Goniometer

In optics

It comprises a fixed part, on which is assembled a moving part. One can have two configurations:

the fixed part is related to the reference frame of the laboratory (on the ground), the moving part pointing in the direction which one wants to determine the angle with the reference axis;
the fixed part is related to the reference frame of the object observed (for example it is placed in a vehicle), the moving part points towards the direction of reference (in general the Northern ).

The goniometer in general comprises a scale in degree S, the Rapporteur, and possibly a sliding gauge to improve the precision.

In Optical, the goniometer is used to determine the deviation of a ray luminous by an optical device (for example a prism; the moving part then comprises a Lunette of aiming.

Use in diffractometry X

In X-ray crystallography, the goniometer is the part of the Diffractomètre which is used to determine the angles. The movements are motorized.

In the major part of the cases, the angles are determined by the order given to the engines (driving step by step): at the time of initialization, the apparatus establishes its zero (compared to a point of reference, for example a notch on the goniometer); the angle corresponding to the zero of the engine is determined by a procedure of alignment.

In the case of a measurement with high-resolution, it becomes very long to stabilize the exact position of the engines (the loop of feedback generates oscillations which are reduced progressively). In this case, it is more interesting (time-saver) to let the engine place itself roughly, then to measure the angle (automatic measurement, for example using an optical sensor).

Goniometer with two circles

In the most general case, there is a goniometer with two circles, i.e. one determines

the angle of incidence of the beam of x-rays on the object, angle called Ω in the general case, and θ in Bragg-Brentano configuration;
the angle enters the incidental beam and the detected beam, called 2θ.

The term of “circle” indicates in fact a motorization allowing a circular motion, the more exact term would be “assembly with two engines”.

In this Bragg-Brentano assembly, the sample has a fixed Orientation compared to the vector of diffraction (Bissectrice between the incidental beam and the detected beam). One associates sometimes a device allowing to make turn the sample around his axis, called spinner , but this device is not regarded an additional circle but as a manner of sweeping a presumedly homogeneous sample completely (the beam with the shape of a feature and clarifies only one reduced portion of the sample).

Two configurations are distinguished (one supposes a measurement in Bragg-Brentano configuration here):

the configuration θ-2θ (“theta-two theta”): the tube with x-rays is fixed, the sample moves of an angle θ and the detector of x-rays of an angle 2θ; it is the most current configuration, indeed, the tube being the heaviest device and most cumbersome, it is simpler to fix it;

the configuration θ-θ (“theta-theta”): the sample remains fixed (horizontal), the tube and the detector moves in a symmetrical way of an angle θ; this makes it possible to measure Poudre S with high angles (the sample remains fixed), and facilitates the installation of certain devices like a furnace around the sample.

Goniometer with three and four circles

In certain cases, one is interested in the anisotropy of the sample; it is for example the case of the Texture (crystalline Orientation preferential), of the Stress measurement, or determination of the crystalline structure starting from a Monocristal. The incidental beam has a specific form here, it informs a small disc on the sample.

In this case, it is necessary to be able to direct the sample in all the directions of space, which supposes three axes of rotation, to which is added the position of the detector; there are thus four engines, the device is known as with “four circles”.

In the case of a polycrystalline sample, the device is called a “cradle of Euler” ( Euler craddle ), by comparison with the Angles of Euler. This cradle of Euler can be centered or excentré, it can be open or closed; the open and offset cradles can accommodate bulkier samples, but they are more complicated to realize.

The sample is set up motorized on a turntable; this one can move according to the three axes ( X , there , Z ) to make it possible to position the center of the sample at the point of reference. In all the cases, the tube is fixed, and in the position of reference, the plan of the sample is vertical (the detector moves according to a horizontal circle).

In the case of a single-crystal sample, this one is stuck on a directional “pinhead”.
In photography opposite, the goniometer is the material in white on the right: it is used to handle by rotation (very precise); a pure microcrystal of a Molécule which one wants to know the structure in the beam of x-ray Monochromatique (frayed tube top) is placed at the end of the needle in the center. A special camera CCC x-rays (on the left with red letters) numerically collects the position and the intensity of the spots of Diffraction S.

Since the position of the spots of diffraction is located by the camera, this one is not directional; there is thus a goniometer with three circles. A video camera (black tube in top on the left) assistance for the first positioning and centering of the crystal in the beam of x-rays.

These measurements treated by computer allow by transformed of Fourier to reconstitute the image of the crystal and especially of the basic molecule which composes it.

Note: one used this instrument to illustrate the models sciences, physics because x-rays had a determining role in the evolution of sciences.

to see a increased image

Out of radio

A goniometer, or more exactly a radiogoniometer, is equipment intended to measure the angle of arrival of a Onde radio.

Historically the goniometers were used much like equipment of assistance to navigation, both for the planes the ships. The advent of GPS seems to make disappear this use. For example:

In the field of VHF and aeronautics, the goniometer is also called VDF ( V HF D irection F inder)
Installé on board an aircraft, when it is used to determine the direction of emission of a fixed beacon in the range HF (High frequencies), it is called ADF ( has utomatic D irection F inder)

The radiogoniometers remain still mainly used by:

armed forces with an aim, either to prevent presence of an enemy broadcasting transmitter (alarm), or to locate the unfavourable transmitters in order to obtain information on the involved forces;
of the government agencies of control of the spectrum, agencies charged to control that the users of radio frequencies are well holders of a license (as the famous licenses UMTS who so much made speak about them);
certain radio amateurs…

The radiogoniometers use various physical principles.

the directivity of the reception antenna.
the Interferometry
the Effect Doppler
…

See too

Internal bonds

External bonds

a video explanatory on the adjustment of a goniometer
a video explanatory on the use of a goniometer with a prism in optics

Random links: