Pneumatic Circuits

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A Regulator in a Circuit

By placing an additional regulator downstream of the main regulator, independent control of the clamping force can be accomplished. The main regulator controls the force of the double acting press cylinder. A procedure such as this could be followed for any multiple force system. Let us now consider function two, energy conservation.

Large amounts of air are wasted when used at pressures higher than are actually required. The higher the pressure of a given volume of air in a cylinder, the greater the consumption of air. In many applications, the pressure used for the “non-work” part of the cycle is the same as the “work” portion. If the “non-work” pressure was less than the required “work” pressure, energy would be saved. There are some methods available to reduce the amount of compressed air used in a circuit, such as differential pressure and dual pressure. First let us look at a differential pressure circuit.

Differential Pressure Circuit
 

Shown is a differential pressure circuit. The work is performed on the extension stroke while little force, hence a low pressure, is needed to retract the cylinder. A venting type regulator is installed in the head end cylinder line allowing a considerable savings in air consumption. The cylinder with this arrangement uses reduced pressure acting as an air spring for returning the piston and rod and some external load. Also, notice that a three-way valve may be used since the pressure to the head end of the cylinder chamber remains constant (Figure 2).

Figure 2

Dual pressure Circuit
 

Another configuration for obtaining dual pressure is shown here. It utilizes the principle that many types of five-ported, 4-way valves may be used with either dual pressure or dual exhaust. Actuation of one solenoid allows one pressure to enter one end of the cylinder. Actuation of the other solenoid reverses the action as it allows a second pressure to enter the other end of the cylinder (Figure 3).

Figure 3

Placement of a Flow Control in a Single Acting Application
 

Pictured in Figure 4 is a single acting cylinder, flow control and three-way directional control valve. The cylinder will be able to move at an unrestricted speed in the upward direction. Upon releasing the directional control valve to the at-rest position, air exhausting from the cylinder must pass through the variable restriction (flow control) and the retraction rate will be controlled. A more common speed control circuit would contain a double acting cylinder.

Figure 4

Controlling the Speed of a Double Acting Cylinder Employing a Four Ported, Four Way Valve Shown are two circuits controlling the rod speed of a double acting cylinder in one direction only. Both systems use a four-ported, four-way valve for directional control. In Figure 5-1, extension speed is controlled; in Figure 5-2, retraction speed is controlled. In both cases the ratio of flow into the cylinder in controlled by how fast the air is allowed to exhaust through the flow control. If independent control in both directions is needed, two controls must be used as in Figure 5-3. Here flow control (1) controls the speed of extension while flow control (2) controls the speed of retraction. Note that flow controls are placed between the cylinder and directional control valve, preferably as close as possible to the cylinder. Greater versatility can be achieved through the use of a five-ported, four-way valve.

Figure 5-1   Figure 5-2     Figure 5-3

Controlling Speed of a Double Acting Cylinder Employing a Five Ported, Four Way Valve A five-ported four-way valve provides two independent exhaust ports. In Figure 6-1, independent flow controls may be mounted in each exhaust port to control cylinder exhaust and thus speed in both directions. When the valve is energized (Figure 6-2), the supply is directed to cylinder port (A) and the exhausting air is passed through a needle valve (1) During retraction (Figure 6-3), supply is directed to cylinder port (B) and needle valve (2) controls the exhausting air. This directional control valve has a definite advantage. First it eliminates the need of a bypass check valve being incorporated in the body of the flow control valve. Second, this circuit also offers the opportunity of fewer connections since the needle valve can be fastened into the valve exhaust ports and not in the connecting lines. On the other hand, the control of the cylinder is not as good as when the control orifice is closely coupled to the cylinder. This is because of the need to bring the air up to the “control pressure” before control can be achieved. The long lines (volume of line), as compared to the cylinder control volume, will greatly affect when the control effect takes place. Also, it may be more difficult to wring the oil out of the exhaust air. Multiple Speed Controls But many times two speeds are required as a cylinder moves through its stroke. For instance, let us say that we have a cylinder with a stroke. For instance, let us say that we have a cylinder with a stroke of 20 inch (508 mm). A fast forward speed of 20 ft/min. (0.10 m/s) is needed for the first 10 inches (254 mm) with a speed of 5 ft/min (0.025 m/s) for the remainder of the stroke. The desired return speed is 25 ft/min (0.125 m/s). It is evident that we need three speed controls and at least one directional control valve. By using a five-ported, four-way valve, the fast forward and return speeds can be obtained by installing needle valves in the two independent exhaust ports. This is shown in Figure 6-3. The slow speed forward remainder must now be obtained. For this section, a valve capable of sensing the cylinder rod position must be added. A normally open, two-way cam operated valve can be used. It is placed in parallel with the slow speed flow control so that as the first 10 inch (254 mm) of forward stroke is completed, flow is directed through the newly placed flow control. It may be difficult task to place the cam in the correct position. This is because the valve must be actuated soon enough to allow for the build up of the back pressure required for the final reverse velocity. Let’s go through the circuit one step at a time. The de-energized circuit is shown in Figure 7-1. When the main valve is energized, air is directed to the “A” port of the cylinder (Figure 7-2). Air exhausting from the “B” port flows through both cam valve “Y” and needle valve “Z”, which is independently set to provide a motion about 5 ft/min (0.025 m/s). Most of the air travels through the open cam valve and then passes through needle valve “1”, which is adjusted after “Z” has been set, so that the two valves exhausting together provide a motion of 20 ft/min (0.10 m/s). When 10 inch (254 mm) of forward stroke is sensed, the cam valve shifts to its closed position (Figure 7-3). This causes tall the air exhausting from the cylinder “B” port to pass through needle valve “Z”. Since the valve was independently set to provide motion of about 5 ft/min (0.025 m/s), the speed of the cylinder is greatly reduced until the new pressure balance is established.   Remember, the exact point of the stroke where this occurs is a variable, depending on the volume in the exhaust lines between the cylinder “B” port, the line to the cam valve, and the line to the “Z” needle valve. The entire volume in these lines must be bought to the new equilibrium pressure before the reduced speed can be realized. For retraction (Figure 7-4), the directional valve released, thus directing supply air to the cam valve “Y” which is closed, needle valve “Z” and the check valve. Since the check has the least resistance, a nearly unrestricted flow is fed into cylinder port “B”. Air exhausting from “A” is passed through the directional control valve and into needle valve (3), which is set at 25ft/min (0.125 m/s), the required retraction speed. Once again, since needle valve (1) is not closely coupled to the cylinder, some problems of control may exist. An improvement is achieved by placing needle valve “1” close to the cylinder (Figure 7-5). Now, even though adjustments are still interactive, problems with control may be diminished because trapped volumes have been reduced. Another change has been made in Figure 7-6. First of all, the two-way valve has been replaced by a three-way cam operated valve. This enables the flow controls to be placed so that minimum interactions take place. Also, they are closer to the cylinder, providing better control. Finally, the circuit in Figure 7-7 will give the best control of all the pneumatic circuits shown. The flow controls are even more closely coupled, and the flow control interaction is virtually nonexistent. (Safety Note: It is extremely difficult to control the speed of a pneumatic cylinder. This is due to compressibility of air. Therefore, each speed control circuit must be examined very closely for all possible failure conditions.) In controlling the cylinder speed there are other problems that must be overcome. One such problem is “jump”. Pneumatic Flow Control Problems In applications, any of the pneumatic controls discussed above may present an objectionable “jump” or rapid partial stroking of the cylinder. This condition may occur when initiating cylinder motion at any position. Supply pressure pushes the piston, which in turn must push out the exhaust air can build up a back pressure, the piston will accelerate; once balanced, steady motion occur. This can occur if the directional control valve is shifted before the cylinder completes its stroke, or if the valve is shifted too soon after the stroke is completed.

Figure 7-1 Fast Forward Figure 7-2 Feed Rate Figure 7-3 Retraction Figure 7-4 Figure 7-5 Figure 7-6 Figure 7-7

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