Influence of working height on the energy consumption of an industrial robot

Electricity consumption is a big problem not only for the economy, but also for the environment. More and more electricity is used, so it is necessary to find a way to save it. This article focuses on the electrical energy consumption of an industrial robot by changing its working height. The goal is to find out at which height it is most suitable to work in order to achieve the lowest possible energy consumption. Research is realized on industrial robot ABB IRB 6700 using simulation tools such as ABB RobotStudio and Process Simulate, where different trajectory heights are simulated. The trajectories are dimensioned according to the size of the robot. Also, the trajectories are designed to maximize the use of the robot’s axis. This method makes it possible to reduce energy consumption even before the robot is physically located in the factory and its position cannot be changed.


Introduction
The demand of high-speed manufacturing and accuracy for repetitive task makes industrial robots highly desired tool in industry.According the report of International Federation of Robotics [1], there is a 3,5 million industrial robots worldwide.The most of them are used in China (1,2 million units).Operational stock in Europe is 680 thousand units, which is 230 thousand more than in America and 280 thousand more than Japanese.Industrial robots mainly work sequentially in the form of cells or on assembly line, preparing subassemblies that are later used in the production line [2].
In the manufacturing industry, industrial robots consume a large amount of energy.For example, the automotive industry consumes electricity by robots in the vehicle manufacturing phase on average about 8% of the total energy required to produce the vehicle [3].The amount of used energy depends on robot size and type, type of performing task, efficiency of components or operating conditions [4].Reducing energy consumption plays an important role not only in reducing the cost of the production process, but also in reducing the impact on the environment due to the reduction of the carbon footprint [5].In addition, research of industrial robot energy consumption can help manufacturers to improve their strategy for energy flexibility in energy supply [6].Lowering energy consumption can be done in different stages of development.First stage is production planning.During this phase, planners are more flexible in optimizing the process and can specify a strategy to reduce energy consumption.Also, this stage allows improvement in efficiency.As an example, may be given improved operation schedule or choosing industrial robot with lower energy consumption.Another thing may be optimizing industrial robots operating parameters and conditions [7].The second stage is commissioning stage.This stage is not so flexible as a previous one for lowering energy consumption because of restriction such as considered manufacturing productivity.Lowering energy consumption can be provided by reduction of waiting and idle time during production phase.The third stage is optimization.At this stage there is a lot of restrictions which makes lowering energy consumption hard to achieve.Hardware components and production rate cannot be changed.Allowed changes are only software nature such as optimization of trajectories or releasing actuator brake earlier [8].Currently, very few robots have additional optimization for lowering energy consumption.The most of optimization processes are focused only to production speed [9].
Many researches are aimed to minimization of energy consumption such as using renewable energy sources, energy efficient equipment, path planning, optimal hardware selection or optimization of the entire production plant [10].However, these strategies involve significant investment and are usable in different time frames.Renewable energies will have a worldwide influence in the medium or long term.New devices may not be easily applied to real systems due to market restrictions such as production rates or cost.Nevertheless, most of the energy saving researches rely on plant modification or path rescheduling and are therefore only realistically used in the product planning stage [11].

Methodology
For the purpose of this investigation, we choose industrial robot ABB IRB 6700.It is a six-axis industrial robot with length of arm 3200 mm and load capacity 150 kg.Total weight of this industrial robot is 1280 kg [12].For this robot was designed specific trajectory (Figure 1), which robot perform in different heights from level base.Every trajectory is defined by same five points with various orientation.These points are designed to use robots joint as much as possible to simulate the maximum load on the joints.Trajectories start from 2400 mm above the ground level to 400 mm under the ground level.Despite the fact that the robot can reach a height above 2400 mm above the ground, due to the preservation of the orientation of the points, it is not possible to use this trajectory at a higher height.Thus, there was created 27 trajectories for every 100 mm.Trajectories were created in Tecnomatix Process Simulate software, where was performed reachability test for every point of all trajectories.Also, maximum joint usage was investigated here.Figure 2 shows graph of usage for every of six robot joints for trajectory in height 2200 mm above ground level.Orange curve represent first joint with range from -42,30° to +42,31°.Yellow curve represents second joint with range from -22,34° to +11,27°.Red curve stands for third joint with range from -16,12° to +14,87°.First three joint range are not so big because they mainly define robot working height.The remaining 3 define orientation of tool during producing process so the range will be significant bigger.Thus, fourth joint is represented by green curve with range from -180,00° to +91,65°.Fifth joint is represented by blue curve with range -109,21° to +109,22°.The Last joint is represented by purple curve with range from -180,00° to +174,75°.Therefore, the velocity was set to max and zone to 0. Since the object of this measurement is not the tool load, the load at the end of the sixth axis is zero.For a more accurate measurement, the robot performs each trajectory 60 times.This represents the standard number of cycles per hour in automotive industry as the average working time per cell is usually around one minute.Energy is measured only while the robot is moving, not the idle times.

Results
The results can be divided into two sections.First Section is aimed to energy consumption during performing trajectories in different height.Figure 4 shows graph of consumed energy depending on working height.Result shows that during performing trajectory at height of 2200 mm above ground level, robot consumes only 748321,5 J. On the other hand, at height 1300 mm above ground level, robot consumes 824373,38 J, which is 9,23 % more, than at height 2200 mm.Second measurement shows total time needed to perform.Results are presented on Figure 6, which shows graph of time spent to perform selected trajectory depending on working height.Result says, that during performing trajectory 50 times at height of 2400 mm above ground level, robot need 480,29 s, but at height 2100 mm need only a 308,66 s.Thus, working at height 2100 mm is faster by 35,73 % than at height 2400 mm.Similar time results as at height 2100 mm are at heights from 2100 mm to 500 mm, where the difference is not so significant.

Conclusion
This paper investigates energy consumption based on working height of industrial robot ABB IRB 6700.Using simulation software Tecnomatix Process Simulate and ABB RobotStudio were performed energy consumption and time measurements of designed trajectory in different heights from 2400 mm above ground level to 400 mm under the ground level.Result shows that lowest energy consumption was at height 2200 mm above ground level, because it was easiest to perform for the robot, even though the TCP tracking shows that robot performed more curved trajectory than in lower heights.Another measurement proves that less curved trajectories spend less time to perform, but spend more energy to perform.Robot performed fastest trajectories in range from 2100 mm to 500 mm above ground level, where was not significant difference between them.Overall, the fastest trajectory was at height 2100 mm.General summary of these measurements is that optimal working height of industrial robot ABB IRB 6700 is at height 2100 mm above ground level because it performs faster trajectory and difference between energy consumption at height 2200 mm (lowest energy consumption) and 2100 mm is not significant.On the other hand, fastest performing trajectory creates longer idle times between work cycles, which should be solved by power saving modes.Results compared to similar measurement made by Rassõlkin [13] shows big difference.According Rassõlkin, robot has lowest energy consumption at 50 % of body height and the highest energy consumption at -9,3 % (150 mm under the ground level).According to our measurements the highest energy consumption was at 61,76 % of body height and the lowest energy consumption at 104,51 %.Difference may be caused by different tested trajectory and robot size.

Future work
To improve results, as a future work authors consider to add another parameter for measurements, which is distance from robot base in X axis in heights with lowest energy consumption.

Figure 4 .Figure 5 .
Figure 4. Energy consumption depending on the working height

Figure 6 .
Figure 6.Working time depending on the working height