NMR SPINNER SPEED CONTROL

Abstract: An NMR sample spinner comprises first rotational torque means to impart a first rotation to a sample rotor and a second rotational torque means for applying a rotational torque of selected magnitude opposite that applied by said first rotational torque means, whereby positive rotational rate control is more precisely and rapidly realized, sense of rotation is controlled, and undesired rotational torques may be compensated.

Description: The invention is in the field of instrumentation for analysis of samples by nuclear magnetic resonance and particularly relates to mechanisms for rotation of a sample in such instrumentation. The study of radio frequency magnetic resonance spectra of samples is facilitated by rotation of the sample about an axis which is usually disposed to coincide with the polarizing magnetic field of the instrument, or at a desired angle thereto. Spinning the sample has the effect of averaging over undesired non-axial magnetic field gradients. Control of the spin rate is an important function and automated spin rate control in a servo arrangement is not uncommon. Whether a manual or closed loop arrangement, the response time for spin rate control from a faster to a slower rotational rate, is often undesirably lengthy due to the very reduced frictional forces operative in the air bearing. Moreover, the very small variations in the structure of the rotor or stator of the spinner frequently give rise to small rotational torques affecting performance. In the prior art it was known to utilize friction braking, aerodynamic damping (with vanes attached to the rotor), and controlled eddy currents for the purpose of controllably reducing the spinning sets of an NMR sample spinner. These are discussed for example in U.S. 3,911,355. EP-A-020085 discloses spinner apparatus for rotating a sample under study in an NMR spectrometer, comprising a rotor for holding said sample, a stator surrounding said rotor, said stator forming a gas bearing in which said rotor is levitated by pressurized gas means incorporated in said stator, said stator comprising first torque means imparting rotation in a first sense to said rotor at a preselected area of the rotor. The present invention improves on this arrangement by providing second torque means selectively imparting rotational torque to said rotor in a sense opposite said first sense at the same preselected area of the rotor. Performance and response of the sample spinner is thereby improved. A positive braking action can be implemented providing a more precise control over the sample spinner, a capability to select the sense of rotation, and to modulate the sample rotation rate and to completely stop sample rotation. Moreover, undesired rotational torques arising from imperfection in practical fluid bearings may be partially or wholly compensated with the selectably controllable torque of either sense. Brief Description of the Drawings : FIG. 1 is a section through a sample spinner of the present invention. FIG. 2 is a transverse section through an embodiment of the present invention. Detailed Description of the Invention : Turning now to FIG. 1, there is shown a common arrangement illustrating the present invention. The sample spinner system is situated within the bore of a magnet, not shown. The sample spinner is surrounded by an RF probe coil 2 which provides RF excitation for resonating nuclei, and subsequently operates to pick up the signal emitted by de-exciting nuclei of the sample. The probe coil 2 is shared for these functions through multiplexer 5 operating to alternately connect probe coil 2 to transmitter 6 and receiver 7 for the separate purposes of excitation and signal acquisition. Transmitter 6 includes modulation apparatus control by processor 8, and receiver 7 includes a digitization means and phase resolving means. Processor 8 controls the multiplexer 5, transmitter 6, and receiver 7 and other apparatus not shown, such as magnetic field stabilization and homogenization equipment, for example. The spinner includes rotor 10, which in turn has keel portion 13 and rotor bearing surface 14. Stator 15 includes stator bearing surface 16. The sample is contained in a sample well 17 in the rotor 10. Stator incorporates jets 19 in the stator bearing surface 16 and radial stabilizing jets 21 directed toward keel portion 13 of rotor 10. FIG. 2 indicates one arrangement for the jets 19, where a first pair of jets 19a are disposed to direct a pressurized gas substantially tangentially directed onto bearing surface 14 of the rotor to bear against scalloped indentations 14a. Those jets 19a produce a torque tending to rotate rotor 10 in one sense (counterclockwise, as seen in FIG. 2). A second set of jets, 19b, cause an oppositely-directed torque. Separate pressurization circuits 22a and 22b for the respective jets 19a and 19b permit the rotation rate in sense to be controlled through relative pressures at the respective jets. A valve 25, and corresponding pressure source, is commonly provided to pneumatically expel the sample and rotor from the system. Although certain embodiments are described above in detail, it will be appreciated that other embodiments are contemplated and that modifications of the disclosed features are within the scope of the invention.

Claims: 1. Spinner apparatus for rotating a sample under study in an NMR spectrometer, comprising a rotor (10) for holding said sample, a stator (15) surrounding said rotor, said stator forming a gas bearing in which said rotor is levitated by pressurized gas means incorporated in said stator, said stator (15) comprising first torque means (19a, 22a, 14) imparting rotation in a first sense to said rotor at a preselected area (14) of the rotor (10), and characterized by second torque means (19a, 22b 14) selectively imparting rotational torque to said rotor in a sense opposite said first sense, at the same preselected area (14) of the rotor. 2. The spinner apparatus of claim 1, said first torque means (19a, 22b, 14) comprising at least one first gas jet (19a) disposed to direct a stream of pressurized gas substantially tangential to the periphery of said rotor (10) in a first direction whereby said rotor (10) is caused to rotate in a first sense. 3. The spinner apparatus of claim 2, said second torque means (19b, 22b, 14) comprising at least one second gas jet (19b) disposed to direct a stream of pressurized gas substantially tangential to the periphery of said rotor (10) in a direction substantially opposite said first direction, whereby said second torque means opposes rotation of said rotor induced by said first torque means. 4. The spinner apparatus of claim 3, said first and second jets (19a, 19b) each comprising respective pressure means (22a, 22b) for selecting the pressure magnitude applied to first and second jets whereby the rotational rate is precisely controlled. 5. The apparatus of claim 4, either said respective torque means are capable of reducing the rotational rate to zero whereby the sense of rotation is selectable. 6. The method of controlling the rotational rate of a rotor containing a sample for a spinner of an NMR spectrometer comprising directing a first rotational torque to said rotor (10) to cause rotation of said rotor at a rotational rate in excess of a desired rate, opposing said first rotational torque by applying a second rotational torque of selected magnitude to said rotor, said second rotational torque opposite in sense said first rotational torque, whereby said rotational rate is reduced.

Inventors: SCHULKE, GEORGE FRANK
Applicants: VARIAN ASSOCIATES, INC.
Application year: 1988
Main classification: G01R33
IPC codes: G01R33