Overview of welding robot motion control system
We are the most commonly used type joints six degrees of freedom, for example, Outlines the structure of the control system, emphasized to clarify with universal joint type robot welding system and the welding parameters of practical computer interface, and the man-machine interface of modern welding robot to do the comprehensive introduction.
As the user of welding robot, for the correct choice, reasonable use and can do regular maintenance, welding robot, the control system of welding robot motion must be have a certain level of understanding.
Welding robot is put on the electrode holder or all kinds of welding torch industrial robots. Industrial robot motion control system involving maths, such as automatic control theory, a lot of content. In shorter length, comprehensively and systematically industrial robot motion control system is introduced in this paper, it is the industrial robot control professionals and, therefore, this chapter content from a user perspective, welding robot concisely in this paper, the general problem of robot motion control system.
1 welding robot moving axis
1.1 the definition of welding robot moving axis
Spot welding and arc welding robot is a typical type 6 joints (also known as 6 axis) industrial robots with electrode holder or welding torch and composition, therefore, discuss the welding robot system, which discuss the typical 6 joints motion system of the industrial robot. As the name implies, six joint industrial robot has six typical movable joints, movement of each joint names are defined, in figure 2, the typical six joint industrial robots is given the serial number of each joint and the action state (number is set at the back of the English capital letter English name the first letter), movement of each joint are driven by a servo electric (dynamic) machine, every machine has its own servo control system. The robots "hand" joint on the installation of the tool center point (TCP) (for point electrode holder and electrode holder TCP point, have been made in the corresponding robot structure regulation) trajectory is the result of the concerted action of joints servo system. The robot motion control system () function is how to command and control according to the programming instructions six servo electric (dynamic) machine concerted action, required to complete the tool center point trajectory.
1.2 welding robot motion axis parameters
Welding robot motion parameters, mainly including: the axis motion of the largest, maximum speed, the allowable torque related shaft, relevant allow the moment of inertia of axis, etc. A kind of typical 6 type joint industrial robot joint movement parameter value shown in table.
As can be seen: 6 type joint industrial robot motion parameters, not only involves the axis itself, also decided to the performance of the whole industrial machine. Among them, the most important performance indicators are: repeat positioning accuracy and load weight.
The composition of 2 welding robot motion control system
2.1 of the general requirements of robot motion control system
Robot control system is an important part of the robot, mainly used for the control of the robot movement, to accomplish a specific task, its basic functions are as follows:
2.1.1 memory functions: storage operation sequence, the path of movement, and movement way, the movement speed and information related to the production process.
2.1.2 teaching function: off-line programming, online teaching, indirect teaching. Online teaching includes teaching box and guiding teaching two kinds.
2.1.3 contact peripheral functions: the input and output interface, communication interface, network interfaces, synchronous interface.
Section 2.1.4 coordinate setting function: the coordinate system, the absolute coordinate system, tools and user-defined four kinds of coordinate system.
2.1.5 human-machine interface: teach box, operation panel, display.
2.1.6 sensor interface: position detection of force sensing, visual, touch, etc.
2.1.7 position servo function: robot multi-axis linkage and motion control, speed and acceleration control, dynamic compensation and so on.
2.1.8 fault diagnosis safety protection function: the runtime system status monitoring, fault condition of safety protection and fault self-diagnosis.
2.2 the composition of welding robot motion control system (hardware)
In the welding robot motion control system hardware generally include:
2.2.1 control computer. The control system of dispatching command structure. Generally for microcomputers, the 32-bit and 64 - bit microprocessor, such as CPU and other types of CPU pentium series;
2.2.2 teaching box. Teach the welding robot work path and parameter setting, and all the man-machine interactive operation.
Teach box have their own independent of CPU and memory cell, and the main computer by serial communication between realize human-machine interactive information;
2.2.3 operation panel. Consists of various operation buttons, lights, only completed the basic function operation;
2.2.4 hard disk and floppy disk storage. Stored welding robot working procedures as well as a variety of welding process parameter database of peripheral memory;
2.2.5 digital and analog input and output. Various state and control input or output of commands.
2.2.6 printer interface. Records need to be the output of all kinds of information.
2.2.7 sensor interface. Information used for the automatic detection, robot control, usually can't sleep, touch and vision sensors. For general spot welding and arc welding robot control system is not set force, tactile and visual sensor.
2.2.8 axis controller. Complete each robot joint position, velocity and acceleration control.
2.2.9 auxiliary equipment control. To control the and welding robot with auxiliary equipment, such as welding power source system, welding torch, electrode holder, welding system, etc.
2.2.10 communication interface. Realize information exchange, robots and other equipment in a serial interface, parallel interface, etc.
2.2.11 network interface
Can be realized by Ethernet 2.2.11.1 Ethernet interface: the number of units or a single robot direct PC communication, data transfer rate is as high as 10 mbit/s, can be used directly on the PC windows9X or windowsNT library function, after application programming support TCP/IP communications protocol, through the Ethernet interface the data and programs into each robot controller.
2.2.11.2 Fieldbus interface: support for a variety of popular Fieldbus specifications, such as Device net, AB Remote I/O, Interbus -s, profibus-dp, M - net, etc.
3 welding axis servo control system of the robot
3.1 axis servo control system structure
Called the motor servo system, is refers to the speed control, servo motor and testing components of three parts; Speed control part and calls the servo unit or device. According to the structure characteristics of servo system, it usually has four basic structure types: open loop and closed loop, half closed loop and mixed loop.
The hardware of the servo unit is generally made up of five parts:
3.1.1 implementation axis servo motor PID control or FUZZY control (FUZZY), servo control or other control law for the single chip microcomputer;
3.1.2 servo control template, its function is to realize the control of single chip microcomputer digital output D/A conversion and input to the microcontroller analog A/D conversion;
3.1.3 servo amplifiers, general robot shaft drive motor power of 100 w ~ 1000 w in more scope, more belong to medium power, therefore, servo control template control signals must be given by the power amplifier to drive motor;
3.1.4 servo motor servo control system of welding robot axis control object.
3.1.5 servo motor speed and position detection device (rotational speed and position sensor). The function of speed and position detection device is real-time detection axis servo motor rotation speed and angular displacement, and the real-time detection results feedback to motor servo system, to form a closed loop of the servo motor or half closed loop control system. Even the open-loop control system, general also need motor speed and motor angular displacement of real-time detection parameters. Therefore, speed, position detection device is the axis of the robot servo control system is a very important link.
As shown in figure 5 axis servo control system of welding robot structure called the master-slave control mode: it is the main, from two levels of control computer system of all the control functions. Main computer implementation axis servo control system of management, coordinate transformation, path generation and diagnosis of the systems, etc.; From computer to realize the coordination of all joint control. Master-slave control system has good real-time performance, suitable for high precision, high speed control, but its poor system scalability, maintenance difficulties. Axis servo control system structure of welding robot also can use the so-called "distributed control system", limited to space, no longer in this paper.
3.2 the requirement of robot motor servo system
Robot (that is, each axis) of each joint movement, ultimately comes down to the corresponding axis drive motor, servo motor rotation,
And puts forward high demands on the robot motor servo system, can be roughly divided into the following four aspects:
3.2.1 and high precision. In order to ensure the quality of welding parts processing and improve efficiency, first of all to ensure that the welding robot positioning accuracy and machining accuracy. Therefore, in the axis position control of the robot requires high positioning accuracy, that is of the order of microns. In speed control, requires high speed control precision, strong ability to resist load disturbance, also called static and dynamic downhill as small as possible;
3.2.2 fast response. System has a good fast response characteristics, which requires command signal tracking faster response and position tracking error (position tracking precision) is smaller;
3.2.3 wide speed range. When it refers to the rated load electric function provides the highest speed and the ratio of the minimum speed. Requirements for general robot motor servo system can be in the range 0 ~ 20 m/min to work;
3.2.4 high torque at low speed. According to machining characteristic of welding robot, mostly work under low load condition (why) spot welding robot, so that requires both at low speeds motor servo system with large output torque and rotation stability requirements.
In order to meet the above four requirements, the motor servo system of the servo motor actuators - also puts forward the corresponding requirements, they are:
Low motor can run smoothly in the whole speed range, smaller torque ripple, especially at low speeds should be still a steady pace without crawling phenomenon;
Low motor should have certain overload ability, in order to meet the requirement of low speed, high torque;
In order to meet the requirements of rapid response, motor must have a smaller moment of inertia and large locked-rotor torque, as small as possible machine, electric time constant and the starting voltage.
3.3 the type of robot servo motor
Robot servo system in open loop stepping motor servo system, dc motor servo system after two stages, has entered the stage of the synchronous motor servo system.
3.3.1 brush dc motor
Dc motor has excellent control performance, the mechanical characteristics of "hard", good speed regulation performance is what kinds of ac motor does not have.
In addition, the dc motor and large starting torque, high efficiency, easy speed regulation, good dynamic characteristics. However, the structure of the dc motor is complex, the field winding on the stator produce a magnetic field, the power of the larger dc motor is often also equipped with commutating pole, in order to improve of the reversing of the motor performance. Dc motor armature winding and been planted on the rotor commutator, complex structure restricts the volume and weight of the dc motor, especially for brush and commutator of the sliding contact caused by mechanical wear and spark, made of dc motor fault, low reliability and short service life, maintenance workload. Commutation spark both caused the commutator electric corrosion, or a radio interference sources, will bring harmful impact on the surrounding of the electrical equipment. Motor capacity, rotational speed is higher, the greater the problem becomes more serious. So, common dc motor brush and commutator limits the dc motor to the development of high speed, large capacity.
3.3.2 rainfall distribution on 10-12 ac asynchronous motor
Compared with the dc motor, ac asynchronous motor has simple structure, reliable operation, long service life, low cost, easy maintenance, etc. But its control performance is poor, small starting torque, overload capacity and low efficiency. Its produce a rotating magnetic field need to absorb reactive power on the grid, so the power factor is low, especially when the light load, which greatly increased the lines and the wastage of the grid. For a long time, without requiring speed, dominant asynchronous motor, but since ac motor variable frequency speed control system developed, the occasion of ac asynchronous motor can also be used to control.
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