TECHNICAL
COMPARISON OF DYNATENSION WITH CONVENTIONAL TENSIOMETERS
1.0 The Conventional Approach
All conventional electronic tensiometers
use the same basic load sensor, a strain
gage load cell. These typically consist
of four small, flat spirals of
resistance wire electrically connected
in a Wheatstone bridge configuration.
The four sensors are tiny and fragile.
All four will fit nicely on a postage
stamp and aren't much thicker. They are
bonded with epoxy to a flat surface on a
steel bar or rod. The assembly is
referred to as a load cell or load pin.
An exception is a vibrating wire load
cell used in Mipeg load monitoring
products. It avoids many of the problems
attendant to conventional load cells. It
measures the frequency of a vibrating
wire or strip that is stressed by the
load being measured. It biggest drawback
is its sensitivity to temperature
changes. For example, its drift with
temperature is specified as better than
+ 0.03% per °C of rated range.
Consequently a system calibrated to zero
error when the temperature is 0 °C (32
°F) will be off 0.6 % if the temperature
changes to 20 °C (68 °F). If rated load
is 100,000 lbs, the error due to
temperature change is + 600 lbs.
Because of their drift with temperature,
Mipeg crane load monitors require
frequent recalibration. In contrast,
DynaTension systems have zero drift with
temperature or time. They never require
recalibration.
Examples of products that utilize the
conventional approach are those made by
W.C. Dillon, MD Totco, Straincert,
Markload and others. Markload, for
example, cites accuracy of their crane
load indicators as + 2% of full
load. A full load of 100,000 lb will be
measured with an accuracy of +
2000 lbs. It will measure a 10,000 lb
load with the same absolute error, or
+ 20%. In contrast, DynaTension
model M2000 measures 100,000 lb with
accuracy of + 1% or 1000 lbs
which is twice as accurate as the
Markload. But the DynaTension M2000 will
measure the 10,000 lb load with accuracy
of + 100 lbs which is 20 times
more accurate than the load cell based
Markload products. Similar reasoning
applies to all load measuring products
that derive loads from strain gage load
cells or load pins.
The load pin is the axis of a sheave
that the wire rope partially wraps
around. As the tension in the line bends
the load pin a few millionths of an
inch, the electronic strain gage bridge
puts out a DC voltage proportional to
that strain, which is proportional to
the line tension. The electrical signal
usually ranges from about 5 millivolts
at full load to about 15 microvolts at
5% of full load.
A frequently used configuration of a
running line dynamometer consists of
three sheaves. The Dillon Running Line
Tensiometer is representative of this
type of sensor. The cable bends around
each of them, and imparts a small
fraction of the load to the one in the
middle. Bending over small radius
sheaves dramatically shortens cable
life. The damaging effect of the bending
is especially severe when the cable is
electromechanical.
The sheaves introduce friction and
hysteresis. Those non-linear forces
approach the magnitude of the quantity
being measured, if the bend is slight.
Therefore, the error they cause is large
and repeatability is poor. The percent
of error can be reduced by increasing
the amount of bend, but that increases
the wear on the cable. The problem grows
as the mechanical parts wear and
corrode.
The bridge output voltage changes as
changes in temperature cause the
electrical gages, and the steel pin to
expand and contract. The percentage of
change is large relative to the output
voltage resulting from small loads. They
are therefore inaccurate and
repeatability is poor. A known load
picked up on a cold day will cause a
different reading from that on a hot
day.
Changes in temperature and humidity
frequently cause the bonding agent
between the electronic gage and the
steel surface to deteriorate. The result
often is, the system appears to work
right, but may actually be off by a very
significant amount, yet not so much it
is obvious to the operator. The
consequence, fairly frequently, is a
bent boom, broken cable or other system
component.
Because output signals from electronic
load cells are so small in amplitude, it
is necessary to locate a "pre-amplifier"
near the sensor. The purpose of the
pre-amplifier is to boost the signal
above electrical noise. Electrical noise
such as navigation transmissions,
walkie-talkie transmissions, radar and
electrical static degrades the output
like a storm degrades the sound quality
of an AM radio when a storm is nearby.
It can render the tension measurement
worthless.
The pre-amplifier is mounted near the
sensor. Therefore, it is subjected to
extremes in temperature and humidity,
plus rain, sleet, snow, vibration and
mechanical shock. The result of all
these deleterious effects is that the
pre-amplifiers are unreliable and also
contribute to error due to drift with
temperature and time.
2.0 The DynaTension Approach
The DynaTension principle senses the
cable tension based on an entirely
different physical principle, known as
the vibrating string equation. It
"infers" tension in a cable by sensing
the frequency at which it vibrates. In a
noisy environment it compares with FM
radio as a load cell compares with an AM
radio.
DynaTension computes the tension based
on the vibration frequency, the cable's
weight per foot and its length between
two "bridgepoints", typically sheaves. A
finite length of string, cable or other
material under tension supported at both
ends in a “hinged-hinged” configuration
will vibrate with a band of natural
resonant frequencies. The fundamental
component is expressed by the equation f
= (1/2L)(Tg/W)1/2. f is the frequency, L
is the span length, W is the weight per
foot, g is the gravity constant and T is
the tension in the cable. The equation
can be rewritten as:
T=K(fL)2 W, Where K is a constant. The
DynaTension principle solves that
equation to derive tension in the
vibrating element.
In DynaTension Dynamometers (DTDM), the
span (L) is established as the distance
between two sheaves. W is determined
from the manufacturer's data on the
cable, or by weighing a sample. The DTDM
plucks the cable with a non-contacting
pulse of magnetic force to establish and
maintain vibration. That vibration
frequency is f in the equation. With f,
L, W and K known, the only variable left
is T, which is calculated
electronically.
The tension is determined without any
physical contact with the cable
whatsoever. An electromagnetic exciter
"plucks" the cable with a magnetic pulse
to make it vibrate. An electromagnetic
sensor senses the vibration and
generates an output voltage analog of
the vibration. The exciter and the
vibration sensor are located nominally
1/2 inch to 1 inch from the cable. There
are no mechanical components to wear,
corrode, or introduce friction and
hysteresis. Repeatability is excellent.
There is no sensitive axis, so angle of
the cable across either sheave is of no
consequence.
Since there is no physical contact with
the cable, there is zero wear on the
cable or the sensor. The exciter and
sensor assembly, ESA, is fully
encapsulated in water impregnable,
non-combustible epoxy, so it is totally
free from problems due to rain, sleet,
snow, or humidity. It weighs about
twenty pounds.
The vibration amplitude is maintained at
a level sufficient to produce an
alternating current (AC) signal out of
the sensor that is about one or two
volts in amplitude. The sensor output is
totally unaffected in any way by
temperature or time. There is no drift
in output voltage that would result in
system inaccuracy, or the need to
recalibrate the system. The AC voltage
waveform is immune to electromagnetic
interference that renders DC load cell
outputs unusable.
The exciter/sensor assembly (ESA) houses
an electromagnetic exciter that plucks
the cable. It also houses an
electromagnetic sensor that senses the
cable vibration and converts it to an
electrical voltage, a sine wave with
frequency f. The signal processor
computes the tension from that
frequency, the span length and the
weight per foot of the cable
The function of the strummer (exciter)
control is to sense the need to pluck to
maintain suitable vibration amplitude
and to appropriately time the pluck. Its
output is a pulse that gates on the
strummer driver circuit, which generates
a current pulse to the electromagnetic
exciter. It pulses only as often as
necessary to ensure the vibration
amplitude does not fall below a minimum
level, set at the time of DTDM
manufacture.
After amplification and filtering, the
sinusoidal vibration analog voltage is
converted to a digital signal. The
period of the signal is measured and
tension is computed in microprocessor-
controlled circuitry by applying the
basic vibrating string equation. The
algorithm also computes and corrects for
a minor source of error that is a
function of the cable construction. The
calculated tension is output as RS 485,
RS 232, 4-20 ma or 0-10 VDC, to be
specified by the customer at the time of
manufacture.
The tension computing circuit board also
provides the capability to input,
process and output other parameters such
as velocity and payout.
The system provides three alarm levels,
low, medium and high. All three levels
are field settable via the operator
interface, which in some cases may be a
Hand Held Terminal (HHT), and in other
cases may be a Graphics Display Unit
(GDU). The medium level operates a set
of normally open and a set of normally
closed relay contacts that may be used
to halt an operation if the load exceeds
a predetermined level. With the
appropriate operator interface, each of
the three levels yields individually
unique aural alarms if its level is
exceeded.
Connectors, as much as practical, are
avoided throughout our system. The
reason is they often experience
intrusion of moisture-laden air, which
condenses and causes the terminals to
corrode, over a period of time,
especially at sea or in chemical plants.
For the same reason, the electronic
circuitry enclosures are sealed. As an
added precaution, all circuit boards are
conformally coated, which forms a
moisture barrier over the surface of all
the electronic components and their
connections
In their twenty-plus years in the field,
our portable tensiometers based on
DynaTension technology have experienced
more than five years
mean-time-between-failures. Based on
that record, and the engineering
dedicated to maximizing reliability,
DynaTension Dynamometers are warranted
against defects in material or
workmanship for two years.
As adjuncts to the core unit, we have
developed four types of sensors that may
be used with it. All but one of them can
be used with running material. One is
the VARI-L which is a non-intrusive
variable inductance sensor used for
sensing vibration in metallic material
such as wire rope or electromechanical
cable. It is extremely rugged and ideal
for many applications in hostile
environments. Another sensor, the EOSENS
is an electro-optic sensor designed for
small, lightweight materials, such as
optical fiber, fine wires, other
filaments, or belts. Lastly is the
ACSENS, which is an acceleration type
sensor that may be used with non-moving
material. A magnet in its base enables
the operator to simply stick it on
ferrous materials.
The portable model P1000 is currently
used to measure tension in drilling
platform mooring cables and chains; in
wire rope; in elevator cables; in flat
belts, in construction rods, in bridge
cables; in opto-mechanical and
electro-mechanical cable; in yacht
rigging rods and cables and others. Its
applications appear nearly limitless.
The M2000 is installed on cranes. It
continuously measures the weight of the
hook load and compares it to the rated
load at all boom angles and load radii.
It provides readouts of the load; the
rated load; the boom angle; the load
radius and percent of rated load. It
provides three alarm levels and relay
operation at the medium level.
DynaTension Crane Load Monitors are
unequalled in terms of accuracy and
reliability. Users include Transocean
Offshore Deepwater Drilling Company,
Pemex, the U.S. Army Corps of Engineers
and others.
The MTMS 2000 is an in-situ system that
continuously monitors and displays
tension in anchor lines mooring offshore
floating platforms. Like all systems
based on the DynaTension principle, they
have demonstrated incomparable accuracy
and reliability. Users of the MTMS 2000
include Transocean Offshore Deepwater
Drilling Company, Diamond offshore
Drilling Company and DeBeers Marine
Mining.
This same unique technology may be
applied to many diverse applications.
The modular hardware/software design
concept makes tailoring a system to
match differing operational needs fast
and economical. If successful operation
depends on accuracy and reliability of
load measurement, there is no better
sensor than DynaTension.
