Air circuits and machine tools

Abstract

To enable air relieved from a pneumatic regulator to be returned to a primary side, without causing a change in air flowing the primary side.SOLUTION: An air circuit 50, for returning air relieved from a pneumatic regulator 30 provided on a main line 20 for feeding high-pressure air from a pneumatic source P to a pneumatic cylinder 10 to a primary side 31 of the pneumatic regulator 30 through a return line 21, comprises a backflow generating unit 51 provided, at an intersection of the return line 21 and the main line 20, to cause a flow countering to a flow of air from the primary side 31 on the main line 20 toward a secondary side 32 and prevent high-pressure air to flow into the pneumatic regulator 30.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a pneumatic circuit having a pneumatic regulator and a machine tool.

Background Art

[0002] Conventionally, for a vertical operating axis that moves up and down, such as a spindle in a machine tool, since its weight affects the operation, a neutral force is obtained so that the same force is achieved during the up and down movement, and then it is operated. A pneumatic cylinder is used as a means for obtaining this force, and a pneumatic circuit for balancing is known. In such a pneumatic circuit, a pneumatic regulator capable of adjusting pressure is used (see, for example, Patent Document 1).

[0003] For example, as shown in FIG. 4, a necessary pressure (the weight of the unit that moves up and down) is supplied to one side of the pneumatic cylinder by a pneumatic regulator 30, and when the vertical operating unit moves up and down, the pneumatic cylinder follows the operation, thereby performing an operation to balance the motor load of the machine tool.

[0004] And every time the vertical operating axis operates, the air supply and exhaust of the following (1) and (2) are performed.

[0005] (1) When the vertical operating axis operates on the supply side of the pneumatic cylinder, an air flow from the primary side 31 to the secondary side 32 of the regulator 30 occurs.

[0006] (2) When the vertical operating axis operates on the discharge side of the pneumatic cylinder, air returns from the secondary side 32 of the regulator 30, and since the pressure of the secondary side 32 increases, the diaphragm 34 inside the regulator 30 is operated, and the air that has returned from the pneumatic cylinder is released to the atmosphere from the relief port 33 through the diaphragm 34.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

[0008] During operation (1), air fills the pneumatic cylinder from the primary side 31 to the secondary side 32 of the regulator 30, and if there are no pressure fluctuations, the impact on operation is minimal. However, if a pressure fluctuation occurs on the primary side 31, the regulator 30's response will cause a pressure fluctuation on the secondary side 32.

[0009] During operation (2), a change occurs in the opening degree of the valve body 37 and diaphragm 34 of the regulator 30, During acceleration and deceleration, the force pressing the diaphragm 34 of the regulator 30 changes, and the fluctuation of the diaphragm 34 affects the shaft and causes a hysteresis. This affects the shaft's movement. • Because the diaphragm 34 is vented to the atmosphere, the pressure drop becomes large. The force on the diaphragm 34 fluctuates, causing changes in air pressure and vibration fluctuations. This affects the shaft movement. • The opening is from the diaphragm 34, which causes an abnormal noise. The conduit is often trumpet-shaped, which easily amplifies the sound. • The compressed air disappears. Such problems arise, and to solve these problems, it is conceivable to return the air from the diaphragm 34 to the primary side.

[0010] However, by returning air to the primary side 31, Backflow occurs from the primary side 31. When the pressure on the primary side 31 of the regulator 30 increases, the regulator 30 determines the relief amount by the difference between the two spring forces. Therefore, for small fluctuations (e.g., around 0.01 MPa), there is no change in the secondary pressure. However, if a larger fluctuation occurs (e.g., a fluctuation exceeding 0.02 MPa), the secondary pressure will also increase, though not proportionally, due to the increase in the primary pressure. Furthermore, if the secondary side 32 is in the relief state, the flow rate will experience an even larger fluctuation due to the addition of weight and fluctuation. • When the operating speed increases, piping alone is insufficient to cope. This creates a new problem.

[0011] The present invention has been made in view of the above, and its purpose is to enable the air relieved from the pneumatic regulator to be returned to the primary side without causing any change to the air flowing on the primary side. [Means for solving the problem]

[0012] To achieve the above objective, this invention incorporates improvements to the piping structure that returns the pneumatic regulator to the primary side.

[0013] Specifically, the first invention relates to an air circuit provided in the main piping that supplies high-pressure air from a pneumatic source to a pneumatic cylinder, which returns the air relieved from the pneumatic regulator to the primary side of the pneumatic regulator via a return pipe having a check valve. The aforementioned air circuit is At the point where the return pipe and the main pipe intersect, a backflow generating section is provided that generates a flow that is contrary to the airflow from the primary side to the secondary side in the main pipe, thereby preventing high-pressure air from flowing into the pneumatic regulator.

[0014] According to the above configuration, when supplying air via a pneumatic regular valve, the operation of the pneumatic cylinder can be stabilized by returning the air relieved from the pneumatic regulator to the primary side of the pneumatic regulator. Since this relieved air is returned to the primary side, it is necessary to prevent pressure fluctuations in the piping leading to the regulator. The backflow generation unit creates a flow that opposes the airflow from the primary to the secondary side in the main piping at the intersection of the return piping and the main piping, preventing high-pressure air from flowing into the pneumatic regulator. This stabilizes the operation of the pneumatic cylinder.

[0015] In the second invention, in the first invention, The aforementioned backflow generation unit is A lid member provided at a portion where the return pipe and the main pipe intersect; A chamfer provided on the inner surface of the primary side of the portion where the return pipe and the main pipe intersect; It has a gap between the lid member and the chamfer.

[0016] According to the above configuration, by using the gap formed between the lid member and the chamfer on the inner surface of the primary side, an effect of creating a flow direction toward the primary side toward the end of the main pipe is achieved, and by forming a flow that presses against the side surface of the main pipe, the eddy current generated from the pipe resistance on the side surface of the pipe is actively utilized to promote the flow toward the primary side (reverse flow). As a result, it is possible to prevent high-pressure air from flowing into the pneumatic regulator.

[0017] In the third invention, in the second invention, The lid member has a plurality of through holes, The inner diameter on the chamfer side of the plurality of through holes is smaller than the inner diameter on the side opposite to the chamfer.

[0018] According to the above configuration, since the inner diameter of the through hole on the chamfer side is smaller than the through-hole inner diameter on the side opposite to the chamfer, the flow rate of the air increases. Since a negative pressure is generated at the tip where the air blown out from the through hole on the chamfer side is blown, it has a function of actively flowing toward the pneumatic source side. On the other hand, by making the through-hole inner diameter on the side opposite to the chamfer larger, the flow rate is reduced. As a result, it is possible to ride on the adjacent flow without hindering the flow generated on the pneumatic source side, and a flow form that flows toward the pneumatic source side is established. This structure can change the exhaust position according to the speed change due to the acceleration and deceleration of the axial movement, and improve the function of the pneumatic regulator.

[0019] In the fourth invention, in any one of the first to third inventions, The main pipe is a pipe that feeds air into the ascending side of the pneumatic cylinder that maintains the neutral position of the vertical operating shaft.

[0020] According to the above configuration, since pressure fluctuations are not generated in the main pipe leading to the pneumatic regulator, the primary side of the pneumatic regulator does not fluctuate, and a stable pressure supply can be continued, so the operation of the pneumatic cylinder is stabilized.

[0021] In the fifth invention, in any one of the first to fourth inventions, A plurality of sub-tanks capable of storing air are arranged in the return pipe via a plurality of on-off check valves that open at a predetermined set pressure. The plurality of on-off check valves are arranged such that the set pressure gradually increases toward the backflow generation section.

[0022] According to the above configuration, while preventing air from flowing into the sub-tank at once, it is possible to process an amount of air that cannot be completely processed by only the return pipe.

[0023] The machine tool of the sixth invention A backflow generation section of any one of the first to fifth inventions, A main pipe, A pneumatic regulator connected to the main pipe, A return pipe for returning air from the pneumatic regulator to the main pipe, And an up-and-down operating shaft, The main pipe is connected to a pneumatic cylinder that maintains the neutral position of the up-and-down operating shaft.

[0024] According to the above configuration, since the operation of the pneumatic cylinder is stabilized, the neutral position of the up-and-down operating shaft can be stably achieved, and machining can be performed stably and with high precision. For example, when the speed of air during acceleration and deceleration of the pneumatic cylinder becomes low, the discharge pressure also decreases. As a result, it becomes possible to change the flow (position) of exhausting to the side closer to the position of the pneumatic regulator. This enables the pneumatic regulator to supply air at a higher pressure to the supply operation from the pneumatic regulator to the pneumatic cylinder that occurs during the change from the exhaust to the supply mode that occurs when the pneumatic cylinder reverses, and the responsiveness of the pneumatic cylinder to the operation is improved.

Effect of the Invention

[0025] As described above, according to the present invention, the air relieved from the pneumatic regulator can be returned to the primary side without causing any change to the air flowing on the primary side. [Brief explanation of the drawing]

[0026] [Figure 1] This is a schematic perspective view of a machine tool having an air circuit according to an embodiment of the present invention. [Figure 2] This is a side view showing a magnified view of the vertical operating axis and surrounding area of ​​a machine tool. [Figure 3] This is an air circuit diagram according to an embodiment of the present invention. [Figure 4] This is a close-up cross-sectional view of a pneumatic regulator. [Figure 5] The backflow generation section is shown, with (a) being a front view and (b) being a cross-sectional view. [Figure 6] Figure 5(b) is an enlarged view from the direction of the arrow VI. [Figure 7] This is a cross-sectional view showing an overview of the pressure distribution within the main piping. [Figure 8] This is a cross-sectional view illustrating the airflow in the backflow generation section. [Figure 9] This diagram shows the circular motion of the vertical operating shafts; (a) is a comparative example in which air is simply returned to the primary side, and (b) is the pneumatic circuit according to the present embodiment. [Figure 10] This shows a pressure-operated check valve, with (a) being a cross-sectional view and (b) being a side view. [Figure 11] These are schematic diagrams showing the configuration of the sub-tank, where (a) has a pressure-operated check valve positioned between the two containers, (b) has a pressure-operated check valve positioned upstream of the container, and (c) has pressure-operated check valves positioned upstream and downstream of the container, respectively. [Modes for carrying out the invention]

[0027] Embodiments of the present invention will be described below with reference to the drawings.

[0028] Figure 1 shows a machine tool 1 comprising an NC machine tool according to an embodiment of the present invention. For example, this machine tool 1 comprises a bed 2, a column 3, a table 4, a chuck 5 for gripping a workpiece W, a spindle motor 6, and a control unit 7 for controlling these. As also shown in Figure 2, the vertical operating shaft 8, which is the rotation axis of the chuck 5 in this embodiment, is provided on the column 3 so as to be movable vertically (in the Y-axis direction), allowing for fine vertical adjustment of the position of the workpiece W. Four pneumatic cylinders 10, for example, are arranged in a row on the column 3 to maintain the neutral position of the vertical operating shaft 8. Note that the configuration of the machine tool 1 is not limited to this, and is not particularly limited in terms of workpiece shape, tool type, etc.

[0029] As shown in Figure 3, these pneumatic cylinders 10 are connected to a main pipe 20 from the source pressure (pneumatic source P). The main pipe 20 uses, for example, a pipe with an inner diameter of 10 mm, but the inner diameter and other configurations are not limited to this.

[0030] In the machine tool 1 of this embodiment, machining can be performed by rotating the workpiece W held in the chuck 5 of the column 3, finely adjusting the position of the workpiece W vertically, and simultaneously finely adjusting the position of the spindle motor 6 on the table 4 to move the position of the tool (not shown). The vertical operating shaft 8 may also be the shaft on which the tool is mounted.

[0031] In this embodiment, the air circuit 50 has a main pipe 20 extending from a pneumatic source P, which is the source pressure of a compressor or the like, and this main pipe 20 also branches to other systems of the machine tool 1. One of these branches is connected via a pneumatic regulator 30 to the upward pressure chamber 11 of the pneumatic cylinder 10, which raises the vertical operating shaft 8 of the pneumatic cylinder 10.

[0032] As shown in an enlarged view in Figure 4, a detailed explanation is omitted, but the pneumatic regulator 30 has a primary side port 31, a secondary side port 32, and a relief port 33, and is equipped with a diaphragm 34, a pressure regulating spring 35, a pressure regulating handle 36, a valve body 37, and a valve spring 38 inside.

[0033] In this embodiment, as shown in Figure 3, the return pipe 21 from the relief port 33 is connected to the primary side 31 of the pneumatic regulator 30 in the main pipe 20 via a check valve 22. The check valve 22 plays a role in preventing backflow from this primary side 31.

[0034] In this way, an air circuit 50 is obtained that returns the air relieved from the pneumatic regulator 30, which is provided in the main piping 20 that supplies high-pressure air from the pneumatic source P to the pneumatic cylinder 10, to the primary side 31 of the pneumatic regulator 30 via a return piping 21 having a check valve 22.

[0035] Furthermore, a feature of this embodiment is that, as shown in Figure 5(a), a backflow generating unit 51 is provided at the intersection of the return pipe 21 and the main pipe 20, which generates a flow that is contrary to the flow of air from the primary side 31 to the secondary side 32 in the main pipe 20, thereby preventing high-pressure air from flowing into the pneumatic regulator 30.

[0036] From another perspective, as shown in Figure 3, the primary main pipe 20 has branches to other systems that consume air elsewhere, as well as internal pipe pressure loss and pressure loss in the pneumatic regulator, resulting in a constant small flow. A backflow generation unit 51 is provided with the aim of making the most of this small flow and guiding the discharged air to the pneumatic source P.

[0037] Specifically, as shown in enlarged view in Figures 5(b) and 6, the backflow generation section 51 includes a cover member 40 provided at the intersection of the return pipe 21 and the main pipe 20, a chamfer 23 provided on the inner surface of the primary side 31 of the intersection of the return pipe 21 and the main pipe 20, and a gap 41 between the cover member 40 and the chamfer 23.

[0038] The lid member 40 has, for example, a plurality of through holes 42, 43, 44, and the inner diameters of these through holes 42, 43, 44 are arranged to gradually decrease toward the chamfer 23 side, from the large diameter through hole 42 on the opposite side of the chamfer 23 to the medium diameter through hole 43 and the small diameter through hole 44. These plurality of through holes 42, 43, 44 and the shielding portion between them can influence the airflow.

[0039] As shown in Figure 7, when there is flow inside the main pipe 20, a pressure pattern is created with a high-velocity portion 20a that is convex in the center. This is because there is pressure loss at the end of the main pipe 20. Furthermore, near the inner circumferential surface of the main pipe 20, contact resistance is present, forming a vortex flow 20b.

[0040] As shown in Figure 8, in the backflow generation section 51, a negative pressure generation section 24 is generated by the flow path created by the gap 41 formed by the chamfer 23 and the shielding section by the cover member 40.

[0041] Furthermore, the small-diameter through-hole 44 located immediately next to the negative pressure generating section 24 on the chamfered 23 side is smaller in diameter than the other large-diameter through-holes 42 and medium-diameter through-holes 43, which increases the flow velocity of the flow coming from the relief port 33. The negative pressure generating section 24 is located at the end of the air blown out from this small-diameter through-hole 44, which allows the air to be carried along by the adjacent flow without actively obstructing the flow generated towards the pneumatic source P, thus establishing a flow configuration toward the pneumatic source P.

[0042] This configuration allows the exhaust position to be changed in response to speed changes caused by the acceleration and deceleration of the vertical operating shaft 8, thereby improving the functionality of the pneumatic regulator 30.

[0043] For example, during acceleration and deceleration, if the air velocity decreases, the exhaust pressure also decreases. This makes it possible to change the exhaust flow to the side closer to the position of the pneumatic regulator 30. This allows the pneumatic regulator 30 to supply air at a higher pressure in response to the supply operation from the pneumatic regulator 30 to the pneumatic cylinder 10 that occurs when the pneumatic cylinder 10 reverses direction, as the form of supply changes from exhaust to supply, thereby improving the responsiveness of the pneumatic cylinder 10 to the operation.

[0044] By creating a flow toward the pneumatic source P side of the air exiting the relief port 33, it becomes possible to utilize the vortex flow 20b within the main pipe 20 to allow the air to penetrate deep into the pneumatic source P side. The air exiting the relief port 33 is saturated due to pressure loss within the pipe and pressure loss due to branching for other uses on the pneumatic source P side. Therefore, high-pressure air is not sent to the pneumatic regulator 30 side, and the pneumatic regulator 30 is not affected.

[0045] As shown in an enlarged view in Figure 8, by providing a chamfer 23 on the backflow generation section 51, a flow is created toward the pneumatic source P. Due to the inclined shape and the pressure difference inside the main pipe 20, this flow flows toward the pneumatic source P. Furthermore, the vortex flow on the inner surface of the main pipe 20 causes the pipe end to flow toward the pneumatic source P. As a result of this phenomenon, negative pressure is generated near the shielding section of the gap 41 and the small-diameter through-hole 44 (air coming out of the gap 41 attracts nearby air), so the air coming out of the small-diameter through-hole 44 is guided by the negative pressure, making it possible to form a flow toward the pneumatic source P only at the pipe end.

[0046] This process is repeated sequentially, allowing the flow to the pneumatic source P to be achieved through approximately half the length of the main piping 20. This configuration eliminates any influence on the pressure changes on the pneumatic regulator 30 side.

[0047] When returning air to the main pipe 20 via the return pipe 21, the pressure and volume on the primary side of the main pipe 20 are adjusted to a level that does not affect the pneumatic regulator 30 before returning the air to the primary side of the main pipe 20. A suitable pressure value is, for example, approximately primary side pressure + 5% when the inner diameter of the main pipe 20 is 10 mm.

[0048] The appropriate flow rate is ≤0.0001 m³ when the inner diameter of the main pipe 20 is 10 mm. 3 It is / sec.

[0049] Furthermore, when returning the air discharged from the relief port 33 of the pneumatic regulator 30 to the primary side 31, the fluid morphology within the piping of the primary side 31 is utilized to create a flow toward the pressure source / pneumatic source P on the pipe wall side. This suppresses changes in the high-velocity section 20a in the center of the main piping 20 of the primary side 31, thereby preventing pressure changes in the primary side 31 connected to the pneumatic regulator 30.

[0050] In this embodiment, the pressure of the returned air is increased, and instead of simply using the pressure difference for return, a groundbreaking return method has been established that achieves a delicate return without compromising the original form of the primary side 31.

[0051] For example, Figure 9 shows the results of evaluating the XY circular motion. The feed rate was several hundred mm / min, and the vertical operating shaft 8 was subjected to circular motion with a radius of 25 mm. The X axis represents the left-right movement of the vertical operating shaft 8, and the Y axis represents the up-down movement of the vertical operating shaft 8. Figure 9(a) shows the circular motion of a comparative example in which air is returned to the primary side 31 of the pneumatic regulator 30 without the backflow generation unit 51. When rising, there is no effect because air is continuously supplied from the pneumatic regulator 30, but when descending, air is discharged from the relief port 33 of the pneumatic regulator 30, so it can be seen that the shaft does not move smoothly along a perfect circle if the air is simply returned as is. Problems such as hunting phenomena due to uneven discharge and stagnation of operation due to poor discharge of the pneumatic cylinder 10 occur.

[0052] On the other hand, as shown in Figure 9(b), when the backflow generation unit 51 of this embodiment is provided, it can be seen that the vertical operating shaft 8 operates without any problems in both left-right and up-down movements.

[0053] Furthermore, if the processing flow rate is high and cannot be handled solely within the main piping 20, several sub-tanks 60-63 are used to gradually increase the pressure. Each sub-tank 60-63 creates a finely graded pressure to suppress changes and fluctuations, thus ensuring a smooth transition between sub-tanks 60-63. This creates statically stable air, improving stability when returning it to the main piping 20 on the primary side 31. Gradually increasing the pressure in small increments is excellent for suppressing fluctuations in flow rate and pressure, and is superior for ensuring a stable air supply.

[0054] The flow rate that can be handled by the return pipe 21 with an inner diameter of 10 mm is approximately 0.0001 m³. 3 Since the rate is per second, it is advisable to install sub-tanks 60-63 to temporarily store the excess if it exceeds this rate.

[0055] For example, with four pneumatic cylinders 10 each with an inner diameter of 80 mm (0.08 m) and requiring a stroke of 50 mm (0.05 m), the required stroke length is 0.04 × 0.04 × π × 0.05 = 0.000251 m 3 That's all you need to save.

[0056] To store this entire quantity, a volume of 0.1m x 0.1m x 0.1m should suffice. However, to prevent a sudden influx of air, sub-tanks 60-63 should be used at progressively higher pressures, and measures should be taken to ensure that the final discharge does not exceed the processing capacity of the main piping 20.

[0057] For example, as shown in Figures 10 and 11, containers 60 to 63, each measuring 0.063m × 0.063m × 0.063m, are connected in series to the main piping 20. A first pressure-operated check valve 65 is placed between the first container 60 and the second container 61. When the primary supply is 0.4 MPa, the first pressure-operated check valve 65 is set to, for example, 0.11 MPa. A second pressure-operated check valve 66 is placed between the second container 61 and the third container 62. The second pressure-operated check valve 66 is set to, for example, 0.21 MPa. A third pressure-operated check valve 67 is placed between the third container 62 and the fourth container 63. The third pressure-operated check valve 67 is set to, for example, 0.31 MPa. A fourth pressure-operated check valve 68 is placed on the primary side of the fourth container 63. The fourth pressure-operated check valve 68 is set to, for example, 0.41 MPa.

[0058] As shown in Figure 10, the first to fourth pressure-operated check valves 65 to 68 are each equipped with a spherical poppet 69, a pressing part 70, and a pressing spring 71, and are designed to allow air to pass through when the pressure exceeds a different set pressure.

[0059] This configuration allows air to flow into multiple sub-tanks 60-63 in stages, rather than all at once into a single sub-tank 60-63.

[0060] The embodiments described above are essentially preferred examples and are not intended to limit the scope of the present invention, its applications, or uses.

[0061] For example, in the above embodiment, sub-tanks 60-63 are provided, but this is not essential and can be omitted. [Explanation of symbols]

[0062] 1 Machine tools 2 beds 3 Columns 4 tables 5 Check 6-axis motor 7 Control Unit 8 Operating shaft 10 Pneumatic cylinders 11. Rising pressure chamber 20 Main piping 20a High speed flow section 21 Return piping 22 Check valve 23 Chamfering 30 Pneumatic regulator 31 Primary side 31st pressure side 32 Secondary side 33 Relief Port 34 diaphragm 35 Pressure regulating spring 36 Pressure Adjusting Handle 37 Valve body 38 Valve spring 40 Lid member 41 gap 42 Large diameter through holes 43 Medium diameter through hole 44 Small diameter through hole 50 Air Circuit 51 Backflow generation section 60 1st container 61 Second container 62 3rd container 63 4th container 65. First pressure-operated check valve 66. Second pressure-operated check valve 67. Third pressure-operated check valve 68. Fourth pressure-operated check valve 68. Fourth pressure-operated check valve 69 Spherical poppet 70 Pressing part 71 Compression spring

Claims

1. An air circuit provided in the main piping that supplies high-pressure air from a pneumatic source to a pneumatic cylinder, which returns the air relieved from the pneumatic regulator to the primary side of the pneumatic regulator via a return pipe having a check valve, At the point where the return pipe and the main pipe intersect, a backflow generating section is provided to create a flow that opposes the airflow from the primary side to the secondary side in the main pipe, thereby preventing high-pressure air from flowing into the pneumatic regulator. An air circuit characterized by the following features.

2. The aforementioned backflow generation unit, A cover member is provided at the intersection of the return pipe and the main pipe, A chamfer is provided on the primary side inner surface of the portion where the return pipe and the main pipe intersect, The gap between the lid member and the chamfer The air circuit according to feature 1.

3. The lid member has a plurality of through holes, The plurality of through holes have an inner diameter on the chamfered side that is smaller than the inner diameter on the opposite side of the chamfer. The air circuit according to feature 2.

4. The aforementioned main piping is a pipe that supplies air to the upward side of the pneumatic cylinder, which maintains the neutral position of the vertical operating shaft. The air circuit according to any one of 1 to 3.

5. The aforementioned return piping has multiple sub-tanks capable of storing air arranged in a row via multiple check valves that open and close at a predetermined set pressure. The plurality of check valves are arranged such that the set pressure gradually increases toward the backflow generation section. The air circuit according to feature 4.

6. The backflow generating unit according to claim 1, Main piping and A pneumatic regulator connected to the main piping, A return pipe that returns air from the pneumatic regulator to the main pipe, Equipped with an up and down operating shaft, The main piping is connected to a pneumatic cylinder that maintains the neutral position of the vertical operating shaft. A machine tool characterized by the following features.

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