TEST VEHICLE

Description

[0001] The invention relates to a testing vehicle which is designed to be placed and moved on an at least partially ferromagnetic substrate, comprising at least one measuring technique designed to detect physical and / or chemical properties of the environment, a drive unit and at least one magnet, in particular a neodymium magnet, which is arranged on the underside of the test vehicle and is designed to hold the test vehicle to the ground.

[0002] The rotor and stator of a generator are subjected to a visual inspection at regular inspection intervals and, if necessary, when unusual events occur, such as load shedding. In particular, the inspection is carried out to identify and locate thermal discoloration, mechanical changes such as scratches, impacts, displacement of parts, loose parts, foreign objects, etc. To enable visual access, the rotor is removed or pulled out of the stator. A visual inspection is carried out by the inspection personnel. Any findings are documented photographically. The generator can only be reassembled and put into operation after the inspection and evaluation of the test results have been completed and any necessary repair work has been carried out. Removing the rotor is associated with considerable effort and high costs.Against this background, it would be desirable to be able to carry out the inspection with the generator assembled, so that if no findings are found, the rotor does not have to be removed.

[0003] One option for visual inspection in the assembled state is generally to use a conventional industrial endoscope. However, this does not apply to generators with rotor lengths of several meters.

[0004] Furthermore, the use of test vehicles for component diagnostics and / or during overhaul work is becoming increasingly popular. These enable a reduction in overhaul times by accelerating the testing process and eliminating the need to disassemble various system components, such as removing a generator's rotor. Furthermore, test vehicles record measurement and test results under similar conditions, thus ensuring consistent quality and excellent long-term comparability. Finally, test vehicles are advantageous from an environmental, health, and occupational safety perspective, as they often avoid entering confined spaces.

[0005] For a comprehensive inspection, it is important that the inspection vehicle can be moved flexibly on the surface, especially the surface of a runner, and that it is also possible to move overhead.

[0006] The applicant is aware of test vehicles for inspecting installed generators, which are designed to be placed on an at least partially ferromagnetic substrate, more precisely on the outer circumference of the rotor of a generator, and to be moved freely in all directions thereon. For this purpose, these test vehicles comprise, in addition to measuring technology and a drive unit modified compared to the previously described crawler system, at least one magnet arranged on the underside of the test vehicle and designed to hold the test vehicle to the substrate. The at least one magnet draws the test vehicle to the substrate with its holding force. Thus, the at least one magnet ensures the contact pressure desired for the propulsion of the test vehicle and ensures that the test vehicle can be moved overhead along a ferromagnetic substrate without falling off.

[0007] If the entire surface on which the test vehicle is to be moved is not ferromagnetic, there will be areas to which a magnet of the test vehicle is extremely attracted, and areas to which a magnet of the test vehicle is hardly attracted or not attracted at all. For example, the majority of the rotor volume of a generator rotor is made of solid steel, with copper rotor slot locking wedges inserted into the surface of the rotor. While copper, with a magnetic permeability of almost 1, is not ferromagnetic, steel has a relatively high magnetic permeability and is therefore ferromagnetic. A magnet of the test vehicle is therefore extremely attracted to a steel area, but not to a copper area.Whether the previously known test vehicle with at least one magnet is held upside down or not depends on the areas of the ground in which the at least one magnet of the test vehicle is currently located.

[0008] To ensure reliable support of the test vehicle regardless of its current position on the ground, it has already been considered to bridge the above-described permeability jumps in the ground by means of a relatively large number of magnets arranged on the underside of the test vehicle. This would ensure that at any time the test vehicle is moving on the ground, a sufficient number of magnets adhere to the ground to hold the test vehicle to the ground. However, this poses the problem that in sections of the ground where exclusively ferromagnetic areas are provided, for example in the pole area of ​​the rotor, where the rotor is not equipped with rotor slot locking wedges, a relatively high magnetic effect occurs, which can lead to the test vehicle coming to a standstill.

[0009] US 2009 / 146680 A1 discloses a test vehicle in which magnets form the drive wheels.

[0010] From the publication by WOLFGANG FISCHER ET AL: "Locomotion System for a Mobile Robot on Magnetic Wheels With Both Axial and Circumferential Mobility and With Only an 8-mm Height for Generator Inspection With the Rotor Still Installed", IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, IEEE SERVICE CENTER, PISCATAWAY, NJ, USA, Vol. 58, No. 12, December 1, 2011 (2011-12-01), pages 5296-5303, ISSN: 0278-0046, DOI: 10.1109 / TIE.2010.2051396, a test vehicle is known that is designed to be placed and moved on an at least partially ferromagnetic substrate.

[0011] Publication DE 10 2009 023194 A1 discloses a device for inspecting a narrow air gap between two surfaces. The device is used particularly in the air gap of generators. Several magnets are movably arranged in a recess on a sensor platform. This allows the magnets to be positioned relative to the ferromagnetic surface and thus guide the sensor platform along the surface to be inspected.

[0012] Based on this prior art, it is an object of the present invention to provide a test vehicle of the type mentioned at the outset which can be reliably moved in all directions on an inhomogeneous ferromagnetic substrate.

[0013] To achieve this object, the present invention provides a test vehicle according to claim 1. In this test vehicle, the at least one magnet is guided within a holder, in particular a rail, so as to be freely movable in one direction of movement. The holder can be produced using an additive manufacturing process, for example with a 3D printer. Due to the magnetic force of attraction between the at least one magnet, which is guided freely movable in one direction of movement, and the ferromagnetic regions of the substrate, the at least one magnet is automatically aligned, i.e. it moves automatically within its holder into a position above the nearest ferromagnetic region of the substrate. In this way, a sufficient contact force or adhesive effect of the test vehicle on the substrate that remains constant within a tolerance range is always ensured.If the test vehicle according to the invention is used on a generator rotor, it can be reliably moved in both axial and radial directions. The test vehicle according to the invention can be used not only on substrates with inhomogeneous ferromagnetism of the type described above, but also on substrates that, in addition to ferromagnetic areas, also have slots, holes, or other interfering contour areas. Several magnets are provided, each of which is freely movable within a holder in a direction of movement. The holders are arranged in pairs in a direction transverse to, in particular perpendicular to, the direction of movement of the magnets and / or offset from one another in the direction of movement of the magnets. This enables the magnets to adhere to as different areas of the substrate as possible, which contributes to a secure hold of the test vehicle on the substrate.

[0014] The strength, arrangement and / or number of the magnet(s) can be selected such that the test vehicle can be moved overhead along the ground.

[0015] Advantageously, the measurement technology comprises at least one camera and a lighting unit for illuminating the image area of ​​the at least one camera, whereby the lighting unit can be integrated into the camera. In addition to or as an alternative to one or more cameras, the testing technology can also comprise other testing devices, for example, ultrasonic testing devices or the like.

[0016] Preferably, the test vehicle comprises a housing, in particular one which is substantially rectangular in shape, which has a housing front side facing the main direction of travel, an opposite housing rear side, longitudinal housing sides arranged opposite one another and connecting the housing front side and the housing rear side, a housing bottom side which faces the ground when the test vehicle is placed on the ground, and an opposite housing top side.

[0017] The direction of movement of the at least one magnet can be transverse, in particular perpendicular, to the main direction of travel. It is also conceivable for the direction of movement of at least one magnet to correspond to the main direction of travel, and for the direction of movement of at least one further magnet to be transverse, in particular perpendicular, to the main direction of travel.

[0018] According to one embodiment of the present invention, the measuring technology is at least predominantly accommodated in the housing.

[0019] Advantageously, the inspection vehicle is designed to be inserted into an air gap between two opposing gap walls and placed and moved on the surface of a first gap wall as a base in order to inspect the first gap wall and / or the opposite second gap wall for damage. The camera is arranged inside the housing and directed at a mirror which is arranged inside the housing between two windows provided opposite one another on the underside and on the top of the housing and is pivotable, in particular by a motor, about a mirror pivot axis in such a way that the camera views the first gap wall through the lower window or the second gap wall through the upper window, depending on the pivot position of the mirror. Accordingly, both the outer peripheral surface of the rotor and the inner peripheral surface of the stator can be inspected with a single camera.

[0020] The camera can be moved back and forth relative to the mirror, particularly by motor. Since the outer peripheral surface of the rotor and the inner peripheral surface of the stator are at different distances from the housing, the camera's focus can be adjusted depending on the area to be inspected.

[0021] Advantageously, the camera is aligned in the main direction of travel, the top of the housing above the camera is provided with a through-opening, and the camera is, in particular, motor-driven and can be moved up and down through the through-opening between a first position in which it is arranged inside the housing and directed towards the mirror, and a second position in which it is arranged outside the housing. In the first position, the camera can therefore be used to inspect the rotor or the stator. In the second position, the camera is aligned in the main direction of travel so that an operator can see where to steer the inspection vehicle. Overall, therefore, only a single camera is required. In principle, it is of course possible, but also possible, to provide several cameras, for example one camera for inspecting the rotor surface, one for inspecting the stator surface, and another for the view in the main direction of travel.

[0022] According to a further embodiment of the present invention, the test vehicle is designed to be inserted into the air gap between a rotor defining the first gap wall and a stator defining the second gap wall of an electrical machine, in particular a generator, and to be placed and moved on the substantially cylindrical outer circumference of the rotor in order to inspect the outer circumference of the rotor and / or the inner circumference of the stator for damage. The drive unit comprises at least four drive groups, which are held in pairs opposite one another on the longitudinal sides of the housing. Each drive group has at least two wheels, which can be driven by separate motors to rotate about wheel axles extending parallel to one another and perpendicular to the main direction of travel. Each drive group is pivotally attached to an intermediate element about a first pivot axis extending parallel to the wheel axles.which in turn is pivotally mounted on the housing about a second pivot axis extending parallel to the first pivot axis, and about a third pivot axis extending perpendicular to the second pivot axis, wherein the pivoting movement of one drive group about its third pivot axis is coupled to the pivoting movement of the opposite drive group about its third pivot axis. This design and arrangement of the drive groups enables the test vehicle according to the invention to be driven, steered, and turned in any direction on a curved surface, for example, on the surface of a rotor. Thus, once placed on the curved surface, it can be moved freely in all directions. This also enables the inspection of the rotor and stator of generators where the air gap between the rotor and stator is only accessible via a one-sided access at the 12 o'clock position.without having to pull the runner. Accordingly, the inspection can be carried out quickly and inexpensively, for example, compared to the previously known crawler system. The operation of the test vehicle and the measuring technology is preferably completely remote-controlled, i.e., wirelessly. Steering is advantageously implemented by driving the wheels of the drive units on both sides at different speeds, similar to a tracked vehicle.

[0023] Conveniently, at least one magnet is arranged on the underside of the housing and / or at least one magnet is arranged on the underside of a drive group, in particular each drive group. A widely distributed arrangement of magnets contributes to a secure hold of the test vehicle on the ground.

[0024] The intermediate elements of oppositely arranged drive groups can be coupled to each other via a lever mechanism such that when one of the two drive groups is pivoted downwards by a predetermined amount about its associated third pivot axis, the other of the two drive groups is pivoted upwards by a corresponding amount about its associated third pivot axis, and vice versa. Accordingly, even during inclined travel, for example, over a runner, it is ensured that the drive groups and thus their respective wheels are always optimally positioned.

[0025] According to a further embodiment of the present invention, each drive group comprises three wheels, each arranged in pairs offset from one another in the main direction of travel and / or in the direction of movement of the magnets, and each driven by separate motors rotating around its wheel axle. This ensures that even if one of the wheels becomes jammed, for example, in an air cooling bore of a rotor, the vehicle's propulsion is maintained via the other wheels.

[0026] The measuring technology and the motors can be supplied with energy via at least one accumulator arranged within the housing.

[0027] The overall height of the test vehicle, when placed on a level surface, is advantageously less than 20 mm, and even better, less than 16 mm. Accordingly, almost all industrial generators can be inspected with the test vehicle according to the invention.

[0028] Further features and advantages of the invention will become clear from the following description of an embodiment of the test vehicle according to the invention with reference to the accompanying drawing. Figure 1 shows a perspective rear view of a test vehicle according to an embodiment of the present invention, obliquely from above, with a camera in a first lower position; Figure 2 shows a view analogous to Figure 1 , with the camera in a second upper position; Figure 3 is a plan view of the Figure 2 test vehicle shown; Figure 4 a front view of the Figure 2 shown test vehicle; Figure 5 is a bottom view of a drive group of the test vehicle shown in Figures 1 to 4 and Figure 6 is a perspective view of the test vehicle shown in Figure 1 The test vehicle shown during an inclined drive over a runner.

[0029] The figures show a test vehicle 1 according to an embodiment of the present invention, which is designed to be inserted into the air gap between two opposite gap walls of an air gap between a rotor 2 and a stator of an electrical machine, in particular a generator, and to be placed and moved on a partially ferromagnetic substrate, in this case on the surface of a first gap wall, more precisely on the essentially cylindrical outer circumference 3 of the rotor 2, in order to inspect the first gap wall, i.e. the outer circumference 3 of the rotor 2, and / or the opposite second gap wall, i.e. the inner circumference of the stator, for damage. The outer circumference 3 of the rotor 2 comprises ferromagnetic rotor body regions 4 made of steel and non-ferromagnetic rotor slot closure wedge regions 5 made of copper.The test vehicle 1 comprises as main components a housing 6, a measuring device 7 which is accommodated in the housing 6, and a drive unit 8 with four drive groups 9.

[0030] The housing 6 is essentially rectangular in shape. It comprises a housing front 11 facing in the main direction of travel indicated by arrow 10, an opposite housing rear 12, opposite longitudinal housing sides 13 connecting the housing front 11 and the housing rear 12, a housing bottom 14 facing the rotor when the test vehicle 1 is in use, and an opposite housing top 15.

[0031] The measuring technology 7 is housed within the housing 6 and is designed to record physical and / or chemical properties of the environment. In this case, it comprises a camera 16 with an integrated lighting unit for illuminating the image area of ​​the camera 16, as well as a mirror 17. The camera 16 is directed in the main direction of travel 10 onto the mirror 17, which is arranged inside the housing 6 between two windows 18 provided opposite one another on the underside 14 of the housing and on the top side 15 of the housing, and is held pivotably by a motor about a mirror pivot axis 19 extending transversely to the main direction of travel 10 in such a way that the camera 16, depending on the pivot position of the mirror 17, optionally views the rotor 2 through the lower window 18 or the stator through the upper window 18 when the test vehicle 1 is mounted on a rotor 2.The camera 16 is motor-driven and can be moved back and forth relative to the mirror 17 in the direction of the arrow 20 along guides (not shown in detail) in order to be able to optimally adjust the focus of the camera 16. Alternatively, such mobility can be dispensed with and a fixed focus can be set. Furthermore, the camera 16 is motor-driven and can be moved up and down between a first lower position, in which it is arranged inside the housing 6 and directed at the mirror 17, and a second upper position, in which it is guided out of the housing 6 through a through opening 21 provided above the camera 16 on the top side 15 of the housing. The first position is in . Figure 1 shown, the second position in Figure 2The up and down movement is realized here via motor-driven pivoting levers 22. In the first position, the camera 16 can thus be used to inspect the rotor 2 or the stator. In the second position, the camera 16 is aligned in the main direction of travel 10, allowing an operator to see where to steer the inspection vehicle 1.

[0032] The drive groups 9 are held in pairs opposite one another on the longitudinal sides 8 of the housing. In this case, each drive group 9 has three wheels 23 arranged offset from one another in the main direction of travel 10, whereby the wheels 23 are partially also arranged offset from one another in a direction perpendicular to the main direction of travel 10. The wheels 23 can each be driven by separate motors 24 to rotate about wheel axles 25 extending parallel to one another and perpendicular to the main direction of travel 10. Each drive group 9 is pivotally mounted on an intermediate element 27 about a first pivot axis 26 extending parallel to the wheel axles 25, which in turn is pivotally mounted on the housing 6 about a second pivot axis 28 extending parallel to the first pivot axis 26 and up and down about a third pivot axis 29 extending perpendicular to the second pivot axis 28 in the main direction of travel 10.The pivoting movement of one drive group 9 about its third pivot axis 29 is coupled to the pivoting movement of the opposite drive group 9 about its third pivot axis 29. In the present case, this coupling is effected via a lever mechanism 30, wherein the lever mechanism 30 of the front two drive groups 9 extends along the housing front 11 and the lever mechanism 30 of the rear two drive groups 9 extends along the housing rear 12. Each lever mechanism 30 comprises two L-shaped levers 31, each of which is rotatably mounted on the housing front 11 and the housing rear 12 about axes of rotation 32 extending parallel to one another in the main travel direction 10. Laterally outward-facing legs of the respective levers 31 are provided with elongated holes 33 into which a free end of an actuating portion 34 of an associated intermediate element 27 engages.On the respective other legs, the levers 31 of a lever mechanism 30 are pivotally connected to one another via a connecting rod 35. Thus, if one of two drive groups 9 arranged opposite one another is pivoted downwards by a predetermined amount about its associated third pivot axis 29, the other of the two drive groups 9 is pivoted upwards by a corresponding amount about its associated pivot axis 29, and vice versa, as shown in particular in FIG. Figure 6 is clearly visible.

[0033] The dimensions of the housing 6 and the drive groups 9 are selected such that the total height H of the test vehicle 1, when the camera 16 is in its first position, is less than 20 mm, better still less than 16 mm.

[0034] In addition, the test vehicle 1 comprises a plurality of magnets 36, in this case neodymium magnets, which are arranged on the undersides of the housing 6 and the drive groups 9 and are designed to hold the test vehicle 1 to the ground. More specifically, three magnets 36 are arranged on the underside of the housing 6 and on the underside of each drive group 9, as shown in Figure 5shown as an example for a drive group 9. According to the invention, the magnets 36 are each guided freely movable within a holder 37, in this case a straight rail, in a direction of movement indicated by the double arrow 38, which in this case runs perpendicular to the main direction of travel 10. The holders 37 are arranged in pairs offset from one another in the main direction of travel 10 of the test vehicle 1 and in the direction of movement 38 of the magnets 36 such that the test vehicle 1 can be reliably moved in all directions on an inhomogeneous ferromagnetic surface.Due to the magnetic attraction force between the magnets 36 and the ferromagnetic rotor body regions 4 of the outer circumference 3 of the rotor 2, the magnets 36 move automatically within their holders 37 into a position above the nearest ferromagnetic rotor body region 4, thus simultaneously moving away from a non-ferromagnetic rotor slot closure wedge region 5. The resulting positions of the three magnets 36 are shown, for example, in FIG. Figure 5 in which a ferromagnetic rotor body area 4 and two non-ferromagnetic rotor slot closure wedge areas 5 are shown as hatched areas for orientation purposes only. In this way, a suitable contact force of the test vehicle 1 against the outer circumference 3 of the rotor 2 is always ensured. The strength, the arrangement and / or the number of magnets 36 is selected such that the test vehicle 1, when it is in accordance with Figure 6is placed on a rotor 2, is held on the rotor 2 by means of the magnets 36 and can also be moved upside down along the outer circumference 3 of the rotor 2.

[0035] The control of the measuring technology 7 and the drive groups 9 is carried out radio- or remote-controlled, i.e. wirelessly.

[0036] Although the invention has been illustrated and described in detail by the preferred embodiment, the invention is not limited to the disclosed examples and other variations may be derived therefrom by those skilled in the art without departing from the scope of the invention.

[0037] During a visual inspection of generators without removing the generator rotor from its housing, the interior of the generator is driven through with a test vehicle.

[0038] This test vehicle is inserted into the generator and runs on the generator rotor.

[0039] The test vehicle inspects both the generator rotor and the generator stator inside the generator.

[0040] For this purpose, the areas to be inspected are illuminated using a light source located on the test vehicle.

[0041] There are cooling holes on the generator rotor through which a significant part of the cooling effect of air-cooled generators is implemented during operation.

[0042] The cooling air holes are accessible from both sides, in particular on one side the cooling air holes are accessible through a groove channel.

[0043] If these cooling holes become blocked by foreign bodies, dirt or other deposits to such an extent that a continuous air flow is no longer guaranteed, there is a risk of local heating or overheating of the generator rotor.

[0044] This can, for example, result in imbalances, which in turn can lead to massive damage to the entire machine.

[0045] In order to inspect these cooling holes using the test vehicle, it is necessary to look very deeply into the cooling holes and the rotor windings behind them in order to be able to assess the complete structure of the slot channel behind them.

[0046] This illumination by the test vehicle alone is particularly difficult because the space inside the generator is very limited.

[0047] By introducing a lighting source into the groove channel, the inspection result of the cooling holes can be significantly improved to the extent that a full overview down to the groove base can be guaranteed.

[0048] The lighting can be realized in particular with a series of light-emitting diodes that are inserted under the generator cap into the groove channel of the generator rotor while the test vehicle drives on the surface of the rotor.

[0049] A standard LED light tube can be used.

[0050] The procedure is carried out by placing an illumination source into the slot channel of the generator while simultaneously driving a test vehicle on the surface of the generator rotor.

[0051] The essential step lies in the idea of ​​illuminating the cooling holes not only "from the front" (from the perspective of the test vehicle), but also from the groove channel.

[0052] This allows a much deeper insight into the internal structure and condition of the windings of the generator rotor.

[0053] Comparing this method, especially with the removal of the rotor ("rotor pull"), which was absolutely necessary before the introduction of robotic systems, it was previously the case that compromises had to be made when inspecting the generator rotor using robot-based systems. However, the backlighting described in this idea eliminates these compromises, ensuring that the inspection results provide an equivalent basis for evaluation.

Claims

1. An inspection vehicle (1) which is designed to be placed onto and moved along a surface (3) that is ferromagnetic at least in part, the inspection vehicle comprising - at least one measurement instrument (7) which is designed to measure physical and / or chemical properties of the surroundings, - a drive unit (8) - at least one holder (37), and - at least one magnet (36), in particular a neodymium magnet, which is located on the underside of the inspection vehicle (1) and is designed to hold the inspection vehicle (1) on the surface (3), wherein the at least one magnet (36) is guided in the holder (37), in particular a rail, so as to be freely movable in one direction of movement (38), wherein multiple magnets (36) are provided, each of which is guided in a holder so as to be freely movable in one direction of movement, characterised in that the holders (37) thereof are each located in pairs offset from one another in one direction transverse to, in particular perpendicular to, the direction of movement (38) of the magnets (36) and / or in the direction of movement (38) of the magnets (36).

2. The inspection vehicle (1) according to claim 1, characterised in that the strength, the location, and / or the number of the magnet(s) (36) is selected such that the inspection vehicle (1) can be moved along the surface (3) upside down.

3. The inspection vehicle (1) according to any one of the preceding claims, characterised in that the measurement instrument (7) has at least one camera (16) and a lighting unit for illuminating the image area of the at least one camera (16), wherein the lighting unit may be integrated into the camera (16).

4. The inspection vehicle (1) according to any one of the preceding claims, characterised in that the inspection vehicle (1) comprises a housing (6), formed in particular substantially rectangular, having a housing front side (11) facing the main direction of travel (10), an opposite housing rear side (12), longitudinal housing sides (13) located opposite one another and connecting the housing front side (11) and the housing rear side (12), a housing underside (14) facing the surface (3) in the state of the inspection vehicle (1) being placed onto the surface (3), and an opposite housing top side (15).

5. The inspection vehicle (1) according to claim 4, characterised in that the direction of movement (38) of the at least one magnet (36) is transverse, in particular perpendicular, to the main direction of travel (10).

6. The inspection vehicle (10) according to claim 4 or 5, characterised in that the measurement instrument (7) is at least predominantly accommodated in the housing (6).

7. The inspection vehicle (1) according to claim 3 and one of claims 4 to 6, characterised in that the inspection vehicle (1) is designed to be inserted into an air gap between two opposite gap walls and to be placed onto and moved along the surface of a first gap wall as surface in order to check the first gap wall and / or the opposite second gap wall for damage, and the camera (16) is located inside the housing (6) and points towards a mirror (17) located inside the housing (6) between two windows (18) provided opposite to one another on the housing underside (14) and housing top side (15) and being held pivotably, in particular by a motor, around a mirror pivot axis (19), such that the camera (16) views the first gap wall through the lower window (18) or the second gap wall through the upper window (18), depending on the pivot position of the mirror (17).

8. The inspection vehicle (1) according to claim 7, characterised in that the camera (16) can be moved back and forth relative to the mirror (17), in particular by a motor.

9. The inspection vehicle (1) according to claim 7 or 8, characterised in that the inspection vehicle (1) is designed to be inserted into the air gap between a rotor (2) defining the first gap wall and a stator of an electrical machine, in particular a generator, defining the second gap wall, and to be placed onto and moved along the substantially cylindrical outer circumference (3) of the rotor (2) in order to check the outer circumference (3) of the rotor (2) and / or the inner circumference of the stator for damage, and in that the drive unit (8) comprises at least four drive groups (9) which are held in pairs opposite one another on the housing longitudinal sides (13), each drive group (9) has at least two wheels (23) drivable, by separate motors (24), to rotate around wheel axles (25) extending parallel to one another and perpendicular to the main direction of travel (10), and each drive group (9) is attached to an intermediate element (27), pivotable around a first pivot axis (26) extending parallel to the wheel axles (25), which in turn is attached to the housing (6), pivotable around a second pivot axis (28) extending parallel to the first pivot axis (26) and pivotable up and down around a third pivot axis (29) extending perpendicular to the second pivot axis (28), wherein the pivot movement of a drive group (9) around its third pivot axis (29) is coupled to the pivot movement of the opposite drive group (9) around its third pivot axis (29).

10. The inspection vehicle (1) according to claim 9, characterised in that at least one magnet (36) is located on the underside (14) of the housing (6) and / or at least one magnet (36) is located on the underside of a drive group (9), in particular each drive group (9).

11. The inspection vehicle (1) according to claim 9 or 10, characterised in that the intermediate elements (27) of drive groups (9) located opposite to one another are coupled to one another via a lever mechanism (30) in such a way that when one of the two drive groups (9) is pivoted downwards around the third pivot axis (29) associated with it by a predetermined amount, the other of the two drive groups (9) is pivoted upwards around the third pivot axis (29) associated with it by a corresponding amount, and vice versa.

12. The inspection vehicle (1) according to any one of claims 9 to 11, characterised in that each drive group (9) has three wheels (23) located in pairs offset from one another in the main direction of travel (10) and / or in the direction of movement (38) of the magnets (36) and is each driven by separate motors (24) rotating about their wheel axles (25).

13. The inspection vehicle (1) according to any one of claims 9 to 12, characterised in that the measurement instrument (7) and the motors (24) are powered via at least one accumulator located within the housing (6).

14. The inspection vehicle (1) according to any one of the preceding claims, characterised in that its total height, when placed onto a flat surface (3), is less than 20 mm, preferably less than 16 mm.