DC motor, lounger with DC motor and method for operating a DC motor

Abstract

DC motor (1) for operation in an external magnetic field comprising: a rotor (2) which is rotatable about an axis of rotation (5) and has at least two windings (10, 11), at least four continuous rotary transformers (3), two rotary transformers (3) being assigned to each of the windings (10, 11) and being designed to supply the respectively assigned winding (10, 11) with direct current, an angle sensor (4) designed to determine the angular position of the rotor (2), and a control unit (15) designed to control the current supply to at least one of the windings (10, 11) as a function of the angular position of the rotor (2), wherein the motor (1) is aligned with an external magnetic field during operation such that the axis of rotation (5) extends transversely to the external magnetic field.

Description

[0001] In a magnetic resonance imaging (MRI) scanner, a motor is used to move the scanner vertically and horizontally. It is known in the art to use an electric servomotor for this purpose. This motor is implemented as a brushless motor, with a stator consisting of multiple windings and a rotor equipped with permanent magnets. Because of the permanent magnets, when using such motors in close proximity to external magnetic fields (such as the magnetic field of an MRI scanner), care must be taken to ensure that they do not get too close to the source generating the external magnetic field.

[0002] Currently, for example, MRI system couches are equipped with a spacer frame that mechanically prevents the couch from getting too close to the MRI magnet. The motors themselves are housed in the rear part of the couch, and a strong anchor is provided to secure the motor in place within the couch's substructure. The torque of such motors is proportional to the magnetic flux density of the permanent magnets in the rotor. Due to the aforementioned distance problem from the MRI magnet, such motors are therefore limited in size and torque to a certain value.

[0003] Current alternatives, such as pneumatic or piezo motors, are either too slow, too weak, or too expensive for use in an MRI system's couch. Purely electric motors therefore appear more advantageous.

[0004] US Pat. No. 4,902,975 A discloses a DC motor with multiple windings on a rotor that can be energized via sliding contacts. The sliding contacts are interrupted at several points, so that commutation occurs mechanically—as is common with DC motors. Furthermore, the motor uses an external magnetic field as the stator field.

[0005] A motor for use in an external magnetic field is also known from US 10 330 754 B2 (and the corresponding German family document DE 10 2017 131 317 A1). The motor has sliding contacts and an inverter rotating with a rotor, via which the windings on the rotor are supplied with current.

[0006] However, for high-precision drive of even power-intensive devices (such as the couches of an MR system), powerful and precisely controllable motors are required whose angle of rotation can be precisely predetermined.

[0007] WO 2007 147 657 A1 discloses a device comprising a stationary part and a rotating part. The stationary part has electrically insulated current transfer surfaces, and the rotating part also has such electrically insulated current transfer surfaces. Each of the current transfer surfaces of the stationary part is associated with a current transfer surface of the rotating part. Between the associated current transfer surfaces is a gap into which a liquid metal alloy is introduced, thereby creating a current transfer contact.

[0008] DE 10 2009 018 655 A1 discloses a liquid metal rotary transformer for transmitting an electrical current in rotating parts. The liquid metal rotary transformer comprises a first contact ring, a second contact ring, and a gap containing liquid metal between the first contact ring and the second contact ring.

[0009] DE 10 2009 060 544 A1 discloses a device for treatment with magnetic fields, which is designed to achieve nuclear magnetic resonances in a tissue to be treated.

[0010] DE 10 2004 036 316 A1 discloses a guard ring for a detector array for direct photo-electron conversion.

[0011] From DE 603 ​​09 207 T2 an irradiation system with inner and outer bearings for precise positioning when rotating the inner bearing is known.

[0012] Therefore, it is an object of the present invention to provide a DC motor for operation in an external magnetic field, which can generate a sufficiently high torque and is precisely controllable.

[0013] The present invention solves this problem with a DC motor having the features of claim 1, with a couch having the features of claim 9 and with a method for operating a DC motor in an external magnetic field having the features of claim 10.

[0014] According to a first aspect of the present invention, a DC motor (DC = Direct Current) for operation in an external magnetic field is provided, comprising a rotor which is rotatable about an axis of rotation and has at least two windings, at least four continuous rotary transformers, two rotary transformers being assigned to each of the windings and being designed to supply the respectively assigned windings with direct current, an angle sensor which is designed to determine the angular position of the rotor and a control unit which is designed to control the current supply to at least one of the windings as a function of the angular position of the rotor, wherein the motor is aligned with an external magnetic field during operation such that the axis of rotation extends transversely to the external magnetic field.

[0015] Compared to the known prior art, the DC motor according to the present invention differs in particular in the continuous rotary transformer, the angle encoder and the control unit. This allows the motor to be used in an external magnetic field and a previously specified or determined angular position of the rotor to be achieved with high precision. A device operated with the motor according to the invention can therefore be driven or adjusted very precisely. For this purpose, the motor can be operated in stepper motor mode or in servo motor mode. In stepper motor mode, the rotor may rotate not continuously, but in individual steps (i.e. stepwise). The step size can determine the achievable accuracy of the motor. In stepper motor mode, the torque may decrease if the speed increases.Preferably, the rotor of the motor used in stepper motor operation has 9-60 windings and thus a step size of 18° to 3°.

[0016] In servomotor mode, the rotor can move proportionally to a control signal, making it infinitely controllable. In servomotor mode, the torque can be provided dynamically depending on the load.

[0017] In contrast, state-of-the-art motors used in external magnetic fields can only be switched on and off without their target position (i.e., their angle of rotation) being precisely defined or predetermined in advance.

[0018] The external magnetic field can, for example, be a B0 magnetic field of a magnetic resonance imaging (MRI) scanner. The external magnetic field can have continuously running field lines. The external magnetic field can serve as a stator magnetic field in the motor according to the invention. Furthermore, a stray magnetic field of an MRI scanner can also be used as the external magnetic field.

[0019] The rotor can be a cylindrical body that is rotatably mounted in a motor housing. The bearing can be a radial bearing, such as a plain bearing or a roller bearing. A plain bearing, for example, can be a Teflon bearing to reduce sliding friction. A roller bearing can be a ball bearing or an industrial bearing. By using standardized bearings, the motor can be manufactured quickly, easily, and at low cost. Ceramic ball bearings, such as those made of zirconium oxide and / or silicon nitride, are preferred. This can increase the longevity of the bearings.

[0020] The rotor further comprises windings, which may comprise conductors for conducting electrical current. The windings may also be referred to as coils. Each winding may be electrically insulated. For example, insulated wires may be used as windings. Each winding may consist of a plurality of wires. A winding may extend substantially along the longitudinal direction of the rotor and be wound several times around the rotor. If the rotor is a cylindrical body, a winding may extend along the vertical direction of the cylinder and at least partially over both cover surfaces. The windings may intersect at the cover surfaces of the rotor. Furthermore, each winding may be connected to the rotary transformer associated with the winding at one of the cover surfaces of the rotor. The windings may be held by retaining elements provided on the circumferential surface of the rotor.For this purpose, the holding elements can each have an undercut so that the respective winding is received therein and held there. The rotor can have a plurality of holding elements at each of the longitudinal ends of the rotor so that a plurality of windings can be provided on the rotor. The holding elements can be arranged evenly distributed over the circumference of the rotor. This enables simple attachment of the windings to the rotor. Furthermore, additional windings can be easily added or windings can be easily replaced. If two windings, each formed from a conductor, are provided, it is advantageous for each conductor to be wound around the rotor several times along the longitudinal direction of the rotor (i.e. along the axis of rotation of the rotor). The two windings can be arranged on the rotor so that they are at an angular distance of substantially 90° from one another.

[0021] The rotary joint can be designed to transfer current from a stationary component (power supply) to a rotating component (rotor). Rotary joints can also be designed as continuous slip rings. In the prior art, it is known that in DC motors, slip rings also serve as commutators, so that - as is usual with DC motors - polarity reversal can occur mechanically. For this purpose, the slip rings are designed with interruptions (i.e., not continuously). Typically, the current in the slip ring is transferred to the rotating rotor by means of carbon brushes. However, when passing from one section of the slip ring to the next (i.e., when the polarity is reversed), so-called brush firing can occur, which can have a negative impact on imaging during MRI operation. Therefore, continuous rotary joints are provided according to the invention.Continuous can mean that there are no interruptions in the rotary joint. Rather, the surfaces of the rotary joint that move relative to each other (i.e., the stationary surface and the rotating surface) are continuous, so that there are no mechanical pole reversals like with interrupted rotary joints. This can prevent brush sparking, ensuring that MRI imaging can operate without interference from the motor. Since there is no polarity reversal in a rotary joint, two rotary joints are assigned to each winding. In other words, one of the two rotary joints can serve as a positive pole and the other as a negative pole of the assigned winding. For this purpose, the rotary joints can be connected in series with the winding.

[0022] The angle encoder can determine the angular position of the rotor using an optical system, a mechanical / electrical system, a magnetic system, or a combination of the aforementioned systems. For example, the angular position can be detected using a potentiometer. Furthermore, the angular position can be determined using a magnetic system using Hall sensors. The angle encoder can also be referred to as an encoder or rotary encoder. Preferably, a purely optical measuring method is used, in which an angular change of the rotor can be determined by means of fiber optic scanning of an encoder disk and subsequent shielded signal evaluation. Regardless of the system used, the angle encoder can output a voltage corresponding to the angular position of the rotor. For this purpose, the encoder disk can be attached to the rotor in such a way that it rotates with the rotor.The angle sensor can be located on the rotor shaft. Preferably, the angle sensor is mounted on the same side of the rotor windings as the rotary transformers.

[0023] The control unit can be designed to supply direct current to the windings individually and separately from one another via the rotary transformers and to variably adjust the current direction of the direct current supplied to the windings. The control unit can be a circuit that can control the operation of the motor by controlling switches and other control devices. Furthermore, the control unit can comprise a CPU for processing signals. The control unit can receive signals (e.g. commands, measured values, detected values, etc.) and output control signals. The control unit can be designed to receive signals from the angle encoder (e.g. a voltage) and, based on these signals (i.e., based on the angular position of the rotor), control the current supply to the windings. Furthermore, the control unit can be designed to receive signals (e.g. control signals, target position of the rotor, etc.) from an interface.Furthermore, the control unit can be configured to compare the signals from the angle sensor with the signals from the interface and, based on this comparison, to control the power supply to the windings. Furthermore, the control unit can comprise a memory unit, which can be configured to permanently or temporarily store an operating program and / or measured values.

[0024] The angular position of the rotor can be expressed as a degree. Furthermore, the angular position of the rotor can be a relative or absolute value. For example, a rotor position where the first winding is oriented horizontally and the second winding is oriented vertically can be referred to as an initial position (angular position of 0°). If the rotor then rotates around its axis of rotation, for example, so that the first winding is oriented vertically and the second winding is oriented horizontally, an angular change or an angular position of 90° occurs.

[0025] During operation of the motor, the motor is aligned with an external magnetic field such that the axis of rotation of the rotor extends transversely to the external magnetic field (i.e., to the field lines of the magnetic field). For an alignment of the axis of rotation of the rotor transversely to the field lines of the magnetic field, it is sufficient that at least one component of the respective field line runs orthogonal to the axis of rotation, i.e., a certain minimum field strength must be present orthogonal to the axis of rotation. In other words, when the axis of rotation is aligned transversely to the field lines, the axis of rotation and the field lines can enclose any angle with the exception of 0° and 180°. The angle is preferably between 30° and 150°, more preferably between 50° and 130°, even more preferably between 70° and 110°. This means that the axis of rotation can be arranged such that it does not extend parallel to the field lines of the magnetic field.The field lines of the magnetic field can be resulting field lines from higher-order magnetic fields. In other words, smaller eddy currents can be disregarded as long as at least one component of the respective field line is essentially orthogonal to the motor's axis of rotation.

[0026] The motor can, for example, adjust a couch using a spindle, threaded rod, and / or a gear device. In particular, the motor can be used to adjust the couch's headrest. Furthermore, the motor can adjust the couch's inclination to the horizontal or advance the couch during an MRI imaging process. In addition, the motor can adjust the couch's height. Since no permanent magnet or similar is provided for the motor, it is not necessary to ensure that the motor is a certain distance from the MR magnet. This allows for increased design freedom in the construction of the couch and the entire MR system. Furthermore, handling of the couch is made easier overall, as there is no need to worry about the couch getting too close to the MR magnet.

[0027] Furthermore, the motor can also be used for calibrating an MRI system. Motion control components can be calibrated and / or tested. For example, a dummy (i.e., a doll simulating a real patient) can be realistically moved with the motor while the magnetic field in the bore is active, and test measurements are performed. This allows motion compensation to be calibrated. The motor can be used to perform repeatable and predetermined movements, simplifying calibration.

[0028] Furthermore, it is conceivable that the motor could be used to homogenize the magnetic field when adjusting the MR system using shim devices. Thus, for example, the motor could be used to automate the rotation of a measuring device in the magnetic field, which is currently performed manually, so that the service technician no longer has to perform this task manually. This can accelerate the tuning of the MR system, leading to cost and time savings.

[0029] The motor could also be used for calibrating Hall sensors. For example, calibrating Hall sensors requires that they be moved in a specific manner within a pre-tuned magnetic field. In this process, known as "elliptical fitting calibration," the motor can be used to advantageously move the Hall sensor to be calibrated in a defined manner, as the motor can be operated easily and with high precision in external magnetic fields.

[0030] The motor is preferably made of plastic, ceramic, and / or copper. The motor preferably contains no ferromagnetic materials. This allows the motor to be easily placed in a strong external magnetic field (i.e., without the need for appropriate anchoring). Furthermore, the motor is also cost-effective to manufacture, as it eliminates the need to use neodymium magnets, as is the case with standard servo motors. This allows for cost savings during production.

[0031] Preferably, each winding lies at least approximately in the same plane as the axis of rotation. In other words, each winding can lie in the same plane as the axis of rotation. The rotor can have a plurality of windings that extend substantially along the longitudinal direction (i.e., along the axis of rotation) of the rotor. Each winding can run on one side of the rotor from a connection on the respective rotary transformer parallel to the longitudinal direction of the rotor, then change over to the opposite side of the rotor at the end of the rotor and there run parallel to the axis of rotation of the rotor back to the connection. In other words, the line from which a winding can be formed can be opposite one another with respect to the rotor (i.e., with respect to the axis of rotation of the motor).In this context, "essential" can mean that the conductors forming the winding are not exactly opposite each other, but rather are located within manufacturing tolerances. For example, these manufacturing tolerances can represent a deviation of approximately 5%. In this case, a plane can be defined by two straight lines running at 90° to each other. Each winding can form a substantially rectangular shape in a plan view. Furthermore, each winding can surround the rotor circumferentially. This can ensure easy access to the winding from the outside.

[0032] Preferably, the ratio between the diameter of the rotor and the length of the winding along the rotor's rotational axis is less than 1, preferably less than 0.3. This ratio ensures that a generated Lorentz force acts over a sufficiently long distance to generate a sufficiently large torque. This allows for the provision of a powerful stepper motor. Furthermore, the ratio between the material used and the effect provided by the motor can be optimized with the defined ratio.

[0033] Preferably, the torque of the motor can be greater than 2.5 Newton meters. Thus, the motor can be designed to be sufficiently powerful, even without a gear transmission, to easily move, for example, a table of an MRI system with a patient on it horizontally and / or vertically.

[0034] Preferably, each rotary transformer has an electrically conductive and continuously extending first and second element. Preferably, the first element and the second element are arranged so as to be rotatable relative to one another. Preferably, during operation of the motor, the second element rotates together with the rotor. Preferably, a gap is formed between the first element and the second element, in which gap an electrically conductive liquid metal alloy, in particular a eutectic alloy comprising gallium, indium and tin, is arranged. The first element can have a first recess on its side facing the second element and the second element can have a second recess on its side facing the first element. An electrically conductive porous material can be arranged in the first and second recess.The liquid metal alloy can establish an electrically conductive connection between the first element and the second element. The porous material can help to securely hold the liquid metal alloy in the gap. The rotary joint can thus be designed to transmit an electrical current from the first element to the second element, regardless of whether the rotor is rotating or not. Thus, current can be transmitted from a stationary power source to the rotating rotor without the need for sliding contact, for example with carbon brushes. A rotary joint such as that described in EP 2498347 B1 can be used as the rotary joint, for example. Furthermore, the content of EP 2498347 B1 is incorporated herein by reference.

[0035] Preferably, the control unit can be designed to short-circuit at least one winding based on the angular position of the rotor, in particular via a transistor cascade, so that rotation of the rotor can be slowed down. In other words, if one of two windings is used to set the rotor in rotation, the unused winding, which also moves through the external magnetic field, can be short-circuited. Due to the movement of the unused winding in the external magnetic field, a voltage is induced in this winding. This can cause a voltage gradient in the winding. If this winding is then short-circuited, i.e. a current flow is generated in this winding corresponding to the induced voltage, a Lorentz force acts against the direction of rotation of the rotor and slows the rotor.The generated Lorentz force depends on the rotational speed of the rotor before the short circuit and becomes zero when the rotor is no longer moving in an external magnetic field. In other words, the generated braking effect can be proportional to the rotational speed of the rotor. This ensures at all times that by short-circuiting an unused winding, a sufficiently high braking effect is generated to stop the rotor in the desired position. The short-circuiting of the unused winding can be effected by a switch. According to the invention, a short circuit can mean that an electrical circuit is closed in such a way that a current can flow essentially without additional resistance (such as consumers or the like). The rotor can therefore be set in rotation by one winding and braked by the other winding. Once the rotor has reached its desired position, the winding previously used for rotation (i.e.The (current-energized) winding can be further energized to increase the motor's holding torque in the desired position. The thus further energized winding creates a radial force component due to the Lorentz force, which prevents the rotor from rotating. Thus, the rotor can be held in the desired position and the holding torque can be increased.

[0036] Braking can also be achieved with more than one winding. For example, the motor can be provided with a plurality of windings, with a specific number of windings used to accelerate the rotor and a specific number used to brake the rotor. Braking is preferably performed with the same number of windings as was used for acceleration.

[0037] In order to limit the current flowing during the short circuit, at least one transistor can be provided which can limit the current so that the braking effect or braking force can be limited. The transistor can also be a controlled transistor so that the current flowing in the event of a short circuit in a winding can be actively controlled by the control unit. This can thus protect the circuit and the motor from damage. The braking effect can be generated in particular due to eddy currents generated during the short circuit. Preferably, the control unit can short-circuit at least one winding at a specific time (i.e. at a specific angular position of the rotor) based on the information it receives from the angle sensor. The short circuit can brake the rotor sharply and cause it to stop immediately. This can ensure that the rotor does not oscillate around the target position, but instead assumes the target position directly.After a 180° rotation of the rotor, the current direction to the windings used for drive can be reversed to continue rotating the rotor in the same direction.

[0038] Braking can be used in particular when the motor is operated in stepper motor mode. In stepper motor mode, a single step performed by the motor can depend on the number of windings. A step can be defined such that it has an angular distance from one winding to the adjacent winding. For example, if two windings are provided, a step can be 90°. If three windings are provided, a step can be 60°. If four windings are provided, a step can be 45°, etc. For high-precision stepper motors, for example, a step size of 1.8° can be set. In this case, 100 windings can be provided on the rotor. Preferably, however, 8 to 10 windings are provided, so that a step size of 22.5° to 18° can result.In stepper motor operation, the control unit can control the current supply to the windings so that the rotor rotates step by step by the angular distance between two adjacent windings.

[0039] The control unit can have a bridge circuit, also known as an H-circuit, H-bridge, or full bridge, to control the current supply to the windings. This makes it advantageously possible to change the current direction by controlling the switches in the bridge circuit. MOSFET switches, transistors, IGBT transistors, or relays can be used as switches. In other words, the control unit can switch the current flow to each winding on and off, as well as change the current direction applied to each winding. Furthermore, the control unit can be configured to change (i.e., adjust) the voltage applied to each winding. This allows the direction of rotation and the torque of the motor to be controlled.

[0040] The rotor can preferably comprise three windings arranged on the rotor. The three windings can be evenly distributed in the circumferential direction of the rotor. In other words, the three windings can be spaced at an angular distance of 60° from one another. Furthermore, even more windings can be arranged on the rotor, which can be evenly distributed in the circumferential direction of the rotor. If the rotor has three windings, the motor can be operated in servomotor mode. A target angular position can be specified (for example, via the interface) which the motor should assume during operation. The position of the rotor can be continuously determined via the angle encoder. For this purpose, the angle encoder can, for example, output a voltage that corresponds to the current angular position. The target position can also be expressed as a voltage.Furthermore, a comparator can be provided which compares the output of the angle encoder with the voltage of the specified target position and operates the motor until the two voltages match, i.e. the rotor has reached the desired position. When the motor is operated in servomotor mode, at least two of three windings can be energized simultaneously. The two simultaneously energized windings can have opposite current flow. This can ensure that a generated Lorentz force causes the rotor to rotate in the same direction, thereby setting the rotor in motion. Furthermore, the current strength in the windings can be adjusted so that the rotor can reach the desired position (i.e. the target position). In other words, the current strength in the individual windings can be varied depending on the position of the windings (i.e. depending on the position of the rotor).This is particularly advantageous for ensuring smooth rotation of the rotor, as a winding can be aligned with the magnetic field such that the Lorentz force generated in this winding only exerts a radial force component on the rotor and thus makes no contribution to rotor rotation. In other words, there is an angular position of the rotor at which a winding cannot generate a tangential force component (due to the Lorentz force) that contributes to rotor rotation. This angular position can occur when a winding lies in the same plane as the axis of rotation and the field lines of the external magnetic field run parallel to the plane (this position can also be referred to as dead center). In contrast, the at least two other windings can each generate a tangential force component, each of which can contribute to rotor rotation.Furthermore, the current supply to the windings can be controlled in servomotor operation such that the rotor slows down before reaching the target position. This can prevent the rotor from oscillating around the target position or overshooting the target position. Preferably, the current supply to the windings can be controlled by the control unit such that the rotor accelerates quickly and then slows down the closer it gets to its target position. Alternatively or additionally, the control unit can brake the rotation of the rotor by short-circuiting one of the windings and / or causing a Lorentz force that opposes the rotation (i.e., by short-circuiting a winding or by selectively energizing a winding).

[0041] Preferably, the control unit can be designed to control the current supply to the windings by means of vector control. In this case, the current supply can comprise a current intensity. Furthermore, the current supply can also comprise the current direction. The forces generated in each winding can be combined using vector control, and a resulting force component can be determined. The current intensity and current direction, which can be applied individually to each winding, can be adjusted such that the resulting force component causes the rotor to rotate in the predetermined direction and with a predetermined torque. In other words, vector control can refer to determining the resulting force vector and can therefore be controlled such that the target position can be reached as quickly as possible and with high precision.

[0042] The angle encoder can preferably be an optical system designed to determine the angular position of the rotor by scanning a magnet wheel. A magnet wheel can rotate together with the rotor during operation of the motor. The magnet wheel can have markings (e.g. recesses and / or elevations) distributed on its surface that can be detected by an optical system. The markings can correspond to an increment or a detection step, so that the more increments are provided, the greater the detection accuracy of the angular position. The angle encoder can thus output a current angular position of the rotor based on the detected markings (i.e. based on the number of detected markings or the type of detected markings). The magnet wheel preferably has twice as many markings as the rotor has windings.The magnet wheel can preferably be designed such that the angle encoder can resolve with a resolution in the range of 800 to 1500 steps, preferably in a range of 900 to 1200 steps, particularly preferably 1000 steps. For this purpose, the magnet wheel can have a corresponding number of markings. Such a high resolution is particularly advantageous for servo motor operation. Preferably, the control unit can control the motor with a closed-loop control based on the information obtained from the angle encoder, especially in servo motor operation. This makes it possible to achieve an optimal balance between manufacturing costs and control accuracy. The angle encoder can, for example, be designed as a light barrier system that can determine the number of interruptions of a light beam by the magnet wheel. Based on the number of interruptions, the angular position of the rotor can be determined.The pole wheel can also be called an encoder disk.

[0043] The motor can therefore be used as a stepper motor or as a servo motor in an external magnetic field, whereas the DC motors known in the state of the art are not designed to work as a stepper motor or as a servo motor in the magnetic field of an MRI.

[0044] According to a further aspect of the present invention, a couch of an MR system is provided, comprising a DC motor according to one of the preceding claims, wherein the couch is adjustable and / or displaceable by the DC motor. The couch can, for example, be formed from a patient table on which a patient can sit, and a substructure that is, for example, movable. The patient table can be movable relative to the substructure. The patient table can, for example, be continuously displaced in different directions (e.g., horizontally and / or vertically) relative to the substructure with the aid of the motor. Such a couch can, for example, be used during an MRI imaging process. A patient can lie on the patient table of the couch and, by moving the patient table, be moved into an image plane in an MRI scanner intended for imaging.The couch can also be located at least partially within the bore. A magnetic field with a strength of approximately 0.5 Tesla to 11 Tesla can prevail within the bore. Furthermore, the height of the couch's patient table can be adjusted using the DC motor, particularly relative to the substructure, so that the patient can be moved into the MRI image planes. Furthermore, the patient table can also be adjusted section by section using the DC motor. For example, a headrest or another part of the patient table can be tilted, inclined, or moved in position to offer the patient the greatest possible comfort while simultaneously positioning the patient optimally in the MRI image plane.

[0045] According to a further aspect of the present invention, a method for operating a DC motor in an external magnetic field is provided, wherein the motor comprises a rotor that is rotatable about a rotational axis and has at least two windings, at least four continuous rotary transformers, wherein two rotary transformers are each assigned to one of the windings and are designed to supply the respectively assigned winding with direct current, an angle sensor that is designed to determine the angular position of the rotor, wherein the method comprises the following steps: placing the motor in an externally generated magnetic field, in particular in a magnetic field of a magnetic resonance imaging scanner, such that the axis of rotation of the motor extends transversely to field lines of the magnetic field, energizing at least one of the windings so that the rotor rotates about the axis of rotation, in particular due to a Lorentz force generated thereby,Determining the current angular position of the rotor using the angle sensor, adjusting the current supply to the windings so that the rotor assumes a predetermined angular position. Positioning the motor can be achieved, for example, by attaching it to a table that can be used to transport and support a patient into the bore of an MRI scanner. It is only necessary to position the motor so that the field lines of the MRI scanner's magnetic field extend perpendicular to the rotor's rotational axis. There is no need to worry about the motor coming too close to the MR magnet.as is the case in the prior art. This allows the motor to be mounted on the couch at positions that are favorable for its function and for the manufacturability of the couch. In other words, only the orientation of the motor needs to be considered, and not its location. This increases the design freedom of the couch and an MR system as a whole. For other uses of the motor in an external magnetic field, the motor can, for example, be manually aligned so that field lines of the external magnetic field extend transversely to the rotor axis. Furthermore, it is conceivable that the motor can be automatically aligned by an alignment device. The field lines can be measured, and based on the measurement result (i.e., the direction of the field lines of the magnetic field), the motor can be automatically aligned so thatthat the field lines extend transversely to the axis of rotation. For example, the motor according to one of the above embodiments can be used in the method.

[0046] Adjusting the current supply can involve selectively applying current to one or more windings. For example, a winding can be energized to cause the rotor to rotate. This can energize precisely the one or more windings that can exert a tangential force component on the rotor due to the Lorentz force. The position of the windings in an external magnetic field can be obtained using the angle sensor. The orientation of the field lines is either known and preset or measured in parallel by a field line measuring device (e.g., with one or more Hall sensors).

[0047] Furthermore, adjusting the current supply can comprise interrupting the current supply to one or more windings. For example, shortly before the rotor reaches a target position, the current supply to one or more windings can be interrupted so that the rotation of the rotor slows down (e.g., due to friction and / or a load). Furthermore, adjusting the current supply can also comprise causing a specific current direction in at least one of the windings so that a force component counteracting the rotation is generated. Finally, adjusting the current supply can also comprise short-circuiting at least one of the windings so that a previously induced voltage leads to a current flow, which in turn causes a Lorentz force counteracting the rotation. The rotor can thus be brought to a stop with pinpoint accuracy. The adjustment can be carried out by a control unit with the features mentioned above.

[0048] Preferably, the step of adjusting the current supply can comprise short-circuiting at least one of the windings or adjusting the current in the windings. In this case, the short-circuiting can be performed in stepper motor mode for a motor with two windings. Adjusting the current in the windings can be performed in servo motor mode for a motor with at least three windings.

[0049] In stepper motor operation, the rotor can have at least two windings. The movement interval (i.e. the step size) of the motor can correspond to the angular distance between two windings. This means that the more windings are provided, the smaller the movement intervals of the rotor. In stepper motor operation, at least one winding can be energized, which ensures the rotation of the rotor. The unused winding can be short-circuited when the target position is reached, so that the rotor comes to a stop. With multiple windings, the winding adjacent to the dead center can be energized to set the rotor in motion. Dead center is the point (angular position) or the position of the rotor at which energizing a winding located there only leads to a radial force component and therefore makes no contribution to rotation of the rotor.The non-energized winding, which is short-circuited to stop the rotor, can be located at any position (i.e., except at dead center). However, since dead center can be the target position of the energized winding, this requirement is automatically met.

[0050] In servo motor operation, three windings are preferably provided on the rotor so that the rotor can rotate continuously in the desired direction depending on the current direction in the respective winding. For example, one winding may be at dead center and therefore not be used to accelerate the rotor. The windings can be energized until the target position is reached. During rotor movement, it may be necessary to reverse the polarity of the current supply so that the rotor continues to rotate in one direction. In contrast to stepper motor operation, none of the windings can be at dead center when the target position is reached in servo motor operation. This means that any target position can be reached through continuous movement. Optionally, braking can also be carried out in servo motor operation using the unused winding in order to brake the rotor in the target position.

[0051] The motor is preferably used in an MRI system. Furthermore, the motor can also be used in production, where it can be used for calibrating Hall sensors and testing so-called "motion control components" (enablers) of magnetic resonance imaging systems.

[0052] Furthermore, the same design forms and advantages apply to the method as to the device and vice versa.

[0053] Further features and advantages are apparent from the following description and the attached figures.

[0054] In the attached figures: Fig. 1 is a perspective view of the engine according to an embodiment of the present invention, Fig. 2 is a perspective schematic view of an engine according to an embodiment of the present invention, Fig. 3 is a schematic sectional view of the rotor of a motor according to an embodiment of the present invention, Fig. 4 schematic sectional views of the rotor from Fig. 3 in an angular position, Fig. 5 schematic sectional views of the rotor from Fig. 3 in another angular position Fig. 6 schematic sectional views of the rotor from Fig. 3 in another angular position Fig. 7 schematic sectional views of the rotor from Fig. 3 in another angular position Fig. 8 is a schematic sectional view of a rotor of a motor according to an embodiment of the present invention, Fig. 9 is a schematic diagram of an engine with further components according to an embodiment of the present invention, and Fig. 10 is a flowchart of a method according to an embodiment of the present invention.

[0055] Fig. 1 shows a perspective view of a motor according to an embodiment of the present invention. The motor 1 has a rotor 2 that is rotatable about a rotational axis 5. The rotor 2 has two windings 10, 11 that run essentially along the rotational axis 5 on the rotor 2. Furthermore, the motor 1 has four rotary transformers 3, two of which are each assigned to one of the windings 10, 11. This means that one rotary transformer 3 represents the positive pole of the first winding 10 and the other rotary transformer 3 represents the negative pole of the first winding 10. Likewise, one of the rotary transformers 3 represents the positive pole of the second winding 11 and another rotary transformer 3 represents the negative pole of the second winding 11. Thus, two rotary transformers 3 are each connected in series with one of the windings 10, 11. The rotary transformers 3 can be supplied with current by a control unit 15.The rotary transformers 3 then transmit the current to the windings 10, 11 associated with the rotary transformers 3. Thus, the control unit 15 is configured to control the current supply to at least one of the windings 10, 11. Furthermore, the control unit 15 can adjust the current direction to the windings 10, 11 individually for each winding.

[0056] The motor further comprises an angle encoder 4 which is designed to determine the angular position of the rotor 2. The angular position is the position of the rotor expressed as an angle around its axis of rotation 5. The angular position can be a relative value or an absolute value. The angle encoder 4 is arranged on the same axial side of the motor 1 as the rotary transformer 3 with respect to the rotor 2. This allows for optimized cable routing, as a common cable can be routed to the angle encoder 4 and to the rotary transformer 3. In an alternative embodiment (not shown), the angle encoder is arranged on the other axial side of the motor 1 with respect to the rotor 2 as the rotary transformer 3. The angle encoder 4 is connected wirelessly to the control unit 15.

[0057] Furthermore, the rotor 2 has holding devices 7, which are evenly distributed around the circumference of the rotor 2 and are designed to hold the windings 10, 11 in a specific position. The holding devices 7 are designed such that they can reliably hold the windings 10, 11 in place even at higher speeds and / or high accelerations. For this purpose, the holding devices 7 have undercuts in which the windings 10, 11 run. Furthermore, the motor 1 has a housing (not shown in the figure) in which the components of the motor 1 are accommodated. Furthermore, the connections of the rotary transmission 3 and the angle encoder are all arranged on the same side of the motor 1. This ensures optimized cable routing and the motor 1 can be designed compactly.

[0058] In the present embodiment, the rotor 2 is a cylindrical body with a shell surface and two cover surfaces. The rotor 2 is rotationally symmetrical with respect to the rotational axis 5. In an alternative embodiment not shown, the rotor 2 is a polyhedral in cross-section with twice as many surfaces as windings.

[0059] The windings 10, 11 are insulated wires that are formed into windings 10, 11 by repeatedly wrapping them around the rotor 2 along the rotational axis 5 of the rotor 2. The wire of each winding 10, 11 is connected at one end to a rotary transformer 3 and at the other end to another rotary transformer 3. The wires cross at the cover side of the rotor. The rotary transformers can also be designed as sliding contacts.

[0060] Fig. Figure 2 is a schematic perspective view of the engine 1, with individual components omitted to simplify the description. Fig. 2, the external magnetic field 6 is shown. In the present embodiment, the external magnetic field 6 is a magnetic field of a magnetic resonance imaging scanner (the B0 field) and is constant with essentially parallel field lines (see arrows in the figure). The external magnetic field can have a strength of 0.5 to 11 Tesla. Furthermore, Fig. 2 the rotation axis 5 is shown in dot-dash. In Fig. 2, the rotor 2 is aligned so that the first winding 10 is at a dead center. A winding is at a dead center when energizing the winding would only cause a force component acting radially to the rotor 2 and thus cannot set the rotor 2 in motion. In other words, in the Fig. In the example shown in Figure 2, energizing the first winding 10 does not cause any movement of the rotor.

[0061] Will be in Fig. 2, on the other hand, the second winding 11 is energized, i.e. connected to a power source such that current flows through the winding 11, a tangential force component acts on the rotor 2 due to the Lorentz force. This tangential force component causes the rotor 2 to rotate clockwise in the present embodiment. On the other hand, if the first winding 10 is energized as in Fig. 2, a radial force component acts on the first winding 10 and thus on the rotor 2, which does not contribute to a rotation of the rotor 2. In other words, the rotor 2 can only be set in rotation if the generated Lorentz force has a tangential force component with respect to the rotor 2.

[0062] The direction of rotation of rotor 2 can be changed by changing the current direction. A control unit 15 is provided and configured to adjust the direction of the current flowing through windings 10, 11. Thus, rotor 2 can be rotated in the desired direction.

[0063] In Fig. 3 is a section through the rotor from Fig. 2 is shown schematically. In contrast to the position of the rotor 2 in Fig. 2, the Fig. 3 is rotated 90° counterclockwise. In this case, the first winding 10 is at dead center, so that the first winding 10 only generates a radial force component when energized and thus cannot cause the rotor 2 to rotate. If, however, the second winding 11 is energized (i.e., connected to a power source), the rotor 2 rotates. If the second winding 11 then reaches a vertical position (in Fig. 3 the up / down direction, i.e. the dead center), the current supply to the second winding 11 does not contribute any force component that could cause the rotor 2 to rotate. The rotor 2 would oscillate back and forth slightly and then come to a stop with the second winding 11 vertically aligned. This oscillating movement is undesirable when the motor 1 is operated as a stepper motor. Therefore, the control unit 15 controls the current supply to the windings 10, 11 such that, in particular, the back and forth oscillation of the rotor 2 is avoided. In this case, the control unit 15 short-circuits the winding not used for the movement (i.e., energized). Thus, the predetermined angular position is reached reliably and precisely (i.e., without the oscillating movement described above). Details of this will be described with reference to Fig. 4 described.

[0064] In Fig. 4 to Fig. 7 are four schematic sectional views of the rotor 2 from Fig. 3 shown in different operating positions. In Fig. 4, none of the windings 10, 11 is connected to the power source and the rotor 2 is stationary. The external magnetic field 6 acts on the Fig. 4 to Fig. 7 shown sectional views always from left to right (for clarity only in Fig. 4 shown by arrows).

[0065] In Fig. 5, the second winding 11 is energized (ie connected to a power source). The right side of the winding 11 in the figure has a positive polarization, and the left side of the winding 11 in the figure has a negative polarization. Accordingly, when a current flows through the second winding 11, the Lorentz force acts tangentially on the rotor 2 as indicated by the arrow in Fig. 5. Consequently, the rotor 2 begins to rotate counterclockwise.

[0066] In Fig. 6, the rotor 2 moves counterclockwise, thus also displacing the windings 10, 11. As the rotor 2 moves along the circular path of the winding 11, the tangential force component contributing to the rotation of the rotor 2 decreases until it becomes zero when the second winding 11 has reached the vertical alignment (i.e., dead center). The control unit 15 has obtained or preset a position of the rotor 2 at which the second winding 11 is vertically aligned as a predetermined angular position. In the present embodiment, this corresponds to one step in stepper motor operation.

[0067] The control unit 15 permanently records the current angular positions of the rotor 2 via the angle sensor 4.

[0068] In the Fig. 7, the rotor 2 has reached the desired and predetermined target position. In order to stop the rotor 2 in this predetermined target position (i.e., to prevent it from oscillating back and forth), the control unit 15 short-circuits the first winding 10 by closing a switch, so that the voltage induced by the movement of the winding 10 through the magnetic field 6 causes a current to flow, which in turn causes a tangential Lorentz force opposite to the direction of rotation of the rotor 2. This causes the rotor 2 to brake suddenly, since the acting Lorentz force (i.e., the braking force) of the first winding 10 depends on the speed at which the first winding 10 was moved through the magnetic field 6. When the rotor 2 comes to a standstill, there is no longer any voltage gradient in the second winding 10, as a result of which no more current flows through the second winding 10 and thus no Lorentz force acts. Consequently, the rotor comes to a stop precisely in the target position.In some embodiments, the second winding 11 continues to be energized even if it is already in the target position (in the case of . Fig. 7 in the vertical orientation). This allows a holding torque of the motor 1 to be generated and the rotor 2 to be held in the target position. In the illustrated embodiment, the pitch is 90°. In other embodiments not shown, the rotor 2 has ten windings and thus has a pitch of 18°.

[0069] The control unit 15 controls the current supply so that a winding located adjacent to or near the dead center moves to the dead center due to the movement. If a rotation of the rotor 2 is to be performed that is greater than the step size of a step, several steps are executed consecutively.

[0070] In Fig. Figure 8 shows another embodiment of the present invention. Fig. Figure 8 is a schematic sectional view through a rotor 2 of a motor 1 according to a further embodiment of the present invention. The present embodiment has a third winding 12 on the rotor 2. The windings 10, 11, 12 are arranged evenly around the circumference of the rotor 2. In other words, the windings 10, 11, 12 are spaced 60° apart from one another. In the present embodiment, at least two of the three windings 10, 11, 12 are energized (i.e., connected to the power source), so that a tangential force component of the Lorentz force of one of the energized windings 10, 11, 12 can always be generated, allowing the rotor to move smoothly into a predetermined angular position.

[0071] In the present embodiment, the motor 1 can be operated in servomotor mode. The motor 1 is controlled such that it assumes any predetermined angular position. The rotor 2 does not move stepwise, but continuously. For this purpose, an additional winding 12 is provided in the present embodiment, so that two windings can always apply a tangential force component to the rotor 2, and one winding can be used, for example, to brake the rotor 2. Thus, the motor 1 of the present embodiment can be operated in servomotor mode for precise positioning tasks.

[0072] The control unit 15 can therefore control the motor 1 of the present embodiment using vector control depending on the position obtained from the angle sensor 4 so that the predetermined angular position of the rotor 2 is reached. In the present embodiment, an unused winding 10, 11, 12 can also be used to stop the rotor 2 in a desired position. A corresponding brake functions as in the previously described embodiment.

[0073] In another embodiment (not shown), multiple windings (preferably 8-10 windings) are provided. This allows for the creation of a sufficiently powerful motor 1. This allows the highest standards of accuracy and strength of the motor 1 to be met for operation as a servomotor for use in an external magnetic field. Thus, the motor 1 of the present embodiment has a torque of at least 2.5 Nm.

[0074] Fig. Figure 9 is a schematic diagram illustrating the motor 1 with additional components. As already mentioned above, the control unit 15 can control the power supply from a power source 16 to the windings 10, 11. The power source 16 can be a power storage device such as a battery or accumulator, or the power grid. Furthermore, the power is supplied to the windings 10, 11 via rotary transformers 3, as described above. The control unit 15 can receive signals from the angle encoder 4. For this purpose, the angle encoder 4 can be connected to the control unit 15 with a cable or wirelessly. The angle encoder 4 outputs a voltage corresponding to the position of the rotor. The control unit 15 can assign an angular position to the voltage. The control unit can output the angular position of the rotor 2 or use it for further processes. An interface 17 is provided, which is either an interface to a user or to another system (computer system).Via interface 17, the control unit obtains the commands for operating motor 1 and can output information about the operating states of motor 1. In particular, the control unit receives the target position of rotor 2 via interface 17.

[0075] In stepper motor operation, the control unit 15 assigns a specific number of steps to be executed (depending on the step size) and a direction of rotation of the rotor 2 to the target position. Based on this, the control unit controls the current supply to the windings 10, 11. The angle encoder 4 is used to precisely execute the individual steps (in particular, the braking intervention described above).

[0076] In servomotor operation, control unit 15 assigns a specific target voltage to the received target position and compares it with the current angular position (i.e., with the voltage output by the angle encoder). This can be done using a comparator. Control unit 15 then controls rotor 2 until both voltages essentially match, i.e., until rotor 2 has reached the target position.

[0077] Fig.10 is a schematic flow diagram of a method according to the invention for operating a DC motor 1 in an external magnetic field 6. A motor 1 according to one of the above embodiments can be used in the method. The method comprises a step S1 of placing the motor in an external magnetic field. The motor 1 is positioned in the external magnetic field 6 such that the rotational axis 5 is aligned transversely to the field lines 6 of the magnetic field. In one embodiment of the present invention, the motor 1 is provided in a couch of an MR system and aligned there such that, when the couch is in an operating state in or on the MR system, the motor is held such that the rotational axis 5 of the motor 1 extends transversely to the field lines 6 of the magnetic field.

[0078] Furthermore, the method comprises energizing S1 at least one of the windings 10, 11, 12 so that the rotor 2 rotates about the axis of rotation 5. As already described in the above embodiments, by targeted energization (i.e., connecting at least one of the windings 10, 11, 12) to a current source, a tangential force component is generated that can cause the rotor 2 to rotate about the axis of rotation 5. Simultaneously or subsequently, in a determining step S3, the current angular position of the rotor 2 is determined using the angle sensor 4. By knowing the angular position of the rotor 2 and thus also the positions of the windings 10, 11, 12, the corresponding winding 10, 11, 12 can be energized, which can exert a desired (for example, the greatest tangential force) on the rotor 2.Furthermore, by knowing the exact angular position of the rotor 2, the winding 10, 11, 12 used to brake the rotor 2 can be determined even during a braking process or immediately before reaching a predetermined target position.

[0079] Furthermore, the method according to the invention comprises a step S4 of adjusting the current supply to the windings 10, 11, 12 so that the rotor 2 assumes a predetermined angular position. For this purpose, the step S4 of adjusting the current supply can be a short-circuiting of at least one of the windings 10, 11, 12 or an adjustment of the current intensity in the windings 10, 11, 12. In other words, for example, in a motor 1 in which two windings 10, 11 are provided, a winding that is not currently being used to drive the rotor 2 can be short-circuited in order to generate a Lorentz force acting counter to the direction of rotation, so that the rotor is abruptly braked.

[0080] In an embodiment in which the rotor 3 has a plurality of windings (for example 3), the current intensity in the respective windings 10, 11, 12 can be controlled such that a resulting tangential force acting on the rotor 2 has a desired magnitude or direction. For this purpose, the windings 10, 11, 12 can also be switched quickly one after the other, for example to pass through a dead center at which one winding only exerts a radial force on the rotor 2. It should be noted that for a rotation after 180° the polarity must be changed so that the force generated in the windings (i.e. the generated Lorentz force) continues to point in the already prevailing direction of rotation. Such a change in polarity can be achieved, for example, using a bridge circuit. However, any other circuit that can alternate the positive and negative poles of a winding 10, 11, 12 is also suitable.In the present embodiments, the motor 1 is formed from materials such as ceramic, plastic and / or copper.

[0081] In one embodiment, the motor 1 according to the invention is used for calibrating Hall sensors. In particular, it can be used in this context for an "elliptical fitting calibration." The motor 1 can move a Hall sensor to be calibrated according to the specifications in an existing magnetic field, so that correction parameters can be easily created based on the known boundary conditions.

[0082] In a further embodiment, the motor 1 according to the invention is used to calibrate a motion correction of an MR system. A dummy is introduced into an MR system and moved by a motor 1 according to the invention. The movements can reflect the actual movements of a patient. Meanwhile, the image planes can be adjusted based on the known movements of the motor 1 so that the movements can be calculated in a subsequent imaging procedure.

[0083] In a further embodiment, the motor 1 according to the invention can be used for measuring a magnetic field, for example, in the bore of an MR system. The motor 1 can replace the otherwise manually performed movements or measurements at predetermined locations in the bore. This can provide automation and simplification of the tuning of an MR system.

Claims

[1] DC motor (1) for operation in an external magnetic field comprising: a rotor (2) which is rotatable about an axis of rotation (5) and has at least two windings (10, 11), at least four continuous rotary transformers (3), two rotary transformers (3) being assigned to each of the windings (10, 11) and being designed to supply the respectively assigned winding (10, 11) with direct current, an angle sensor (4) designed to determine the angular position of the rotor (2), and a control unit (15) designed to control the current supply to at least one of the windings (10, 11) as a function of the angular position of the rotor (2), wherein the motor (1) is aligned with an external magnetic field during operation such that the axis of rotation (5) extends transversely to the external magnetic field. [2] DC motor (1) according to claim 1, wherein each winding (10,11) lies at least approximately in a plane with the axis of rotation (5). [3] DC motor (1) according to one of the preceding claims, wherein a ratio between a diameter of the rotor (2) and an extension length of the windings (10, 11) along the rotational axis (5) of the rotor (2) is less than 1, preferably less than 0.

3. [4] DC motor (1) according to one of the preceding claims, wherein each rotary transformer (3) comprises an electrically conductive and continuously extending first and second element, wherein the first element and the second element are arranged to be rotatable relative to one another, wherein, during operation of the motor (1), the second element rotates together with the rotor (2), and wherein a gap is formed between the first element and the second element, in which gap an electrically conductive liquid metal alloy, in particular a eutectic alloy comprising gallium, indium and tin, is arranged. [5] DC motor (1) according to one of the preceding claims, wherein the control unit (15) is designed to short-circuit at least one of the windings (10, 11) based on the angular position of the rotor (2), in particular via a transistor cascade, so that a rotation of the rotor (2) can be braked. [6] DC motor (1) according to one of the preceding claims, wherein the rotor (2) comprises three windings (10,11,12) arranged on the rotor (2). [7] DC motor (1) according to claim 6, wherein the control unit (15) is designed to control the current supply to the windings (10, 11, 12) by means of a vector control. [8] DC motor (1) according to one of the preceding claims, wherein the angle sensor (4) is an optical system designed to obtain the angular position of the rotor (2) by scanning a pole wheel. [9] Couch of an MR system comprising a DC motor (1) according to one of the preceding claims, wherein the couch is adjustable and / or displaceable by the DC motor (1). [10] Method for operating a DC motor (1) in an external magnetic field, the motor (1) comprising: a rotor (2) which is rotatable about an axis of rotation (5) and has at least two windings (10, 11), at least four continuous rotary transformers (3), two rotary transformers (3) being assigned to each of the windings (10, 11) and being designed to supply the respectively assigned winding (10, 11) with direct current, an angle sensor (4) designed to determine the angular position of the rotor (2), the method comprising the following steps: Placing (S1) the motor (1) in an externally generated magnetic field, in particular in a magnetic field of a magnetic resonance imaging device, so that the axis of rotation (5) of the motor (1) extends transversely to field lines of the magnetic field, Energizing (S2) at least one of the windings (10, 11) so that the rotor (2) rotates about the axis of rotation (5), in particular due to a Lorentz force generated thereby, Determining (S3) the current angular position of the rotor (2) using the angle sensor (4), Adjusting (S4) the current supply to the windings (10,11,12) so that the rotor assumes a predetermined angular position. [11] Method according to claim 10, wherein adjusting the current supply (S4) comprises short-circuiting at least one of the windings (10, 11, 12) or adjusting the current intensity in the windings (10, 11, 12).

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