Valid Measurements
papers on the Anderson Loop
Anderson Loop equations and wiring diagrams
Valid Measurements papers on the
Ultraguard and Charge Channels
NASA papers on the
Anderson Loop
WRSGC/Boeing paper on the Anderson Loop
NASA Patents
Ultraguard Patent
All of the technical papers listed below and more are available complete and unabridged under one cover in the Current Loop Compendium from Valid Measurements.
THE LOOP TECHNIQUE FOR STRAIN GAGE ROSETTE SIGNAL CONDITIONING, Karl F. Anderson, January 2000.
COMMENT:
This is the most recent paper on the Anderson loop. It is based on an article in the January/February 2000 issue of Experimental Techniques magazine, published by the Society for Experimental Mechanics (SEM).
ABSTRACT
The Anderson loop is presented as an advantageous method of signal conditioning strain gage rosettes, with and without the use of temperature compensating gages in the thermal vicinity of the strain rosette.
A 34K PDF file is available to download.
COMMENT:
For most people, this is the best "first paper" to read. It is based on the cover story in the premiere March 1998 issue of the IEEE Instrumentation and Measurements Magazine and the lead paper in the October 1997 IEEE Transactions on Instrumentation and Measurement.
ABSTRACT
The Anderson loop analog measurement circuit topology is presented that significantly outperforms the classic Wheatstone bridge in many instrumentation, measurement and control applications. Using active subtraction as the key enabling technology, the Anderson loop can provide greater accuracy with less excitation power and fewer connecting wires. Several sensor elements can exist in the same current loop with their independent and combined outputs available simultaneously. Measurement functions beyond what an equivalent Wheatstone bridge can provide have been accomplished. The limitations of the Wheatstone bridge are recalled and the new current loop theory is presented along with test data acquired using an elementary single-component active subtractor. .
A 229K PDF file available to download.
THE ANDERSON LOOP: NASA'S SUCCESSOR TO THE WHEATSTONE BRIDGE, Karl F. Anderson, October 1997.
COMMENT:
This paper is similar to the one above, but does not include test data or as many example circuits. It is based on a paper presented at ISA Tech97 in Anaheim, CA. This paper was honored with the ISA's 1998 Excellence in Documentation Aware as their best paper of the year.
ABSTRACT
NASA's new Anderson loop analog measurement circuit topology significantly outperforms the classic Wheatstone bridge in many measurement and control applications. Using active subtraction as the key enabling technology, the Anderson loop can provide greater accuracy with less excitation than the Wheatstone bridge and accomplish additional measurement functions. The limitations of the Wheatstone bridge are recalled and the fundamentals of Anderson loop theory are presented.
A 66K PDF file available to download.
TRIFERENTIAL SUBTRACTION IN STRAIN GAGE SIGNAL CONDITIONING, Society for Experimental Mechanics Western Regional Strain Gage Committee Paper, Karl F. Anderson, August 1998
ABSTRACT:
This paper introduces the triferential subtractor which has significant
advantages for the typical case of adjacent gages in an Anderson loop signal
conditioning circuit. The triferential and dual-differential subtractors
are compared and a variety of signal conditioning circuits based on the
new triferential subtractor concept are presented.
A 69K PDF file available to download.
CONTINUOUS MEASUREMENT OF BOTH THERMOELECTRIC AND IMPEDANCE-BASED SIGNALS USING EITHER AC OR DC EXCITATION, Measurement Science Conference Paper, Karl F. Anderson, January 1997
ABSTRACT:
This report describes a technique based on the Anderson loop measurement circuit topology that continuously observes both impedance change and thermoelectric signals while using either alternating or direct current excitation of the impedance. Alternating current excitation provides better separation of the signals while direct current excitation results in wider bandwidth. Anderson loop circuit topology benefits are maintained. The technique is illustrated with the simultaneous measurement of resistance variations and temperature by connecting to a strain gauge with thermocouple wire.
A 73K PDF file available to download.
LOOKING UNDER THE BRIDGE, Measurement Science Conference Paper, Karl F. Anderson, January 2001
ABSTRACT:
A technique is present for observing the individual contributions of each element within a transducer based on the Wheatstone bridge measurement circuit topology. An analog signal representing the appropriate sum of all the individual analog elements is simultaneously available. The approach involves opening any convenient corner of the bridge and observing the resulting series string of varying impedances by using the Anderson loop measurement circuit topology. This technique finds application in the development and testing of transducers and can also be used to cause each element within a transducer to have the designer's preferred influence on the output of the transducer..
An 84K PDF file available to download.
Equations and Wiring Diagrams:
EQUATIONS FOR ANDERSON LOOP SIGNAL CONDITIONERS
The fundamental equations for Anderson loop signal conditioning are available for download as a 9 KB PDF file.
EQUATIONS FOR ANDERSON LOOP SIGNAL CONDITIONERS
The fundamental equations for Wheatstone bridge signal conditioning are available for download as a 23 KB PDF file.
INPUT WIRING DIAGRAMS FOR THE ACL-2 ANDERSON LOOP SIGNAL CONDITIONERS
ACL-2 (Trig-Tek 5095) The fundamental equations and set of input wiring diagrams for using this signal conditioner are available for download as a 210 KB PDF file. Block diagrams displaying input wiring and associated circuit features are found in a 218 KB PDF file.
INPUT WIRING DIAGRAMS FOR THE ACL-8 ANDERSON LOOP SIGNAL CONDITIONERS
ACL-8 (Trig-Tek 251B) The fundamental equations and set of input wiring diagrams for using this signal conditioner are available for download as a 423 KB PDF file.
The Ultraguard, Noise Containers, Multi-Potential Gradient Guards and Charge Channels:
A CONTAINER FOR ELECTRICAL NOISE: ULTRAGUARD THEORY AND PRACTICE, Measurement Science Conference Paper, Karl F. Anderson, January 2000
ABSTRACT:
A theory for active containment of electrical noise within a region is presented. A technique called the ultraguard, based on this theory, is presented and experimentally verified. The ultraguard is demonstrated to prohibit undesired charge flow in a signal-carrying conductor from arising due to noise signals that develop between a region’s conductive boundary and a signal-carrying conductor passing through the region. The technique is experimentally demonstrated to operate despite distributed and randomly varying impedance between the signal conductor and the conductive region boundary. The region can be volumetric (i.e. within the shield of a signal cable or within an enclosure surrounding a system or subsystem) semi-volumetric (i.e. within a partially-open container) or planar (i.e. within a conductive boundary such as a guard ring or channel on a printed circuit board surface). The contained noise can arise from various causes, such as the triboelectric effect, radiation- and thermally-induced charge separation in practical insulation, ion migration between the signal conductor and shield, etc. Impedance variations can arise from insulation degradation between the signal conductor and its surroundings, mechanical variation of the capacitance between a conductor and its shield, etc.
An 90K PDF file available to download.
The Ultraguard Patent:U.S. Patent No. 6,147,851, Nov. 14, 2000, METHOD FOR GUARDING ELECTRICAL REGIONS HAVING POTENTIAL GRADIENTS.
[This is the fundamental Ultraguard method patent. A 664 KB PDF file is available for download.]
ABSTRACT:
A multi-potential guarding technique for preventing electrical charge exchange to or from an electrical system containing potential gradients and sources of electrical noise. An inner electrically conductive guard shield surrounds the system to be guarded. The electrical potential of the inner electrically conductive guard shield is such that there is ideally no net current flow to the guard shield from the electrical system therewithin. An outer electrically conductive guard shield surrounds the inner electrically conductive guard shield. An operational amplifier drives the outer electrically conductive guard shield to an electrical potential that is substantially equal to that of the inner electrically conductive guard shield so that there is substantially no potential difference between the inner and outer guard shields which has the effect of producing a high insulation impedance around the guarded system. The multi-potential guarding technique of this invention has application in a gradient-guard configuration and a charge-channel configuration.
The following paper is available courtesy of the Western Regional Strain Gage Committee and The Boeing Phantom Works, Seattle:
DAMAGE DOSIMETER: A PORTABLE BATTERY POWERED DATA ACQUISITION COMPUTER, Western Regional Strain Gage Committee Paper, Dave Smith and Ian Searle, February 1998.
COMMENT:
Boeing's Damage Dosimeter uses the Anderson loop to minimize the power required to excite transducers while acquiring dynamic data from strain gages and to eliminate the need to correct the resulting data for wire-length effects.
ABSTRACT:
The Damage Dosimeter has been designed to fill
a void in commercially available data recorders. The United States Air
Force often has the need to measure structural strains and temperatures
on in-service aircraft in order to diagnose difficult-to-analyze structural
conditions, such as high cycle fatigue. To perform these functions a rugged,
small, and lightweight data acquisition unit
called the Damage Dosimeter has been constructed.
Running off of battery power in an autonomous fashion the Damage Dosimeter
measures 3 channels of strain at sample rates as high as 15 kilo-samples
per second and a single channel of temperature. The Damage Dosimeter merges
the functionality of both the analog signal conditioning and a digital
single board computer on a single 3.5 inch by 5 inch card. The entire unit
(sans battery) weighs less than 1.5 lb. and fits in the palm of your hand.
This paper will discuss the motivation and requirements for the Damage
Dosimeter, along with an overview of the design and construction. Finally,
the software, operational modes, and sample data will be presented.
A 150K PDF file available to download.
THE CONSTANT CURRENT LOOP: A NEW PARADIGM FOR RESISTANCE SIGNAL CONDITIONING, NASA Technical Memorandum 104260, Karl F. Anderson, October 1992.
TM-104260 is NASA's fundamental Anderson current loop report. It the most comprehensive comparison of the Anderson loop to the Wheatstone bridge.
SIMULTANEOUS MEASUREMENT OF TEMPERATURE AND STRAIN USING FOUR CONNECTING WIRES, NASA Technical Memorandum 104271, Allen R. Parker, Jr., November 1993.
Parker's award-winning report details the first method developed for simultaneously separating self-generating (thermoelectric) and non-self-generating (resistance change) signals.
CURRENT LOOP SIGNAL CONDITIONING: PRACTICAL APPLICATIONS, NASA Technical Memorandum 4636, Karl F. Anderson, January 1995.
TM- 4636 is the most comprehensive report published to date on Anderson loop implementation alternatives and their relative merits. It appears in the literature as presented at various technical conferences, formatted according to the technical society editorial guidelines.
PRACTICAL APPLICATIONS OF CURRENT LOOP SIGNAL CONDITIONING, SPIE Conference Paper, Karl F. Anderson, January 1994.
This SPIE conference paper is a version of TM-4636 in the format preferred for SPIE conference proceedings. The information is the same, but you may find the format to be more readable.
A CONVERSION OF WHEATSTONE BRIDGE TO CURRENT-LOOP SIGNAL CONDITIONING FOR STRAIN GAGES, Technical Memorandum 104309, Karl F. Anderson, April 1995.
TM-104309 describes replacing over 300 bridge completion and calibration plug-on cards with Anderson loop cards. No other hardware or software changes were required in this large-scale computer-controlled data acquisition and system.
ABSTRACT
TM-107416 describes a successful application of the Anderson loop and RTDs to measure the temperature of flow in a wind tunnel with a greater degree of accuracy than is possible with thermocouples. The Anderson current loop has become the preferred method used for signal conditioning. This scheme has been used in NASA Lewis Research Center's 9x15 Low Speed Wind Tunnel.
TM-104331 describes the first Anderson loop-based measurements made in supersonic flight research. Adjacent sets of strain gages were connected to classic Wheatstone bridge and the new Anderson loop signal conditioning circuitry. Data from various configuration are presented with essentially identical engineering unit results from the two methods. Of particular interest is the discussion of the Wheatstone bridge and Anderson loop noise floors experienced in flight and the excellent work in resolving the difference between flight experience and theoretical expectations.
U.S. Patent No. 5,371,469, Dec. 6, 1994, CONSTANT CURRENT LOOP IMPEDANCE MEASURING SYSTEM THAT IS IMMUNE TO THE EFFECTS OF PARASITIC IMPEDANCES.
[This is the fundamental Anderson loop method patent. It underlies the entire Anderson loop measurement circuit topology. A 707K PDF file is available to download or you can search for this patent by number through the facilities of the U.S Patent and Trademark Office.]
ABSTRACT:
A constant current loop measuring system is provided for measuring a characteristic of an environment. The system comprises a first impedance positionable in the environment, a second impedance coupled in series with said first impedance and a parasitic impedance electrically coupled to the first and second impedances. A current generating device, electrically coupled in series with the first and second impedances, provides a constant current through the first and second impedances to produce first and second voltages across the first and second impedances, respectively, and a parasitic voltage across the parasitic impedance. A high impedance voltage measuring device measures a voltage difference between the first and second voltages independent of the parasitic voltage to produce a characteristic voltage representative of the characteristic of the environment.
[This is the Anderson/Parker patent on the NASA Thermostrain Gage. You can obtain a clean copy from Valid Measurements or you can view this patent through a search for this patent by number through the facilities of the U.S Patent and Trademark Office. Another approach to this same problem is also available in a 73K PDF file.]
ABSTRACT:
A constant current loop measuring system measures a property including the temperature of a sensor responsive to an external condition being measured. The measuring system includes thermocouple conductors connected to the sensor, sensing first and second included voltages responsive to the external condition. In addition, the measuring system includes a current generator and reversor generating a constant current, and supplying the constant current to the thermocouple conductors in forward and reverse directions generating first and second measured voltages, and a determining unit receiving the first and second measured voltages from the current generator and reversor, and determining the temperature of the sensor responsive to the first and second measured voltages.