Dynamic measurement of density using terahertz radiation with real-time thickness measurement for process control

1. A method of determining a density of a roller compacted ribbon, said method comprising:

compacting dry pharmaceutical powder between press rollers of a roller compactor to produce a compact ribbon;

determining a thickness at a point on said compact ribbon in a non-invasive manner after it has rolled out from in between said press rollers;

passing said compact ribbon through a gap in between a terahertz emitter and a terahertz detector;

determining a refractive index at said point on said compact ribbon using a measurement value from said terahertz emitter and said terahertz detector and a measured value for said thickness at said point; and

computing a density of said compact ribbon at said point using a value of said refractive index and a calibration equation.

2. The method of claim 1 , wherein said determining a thickness comprises measuring said thickness using a laser micrometer.

3. The method of claim 2 , wherein said determining a thickness further comprises:

passing said compact ribbon through a gap between two laser micrometers; and

obtaining simultaneous readings from said two laser micrometers to determine said thickness.

4. The method of claim 1 , wherein said determining a refractive index comprises:

determining a time-of-flight duration of a radiation pulse emitted by said terahertz emitter and detected by said terahertz detector for said point on said compact ribbon; and

computing said refractive index at said point using said time-of-flight duration and said thickness.

5. The method of claim 1 , wherein said determining a refractive index further comprises:

translating said terahertz emitter and said terahertz detector in an x-direction to obtain refractive index values for a plurality of points on said compact ribbon, wherein the x-direction is a direction perpendicular to a direction said compact ribbon is moving.

6. The method of claim 5 , wherein said determining a thickness further comprises:

translating a pair of laser micrometers in the x-direction across said compact ribbon to obtain thickness values corresponding to said plurality of points.

7. The method of claim 6 , further comprising:

computing density values for said plurality of points using respective thickness and respective refractive index values corresponding to said plurality of points.

8. An apparatus for determining a density of a roller compacted ribbon, said apparatus comprising:

a roller compactor operable to compact dry pharmaceutical powder between press rollers of said roller compactor to produce a compact ribbon;

at least one laser micrometer operable to determine a thickness at a point on said compact ribbon in a non-invasive manner after it has rolled out from in between said press rollers;

a terahertz emitter operable to emit a terahertz radiation pulse through said point on said compact ribbon;

a terahertz detector operable to detect said terahertz radiation pulse;

a memory; and

a processor configured to:

determine a refractive index at said point on said compact ribbon using measured values from said terahertz emitter and said terahertz detector and a measured value for said thickness; and

compute a density of said compact ribbon at said point using a value of said refractive index and a calibration equation.

9. The apparatus of claim 8 , wherein said at least one laser micrometer comprises two laser micrometers, wherein each laser micrometer is disposed on either side of said compact ribbon, and wherein thickness measurements are obtained from said two laser micrometers simultaneously.

10. The apparatus of claim 9 , wherein a first of said two laser micrometers is disposed adjacent to said terahertz emitter and a second of said two laser micrometers is disposed adjacent to said terahertz detector.

11. The apparatus of claim 8 , wherein said measured values from said terahertz emitter and said terahertz detector comprises a time-of-flight duration of a radiation pulse emitted by said terahertz emitter and detected by said terahertz detector for said point on said compact ribbon.

12. The apparatus of claim 8 , wherein said terahertz emitter and said terahertz detector are operable to be translated in an x-direction to obtain refractive index values for a plurality of points on said compact ribbon, wherein the x-direction is a direction perpendicular to a direction said compact ribbon is moving.

13. The apparatus of claim 12 , wherein said at least one laser micrometer is operable to be translated in the x-direction to obtain thickness values corresponding to said plurality of points on said compact ribbon.

14. The apparatus of claim 13 , wherein said processor is further configured to compute density values for said plurality of points using respective thickness and respective refractive index values corresponding to said plurality of points.

15. A tester system comprising:

a roller compactor operable to compact dry pharmaceutical powder between press rollers of said roller compactor to produce a compact ribbon;

a plurality of laser micrometer pairs operable to determine a thickness at a plurality of points on said compact ribbon in a non-invasive manner after said compact ribbon has rolled out from in between said press rollers, wherein said plurality of laser micrometer pairs are disposed adjacent to each other in a first axis direction in order to scan dedicated tracks along a ribbon flow in a second axis direction, and wherein each pair of laser micrometers is operable to determine a thickness at a single point from said plurality of points on said compact ribbon;

a plurality of terahertz emitters, wherein each terahertz emitter is operable to emit a terahertz radiation pulse through a respective point from said plurality of points on said compact ribbon;

a plurality of terahertz detectors, wherein each terahertz detector is operable to detect said terahertz radiation pulse passing through a respective point from said plurality of points on said compact ribbon;

a memory; and

a processor configured to:

determine a refractive index at each of said plurality of points on said compact ribbon using respective measured values from said terahertz emitter and said terahertz detector and a respective measured value for said thickness; and

compute a density of said compact ribbon at each of said plurality of points using a respective value of said refractive index and a calibration equation.

16. The tester system of claim 15 , wherein each pair of laser micrometers comprises two laser micrometers, wherein each laser micrometer is disposed on either side of said compact ribbon, and wherein thickness measurements are obtained from said two laser micrometers simultaneously.

17. The apparatus of claim 15 , wherein measured values from each of said plurality of terahertz emitters and each of said plurality of terahertz detectors comprises a time-of-flight duration of a radiation pulse emitted by a terahertz emitter and detected by a respective terahertz detector for a respective point on said compact ribbon.

18. The apparatus of claim 15 , wherein said plurality of laser micrometer pairs are disposed adjacent to said plurality of terahertz emitters and said plurality of terahertz detector pairs.

19. The apparatus of claim 15 , wherein said terahertz radiation pulse is transmitted using a terahertz beam substantially in the range between 1 to 6 mm in diameter.

20. The apparatus of claim 15 , wherein said processor is further configured to record a plurality of density values corresponding to measured values at said plurality of points, and wherein said tester system further comprises a display to display said plurality of density values.