuits.

Characteristics and functions of servo system

The servo system is essentially different from the feed system of general machine tools. It can accurately control the movement speed and position of the actuator according to the command signal. The servo system is the link between the CNC device and the machine tool, and is an important component of the CNC system. It has the following characteristics:

It is necessary to have a high-precision sensor that can accurately give the electrical signal of the output quantity.

Both the power amplifier and the control system must be reversible.

Sufficiently large speed regulation range and strong enough low-speed load performance.

Fast response and strong anti-interference ability.

Types of Servo Systems

According to the control principle: there are three forms: open loop, closed loop and semi-closed loop

According to the nature of the controlled quantity: there are servo system forms such as displacement, speed, force and torque

According to the driving mode: There are electrical, hydraulic and pneumatic servo drive forms

According to the actuator: There are stepper motor servo, DC motor servo and AC motor servo

Actuators of servo systems

1. Types and characteristics of actuators

(1) Electrical actuators Electrical actuators include direct current (DC) servo motors, alternating current (AC) servo motors, stepper motors, and electromagnets, which are the most commonly used actuators. In addition to requiring smooth operation, servo motors are generally required to have good dynamic performance, be suitable for frequent use, and be easy to maintain.

(2) Hydraulic actuators Hydraulic actuators mainly include reciprocating cylinders, rotary cylinders, hydraulic motors, etc., among which cylinders are the most common. Under the condition of equal output power, hydraulic components have the characteristics of light weight and good speed.

(3) Pneumatic actuators Pneumatic actuators are no different from hydraulic actuators except that they use compressed air as the working medium. Although pneumatic drive can obtain greater driving force, stroke and speed, it cannot be used in situations where high positioning accuracy is required due to the poor viscosity and compressibility of air. The differences between the three types

2. Commonly used control motors

The control motor is the power component of the electrical servo control system. It is an energy conversion device that converts electrical energy into mechanical energy. The control motor commonly used in mechatronic products refers to a servo motor that can provide correct movement or more complex movements. The control motor includes rotary and linear drive motors, which are controlled by voltage, current, frequency (including command pulses), etc. to achieve constant speed, variable speed drive, or incremental drive with repeated start and stop, as well as complex drive. The drive accuracy varies with the drive object. (1) Servo drive motors generally refer to: stepping motor, DC servo motor, AC servo motor

(2) There are three common control methods for servo control motors: open-loop control, semi-closed-loop control, and closed-loop control. The drive system of a closed-loop system has a position (or speed) feedback link; an open-loop system does not have a position and speed feedback link. a. An open-loop CNC system does not have a position measurement device, and the signal flow is unidirectional (CNC device → feed system), so the system has good stability.

Without position feedback, the accuracy is not high compared to the closed-loop system. Its accuracy mainly depends on the performance and accuracy of the servo drive system and the mechanical transmission mechanism. Generally, a power stepper motor is used as a servo drive element. This type of system has the advantages of simple structure, stable operation, convenient debugging, simple maintenance, and low price. It is widely used in occasions where the accuracy and speed requirements are not high and the driving torque is not large. It is generally used for economical CNC machine tools. b. Semi-closed-loop CNC system The position sampling point of the semi-closed-loop CNC system is shown in the figure. It is derived from the drive device (commonly used servo motor) or the lead screw, and the sampling rotation angle is detected, rather than directly detecting the actual position of the moving part.

The semi-closed loop does not include or only includes a small number of mechanical transmission links, so stable control performance can be obtained. Although the stability of the system is not as good as that of the open-loop system, it is better than that of the closed loop. The motion error caused by the pitch error of the screw and the gear clearance is difficult to eliminate. Therefore, its accuracy is worse than that of the closed loop and better than that of the open loop. However, this type of error can be compensated, so satisfactory accuracy can still be obtained. The semi-closed loop CNC system has a simple structure, is easy to debug, and has high accuracy, so it has been widely used in modern CNC machine tools. c. Fully closed-loop CNC system The position sampling points of the fully closed-loop CNC system are shown in the dotted line in the figure, which directly detects the actual position of the moving parts.