3.1. TIME OF THE ASSEMBLY PROCESS
In variant I, a single-arm YuMi robot was used to assemble the computer motherboard. The total simulation time is 20.6 seconds. During the simulation, the robot performs all necessary tasks related to motherboard assembly, such as picking up components and placing them on the board. The times of individual tasks performed by the robot are presented in Table 2 and in the chart in Fig. 12.
Table 2. The time needed to complete individual assembly tasks by a single-arm robot
Task
|
RAM 1
|
RAM 2
|
Pressing the RAMs
|
Processor
|
Free movements
|
Total
|
Time [s]
|
2.1
|
2
|
3.6
|
4.2
|
8.7
|
20.6
|
The largest part of the time involves free movements within the workstation, mainly reaching the gripper to the place where the part is picked up. These movements account for 43% of the time of the entire assembly process. This indicates that the robot performed activities related to the manipulation of elements for 8.7 s. The next longest task was mounting the processor due to the longest distance the robot arm had to move. The operation of pressing the RAMs took 3.6 s, or about 17% of the time of the entire operation, and mounting the RAMs was 10% for each of them.
In variant II, a dual-arm YuMi robot was used. The total simulation time is 13.6 s. Both robot arms are able to perform tasks simultaneously, which speeds up the motherboard assembly process. During the simulation, one robotic arm can pick up parts while the other places them on the board. All values related to time analysis are presented in Table 3 and charts in Fig. 13 and 14. In the time analysis in the variant II, the percentage of time of each task on both arms was considered and, additionally, as in variant I, the ratio of the robot's work to free movements.
Table 3. The time needed to perform individual assembly tasks by each arm of a dual-arm robot
Task
|
RAM 1
|
RAM 2
|
Pressing the RAMs
|
Processor
|
Free movement
|
Total
|
Left arm
Time [s]
|
2.6
|
-
|
-
|
3.9
|
7.1
|
13.6
|
Right arm
Time [s]
|
-
|
2.4
|
2.1
|
-
|
9.1
|
13.6
|
The operating time of the left robot arm in variant II includes a large amount of free movements of over 50%. This arm places the first RAM in its appropriate slot, and then reaches for the processor and mounts it in the designated place. These activities take 2.6 s and 3.9 s respectively. For the right arm, the free movements are much higher and amounts to 68%. This arm is responsible for mounting the second RAM and for pressing RAM No. 1 and 2. Both of these tasks took a total of 4.5 seconds. Taking into account the entire operation performed by both robot arms in variant II, summing up the assembly times of each part, free movements took only 20% of the time of the entire process.
3.2. SPEED AND ACCELERATION SIGNALS
The sequence of movements of both compared robots (single- and dual-arm) in relation to time is presented in Table 4.
Table 4. Robot movement sequences
The course of the actual TCP speeds of the gripper of a single-arm robot and two arms of a dual-arm robot is shown in Fig. 15. Due to the fact that the programmed speeds may differ from the real ones due to the kinematics of the robot or the length of the movement path and the inability to achieve the programmed speed on a given section of the path.
ABB YuMi robots have no difficulty in achieving a maximum speed of 200 mm/s on programmed paths. It is not achieved only when pressing the RAM bone with the right arm of the robot, because the section of the path between individual points is too short and it is not possible to develop the programmed speed. The speed of the TCP of the gripper to the position of picking parts from the container and placing them in the computer motherboard is reduced to 50 mm/s to minimize the risk of collisions and possible displacements of moved assembly parts.
Figure 16 shows the accelerations achieved by the gripper's TCP during the process.
The graph of the acceleration of the robot's arms at the TCP point versus time suggests that the largest acceleration peaks for all arms occur between 8 and 12 s. For a single-arm robot, it is the movement it performs over the processor in preparation for lifting it, and for a dual-arm robot, it is the movement associated with moving the processor to the appropriate place. The upward trends in most cases are related to starts after stops in the parts pick-up and put-down zones. Downward trends appear as the gripper approaches the target points where direct assembly is performed.
3.3. ENERGY CONSUMPTION
Industrial robots are an integral part of modern production lines, contributing to increased efficiency and precision of processes. However, their growing importance requires taking into account energy consumptions, because energy is one of the key factors influencing the costs and sustainable development of the industry. In this case, two stations were analyzed - with a single- and dual-arm YuMi robot. These results are presented in Table 5 and in the graph in Fig. 17.
Table 5. Energy consumption
|
Time [s]
|
Energy consumption [J]
|
Variant I – single-arm robot
|
20.6
|
205.8
|
Variant II – dual-arm robot
|
13.6
|
260
|
During one assembly cycle, a single-arm robot consumes 205.8 J of energy to power the robot in 20.6 s. Whereas a dual-arm robot consumes 260 J in 13.6 s. Based on these results, it can be concluded that variant I - robot single-arm is more energy-efficient. However, a two-arm robot performs the assembly operation faster and using 20% more energy, which could turn out to be a much more financially profitable option in the case of large-scale production. When comparing collaborative robots, it should be noted that the operating time is not proportional to the energy consumption.
3.4. 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 an 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 3 shifts for 22 days a month (average value of working days). Based on these data, the results are presented in Table 6. The man-hour values were adopted on the basis of possibility of producing a given number of products per hour, cost of purchasing the robot, system maintenance and costs of an employee monitoring work.
Monthly profit is calculated based on the equation (1):
Monthly profit
= (No. of cycles per month * Cost of producing 1 product)
- (No. of shifts * No. of hours per shift * No. of working days per month * cost per man hour)
- (Energy consumed in a month * cost per kWh) (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 (2):
where:
Kj – 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 €/pcs. and for dual-arm robot 0.058 €/pcs.
Table 6. Cost and profit estimation of the robotic assembly (estimated data for 2023) [35, 36]
|
Single-arm robot
|
Dual-arm robot
|
Unit
|
Purchase cost
|
37369 [37]
|
61968 [38]
|
€
|
Cycle time
|
25.60
|
18.60
|
s
|
Energy consumed per cycle
|
206
|
260
|
J
|
Number of shifts
|
3
|
|
Number of hours per shift
|
8
|
|
Number of cycles per hour
|
141
|
194
|
pcs
|
Energy consumed per hour
|
28941
|
50323
|
J
|
Energy consumed per number of shifts
|
694575
|
1207742
|
J
|
Number of working days/month
|
22
|
|
Energy consumed in a month
|
15280650
|
26570323
|
J
|
Energy consumed in a month
|
4.24
|
7.38
|
kWh
|
Number of cycles per month
|
74250
|
102194
|
pcs
|
Energy consumption in the year
|
51
|
89
|
kWh
|
Number of cycles per year
|
891000
|
1226323
|
pcs
|
Cost per man hour
|
10.78
|
21.55
|
€
|
Cost of production pcs.
|
0.32
|
€
|
Cost per kWh
|
0.27
|
€
|
Monthly profit
|
18312
|
21655
|
€
|
Unit cost
|
0.047
|
0.058
|
€/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. Figure 18 shows a graph of the cumulative profit of a dual-arm robot versus a single-arm robot.