DE202007014486U1 - Drive device for the fan of a device that can be introduced into an MR tomograph - Google Patents

Abstract

driving means for the Fan one in an MR tomograph insertable device, characterized in that it comprises a piezomotor, via a transmission gear the fan drives.

Description

The The invention relates to a drive device for the fan of a device that can be introduced into an MR tomograph Device.

Of the Room in which the investigations were made with the MR-Tomografen is to be, is naturally very cramped, because it is very expensive is to generate the required high magnetic field over larger volumes and to maintain. It is therefore often necessary ventilation. This is especially true for Incubators, with whom premature births for examinations be introduced into an MR tomograph. Here is a fan for Generation of the controlled climate required in the patient room required.

One previously very common concept for the generation of a defined airflow in the incubator is the drive of a radial or Tangentiallüfterrads means an electrodynamic engine. This engine works mostly at a constant speed, either by its own mode of action (e.g. Synchronous motor, stepper motor) or by measuring the speed the drive shaft as an actual value for a switched-on control circuit (regulated DC motor).

In In any case, the conversion of electrical into kinetic energy is based on temporal and / or spatial variable Magnetic fields. It's surprising not that this drive concept in the environment of strong external magnetic fields reaches its limits, in the the useful field of the motor is superimposed with the external interference field and the required continuous Rotational motion disturbed or prevented. That's exactly what happens when you bring in an incubator with a conventional fan drive into the homogeneous field of the MRI magnet of the case.

• 1. MRT Magnetfeldstärken ab 3 Tesla
• 2. Untersuchung z.B. des Spinal-/Abdominalbereichs anstelle des Kopfbereichs des Neonaten und dem damit notwendigen weiteren Hineinfahrens des Inkubators in den Magneten.
• 3. MRT-Modellen mit „langer" Magnetanordnung und einem damit räumlich aufgeweiteten Streufeld.

Known technically possible solutions are limited to, for example, magnetic shielding of the motor as well as spatial displacement of the motor in a position of lower interference field strength, so far as possible away from the homogeneous field and stray field of the magnet in order to avoid these problems. However, none of the measures provides satisfactory results when using an incubator in terms of:

• 1. MRI magnetic field strengths from 3 Tesla
• 2. Investigation of, for example, the spinal / abdominal area instead of the head area of the neonate and the thus necessary further driving in of the incubator in the magnet.
• 3. MRI models with "long" magnet arrangement and a spatially expanded stray field.

The The object of the invention is to provide a drive for a Fan, in which the problems of the magnetic components are avoided.

The inventive solution draws characterized by the fact that they completely dispensed with magnetic components and for the drive of the fan a piezomotor used over a transmission gear the fan drives.

Of the known piezoelectric effect is due to displacement of the spatial Crystal structure of certain crystals and ceramics when applied an electrical voltage. He becomes industrial and commercial used, especially for quick and accurate positioning, or as a linear drive for relatively slow processes.

The Use as a universal, rotary drive was not long feasible due to limited lifetime, poor efficiency, too low speed and a relatively small available torque on the drive shaft. In most cases, the linear motion of the piezo motor was with mechanical elements transformed into a circular motion.

Recent developments in materials and technologies have mitigated all the above limitations. An industrially available piezomotor according to the so-called. Ultrasonic traveling wave principle is provided by the company Shinsei / Japan. A specially adapted to the engine control electronics and an optical encoder as an encoder provide for a speed stabilization regardless of operating point and load conditions. Via an external control voltage U _S , the speed of the motor shaft can be set in a range of 15 to 150 l / min. When load limits are exceeded, the controller shuts off the motor to prevent it from being damaged by overheating.

According to the invention was now found out that such a piezomotor in spite of its above disadvantages in an advantageous way for the drive of a fan in devices can be used in the magnetic field of an MR tomograph introduced should be. He drives the fan via a gearbox.

One An essential feature of this invention is the integration of the piezo motor in an MR-compatible incubator as a replacement for the previously used DC motor while maintaining both the MR compatibility as well as the constancy of the climate with regard to temperature distribution and airflow in the patient room.

The latter means that the previous, proven flow profile and the geometry of the Lüf request that the fan still with a rated speed of N _L = 1280 (± 100) l / min must be driven.

constructive let yourself accomplish this by the relatively small speed at the drive shaft of the Piezo motor translated by means of a gearbox to the rated speed required above.

in the Following are the functional structure as well as demarcations and Differences to usual Standard transmissions based on advantageous embodiments with reference on the attached Illustrated drawings, for example. Show it:

1 the basic structure of an advantageous embodiment;

2 an electrical circuit with source and sink;

3 the circuit of 2 with a blocking filter; and

4 the frequency response of the blocking filter the 3 ,

The MR compatibility From the viewpoint of electrical properties is generally achieved through spatial Separation of the magnetically sensitive control unit of the non-magnetic piezo motor. The required connection cable is shielded and earthed, and the signals carried therein are filtered. These measures are described in detail below.

Achieving mechanical adaptation

• i. Die Mechanik der Getriebeeinheit wird ohne ferromagnetische oder elektrisch leitende Wellen, Räder oder sonstige schnell rotierende Komponenten mit diesen Eigenschaften ausgelegt, um somit MR-Bildstörungen durch auftretende Wirbelströme zu vermeiden.
• ii. Die Übersetzung wird so ausgelegt, dass sich bei einer Drehzahl an der Antriebswelle des Piezomotors von N = 57 l/min die gewünschte Nenndrehzahl von N_L = 1280 l/min an der Abtriebswelle des Lüfterrades ergibt. Das Verhältnis wird daher mit n = 1:22,5 festgelegt. Dies hat folgenden Grund. Der Arbeitspunkt des Piezomotors ist in dem Bereich um ca. 60 l/min optimal, d.h. die Stromaufnahme ist bei vergleichbarer Last minimal. Außerdem verlängert sich durch diesen niedertourigen Betrieb die Lebenszeit des Motors erheblich (D > 2000 h, der Hersteller gibt bei einer Maximaldrehzahl von 150 l/min eine Lebensdauer von 1000 h an). Bei der Umsetzung ist entscheidend, dass die gesamte Übersetzung n in drei Teilstufen aufgeteilt wird: n = n1 × n2 × n3 = 1:3 × 1:5 × 1:1,5 = 1:22,5 = NP:NL Die einzelnen Stufen bilden sich nach dem Prinzip „Zahnrad-Riemen-Zahnrad" und die Kopplung dazwischen nach dem Prinzip „Zahnrad-Welle-Zahnrad" (starre Verbindung). Begründung: Die Stufenanordnung wirkt sich verringernd auf Verschleiß und verbessernd auf Wirkungsgrad aus. Die Riemenkopplung verringert die Wälzgeräusche im Vergleich zur direkten Kopplung „Zahnrad-Zahnrad".
• iii. Eine Umdrehung der Piezomotorwelle erzeugt n Umdrehungen an der Lüfterradwelle. Um Reibung und Trägheitsmomente beim Einschalten des Motors schonend zu überwinden, wird die Drehzahl nicht sprungartig auf Nenndrehzahl gestellt, sondern per linearer Rampenfunktion über eine Zeitdauer von 60 Sekunden hochgefahren.
• iv. Der Piezomotor bietet beim Ausschalten (der Versorgungsspannung) sofort das volle Haltemoment bei N_P = 0. Um die in dem Lüfterrad und Getriebe gespeicherte Rotationsenergie zerstörungsfrei aufzubrauchen, wird zwischen Antriebswelle und erster Getriebestufe ein Freilauf eingebaut. Auch diese Komponente wird ohne rotierende magnetische bzw. metallische Komponenten aufgebaut.

The transmission has the following characteristics to meet the requirements:

• i. The mechanism of the gear unit is designed without ferromagnetic or electrically conductive shafts, wheels or other fast rotating components with these properties, so as to avoid MR image interference caused by eddy currents.
• ii. The ratio is designed so that at a speed of the drive shaft of the piezo motor of N = 57 l / min, the desired nominal speed of N _L = 1280 l / min results on the output shaft of the fan wheel. The ratio is therefore set with n = 1: 22.5. This has the following reason. The operating point of the piezo motor is optimal in the range of about 60 l / min, ie the power consumption is minimal at comparable load. In addition, the life of the engine is considerably extended by this low-speed operation (D> 2000 h, the manufacturer indicates a service life of 1000 h at a maximum speed of 150 l / min). In terms of implementation, it is crucial that the entire translation n is divided into three sub-levels: n = n 1 × n 2 × n 3 = 1: 3 × 1: 5 × 1: 1.5 = 1: 22.5 = N P : N L The individual stages are formed according to the principle "gear belt gear" and the coupling between them according to the principle "gear shaft gear" (rigid connection). Reason: The step arrangement reduces wear and improves efficiency. The belt coupling reduces the rolling noise compared to the direct coupling "gear wheel".
• iii. One revolution of the piezo motor shaft generates n revolutions on the fan wheel shaft. In order to gently overcome friction and moments of inertia when the motor is switched on, the speed is not suddenly set to rated speed, but ramped up by a linear ramp function over a period of 60 seconds.
• iv. When switched off (the supply voltage), the piezomotor immediately provides the full holding torque at N _P = 0. In order to use up the rotational energy stored in the fan wheel and gearbox nondestructively, a freewheel is installed between the drive shaft and the first gear stage. This component is also constructed without rotating magnetic or metallic components.

The calculation of the total efficiency η relates the mechanical power Pme delivered to the fan wheel to the consumed electric power Pel. In the exemplary operating point, the drive shaft is loaded with a torque of M = 0.04 Nm at the speed N _L ; The motor consumes a current of I = 1.5 A at a supply voltage of U = 12 V. This results in: Pme = M × ω ≈ 0.04 Nm × 2π × 21.33 l / s ≈ 5.36 W Pel = U × I = 12V × 1.5A = 18W η = Pme / Pel ≈ 0.298 → 29.8%

The system control SYSTEM of the incubator monitors the measured shaft speed N _P and ensures that the above efficiency is largely maintained during normal operation. In the event of a deviation or error, SYSTEM will alarm; in the extreme case of a permanent overload or a forced blockade of piezo motor, impeller or gearbox, the control electronics PSTEUER of the piezo motor switches this off; the monitoring SYSTEM detects N _P = 0 and alerts.

Reaching the electrical adaptation

• i. Shielding: As a basic measure will the shielding of all involved components and their connections performed.
• ii. Grounding: Each enveloping Shield ends at a grounding point; the presence of ground loops impaired the imaging and is therefore absolutely to be avoided.
• iii. Filtering: The third and most important measure is to filter the signals between PSU, SYSTEM and PMOTOR. General: Each signal is routed between source (Q) and drain (S) along a path (usually an electric cable), s. 2 , The source is on the left, the sink on the right. The frequency spectrums applied by the MRT are narrow-band in the respective device class, so that by looping in a selective barrier filter of higher order along each signal path between PSU, SYSTEM and PMOTOR the interferences mentioned above are minimized. Such a notch filter can be easily used as an LC element (2nd order passive filter) as in 3 shown, realize. In the case of piezo technology, it is advantageous that in this application the useful frequency range (approximately 40 kHz) is far enough away from that of the MRT (42 ... 300 MHz) that the filtering does not cause any side effects. 4 shows the frequency response when using the notch filter 3 , The resonance frequency was tuned for a 1.5T magnetic field strength MRI system, which corresponds to a Lamor frequency of 63.9 MHz. In this range, the insertion loss is better than 40 dB. This filtering is now to be applied to each of the above signal paths on both ends of the VKABEL and on the SYSTEM side end of AKABEL.

1 shows the arrangement of the above-mentioned components and terms.

NKABEL: Shielded power cable (3-core: L, N, PE) Length approx. 30 cm. Already present, as necessary as AC supply cable for the incubator.

POWER ADAPTER: Shielded box containing the switching power supply. Is spatially separated from the rest of the device and is operated in an area where the residual stray field of the MRI is very weak (flux density in the air B <20 Gauss). Thus no impairment the function of the switching power supply. Is already available, there as DC voltage generation (12 V) for the incubator necessary.

PSTEUER: control electronics for the piezomotor. This contains - like the switching power supply - ferritic components, which may only be exposed to a weak field. This electronics is supplied directly from the power supply with 12V. The output via VKABEL are the power signals for the motor (3-wire: USIN, UCOS, GND), the input is the two square signals from the optical encoder (2-core: APHASE, BPHASE) and the control voltage for the motor speed (UMOT). All lines of input and output are replaced by a blocking filter 3 Mistake.

VKABEL: Shielded connection cable, which is described under NETWORK PART Lines included. This cable has a sufficient length (L = 4.5 m) to the spatial Disconnecting the PSU from the device to reach.

PMOTOR: The actual piezomotor. This takes the power signals (USIN, UCOS, GND); the angle encoder placed on the drive axle returns the RPM signals (APHASE, BPHASE) for PSTEUER and SYSTEM.

AKABEL: Shielded connection cable from the piezomotor. Performs the signals described under PMOTOR to the SYSTEM.

SYSTEM: The system electronics of the incubator. Those arriving from PMOTOR Speed signals (APHASE, BPHASE) are evaluated, and in case of deviations the user is alerted by the device.

In addition, will over here an existing digital / analog converter generates the control voltage UMOT and to PSTEUER to realize the linear voltage ramp for the start phase.

Incidentally, the signals are looped through between PMOTOR and PSTEUER and each with blocking filters after 3 Mistake.

Claims (12)

Drive device for the fan of an insertable into an MR tomograph device, characterized in that it comprises a piezomotor, which drives the fan via a transmission gear. Drive device according to claim 1, characterized in that that the gear ratio of the transmission gear between 1:10 and 1:30 Drive device according to claim 1 or 2, characterized characterized in that the gear ratio of the transmission gear 1: 22.5 is. Drive device according to Ansprü che 1 to 3, characterized in that the transmission ratio of the transmission gear is divided into three sub-stages. Drive device according to claim 4, characterized in that the transmission ratio of the transmission gear n = n 1 × n 2 × n 3 = 1: 3 × 1: 5 × 1: 1.5 = 1: 22.5 = N P : N L is. Drive device according to one of claims 1 to 5, characterized in that the rotational speed on the piezo motor shaft is detected optically / electronically and this value is both monitored as well as being regulated to a constant speed at the fan wheel shaft to ensure. Drive device according to one of claims 1 to 6, characterized in that the transmission gear non-magnetic is and exclusively contains electrically non-conductive moving parts. Drive device according to one of claims 1 to 7, characterized in that it comprises a freewheel. Drive device according to one of claims 1 to 8, characterized in that it comprises a controller, with the speed after switching electronically with a linear Ramp function is increased to the final speed. Drive device according to one of claims 1 to 9, characterized in that they are selective electrical filters along the supply and signal lines from / to control electronics and motor, especially in the vicinity of connectors. Drive device according to one of claims 1 to 10, characterized in that it comprises shields. Drive device according to one of claims 1 to 11, characterized in that it has an alarm device for reporting a malfunction of the fan or fan drive having.