Presentation NotesRobot Cell Design Nyasha Mukudu and Romeo Samhungu . loading and unloading of parts. This arrangement was organized because in 1960’s a single robot was used to perform only one function. either carrying out a production process or examining a production machine. Robot-centered cell was introduced to maximize the usage of robots. and 3) Mobile-robot cells (generalization of in-line robotic cells and robot-centred cells) (Logendran and Sriskandarajah 1996). a robot remains inactive for a longer period. During this process. In this arrangement. the robots were mostly employed in several operations like die casting. As a result. a single robot will be incorporated at the center of the work cell for performing operations on several machines that are set in a semi-circle form. etc. Robot-centered cell: Robot-centered cell is one of the commonly used layouts in the industrial applications. where the usage of robots was very low when compared with the machine usage times. instead of examining the production machine. Three different cell layouts have been examined. In this process.Robot cell design A robotic cell is a manufacturing cell in which loading and unloading operations are performed by robots. This inequity in performance was highly occurred in the metal machining operations. 2) in-line robotic cells (where the robot moves linearly). Those times. a robot is used to perform the production process. This type of work cell is shown in the below figure. Application of Robot-centered cell: Arc welding operation is an application of the Robot-centered cell. namely: 1) robot-centred cells (where the robot movement is rotational). Human workers are also employed for performing the part loading and unloading tasks. conveyors. the robots are placed beside the assembly line to carry out some operations like spot welding. This conveyor travels near the robots for performing a function on the work parts. It can therefore be concluded that changing the robotic cell layout from an in-line to a robot-centred cell can improve the effectiveness of these systems. The electrode will emit sufficient electric current. To be more specific. In the in-line robot cell design. These devices will place the work parts in a preferred area for appropriate lifting. It is a continuous process used to weld long joints. This type of arrangement is mostly used in the automobile industries for assembling the car bodies. which will be in a range of 5 – 500 amps or 10 – 30 volts.Additionally. and other devices for providing the work parts to the cell. anyone of the following transfer system can be used: Synchronous or intermittent transfer system Continuous transfer system Non synchronous transfer system It is generally known that robot-centred cells are preferred in practice because they reduce the required physical space. In-Line robot work cell An In-Line robot cell arrangement has a movable conveyor with the work parts. What is arc welding The purpose of an arc welding robot is to fuse two metals by creating an electric arc between the work piece and electrode. this operation requires pallets. . Design of mobile robot work cells: If a robot examines several machines for a longer period. During this process. which causes less usage of the robot. As a result. the robot will be mostly employed in the tasks than being in idle times. A mobile robot cell has a problem in its design for identifying the best possible machines that a robot requires to examine. It is said so because each robot should wait until the process gets finished in the production machine. this system allows the robot to move equivalent with the path of work parts. Continuous transfer system: The continuous transfer system transfers the work parts continuously at a constant time along a conveyor. Non synchronous transfer system: . If this process is not applied. the robots are arranged to move at different places in the cell. this problem helps to increase the machines in the work cell without providing any idle times to the machines. Moreover. this system uses the manipulator for tracking the movable work parts. two things can be described: The overhead rail system looks to be the best one in terms of floor space. tracking refers to the ability of a robot to sustain the locations of the programmed points relative to the work part even during the movement of the conveyor. It is made possible by attaching the robot in a movable bottom. Here. This system can be done by either anyone of the following methods: Overhead rail system Floor track system When comparing the floor track system and overhead rail systems. The cost for constructing the floor track system is very lower than the overhead rail system.Mobile robot work cells In the mobile robot work cells. The continuous movement of work parts makes the robots tough to operate in a fixed position. A stationary baseline tracking system: In this method. Hence. The reason is that it requires less floor space than the floor track system. Instead of moving the robot. the robot is placed in a fixed position along a line. the traveling position of the robot and work parts will remain the same. which is connected to the rail system. there are two tracking system that helps to solve this problem such as: A moving baseline tracking system: During an operation. then separate robot will be required to perform this process. then mobile robot work cell will be the perfect choice. With the help of start and stop motion. this system also moves with a start and stop motion. Apart from in-line robot cell. there are other two robot cell arrangements such as robotcentered cell and mobile robot cell. Sometimes. Synchronous or Intermittent transfer system: In this transfer system. there is the possibility of uneven arrival timings. Therefore. The robots are placed in a fixed position near the moving line in which the work parts travel. As like synchronous transfer. . every work part is transported at the same time from one workplace and stored in another workstation. This transfer system is also called as power – and – free system. the work parts are stopped next to a robot for performing the operations on it.The non-synchronous transfer system allows the work parts to transfer separately along a conveyor. sensors are used for specifying the starting time of the process to a robot. is important for efficient operation. Skipping any one of these tasks. can lead to the abortion of the entire installation. completing each task sequentially. i. in order to improve the installation plan. The primary flow through this chart is along the large arrows. the layout of the machines on the factory floor. the group of machines which includes a robot and all other machines with which the robot works directly is called a robot cell. The design of the cell. efficiency means keeping the most expensive machines busy doing useful work for as much of the time as possible. In automation. At each stage. Design Sequence A set sequence of tasks should be followed to ensure the successful installation of the robot work cell. This particular chart was first illustrated by Asfahl [1]. remembering that whatever can go wrong will go wrong.In automated manufacturing. This sequence can be best described with the aid of a flow chart depicted in Figure 1. all of which can be aborted at any time. The essential point to this chart is to plan for every eventuality.e. . it is possible to reconsider the last or even first task. At this point. The workforce in a factory are the most important resource and should not be overlooked. If the work force sees the robot work cell as an aid to their work. planning. . then they are more likely to accept it. the education of these figures is essential for the overall acceptance of the new work cell. The first stage. The key is. union representatives and some of the labour force whom the robot will replace. where possible. or displace. can be further subdivided into smaller tasks as shown below in Figure 2.Planning The planning stage of a robot work cell design should involve important engineering personnel. re-educate the current work force in the use of robots or if this is not possible ensure that the jobs that they are displaced to are seen as an improvement over their currentctasks. or that none of the tasks are suitable candidates for the introduction of a robot cell. many of which given fit into the following list: -Spot welding -Injection moulding -Arc welding -Machine loading -Assembly -Surface coating . Typical industrial applications of robots were discussed in previous lectures. It may become apparent at a later stage in the planning process that the chosen task should be disregarded in favour of another application.Application Isolation of the application requires a study of the current manufacturing operations to identify a suitable candidate for automation. -Handling and palletizing -Press loading -Grinding and deburring -Die casting -Inspection -Gluing and sealing -Casting. . and -Laser cutting. What effect will a power cut have. More likely reasons include increasing current productivity. If the quality of the parts used in assemblies deteriorate. Bad for one employee. food production etc.The tasks involved in the applications help the designer to decide which robot is suitable. even if some of the employees are displaced during the transition. will the system cope? . i. but hardly a productive approach for a manufacturer. If the robot work cell is down can production be maintained at a reduced rate by manual techniques? 4. This influence can be: 1. the final robot cell may increase the production rate. and realise the restrictions of current technology. work volume. it is easy to see that number four is the most beneficial.e. an hydraulic drive system would hardly be suitable for a ‘clean’ environment i. The most beneficial displacement. It may be to ‘try’ new technologies. The company must also consider the impact of the proposed cell on the current production facility. These objectives should then be realistic. but good for most employees 4. why does the company want to introduce a robot work cell. the downtime required for the introduction of that cell may eliminate any possible cost benefit from the increased production rate. either process end-effectors or grippers. will the robot be able to reset. The next step is the consideration of the possible benefits of introducing a robot cell. speed. The task also determines the type f end-effector.g. Caution should however be exercised when stating the project objectives. but bad for employees 2. This is also beneficial to the company by increasing productivity through reduced sick leave. will the payload be dropped. is the removal of possibly hazardous tasks. Performance characteristics that influence the choice of a robot arm include payload. only replace the human were the labour is almost robotic. will the robot halt immediately? 2. If required can the robot cell be removed in favour of manual methodslater? 3. a valid enough reason. However.e. consideration of their new role in the company may result in a benefit to the employee/employees. One of the first questions should be. and repeatability.. Other considerations include the robot drive system. reducing labour costs. e. Of course. Each of these reasons can then be used to describe the objective or objectives of the project. or improving working conditions. and the possible drawbacks of such a move. If current production is nearing the end of the products life then the introduction of a robot cell may be questionable.e. 1. Possible Drawbacks The introduction of a robot can influence individual workers and the entiremanufacturing operation. and perhaps the easier of the four to implement. Other drawbacks can be found in the listing of what-if scenarios. Good for the company. simply trying to reduce labour costs by the removal of the workers may ultimately result in the failure of the project.. i. from the employees viewpoint. Good for the company and good for all employees. Possible Benefits The possible benefits of the proposed robot work cell should be determined at an early stage in the planning process. clean rooms. Bad for both company and employees 3. reducing cycle times. From this list. It may have become apparent from the documentation process that current robot technology is not sufficient for the completion of the task. speed. Manual production is used for low production rates and requires only a small initial capital investment. However increasing the production rate of a hard automation system reduces the unit cost in a linear fashion. The main factors here are payload. manual operation times. maintenance histories and quality records. hard automation may be the better choice. the robot system illustrates a non-linear reduction in cost as the production rate increases. The cost of producing one unit will remain invariable as the worker can only produce a single unit in a set time regardless of the overall production rate. The latter can be used to produce a what-if scenario list in the case of maintenance histories for current machinery while the quality records highlight problems with the current manual methods. Finally. The non-linearity can be attributed to the requirement for additional equipment as the production rate increases. i. work volume. The documentation should include the cell layout. the detail required to make the final decision did not exist until the operation was fully documented.e. the current manual task must be well documented. Robots are used for mid range production rates that require a slightly higher capital investment. Hard automation is used for high production rates incurring a high initial capital investment but ultimately resulting in a reduced unit cost. and repeatability. If on the other hand the product life cycle is long and the production rate is high. The steps also illustrate major changes to the system through the introduction of more robots. Much of this work would have been carried out when deciding upon the operation however. . Hard or Flexible Automation This decision is part of the robot feasibility review. A simple graph shown in Figure 3 illustrates how the unit costs will vary with the production rate and method used. dexterity required for handling awkward parts. Unlike hard automation.5. machine cycle times. Is there sufficient flexibility in the robot system for alternative tasks if the current production task is eliminated? Document Current Operation To detail the task that the robot will ultimately complete. Scrutiny of the current task will reveal any possible problems for the robot. Robot Feasibility Review Now that a detailed description of the current manual operation exists the suitability of the operation for the introduction of a robot work cell must be evaluated. It may be that the product life is short and therefore flexible automation that can be adapted to other products will be more cost effective than a highly specialised production process. but result in lower unit costs. Information gained from this study will be used to plan the robot task and evaluate the final production operation. however requires additional equipment for a robot work cell. The complex interaction of all these parameters results in an iterative approach to the cell design. The interaction of the robot with the conveyor system must also be analysed. tooling. For instance. This CAD/CAM approach is a useful and cost-effective design technique for the production of the cell layout. These accurate descriptions can be used to position the tooling relative to the robot. e. The majority of these systems are used by consultants and robot manufacturers who provide a tailor made system. Simulation packages also provide facilities for collision checking. The flow chart shown in Figure 3 shows how the design process can still be completed effective without the used of complex simulation packages. including the work volume. motion timing and off-line programming. including details of the machines in the work cell as well as the work cell layout. The current layout may have to be modified in favour of the new robot system. This iterative part of the design process is illustrated in Figure 2. This diagram illustrates how major elements including services. Robotics companies . a decision as to whether the robot manipulates the part or processes the parts is required. robot work volume. the manual handling of screw fasteners is not a problem. the supply of parts may require some form of buffering before the robot transports the parts. The conveyor system may also have to be moved to accommodate the robots work volume.Proposed System Layout The current work cell used for the manual production task has been well documented. If the part is to be processed. The advances in the robot simulation software’s now enable cell designers to call upon kinematically accurate drawings of robots. conveyor system and work cell perimeter can be designed and visualized within a simulation package. providing a visual and mathematical check of the cell layout as shown in Figure 4.g. also provide CAD files of their robot product ranges that can be used to design the cell layout using standard CAD software’s. but near enough to the robot to allow loading and unloading without demanding maximum reach. Regardless of which system is used to design the cell layout. a radial layout is common. Tooling Radial Layout Most industrial robots have a revolute waist joint. Each machine is far enough away from the robot to allow the robot to move around the cell. there exist standard work cell layouts. and for these machines. . All the machines with which the robot has to work are arranged in a circle around the robot. Figure 5. A robot work cell for the machining of truck differential castings is shown in Figure 5. The parts are then transferred in an anti-clockwise radial path finally arriving at the out conveyor. Here. . A radial layout of a robot cell may include smaller robot cells within it as shown in Figure 5. which transfers the machined castings to the next work cell. The path the component takes starts with the indexible conveyor system which only increments each time a component is lifted and transferred to the first boring machine. This particular configuration uses a radial layout of machines around a cylindrical robot. it is important that jigs on which work is mounted can be reached by all the robots that require access. The layout often includes feeders around the outside of the cell so that they can be accessed by operators without having to enter the robot cell. A typical in-line work cell layout is illustrated in Figure 8. or the use of software to locate a new world axis relative to the conveyor. it is possible for a robot to access many machines arranged in a straight line as shown in Figure 7 (a). as shown in Figure 7(b). and when all robots have finished the conveyor indexes to the next position. There are two methods used for this task. A further consideration for this configuration is the possibility of collision.e. for example painting. or by putting another configuration of robot on rails. The latter method reduces the work volume available to the robot to accomplish the task. Robots may work on parts mounted on continuously moving conveyors. Robots frequently work on parts mounted on conveyors. an additional rectilinear motion at the base. Since the robot work volumes overlap timing is critical. Usually the conveyor indexes. i. .In-line Layout By using a Cartesian robot. the conveyor stationary while the robots are at work. This arrangement is suitable when accuracy is not critical. Robot Safety There are two viewpoints on robot safety. If the arm is not moving. and is a major safety issue for hydraulic machines that are capable of . A slow moving arm will continue to move slowly 4. The third assumption does not apply to robots in general. The fence has been added to this system in the interests of safety. The robot cell shown in Figure 4 has a perimeter fence that houses the entire cell. or the operators safety. The arm will obey all of the commands given. Safety is a major consideration in the design of a robot work cell. either that of the machines safety. the robot path may only be repeated a few times before the robot commences a second operation.Other robot cell layouts exist. The arm will continually repeat a single pattern of movement 3. the majority of which are simply variations on a theme. There are several assumptions on the part of the operator. it is not going to move 2. which cause accidents [5]: 1. both of which must be considered. The second assumption is incorrect. Several ways exist for the operator to become trapped by the robot including trapped between. A typical robot cell is shown in Figure 9. A pressure sensitive mat is used at the loading side of the robot cell to determine if the operator is near the rotary fixture. the links of the robot arm. A robot can cause injury from impact in two ways. There are several sources of hazards to the operator from the application itself and the robot. The back of the fence has a gate to access the robots.high-speed changes. The main dangers from the robot are impact and trapping. The work cell perimeter is therefore normally designed to be at least 1 m from the robot work-volume limits. Finally. or by colliding with another solid object causing parts to fly from the gripper. which if released will halt the robot motion. The teach-pendent used during the programming of the robots contains a dead man stop. and if activated will halt the rotation of the fixture. A second emergency-stop button is also available to the operator at the loading side of the cell. either by colliding directly with the operator. The welding robot cell that contains two robots and a rotary fixture is surrounded by a mesh perimeter fence. the arm and the safety barriers or other solid items in the work cell. . The latter is a bigger problem for high-speed operations. This gate contains a safety interlock switch that halts the operation of the cell if open. the operator is shielded from the welding operation by a protective curtain. These could have been used in the welding cell rather than pressure sensitive mat to detect the presence of an operator. .Other devices not shown in Figure 9 are light curtains. Whisker sensors can also be used on the robot arm to detect contact and halt the operation.