There are four major components of industrial robots, which are the body, servo, reducer and controller. Among them, the general structure of the electric servo system of industrial robots is three closed-loop control, i.e. current loop, speed loop and position loop. In general, for the AC servo drive, the position control, speed control, torque control and other functions can be realized through the manual setting of its internal function parameters.
Servo system (servomechanism), also known as the following system, is a feedback control system used to accurately follow or reproduce a process. Servo system makes the object's position, orientation, state and other output controlled quantities can follow the input target (or given value) of any change in the automatic control system.
Servo system is a product developed on the basis of frequency conversion technology, and it is an automatic control system with mechanical position or angle as the control object. In addition to speed and torque control, servo system can also perform precise, fast and stable position control.
A broad servo system is a control system that accurately tracks or reproduces a given process, and can also be called a follower system.
Narrow servo system, also known as position follower system, the controlled quantity (output quantity) is the line displacement or angular displacement of the spatial position of the loaded machinery, when the position of the given quantity (input quantity) for any change, the main task of the system is to make the output quantity quickly and accurately reproduce the given quantity of change.
Mechatronics servo control system has a wide variety of structures and types, but from the point of view of automatic control theory to analyze, the servo control system generally includes the controller, the controlled object, the execution link, the detection link, the comparison link and other five parts.
1. Comparison link
Comparison link is the input command signal and system feedback signal comparison, in order to obtain the output and input deviation signal link, usually by the special circuit or computer to realize.
2. Controller
The controller is usually a computer or PID (Proportional, Integral and Differential) control circuit, whose main task is to transform the deviation signal output from the comparison element, in order to control the executive element according to the required action.
3. Execution link
The role of the executive link is according to the requirements of the control signal, the input of various forms of energy into mechanical energy, drive the controlled object work. Mechatronics system in the implementation of elements generally refers to a variety of motors or hydraulic, pneumatic servo mechanism.
4. Controlled object
Controlled object refers to the object to be controlled, such as a mechanical arm, or a mechanical work platform.
5. Detection
Detection link refers to the output can be measured and converted into a comparative link required for the outline of the device, generally including sensors and conversion circuits.
Servo system features and functions
Servo system and general machine tool feed system is fundamentally different, it can be based on the command signal to accurately control the speed and position of the movement of the executive part. The servo system is the link between the CNC device and the machine tool, and is an important component of the CNC system with the following characteristics:
It must have high-precision sensors that can accurately give electrical signals for the output quantity.
The power amplifier as well as the control system must be reversible.
Sufficiently large speed range and strong enough low-speed with load performance.
Fast response capability and strong anti-interference capability.
Types of servo systems According to the principle of control: open-loop, closed-loop and semi-closed-loop in three forms
According to the nature of the amount to be controlled: displacement, speed, force and torque and other forms of servo systems
According to the drive mode: there are electrical, hydraulic and pneumatic servo drive form
According to the executive element: there are stepper motor servo, DC motor servo and AC motor servo form
Servo system actuators
1, the types of actuators and their characteristics
(1) Electrical actuators
Electrical actuators include direct current (DC) servo motors, alternating current (AC) servo motors, stepping motors, and solenoids, which are the most commonly used actuators. Servo motors are the most commonly used actuators. In addition to the requirement of smooth operation, they are generally required to have good dynamic performance, suitable for frequent use and easy maintenance.
(2) Hydraulic actuators
Hydraulic actuators mainly include reciprocating cylinders, rotary cylinders, hydraulic motors, etc., of which cylinders are the most common. In the case of the same output power, the hydraulic components have a light weight, good speed and so on.
(3) Pneumatic actuators
Pneumatic actuators in addition to compressed air as a working medium, and hydraulic actuators are no different. Pneumatic drive can get a larger driving force, stroke and speed, but because of the poor viscosity of the air, compressibility, so can not be used in the positioning accuracy requirements of high occasions.
Differences between the three types
| type | specificities | vantage | drawbacks |
| Electropneumatic | Available commercial power supply; signals are transmitted in the same direction as power; there is a difference between AC and DC; pay attention to the voltage and power used. | Easy to operate; easy to program; can realize positioning servo control; fast response, easy to connect with computer (CPU); small size, high power, no pollution. | High instantaneous output power; poor overload; once jammed, it will cause burning accidents; affected by external noise. |
| Pneumatic | Gas pressure source pressure 5~7×Mpa; skilled operators are required. | Convenient gas source, low cost; no leakage to pollute the environment; fast and easy to operate. | Small power, large size, difficult to miniaturize; unsmooth movement, difficult to transmit over long distances; high noise; difficult to servo. |
| Hydraulic | Liquid pressure source pressure 20~80×Mpa; requires skilled operators. | High output power, fast speed, smooth movement, can realize positioning servo control; easy to connect with computer (CPU). | The equipment is difficult to miniaturize; the hydraulic source and hydraulic oil requirements are strict; easy to produce leaks and pollute the environment. |
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. Control motors commonly used in mechatronic products are servo motors that can provide the correct movement or more complex action.
There are rotary and linear drive motors for control, which are controlled by voltage, current and frequency (including command pulse) to realize fixed-speed, variable-speed drive or incremental drive with repeated starting and stopping, as well as complex drive, and the precision of drive varies with the drive object.
(1) Servo drive motor generally refers to: stepping motor (Stepping Motor), direct current servo motor (DC Servo Motor), AC servo motor (AC Servo Motor)
(2) commonly used servo control motor control methods are: open-loop control, semi-closed-loop control, closed-loop control three.
Closed-loop drive system with position (or speed) feedback link; open-loop system does not have position and speed feedback link.
a, open-loop CNC system does not have a position measuring device, the signal flow is unidirectional (CNC device → feed system), so the system stability is good.
Without position feedback, the accuracy is not high compared to the closed-loop system, and its accuracy mainly depends on the performance and accuracy of the servo drive system and mechanical transmission mechanism. Generally take the power stepping motor as the servo driving element.
This kind of system has the advantages of simple structure, stable work, easy debugging, simple maintenance, low price, etc., in the precision and speed requirements are not high, the driving torque is not big occasions are widely used. Generally used for economic CNC machine tools.
b, semi-closed-loop CNC system semi-closed-loop CNC system position sampling point as shown in the figure, from the drive (commonly used servo motor) or screw lead, sampling rotation angle detection, not directly detect the actual position of moving parts.
Semi-closed loop loop does not include or only include a small number of mechanical transmission links, so you can get a stable control performance, the stability of the system is not as good as the open loop system, but better than the closed loop. Due to the pitch error of the screw and the motion error caused by the gear gap is difficult to eliminate. Therefore, its accuracy is worse than closed loop and better than open loop. However, such errors can be compensated, and thus satisfactory accuracy can still be obtained.
Semi-closed-loop CNC system structure is simple, easy to debug, and high precision, and thus widely used in modern CNC machine tools.
c, full closed-loop numerical control system full closed-loop numerical control system position sampling point as shown in the figure of the dotted line, directly on the actual position of the moving parts detection.
Theoretically, it can eliminate the error, clearance and loss of motion of the whole drive and transmission link. It has high position control accuracy. Since the friction characteristics, rigidity and clearance of many mechanical transmission links within the position loop are nonlinear, it is easy to cause system instability, making the design, installation and commissioning of the closed-loop system quite difficult.
The system is mainly used for boring and milling machines, ultra-precision lathes, ultra-precision grinding machines and larger CNC machine tools with high precision requirements.
Servo systems for robots
Typically, by robot servo system, we mean a precision servo system used for multi-axis motion control. A multi-axis motion control system is composed of a high-level motion controller and a low-order servo drive, the motion controller is responsible for motion control command decoding, the relative motion of the various position control axes, acceleration and deceleration contour control, etc., and its main role is to reduce the overall system motion control path error; servo drive is responsible for the position control of the servo motors, and its main role is to reduce the servo axis of the tracking error. The servo drive is responsible for servo motor position control, and its main role is to reduce the servo axis following error.
Robot servo system consists of servo motors, servo drives, command mechanism of the three major components, servo motors are actuators, is to rely on it to realize the movement, servo drive is the servo motor power supply, the command mechanism is to send a pulse or give the speed used to cooperate with the servo drive to work properly.
Robot on the servo motor requirements than the other two parts are high. First of all, the servo motor is required to have a fast response. The motor from the command signal to complete the instruction required by the working state of the time should be short. Response to the command signal the shorter the time, the higher the sensitivity of the electric servo system, the better the fast response performance, generally to the servo motor electromechanical time constant of the size of the servo motor to illustrate the performance of the fast response. Secondly, the starting torque inertia ratio of the servo motor should be large. In the case of driving loads, the robot's servomotor is required to have a large starting torque and a small moment of inertia. Finally, the servo motor should have the continuity and linearity of the control characteristics, with the change of the control signal, the motor's speed can be continuously changed, and sometimes the speed is also required to be proportional or approximately proportional to the control signal.
Of course, in order to match the robot's body shape, the servo motor must be small in size, small in mass and short in axial dimension. Also withstand harsh operating conditions, can be very frequent forward and reverse and acceleration and deceleration operation, and can withstand several times the overload in a short period of time.
Servo drive is a servo motor can be used to generate torque and force, directly or indirectly drive the robot body to obtain a variety of robot motion actuator, with high torque moment of inertia ratio, no brush and commutation sparks and other advantages, in the robot is more widely used.