# Comparison of Single-Arm and Dual-Arm Collaborative Robots in Precision Assembly
## 3 Results and Discussion
### Costs  
In the comparative analysis of the costs of two variants of collaborative robots, two points of view are taken into account: the profit and loss account from the application of robots in assembly, as well as the unit cost of robotic assembly.  
In order to assess which robot would be more profitable during production, a cost estimate was prepared. The first expense that must be incurred when implementing a robotic station is the cost of purchasing the robot. This cost estimate does not include the purchase prices for the stand's equipment, as it would be the same in both variants. The second important aspect is the efficiency of robots, i.e., the number of products they can assemble per month. The robots are located next to the assembly line and it was necessary to take into account the time during which the assembled frame with parts moves to the place where the process will start. The value of this allowance was set at 5 s and cycle times of 25.6 s for a single-arm robot and 18.6 s for a dual-arm robot. This cost estimate assumes that the robot will work in three shifts for 22 days a month (average value of working  
Figure 16: Energy consumption.  
\begin{table}
\begin{tabular}{l c c} \hline \hline  & **Time [s]** & **Energy Consumption [J]** \\ \hline Variant I—single-arm robot & 20.6 & 205.8 \\ Variant II—dual-arm robot & 13.6 & 260 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Energy consumption.  
days). Based on these data, the results are presented in Table 6. The man-hour values were adopted on the basis of the possibility of producing a given number of products per hour, cost of purchasing the robot, system maintenance and costs of an employee monitoring work. The monthly profit is calculated based on the Equation (1):  
\[\begin{array}{c}\text{Monthly profit =}\\ \text{(No. of cycles per month * Cost of producing 1 product) -- (No. of shifts *}\\ \text{No. of hours per shift * No. of working days per month * cost per man}\\ \text{hour) -- (Energy consumed in a month * cost per kWh)}\end{array} \tag{1}\]  
It is important in cost analysis to determine the unit cost. Using division calculation, which is one of the basic ways of determining unit costs and calculated as the quotient of the sum of production costs and the production volume--Equation (2):  
\[K_{j}=\frac{K}{P} \tag{2}\]  
where:  
\(K_{j}\)--unit cost,  
\(K\)--total costs (purchase cost, energy, labor costs, etc.),  
\(P\)--production volume.  
Taking into account the data in Table 6, the unit cost for a single-arm robot is 0.047 \(\up\up\)/pcs., and for a dual-arm robot it is 0.058 \(\up\)/pcs.  
For the analyzed assembly process, robots bring profit, and their implementation would pay off after about 3 months. A two-arm robot is a more advantageous economic solution due to higher profits per month. Cumulative profit is the difference between the financial results (profit/loss) of the dual-arm robot and the financial results (profit/loss) of the single-arm robot, taking into account subsequent months. In this way, it can be seen that from the perspective of time, the dual-arm robot brings more benefits. Comparing the profits for both solutions and assuming that we allocate all the profit to cover the purchase costs of the robots, it can be seen that the solution with the dual-arm robot will yield more profit after eight months than the solution with the cheaper single-arm robot. Therefore, in the long run, it is beneficial to use a dual-arm robot. Figure 17 shows a graph of the cumulative profit of a dual-arm robot versus a single-arm robot.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Single-Arm Robot** & **Dual-Arm Robot** & **Unit** \\ \hline Purchase cost & 37,369 [47] & 61,968 [48] & \(\up\up\