**3.13. Optimising the control system at the point of use**

164 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

**3.12. Optimisation of devices that consume compressed air** 

generate some leakage and create unnecessary loses.

elements, and better use of existing components.

in the total cycle of material handling.

Fig. 6 right) is 23.28 l/min.

*Abandoned equipment -* From time to time, reconstructions occur in factories that often lead to abandonment of some parts of compressed air equipment leaving the air supply pipeline intact. The airflow going through the pipeline to the abandoned piece of equipment should be interrupted, as close as possible to the air supply source, because it will inevitably

Many devices that consume compressed air can be used in a more energy efficient manner. The optimisation of devices that consume compressed air is one aspect of systemic approach (Šešlija, 2003) to designing a compressed air system. The optimisation can be achieved by: replacing the existing components with more energy efficient ones; installing the additional

For example, in the case of applying a vacuum generator, the savings in the compressed air are realised by using more energy efficient components, which have an integrated vacuum switch with an air saving function - example is Air saving circuit (Festo, 2011). The vacuum range is set on the vacuum switch. The switch generates a pulsating signal which actuates the solenoid valves for vacuum when the vacuum pressure has fallen below the selected upper limit value (due to leakage etc.). At all other times, the vacuum is maintained with the non-return valve, even when the vacuum generator is not switched on. Fig. 5 presents the operational diagram for vacuum pump and vacuum generator with implemented Air saving function. Since the price of vacuum produced in this way is too high and vacuum suction elements represent significant consumers of compressed air, this option contributes to the increase of energy efficiency of the system. The savings are proportional to participation of time Δt, shown in Fig. 5, within a total time of holding the working object. This solution is especially suitable for application in which time of holding an object significantly participates

Using contemporary engineering tools for supporting the design of vacuum applications, it can be analysed the change of system parameters or parameters of devices that are installed in the system. For example, by using the FESTO vacuum engineering module, for manipulation of an object with a cylindrical shape whose dimensions are: diameter of 150 mm, height of 40 mm and weight of 200 g, a total of 6 vacuum cups are needed. The change of operation pressure enables the usage of highest vacuum level. Air consumption, in the case of 6 bar pressure (see Fig. 6 left) is 27.60 l/min, while with the pressure of 5.4 bar (see

**Figure 5.** Operation of vacuum pump and vacuum generator with implemented Air saving function

In traditional design of pneumatic control system there were no concerns about energy efficiency. Several approaches are developed for energy efficient control of pneumatic systems. Here we will stress only two: Optimising servo-pneumatic systems using PWM and Recycling of used compressed air.

#### *3.13.1. Optimising servo-pneumatic systems using PWM*

If servo control is required by means of pneumatic actuator, it is necessary to use proportional valve in order to control pressure in cylinder chambers. Regardless of the type, the proportional valve is the most expensive component of pneumatic servo system (Liu and Bobrow, 1988; Lai et al., 1990).

Instead of proportional valves and servo valves, on/off electromagnetic valves (2/2 or 3/2 way) are being investigated in order to develop cheap pneumatic servo systems. On/off electromagnetic valves take either entirely open or entirely closed position according to electric command. A pneumatic actuator with on/off electromagnetic valves can be controlled by Pulse Width Modulation (PWM) (Barth et al., 2003; Shen et all., 2004).

The control of pneumatic actuator by means of PWM enables servo control by on/off electromagnetic valves at significantly lower cost than the cost of the control done by proportional valves. If response rate and positioning accuracy are taken into account, the results obtained by PWM control are approximately the same as the results obtained by proportional control.

In the case of proportional valve based systems, the fluid flow is continuously varied. In the case of PWM-controlled systems, the valve is entirely open or entirely closed while the control is done by time of keeping the valves in final positions. Thus, the valve delivers discrete quantity of fluid mass whose size depends on control signal. If the frequency of valve opening and closing is much higher than boundary frequency of the system, the system responds to mean value of discrete flow which is the case of continuous flow, too. With

on/off electromagnetic valves controlled by PWM it is possible to develop a multifunctional pneumatic system having acceptable price and performances (Šitum et al., 2007).

Increasing the Energy Efficiency in Compressed Air Systems 167

**3.14. Measuring and recording the system performance** 

processes necessary for improving the system.

because of two primary reasons:

and Foussias, 2002).

of components (Jocanović et al., 2012).

Measuring and recording the system performance, by itself, does not increase the energy efficiency but usually presents a first step towards improvements in energy efficiency

• Measuring the air consumption and energy used for its production is of essential importance for determination, whether the changes in maintenance practice or equipment investments have justified their costs. As long as the price of a unit of consumed compressed air remains unknown, it is difficult to initiate the managerial

• Recording the system performance is a valuable tool for discovering the degradation in

Three basic parameters – airflow, air pressure and consumption of electrical energy, must be measured and recorded in order to evaluate the system performance. Although this might look simple, in principle, the interpretation of these data can present a difficulty, especially under conditions of variable consumption. Measurement of flow of compressed air also involves certain technical problems and retrofitting reliable measurement instruments can be difficult if not impossible task, unless it was taken care of during the phases of system design and installation. For example, most frequently used type of flow meter must be installed into the pipeline sections that are free of turbulence and whose length must be 10 times greater than its diameter. In some systems, there is not an adequate place for installing of a flow meter. Because of that, it is suggested that large systems and medium size systems must be designed and installed in such a way to enable air flow measurements. If the flow data is not available, the cheaper alternative equipment used for pressure measurement can be very useful to, for example, measure the pressure drop over filters or pressure loss along the pipeline, or for the purposes of detecting larger variations of pressure within the system.

Increase of energy efficiency in compressed air systems in often connected with optimisation of the whole automated system. That is particularly the case with the complex automated systems such as robotic cell and robotic line. In such applications are often integrated, besides robots, pneumatic and hydraulic systems. Minimisation of energy consumption is done in most cases up to now, separately for the each subsystem. There are numerous references dealing with trajectory optimisation of industrial robots, influence of the robot velocity on energy efficiency and design of the optimal components layout within the robotic cell (Šešlija, 1988; Tomović et al., 1990; Yoshimura, 2007; Guilbert et al., 2008; Kamrani et al., 2010; Wenger and Chedmail, 1991; Dissanayake and Gal, 1994; Aspragathos

As well for the optimisation of operation of hydraulic systems concerning efficiency increase it is of significans to monitor the system operating parameters and to increase the reliability

performance or changes in quantity or quality of used compressed air.

**4. Increasing energy efficiency in complex robotic cell** 

The results obtained in (Čajetinac et al., 2012) show that PWM principle of control gives a good quality of the signal tracking at the considerably lower costs and that PLC with standard program support can be used for its realization. It has been shown that it is possible to achieve control performances which are comparable to those achieved by proportional or servo valve but at rather lower cost and with better energy efficiency. Thus these methods of control may be applied as well as pneumatic servo systems since the described system is relatively new and expanding.

#### *3.13.2. Recycling of used compressed air*

Pneumatic actuators are widespread in many branches of industry and a lot of efforts are done in order to increase energy efficiency in their operation. It is a known fact that pressure inside a piston chamber can reach a final value that is equal to supply pressure, only after some time being at the end position of the cylinder piston has been reached. When the direction of movement of the cylinder piston is reversed, all the compressed air contained within a working volume is released into the atmosphere. This represents a significant loss of compressed air that possesses enough potential energy to perform some other kind of work. There is a possibility to reuse this kind of compressed air, see Fig. 7 (Blagojević et al., 2011).

The performance of the system with restoring energy can be improved in comparison with the traditional control system. Compressed air consumption saving of the conventional pneumatic system with restoring energy is from 33.3% to 44.3% or average 38.8%, for 200 to 600 kPa range of pressure supply. Compressed air consumption saving of the pneumatic servo system with restoring energy is from 20.6% to 33.3%, or average 28.6% for 200 to 600 kPa range of pressure supply. If the system is using pressure of nominal operating value that is 600 kPa than the average compressed air consumption saving is 41.9 % for conventional system and 30.6% for servo system. Return of investment periods of the proposed conventional pneumatic system with restoring energy are average 2.45 years. If the actuator of the conventional system is rodless or trough rod cylinders or semi-rotary drive, and if there is no load, which can be used for support of actuator movement, the saving is less than 5%.

**Figure 7.** Pneumatic circle a) with restoring energy, b) without restoring energy (Blagojević et al., 2011)

#### **3.14. Measuring and recording the system performance**

166 Energy Efficiency – The Innovative Ways for Smart Energy, the Future Towards Modern Utilities

described system is relatively new and expanding.

*3.13.2. Recycling of used compressed air* 

pneumatic system having acceptable price and performances (Šitum et al., 2007).

on/off electromagnetic valves controlled by PWM it is possible to develop a multifunctional

The results obtained in (Čajetinac et al., 2012) show that PWM principle of control gives a good quality of the signal tracking at the considerably lower costs and that PLC with standard program support can be used for its realization. It has been shown that it is possible to achieve control performances which are comparable to those achieved by proportional or servo valve but at rather lower cost and with better energy efficiency. Thus these methods of control may be applied as well as pneumatic servo systems since the

Pneumatic actuators are widespread in many branches of industry and a lot of efforts are done in order to increase energy efficiency in their operation. It is a known fact that pressure inside a piston chamber can reach a final value that is equal to supply pressure, only after some time being at the end position of the cylinder piston has been reached. When the direction of movement of the cylinder piston is reversed, all the compressed air contained within a working volume is released into the atmosphere. This represents a significant loss of compressed air that possesses enough potential energy to perform some other kind of work. There is a possibility to reuse this kind of compressed air, see Fig. 7 (Blagojević et al., 2011).

The performance of the system with restoring energy can be improved in comparison with the traditional control system. Compressed air consumption saving of the conventional pneumatic system with restoring energy is from 33.3% to 44.3% or average 38.8%, for 200 to 600 kPa range of pressure supply. Compressed air consumption saving of the pneumatic servo system with restoring energy is from 20.6% to 33.3%, or average 28.6% for 200 to 600 kPa range of pressure supply. If the system is using pressure of nominal operating value that is 600 kPa than the average compressed air consumption saving is 41.9 % for conventional system and 30.6% for servo system. Return of investment periods of the proposed conventional pneumatic system with restoring energy are average 2.45 years. If the actuator of the conventional system is rodless or trough rod cylinders or semi-rotary drive, and if there is no load, which can be used for support of actuator movement, the saving is less than 5%.

**Figure 7.** Pneumatic circle a) with restoring energy, b) without restoring energy (Blagojević et al., 2011)

Measuring and recording the system performance, by itself, does not increase the energy efficiency but usually presents a first step towards improvements in energy efficiency because of two primary reasons:


Three basic parameters – airflow, air pressure and consumption of electrical energy, must be measured and recorded in order to evaluate the system performance. Although this might look simple, in principle, the interpretation of these data can present a difficulty, especially under conditions of variable consumption. Measurement of flow of compressed air also involves certain technical problems and retrofitting reliable measurement instruments can be difficult if not impossible task, unless it was taken care of during the phases of system design and installation. For example, most frequently used type of flow meter must be installed into the pipeline sections that are free of turbulence and whose length must be 10 times greater than its diameter. In some systems, there is not an adequate place for installing of a flow meter. Because of that, it is suggested that large systems and medium size systems must be designed and installed in such a way to enable air flow measurements. If the flow data is not available, the cheaper alternative equipment used for pressure measurement can be very useful to, for example, measure the pressure drop over filters or pressure loss along the pipeline, or for the purposes of detecting larger variations of pressure within the system.

## **4. Increasing energy efficiency in complex robotic cell**

Increase of energy efficiency in compressed air systems in often connected with optimisation of the whole automated system. That is particularly the case with the complex automated systems such as robotic cell and robotic line. In such applications are often integrated, besides robots, pneumatic and hydraulic systems. Minimisation of energy consumption is done in most cases up to now, separately for the each subsystem. There are numerous references dealing with trajectory optimisation of industrial robots, influence of the robot velocity on energy efficiency and design of the optimal components layout within the robotic cell (Šešlija, 1988; Tomović et al., 1990; Yoshimura, 2007; Guilbert et al., 2008; Kamrani et al., 2010; Wenger and Chedmail, 1991; Dissanayake and Gal, 1994; Aspragathos and Foussias, 2002).

As well for the optimisation of operation of hydraulic systems concerning efficiency increase it is of significans to monitor the system operating parameters and to increase the reliability of components (Jocanović et al., 2012).

Having in mind the fact that compressed air is widely used in robot applications, it could be naturally that there are many possibilities for savings and its efficient usage. In that manner we can describe an example of a robotic cell (Fig. 8) optimisation with installed electric and pneumatic devices (Mališa et al., 2011).

Increasing the Energy Efficiency in Compressed Air Systems 169

pneumatic devices which would not be able to serve the process, especially if pneumatic devices are set to be the lowest energy consuming. Theoretically, the minimum pressure gives the minimum energy consumption and this could be a pressure of 3 bar, because most of the nowadays devices are able to work on low pressures. The importance of problem could be described with example of vacuum application. For the high robot's velocities, working pressure of 3 bar could not provide a sufficient vacuum level for the amount of

After the numerous experiments and analysis it is concluded that complex robotic cells that use electricity and compressed air for their operation should be optimised as follows. Firstly, parameters that influence the electricity consumption should be optimised. Secondly, parameters related to compressed air consumption should be adjusted according to constraints given by the robot and working regime. Applying the suggested optimisation method on the complex robotic cell, including the various parameters within it, is possible to significantly reduce the overall energy consumption. For example, in the considered complex robotic cell, with the reduction of the supply pressure from 6 to 4 bar, it is possible

**5. The potential of applying the measures for energy efficiency increase** 

Measures for increasing energy efficiency of compressed air systems are related to different

The greatest potential for achieving the savings exists in times of conceiving a new system because at that moment a great spectrum of energy saving measures, described in the table below, is available. This kind of situation is relatively rare, because new factories are not continuously built so even the best opportunity for systematic design becomes rarely available (first column in table 4). Table 4 gives approximate indications of phases in which

Much frequently encountered is the case of replacing the main components of the existing system. In this kind of situation, it is possible to implement many measures, some of which are faced with greater difficulty especially the ones that are related to system design: compressed air distribution network, systems with multiple pressure levels, selection of the type of end consumer, etc. It is estimated that the possibility for savings in the existing systems, in the time of replacement of main components, amounts to 2/3 of the efficiency increase that could be realized in a new system that would be designed with initially having

time determined by the process.

to decrease total air consumption by 15%.

• System installation,

phases of the life cycle of a compressed air system:

• Preventive and corrective maintenance.

each of described measures can be applied.

energy efficiency in mind (Radgen and Blaustein, 2001).

• System design, gathering of offers or purchasing,

• Significant changes in components or system improvement,

The key step is to define the parameters within a robotic cell that affect electricity and/or compressed air consumption, e.g. robot's velocity, device activity, movement trajectories, position of the robot relatively to working area; pressure of compressed air; suction capacity (in the case of vacuum); length of the supply tubes, etc. As various factors are present there are three different ways for optimisation of such complex robotic cell. First approach is the most common case when two independent experts (usually robotic expert and pneumatic expert) optimise the parameters in their domain in parallel. Second approach encompasses firstly the optimisation of the parameters that influence the compressed air consumption, and than parameters that influence the electricity consumption. The third method includes optimisation of the parameters influencing the electricity consumption, and after that, adjusting the parameters related to compressed air.

**Figure 8.** Complex robotic cell

Doing the experiments that would have confirmed one of the mentioned approaches, it was realised that, electric and compressed air parameters could not be observed separately, because most of them influence each other. For instance the adoption of the lowest robot velocity, as the lowest electricity consuming, would disregard the most important principle of a production system: productivity. On the other hand, the highest robot velocity would ensure the highest productivity of the robotic cell, but it would induce the problems with pneumatic devices which would not be able to serve the process, especially if pneumatic devices are set to be the lowest energy consuming. Theoretically, the minimum pressure gives the minimum energy consumption and this could be a pressure of 3 bar, because most of the nowadays devices are able to work on low pressures. The importance of problem could be described with example of vacuum application. For the high robot's velocities, working pressure of 3 bar could not provide a sufficient vacuum level for the amount of time determined by the process.

After the numerous experiments and analysis it is concluded that complex robotic cells that use electricity and compressed air for their operation should be optimised as follows. Firstly, parameters that influence the electricity consumption should be optimised. Secondly, parameters related to compressed air consumption should be adjusted according to constraints given by the robot and working regime. Applying the suggested optimisation method on the complex robotic cell, including the various parameters within it, is possible to significantly reduce the overall energy consumption. For example, in the considered complex robotic cell, with the reduction of the supply pressure from 6 to 4 bar, it is possible to decrease total air consumption by 15%.
