Scanning monochromator with direct drive grating
A novel scanning monochromator uses a PM stepper-motor to directly drive a diffraction grating. By employing interpolated encoder feedback in combination with the PM stepper-motor feedback, a resolution of over 250,000 pulsed steps is available for each revolution of the PM stepper-motor. This translates into more than 20,000 incremental angular-displacement steps over a usable 30° range of dispersion-element rotation. High field accuracy is achieved by a direct PM stepper-driven diffraction grating, and a unique calibration approach based on Wood's anomalies. A plurality of diffracted light beams emerge from the oscillating grating, and these are scanned past a detector for detection, whereby the relative rotation information of the grating can be detected with great accuracy. A number of tolerance-correcting measures are also included to yield an extremely accurate, self-lubricating scanning monochromator that can be economically produced.
1. An apparatus for high speed spectrescopic sample analysis, comprising:
a sample tray for holding a sample in position for spectroscopic scanning;
a light source for generating broadband light for use in illuminating said sample;
a collimating entrance optic assembly in advance of said light source for collimating light therefrom;
an entrance slit in advance of said entrance optic assembly;
a shutter mechanism for selectively shuttering said entrance slit;
an oscillating diffraction-grating mounted on a rotatable shaft for breaking said broadband light from the light source into spectrographic constituent wavelengths;
a programmable stepper motor having a shaft connected directly to said diffraction grating for direct drive thereof, and controlled incremental rotation of said diffraction-grating over an angular range in order to scan each of said constituent wavelengths;
an encoder mounted proximate said diffraction-grating for outputting pulses corresponding to an angular position of said diffraction-grating within said angular range;
a processor in communication with said programmable stepper motor and said encoder for incremental control of the rotation of said stepper motor and diffraction-grating within said angular range in accordance with said encoder output pulses;
an exit slit in an optical path of said diffraction grating;
an exit optic assembly for directing light from said exit slit toward said sample tray; and
an optical sensor for collecting said constituent wavelengths reflected by the sample in said sample tray.
2. The apparatus for high speed spectroscopic analysis according to claim 1 , wherein said processor time-stamps each outputted encoder pulse within said angular range and computes a rotational velocity of said diffraction grating for each outputted encoder pulse.
3. An apparatus for high speed spectroscopic sample analysis, comprising:
a sample tray for holding a sample in position for spectroscopic scanning;
a light source for generating broadband light for use in illuminating said sample;
a collimating entrance optic assembly in advance of said light source for collimating light therefrom;
an entrance slit in advance of said entrance optic assembly;
a shutter mechanism for selectively shuttering said entrance slit;
an oscillating diffraction-grating mounted on a rotatable shaft for breaking said broadband light from the light source into spectrographic constituent wavelengths;
a programmable motor having a shaft coaxially coupled to said diffraction grating via a flexible non-dampening coupling for direct drive thereof, and incremental rotation of said diffraction-grazing in order to scan each of said constituent wavelengths;
an exit slit in an optical path of said diffraction grating; an exit optic assembly for directing light from said exit slit toward said sample tray; and
an optical sensor for collecting said constituent wavelengths reflected by sample in said sample tray.
4. An apparatus for high speed spectroscopic sample analysis, comprising:
a sample tray for holding a sample in position for spectroscopic scanning;
a light source for generating broadband light for use in illuminating said sample;
a collimating entrance optic assembly in advance of said light source for collimating light therefrom;
an entrance slit in advance of said entrance optic assembly;
a shutter mechanism for selectively shuttering said entrance slit;
an oscillating diffraction-grating pivotally-mounted on a rotatable shaft for breaking said broadband light from the light source into spectrographic constituent wavelengths, said shaft being rotatably anchored between two opposing platforms in self-lubricating bushings mounted in the respective platforms;
a programmable motor having a shaft connected directly to said diffraction grating for direct drive thereof, and incremental rotation of said diffraction-grating in order to scan each of said constituent wavelengths;
an exit slit in an optical path of said diffraction grating;
an exit optic assembly for directing light from said exit slit toward said sample tray; and
an optical sensor for collecting said constituent wavelengths reflected by sample in said sample tray.
5. The apparatus for high speed spectroscopic analysis according to claim 4 , wherein periodic rotation of said diffraction-grating through at least one complete revolution to maintain a proper grease distribution within said self-lubricating bushings, thereby increasing the useful life of said bushings.
6. A method of calibrating a scanning monochromator comprising the steps of:
taking a background scan at periodic intervals;
establishing a position of at least one Wood's region in each background scan to provide a calibration point;
storing each established Wood's region calibration point;
comparing each successive Wood's region calibration point from each background scan to one of prior-stored Wood's region calibration points and baseline calibration points established at factory calibration; and
calculating calibration offset values for adjustment of a live scan.