Technology Profile |
International standard or a reliable
WIM system: what comes first?
When it comes to the
backwardness of
implementing weigh-inmotion
(WIM) systems for direct
enforcement, manufacturers
offer several explanations.
An international standard, if it
existed (and it doesn’t), could act
as a warranty for the future
investments into developing
reliable WIM systems that
are capable of achieving the
level of accuracy expected by
road authorities.
Such a concept does not even
exist for static weighing
systems. In some countries, if a
system has not been approved
locally, then it cannot be used,
even if it has been certified
according to OIML, EU or NIST.
There should be a consensus
about the standard. Some
believe it should be the OIML
R134 recommendation, others
back the ASTM standard
specification or self-made
standards, such as the one
from NMI Nederland. Local
legislations are also a topic
of discussion.
It is very hard to achieve the
level of accuracy in WIM systems
that is required for direct
enforcement. Nevertheless, this
is not a reason to soften the
rules and neglect the quality
of enforcement.
Controlled testing
A system certified in ideal
conditions is not necessarily
reliable when used outside of
those conditions in the real
world. WIM sensors are
perfectly calibrated in the
factory in which they are made
and their intrinsic error is
determined using a test machine
or dead weights. Depending on
the technology, the calibration
could be influenced by the
following factors: temperature,
incorrect loading, tilting,
bending, lateral forces,
| Need to know
Benefits of Haenni
Instruments wheel
load scales include:
068 Traffic Technology International July/August 2019
www.TrafficTechnologyToday.com
> A weight of less than
100kg (220lb) for a
complete set, comprising
two scales, a processing
unit, and cabling and
levelling mats
> Can operate at
temperatures from
-20 to 60ºC (-4 to 140ºF)
repeatability, creep, warm up,
moisture, electromagnetic
susceptibility, etc. Intrinsic
errors represent just 20% of
the overall error of a WIM
system. The rest (80%) is not
related to the measurement
instrument itself.
The reality of real dynamic
measurement is that the overall
error is influenced by several
external factors such as the
inclination of the site, oscillation
Left: Haenni
Instruments,
a leader in
portable static
scales for weight
enforcement, also
manufactures
WIM technologies
Certification officers and
national institutes are aware
of this and hence careful when
reviewing and accepting
international certificates to
avoid legal issues for the
end users.
OIML is planning to revise
its recommendation in the
coming years. It seems that it
will add some rules for inroad
systems, because they are not
explicitly covered by the
recommendation. However,
manufacturers still have work
to do. Testing rules will need
to be better defined for an
international standard to be
achieved but this will not
facilitate or simplify the testing
processes, because a high level
of accuracy will still be
required. In short, the product
must fit the standard and not
the opposite.
of the vehicle, vehicle
unevenness, break reaction
forces, suspension, load
distribution, wind, speed,
temperature and road structure.
It is factors like these that are
holding the expansion of WIM
systems for weight enforcement
back and nobody could expect
them to be minimized at all
times by end users and road
authorities. An international
standard can only be reliable
if it considers all of these
varying factors.
“The rules for certifying
a system for dynamic
measurements must be as strict
as the ones for the static
weighing,” says Daniel
Kneubühl, managing director of
Haenni Instruments.
Kneubühl believes that there
are too many legal loopholes in
the approvals and testing
process. “The problem is not
only finding a good test site for
the installation, but how the
measurements are done,” he
says. Tests are carried out in
good conditions in limited
geographical areas with just
a handful of vehicle types.
| Free reader
inquiry service
Haenni Instruments
To learn more about this advertiser, please
visit: www.magupdate.co.uk/ptti
/www.TrafficTechnologyToday.com
/ptti