Automated Production systems can best be classified into three basic types
- Fixed Automation
- Programmable automation
- Flexible Automation
Fixed automation: is system in which the sequence of processing ( or assembly) operations is fixed by the equipment configuration. The operation in the sequence are usually simple. It is the integration and coordination of many such operations into one piece of equipment the makes the system complex. The typical features of fixed automation are:
- High initial investment for custom-engineered equipment
- High production rates
- Relatively inflexible in accommodating product changes
The economic justification for fixed automation is found in products with very high demand rates volumes. The high initial cost of the equipment can be spread over a very large number of units, thus making the unit cost attractive compared to alternative methods of production. Example of fixed automation include mechanized assembly lines (starting around 1913- the product moved along mechanized conveyors, but the workstations along the line were manually operated) and machining transfer lines beginning around 1924).
In programmable automation, the production equipment is designed with the capability to change the sequence of operations to accommodate different product configurations. The operation sequence is controlled by a program, which is a set of instructions coded so that the system can read and interpret them. New programs can be prepared and entered into the equipment to produce new products. Some of the features that characterize programmable automation include:
- High investment in general-purpose equipment
- Low production rates relative to fixed automation
- Flexibility to deal with changes in product configuration
- Most suitable for batch production
Automated production systems that are programmable are used in low and medium-volume production. The parts or products are typically made in batches. To produce each new batch of a different product, the system must be reprogrammed with the set of machine instructions that correspond to the new product. The physical setup of the machine must also be changed over: Tools must be loaded, fixtures must be attached to the machine table, and the required machine setting must be entered. This changeover procedure takes time. Consequently, the typical cycle for a given product includes a period during which the setup and programming takes place, followed by a period in which the batch is produced. Example of programming automation include numerically controlled machine tools ( first prototype demonstrated in 1952) and industrial robots (initial applications around 1961), although the technology has its roots in the Jacquard loom (1801).
Flexible automation is an extension of programmable automation. The concept of flexible automation has developed only over the last 15 or 20 years, and the principles are still evolving. A flexible automated system in one that is capable of producing a variety of products ( or parts) with virtually no time lost for changeovers form one product to the next. There is no production time lost while reprogramming the system and altering the physical setup (tooling, fixtures, machine settings). Consequently, the system can produce various combinations and schedules of products, instead of requiring that they be made in separate batches. The features of flexible automation can be summarized as follows:
- High investment for custom-engineered system
- Continuous production of variable mixtures of products
- Medium production rates
- Flexibility to deal with product design variations
The essential features that distinguish flexible automation from programmed automation are (1) the capacity to change part programs with no lost production time and (2) the capability to change over the physical setup again with no lost production time. These features allow the automated production system to continue production without the downtime between batches that is characteristic of programmable automation. Changing the part programs is generally accomplished by preparing the programs off-line on a computer system and electronically transmitting the programs to the automated production system. Therefore the time required to do the programming for the next job does not interrupt production on the current job. Advances in computer system technology are largely responsible for this programming capability in flexible automation. Changing the physical setup between parts is accomplished by making the changeover off-line and then moving it into place simultaneously as the next part comes into position for processing. The use of palled fixtures that hold the parts and transfer into position at the workplace is one way of implementing this approach. For these approaches to be successful, the variety of parts that can be made on a flexible automated production system is usually more limited that a system controlled by programmable automation. Examples of flexible automation are the flexible manufacturing systems for performing machining operations that date back to the late 1960s.
The relative positions of the three types of automation for different production volumes and product varieties are depicted in figure 1.1.