ROBOT SELECTION CRITERIA FOR INDUSTRIAL APPLICATIONS

21 Temmuz 202021 Temmuz 2020 tarihinde gönderilmiş omer tarafından

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INTRODUCTION

The developing technology has caused serious changes on the production methods. Over time, there have been developments to use manpower more efficiently in production processes. Realizing that he could not rule over to nature with his wrist strength, mankind tended to do things with his mind. In the first processes, human started using simple machines based on general laws of physics. Subsequently, he began to benefit from animal power and nature power. In the following processes, he started to discover and control energy resources and use them for his purposes. Steam power was added to human power. This was a term where production and industry processes were speeding up. Over time, the use of electricity has progressed. Electric engines were added next to the steam engines. Manpower was used more efficiently than ever before. Now, in many processes, people took advantage of machines that work with electric motors, and they could do many things they couldn’t do with their body power with these machines.

Developments continued day by day. Ways of controlling the machines independently began to be discovered. Mankind discovered the transistor. These seemingly simple components were able to do great work when combined. Microcontrollers and microprocessors began to be used to control machines. Now people have started controlling many machines with these microprocessors. As electronic developments progressed, machines started to be controlled much better.

Sensor technology was another area that went in parallel. When the sensors and microcontrollers were brought together, the machines started to be moved by receiving data from the external environment. These machines started to be called “robots” in general. Now, robots that are independent of human control have started to take part in production processes.

In this study, robots, industrial robots, motion areas of robots, general characteristics of robots will be discussed. Then, “industrial robot selection”, which constitutes the main purpose of the study, will be emphasized. The criteria to be considered when choosing the robots that will take place in the production processes will be mentioned.

WHAT IS ROBOT?

“Word robot was coined by a Czech novelist Karel Capek in a 1920 play titled Rassum’s Universal Robots (RUR). Robot in Czech is a word for worker or servant” [1]

If we make a more technical definition, robots are machines that can collect data from the external environment and make changes in their movements according to these data.

WHAT IS INDUSTRIAL ROBOT?

Industrial robots can be defined as robots used in production processes. These robots are equipped with different sensors according to their usage areas.

Today, industrial robots are located in many production facilities. Many operations such as assembly and joining processes, welding process, packaging, relocation of parts can be done with industrial robots.

GEOMETRIC CONFIGURATION

Industrial robots make their movements with joint connections that can make linear movement or rotary movement. The relationship of these joining points with each other determines the geometric properties of the robot. The geometric shape of the robot is important in terms of the environment it will work in and the work it will do.

WORK ENVELOP

Industrial robots make certain movements. These movements create a three-dimensional volume. This volume is called the robot’s “work envelope” or “work area”. The robot performs its movements in this closed volume. Therefore, the process envelope should be taken into consideration when choosing a robot.

The work envelope varies according to the robot types in general. In Cartesian robots, the work envelope is in the form of a rectangular prism, while SCARA type robots have a hollow, truncated cylinder geometry. The work envelope of different robot types is given in figure 1.

Figure 1; Work envelope of different type of robots.

POWER SOURCE

Robots use energy in the working process. The energy or power sources used by robots may differ. Robots provide their basic movements with these power sources. Industrial robots’ power supplies are three main types:

Pneumatic Power Supply
Hydraulic Power Supply
Electric Power Supply

1- Pneumatic Power Supplies

Many operations in industrial plants are carried out thanks to pneumatic systems. In robots operating with pneumatic power sources, linear and rotary movements are provided with pneumatic systems.

Advantages of pneumatic systems;

It is suitable for linear movements.
Working speed is too high
Energy can be stored in pneumatic systems
Air is an accessible resource [2]
It is possible to reach high speeds. [3]

Disadvantages of pneumatic systems:

Energy costs are high
Very sensitive to temperature changes [3]
Positioning accuracy is low-rg
Speed control is difficult [2]
Fluid operates in an open system

2- Hydraulic Power Supplies

They are the power sources used by robots working with hydraulic systems. Hydraulic systems are systems where incompressible fluids are used and power transmission is achieved by making use of this feature of the fluid. [3]

Advantages of hydraulic power supplies:

It has better positioning accuracy than pneumatic systems.
High powers can be obtained in rotary and linear movements.
Suitable for linear movements
Speed adjustment is relatively easy
Suitable for the transportation of heavy loads

Disadvantages of hydraulic power supplies

Energy cost is high
Working speed is lower than pneumatic
Sensitive to temperature changes
Energy storage limited

3- Electric Power Supplies

Industrial robots can also move using electricity. In such robots, actuators that convert electrical energy into motion energy are used. These actuators produce rotary or linear movements.

Advantages of electric power supplies:

Energy transmission easy
Easy to store energy
Energy cost is cheap
It is not affected by temperature changes
Strong in rotary movements
Very precise position and speed control

Disadvantages of electrical power supplies:

Linear movements are difficult [3]
There is a risk of electric shock.
Linear power generation is difficult compared to hydraulic system

PERFORMANCE SPECIFICATION OF INDUSTRIAL ROBOTS

The Machine Tool Industry Research Association (MTIRA), operating in the UK, has conducted a number of tests on the performance characteristics of industrial robots. These tests are ongoing. Although the performance parameters for industrial robots are not exactly certain, the following parameters are recommended:

The amount of load the robot can carry-pay load
Ability to repeat positioning-repeatability
Ability to follow the road
Speed regularity / consistency
Positioning time
Dynamic and static stiffness characteristics
Vibration features [4]

PRECISION

It is a term that contains three different features in precision. In order to talk about the precision of an industrial robot, three important features are examined; Resolution, repeatability and accuracy.

Resolution

It is called the minimum amount that can be controlled. This amount of increase is specified as length for linear movements and angle for rotational movements. Resolution is an important parameter for the work that the robot will accomplish.

Repeatability

It is simply the ability of the robot to repeat the movement between two different linear or angular positions. For example, let’s consider a robot arm at the joining station. This is when the robot arm always goes to the programmed locations in the same way. While repeating this movement, the measure of how similar each repetition is to each other is expressed as being able to repeat.

Accuracy

Industrial robots are programmed to perform a task in production processes. Depending on the type of these tasks, they must act more or less within an accuracy range. Accuracy is the difference between the position it should go and its position when the robot goes to the programmed location.

See Figure 2 for a better understanding of the concepts of accuracy and repeatability:

Figure 2; Repeatability and accuracy

It is generally desired to have high accuracy and repeatability. In addition, in many robotic applications, the repeatability becomes important than the accuracy of the industrial robot. A robot with high repeatability can be programmed to perform the task as desired, since the error limits are well known.

INDUSTRIAL ROBOT SELECTION

Today, many companies that make industrial production want to take advantage of robots. Because, thanks to the robotic applications, the production process can be realized at lower costs, although the production quality and capacity increase. Because robots do not get tired, their emotional state does not affect production processes.

Decision making is very important in the selection of industrial robots. Because a wrong decision will cause more harm than profit. Firms’ high quality, higher production volumes and lower cost expectations will not be realized. On the contrary, maybe they will suffer damage.

INDUSTRIAL ROBOT SELECTION CRITERIA:

Industrial robots are generally selected specifically for the production process. The work to be done by the robot is determined and a robot is selected in line with that work. At this stage of preference, the criteria listed below should be considered;

The way of doing the work to be done

What kind of geometric path does the movement expected from the robot follow? The endpoints of this path on the xyz axes should be measured. In accordance with the measured values, the volume geometry of the robot is determined. Then the robots that contain this geometry are examined.

Load weight to be lifted-pay load

Robots lift certain weights during operation. The largest load that an industrial robot can move within the range of certain precision tolerances is called “pay load”. As the robot carries loads of this weight, it moves within the precision values of its catalog. If the load carried by the robot exceeds the pay load limit, it goes out of the desired precision range.

Other disadvantages of industrial robots lifting weights greater than the specified amount are the frequent maintenance periods and reduced working life. Since the robot’s movement mechanisms and loading systems will exceed the projected amounts, the possibility of encountering unexpected problems will increase.

In the industrial robot selection, the power that the robot must move must be determined. As an example, let’s assume that we will prefer a robot to connect to screw. The end effector part of the robot is a mechanism that will perform screwing. This end effector weight is determined. Then, the linear force to be applied for screwing should be determined. Finally, the moment effect that will occur during screwing is also taken into account. These three forces are summed up. The summerized force gives the minimum payload of the robot to be selected. It may be recommended to use a dynamometer in force determination.

Precision

We have mentioned that the concept of precision includes three different features. At which precision intervals can the robot’s work be performed in the choice of industrial robots? Let us give examples of operations that require and do not require precision with two different examples; In our first example, let’s consider a robot that places a bearing on the shaft. The concept of sensitivity is very important for this process. The robot to be preferred should have high repeatability and accuracy. As a second example, let’s consider a robot arm that transfers parts between two conveyor belts. Conveyor belt widths are 500mm and the widest part of the piece is 200mm. In this case, the precision range of the robot that will perform the transfer process can be kept wider.

Current power supply

The power supply of the robot is also an important criterion in the choice of industrial robots. The type of power source can affect the robot’s load handling and precision conditions, as well as its cost and operating costs. For robots that can perform the same task but operate with different power sources, a preference can be made in line with the facility’s possibilities.

Compatibility with other robots

There may be other robots already used in the plant in industrial production processes. In this case, the new robot to be preferred must be compatible with existing robots. This compatibility should be in the following aspects;

Programming method; If there is a team programming robots in the facility, robots that can be programmed in a similar way to the programming method of other robots should be selected.

Maintenance and control; Robots that can be serviced similarly to existing robot maintenance methods should be preferred. For example, if the power supply of existing robots is electricity, the power supply of the new robot should be electricity if possible. Because maintenance of electrical systems and maintenance of hydraulic or pneumatic systems are very different.

Lifetime

Industrial robot preferences should be made in line with the production time. How long will the production involving the robot to be preferred take? For example, it will not be economical to choose a robot with a longer lifetime for a production that will last for 4 years.

Another point to be noted at this point is that industrial robots are not produced for a purpose only. That is, a robot can be used for other jobs if its life time has not expired. In this case, the company should consider that it can adapt its selected robot to other production processes.

Operation and maintenance cost

Industrial robots, like other industrial machines, need maintenance and calibration periodically. In addition to maintenance, there will be operating costs, especially energy, during the operation process. Therefore, these expenses must be taken into consideration.

As a method, operating and maintenance expenses can be brought to today’s values and included in the total cost amount.

Scrap value

Machines used in the production process generally use large amounts of metal materials. Metals are materials that are easy to recycle. So, like any machine, robots have a scrap value. In this case, it would be appropriate to make a calculation. When choosing between robots that will do the same job, scrap value should also be taken into consideration in price comparison. The scrap values of the robots can be calculated according to the present value and included in the price calculation.

Economic status according to feasibility / other methods

One of the most basic criteria for making a robot selection in industrial processes is to compare it economically with other methods. For example, let’s assume that an industrial robot will be preferred for a painting process. If this is done using staff, how many staff can be done? How long can it be done? How much salary is paid to these employees? These expenses should be calculated and brought to today’s values and compared with the robot alternative.

Environmental aspects

Industrial plants can seriously cause environmental pollution. In robot preferences, its effects on the environment must be evaluated. The main purpose of engineering works is to make the world more livable. An engineer making calculations in line with this philosophy should add environmentalism to their accounts as an important parameter in addition to the economy.