Mrtomobile suspension assembly known as a Crosstube. They are considering moving from manual to robotic production in either a single cell/single robot or line/multi robot layout. The following report recommends the use of the Fanuc Arc Mate robot, which is a compact, six axis, modular-built, electric servo-driven robot which is controlled by the a R-J3 controller with ArcTool application software. All ancillary equipment options are considered.
The two layouts are considered and all health and safety implications are detailed. The report concludes with a discussion on the validity of each layout. For low volume manufacture it is recommended that a single-cell layout is utilised. For economic production of high volumes a line layout would be the primary choice.
3.0Specification Of Ancillary Equipment
BL Cars Ltd manufactures a part of an automobile suspension assembly. It is known as a Crosstube. The complete part consists of four thin sheets of steel presswork. Two parts are identical and are positioned either end of the larger cross brace, which has been formed from two larger pieces of sheet steel..
The diagram below illustrates the complete assembly:
Presently the two parts making up the cross brace are secured using specific jigs. Once secure they are spot welded along the flange edges. The next step is to secure the two mounting plates to the presswork. The process is completed by arc welding the component to add extra strength to the spot welds and secure the mounting plates. N.B. When the parts are pressed from the sheet steel locating points are incorporated. This ensures each part is located in the correct position in the jig.
The use of industrial robots is ever increasing. The robot usually does a secondary machinery operation after a primary shaping operation. When performed manually, this secondary operation often requires the operator to work in an unpleasant environment, to carry out a repetitive and tiring task with bulky tools.
BL Cars Ltd is considering using robots to automate the Crosstube assembly process. The robots will be used for material handling, spot and arc welding in one of two contrasting layouts:
1.Single-cell, single robot – low volumes
2.Line-layout, multi-robot – high volumes
Compared to manual welds, robot welds provide the following benefits:
Increase arc on time as a proportion of total time
Automatic weld quality monitoring.
Compared to manual handling, robots handling provides the following benefits:
BL Cars Ltd have decided to use the Fanuc Arc Mate 50il. This robot is a compact, six axis, modular-built, electric servo-driven robot which is controlled by the R-J3 controller with ArcTool application software. The robot has been designed to maximise throughput and use of floor space by providing a compact, high speed design in a proven and reliable mechanical unit.
The robot has six axes of movement. The diagram below illustrates axis movement:
The diagram below illustrates the floor space required for mounting:
The majority of MIG welding jobs are conducted manually. It is relatively recently that machines have come on to the market with sufficient positional repeatability to automate the process reliably and flexibly. Arc welding using MIG is a complex multi-variable control task. Therefore is imperative that an effective controller is chosen.
The controller BL Cars LTD has chosen to use is the R-J3 shown below:
FANUC Robotics’ SYSTEM R-J3 Controller uses advanced technology packaged in a proven, reliable third generation controller design. Process capability and open architecture features improve application and motion performance while simplifying system integration.
SYSTEM R-J3 incorporates FANUC Robotics’ unique “plug-in-options” concept which allows the flexibility for applications specific configurations while maintaining a commonality for all users of the system.
32-bit main CPU with dual processor architecture permits fast calculations, reduces program execution times and increases path accuracy
Provides extensive line of compact I/O modules for both digital and analogue signals
Allows for fast power-up and program execution with auto resume after cycle start
High-speed, precision control of up to 6 axes of motion
Auxiliary axes options can support up to three separate motion groups, each with its own control program and simple kinematic models
Advanced communications and networking capabilities include built-in Ethernet and PCMCIA interface
AccuPath provides enhanced path tracking during linear and circular motion while minimizing speed variations
Collision detection minimizes potential damage to the robot or end-of-arm tooling
Collision detection minimizes potential damage to the robot or end-of-arm tooling
Use of surface mounting and 3-D packaging reduces components and increases reliability
Quick change servo amplifier reduces maintenance time
Distributed I/O options reduce cabling costs and simplify troubleshooting
Increased use of fiber optics simplifies connections and enables faster communications
Increased use of fibre optics simplifies connections and enables faster communications
Multi-tasking operating system allows execution of several concurrent user programs
Instant trigger response (*4 ms) increases repeatability and improves process control.
Reduces capital expenses: -Modular package allows rotator to be added only if needed -Eliminates expensive tools typically required to trim parts -Reduces number of robots required to achieve a target throughput
Increases throughput and quality up to 35% over competitive robotic tracking systems and manual welding
Adaptive well bias optimises weld wire positioning to fill lap joint gaps without reducing travel speeds (100 inches per minute)
Root Pass Memorization (RPM) allows users to perform multi-pass welds without retracking.
3.0Specification Of Ancillary Equipment
1.Use senseless robots and relatively precise tooling, better jigs, and more efficient transportation. This method reduces dimensional variation.
2.Use a robot and sensor system to locate the start and end points of each weld. This method also traces the seam steering the weld torch, and as a result reduces the error.
Precise tooling is very expense. Taking into account capital and recurrent costs BL Cars will utilise robots with sensors.
‘The MIG EYE seam tracking sensor equips FANUC Robotics’ welding systems with a laser-based tracking sensor. Designed specifically for arc welding sheet metal stampings, MIG EYE locates weld seams and provides real time control of the robot’s path to maintain proper alignment between weld wire and joints.’ The aim of this equipment is to maximise the quality and the speed of the weld.
Assuming BL Cars Ltd choose to integrate this tool with an ARC Mate robot and Arc Tool software, MIG EYE reduces total programming time, enhances uptime and increases throughput for a wide variety of welding systems.
The illustration below shows a tool which incorporates a sensor and MIG welder.
Figure 2 torch/sensor package (57mm Diameter)
To automate the line layout a handling tool is required. The tool will be used to hold the job whilst it is welded. It is necessary to consider the shape, orientation, weight, and the centre of gravity of the part. Once these variable have been determined an appropriate gripper can be selected.
For photograph of chosen tool, and further information on handling tool application software please refer to appendix 1.
A robotic assembly cell is an independent unit consisting of one or more robots and associated peripheral equipment, by means of which a complete product can be assembled as far as possible.
Relatively large number of various parts assembled per robot
In the automotive industry the ratio of assembly cells to assembly lines is approximately 2:1.
The diagram below illustrates a simple cell layout.
A line layout comprises two robotic stations which are installed in series. The cycle times are relatively short, and also a limited number of parts per station and product transport between stations.
The diagram below illustrates possible line layout:
Set-up costs are greater in a line layout since there are two robots and extra ancillary equipment. However all line layout is capable of significantly higher throughput levels. Therefore if high volume manufacture is required a line layout will be the most economic option, providing a lower overhead cost/unit.
When implementing any form of system within a manufacturing environment it is imperative that all health and safety implications are taken into account and the appropriate precautions are incorporated.
The majority of accidents involving robots have occurred:
1.Whilst an operator is programming
2.Whilst conducting experimental robot sequences
3.Whilst an operator is adjusting and/or maintaining peripheral devices
4. As a result of poor provision or installation of safety equipment
From the above information it is apparent that when setting up a robotic cell it is important to consider
1.Operator safety whilst programming and/or maintaining the robotic system
With robot systems, the type of robot, its use and its relationship to other plant will all influence the design and selection of safeguards. Any safeguard chosen must permit the required operations to be conducted and where necessary, teaching, programming, setting, maintenance and trouble-shooting operations to be carried out.
Prior to safeguard design all hazards and injury risks must be identified. In this case a single robot cell will require a different level of safeguarding to a multi robot line layout.
Perimeter fencing will be used to protect all members of staff from the equipment line. The fencing will consist of a hollow section steel framework in filled with mesh. All sections will be 2m high and securely fastened to the framework
Where there is a danger of molten metal and welding flash infringing regulations, the filling between the framework will be manufactured from sheet steel. A sliding access gate will be located either side of the line to allow for regular access. A presence sensing device will be linked to the control system to instantly cut power to all machinery within the restricted area.
An operator will be working with the robot within the cell. Therefore it is important that suffient safety devices are incorporated within the cell to ensure operator safety.
Four emergency stop buttons will be strategically positioned in and around the cell to allow emergency shut down in the event of a problem.
An escape path and guidelines will be displayed outside the cell to highlight to the operator the action that should be taken if the robot goes out of control.
Pressure mats will be placed around the robot. When pressure is applied to the mat the system is shut down. An alarm system will also be incorporated within the system to indicate human presence.
The operator within the cell will always have ultimate control over robot activity.
Infra-red curtains are flexible and reliable and make an excellent method of operator protection.
Whilst operating under normal conditions its is unlikely that a robot will require manual human input.
In addition to the general safety precautions, it is necessary to consider the possibility of the two robots damaging themselves and other equipment. This will be achieved by using positive stops which limit the movement of the robot to part of its envelope. Trip devices will be utilised to stop the robot if it comes into contact with people or other equipment
A trapped key exchange will be used to secure all gates. This system will positively isolate the power supply to all parts of the robot installation. There is a lock on all perimeter gates and a lock on an isolator controlling the robot actuators. The key cannot be removed from the lock to open the gate unless the system is in a safe state.
The crosstube is mass produced, i.e. high volumes are required. The most cost effective layout choice would be the line-layout. Although higher set-up costs are involved initially the cost of implementing cells with an equivalent rate of throughput would be greater.
However it is likely that this product will move into decline and the number of these products demanded will fall. When this occurs the line layout could be abandoned, the space utilised for a part in growth, and a single cell could be established for dedicated production of the cross tube.
BL Cars Ltd must consider using robots cell for the economic manufacture of prototype and growth products. As growth slows and a product is nearing maturity a line-layout can be considered (volume determined)This method of assembly would be partially useful in the decline period of the products life cycle, as the cell would be capable of producing other similar parts or parts within the product family.
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