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, or 4-20 ma, 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 laptop PC, 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 three 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.

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.


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