rel="nofollow" href="#ulink_4a3474d4-5e00-5924-9757-545ccb763c13">Figure 1-9.
Figure 1-8 Programmable automation
Robotic technology is very similar to CNC technology in that it utilizes mechanical, electrical, and computer technology to move a manipulator in three-dimensional space. Also, in many applications it uses a tool to perform processing on a workpiece, an example of which is a welding robot: the robot moves the welding tool through a specific path over the workpiece. However, in many other applications the robot does not use a tool. It merely provides material handling capabilities such as moving a workpiece from one machine to another and/or stacking the workpiece in a specific pattern on a pallet. In either case, the robot is performing a task that could also be performed by a human. In fact, the origin of the term “robot” is credited to a play, which premiered in 1921, about a factory that made artificial people devoid of feelings. These artificial people were called robots. The word was derived from the Czech word robota, meaning serf labor, thereby implying servitude and hard work. Thus, robots are often distinguishable from other types of automation in that they possess humanlike characteristics (e.g., a robot arm) and perform tasks often completed by humans. Robotic technology examples are shown in Figures 1-10 and 1-13.
Figure 1-9 CNC technology
Figure 1-10 Robotic technology
Whereas CNC and robotic technologies provide motion control, programmable logic control (PLC) technology imparts automatic control over tasks or events through the use of electrical and computer technology. This is accomplished by monitoring the status of a given system through sensors that input information to the PLC. Based on the status of these inputs, the PLC will make decisions and take appropriate action on the system by outputting information to actuators. The output to the system is based solely on the status of the inputs. This is called a discrete process control system. A discrete system has inputs and outputs that are binary with two possible values: on or off. The status of these inputs and outputs change at discrete moments in time. Thus, PLC technology provides control over event-driven changes to the system. As events occur that change the status of the inputs, the outputs automatically change. Figure 1-11 shows an example of a PLC.
Figure 1-11 PLC technology
These three technologies are the foundation upon which modern automation is built. The automation system shown in Figure 1-12 makes good use of all three technologies. The figure depicts a typical manufacturing cell. A manufacturing cell is defined as an interconnected group of manufacturing processes tended by a material handling system. In this particular case, the manufacturing processes consist of a CNC lathe and a CNC mill. These two machines are tended by a robot, which loads raw material into one machine, transfers it to the next, and then unloads and stacks the processed material. A programmable logic controller (PLC) controls and coordinates activities between the CNC machines and robot. (Note that PLC is the acronym for both programmable logic control and programmable logic controller.)
Figure 1-12 Manufacturing cell
The CNC equipment, robot, and PLC must possess intelligence in order for the cell to function properly. The intelligence is expressed in terms of decision-making ability. Each machine in the cell must be able to accept input, make a decision based on that input, and implement the decision. For example, consider the CNC lathe. When it is time to process material, it must first recognize that it is ready to process more material and then prepare itself to receive material by opening safety gates, moving tooling out of the way, and opening the lathe chuck. Next, it must inform the PLC that it is ready to accept material for processing. Once the material is loaded, the CNC lathe must recognize that it is loaded and then process the material. When the processing is complete, it must prepare itself to be unloaded. Finally, it will inform the PLC that it is ready to be unloaded. The robot functions similarly. It receives input from the PLC that the lathe is ready to be loaded. It then executes a sequence of movements to pick up the raw material and load it into the lathe. It will then inform the PLC that it has loaded the material and is ready for the next instruction. Thus, the PLC is, essentially, the brain of the cell. It controls the timing and sequence of all events that occur within the cell. It monitors the status of the cell and informs each piece of equipment when and where actions are to be performed.
Figure 1-13 depicts another manufacturing cell. This cell consists of a hydraulically actuated press, a shuttle system, and a robot. In this cell, products (round disks) are molded inside the press then moved outside the press (with the shuttle system), where the robot unloads and finishes the product. A PLC controls the cell. Note that even though a CNC machine was not part of this particular cell, CNC technology had an impact on the cell. A CNC machine was used to produce the mold in the press and the gripper on the end of the robot arm.
Figure 1-13 Press manufacturing cell
Thus, as shown in these two examples, it is easy to envision how programmable automation is present, either directly or indirectly, in almost all modern automation systems.
1.4 Manufacturing Performance Measures
In order to understand when and where to apply automation in general and programmable automation in particular, it is essential to comprehend how manufacturing production performance is measured. From these performance measures one can evaluate and justify the use of automation. As will be seen in the next chapter, the performance measures used most often to quantify production include:
Production rate—measure of products per hour (pc/hr).
Setup time—measure of the amount of time to prepare a machine or process to make a product (hrs).
Production capacity—measure of the maximum amount of product that can be produced by a manufacturing facility, system, cell, or process in a specified period of time (output units/time period).
Utilization—ratio of the actual amount of output from a manufacturing facility, system, cell, or process in a specified period of time to the production capacity over the same time period (%).
Manufacturing lead time—total time to process a product through a manufacturing facility, system, cell, or process.
Each of these measures provides a picture of how certain individual aspects of the manufacturing process and system are performing. These are vital and critical measures to be evaluated when one considers automation. However, each only provides a small segment of the overall picture. Thus, the measures need to be evaluated collectively to effectively evaluate and justify the use of automation. There are other factors as well that should be considered, including burden rates of equipment and labor costs. Thus, it may be difficult to
