How Does a Hydraulic Servo System Work?

How Does a Hydraulic Servo System Work?

How Does a Hydraulic Servo System Work? What Are The Hydraulic Equipments Used?

TYPE OF ENERGY USED IN HYDRAULIC SYSTEMS

Fluid power (hydraulic energy) is used as energy source in hydraulic systems. This energy comes from the combination of potential energy, hydrostatic energy and hydrodynamic energy (Bernoulli equation). Here, besides hydrostatic energy as a function of pressure, potential energy as a function of weight or fluid height and hydrodynamic energy effects as a function of flow velocity are very small. For this reason, hydrostatic energy generated by the effect of pressure in hydraulic systems is taken into consideration. Hydraulic energy is obtained by driving a hydraulic pump by an electric motor or an internal combustion engine. In order for this hydraulic energy to fulfill the expected functions, it must be controlled by control elements (pressure, direction and flow control valves) and directed to the desired elements (hydraulic cylinder or hydromotor). Hydraulic systems are systems in which the pressure, flow rate and direction of the fluid can be controlled by using incompressible fluids, and linear, circular and angular movements can be produced with this hydraulic energy, and precise and controllable movements can be achieved. A hydraulic circuit diagram shows how a hydraulic system circuit is formed. Each element used in the circuit is represented by standard symbols and connected by pipe connections. The function sequence of the hydraulic system can be seen in the circuit diagram. In large circuit diagrams, motion and control diagrams can be used to determine the exact times of work sequences. If the circuit diagrams that seem to be large and complex are examined carefully, it can be seen that the circuit shown in Figure 1 consists of many repetitions. For this reason, the circuits of hydraulic systems can be established easily and circuit problems that may arise can be easily overcome by knowing the basic circuit elements well.

HYDRAULIC SYSTEM ELEMENTS

Brief Representation of Elements in Hydraulic Servo Mechanisms;
Hydraulic Pumps Hydromotors
Unidirectional current
conveying pump
One Way
hydromotor
Bidirectional current
conveying pump
Two sided
hydromotor
Flow
interchangeable
pumps
Hydraulic Cylinders
Single effect
Double effect
Energy-carrying lines
Control lines
Leak lines
Hydraulic Hose
Manometer
Disconnected crossing lines
Connected hydraulic lines
Filter
Accumulator
Electric motor and pump
Hydraulic oil tank
Throttle (speed) valve (adjustable)
Non-return valve. (Check Valve)
(With spring and without spring)
Pressure limiting valve (adjustable).
Hydraulic directional control valves sample symbols
4/3 valve (4 Way 3 Position)
4/3 valve (4 Way 3 Position) spring
Centering electromagnetic operated gate valve.
Manually operated spring return 3/2 valve.
Solenoid operated single side spring return 3/2 valve.
Pilot operated spring return 3/2 valve.
Seated (positioned) type hand operated 4/3 valve
Button spring return 4/2 valve

Hydraulic Pumps

The devices that convert the energy received by the motor driving them into hydraulic energy are called hydraulic pumps. Pumping process in all pumps is based on the same principle. Pumping is provided by creating an increasing volume at the suction nozzle and a decreasing volume at the discharge nozzle. Although there are many different types of pumps, the most common are vane, gear and piston pumps.

Vane Pump
The vane pump consists of a rotor, blades, ring and distribution plate with inlet and outlet holes on it, (Figure 2). The pump rotor with blades in the slots on it takes its motion from the drive shaft to which it is connected. The blades on the rotating rotor are thrown outwards by the centrifugal effect and follow the non-rotating ring wall. Thus, when the rotor rotates, the blades form volumes along the ring that gradually grow and then decrease. The suction hole through the distribution plate is in a shape corresponding to the growing volume and the discharge hole to the decreasing volume. Thus the pumping process
takes place.

Gear pump

It consists of a body with inlet and outlet holes on it and a rotating gear connected to the drive motor and a rotating gear (Figure 3). The engagement and disengagement of the gears creates increasing and decreasing volumes. In this way, the pumping process takes place.

Piston Pump
Piston pump mainly consists of cylinder block, pistons and piston shoe inclination plate. Pistons are placed circumferentially parallel to the pump axis, (Figure 4). Pumping in piston pumps is provided by the movement of the piston back and forth in the cylinder. The movement of the pistons is carried out by an angled tilt plate connected to a bearing shaft (Figure 5). With the rotation of the shaft, in one half of the rotation, the piston moves out of the cylinder block, providing increased volume (suction). In the second half of the rotation, the volume decreases as the piston moves into the cylinder block.
(pressing) performs.

Hydraulic Valves
Pressure Control Valves
In order to generate force in hydraulic cylinders and momentum in hydraulic motors, valves that keep and adjust hydraulic fluid pressure at desired values ​​are called "pressure control valves." Pressure control valves are mainly used to adjust and classify the pressure of hydraulic fluid on the circuit. Pressure control valves are divided into the following classes in terms of their duties;

1-Pressure relief valve.
2- Pressure limiting valve.
3-Pressure Reducing (or pressure regulating) valve.
4-Pressure sequencing valve.
5-Pressure balancing valve.
6-Discharge valve.
7-Braking Valve.

Flow Control Valves
Valves that allow or prevent the passage of the fluid going to or from the actuator or change the speed of the actuator (cylinder or engine) by changing the flow rate of the fluid are called “flow control valves”.

Directional Control Valves
Valves that change, close or open the flow direction (path) of the fluid in hydraulic circuits are called "directional control valves". The boxes in the eml of the directional control valves show how many positions (positions) the hydraulic valve can take.

HYDRAULIC SYSTEM CIRCUIT EXAMPLES

The hydraulic circuit of an injection press is shown. Basically the operation of the press is as follows:
- The closing unit locks the mold with the help of lever arms.
- The injection nozzle is pushed towards the mold
- Injecting the material to be injected at a constant speed
- In order to equalize the amount of goods that decrease during the cooling of the melted good,
Keeping the pressure to push from behind at the same value at all times.
- Hot flowing from the hot nozzle due to the injection nozzle being pulled from the mold
the fluid is cut off from the cooled mold.
- Opening the mold by providing return pressure after the cooling period.
- Removal of the ejected part from the machine.

Hydraulic Circuit
A pair of vane pumps are used in this circuit. Pressure monitoring for both pumps in different operating ranges 3 and 4 no. Netting with lu safety valves.
In addition, different pressures are provided by controlling the valves no. 8, 9, 10, 11 and direction valves no. 5, 7, 6. By means of safety valves, the pumps discharge into the tank without pressure while the system is idle. Valve no. 12 connects the pump no. 2 to the no. 26 motor during the operating period.
Direction Control and Speed ​​Adjustment Valves numbered 19, 14, 15, 16, 17 are used for direction control. Its speeds are controlled by valves 18, 19, 20, 21, 22 and 23, 23, 24. Here, the desired pressure in the injection nozzle has a characteristic. This pressure should always remain constant during the injection and compression phases of the nozzle docking. To achieve this, pressure reducing valve 25 is used. Hydroic engine is used for plasticizing process. Hydraulic motor no.2
It is fed from the pump and the rotation number is adjusted by the speed regulating valve 27. The pressure must be kept at a certain value in the back movement of the plasticizing screw, and this is provided by valve 28. The value of the pressure is set from 29. This pressure is called "Absolute Pressure".

RESOURCES
1-Özcan F., "Hydraulic Fluid Power",
2-Schmitt A., Aykun H. (Trans.), "Industrial Hydraulics Education Oil Hydraulics Education and Advisory Book",
3Karacan İ., "Industrial Hydraulics",
4-Karacan İ., "Hydraulic and Pneumatic",
5-Demirel K. "Hydraulic and Pneumatic",