The Effect of Hydraulic Accumulator on the Performance of Hydraulic System

T he purpose of this paper is to depict the effect of adding a hydraulic accumulator to a hydraulic system. The experimental work includes using measuring devices with interface to measure the pressure and the vibration of the system directly by computer so as to show the effect of accumulator graphically for real conditions, also the effects of hydraulic accumulator for different applications have been tested. A simulation analysis of the hydraulic control system using MATLAB.R2010b to study was made to study the stability of the system depending on the transfer function, to estimate the effect of adding the accumulator on stability of the system. A physical simulation test was made for the hydraulic system using MATLAB to show the effect of the accumulator when it's connected to the system for different parameters and compare it with a PID controller. The hydraulic system has been simulated and tested using Automation Studio (AS) to measure different data such as the linear speed of hydraulic cylinder and the effect of connecting the accumulator to the system. All the results showed that the hydraulic accumulator has a great benefits and a large enhancement to the hydraulic system


INTRODUCTION
In the modern world of today, hydraulics plays a very important role in the day-to-day lives of people. Any device operated by a hydraulic fluid may be called a hydraulic device, but a distinction has to be made between the devices which utilize the impact or momentum of a moving fluid and those operated by a thrust on a confined fluid, i.e. by pressure. This leads us to the subsequent categorization of the field of hydraulics into: • • Help to reduce pump ripple and pressure transients.
• Absorb hydraulic shock waves (due to valve closures or actuators hitting stops).
• Used as a primary power source for small (low demand) systems.
• Help system to accommodate thermal • Compensate for system leakage expansion of the fluid ,Isaiah 2009. There are three main types of accumulators as shown in Fig .1 bladder, diaphragm bladder and piston The choice of accumulator to use in a given application depends on required speed Of accumulator response, weight, reliability and cost. The bladder accumulator is commonly used in hydraulic systems because of the main advantages of a bladder accumulator such as, fast acting, no hysteresis, not susceptible to contamination and consistent behavior under similar conditions. Hence, bladder accumulators are the best choice for pressure pulsation damping. web1,2012. So we choose this type of accumulator to be used in my experimental branch test. Yudong X., Y. 2009, presented a dynamic design of electro -hydraulic control valve with accumulator based on a physical simulation model, he found That velocity oscillation of the electrohydraulic actuator results from the inter coupling effect of the flow pressure pulsation. In order to reduce the velocity overshoot of the hydraulic actuator, an accumulator can be used to absorb the pressure pulsations to weaken the inter coupling effect. Xiangdong, k., 2010, presented a simulation and experimental study on the effects of adding accumulator to the fast forging hydraulic control system. By using the mathematical model of fast forging system and do simulation study, Minav,T.A et.al. 2012, presented how to use the hydraulic accumulator as a energy storage device that recovered from an electro-hydraulic forklift truck. The braking energy can be stored in the hydraulic accumulator for a long time, and the efficiency of the system increase from 5% to 32%.

THEORETICAL MODELING AND SIMULATION
Theoretical model for the main parts of the hydraulic system will be studied, to determine the transfer function for the system.i-Four-way directional valve controlled cylinder modeling: The directional valves are one types of the spool valve; the general relations and performance for the valve have been derived and studied.
Considering power matching of hydraulic cylinder and directional valve, Fig. 2 shows a schematic diagram was made using (Auto CAD 2012) of a valve-piston combination. If orifices area of slide valve is matching and symmetrical as shown in Fig.2, with zero lap, then the flow pressure equation in the valve is: Herbert, 1967.
Where, the sum of the line pressures (P 1 ) and (P 2 ) is approximately equal to the supply pressure (P s ). and the same for the supply flow. Valdmier, 2006. Applying the continuity equation to each chamber of the cylinder yields, The equations above solved simultaneously gives, Now by applying Newton's second law to the forces on the piston, the resulting force equation Laplas transformed, is Now substitute Eq. (7) into Eq. (5) gives, The hydraulic natural frequency And the damping ratio, is Now substitute Eqs. (9) and (10) into Eq. (8) and simplify, the transfer function of the valve controlled cylinder is given by :

3.THE LONG PIPE LINE MODELING
The precise model of fluid transmission pipeline is a dissipative friction model which is related to the frequency, and it includes a complex Bessel function and a hyperbolic function, as a result, it is very difficult to get accurate analytical solutions. Therefore, in engineering, the influences of pipeline for hydraulic system dynamic dynamic behavior are always neglected, which is unfavorable for system control under the situation of long pipeline. Considering fluid motion feature and physical properties in pipeline such as mass, damping and pressure, Jiang, 2006. So that the simple mass-springdamping dynamic model can be used to simulate the liquid in pipeline. The model is shown in Fig.3.
denotes liquid mass, damping coefficient, ( spring rate, external force and displacement. The transfer function model is derived as follows, Where, ( is natural frequency of long pipeline, =√ , ) damping ratio, √ . Fig. 4 shows a schematic diagram was made using, Auto CAD, 2012. of a single-stage pressure control valves (relief valve). The equations describing spool motion, Herbert, 1967. is,

PRESSURE RELIEF VALVE MODELING
Now, by combining Eqs. (11), (12), (16) and (22), the Transfer function for the hydraulic system can be derived: 1-Transfer function for the hydraulic system with accumulator for short pipe.
2-Transfer function for the hydraulic system without accumulator for short pipe 3-Transfer function for the hydraulic system with accumulator for long pipe.
4-Transfer function for the hydraulic system without accumulator for long pipe. Fig. 6 shows a simple block diagram of the system.

RESULT AND DISCUSSION
The hydraulic accumulator has many benefits to the hydraulic system and some of these benefits which have been tested experimentally are: 1-The use of accumulator as an storage device energy Fig.7 represents the time required the hydraulic cylinder to extend for both cases with and without accumulator and it's clearly shows that the time is decreased when the accumulator is used. The time required the hydraulic cylinder to extend at system pressure (40 bar) is (2.26 sec), without using the accumulator and at the same pressure but with connecting the accumulator to the system the time is  (1.72 sec). the decreasing in time required is about (33%).the same result shown at system pressure (30, 20 and 10 bar). Fig 8 represents the speed of extending of hydraulic cylinder with pressure for both cases with and without accumulator, and it clearly shows that the speed increased when the accumulator connected to the system. This has been happened because the potential energy stored in accumulator

2-The use of accumulator as a leakage compensator:
The accumulator acts as a compensator, by compensating for losses due to internal or external leakage that might occur during the operation. Also pressure losses happened due to the friction in pipes and connections, also due to the increase in oil temperature which affect on the performance of the hydraulic system. At pump pressures of 40, 30, 20 and10 bars, the decreasing in pressures drop are about 13.3%, 13 %, 12.7% and 12.5%respestivly ,as shown in Fig.9.

3-The use of accumulator to cushion the vibration of the system:
In this test the effect of adding the accumulator on the vibration of the system have been studied. Figs. 10 and 11 show the velocity and acceleration of vibration with and without using the accumulator .At system pressure 40 bar, the velocity and acceleration of vibration is 1.6 mm/sec and 4.3m/ respectively, So it's clearly that the accumulator cushions the vibration of the system, also the same results at system pressures (30, 20 and 10 bar). Also a graphical test using vibration meter with interface software (sw-u801wn) by (lutron company). was made at point before the relief valve with system pressure 10 bars, Figs. 12 and 13. From the results above it's clearly that the accumulator reduced the vibration of the system.

4-The use of accumulator as shock absorber:
One of the most important industrial applications of accumulators is in the elimination of highpressure pulsations or hydraulic shocks. Quan, 2007. To test this Phenomena using a graphical chart display using the pressure meter with interface software (sw-u801 wn) to view the behavior of pressure at the cylinder when it's suddenly stops at the end of the stroke. The set pressure is 30 bars, as shown in Fig.14. When the cylinder reached the end of the stroke, the stop without using the accumulator is suddenly happened and very fast and causes a hydraulic line shock, but with using the accumulator and from Fig.15. it can be seen that the cylinder stops at the end of the stroke fluently. So the benefits of adding accumulators to the system are to damp pressure spikes from pumps . 7. THE SYSTEM STABILITY TEST USING MATLAB PACKAGE V1.1(R2010B): Fig .16 represents the Bode diagram for the system without connecting the accumulator for short pipeline, and the result shows that the system is unstable because of the pulsation at the system response and the over shoot is big. Fig.17 represents the Bode diagram for the system with connecting the accumulator for the short pipeline, and the result shows that the system is stable with phase margin of 87.2 deg at frequency of 0.06 rad/sec .Also for a long pipeline Fig.18 represents the bode diagram for the system without connecting the accumulator and the result shows that the hydraulic system is unstable. Fig.19 represents the bode diagram for the system with connecting the accumulator, and the result shows that the system is stable with phase margin of 180 deg. at frequency of 10.7 rad/sec.

The physical simulations for the hydraulic system using Matlab V7.11 (R2010b):
A simulation Fig.20 shows the effect of the accumulator when it's connected to the system for different parameters like cylinder pressure, cyl-inder load, and displacement of cylinder, and the simulated results are shown in Figs. 21, 22 and 23 respectively. To compare the effect of accumulator and a PID controller to the system the effect of PID controller are shown in Figs.24, 25 and 25 which makes a self tuner to the system. The self -tuned parameters are found using MATLAB as P= 0.9, I= 1.2 and D= 0.1. But when we connected the accumulator the system become more stable and less fluctuation, as shown in Figs. 7, 28 and 29. From the figures above it's clearly that the accumulator makes the system steadier than the PID controller; for this case.
The Simulation analysis with automation studio package V5.2 The hydraulic system has been built with Automation Studio Package V5.2(AS 2008) to measure different data, such as the linear speed of hydraulic cylinder and to study the effect of connecting the accumulator to the system. Figs . 30 and 31. show a compression between the effect of connecting or disconnecting the accumulator on the linear speed at set pressure of 40 bars. The results of simulation shows that the liner speed increases by (31%) and the response become much faster. Also to estimate the effect of using the accumulator as shock absorber, Fig. 32 shows the response of the cylinder pressure at the end of stroke. It's clearly that the hydraulic cylinder stops suddenly and so fast which causes a pressure shock at the cylinder, but when the accumulator is connected to the system the rise of the pressure until the cylinder reached to the end of the stroke become more smoothly as shown in Fig.33 the set pressure is 10 bar.

CONCLUSIONS
The present, theoretical simulation analysis and experimental investigation show several conclusions these conclusions can be summarized as below:-1-The experimental tests showed that the performance of the hydraulic system clearly improvement by connecting the accumulator to the system and the results showed that the accumulator can be used in a wide variety of applications such as: a-Energy storage b-Leakage compensation c-Cushion the vibration d-Emergency operation e-Shock absorption It's found that the effect of accumulator as an energy storage is the most common application than the others. 2-A model equation for the hydraulic system combination has been derived. The theoretical simulation analysis using the bode diagram showed that the system become stable with connecting the accumulator.
3-A physical simulation test using Matlab V7.11 (R2010b) was made for the hydraulic system to show the effect of the accumulator when it's connected to the system for the different parameters, the results showed that when the accumulator is connected, the system become more stable and less fluctuation also than the PID Controller, for this case. 4-The practical results and simulation using (AS) program are clearly convergence. This leads us to the possibility of using this program for testing and analysis and design of any hydraulic system.

Figure12.
Acceleration of vibration at set pressure (10) bar without connecting the accumulator.

Figure13.
Acceleration of vibration at set pressure (10) bar with connecting the accumulator.

Figure14
. Pressure measured at the cylinder without connecting the accumulator, the set pressure is 30bar.

Figure15.
Pressure measured at the cylinder with connecting the accumulator.

Figure21
. The cylinder pressure of hydraulic system without the pid controller or the accumulator.

Figure22
. The cylinder load of hydraulic system without the pid controller or the accumulator.

Figure23.
The cylinder displacement without connecting the pid controller or the accumulator.

Figure24
. The cylinder pressure of hydraulic system with connecting the pid controller.

Figure25
.The cylinder load of hydraulic system with connecting the pid controller.

Figure26.
The cylinder displacement of hydraulic system with connecting the pid controller.

Figure27
. The cylinder pressure of hydraulic system with connecting the accumulator.

Figure28
. The cylinder displacement of hydraulic system with connecting the pid controller.
Figure29. The cylinder displacement of hydraulic system with connecting the accumulator.

Figure30.
The linear speed and acceleration of cylinder without connecting the accumulator using (as).

Figure31.
The linear speed and acceleration of hydraulic with connecting the accumulator using (as ).

Figure32.
The cylinder pressure at the end of stroke without connecting the accumulator using (as).

Figure 33.
The cylinder pressure at the end of stroke with connecting the accumulator using (as).