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General operation The majority of AMTI’s multiaxis transducers
utilize strain gage sensing technology to decompose the applied loads into
orthogonal components (components at right angles to one another). Strain gages, configured in full four arm
bridges, are mounted on one or more sensing elements which carry the applied
load from one mounting surface of the transducer to the other mounting
surface of the transducer. The strain
gage bridges are designed to isolate the strains caused by loading, from
either forces or moments, acting along or about the three axes of the
transducer. AMTI’s multiaxis loadcells (or multiaxis transducers) have up to six degrees of freedom (DOF) which are the three
forces Fx, Fy and Fz and the three moments Mx, My
and Mz. The four arm bridge strain gage circuits
require a source of bridge excitation (a stable voltage applied across the
bridge arms) as well as output amplification in order to produce usable
voltage outputs. AMTI’s MiniAmp
and DigiAmp
products can be used to provide the required excitation and amplification and
both offer additional signal conditioning benefits as well. The amplified output signals of the
transducers are voltages which are proportional to the applied forces (Fx, Fy and Fz)
and applied moments (Mx, My and Mz). These voltages may be digitized and stored
using a PC with an installed A/D card or in the case of the DigiAmp (which contains its own internal 16 bit A/D
converter) simply transmitted to a PC via an Ethernet cable. Sensitivity and Calibration AMTI’s
transducers are factory calibrated to determine the sensitivity of each
channel prior to shipment. A multipoint,
multiload calibration procedure is used to
accurately determine the sensitivity of each of the channels of the
transducer. The results of this
calibration procedure are delivered to our customers on a calibration report
and in a text file on a calibration diskette.
The sensitivities are reported in a calibration matrix, the main
diagonal terms of which describe the on axis sensitivities while the off
diagonal terms describe the cross talk sensitivities. In many cases the main diagonal terms alone
are sufficient for satisfactory accuracy.
However in applications where large moments are applied to the
transducer it is often desirable to use the full matrix to determine the
loads applied to the transducer with increased accuracy. This procedure is described in the white
paper Calculating Forces and
Moments from Biomechanics Platform, and Transducer Outputs.
AMTI
provides an NIST traceable recalibration service for our multiaxis load
cells. Please note that we provide
this service to our multiaxis force platform and multiaxis transducer
customers only. Please contact our sales office for your recalibration
needs.
Coordinate system
The coordinate system axes passes through
the approximate geometrical center of the load cell. During calibration the exact location of
the axes origin is calculated and is distributed in the loadcell
calibration sheet. |
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