AUTOMATION AND ROBOTICS:PERFORMANCE EVALUATION AND ECONOMIC JUSTIFICATION IN SELECTING THE ASSEMBLY SYSTEM

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PERFORMANCE EVALUATION AND ECONOMIC JUSTIFICATION IN SELECTING THE ASSEMBLY SYSTEM

Various methods have been developed and proposed for performance evaluation and system selection. Simplified mathematical models have been developed to describe economic performance, and the effects of several important variables have been analyzed:

1. Parts quality (PQ), represented by this factor, is the average ratio of defective to acceptable parts.

2. Number of parts in each assembly (NA).

3. Annual production volume per shift (VS).

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4. Product style variation (SV) is defined as the ratio of the total number of parts available (NT) to the number NA actually used in each product.

5. Design changes (ND) is the number of parts that are changed (necessitating new feeders and workheads, etc.) during the life of the assembly system.

6. Number of products to be assembled (NP).

7. Economic climate (RI). Economic market conditions are expressed by the capital cost of the equipment relative to an equivalent operator rate.

8. QE is defined as the cost of the capital equipment that can economically be used to do the work of one operator on one shift per year.

The economic climate for investment in automation can be expressed by the ratio RI:

Automation and Robotics-0120

where SH = number of shifts

WA = annual cost of one assembly operator

When this ratio is low, investment in automation is less likely to be profitable.

In addition to the eight variable factors above, it should be remembered that manual assembly systems will be necessary if special fitting or adaptation of parts is required or if there are wide fluctuations in demand for the product, such that its automated system will not be able to respond in time to the new requirements. In addition, intangible benefits of automation, which are difficult to quantify (e.g., shortage of qualified workers) or high consistency requirements must be taken into account.

The cost of assembly (CA) for a complete assembly is given by

Automation and Robotics-0121

where TP = average time between delivery of complete assemblies for a fully utilized system WT = total rate for the machine operators

CE = total capital cost for all equipment including engineering setup and debugging cost For the purpose of comparing the economics of assembly systems, the cost of assembly per part will be used and will be nondimensionalized by dividing this cost per part by the rate for one assembly time per part, TA. Thus, the dimensionless assembly cost per part (CD) is given by:

Automation and Robotics-0122

and is the ratio of the cost of all operators compared with the cost of one manual assembly operator and expressed per part in the assembly. The dimensionless assembly cost per part for an assembly operator working without any equipment will be one unit, which forms a useful basis for comparison purposes. For a particular assembly system Eq. (4) holds true only if the required average production time (TQ) for one assembly is greater than or equal to the minimum production time (TP) obtainable for the system. This means that if TP ::: TQ (because the system is not fully utilized), then TQ must be substituted for TP.

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