Automated Hydraulic Control Systems

INTRODUCTION Control engineering is one of the most rapidly growing fields of applied sciences in the world. Almost concivable type of machine requires the use of some form of control not only for starting and stopping but also for regulating the motion and providing the means by which the machine can think it self in various degrees. The increased emphasis on the mass production or as a means or strengthening our economy and increasing our standard of living has necessitated more, automation in every field dealing with consumers goods, since the machine tool industry is one of the basic industries, controlling and automation of them is of much importance. There are three major control systems. 1) Mechanical. 2) Electrical. 3) Fluid. Fluid control means both gas and liquid controls. In making decision for selecting the control systems the advantages and disadvantages. Limitations of all system should be considered. The limitations of mechanical controls are the limitation on speed variation, lubrication problems, space problems, uneven load application and inclusion of fall safe control. Utilization of fluid power for controlling and transmitting motion and loads has certain advantages. The fluid control system provides mechanically stiff systems that can be designed to give very fast operation and more enormous loads. They

can be employed more economically over greater distances than mechanical systems, but they are restricted to shorter distances than the electrical systems. It's advantages are as follows: 1)

It permits large power and forces to be transmitted i.e. power t weight ratio is

high. 2)

It provides various degrees of cushioning for shock loads.

3)

It offers reversible means of obtaining stepless and infinity variable speed and load.

4)

It is capable of producing completely automatic operation.

5)

It provides very accurate position control for both rotary and linear elements.

6)

Smooth working increases the tool life and allows high speed and feed rates.

7)

It provides power linkage where kinematic linkage is not possible.

8)

Wear is less due to self lubricating actions of transmission flu'ds

9)

It provides safety system for both machine and operator.

10) It offers simple and flexible means of machine operation. 11) Extremly fast response is given to the demand of operator. But there are also some limitations of this system. Periodic maintenance is essential to prevent it from getting contaminated with dirt and other foreign material. Hence good filtration system is also essential. Another disadvantage is it can not be made completely leak proof. Hydraulic systems have been used for many decades in industrial and military applications. Their high power to weight ratio results in it's wide use in machine tools, speed governing systems and position control systems.

2. REQUIREMENTS Requirements of this system are as follows : 1)

Hydraulic fluid.

2)

Hydraulic power pack.

1) Hydraulic fluid :Fluids for hydraulic systems are subjected to a wide range of pressures, velocities and temperatures. Water was the earliest hydraulic fluid used, but finds little application in modern high performance control systems. It has many limiting features which include low lubricity, danger of rusting and its useful working temperature range is restricted. The largest range of hydraulic fluids comprises oils with pertrolium base, notably five risk. This includes phosphate or silicate ester compounds halogenated fluid give off toxic vapours. Care has t be taken in. designing equipment for synthetic compounds, as packing materials used in glands and joints may to be compatible with the fluid. The lubricity of silicone fluid is less than that of petrolium products and it is poor lubricant for farous materials sliding on farous material.

Properties required for deal Hydraulic fluid are as follows :1) Viscocity. 2) Viscocity index. 3) Lubricating properties. 4) Chemical stability.

5) Compressibility. 6) Low pour point temperature 8) High flash point. 9) Low toxicity. 10) Antirust properties. 11) Resistance to foaming . Viscocity :- The viscocity of the fluid is the measure of its resistance to shearing displacement. Fluids used in hydraulic control systems must have adequate viscocity to give good seal at pistons, glands, valves but not to viscous that it adds excessive load to the moving parts wearing rapidly and loss of fluid from the system. Viscocity index :- The degree of dependence of viscocity on temperature is given by viscocity index. It should as low as possible.

Lubricating properties :It must have sufficient film strength to prevent metal to metal contact between moving parts. Lubrication breaks down when the film of fluid is punctured, resulting in damage to the surfaces between which relative movements takes place. Chemical stability :Chemical stability of hydraulic fluid refers to their resistances of formation of sluge and gum. The fluid is churned when it passes through the pumps, valves, pipings and motors and absorbs oxygen in the process. This results information

sluge and gum. Some hydraulic fluids form emulsions in use which reduces their lubricity and often causes rusting. Compressibility :- Hydraulic fluids are normally though of being incompressible. In practice fluids exhibits some degree of compressibility due to absorbed gases and aeration. The compressibility is expressed in terms of bulk modulus B and is given by, P B = ---------dv/v

Kg/Cm2

The above explained four properties are of most importance.

3. BASIC HYDRAULIC CONTROL CIRCUITS : It is as shown in block diagram consisting of control valve and actuator and power pack. Due to the flexibility of electrical equipment and the fact that large physical distance between output and input point signals are usually electrical. As shown in block diagram, a Hydraulic power pack suplies high pressures fluid. Actuator is the element of the circuit which is directly connected to that part of Machine tool of which movement is to be control. Control valve is a valve which control the flow and pressure of the fluid depending on the signal. The fluid always flows through a path of minimum resistance, in the diagram is shown the actuator will be having some load depending on the load of the machine tool to be operated. When the control valve is completely open, the whole flow will pass through the valve to the sump and the actuator will be remain at its original position. Now if the ball is given a signal in such a way so that it is partially open, then the flow through the valve will be proportional to the area of the opening. Due to control flow through the valve some pressure is built up in the system which acts on the actuator and operating the actuator, when the vlave is completely closed, then whole power from power is utilised in moving the load operating the actuator. High pressure for the hydraulic is provided by the power pack . there are one main types of power pack. 1) Constant flow power pack CONSTANT FLOW POWER PACK :Constant flow power pack have closed loop pipe line with controlled leakage as shown in fig. The fluid drawn from the reservoir by the pump

usually of cnstant displacement type, to provide a constant flow. Any foreign particles is filtered out before it passes on to the control valve of own center type which permits fluid to back to the sump. When the controlled system imposes no load on the power pack all the fluid is returned to the reservoir by control valve. This type of power pack can control only one load, and should be individually matched to its requirements Constant flow supplies are most efficiently utilised where the load uses the full flow of pump for long period.

4. COMPONENT SELECTION AND TYPES In order t make the proper selection, there must be complete familiarity with various types of hydraulic control characteristics. Now we will see some details of the components used. 1) Pump ;- It has been already seen that for the supply of high pressure fluid pumps are used and reciprocating and rotary are only used. The different type of pumps used are as follows. a) Gear pump b) vane pump c) Piston pump a) Gear pump :- These are of two types. i) External gear pump ii) Internal gear pump For low and medium pressure range i.e. upto 1500 psi, and capacity upto 120 gpm, gear pumps are used. Their volumetric efficiency is 96% and mechanical efficiency is 90%. Helical gear pumps can be operated at high speeds but they are limited by end thrust hence used upto 100 psi and herring bone upto 500 psi, different types of gear pumps are shown m fig. b) Vane pump :- These are also of two types i) Balance ii) Unbalanced load

In balanced load type pump there are two inlet and out lets and hence the casing is symmetrically accentric. Hence as pressure n both sides balances each other, it is called as balanced load vanes piimp. Balanced load pump operates at max. 200 psi and unbalanced load lipto 1000 psi. The maximum discharge capacity is 80 gpm at 1200 to 1800 rpm. c) Piston pumps :- These are of there types. i) Axial piston pump ii) Radial piston pump iii) Rotating piston pump Axial piston pump are used at above 5000 psi and some of them gives discharge above 100 gpm Their volumetric efficiency is 95% and mechanical efficiency is 90% Radial piston pump is used upto 25000 psi and 20 to 150 hp; with cylindrical piston 6000 psi max. with 1.5 gpm; with ball shaped piston. 7000 rpm fro 35 gpm, 20,000 rpm for 2 to 8 gpm Single piston pups are not generally used because they do not gives continuous pressure. Multi piston or plunger pumps are used which gives continuous discharge. Valves :- Types of valves are already given before. So now see some details about them. 1) Relief valves 2) Pressure reducing valves 3) Check valves

4) Flow control valves 5) Direction control valves Types of valves used :Pressure limiting valves :Just as electrical devices are fuse to protect the generators from over load, pumps must be provided with devices to protect them and their associated circuits from over pressure. In case of fixed displacement pumps, for example that the piston has clamped a wokr piece and that pressure should be maintained but flow reduced to a trickle to make up clearance losses. The pump would try to force more fluid into the pipe lines, and pressure rise unchecked unlimited pressure may destroy pump or cylinder or more likely may spring leaks in. Fitting or seals. Over pressure may also occurs in isolated parts of a circuits in components remote from pump. Relief valves, safety valves and hydraulic fuses set up varying degree of protection and insurance against destruction for these components. Relief Valves :Relief valves are probably the most important of the pressure limiting devices; since every pump should have the protection this valve offers. The relief valve automatically creates a passage way between the pressure line and reservoir, large enough to diver all the excess fluid supplied by a pump back to the reservoir when the pressure exceeds a chosen value.

"A ideal relief valve would permit the pressure to rise to set value and then hold that pressure constant regardless of how much oil must be diverted to the reservoir" This ideal is usually not economically feasible, and so most direct spring loaded valves will permit the pressure to rise some what when larger flows must be handled. The simplest true relief valve is sliding spoor two way

valve spool

movement gradually open a passage or orifice between the pressure input port and the port connected to the reservoir. One end of the spool is exposed to the input pressure and the other end is loaded with a coil spring. The pocket enclosing the spring is drained to the reservoir either directly or through he reservoir connection. In some circuits the main reservoir connection is not always at reservoir pressure because of special circuits configurations. In this case spring pocket pressures in spring pocket would add the forces exerted by the spring effectively raising the relief valve pressure setting.

Pressure reducing valves :Fluid pressure application often requires several different pressures from single pressure source. The pressure reducing valve is designed to supply low pressure oil to a branch circuit from high pressure line. These low pressure circuits may be used for hydraulic clamps or other mechanical devices they would be damaged by the application of full line pressure. Pressure reducing valve :-

Fig. shows a direct spring operated reducing valve in which nil from the low pressure connection is directed through an orifice to the back of spool. As secondary pressure increases the valve is closed. Another fig. shows piloted pressure reducing valve with integral pilot to control oil pressure in the spring pocket. Tightening the pilot valve spring permits higher pressure be developed in the spring pocket requiring higher secondary pressure pressure to close off main spool passage. Check valves :A valve which permits flow of fluid in one direction only and self closes to prevent any flow in opposite direction. In its simplest form this valve consists of a drilled inlet port that is closed off by a spring of gravity held ball. Flow to the inlet port lifts the ball off its seat, and oil flows freely to the outlet ports. Reverse pressure forces the ball against the seat and closes off flow. The primary disadvantages of the ball element is a poor wear characteristics. If the ball always seats in the same position, it can be expected t seat satisfactorily. If it reseats in the one position often enough to wear a groove at the seals line, any later rotation of the ball with the seal. For this reason balls are of the guided or poppets are substituted The poppet design always maintains the relationship with its seat. Flow control valve :Many advances in the art of metal working can be traced to the fine feeds made available by hydraulic transmission of power.

Variable displacement pumps are excellent for this type of services. But they may be too expensive so warrant installation is some circuits. Flow from constant delivery pumps may be controlled by an orifice effectively and at reasonable cost. The basic problem with orifices is that flow is a function of pressure drop as well as areas when pressure at the cylinder or fluid motor fluctuates in responses to load changes, flow rate changes also. Some additional pressure sensitive device must be used to compensate. The simplest form of compensator is a reducing valve installed between the high pressure lien and the orifice. The reducing valve controls flow to the orifice sod that pressure drop across the orifice is maintained constant. Needle valves and other non compensated orifices may be used to meter fluid in or out of cylinder. The flow will vary with temperature and pressure even thought he orifices locked in one position. Non compensated flow control devices are not recommended for by pass or bled off service except in very simple gravity return circuits. Broaches, planner, and other more sensitive machine use compensated flow control valves in bleed off circuits. Directional control valves :- (Two ways) A two way valve is intented to open and close two functional flow ports connected to the valve body. Many of the valves described above may be termed as two way valves. However, relief valves, pressure limiting or reducing valves, and check valves are ordinarily not reversible, that is flow between the two posts is dependent on direction and their primary function goes beyond simply stopping or permitting flow.

To way directional valve opens or closes flow between its two ports in response to some external signal, either mechanical, as with can or lever operating the steam, or electrical, as with a solenoid, or air or oil pilot operator. Pilot operated two way valves may be solenoid actuated if a small pilot valve is added. In small pipe size two way valves this solenoid may even be connected directly to the main two way spool The big advantage of the solenoid pilot control is its ability to control speed of the main two way spool movement independent of the speed of signal to the solenoid

Controlled can actuation is impracticable in many cases. No controlled opened to closed actuation would be possible in the solenoid were directly connected to the main two way spool. However, with a pilot valve connected to the solenoid the degree of spool shift speed and hence acceleration and deceleration of main two way flow will be a function of the size of pilot spool and physical configuration of main spool. Ground angles on the pilot spool or notches of various kinds can also be used to establish a delay in opening or closing of the main two way valve. This type of control is used in. glass heat treating installation. The term three or four way directional valve indicate the number of functional flow ports a valve is intented to control. Porting in three way valve consists of pressure port, one cylinder port and tan port. Most of the cylinders and fluid motors requires flow controls of both sides of the piston, and so fourth connection is added. The arrows in the symbol

indicate that the pressure may be applied to either cylinder and by changing the vlave position. The opposite cylinder connection is always ported to the tank, possibly through other financial functional control devices. As the valve shifts between none extreme position and the other, it goes through a highly neutral position. If the spool is held on center by ball retainer, balance of spring

5. ANALYSIS OF THE CIRCUIT Moulding Thermo-plastics with fluid power In manufacturing of thermo-plastics product, conversion of plastic material into finished product is very impotent step. The preference requirement of finished products largely depend on processing condition and material properties .The moulder can only use the raw material as of finished and hence the processing parameters and specifications become ofprime importance to molder. In this report I would like to explain the importance of fluid power on manufacturing of Thermo-plastics products by moulding operation.

SCREW TYPE MOULDING MACHINE AND ITS OPERATION : The present generation of screw type moulding machines are all hydraulically operated. These moulding machines have to meet specific moulding processing parameters like time, temperature and pressures. A typical mould cavity pressure and time plot is shown in fig (A). The moulding operator feeds the plastic material into the hopper which is normally dried to a prescribed temperature. The injection unit receive for further processing. The schematic diagram of injection moulding unit is shown in fig. (B). The injection unit consisting of a reciprocating screw, injection ram, drive shaft and hydraulic motor. The screw is inserted it no a hard barrel that becomes a part of the stationary feed chamber. A material hopper is mounted on top of feed chamber. The screw is inserted and keyed in to the injection ram.

The screw remains in the forward position after injection. The rotation of the screw causes the plasticized material to move forward while absorbing heat. The could and pattern are locked tighted to a predetermined high pressure. The hydraulic fluid is supplied to the injection ram for injection material. The screw moves forward injecting the plastic material into the mold.

OPEN LOOP AND CLOSE LOOP MOULDING MACHINES : The open loop moulding machines are relatively older machines. Fig (c) shows the basic hydraulic circuit normally used in open loop control moulding machines. The open loop hydraulic system uses separate flow control valves to control injection and pressure control valves to control the injection unit. These valve are set manually to reference pressure. The open loop control systems are therefore unable to adjust in relation to actual moulding machine performance desired by mould designer. The compensation for draft due to wear, temperature variations, load and plastic viscosity operation can not be accommoded. The

closed

loop

control

molding

machines

include

microprocessor control system, electro-hydraulics and feed back transducers. These elements monitor molding process through feed back and automatically adjusting the processing parameters t maintain the repeatability in quality production. The typical hydraulic circuit in closed loop machines is shown in fig (D). The actuation speed is one of the important parameters while designing the hydraulic circuit for clamp unit. A simplified hydraulic circuit for clamp

unit is shown in fig (E), The faster the ram moves, the more critical it becomes to have an accurate and repeatable stroke to avoid mold damage. Hence the clamp hydraulic circuit offens two operational speeds. Initial closing stroke is faster to save time and finally mould closes slowly to protect the mold. Design of hydraulic circuit for moulding thermo-plastic material. IMPORTANCE :- The performance requirements of finished products are largely depend on processing conditions and material properties. The moulder can only use the raw material as furnished and hence the processing parameters and specification become of prime importance to moulder. In this experiment we will study the moulding cycle and designing of hydraulic circuit for moulding thermo-plastic material. SCREW TYPE MOULDING MACHINE :The present generation of screw type moulding machines are all hydraulic. These machines have to meet specific moulding processing parameters like time, temperature and procedures. The schematic diagram of injection moulding machine are shown in the fig. The operation of screw type injection moulding machines are as follows. 1.

The injection unit consist of a reciprocating screw, injection ram, drive shaft and

hydraulic motor. 2.

The screw remains in the toward position after injection.

3.

Due to rotation of the screw, the plasticised material move forward while absorbing

heat from heat band.

4.

As the plasticised material enters the extruder front-end it displaces the screw itself

by moving the whole assembly backward against on adjustable pressure until the volume needed for the moulded product is reached. 5)

At this instant, the screw stops rotating and is ready for injection.

6)

The mold and platters are locked tight to a predetermined high pressure and calls for

injection to take place. 7)

The hydraulic fluid is supplied to the injection ram and brings about the forward

movement of the screw to inject the plasticised material into the mold. 8)

The contact between injection unit and mold is made by a nozzle assembly.

9)

The screw moves forward injecting the plasticised material into the mold.

10)

Injection pressure is held on the material as per the time set on injection timer.

Operation of mold unit performed as follows. i)

Opening mold.

ii)

Fast close due to high volume pump acts n it.

iii)

Clamping pressure - A low pressure pump controlled the damping pressure.

iv)

Mould breakaway - When mold is ready a low volume high pressure pump

remove mould. v)

Fast return - Rapid opening due to high volume high pressure pump.

HYDRAULIC SYSTEM USED TO OPERATE INJECTION MOULDING MACHINE HARDWARE USED IN HYDRAULIC CIRCUIT :1. 2.

Double acting, single end rod cylinder Pressure relief valves.

3.

3/4 direction control valve.

4.

Filter , Hydraulic pump,, electric motor , oil tank

5.

Double acting, double end and cylinder

6.

2/3 direction control valve (pilot valve operated by hydraulically)

7.

4/4 direction control valve.

8.

Hydraulic motor, microprocessor, pressure sensors, 9. Pressure resistant

position sensor. Inductive position sensor 10.

Temperature sensor. Ultrasonic distance sensor,

11.

Inductive speed sensor.

WORKING OF HYDRAULIC CIRCUIT 1.

Initially electrical signal pass to the electric motor (#l ) through set point signal

pannel. 2.

Due to rotation of electric motor <#l) , hydraulic pump (#3) pump oil from tank,

through filter to the 3/4 solenoid operated direction control valve. 3.

Due to actuation of solenoid, part 'p' connect with port 'A' and port 'T connect with

part 'B'. 4.

Therefore oil pump into the piston moves to the right hand side. Oil from rod end

side directly goes to the tank, through part 'B ' connect with port 'T' 5.

Due t this action, mold closed and ready for injection of plasticised material into if.

6.

Now plasticised material are poured into the hopper.

7.

Due to electric signal pass to the hdyraulic motor (#5), rotation of screw take place.

8.

Due to rotation of screw, electrically heated homogenous plasticised grannules are

move forward and collected of the junction of the nose of nozzle.

9.

Screw itself move backward, due to back pressure of material collected at the

junction of the nozzle. 10.

Due to backward movement of screw, oil from piston end are directly goes to the

tank through part 'A' connect with part 'T of 2/3 direction control valve (#6), as shown in the hydraulic circuit. 11.

Now sensor gives signal to the setpoint signal pannel and rotation hydraulic motor

(#5) stop. 12. 13.

Now signal goes to the electrical motion (# 2). Due to rotation of electrical motor (# 2) , hydraulic pump (#4), pump oil from tank

through filter to the 2/3 direction control valve (#6) and (#7) and 4/4 direction control valve 0#8). 14. ] 5.

However, as per hydraulic circuit their is blockage for pumped oil. Due to blockage for pumped oil. Now hydraulic signal goes to the hydraulically

operated pilot valve of 2/3 direction control valve (#7) and part 'p' connect with part 'A' and oil goes to the empty side of double acting, double ended, rod end side injection unit cylinder. 16.

Now by continous flow of oil, empty side fill up with oil and their is again

blockage for oil movement. 17.

Now electrical signal goes to the microprocessor (#9), which give signal to the

electrically operated solenoid valve or 4/4 direction control valve (#8). 18.

Due to this, part 'P' connect with part 'A' and part 'B' connect with part 'T' of 4/4

direction cntrol valve (#8) and oil flow and give signal to the hydraulically operated pilot valve of 2/3 direction control valve (#6).

19.

Due to this signal, port 'P' connect with port 'A' of the 2/3 direction control valve

(#6) and oil flows into the piston end side of double acting, double ended injection unit cylinder. 20.

By entrance of oil into cylinder, screw moves to the left hand side and electrically

heated homogenous material forcely moves into the mold cavity and achieve the required shape of mold. 21.

Remaining oil from rod end of double acting cylinder directly goes to the tank

through port 'A' connect with port 'T' of the 2/3 direction cntrol valve (#7). 22.

After predetermined time signal goes to the microprocess (# 10) which transmit

signal to the electrically operated solenoid of 3/4 direction control valve. 23.

Now oil goes to the piston end cylinder of mold through port ' P' to part 'B' &

remaining oil goes to the tank through port 'A' to part 'T' of 3/4 direction control valve (# 11). 24.

Molded part removed from mold.

25.

Recycle take place.

ADVANTAGES OF HYDRAULIC POWER 1. Fast control over ram movement. 2. Slow closing of mould of low pressure. 3,. Rapid build up of clamping force. 4. Hold on clamping force during injection, packing and cooling time 5. Slow opening of mould under high pressure 6. Fast opening 7. Decelerate to stop mold opening.

REFERENCES 1. Hydraulics and Pneumatics By -Harry L. Stewart 2) Hydraulic Control on Machine Tool By Flint, Michigan 3) Hydraulic System and Equipments By Hadeke R. 4) Industrial Hydraulics By John Pippeger, Tyier Hicks 5) Moulding Thermo-plastics with fluid power By S.S. Patil ( The economics Times fluid power joemal on page No. 8 to 18) 6) Foundry Technology - By O.P. Khanna.