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Submitted on 1 Jan 1984
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A remote control eucentric goniometer head for use within a top loading helium cryostat
M. Berneron, A. Filhol, J.J. Vernier, M. Thomas
To cite this version:
M. Berneron, A. Filhol, J.J. Vernier, M. Thomas. A remote control eucentric goniometer head for use within a top loading helium cryostat. Revue de Physique Appliquée, Société française de physique / EDP, 1984, 19 (9), pp.795-797. �10.1051/rphysap:01984001909079500�. �jpa-00245263�
795
A remote control eucentric
goniometer
head for usewithin a top
loading
helium cryostatM. Berneron, A. Filhol, J. J. Vernier and M. Thomas Institut Laue Langevin, 156X, 38042 Grenoble Cedex, France
Résumé. 2014 La tête goniométrique eucentrique est de forme cylindrique et est assez petite (par exemple : ~ 38 mm, H = 80 mm pour le prototype) pour être fixée à la canne centrale d’un cryostat et descendue dans la zone froide de ce dernier. La rotation de deux tiges de contrôle, à partir du sommet du cryostat, commande le réglage de la tête.
Une fois monté, l’échantillon peut être incliné de ± 6° dans toutes les directions. L’orientation de l’échantillon selon la direction souhaitée est manuelle ou assistée par un ensemble moteurs-codeurs. La possibilité du contrôle de l’ensemble par ordinateur est envisagée.
Abstract. 2014 The eucentric goniometer head is cylindrical in shape and is small enough (e.g. ~ 38 mm, H = 80 mm
for the prototype) to be fixed to the central stick of a cryostat and lowered to the cold zone. Adjustments are made through the rotation of two control rods, from the top of the cryostat. Once mounted the sample may be inclined
± 6° in any direction. The sample setting to the required orientation is manual or assisted by a motor encoder
device. Full computer control is foreseen.
Revue Phys. Appl. 19 (1984) 795-797 SEPTEMBRE 1984,
.1. Introduction.
The precise alignment of the axis of a sample placed
within a top loading cryostat is conventionally achiev-
ed by using an orthogonal arc table [1] capable of supporting the 30 to 40 kg of the loaded cryostat. The table may have an eucentric design and provide accu-
rate angle setting (0.01° or less). The draw-backs to this solution are its high cost (~ 80 000 F), the fact
that it must be included in the instrument design and,
more over, the cryostat is inclined and precesses around the instrument axis with a large overhang.
This paper describes a novel device consisting of a remotely-controlled goniometer head placed within
the cryostat and optionally motorised and computer controlled. Its cost is much lower (1), it may be easily
installed on or removed from an existing instrument
and the cryostat remains vertical and co-axial. The accuracy is lower (~ 0.020 in the present design)
but sufficient for most uses.
2. Description.
The general design of the goniometer head and its motor-encoder device is shown in figure 1.
(1) Cost of the prototype (Fig. 1) : - 5 000 F for the eucentric arc module ; - 4 000 F for the motor-encoder
device ; - 5 000 F for the electronics.
2. 1 THE EUCENTRIC ARCS. - The inclination of the
sample axis in a given direction is the result of the combined displacement of two right angled levers (Ex and Ey) controlled by the rotation of two push rods (Cx and Cy) (Fig. 2). The locking of the rods locks the
aligned sample.
The prototype module is all in stainless steel except for a bronze-beryllium spring and thus may be used down to 4.2 K without jamming. Its dimensions (38 mm in diameter ; 80 mm in height) allow an angular
inclination range of + 60.
2.2 CRYOSTAT CENTRE-STICK. - Figure 1 shows
the arc module mounted at the end of a modified centre-stick of an ILL standard o orange » helium
cryostat [2].
2.3 MOTOR-ENCODER SYSTEM. - The arc module may be easily set by hand and charts of « rod turns » versus « inclination (4)) and direction (m) angles » (Fig. 2) have been produced [3]. However, as a first
step towards a fully computer controlled assembly ; as motor-encoder system and its electronics have been
developed (Fig. 3). The prototype encoder relies on a perforated disk between a LED and photocell. Low
cost d.c. motors are used. In the present design :
one encoder step = 1/36 control rods turn one encoder step = 0.019° of inclination.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01984001909079500
796
Fig. 1. - Schematic drawing of the goniometer head assembly. 1) sample position, 2) eucentric arc module, 3) control rods, 4) cryostat centre-stick, 5) motor-encoder system, 6) motor-encoder control electronics.
Fig. 3. - The goniometer head assembly installed on a top loading helium cryostat 1) helium top loading cryostat, 2) motor-encoder system of the goniometer head, 3) control
rods (Cx and Cy), 4) control electronics.
Fig. 2. - The eucentric arcs. (a) The arcs : (A) inclination centre ; (B) spherical part ; (Cx and Cy) control rods ; (Dx and Dy) screw thread ; (Ex and Ey) angled levers ; (F) cross-cut guide ; (G) free running « U » bracket ; (H) bronze-beryllium spring. (b) Definition of the incli- nation (0) and direction (co) angles.
2.4 COMPUTER CONTROL. -Control by the instrument
computer may be of interest for some uses (e.g. auto-
mated sample alignment). This will be done through
an interface (under design) including a microprocessor
dedicated to the « rod turns »/« inclination angles » conversion, operating the motor-encoder system and giving set status in reply to ASCII or IEEE488 com-
mand strings.
3. Tests of the prototype assembly.
Accuracy and reproducibility tests have been per- formed at room temperature (optical tests with a laser)
797
and at 6 K on the neutron diffractometer D13A [4].
We observed no jamming at low temperatures and no
changes in backlash and accuracy. The setting accu-
racy for the inclination angle (ql) is better than the encoder step width (0.019°) ; the inclination (ql) and
direction (co) backlash were found to be of ± 0.060 and ± 0.070 respectively. The first one is partly due to repeated mounting/dismounting of the prototype mechanism and may be kept much smaller. The second
one is due to an under dimensioning of part « F »
(Fig. 2) of the arc module which will be twice as large
in a subsequent design.
The « hole/photocell » encoder operates very satis-
factorily. The limit-switches are precise and reliable whereas the zero point detection is not and will be
redesigned
The goniometer head assembly has been success-
fully used at 4 K to set in the equatorial plane Bragg
reflections from a single crystal of Cd1- xMnx Te,
in a paramagnetic and magnetic scattering experi-
ment [5] with polarisation analysis, on the neutron TOF-spectrometer D7 [4] at the ILL.
4. Future developments.
The present design (Fig. 2) is in fact more suitable in the case where the sample rests on the arc module,
If the sample is suspended then a sample weight limi-
tation exist due to the spring « H ». This will be easily
corrected in a subsequent design in which the spring
is placed below the spherical part of the goniometer
head instead of above as presently.
A room temperature assembly will be developed
to support the monochromator of a neutron 4-circle diffractometer for biology (ILL instrument DB21 [4]).
References
[1] e.g. GOBERT, G., Ensembles goniométriques, Catalogue of
ILL assemblies (1983).
[2] ILL patent, AS Scientific, Abingdon, Oxon., U.K.
[3] FILHOL, A., SIMMS, P. and WRIGHT, A. F., ILL internal report 82FI16T (1982).
[4] Neutron Research Facilities at the ILL High Flux reactor,
ILL internal report available from the Scientific
Secretary.
[5] STEIGENBERGER, U., (1984) private communication.
