Vortex damper and vacuum pump

Abstract

An eddy current damper for a vacuum pump includes: an electrically conductive ring connectable to a housing; and a magnetic element connectable to the rotor shaft for rotation relative to the conductive ring. Wherein the conductive ring is made of a conductive material and includes an axially extending portion and a radially extending portion. Further, the magnetic element comprises at least one annular magnet, where the radially extending portion of the conductive ring is arranged axially proximate to the at least one annular magnet.

Description

Technical Field

[0001] The invention relates to an eddy current damper for a vacuum pump, in particular for a turbomolecular vacuum pump. Furthermore, the invention relates to a vacuum pump comprising such an eddy current damper. Background Art

[0002] A common vacuum pump includes a housing having an inlet and an outlet. A rotor is disposed in the housing and rotatably supported by at least one bearing. The rotor includes a rotor shaft and at least one pump element. When rotated by an electric motor, the pump element transports a gaseous medium from the inlet to the outlet.

[0003] It is known to use frictionless magnetic bearings in order to support the rotor. However, particularly when using magnetic bearings, it is important to avoid radial vibrations of the rotor, for example caused by an imbalance of the rotor or by inaccuracies of the magnetic bearings.

[0004] It is known to use eddy current dampers to reduce radial vibrations of the rotor. The eddy current damper comprises a magnetic element connected to the rotor and rotating with the rotor and at least one conductive element that does not rotate and is connected to the housing. By radial vibrations, eddy currents are induced in the conductive element, which generates a magnetic force acting in the opposite direction of the vibrating motion, resulting in a restoring force on the rotor, thereby damping the radial vibrations of the rotor. However, common eddy current dampers are bulky and, due to the additional elements of the eddy current damper, may not meet the space requirements, thereby increasing the overall size of the vacuum pump. Summary of the invention

[0005] It is therefore an object of the present invention to provide an eddy current damper which is compact in size and highly efficient.

[0006] This problem is solved by an eddy current damper according to claim 1 and a vacuum pump according to claim 20 .

[0007] The eddy current damper (EDC) for a vacuum pump according to the present invention includes a conductive ring, which is stationary and can be connected to the housing of the vacuum pump. Therein, the conductive ring is made of a conductive material, such as copper, aluminum, etc. The conductive ring also includes an axial extension portion and a radial extension portion, which are preferably integrally constructed as a single piece. Therefore, the cross-section of the conductive ring can be L-shaped or T-shaped. Therefore, the axial extension portion and the radial extension portion can be connected to each other at their respective ends to form an L-shaped cross-section. Alternatively, the radial extension portion can be connected to the axial extension portion to form a T-shaped cross-section by connecting the end of the radial extension portion to a position between the two ends of the axial extension portion.

[0008] According to the present invention, the ECD further comprises a magnetic element having at least one annular magnet, preferably constructed as a permanent magnet. The magnetic element can be connected to the rotor shaft of the vacuum pump to rotate relative to the conductive ring. Therein, the radially extending portion of the conductive ring is arranged to be axially adjacent to the at least one annular magnet. Therefore, the magnetic field of at least one annular magnet induces eddy currents in the radially extending portion of the conductive ring. At the same time, the axially extending portion of the conductive ring acts as a conductor so as to allow efficient flow of eddy currents within the conductive ring so as to efficiently generate a magnetic field by the eddy currents. Therefore, by the axially extending portion of the conductive ring, the ohmic resistance of the conductive ring is reduced so as to reduce the attenuation of the eddy currents within the conductive ring. Therefore, by the specific shape of the conductive ring, a compact and efficient design can be provided, thereby allowing the radial vibration of the rotor of the vacuum pump to be reliably and efficiently damped.

[0009] Preferably, the magnetic element comprises a second annular magnet, which is arranged opposite to the first annular magnet with respect to the conductive ring. Therefore, the magnetic field applied to the radially extending portion of the conductive ring is enhanced by the second annular magnet. In particular, if the second annular magnet is included in the magnetic element, the cross section of the conductive ring is preferably T-shaped.

[0010] Preferably, the first annular magnet and the second annular magnet have the same magnetic pole orientation so as to enhance the magnetic field at the location of the radially extending portion of the conductive ring.

[0011] Preferably, the second ring magnet can be connected to the housing and is therefore stationary and does not rotate. Alternatively, the second ring magnet can also be connected to the rotor shaft like the first ring magnet and rotate relative to the conductive ring.

[0012] Preferably, a radially extending gap is formed by the first annular magnet and the second annular magnet, wherein the radially extending portion of the conductive ring at least partially extends into the gap. Therefore, the first annular magnet and the second annular magnet are arranged to be directly adjacent to the radially extending portion of the conductive ring in the axial direction. By creating a gap between the first annular magnet and the second annular magnet, the magnetic field at the position of the radially extending portion of the conductive ring is enhanced, and thereby eddy currents are efficiently induced in the conductive ring.

[0013] Preferably, a yoke made of a magnetic material is provided, the yoke magnetically connecting the first annular magnet and the second annular magnet so as to form a magnetic circuit and further enhance the magnetic field at the location of the radially extending portion of the conductive ring.

[0014] Preferably, the yoke comprises two radially extending portions, which are magnetically connected to the first and second annular magnets, respectively, and comprises an axially extending portion, which connects the two radially extending portions. Thus, the yoke is substantially U-shaped.

[0015] Preferably, the yoke is connected to the rotor shaft and rotates together with the first and second ring magnets.Thus, the yoke and in particular the two radially extending parts of the yoke are directly attached to the first and second ring magnets.

[0016] Instead, the yoke is stationary and connected to the housing. Therefore, the yoke and in particular the two radial extensions of the yoke are not directly attached to the first and second annular magnets, and the magnetic field passes through the gap between the stationary yoke and the rotating annular magnet to generate a magnetic circuit.

[0017] Preferably, the radial width of the axially extending portion of the yoke is smaller than the axial width of the first ring magnet. Alternatively or additionally, the radial width of the axially extending portion of the yoke is smaller than the axial width of the second ring magnet. Therein, it is surprisingly found that it is beneficial to the damping efficiency if the radial width of the axially extending portion of the yoke is smaller than the axial width of the first ring magnet and / or the radial width of the axially extending portion of the yoke is smaller than the axial width of the second ring magnet.

[0018] Preferably, the axial width of at least one radially extending portion and preferably both radially extending portions of the yoke is smaller than the axial width of the first ring magnet. Alternatively or additionally, the axial width of at least one of the two radially extending portions of the yoke and preferably both radially extending portions is smaller than the axial width of the second ring magnet. Therein, it was surprisingly found that it is beneficial to the damping efficiency if the axial width of at least one radially extending portion and preferably both radially extending portions of the yoke is smaller than the axial width of the first ring magnet and / or the axial width of at least one of the two radially extending portions and preferably both radially extending portions of the yoke is smaller than the axial width of the second ring magnet.

[0019] Preferably, the radial distance between the radially outermost surface of the axially extending portion of the yoke and the first annular magnet is between 1 and 5 times the axial width of the first annular magnet. Alternatively or additionally, the distance between the radially outermost surface of the axially extending portion of the yoke and the second annular magnet is between 1 and 5 times the axial width of the second annular magnet. Therein, it is surprisingly found that if the radial distance between the radially outermost surface of the axially extending portion of the yoke and the first annular magnet and / or the second annular magnet is between 1 and 5 times the axial width of the first annular magnet and / or the second annular magnet, it is beneficial to the damping efficiency.

[0020] Preferably, the first ring magnet extends in the axial direction beyond the axially extending element of the conductive ring. Alternatively or additionally, the second ring magnet extends in the axial direction beyond the axially extending element of the conductive ring. Therefore, it was surprisingly found that it is beneficial to the damping efficiency if the axially extending element of the conductive ring partially surrounds the first and / or second ring magnet in the radial direction.

[0021] Preferably, the conductive ring comprises a second axially extending portion, which is arranged at the opposite end of the radially extending portion, opposite to the first axially extending portion. Thus, the cross section of the conductive ring is substantially H-shaped or U-shaped, wherein the first annular magnet is arranged in a groove (trough) of the U or H-shaped cross section. In particular, if the second annular magnet is included, the conductive ring has an H-shaped cross section, wherein the second annular magnet is arranged in the second groove of the H-shaped cross section of the conductive ring. By means of the second axially extending element of the conductive ring, the conductivity of the conductive ring is further enhanced by increasing the cross section of the conductive ring, thereby reducing the ohmic resistance of the conductive ring and allowing efficient flow of eddy currents within the conductive ring.

[0022] Preferably, the conductive ring is divided into two parts along the circumferential direction of the conductive ring. Therefore, the conductive ring can be easily assembled around the rotor shaft and around the first annular magnet and preferably the second annular magnet in an interlocking manner nested in each other. Since the field lines of the magnetic field are arranged in the axial direction and the intersections of the parts of the conductive ring are also arranged in the axial direction, dividing the conductive ring into two or more parts has no effect or only a small effect on the magnetic field. Preferably, a conductive reinforcing material, such as a conductive paste, etc., can be placed in the intersection.

[0023] Preferably, the conductive ring and the magnetic element are rotationally symmetric.

[0024] Preferably, the distance between the outermost radial surface of the conductive ring and the corresponding outermost radial surface of the first annular magnet is smaller than the radial width of the first annular magnet and / or smaller than the radial width of the second annular magnet. Alternatively or additionally, the distance between the innermost radial surface of the conductive ring and the corresponding innermost radial surface of the second annular magnet is smaller than the radial width of the first annular magnet or the radial width of the second annular magnet. Therein, it was surprisingly found that if the distance between the outermost / innermost radial surface of the conductive ring and the corresponding outermost / innermost radial surface of the first annular magnet is smaller than the radial width of the first annular magnet and / or smaller than the radial width of the second annular magnet, it is beneficial to the damping efficiency.

[0025] Preferably, the axial width of the radially extending portion of the conductive ring is smaller than the axial width of the first ring magnet and / or smaller than the axial width of the second ring magnet. Therein, it is surprisingly found that it is beneficial to the damping efficiency if the axial width of the radially extending portion of the conductive ring is smaller than the axial width of the first ring magnet and / or smaller than the axial width of the second ring magnet.

[0026] Preferably, the radial width of the first ring magnet is between 2 mm and 20 mm and more preferably between 3 mm and 10 mm. Alternatively or additionally, the radial width of the second ring magnet is between 2 mm and 20 mm and more preferably between 3 mm and 10 mm. Therein, it was surprisingly found that if the radial width of the first ring magnet is between 2 mm and 20 mm and more preferably between 3 mm and 10 mm, this is beneficial for the damping efficiency while maintaining the space requirements.

[0027] Preferably, the axial distance between the radial extension of the conductive ring and the first annular magnet is between 0.05 mm and 2 mm, and more preferably between 0.1 mm and 1 mm. Alternatively or additionally, the axial distance between the radial extension of the conductive ring and the second annular magnet is between 0.05 mm and 2 mm, and more preferably between 0.1 mm and 1 mm. Therein, it was surprisingly found that if the axial distance between the radial extension of the conductive ring and the first annular magnet and / or the second annular magnet is between 0.05 mm and 2 mm, and more preferably between 0.1 mm and 1 mm, it is beneficial to the damping efficiency while maintaining the space requirements.

[0028] Preferably, the radial distance between the inner surface of the first axially extending portion of the conductive ring and the first and / or second annular magnet is between 0.05 mm and 2 mm and more preferably between 0.1 mm and 1 mm. Therein, it was surprisingly found that if the radial distance between the inner surface of the first axially extending portion of the conductive ring and the first / second annular magnet, respectively, is between 0.05 mm and 2 mm and more preferably between 0.1 mm and 1 mm, it is beneficial to the damping efficiency while maintaining the space requirements.

[0029] Preferably, the radial distance between the outer surface of the second axially extending portion of the conductive ring and the first and / or second annular magnet is between 0.05 mm and 2 mm and more preferably between 0.1 mm and 1 mm. Therein, it was surprisingly found that if the radial distance between the outer surface of the second axially extending portion of the conductive ring and the first / second annular magnet, respectively, is between 0.05 mm and 2 mm and more preferably between 0.1 mm and 1 mm, it is beneficial to the damping efficiency while maintaining the space requirements.

[0030] Preferably, the eddy current damper comprises at least one second conductive ring that can be connected to the housing and at least one corresponding second magnetic element that can be connected to the rotor shaft to rotate relative to the conductive ring. Therefore, the eddy current damper comprises a stacked arrangement structure along the axial direction of the rotor shaft, the stacked arrangement structure comprising more than one conductive ring and a corresponding number of magnetic elements. Therein, at least one second conductive ring and at least one second magnetic ring are constructed according to the above-mentioned features regarding the conductive rings and the corresponding magnetic elements. Therein, the conductive rings and their corresponding magnetic elements are constructed identically or may be constructed differently.

[0031] Preferably, the annular magnet of one conductive ring is simultaneously the annular magnet of the second conductive ring. Thus, one annular magnet simultaneously acts as a magnetic element to induce eddy currents in two conductive rings that are arranged directly adjacent to each other in a stacked or alternating manner.

[0032] In another aspect of the present invention, a vacuum pump is provided, comprising a housing, a rotor disposed in the housing, and an eddy current damper as described above, wherein the first annular magnet is connected to the rotor and rotates together with the rotor, wherein the conductive ring is connected to the housing.

[0033] Preferably, the vacuum pump is constructed according to the features described above in conjunction with the eddy current damper. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Hereinafter, the present invention will be described in more detail with reference to the accompanying drawings.

[0035] The accompanying drawings show:

[0036] Figure 1 is a vacuum pump according to the present invention,

[0037] Figure 2 A first embodiment of the eddy current damper according to the present invention is shown in FIG.

[0038] Figure 3 A second embodiment of the eddy current damper according to the present invention is shown in FIG.

[0039] Figure 4 is a detailed view of a conductive ring according to the present invention, and

[0040] Figure 5 is a detailed view of a magnetic element according to the present invention. DETAILED DESCRIPTION

[0041] refer to Figure 1, shows a vacuum pump constructed as a turbomolecular pump. The vacuum pump comprises a housing 10, which comprises an inlet 12 and an outlet 14. The rotor 16 is arranged in the housing and supported by a first radial bearing 18 constructed as a permanent magnetic bearing and a second radial bearing 20 also constructed as a permanent magnetic bearing. The first radial bearing 18 and the second radial bearing 20 comprise a plurality of magnetic rings 22, 23. Therein, the stationary magnetic ring 23 of the first radial bearing 18 is attached to a trunnion 24, which extends into a recess 26 of the rotor shaft 16. The rotating magnetic ring 22 is arranged at the inner surface of the recess, radially adjacent to the stationary magnetic ring 23. For the second radial bearing 20, the rotating magnetic ring 22 is attached to the inside of a bell-shaped element 28, radially adjacent to the stationary magnetic ring 23 connected to the housing. Therein, the stationary magnetic rings 23 of the first radial bearing 18 and the second radial bearing 20 repel each other with the rotating magnetic rings 22 of the first radial bearing 18 and the second radial bearing 20 , respectively, thereby providing rotatable support for the rotor 16 in the housing 10 .

[0042] Furthermore, the first radial bearing 18 and the second radial bearing 20 comprise an emergency running bearing 30 which is constructed as a ball bearing. The rotor shaft 16 is driven by an electric motor 32. A plurality of pump elements 34 are attached to the rotor shaft 16, which are constructed as blades and interact with stator elements 36 which are connected to the housing 10 of the vacuum pump and are arranged alternately with the pump elements 34. Furthermore, Figure 1 The vacuum pump of WO 2004 / 056644 1 comprises a Holweck stage 38 comprising a rotating cylinder 40 interacting with a threaded stator 42 connected to a housing. By the rotation of the rotor shaft 16, a gaseous medium is conveyed from the inlet 12 to the outlet 14 of the vacuum pump.

[0043] also, Figure 1 The vacuum pump shown in FIG. 1 includes an eddy current damper (ECD) 100. The ECD 100 includes a first annular magnet 104 and a second annular magnet 106, which are connected by a yoke 102 to form a magnetic circuit. A gap 105 is formed by the first annular magnet 104 and the second annular magnet 106. The ECD 100 also includes a conductive ring 108, which includes an axially extending portion 110A and a radially extending portion 112.

[0044] Therein, the radial extension portion 112 extends into the gap 105 between the first annular magnet 104 and the second annular magnet 106. Therein, the first annular magnet is connected to the rotor shaft 16 and rotates together with the rotor shaft 16. In addition, the conductive ring 108 is connected to the housing and is stationary. Through the radial vibration of the rotor shaft 16, eddy currents are induced in the conductive element, and the eddy currents generate magnetic forces acting in the opposite direction of the vibration motion, resulting in a restoring force on the rotor, thereby damping the radial vibration of the rotor.

[0045] Reference now Figure 2 , shows a detailed view of the construction of an ECD, which is rotationally symmetrical about axis 101. Identical or similar elements are denoted by the same reference numerals. Figure 1 In the description, reference is made to the axial direction 99 and the radial direction 98 of the vacuum pump, such as Figure 2 as shown in .

[0046] Simulations and experiments have shown that it is advantageous to use certain dimensions to provide efficient damping with minimal space requirements. In order to enhance the magnetic field at the location of the radial extension 112 of the conductive ring 108 to generate larger eddy currents, the first ring magnet 104 and the second ring magnet 106 should be as close together as possible, thereby reducing the size of the gap 105. l and the axial width h of the radially extending portion 112 of the conductive ring 108 k2 However, when the axial width h of the radially extending portion 112 of the conductive ring 108 is reduced k2 When the conductive ring 108 is connected to the radial extension 112 of the conductive ring 108, the cross section of the conductor formed by the conductive ring 108 is reduced, thereby increasing the ohmic resistance, resulting in the attenuation of the eddy current induced in the conductive ring 108. Therefore, the axial extension 110A is connected to the radial extension 112 of the conductive ring 108 to increase the cross section and reduce the ohmic resistance, so as to efficiently generate eddy currents in the conductive ring 108, which generate magnetic forces in completely opposite directions of the vibration motion on the first annular magnet 104 and the second annular magnet 106, acting as a restoring force for the rotor shaft 16. Figure 2 As shown in FIG. 1 , the second axially extending portion 110B may be connected to the opposite end of the radially extending portion 112 of the conductive ring 108 to further enhance the flow of eddy currents within the conductive ring 108 . Figure 2 The conductive ring 108 has an H-shaped cross section.

[0047] Therefore, it can be shown that the most efficient and compact ECD can be constructed using one or more of the following conditions (see Figure 2 ):

[0048] - For conductive ring 108:0 <d kma <d mAnd 0 <d kmi <d m ;

[0049] - In addition, for the conductive ring 108:0 <h k2 <h m ;

[0050] The conductive ring 108 is shaped so that the axially extending portions 110A / 110B radially surround the annular magnet at a larger and / or smaller diameter, i.e. <h k1 <h l +h m ;

[0051] - For the first ring magnet 104: 3 mm <d m <10mm;

[0052] - For the second ring magnet 106: 3 mm <d m <10mm;

[0053] - For the gap between the first annular magnet 104 and the radially extending portion 112 of the conductive ring 108: 0.1 mm <h l <1mm;

[0054] - For the gap between the second annular magnet 106 and the radially extending portion 112 of the conductive ring 108: 0.1 mm <h l <1mm;

[0055] - For the gap between the first annular magnet and / or the second annular magnet and the first axially extending portion 110A of the conductive ring 108: 0.1 mm <d ka1 -d ma <1mm;

[0056] - For the gap between the first annular magnet and / or the second annular magnet and the second axially extending portion 110B of the conductive ring 108: 0.1 mm <d mi -d ki2 <1mm.

[0057] -For Yoke: 0 <h r <h m ;

[0058] -For Yoke: 0 <d r <h m ;as well as

[0059] - For yoke: h m <d mi -d ra <5h m .

[0060] refer to Figure 3 , shows an alternative embodiment of the eddy current damper, wherein the cross section of the conductive ring 108 is T-shaped. In addition, it is shown that the yoke 102 is divided into a first yoke part 102A and a second yoke part 102B in the axial direction for easy assembly.

[0061] refer to Figure 4 , a conductive ring 108 is shown in a plan view, wherein an axially extending portion 110 of the conductive ring extends out of the paper, and a radially extending portion 112 of the conductive ring extends in the paper. Therein, the conductive ring 108 is constructed by two parts 108A and 108B that separate the conductive ring in the circumferential direction. Therefore, the conductive ring 108 can be easily assembled around the rotor shaft 16 with the first and second annular magnets 104, 106 in an interlocking manner.

[0062] Reference Figure 5 , shows the first ring magnet 104 and also includes a portion 102A of the yoke, wherein the first ring magnet 104 is surrounded by a reinforcing element 118 to withstand the rotational force during the operation of the vacuum pump.

[0063] Thus, using the discovered ECD configuration, a compact and efficient ECD can be achieved, which can be implemented in different types of vacuum pumps.Although the ECD according to the invention is shown in a turbomolecular pump, other types of vacuum pumps and pump stages may also benefit from the ECD according to the invention.

Claims

1. Eddy current damper for vacuum pump, comprising: a conductive ring capable of being connected to the housing; a magnetic element capable of being connected to the rotor shaft for rotation relative to the conductive ring; Wherein, the conductive ring is made of conductive material and comprises an axially extending portion and a radially extending portion, and The magnetic element comprises at least one annular magnet, wherein the radially extending portion of the conductive ring is arranged axially adjacent to the at least one annular magnet.

2. The eddy current damper according to claim 1, wherein: The magnetic element includes a second annular magnet disposed opposite the first annular magnet relative to the conductive ring.

3. The eddy current damper according to claim 2, wherein: The first ring magnet and the second ring magnet have the same magnetic pole orientation.

4. The eddy current damper according to claim 2 or 3, wherein: The second ring magnet can be connected to the housing or can be connected to the rotor shaft.

5. The eddy current damper according to any one of claims 2 to 4, wherein: A radially extending gap is formed by the first ring magnet and the second ring magnet, wherein the radially extending portion of the conductive ring extends at least partially into the gap.

6. The eddy current damper according to any one of claims 2 to 5, wherein: A yoke made of a magnetic material is provided, the yoke magnetically connecting the first ring magnet and the second ring magnet.

7. The eddy current damper according to claim 6, wherein: The yoke includes two radially extending portions, which are magnetically connected to the first and second annular magnets, respectively, and includes an axially extending portion connecting the two radially extending portions.

8. The eddy current damper according to claim 7, wherein: The radial width of the axially extending portion of the yoke is smaller than the axial width of the first and / or second annular magnet.

9. The eddy current damper according to claim 7 or 8, wherein: The axial width of at least one of the two radially extending portions of the yoke is smaller than the axial width of the first and / or second annular magnet.

10. The eddy current damper according to any one of claims 7 to 9, wherein: The distance between the radially outermost surface of the axially extending portion of the yoke and the first and / or second annular magnet is between one and five times the axial width of the first and / or second annular magnet.

11. The eddy current damper according to any one of claims 1 to 10, wherein: The first and / or second annular magnets extend in the axial direction beyond the axially extending element of the conductive ring.

12. The eddy current damper according to any one of claims 1 to 11, wherein: The conductive ring includes a second axially extending portion disposed at an opposite end of the radially extending portion, opposite to the first axially extending portion.

13. The eddy current damper according to any one of claims 1 to 12, wherein: The conductive ring is divided into at least two parts along a circumferential direction of the conductive ring.

14. The eddy current damper according to any one of claims 1 to 13, wherein: The conductive ring and the magnetic element are rotationally symmetric.

15. The eddy current damper according to any one of claims 1 to 14, wherein: The distance between the outermost or innermost radial surface of the conductive ring and the corresponding outermost or innermost radial surface of the first and / or second annular magnet is smaller than the radial width of the first and / or second annular magnet.

16. The eddy current damper according to any one of claims 1 to 15, wherein: The axial width of the radially extending portion of the conductive ring is smaller than the axial width of the first and / or second annular magnets.

17. The eddy current damper according to any one of claims 1 to 16, wherein: The radial width of the first and / or second annular magnet is between 2 mm and 20 mm, preferably between 3 mm and 10 mm.

18. The eddy current damper according to any one of claims 1 to 17, wherein: The axial distance between the radially extending portion of the conductive ring and the first and / or second annular magnet is between 0.05 mm and 2 mm, preferably between 0.1 mm and 1 mm.

19. The eddy current damper according to any one of claims 1 to 18, wherein: The radial distance between the inner surface of the axially extending portion of the conductive ring and the first and / or second annular magnet is between 0.05 mm and 2 mm, preferably between 0.1 mm and 1 mm.

20. A vacuum pump comprising a housing, a rotor disposed in the housing, and an eddy current damper according to any one of claims 1 to 19, wherein: The first annular magnet is connected to the rotor and the conductive ring is connected to the housing.

Images