[机械模具数控自动化专业毕业设计外文文献及翻译]【期刊】多功能的六自由度机器人-外文文献

A Versatile Six Degree of Freedom Robot C. B. Besant, K. J. Gilliland, M. Risti and L. P. Williams Department of Mechanical Engineering, Imperial College, London S W7 2BX, United Kingdom ThLs paper describes a prototype six degree of freedom 8kg capacity robot constructed to meet the needs of robotics research at Imperial College. With ,itture robot requirements in mind, the robot control system was developed in collaboration with R.D. Projects Ltd. 1. INTRODUCTION The market for robots capable of pick and place operations and continuous path applications such as spray painting, welding, grinding and polishing, is well established, whereas assembly operations still represent a future major growth area for the robotics industry. The latter follows from the fact that at present assembly operations constitute a major proportion of the use of human labour in manufacture. An estimated 40% of the working population in engineering is involved in assemblytq and therefore potentially considerable cost savings could be made by automation in this area. Furthermore it has been observed that the majority of components in such work are relatively small and light, for example over 90% of the components in a car weigh less than 3 kg.itl The Institute of Mechanical Engineering and Imperial College have performed market surveys I8.91 to determine the most required robot sizes and capacities. The indication from these surveys are that a robot of around 8kg capacity with a reach of 1.2m would have a large market. Furthermore, there is the requirement that such a robot should have a powerful programming language in order that it can be quickly programmed to perform a wide range of tasks. This requirement comes from the fact that in many applications the batch sizes are relatively small. For example, Groover 71 states that 75% of all engineering products in the UK are manufactured in batch sizes of 50 or less. In order to be economically justifiable, automation of batch manufacturing must obviously be much more flexible than dedicated hard automation currently in use in mass production systems. The flexibility required can be illustrated by the needs of Flexible Manufacturing Systems where robots have to work in a close and co-ordinated manner with other machines such as CNC systems. A second example of the need for great flexibility lies in assembly applications where a robot will need access to sensors such as vision, force and tactile. Thus future robots will require control systems with more intelligence, than most existing commercially available robots, with the ability to communicate with other systems and have the capability of expansion of computational power to cope with sensor technology. With future robot requirements in mind, a robot control system was developed in collaboration with R.D. Projects Ltd. and a prototype six degree of freedom 8kg capacity robot constructed to meet the needs of robotics research at Imperial College. Some of the research areas are real time dynamic compensation for accurate path following, adaptive control for deburring, assembly task applications with new intelligent grippers and the use of AI languages for collision avoidance. 2. DESCRIPTION OFTHE MECHANICAL ARM The configuration chosen for the robot was anthropomorphic on the basis that this configuration offered a good compromise between adequate working envelope, good The International Journal of Advanced Manufacturing Technology, I (3)75-107, 1986, )IFS (Publications) Ltd, 0268-3768 75 The International Journal of Advanced Manufacturing Technology strength to weight ratio and finally ease of construction. The specification was determined on the basis of the market surveys 8.91 and on the planned research applications for the robot. It was necessary to specify a six degree of freedom robot to meet the needs of some of the continuous path applications such as welding and plasma metallic spraying. Furthermore, some assembly applications also require up to six degrees of freedom. The market surveys indicated that a maximum load capacity of 8kg at a reach of 1.25m offers a solution to many applications for robots. At the start of the project a maximum gripper speed of l m/s with a maximum acceleration of2m/s 2 was specified although these values were later uprated to 2m/s and 4m/s 2 respectively. Accuracy and repeatability definitions for robots often promote much discussion. The actual position achieved by the end of the arm in a series of movements to a programmed point will be subject to statistic variation. Positional accuracy is usually taken as the mean deviation from the programmed point or as the value within which 90% of the points achieved lie. Similarly repeatability can be defined as half the range or the maximum deviation from the mean. A target accuracy specification of 0.1 mm was laid down for the robot, giving a repeatability of 0.05mm. Although this value of repeatability may not be adequate for some assembly applications, it was thought that this limitation could be overcome by an intelligent gripper with compliance capability. Six-axis robot arm f2 Hand-held teach pendant IEEE 488 bus to intelligent peripheral Interlink cables RS232+power Terminal RS232 Rack with axis cards and UO interfaces Host processor (3 0 Fuses Digital l/O to dumb peripherals Robot controller i Keyswitch 1 . Rack PSU / Servo amplifiers m Interlocked - cable II Single phase Three-phase - , ower box power box p / sAXitSc ihnehi bit ,/ Figure 1. Hardware organisation. 76 A versatile six degree of freedom robot 2.1 Mechanical Design of the Arm The mechanical design was undertaken by Bamforth and Parr-Burman/l as part of postgraduate and undergraduate project work. The anthropomorphic type of design was chosen at an early stage because, in general, it offers the following advantages over other types: 9 A mechanically simpler design results since rotary bearings can be used which are easy to seal. O A large working volume can be achieved for its size, giving good reach and therefore flexibility in use. O It is easier to achieve a stiffdesign than with most other types. The overall configuration chosen for the design is shown in Figure I. It consists of a lower arm consisting of the wrist at one end and the drive motors for the wrist at the other end. The wrist is compact and constructed using proprietary bevel gears. Drive to the wrist is by three 200W Mavilor DC servo electric motors via 80 : 1 harmonic reduction gearboxes. and three concentric steel tubes. Mounted on the back of each motor is a tachogenerator for velocity feedback and a 1000 pulse per revolution optical encoder for position sensing. The wrist has the following angular movements: Rotate 360 Pitch 210 and Roll 360 Each axis of the wrist is capable of a maximum angular velocity of 180 The lower arm or forearm is mounted at the elbow joint, at its centre of gravity. The upper arm pivoted at the shoulder which is at the top of the trunk. Rotation of the robot about the elbow and shoulder is powered by two Mavilor 700W DC servo electric motors via precision ballscrews and the conventional parallelogram linkage system. This arrangement, although it limits the angular movements about the elbow and shoulder joints, does give stiffness about these joints with no inherent vibration. Use of a harmonic gearbox to give a direct drive onto these joints does permit greater angular movements as well as motors with less torque than with an equivalent ball screw system. However the harmonic drive gearbox does have less stiffness than a ballscrew and at certain speeds the natural frequency mode can be excited resulting in arm vibrations. The motors powering the elbow and shoulder contain an electromagnetic disk brake, a tachogenerator and a 1000 pulse per revolution optical encoder in a piggy-back formation. The base unit supports the trunk on a slewing ring. Rotation of the trunk is achieved by a 700W DC motor unit with brake, tachogenerator and encoder similar in layout to the shoulder and elbow units. The motor drives the trunk through a harmonic reduction gearbox. The working envelope for the elbow and shoulder axes is indicated in Figure 2 and rotation about the base is 360 . Limit switches are situated on all axes and act as datum positions as well as limits. 3. THE CONTROL SYSTEM 3.1 Overview The control system is the link between the robot programmer and the robot itself. Conventional robot controllers allow the user often sophisticated control of robot actions via a teach pendant and programming language but are often inaccessible in terms of modification. In a research environment this is a great limitation on the work that can be carried out. (Far more can be achieved with a robot and control system built in-house since both can be tailored to the requirements of the research work and all technical specifications are readily available.) The controller of ttle present robot is based on a distributed network of microprocessors which perform various functions in parallel and communicate with one another via an industry standard bus under instructions from a host processor. In this way computing power is achieved through the use of several, inexpensive processors instead of a single, powerful, 77 The International Journal of Advanced Manufacturing Technology ,t ! I I Figure 2. Working envelope (shoulder and elbow). expensive processor. The use of a well documented industry standard bus (IEEE 488) means that further intelligent devices can be made to communicate with the controller without specialised interface electronics and major hardware alterations. The robot language runs on a Motorola 68000 based SAGE II which also acts as the controlling device for inter-processor communication. All other intelligent devices currently on the bus are 6809 based and at present consist of six-axis control cards. These perform the servo loop closure for each axis and pass analogue demands to the axis servo amplifiers to move each axis. The controller also contains simple interface electronics for digital input/ output between the controller and robot or other dedicated equipment. The axis servo- amplifiers and all power supplies are housed in the control cabinet. The main transformer which provides power from the three-phase wall supply is housed separately. A hand held teach pendant allows the robot to be manoeuvred and taught by an operator under joystick control and menu driven software. Cabinet temperature is controlled by an air conditioning unit. Figure 1 shows the general arrangement of the hardware while Figure 3 shows how the various devices communicate. 78 A versatile six degree of freedom robot r dcw 61110I lJ P Intcri;tcc i TM To peripherals - t IEEE 4i hu Additional intclligcnt -, r -!-1 . r-t-7 II Gripper I I Force I | conlro |I J sensing J _J L_J Encoder signals Velocity feedback from acho Molor power L.mtl witch slalus Brake eomrol I Figure 3. Interdevice communication, 3.2 Power Supplies and Interlocks The three-phase power box is supplied from a wall mounted switchbox and is equipped with interlocks so that no mains voltage appears at the cabinet interlink cables unless the cable is properly connected. Contactors within tile power box isolate the three-phase transformer if the keyswitch is off, the cable is disconnected or the three-phase power box cover is removed. Under these circumstances the only accessible live supply is a low voltage interlock circuit which can be considered harmless. All three-phase wiring in the power box conforms to lEE regulations. Provided that the power box connections are correct and the keyswitch is in the on position, the cabinet will receive both single and three-phase supplies. These are: 240 vac single phase- termed auxiliary power 90 vac three phase 18 vac three phase - termed mains power The mains power supplies are switched via contactors within the cabinct and are fused, rectified and smoothed to provide high current DC supplies to the serve-amplifiers. Tile auxiliary supply is the source of all other robot and controller supplies. The various supplies and their sources are shown diagrammatically in Figure 4. When auxiliary power is available the only supply that remains isolated is the mains supply to the amplifiers. Without this supply the robot cannot be moved under power so a number of interlocks must be complete before it is made available. All emergency stop switches must be in their sale position, all robot interlink cables must be correctly connected, the rack PSU must be switched on, the teach pendant must be connected and all cabinet panels must be secured before a motor receives power. The low voltage interlock supply is derived from the same source as the brake and limit switch supply thus ensuring that loss of brake voltage at source automatically disconnects the motors from drive current. All safety circuitry is designed to be thil-safe, that is, current must flow in a circuit if it is sale. This method ensures that dangerous equipment cannot be operated unless the safety 79 The International Journal of Advanced Manufacturing Technology interlocks themselves are functioning correctly. Broken wires, intermittent faults due to dry joints or power supply failure in interlock circuitry result in robot shut down. These interlocks are designed to protect the operator from dangerous supply voltages that may be exposed within the cabinet or at the ends of cables and physical harm inflicted by the robot should it not be under complete control. In this way it is impossible to servo the robot without complete servo loop closure or some rapid means of removing power. The brake supply is switched by the same contactors that switch the mains supply in the cabinet. This means that a motors brake cannot be released unless the amplifier has power with which to servo the joint. This is very important since gravitational forces can easily back-drive the high efficiency ball screws when no brake is applied. Front panel switches are provided which enable the operator to inhibit selected axes while others are free to run. Here again, safety is built into hardware. If an axis is inhibited manually, its brake is applied regardless of software demands. All DC supplies are single ended to avoid possible problems caused by multiple ground loops. This also tends to reduce the number of wires required to connect the robot and controller. There is a small amount of circuitry located within the robot itself, mainly associated with the optical encoders. The encoders output TTL level signals which are transmitted to the cabinet some metres away. To reduce the effect of electrically generated noise on the signals during transmission, differential line drivers and receivers are used. Both these devices and the encoders require a stabilised supply and this is achieved by regulating the supply from the rack at the robot. A similar procedure is carried out to produce reliable supplies within the teach pendant which is also remote from the cabinet. I inlerhk I SAGE Auxitiar Wdll ,upfll I 415 vac I, I I I o&,: . 2.1 vdc I l.imil ,llchcs. brakes, inlc rik npt, llO I vdc i Rck TTL v I contct+ I Lamp upply S,tch ml PSU I I I I 14 vd - 15 dr 15 v 5 vdc ,I 5 vde Pcnd;int 5tlpply i ,uppl I I vdc I umplifle= (MOS Figure 4. Power supplies layout. 80 A versatile six degree of freedom robot 3.3 Controller Rack The rack is where most of the intelligent devices in the controller reside. With the exception oftacho feedback, motor drive and pendant communication all I/O signals pass through the rack. It is from here that amplifier demands are issued and all brake, limit switch, I/O and encoder signals are processed. The rack has its own in-built switch mode supply and can be removed as a unit from the controller simply by unplugging inputs and outputs. The rack accepts standard double eurocard size boards and DIN indirect edge connectors. An IEEE 488 link is bussed to all card locations in the rack for use by intelligent devices. It is here that the six-axis cards reside together with various digital 1/O interface boards. 3.4 General Purpose Axis Control Card An axis card is required for each driven degree of freedom and its main purpose is to close the position servo loop with a suitable control strategy (see Section 5). IEEE communication protocol is handled via Texas Instruments interface devices. These enable the on-board Motorola 68B09 microprocessor to receive commands from the host processor. In this particular application the axis cards are not fully populated. The axis card is a general purpose axis control card and contains many features that are redundant for simple servo loop closure, full address decoding provides the microprocessor with access to eight I/O devices, 2K bytes of RAM and up to 8K bytes of EPROM. The I/O devices provide the following functions: O 16 digital I/O lines O Six programmable down counters O One 12 bit digital to analogue converter O One eight input, eight hit analogue to digital converter O One IEEE interface O One hardware interpolation chip O One voltage controlled oscillator In this application the D/A converter is used to set the demand to the servo amplifier. A peripheral interface adaptor receives signals regarding the status of limit switches and brake for that particular axis. It is also used to drive card edge LEDs which indicate axis card status. Joint position is determined in terms ofencoder pulses by comparison of the values held in two of the programmable down counters. Software in EPROM can then be made to execute classical PID control to close the servo loop. 3.5 Encoder Signal Decoding Axis position feedback is supplied via 1000 line optical incremental encoders mounted on the free-end of each motor. These encoders provide two TTL compatible square w