XIX IMEKO World Congress [603463]

XIX IMEKO World Congress
Fundamental and Applied Metrology
September 6 −11, 2009, Lisbon, Portugal

NEW AUTOMATIC CALIBRATION SYSTEM FOR LARGE MASSES

Mă Nieves Medina Martín 1, José A. Robles Carbonell 2, Alfonso Lobo Robledo3

1 Centro Español de Metrología, Ma drid, Spain, [anonimizat]
2 Centro Español de Metrología, Madrid, Spain, [anonimizat]
3 Centro Español de Metrología, Ma drid, Spain, [anonimizat]

Abstract − An automatic weight handler system for
weights with masses up to 1000 kg has been designed and
constructed. The system can be used with different
commercial top loaded weighing platforms. This designed
device has been focused on improving the eccentricity
behaviour as well as the complete automation of the facility. This paper describes their main technical and metrological
features.
Keywords : weight handler system, balance, automation.
1. INTRODUCTION
Centro Español de Metrología (CEM) has developed an
automatic weight handler system for large standard weights.
This project started in 2007 and will be finished at the end
of 2008, beginning 2009.
In the 1000 kg range there is no automatic calibration
system commercially available. On the other hand, manual
calibrations on top load weighing platforms are very time consuming and require a lot of manpower. Besides, any
measuring automation process usually improves
repeatability and reproducibility of measurement results. That is the reason why the development of an automated
calibration system is essential in order to improve
measurement accuracy.
The aim of the project is the improvement of CEM
calibration and measurement capabilities for 1000 kg
standard weights, although this system will also be used for
200 kg and 500 kg standard weights and other masses in this
range.
2. DESCRIPTION
This facility will be installed on a laboratory with
temperature controlled in the interval 20 șC ± 0,5 șC.
It has been designed to allow the use of different top
loaded weighting instruments that can be easily changed as required.
It consists of a robust aluminium fixed frame (a), which
supports the weighing platform (c), a hanging loading plate (b) and a vertically and horizontally movable table (d) that
allow the positioning of the weights on the loading plate and its alternation between test and standard weights. The
top loaded weighting instrument rests on a thick stone plate
(e).

Fig. 2. Weight handler sy stem photograph (side view)
a
bc
de
gf

Fig. 1. Weight handler system (side view)

The loading plate is hanging from the central point of the
weighing platform by means of a knife edge system and a
loading pad (f). The loading pl ate is provided with special
profiles that pass through the movable table profiles.
The alternation between test and standard weights is
achieved by means of a movable table, which allows resting the appropriate weight on the lo ading plate as required. This
table can drive horizontally to exchange the weights by
means of an electric motor and vertically to position the weight on the loading plate by means of four ballscrew lifts
(g) commanded by an electric motor.
The loading sequence is described as follows. When the
movable platform is in its upper position, both weights rest
on the movable table. When the movable table is its lower position, the weight which is wanted to be measured rests on
the loading plate while the other weight rests on the
movable table.
b
dg

Fig. 3. Weight handler system (aerial view)
The facility is provided with an auto-centring system to
reduce the pendulum movement of the hanging device. This system consists of four shock absorbers ended in steel
fingers. The loading plate is provided with four holes in
which the steel fingers are in. When the steel finger is pushed by the loading plate, this movement is absorbed.
Different fitting can be inserted in the fingers in order to
reduce the space inside the hole and make the movement absorption more efficient. In order to reduce the system weight special aluminium
has been used as much as possible. This is very important
for the loading plate and its hanging system, which weight acts as a tare weight for top loaded weighing instrument and
reduces its maximum capacity.
The facility will act as a completely automated
comparator and the operator will only have to put the
weights on the movable platform with the help of a crane.
a
bdc
e
gf
Fig. 4. Weight handler system (front view)

Figures 1 to 5 are different views of this weight handler
system. Their main technical f eatures are listed as follows:
• Weight range from 200 kg up to 1000 kg
• Testing load space: 800 mm diameter and 960 mm
height
The software allows progra mming load test sequences,
setting speed of load vertical a nd horizontal translation, and
stabilization time and data acquisition. The electric motors include frequency drives which will be directly commanded
by the computer.
This software allows operating in fully automatic or
manual way. All commands, the programming and loading
routines are activated by easy intuitive graphic mode via
keyboard or mouse-click.
3. TEST RESULTS
Some studies of this system have being carried out with
a 1000 kg top loaded weighing platform with 0.5 g resolution.

Fig 5. Weight handler system general view

The system repeatability has improved a lot as shown in
table 1. The number of measurements which has been used
in order to evaluate the repeatability is 10.

Repeatability
declared by the
manufacturer Repeatability
(standard
deviation)
obtained during
manual
calibration Repeatability
(standard
deviation)
obtained with
our new
automatic weight
handler system
200 kg 2 g 0.64 g 0.5 g
500 kg 3 g 0.99 g 0.6 g
1000 kg 3 g 1.74 g 0.7 g
Table 1. Improvement in repeatability

On the other hand, it is remarkable that, as a
consequence of weight handle r system design, eccentricity
is completely avoided. Another important advantage is the
fact that system automation, which allows loading and
unloading times remain the same, makes creep effects completely compensated.
Actually the time required for a calibration has
decreased more than 50 %, but the main advantage it the fact
that the operator does not have to be present during tests.
Figures 6 and 7 show the 1000 kg top loaded weighing
platform performance during the comparison of two 1000 kg weight for the calibration of one of them. It is clear that the indication variation performance is much better with the
new automatic weight handler system.

Indication variation during a 1000 kg weight calibration just with the top
loaded weighing platform
-6-4-20246810
12345678g
Standard
Test
Fig 6 . Performance of the 1000 kg top loaded weighing
platform alone

Indication variation during a 1000 kg weight calibration with the new
automatic calibration s ys tem
-6-4-20246810
123456789 1 0 1 1 1 2g
Standard
Test
Fig 7. Performance of the 1000 kg top loaded weighing
platform with the automatic weight handler system

4. CONCLUSION
This paper describes the relevant design features and
metrological performance of the 1000 kg automatic
calibration system that will improve CEM calibration and measurement capabilities in this range and reduces the time
and manpower required for large weights calibration.
This system has been designed in order to avoid
eccentricity, compensate cree p and improve repeatability
and, fortunately, this goal has been achieved. Results are
especially good for 1000 kg where repeatability error has decreased up to 40% its initial value.
REFERENCES
[1]
EURAMET/cg-18: Guidelines on the calibration of non-
automatic weighing instruments (January 2009).

[2] OIML R 76 Non-automatic weighting instruments (2006).

[3] Automatic system for data analysis. A. Lumbreras et al.
IMEKO-96 Volume 1 (369-371).

[4] Automatic calibration of weight s up to 500 kg, K. Riski et al,
Proceedings of the 13th International Conference on Force and
Mass Measurement (1993).