Hydraulic control systems Subject: Control Engineering Guided by: S.P. Joshi Sir Group 13 Name Enrollment number Nikunj Rana 130080119026 Aashal Shah 130080119027 Kavan Shah 130080119028 Ravin Shah 130080119029

COMPONENTS OF HYDRAULIC SYSTEMS:   COMPONENTS OF HYDRAULIC SYSTEMS: The major components are: Prime mover Pump Control valves Actuators Piping system Fluid

Few supporting components are: Filters Strainers Storage tank Heat exchangers Pressure gauges Sensors Protective devices Control devices

Major components Prime mover : It is the device which develops the mechanical power. It is a power producing device. The type of prime mover will depend on the system. After passing through fluid power system this power is again available as mechanical power.

PUMP Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy (hydrostatic energy i.e. flow, pressure). It generates flow with enough power to overcome pressure induced by the load at the pump outlet. When a hydraulic pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir into the inlet line to the pump and by mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.

Control valves: The pressurised fluid supplied by the pump is required to diverted to various parts of the system. Also controls various parameter of flowing fluid. Classified into three types: Pressure control valves Flow control valves Direction control valves As name suggests , these valves control the respective parameter of fluid.

Actuators Actuators convert the fluid power contained in pressurized fluid to mechanical energy. They are the muscles of the system. They provide the mechanical motion to the desired part and the desired actuating force. The actuators can be divided into linear and rotary actuators. Example of linear actuators is single acting cylinders and rotary actuator is limited rotation motor.

Piping system: They carry fluid containing the energy to various parts of system. After transmitting energy, the return oil is brought back to the reservoir. Due to the high pressure involved, design of piping system require extreme care. Bursting of piping could prove to be a serious matter leading to damage of equipment or injury to personnel.

The supporting components Filters: Hydraulic filters remove dirt and particles from fluid in a hydraulic system. A hydraulic filter helps to remove these particles and clean the oil on a continuous basis. The performance for every hydraulic filter is measured by its contamination removal efficiency, i.e. high dirt-holding capacities. Almost every hydraulic system contains more than one hydraulic filter. Accumulators: These are storage devices. They can cater for small time fluctuations in the energy. This may be due to power failures. They provide a small pool of pressurized oil which can be used during emergencies.

Electrical devices: They provide much flexibility in operation. They are used in control of fluid power. Electrical control enables us the remote operation, automation and sequencing. Some of the electrical components include solenoids, torque motors, and limit switchers.

Pressure-control valves Pressure-control valves are found in virtually every hydraulic system, and they assist in a variety of functions, from keeping system pressures safely below a desired upper limit to maintaining a set pressure in part of a circuit. Types include relief, reducing, sequence, counterbalance, and unloading. All of these are normally closed valves, except for reducing valves, which are normally open. For most of these valves, a restriction is necessary to produce the required pressure control. One exception is the externally piloted unloading valve, which depends on an external signal for its actuation

Pressure Relief valves Most fluid power systems are designed to operate within a present pressure range. This range is a function of the forces the actuators in the system must generate to do the required work. Without controlling or limiting these forces, the fluid power components (and expensive equipment) could be damaged. Relief valves avoid this hazard. They are the safeguards which limit maximum pressure in a system by diverting excess oil when pressures get too high.

Flow control valves Flow control valves manage the flow by decreasing or increasing the opening at the throttling point. This helps to determine speed of movement for the actuators. The simplest design for a flow control valve is a needle or longitudinal slot mounted in the pipeline and connected to a screw that adjusts the opening at the throttling point. These are called throttle valves and they are regularly used in combination with a check valve, i.e. the throttle check valve for speed control in one direction of flow. A disadvantage of throttle valves is that at varying loads a change in pressure drop will change the flow; thus, the speed of the moving actuator will also be affected.

Classification of flow control valves Needle valves : Needle controls the area of orifice , which causes change in flow rate through the valve. Globe valves : Controlling element is disc or globe. Gate valves : Flow control achieved by the movement of the gate.

Directional Control Valves 2 POSITION, FOUR WAY DCV

Hydraulic Integral Controller: This is shown in fig. the components are similar to that of proportional control. But the pilot valve in this case can divert the oil to two ports. Each going to each side of a double acting cylinder.

The input and feedback link are the same The input and feedback link are the same. If an error input e is given to input link, it moves the spool of the pilot valve. Oil would be send to the corresponding port of the double acting cylinder when pressurized oil goes to the double acting cylinder. This movement of the piston is feedback to the feedback link. Thus an output motion of the cylinder is produced corresponding to the error input e. To show that the control action is integral. For an input displacement e, a proportional discharge Q is produced by the pivot valve Above Eel is the equation for an integral control. Thus the above system gives a hydraulic integral control.

Derivative Controllers: Basic definition: Derivative control is the type of control action in which, the controller output is proportional to the rate of change of the deviation. This mean that the controller output is related to the rate of change of deviation. If the deviation is changing fast, then the controller output will be high. If the deviation is changing at a slow rate, the value of controller output would be low. The deviation control action start even before the error has actually changed by that much. The slope of the change or the trend is sufficient to initiate control action.

Mathematical representation: As per the definition, in derivative control, the controller output m is proportional to rate of change of error. Where is called the derivative time. If express above eel in the Laplace domain we get,

The block diagram will look as shown

Graphical representation Derivative control action is shown graphically in fig. the error is given by a curve as shown in the error characteristics. The error increases at uniform rate, remains constant and thereafter drops at a constant rate. When error is increasing at a uniform rate, the controller output has a constant value as shown by the controller characteristics. This is because, for derivative control, the controller output is proportional to the rate of change of error. When the error curve becomes parallel to the time axis we could see that the controller output drops to zero. So though is an error. But there is no corrective action.

Hydraulic proportional integral controller: This combines, proportional as well as integral actions combined fig. shows the constructional details of a hydraulic proportional controller.

In this, instead of a pilot valve we have a distributor block and a swinging nozzle. The swinging nozzle is connected to the input link. When the input link is moved due to an input, the nozzle swings. This changes oil flow rate through port A and B The swinging nozzle produces the proportional action and the needle valve and the feedback cylinder produces the integral action. Thus the combined output is a proportional – integral action.

Mathematical representation: The three modes of control which we have seen now that is, proportional, integral and derivative mode of control are not always used in single mode. Proportional mode can be additively combined with integral mode to get the benefits of both the modes. This is called proportional – integral control. Proportional action Integral action

When two are combined It can be represented as:

Hydraulic proportional derivative controller: Given fig. shows the schematic diagram of a proportional derivative controller. It consists of an input arm to which the input displacement x is given. Pilot cylinder has five ports. Pressurized oil comes to the pilot cylinder through the center port. Return oil goes through the two end ports. Oil goes to the two sides of the power cylinder depending up on the position of the spools. The spool has two pistons connected to the common rod. Depending on the input displacement and the error signal, the spools move to the right or left. The power cylinder has one piston connected to the piston rod. Depending up on the oil flowing in to the power cylinder the piston will move in appropriate direction. This will give the output displacement y to the piston rod.

Working: When an input displacement is given to the input lever say to the right the spool move to the right. This will cause the oil to flow to the left side of the power cylinder. This will push the piston to right by an amount y. the amount of piton movement will be proportional to the input displacement x. This is the proportional part of the controller. Simultaneously the rod of the power cylinder will move the piston in the feedback cylinder. The movement of the piston in the feedback cylinder will push the fluid to the right pressurizing the fluid in the right side of the cylinder. This in turn will cause a flow rate of oil from the right side to the left side. This is nothing but a rate feedback.

Graphical representation: A proportional –derivative action cannot be adequately described by a step change. Because as we saw earlier, in pure derivative the controller output is present only during the rising part of the step. There after the output drops to zero at the flat portion of the step. So we will use a ramp signal as the error input and see how the output would be shown in fig. For a ramp signal

Block diagram I s shown in fig.

Proportional – Integral – Derivative Control (PID): This is also called the three mode control. It combines the merits of all the three modes and is widely used in processes. It is commonly known as PID Control. It is nothing but the additive combination of proportional, integral, and derivative control actions.

mathematically a PID control action is represented as In Laplace form,  

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