1. INTRODUCTION
Here we have fabricated the pneumatic forging machine; it's a new innovative concept. Forging is the term for shaping metal by using localized localiz ed compressive forces. Cold forging is done at room temperature or near room temperature. Hot forging is do ne at a high temperature, which makes metal easier to shape and less likely to fracture . Warm forging is done at intermediate temperature between room temperature and hot forging temperatures . This machine has been mainly developed dev eloped for metal forming to the required shape and size. In this machine we have using the pneumatic cylinder for forging the specimen. Pneu matic is air operated device. By doing the manual process it consumes more time and large amount of man power required for forging. By using this machine we can save the time and man power requirement in the industries Here we have fixed the pneumatic cylinder on the column of the machine which is fixed on the base table. The forging hammers are fixed at the end of the pneumatic cylinder piston rod. The pneumatic cylinder is operated through the pneumatic energy (air). The air stored in a compressor the compressed air is passed to the pneumatic cylinder with the help of the solenoid valve. The solenoid valves are operated through the control unit. The air enters on port one in the pneumatic cylinder to moves the forging hammer in downward direction. After the forging operation takes places the forging hammer will moves in upward direction. While on the hammer moves in upward direction the port one will release and the air will be forced into the port number two. The two directions are controlled by control unit. During the movement the forging hammer forged the specimen to make it to the required shape and size. After the required number of strokes completed the forging process is stopped by the help of the control unit
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2. BLOCK DIAGRAM
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3. Pneumatic Actuator (Air Cylinder) Basics
There are thousands of industrial applications that require a linear motion during their operation sequence. One of the simplest and most cost effective ways to accomplish this is with a pneumatic actuator. Pneumatic actuators are also very clean operating because the operating fluid is a gas, which prevents leakage from dripping and contaminating the surroundings. This section will discuss the basic construction and function of a pneumatic actuator, the relationship with a fluid power system and the selection guidelines for pneumatic actuators or air cylinders. Basic Styles
Pneumatic actuators convert compressed air into rotary or linear motion. There are many styles of pneumatic actuators: diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders.
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he most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. Even so, there is a large variety of construction techniques and materials to fit a wide range of applications and user preferences. Body materials can be aluminum, steel, stainless steel and even certain polymers. Construction can be either non-repairable or repairable. This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders have a single port to allow compressed air to enter the cylinder to move the piston to the desired position. They use an internal spring or sometimes simply gravity to return the piston to the “home” position when the air pressure is removed. Single-acting cylinders are a good choice when work is done only in one direction such as lifting an object or pressing an object into another object. Double-acting cylinders have a port at each end and move the piston forward and back by alternating the port that receives the high pressure air. This uses about twice as much energy as a single-acting cylinder, but is necessary when a load must be moved in both directions such as opening and closing a gate.
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In a typical application, the actuator body is connected to a support frame and the end of the rod is connected to a machine element that is to be moved. A control valve is used to direct compressed air into the extend port while opening the retract port to atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the area of the piston. If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element. Changing the valve to direct compressed air to the retract port while opening the extend port to atmosphere will cause the cylinder assembly to retract back to the “home” position.
Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, there are compressors, filters, pressure regulators, lubricators, control valves and flow controls. Connecting all of these together is a network of piping or tubing (either rigid or flexible) and fittings.
Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure the desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly or even make it unable to move its load at all.
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Force
The above Figure shows a basic system to power and control a pneumatic actuator. When selecting an actuator it is important to properly match the cylinder to the job.
A typical pneumatic system configuration is shown in Figure 4C. The theoretical force available in the actuator is the piston area multiplied by the supplied air pressure. Spring force must be subtracted from this value for single-acting cylinders. The actual force of the actuator will be 320 percent less due to pressure losses in the system. A good rule to use when sizing an actuator is to select an actuator that has about 25% more force available than needed for the job, and the following formula can help with determining size requirements. Take a look at our Interactive Cylinder Bore Calculator here.
Speed
When the cylinder force (F) is known, the bore diameter(d) can be found by the above formula. F is the force required (lbs) and P is the supply pressure (psi). Stroke length is determined by the required travel of the machine element driven by the actuator. The speed at which the cylinder can move a load is directly related to the rate that the compressed air can flow through the pneumatic system to the piston to make it move.
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This can often be a little tricky to calculate, since as the flow rate increases, system resistance (basically friction of the air moving through pipes and components) will increase in a non-linear fashion. The result is a larger pressure drop from the supply (air compressor) to the cylinder. When the pressure drop is so large that the available pressure at the cylinder cannot move the load, the cylinder will stall. When speed is critical to a machine operation, it may require testing two or three combinations of valves, tubing and cylinders to get the desired per formance. Let’s look at a practical example of how you would figure out your requirements. See our “Rules of Thumb” for fast cylinder action along with our Interactive Theoretical Speed Table here.
For example:
It is desired to move a 200lb load 12 inches at a rate of 20 cycles per minute. Using a 2” bore cylinder, about 64 psi is required to move the load. Adding 25% gives an operating pressure of 80 psi. At the desired cycle rate and using 1/4” OD tubing (0.156” ID), pressure losses in the tubing are about 1.5 psi per foot. It can be seen that the tubing run total (extend and retract lines) needs to be less than 10 feet or else the pressure losses due to friction will drop the available pressure at the cylinder below 64 psi and the cylinder will stall. Once the cylinder stops moving however, the friction losses go away and the pressure builds back up to 80 psi. This situation results in a jerky motion of the cylinder as it moves the load. Several factors could overcome this problem:
System pressure can be increased to overcome friction losses
Larger tubing can be used to reduce friction losses
Different size cylinder could be tried that will reduce the flow
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Mounting
The final bit of basic selection criteria is the cylinder mounting arrangement. There are many different configurations available from various manufacturers. The more common ones include rigid nose or tail mount, trunnion mount, rear pivot mount and foot mount. A study of the machine motion required usually will show which mounting configuration is the best choice.
Once the basic actuator size and configuration is known, other options such as end-of-stroke cushions, magnetic piston (for position detection switches) or special seals should be considered when making the final selection.
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How Pneumatic Cylinder Work
The pneumatic cylinder is a device that has been mechanically engineered to produce force (in linear motion) from compressed gas. They are also k nown as air cylinders.
The air cylinders come in various appearances and sizes and are meant to perform different functions depending on the rising needs of the market. Depending on your ready market, you can choose to go for Single-acting Cylinder
This type of air cylinder is quite small in size as compared to the other types. They create driving linear force (precisely &lout’) from the pressure imparted by the compressed air by the piston which then springs back to the original position. This type of air cylinder will be best suitable for the customers who need little application since it has a limited extension due to the small space for compressed air.
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Double acting cylinders
With expanded application, a customer can consider buying this machine. It has two ports that enable it to perform both extend and retract strokes. Its stroke length is also not limited a fact that presents the double acting cylinder a better choice of pneumatic cylinder that should never miss in your hardware shop stock. 3) Multiple stage telescoping cylinder
As the name suggests, it is a combination of both sing-acting cylinder and the double acting cylinder in performance capability. With both piston rod nested with series of hollow stages in an increasing diameters; this machine allows notable longer strokes. lt is the best tool for customers who deal with applications that have minimal side loading.These are the main types of air cylinders that are well known and widely used whose demands suit the market. However, there are other types too.Why should you stock the pneumatic cylinders? The air cylinders work with the basic physics principles which state that force produced by the cylinder is directly proportional to the surface area of the piston acting on it with other factors like the friction and the turbulence kept constant. This theoretically implies that the machine performs reliably between 50% and 70%. Their extensive usage makes them more marketable since. Many engineers choose to use the pneumatic cylinder since they are cleaner, produce less noise,and they do not need large space for fluid storage.ii. In addition, pneumatic cylinders are also the most preferred in the market since their operating fluid is a gas; and by the fact that gases do not drip during leakages keeps the operating environment cleaner. The air cylinders are also worthy in the stock since their choice of use is wider depending on the job specification, the level of loads, temperature, and stroke length needed. Air cylinder installed with quick exhaust valves increases the air cylinder cycle. This makes your stock more marketable and allows quick replenishing of stock as these will be the most preferred machines. When going to shop for the pneumatic cylinders, the customer is often driven by the quality of the product. As an entrepreneur of air cylinders, you need to win the trust of your customer by stocking standard air cylinders that have a wide range of bone sizes of 16 and 18 inches of bore sizes and metric sizes respectively.
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The cylinders must also have many mounting configurations and standard switch capability that relies on the location of the application and mach-inability of the air cylinder. For the double-acting pneumatic cylinders with two pressure setup, you can consider stocking the types with air pressure and exhaust valve installed which during operation leads to a higher pressure production for the work force and low pressure for return force. This modification systematically reduces the operational cost; features that have made air cylinders more marketable as such are the machines that can be multipurpose. When operating with the pneumatic cylinder, customers are often pissed up by the noise it produces from the impacting air cylinder end caps. The noise is not only irritating to the user but also harmful to the machine itself and this could be a turnoff to the operators. Therefore to make small but significant changes to your models in stock will be to your business’ advantage. You can either add to your stock the internal cushions 11
or customize your appliances by fixing the cushions that will add to the stroke time and reduce noise produced by the stocked air cylinders. Cushions are always adjustable hence are less expensive. This will significantly increase the demand from your customers as well as making the air cylinder more suitable for use even to customers in noise-restricted areas. In your stocking, you can also consider having a continuous feedback sensor and closed-loop valve controller incorporated air cylinders in stock thus making them both fully extended and fully retracted. These machines will effectively provide linear electric drives a quality looked for in the market. The features air cylinders definitely present uniqueness an increased demand for your stoked machines that will make them highly competitive in the market. Why the pneumatic cylinder suits a wider market
The varied sizes of the gadget with the ability to perform various jobs that vary from picking very small objects to larger ones make them more applicable for industrial use widely in different fields. You will never again suffer a dead-stock with these kinds of flexible air cylinders since they allow the operator to have full control over it as it follow a commanded motion or index to various adjustable locations of the operator’s choice.
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ln addition to that, the design of air cylinder suits the vast needs of your customers in terms of configuration and scope of application. Therefore, your clients will comfortably offer to get the exact value for their money. The durability of a machine is also one of the key driving force to many customers and having air cylinder stock with command start and stop points of motion less than the full stroke of a cylinder makes the pneumatic cylinders more durable. With these features, modifications and broad application of pneumatic cylinder directly translate into their demand in the market with assured moving-stock.
4. Pneumatic valves
compressed air pneumatic systems require methods of safe and precise control of the actuators unique to their accoutrement. Although the medium is fluid, just as hydraulic or process water systems, the execution of control is different in many ways than with a liquid. What is shared in the conduction of any fluid power medium is the need for valves to control force, velocity and direction of movement. Air preparation
Pressure relief valves will control pressure at their inlet port by exhausting pressure to atmosphere. Relief valves are typically used only in receivers or air storage devices, such as accumulators, as a means to prevent excessive pressurization. As such, relief valves are often called safety valves and are not typically appropriate for use anywhere but the air preparation stage.
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Pressure regulators in pneumatic systems limit pressure downstream of the unit by blocking pressure upstream at the inlet. Regulators are used in the air preparation stage, as well as in control of cylinders and motors. The letter R in the acronym FRL stands for regulator, which is installed downstream of the receiver tank, but be fore the circuit they are regulating pressure for. Sometimes multiple stages of pressure reduction are required, especially with a large centralized compressor and receiver feeding various workstations. A regulator can control pressure within the main grid of distribution plumbing, but sometimes air is piped directly to an FRL at each workstation or machine. Pressure at this main header could be 120 psi or more, but a branch circuit could be regulated at 90 psi, for example. Most regulators are capable of relieving downstream pressure, which prevents that downstream pressure from elevating as a result of load-induced pressure or thermal expansion. Pressure regulators can be had as stand-alone units, but sometimes a filter is attached to kill two birds with one stone. Regulators are most often available as a component of a modular set, with a filter, regulator, lubricator or dryer etc., and can be assembled in any combination. The regulator will have an inlet port, outlet port and a port for the pressure gauge, which they are most often included with. Pressure regulators can also be used to control pressure for individual actuators, such as an inline regulator or work-port mounted regulator. These are typically quite small and included with reverse flow check valves, as would be required for double acting function of a cylinder, for example. Further still, differential pressure regulators are offered by some manufacturers, to maintain a set pressure differential between the two ports, rather than just maintaining downstream pressure. It should be noted that all pressure regulators are adjustable, most often with screws or knobs
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6. Flowcontrols
Also common in pneumatic systems are valves to control flow. There are fewer available types of flow valves compared to pressure or directional valves, but most circuits apply them to make for easy adjustment to cylinder or motor velocity. Controlling velocity in pneumatic systems is more complex than in a hydraulic system, because pressure differential between the work ports of a cylinder plays a larger part.
Flow control valves for pneumatic systems are quite simple, usually available in two configurations used in two different ways. One configuration is merely a variable restriction, with a screw or knob adjustment to open and close a variable orifice, which is also often referred to as a needle or choke valve. The other type introduces a check valve, which allows free flow in one direction, and restriction in the opposing direction. For whatever reason, this valve has hijacked the name flow control all for itself.
Flow control valves are applied in two different ways; meter in or meter out. Meter in is the method of controlling the rate of airflow as it enters a motor or cylinder. When metering in, a cylinder will move rapidly with high force and efficiency, but the motion of the piston is prone to spongy and unpredictable movement. When metering out, the cylinder velocity is more stable and repeatable, but efficiency and dynamic force are lost to the energy required to push past the flow control. Regardless, most pneumatic applications operate using meter out flow controls, because the disadvantages are easy to overcome by increasing upstream pressure. A method of increasing cylinder velocity, typically for double acting or spring-return cylinder retraction functions, is to add a quick exhaust valve to the cap side work port. Because cylinders retract faster than they extend as a result of differential air volumes, it is harder to evacuate the cap side air volume without oversized valves or plumbing. A quick exhaust valve vents directly to air from the cap side work port, and massively reduces the backpressure created upon retraction, permitting very rapid piston velocity.
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7. Directional control valves
Pneumatic directional valves are available in many sizes, styles and configurations. At the basic end of the spectrum is the simple check valve, which allows free flow in one direction and prevents flow in the reverse direction. These can be installed anywhere from right after the receiver to within a flow control valve itself.
As directional valves grow in complexity, they are specified under a general naming practice related to the number of positional envelopes of the valve and the number of work ports in the valve, and specifically in the order described. For example, if it has five ports, port 1 will be for pressure inlet, ports 2 and 4 for work ports, and 3 and 5 for the exhaust ports. A valve with three positions will have a neutral condition, extend condition and retract condition. Putting it all together, this describes a five-way, three-position valve, also referred to as a 5/3 valve. The common configurations seen in pneumatics are 5/3, 5/2, 4/2, 3/2 and sometimes 2/2 valves.
Also part of the description of a directional valve is its method of both operation and positioning. The valve operator is the mechanism providing the force to shift the valve between its positions. The operator can be a manual lever, electric solenoid, an air pilot, or cam mechanism, to name a few. Some valves are a combination of these, such as a solenoid pilot valve, which is a tiny valve providing pilot energy to move the main-stage valve. Positioning of any valve is achieved by either a spring, such as with a 5/2 spring-offset valve, or with detents in 5/2 detented valves.
A 5/2 spring-offset valve will return to its starting position when energy is removed from its operator, like de-energizing the coil, or removing pilot pressure. A 5/2 detented valve will stay in the position it was last activated to until the operator switches it again.
Pneumatic valves are manufactured in various incarnations. Poppet valves are simple, using a spring to push a face of the poppet down on its seat. Construction can be metal-to-metal, rubberto-metal or even with diaphragms. Poppet valves can often flow in one direction, just as a check valve, but need to be energized to flow in reverse. They are limited to two- or three-way port configurations, although they can mimic four- or five-way valves when used in parallel. They offer typically high flow conductance for their size, and are generally very resistant to contamination.
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Spool valves use a notched metal cylinder that slides within a precisely machined body, drilled with three to five ports, or even seven ports if the valve is pilot operated. Low-end valves consist of only a spool and body, and are prone to internal leakage. Better valves use seals in the body or spool to prevent leakage between ports. High-end spool valves are constructed with precision, often requiring fine lapping procedures during manufacturing, and with their tight tolerances, often require few seals, improving reliability and longevity. Other forms of high-end valves use a sliding block of metal or ceramic, which is not only efficient, but also extremely resistant to contamination, making them great for dirty environments.
Mounting considerations
Pneumatic directional valves come in both standard and non-standard mounting configurations. The non-standard valve is constructed at the whim of the manufacturer, with port layout, operator style and mounting options unique to their product. They can be inline, subplate mounted or sectional stacks mounted in a row. Because each manufacturer does mounting differently, it is best to research the product appropriate for your application. Luckily, most manufacturers have lines of standardized valves suiting one or more specification, such as ISO 5599-1, with its staggered oval ports; this means one manufacturer’s valve will fit the subplate or manifold of another manufacturer’s. Port and electrical connections are standardized with most valves as well. NPT ports are common, but many new valves come with push lock fittings on the subplate itself. Electrical connectors for standardized valves are frequently DIN, mini-DIN or with field bus connection, making the operation of a dozen valves as easy as one connector
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8. Timer control
We have seen that Multivibrators and CMOS Oscillators can be easily constructed from discrete components to produce relaxation oscillators for generating basic square wave output waveforms. But there are also dedicated IC’s especially designed to accurately produce the required output waveform with the addition of just a few extra timing components.
One such device that has been around since the early days of IC’s and has itself become something of an industry “standard” is the 555 Timer Oscillator which is more commonly called the “555 Timer”.
The 555 timer which gets its name from the three 5kΩ resistors it uses to generate the two comparators reference voltage, is a very cheap, popular and useful precision timing device that can act as either a simple timer to generate single pulses or long time delays, or as a relaxation oscillator producing stabilized waveforms of varying duty cycles from 50 to 100%.
The 555 timer chip is extremely robust and stable 8-pin device that can be operated either as a very accurate Monostable, Bistable or Astable Multivibrator to produce a variety of applications such as one-shot or delay timers, pulse generation, LED and lamp flashers, alarms and tone generation, logic clocks, frequency division, power supplies and converters etc, in fact any circuit that requires some form of time control as the list is endless.
The single 555 Timer chip in its basic form is a Bipolar 8-pin mini Dual-in-line Package (DIP) device consisting of some 25 transistors, 2 diodes and about 16 resistors arranged to form two comparators, a flip-flop and a high current output stage as shown below. As well as the 555 Timer there is also available the NE556 Timer Oscillator which combines TWO individual 555’s within a single 14-pin DIP package and low power CMOS versions of the single 555 timer such as the 7555 and LMC555 which use MOSFET transistors instead.
A simplified “block diagram” representing the internal circuitry of the 555 timer is given below with a brief explanation of each of its connecting pins to help provide a clearer understanding of how it works.
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555 Timer Block Diagram
• Pin 1. – Ground, The ground pin connects the 555 timer to the negative (0v) supply rail. • Pin 2. – Trigger, The negative input to comparator No 1. A negative pulse on this pin “sets” the internal Flip-flop when the voltage drops below 1/3Vcc causing the output to switch from a “LOW” to a “HIGH” state. • Pin 3. – Output, The output pin can drive any TTL circuit and is capable of sourcing or sinking up to 200mA of current at an output voltage equal to approximately Vcc – 1.5V so small speakers, LEDs or motors can be connected directly to the output. • Pin 4. – Reset, This pin is used to “reset” the internal Flip-flop controlling the state of the output, pin 3. This is an active-low input and is generally connected to a logic “1” level when not used to prevent any unwanted resetting of the output. • Pin 5. – Control Voltage , This pin controls the timing of the 555 by overriding the 2/3Vcc level of the voltage divider network. By applying a voltage to this pin the width of the output signal can be varied independently of the RC timing network. When not used it is connected to ground via a 10nF capacitor to eliminate any noise.
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• Pin 6. – Threshold , The positive input to comparator No 2. This pin is used to reset the Flip-flop when the voltage applied to it exceeds 2/3Vcc causing the output to switch from “HIGH” to “LOW” state. This pin connects directly to the RC timing circuit.
• Pin 7. – Discharge , The discharge pin is connected directly to the Collector of an internal NPN transistor which is used to “discharge” the timing capacitor to ground when the output at pin 3 switches “LOW”. • Pin 8. – Supply +Vcc, This is the power supply pin and for general purpose TTL 555 timers is between 4.5V and 15V.
The 555 Timers name comes from the fact that there are three 5kΩ resistors connected together internally producing a voltage divider network between the supply voltage at pin 8 and ground at pin 1. The voltage across this series resistive network holds the negative inverting input of comparator two at 2/3Vcc and the positive non-inverting input to comparator one at 1/3Vcc. The two comparators produce an output voltage dependent upon the voltage difference at their inputs which is determined by the charging and discharging action of the externally connected RC network. The outputs from both comparators are connected to the two inputs of the flip-flop which in turn produces either a “HIGH” or “LOW” level output at Q based on the states of its inputs. The output from the flip-flop is used to control a high current output switching stage to drive the connected load producing either a “HIGH” or “LOW” voltage level at the output pin. The most common use of the 555 timer oscillator is as a simple astable oscillator by connecting two resistors and a capacitor across its terminals to generate a fixed pulse train with a time period determined by the time constant of the RC network. But the 555 timer oscillator chip can also be connected in a variety of different ways to produce Monostable or Bistable multivibrators as well as the more common Astable Multivibrator. The Monostable 555 Timer
The operation and output of the 555 timer monostable is exactly the same as that for the transistorised one we look at previously in the Monostable Multivibrators tutorial. The d ifference this time is that the two transistors have been replaced by the 555 timer device. Consider the 555 timer monostable circuit below.
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Monostable 555 Timer
When a negative ( 0V ) pulse is applied to the trigger input (pin 2) of the Monostable configured 555 Timer oscillator, the internal comparator, (comparator No1) detects this input and “sets” the state of the flip-flop, changing the output from a “LOW” state to a “HIGH” state. This action in turn turns “OFF” the discharge transistor connected to pin 7, thereby removing the short circuit across the external timing capacitor, C1. This action allows the timing capacitor to start to charge up through resistor, R1 until the voltage across the capacitor reaches the threshold (pin 6) voltage of 2/3Vcc set up by the internal voltage divider network. At this point the comparators output goes “HIGH” and “resets” the flip-flop back to its original state which in turn turns “ON” the transistor and discharges the capacitor to ground through pin 7. This causes the output to change its state back to the original stable “LOW” value awaiting another trigger pulse to start the timing process over again. Then as before, the Monostable Multivibrator has only “ONE” stable state. The Monostable 555 Timer circuit triggers on a negative-going pulse applied to pin 2 and this trigger pulse must be much shorter than the output pulse width allowing time for the timing capacitor to charge and then discharge fully. Once triggered, the 555 Monostable will remain in this “HIGH” unstable output state until the time period set up by the R 1 x C1network has elapsed. The amount of time that the output voltage remains “HIGH” or at a logic “1” level, is given by the following time constant equation.
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Where, t is in seconds, R is in Ω and C in Farads. 555 Timer Example No1
A Monostable 555 Timer is required to produce a time delay within a circuit. If a 10uF timing capacitor is used, calculate the value of the resistor required to produce a minimum output time delay of 500ms. 500ms is the same as saying 0.5s so by rearranging the formula above, we get the calculated value for the resistor, R as:
The calculated value for the timing resistor required to produce the required time constant of 500ms is therefore, 45.5KΩ. However, the resistor value of 45.5KΩ does not exist as a standard value resistor, so we would need to select the nearest preferred value resistor of 47kΩ which is available in all the standard ranges of tolerance from the E12 (10%) to the E96 (1%), giving us a new recalculated time delay of 517ms. If this time difference of 17ms (500 – 517ms) is unacceptable instead of one single timing resistor, two different value resistor could be connected together in series to adjust the pulse width to the exact desired value, or a different timing capacitor value chosen. We now know that the time delay or output pulse width of a monostable 555 timer is determined by the time constant of the connected RC network. If long time delays are required in the 10’s of seconds, it is not always advisable to use high value timing capacitors as they can be physically large, expensive and have large value tolerances, e.g, ±20%. One alternative solution is to use a small value timing capacitor and a much larger value resistor up to about 20MΩ’s to produce the require time delay. Also by using one smaller value timing capacitor and different resistor values connected to it through a multi-position rotary switch, we can produce a Monostable 555 timer oscillator circuit that can produce different pulse widths at each switch rotation such as the switchable Monos table 555 timer circuit shown below.
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A Switchable 555 Timer
We can manually calculate the values of R and C for the individual components required as we did in the example above. However, the choice of components needed to obtain the desired time delay requires us to calculate with either kilohm’s (KΩ), Megaohm’s (MΩ), microfarad’s (μF) or picafarad’s (pF) and it is very easy to end up with a time delay that is out by a factor of ten or even a hundred. We can make our life a little easier by using a type of chart called a “Nomograph” that will help us to find the monostable multivibrators expected frequency output for different combinations or values of both the R and C. For example,
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Monostable Nomograph
So by selecting suitable values of C and R in the ranges of 0.001uF to 100uF and 1kΩ to 10MΩ respectively, we can read the expected output frequency directly from the nomograph graph thereby eliminating any error in the calculations. In practice the value of the timing resistor for a monostable 555 timer should not be less than 1kΩ or greater than 20MΩ. Bistable 555 Timer
As well as the one shot 555 Monostable configuration above, we can also produce a Bistable (two stable states) device with the operation and output of the 555 Bistable being similar to the transistorised one we look at previously in the Bistable Multivibrators tutorial. The 555 Bistable is one of the simplest circuits we can build using the 555 timer oscillator chip. This bistable configuration does not use any RC timing network to produce an output waveform so no equations are required to calculate the time period of the circuit. Consider the Bistable 555 Timer circuit below.
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Bistable 555 Timer (flip-flop)
The switching of the output waveform is achieved by controlling the trigger and reset inputs of the 555 timer which are held “HIGH” by the two pull-up resistors, R1 and R2. By taking the trigger input (pin 2) “LOW”, switch in set position, changes the output state into the “HIGH” state and by taking the reset input (pin 4) “LOW”, switch in reset position, changes the output into the “LOW” state. This 555 timer circuit will remain in either state indefinitely and is therefore bistable. Then the Bistable 555 timer is stable in both states, “HIGH” and “LOW”. The threshold input (pin 6) is connected to ground to ensure that it cannot reset the bistable circuit as it would in a normal timing application. 555 Timer Output
We could not finish this 555 Timer tutorial without discussing something about the switching and drive capabilities of the 555 timer or indeed the dual 556 Timer IC . The output (pin 3) of the standard 555 timer or the 556 timer, has the ability to either “Sink” or “Source” a load current of up to a maximum of 200mA, which is sufficient to directly drive output transducers such as relays, filament lamps, LED’s motors, or speakers etc, with the aid of series resistors or diode protection. This ability of the 555 timer to both “Sink” (absorb) and “Source” (supply) current means that the output device can be connected between the output terminal of the 555 timer and the supply to sink the load current or between the output terminal and ground to source the load current. For example. 25
Sinking and Sourcing the 555 Timer Output
In the first circuit above, the LED is connected between the positive supply rail ( +Vcc ) and the output pin 3. This means that the current will “Sink” (absorb) or flow into the 555 timer output terminal and the LED will be “ON” when the output is “LOW”. The second circuit above shows that the LED is connected between the output pin 3 and ground ( 0v ). This means that the current will “Source” (supply) or flow out of the 555 timers output terminal and the LED will be “ON” when the output is “HIGH”. The ability of the 555 timer to both sink and source its output load current means that both LED’s can be connected to the output terminal at the same time but only one will be switched “ON” depending whether the output state is “HIGH” or “LOW”. The circuit to the left shows an example of this. the two LED’s will be alternatively switched “ON” and “OFF” depending upon the output. Resistor, R is used to limit the LED current to below 20mA.
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We said earlier that the maximum output current to either sink or source the load current via pin 3 is about 200mA at the maximum supply voltage, and this value is more than enough to drive or switch other logic IC’s, LED’s or small lamps, etc. But what if we wanted to switch or control higher power devices such as motors, electromagnets, relays or loudspeakers. Then we would need to use a Transistor to amplify the 555 timers output in order to provide a sufficiently high enough power to drive the load. 555 Timer Transistor Driver
The transistor in the two examples above, can be replaced with a Power MOSFET device or Darlington transistor if the load current is high. When using an inductive load such as a motor, relay or electromagnet, it is advisable to connect a freewheeling (or flywheel) diode directly across the load terminals to absorb any back emf voltages generated by the inductive device when it changes state. Thus far we have look at using the 555 Timer to generate monostable and bistable output pulses. In the next tutorial about Waveform Generation we will look at connecting the 555 in an astable multivibrator configuration. When used in the astable mode both the frequency and duty cycle of the output waveform can be accurately controlled to produce a very versatile waveform generator.
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8. Conclusion
The manually controlled press is converted into automatic machine by which maximum operating time will be saved. Thus the output will be more. In this project the human intervention is for loading and unloading the plate. It may be called as semiautomatic machine. This machine can be converted into a fully automatic machine where loading and unloading of the plate can be done automatically. To conclude, this project is made keeping in mind that any manually operated machine can be converted to automatic machines by using pneumatic, electrical and electronic devices. For these purpose one should have the full know how of the devices which are being used. By doing so the existing old machines can be modified and made automatic by which the initial cost, to procure new automatic machines may be minimized. Thus there is a lot of scope in this area (automation). Further in this project the wiring is very much complicated, if any troubleshoot occurs then the fault cannot be easily found, for this the interface with the PLC can be used, by which the wiring is minimized and the fault can be easily detected without waste of time.
9. REFERNCES
1. Design of a Pressure Observer and its Application to a Low Cost Pneumatic Control System”, Takahiro Kosaki and Manabu Sano, year 2011 , Int. J. of Automation TechnologyVol.5No 4. 2. Design and applications of a pneumatic accelerator for high speed punching” ,Su¨leyman Yaldız , Hacı Sag˘lam, Faruk U nsacar, Hakan Isik , 2007, Materials and Design 28. 889– 896. 3. Development of a micro-forming system for micro-punching process of micro-hole arrays in brass foil” , Jie Xu, Bin Guo, Debin Shan, Chu nju Wang, Juan Li , Yanwu Liuc, Dongsheng Qu , 2012 ,Journal of Materials Processing Technology 212 ,2238 – 2246. 4. A new design method for single DOF mechanical presses with variable speeds and lengthadjustable driving links” , Ren-Chung Soong ,2010 ,Mechanism and Machine Theory 45, 496 – 510.
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