unit 3 fluid power control · 03/07/2018 · control valves-in fluid power, controlling elements...

Unit 3

Fluid Power Control

Introduction-

Most important consideration in any fluid power system is

Control.

If control components are not properly selected the entire

system will not function as required.

The fluid power is controlled primarily through use of control

devices called Valves.

Control Valves-In fluid power, controlling elements are called valves.

There are three types of valves:A. Directional control valves (DCVs) : They determine the path

through which a fluid traverses a given circuit.

Eg- Check valve, 2-way, 3-way, 4-way DCV etc

B. Pressure control valves (PCVs) : They protect the system

against over pressure, which may occur due to a sudden surge

as valves open or close or due to an increase in fluid demand.

Eg- Pressure Relief, Pressure reducing, Sequencing, Unloading

etc.

C. Flow control valves (FCVs) : To control flow rate in various

lines of circuit. The control of actuator speed depends on flow

rate.

Eg- Globe valve, etc

A.DCVs:Receives an external signal (mechanical, fluid pilot signal,

electrical or electronics) to release, stop or redirect the fluid

that flows through it.

Function to control the direction of fluid flow in any

hydraulic system. A DCV does this by changing the position of

internal movable parts.

Classification of DCVs:Based on fluid path,

1. Check valves.

2. Shuttle valves.

3. Two-way valves.

4. Three-way valves.

5. Four-way valves.

1. Check valves:Allows flow in one direction, but blocks the flow in the opposite

direction.

Two-way valve because it contains two ports.

Ball-type check valve.

2.Shuttle valvesAllows two alternate flow sources to be connected in a one-branch circuit.

a. Forward Stroke b. Return Stroke

3. 2/2-Way DCV

A pair of two-way valves is used to fill and drain a vessel.

4. 3/2-Way DCV

P

A

5. 4/2 DCV.

4/3 directional control valve

PRESSURE-CONTROL VALVES

B.PRESSURE-CONTROL VALVES

PCV used in hydraulic systems to control actuator force

(force = pressure × area) and to determine and select

pressure levels at which certain machine operations must

occur.

Functions:

1.Limiting maximum system pressure at a safe level.

2.Regulating/reducing pressure in certain portions of the

circuit.

3.Unloading system pressure.

4.Assisting sequential operation of actuators in a circuit with

pressure control.

5.Any other pressure-related function by virtue of pressure

control.

6.Reducing or stepping down pressure levels from the main

circuit to a lower pressure in a sub-circuit.

Valves used for pressure control:

1.Pressure-relief valve.

2.Pressure-reducing valve.

3.Unloading valve

4.Counterbalance valve.

5.Pressure-sequence valve.

1.Pressure-Relief Valves (PRV)

Functions:

1. PRV limit the maximum pressure in a hydraulic circuit by

providing an alternate path for fluid flow when the pressure

reaches a preset level.

2. All fixed-volume pump circuits require a relief valve to protect

the system from excess pressure.

Types-

1. Direct Acting Relief Valve.

2. Pilot Operated Relief valve.

1. Direct Acting Relief Valve-

Function is to limit the pressure to a specified maximum value by

diverting pump flow back to the tank.

Construction & Working-

It consists of a valve body, poppet, spring and adjusting screw.

Poppet held on valve seat by spring compression, which is

adjusted by screw.

When pressure at inlet is insufficient to overcome the force of the

spring, the valve remainder on reaching closed. The pre-set

pressures reached the oil pressure over came the spring forces

and force off the poppet from its seat and by-passes sits on its

seal to tank. And poppet will remain lifted till pressure is above

pre-set pressure. As pressure reduces poppet again block

bypassing of oil to tank.

Advantage of direct-acting relief valves over pilot-operated relief valves

is that they respond very rapidly to pressure buildup. Because there is

only one moving part in a direct-acting relief valve, it can open rapidly,

thus minimizing pressure spikes.

1.2.Compound Pressure Relief Valve(Pilot-Operated Pressure Relief

Valve):

Pilot-Operated Pressure Relief Valve (cont…)

Construction & Working-

In case of direct acting relief valve the pressurized fluid is stop by spring

loaded poppet against by-passing to the reservoir. While in case of pilot

operated relief valve a balanced piston or spool block the passage of

pressurized fluid against bypassing to reservoir. Excess pressure cause

unbalances of the piston, because of which it slides to one side allowing

pressurized fluid to by-pass to the reservoir. One side chamber of piston is

connected to pressure port of direct acting relief valve.

And other side of the chamber to the pressure line of pump. Both the

chambers are connected by small orifice. Because of which pressure on both

the sides are same, and there is no continuous flow of fluid in either side of

balance piston across orifice. As pressure increase, the poppets of direct acting

relief valve get lifted and pressurized fluid bypasses to reservoir.

This causes a flow of fluid across the orifice, when fluid flow through small

orifice, their pressure drops due to throttling. Hence on this side of chamber

there is less pressure than pump side. Hence this difference in pressure cause

an unbalance force on piston and it slides to low-pressure side. This side

shifting of piston open path to pressurized fluid of pump to bypass to

reservoir.

Remote Control Pilot Operated relief Valve:A direct acting relief valve connected to pilot operated relief valve at vent port.

Adjusting main valve is very risky, as even if by mistake or oversight operator

increase pressure beyond safe limit of system, press will get damage. Hence

another addition relief valve should be fitted on control panel.

Solenoid Operated Pilot Operated Relief Valve:

2.Pressure-reducing valveUsed to maintain reduced pressures in specified locations of hydraulic

systems.

The disadvantage of this method is that the pressure drop across the

reducing valve represents the lost energy that is being converted into heat.

If the pressure setting of the reducing valve is set very low relative to the

pressure in the rest of the system, the pressure drop is very high, resulting in

excessive heating of the fluid.

3. Unloading valve:

Used to dump excess fluid to the tank at little or no pressure.

A common application is in high flow-low pressure pump circuits where two

pumps move an actuator at a high speed and low pressure

The circuit then shifts to a single pump providing a high pressure to perform

work

When punching operation begins, the increased pressure opens the

unloading valve to unload the low-pressure pump.

The check valve protects the low-pressure pump from high pressure, which

occurs during punching operation

Application of Unloading valve in a punching press (high–low circuit).

4. Counterbalance valveUsed to maintain a back pressure on a vertical cylinder to prevent it from

falling due to gravity.

They are used to prevent a load from accelerating uncontrollably.

Counterbalance valve (cont..)

Application of a Counterbalance Valve

5. Pressure Sequence Valve

Used to force two actuators to operate in sequence

Flow-control valves:Control the rate of flow of a fluid through a hydraulic circuit.

Their function is to provide velocity control of linear

actuators, or speed control of rotary actuators.

Typical application include regulating cutting tool speeds,

spindle speeds, surface grinder speeds, and the travel rate of

vertically supported loads moved upward and downward by

forklifts, and dump lifts.

FCV also allow one fixed displacement pump to supply two or

more branch circuits fluid at different flow rates on a priority

basis.

Flow-control valves can be classified as follows: 1.Non-pressure compensated. a. Gate FCV

b. Globe FCV

c. Needle FCV

2. Pressure compensated.

1.Non-pressure-compensated flow-control valves Used when the system pressure is relatively constant

and motoring speeds are not too critical.

The operating principle behind these valves is that the

flow through an orifice remains constant if the pressure

drop across it remains the same.

Disadvantage-

The inlet pressure is the pressure from the pump that

remains constant. Therefore, the variation in pressure

occurs at the outlet that is defined by the work load.

This implies that the flow rate depends on the work

load.

Hence, the speed of the piston cannot be defined

accurately using non-pressure-compensated flow-control

valves when the working load varies.

Non-pressure-compensated flow-control valves: Cont….

Pressure-compensated flow-control valve.Pressure compensated flow control valves are used to limit or

regulate flow.

P3 is conjunction with spring pressure causes downward force and

P2 causes upward force

Piston is moves up and down until pressure differential between P2

and P3 matched spring compressive force, hence termed as Pressure-

compensated flow-control valve

Non-pressure-compensated flow-control valves

1. Gate ValveA valve that opens by lifting a round or rectangular gate/wedge out of

the path of the fluid.

The distinct feature of a gate valve is the sealing surfaces between the

gate and seats are planar, so gate valves are often used when a straight-

line flow of fluid and minimum restriction is desired.

Primarily used to permit or prevent the flow of liquids, but typical

gate valves shouldn't be used for regulating flow, unless they are

specifically designed for that purpose.

Gate Valve

2. Globe valve Used for regulating flow in a pipeline, consisting of a movable disk-type

element and a stationary ring seat in a generally spherical body.

Globe valves are used for applications requiring throttling and frequent

operation

They are not recommended where full, unobstructed flow is required.

Globe valve

Needle valveA type of valve having a small port and a threaded, needle-

shaped plunger.

It allows precise regulation of flow, although it is generally

only capable of relatively low flow rates.

The solenoid valve is therefore acting much like

a switch in an electrical circuit.

In one position the light is switched off...

... and in the other position it is switched

on but there are no intermediate states.

However, another type of switch can be used

for controlling a light bulb known as a

dimmer switch.

In this case, the switch can be turned to any

position between fully off and fully on to vary

the brightness of the bulb.

SWITCHING SOLENOID VALVE

A conventional solenoid valve can be thought of

as a simple switching valve.

It is controlled by some form of electrical device

which simply switches the electrical current on

or off.

PROPORTIONAL VALVE

A proportional directional valve however will

be controlled by an electrical device more

like a dimmer switch.

PROPORTIONAL VALVE

By varying the current to either solenoid, the

amount of spool movement can be varied and

hence the amount of flow through the valve can

be controlled.

LIFT EXAMPLE - CONVENTIONAL SYSTEM

The reason why it is useful to be able to control the speed of spool movement of a valve is to

reduce shock in a system. This is achieved by controlling the acceleration and deceleration of the

actuator. Suppose, for example, that the simple hydraulic system described earlier is used to operate

a passenger lift in a hotel.


When the solenoid valve is energised to lower the lift, the valve spool will move across very

rapidly. This means that the cylinder will accelerate very quickly up to its maximum speed

(determined by the setting of flow control valve F). This sudden starting of the lift provides a very

uncomfortable ride for its occupants.

F


Similarly, when the lift reaches its destination, the solenoid valve will shut off very rapidly causing

a sudden stopping of the lift and again a very uncomfortable situation for the occupants. In real

hydraulic systems, the shocks generated by sudden starting and stopping of actuators create high

peak pressures which are one of the principle causes of fluid leakage.

LIFT EXAMPLE - PROPORTIONAL SYSTEM

If the solenoid valve and flow control valve are replaced with a proportional valve then not only

can the speed of the lift be adjusted electronically, but also its stopping and starting can be

controlled.


The proportional valve can be opened sufficiently slowly to provide a smooth acceleration of the

lift up to its maximum speed.


And likewise the deceleration can be controlled by slowing down the speed of spool movement

back to the centre condition.

time

Dis

tan

ce

MOTION CONTROL

In general therefore, proportional valves are capable of providing full motion

control in terms of:

time

Acceleration

Dis

tan

ce

MOTION CONTROL

1. A smooth and controlled acceleration of an actuator up to its maximum speed.

time

Acceleration

Velocity

Dis

tan

ce

MOTION CONTROL

2. Control of the actuator velocity and if necessary maintaining it constant with

varying loads.

time

Acceleration

Deceleration

Velocity

Dis

tan

ce

MOTION CONTROL

3. A smooth deceleration with minimal pressure peaks.

time

Force

FORCE CONTROL

Proportional valves can also be

used to control the force output

from an actuator (for example

in press or plastic injection

moulding applications) by

controlling the pressure applied

to the actuator.

time

Force

FORCE CONTROL

In such cases it is often

necessary to control not only

the maximum actuator pressure

but also the rate at which the

pressure is applied or removed.

time

Force

FORCE CONTROL

In fact the machine cycle may

consist of a series of ramps and

holding periods all of which

can be achieved with just the

one proportional valve.

time

Force

FORCE CONTROL

At the end of the machine cycle

the rate at which the pressure is

reduced is also critical in many

processes.

FORCE CONTROL

Motion and force control can

thus be achieved using

proportional valves, and in

some cases the same valve can

be used for both motion and

force control. This is usually

referred to as ‘PQ’ control ie.

the control of both pressure (P)

and flow (Q).

Furthermore, all of these

control functions can be

achieved using electronic

inputs to the valve thus

providing a simple interface to

the machine controller.

PROPORTIONAL CONTROL VALVES

Proportional control valves can be operated easily using a

solenoid.

Solenoid controls have a digital control system: A valve is

opened when the solenoid is energized and is closed when it is

de-energized or vice versa.

They are very quick in their operation and thus give rise to

pressure and flow surges in the fluid power control units

The advantage of these valves is that they give greater

flexibility in the system design and operation.

They also decrease fluid power circuit complexity especially

for processes requiring multiple speed or force outputs.

Notched spool proportional valve.(a) Valve construction;(b) electrical control diagram

Cartridge valves

The term cartridge valves commonly refers to pressure,

directional, and flow control valves that screw into a

threaded cavity.

These valves are mostly rated for low flows - 40 gpm or

less

They are available in many configurations; on/off,

proportional, pressure relief, etc. They generally screw into a

valve block and are electrically controlled to provide logic

and automated functions.

Advantages:1.Leakage problem minimum

2.Small size and light in weight

3.Operating noise level is less

Pre-fill valve

Pre-fill valves operate similarly to pilot-operated check valves, but they are usually much larger.

Their normal function is to fill and exhaust a large bore cylinder as it

travels to and from contact with the work piece. Large, high-tonnage

presses -- both vertical and horizontal -- use pre-fill valves to reduce

pump size while maintaining cycle time.

A large main-flow poppet seals the path between the tank and the

cylinder ports. As the piston advances, vacuum in the void behind it

allows atmospheric pressure to push the main-flow poppet open so

fluid from the tank can fill this void. On the retraction stroke, a signal

to the pilot piston pushes the main-flow poppet open so fluid can return

to tank. While a pilot-operated check valve’s pilot piston is larger than

the poppet it opens, the main-flow poppet in a pre-fill is much larger in

diameter than the pilot piston. Thus it is impossible to open the main-

flow poppet against high backpressure. This keeps decompression

shock from damaging pipes and components.

The circuit in operates a vertical single-acting hydraulic ram press with pullback cylinders for the

retraction stroke. The press has a poppet-type pre-fill and gets a fast stroke from only filling the pullback

cylinders during the approach stroke. A sequence valve keeps pump flow from going to the ram until

pressure reaches a preset level.

During the approach part of the stroke, atmospheric pressure pushes fluid into the large-bore ram through

the pre-fill valve because there is vacuum behind the extending ram. When it contacts the work, the ram

stops and the pre-fill valve closes. Pressure starts to rise and when it is high enough to open the sequence

valve, pump flow goes to the pullback cylinders and the ram. Extension speed slows and tonnage increases

to do the work required.

A signal that the work is complete shifts the directional control valve to send pump flow to the rod ends of

the pullback cylinders and to the pilot signal of the pre-fill valve. The pre-fill valve’s pilot piston moves

forward and contacts the decompression poppet. This lets trapped fluid flow out at a controlled rate.

Pressure in the ram drops quickly and smoothly. When pressure is low enough, the pilot piston opens the

main poppet to let fluid from the ram return to tank. When the ram loses pressure, the pullback cylinders

can raise the platen and push fluid from the ram back to tank.

Brake valve

A brake valve performs the same function as a counterbalance valve, but it is designed to

overcome a key disadvantage . The pressure drop across the counterbalance valve is converted

to heat; consequently, half the hydraulic power is wasted.

A brake valve has an internal pilot passage and a remote pilot passage. Suppose the spring is

set for 1000 psi. When pressure at the internal pilot reaches 1000 psi, the piston pushes the

spool upward to open the valve.

The area of the piston is much less than the area of the bottom of the spool. A typical area ratio

might be 10:1. The remote pilot applies pressure directly to the bottom of the spool;

consequently, only 100 psi is required to compress the spring and open the valve. Pressure

required to open the valve is 1000 psi at the internal pilot and 100 psi at the remote pilot.

It requires 100 psi at the motor inlet to keep the valve open. As long asthe load on the motor requires more than 100 psi, the brake valve doesnot affect circuit efficiency. If the load starts to overrun, and the pressuredrops below 100 psi, the brake valve closes. It requires 1000 psi at thedirect (or internal) pilot to open the valve. This 1000-psi pressure dropacross the brake valve converts the mechanical energy of the overrunningload to heat energy and slows the load. When pressure at the inlet buildsto 100 psi again, the brake valve opens

Hydraulic Servo valves

The hydraulic systems, subsystems and hydraulic components that have been discussed so far have

had open-loop control or in other words power transfer without feedback. We shall now take a

look at servo or closed loop control coupled with feedback sensing devices, which provide for a

very accurate control of position, velocity and acceleration of an actuator.

A servo valve is a direction control valve, which has an infinitely variable positioning capability.

Thus, it controls not only the direction of the fluid flow but also the quantity. In a servo valve, the

output controlled parameter is measured with a transducer and fed back to a mixer where the

feedback is compared with the command. The difference is expressed in the form of an error

signal which is in turn used to induce a change in the system output, until the error is reduced to

zero or near zero. A typical example is the use of a thermostat in an automatic furnace whose

function is to measure the room temperature and accordingly increase or decrease the heat in

order to keep it constant. Let us now discuss in brief, the various components that comprise a

servo system.

This valve is essentially a mechanical force amplifier used for positioning control. In this

design, a small impact force shifts the spool by a specified amount. The fluid flows through

port P o retracting the hydraulic cylinder to the right. The action of the feedback link shifts

the sliding sleeve to the right until it blocks off the flow to the hydraulic cylinder. Thus, a

given input motion produces a specific and controlled amount of output motion. Such a

system where the output is fed back to modify the input, is called a closed loop system.

One of the most common applications of this type of mechanical hydraulic servo valve is in

the hydraulic power steering system of automobiles and other transport vehicles.

unit 3 fluid power control · 03/07/2018 · control valves-in fluid power, controlling elements...

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