Displacement measurement sensor head and system having measurement sub-beams comprising zeroth order and first order diffraction components
View Patent ↗A sensor head for use with a measurement grating is described. The sensor head comprises: a splitter grating configured to split a light beam into first and second measurement beams; a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating. In one embodiment the second measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams comprises a zeroth order diffraction component and a first order diffraction component. In another embodiment, the first and second sub-beams each comprise a zeroth order diffraction component and a first order diffraction component.
1. A sensor head for use with a measurement grating, the sensor head comprising:
a splitter grating configured to split a light beam into first and second measurement beams;
a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and
a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating, wherein the second measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams comprises a zeroth order diffraction component and a first order diffraction component.
2. A sensor head as claimed in claim 1 , wherein the first measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams of the first measurement beam comprises only zeroth order diffraction components.
3. A sensor head as claimed in claim 1 , wherein the first measurement beam is diffracted by the measurement grating to form first and second sub-beams and one of the first and second sub-beams of the first measurement beam comprises only first order diffraction components.
4. A sensor head as claimed in claim 1 , wherein the other of the first and second sub-beams of the second measurement beam comprises a zeroth order diffraction component and a first order diffraction component.
5. A sensor head as claimed in claim 1 , wherein the other of the first and second sub-beams of the second measurement beam comprises only zeroth order diffraction components or first order diffraction components.
6. A sensor head as claimed in claim 1 , further comprising a first detector array and a second detector array, wherein the first and second detector arrays are adapted to receive the beams diffracted from the measurement grating.
7. A sensor head as claimed in claim 6 , wherein the detector arrays measure a phase difference for the first and second measurement beams represented as N A and N B and an x-displacement is calculated as:
x
=
p
4
(
N
B
-
N
A
)
;
and a z-displacement is calculated:
z
=
p
4
(
N
B
+
N
A
)
cot
1
2
(
β
-
α
)
,
wherein p is a pitch of the measurement grating, α is an angle of incidence and β is an angle of diffraction.
8. A measurement system for measuring a displacement comprising a sensor head as claimed in claim 1 .
9. A measurement system for measuring a displacement comprising a sensor head as claimed in claim 2 .
10. A measurement system for measuring a displacement comprising a sensor head as claimed in claim 3 .
11. A sensor head for use with a measurement grating, the sensor head comprising:
a splitter grating configured to split a light beam into first and second measurement beams;
a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and
a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating, wherein the first and second measurement beams are diffracted by the measurement grating to form respective first and second sub-beams of each of the first and second measurement beams and the first and second sub-beams each comprise a zeroth order diffraction component and a first order diffraction component.
12. A sensor head as claimed in claim 11 , further comprising a first detector array and a second detector array, wherein the first and second detector arrays are adapted to receive the beams diffracted from the measurement grating.
13. A sensor head as claimed in claim 12 , wherein the detector arrays measure a phase difference for the first and second measurement beams represented as N A and N B and an x-displacement is calculated as:
x
=
p
4
(
N
B
-
N
A
)
and a z-displacement is calculated:
z
=
p
4
(
N
B
+
N
A
)
cot
1
2
(
β
-
α
)
wherein p is a pitch of the measurement grating, α is an angle of incidence and β is an angle of diffraction.
14. A sensor head for use with a measurement grating, the sensor head comprising:
a splitter grating configured to split a light beam into first and second measurement beams;
a first retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating; and
a second retroreflector configured to retroreflect the first and second measurement beams toward the measurement grating, wherein the first and second measurement beams are diffracted by the measurement grating to form respective first and second sub-beams of each of the first and second measurement beams and the first and second sub-beams each comprise only zeroth order diffraction components or first order diffraction components.
15. A sensor head as claimed in claim 14 , further comprising a first detector array and a second detector array, wherein the first and second detector arrays are adapted to receive the beams diffracted from the measurement grating.
16. A sensor head as claimed in claim 15 , wherein the detector arrays measure a phase difference for the first and second measurement beams represented as N A and N B and an x-displacement is calculated as:
x
=
p
4
(
N
B
-
N
A
)
;
and a z-displacement is calculated:
z
=
p
4
(
N
B
+
N
A
)
cot
1
2
(
β
-
α
)
,
wherein p is a pitch of the measurement grating, α is an angle of incidence and β is an angle of diffraction.
17. A measurement system for measuring a displacement comprising a sensor head as claimed in claim 14 .
18. A method of measuring a displacement, the method comprising:
splitting a light beam into first and second measurement beams;
directing the first measurement beam toward a measurement grating;
directing the second measurement beam toward the measurement grating; and
diffracting the first and second measurement beams into respective first and second sub-beams, wherein one of the sub-beams comprises a zeroth-order diffraction component and a first order diffraction component.
19. A method as claimed in claim 18 , further comprising diffracting the first measurement beam to form first and second sub-beams wherein one of the first and second sub-beams of the first measurement beam comprises only zeroth order diffraction components.
20. A method as claimed in claim 18 , further comprising diffracting the first measurement beam to form first and second sub-beams wherein one of the first and second sub-beams of the first measurement beam comprises only first order diffraction components.
21. A method as claimed in claim 18 , wherein the other of the sub-beams of the second measurement beam comprises a first order diffraction component and a second order diffraction component.