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Getting started During your initial gait or balance
laboratory design some thought should be given to the following general
questions:
If portability is a primary concern then
please see our AccuSway system for balance and our AccuGait system for gait
as these systems are highly portability turnkey balance and gait solutions
respectively. If portability is low on the list of
priorities then AMTI’s high precision strain gage platforms are the
logical choice. These platforms
provide the highest possible accuracy, our high natural frequency, low cross
talk designs out perform all other platforms on the market today. Platform selection AMTI manufactures a large variety of strain gage based force
platforms suitable for gait, athletic performance and balance studies. Please review our force platform selection guide to help
select the platform most suitable for your application from our many platform
designs. A strain gage based platform system requires several system
components in order to accomplish the measurement task. The system components include:
Platform mounting systems are discussed in our Mounting System page (click for more
information). Force platform selection
is covered in our Platform Selection
page. The following section discusses
some of the options available for signal conditioning and data acquisition. Typical analog and Ethernet installations are shown in the two
figures below. For more information on
each of the components click on the component image. |
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System Operation AMTI’s platforms use strain gages
mounted on precision metal sensing elements located within the platform to
perform the force and moment measurement task. The strain gages are electrically wired in
full four arm bridge arrangements to provide thermal stability and to isolate
the strains caused by forces applied in the several directions. In order to function a strain gage bridge
requires a source of stable excitation voltage. When this voltage is applied across two
terminals of the bridge the alternate two terminals of the bridge will be
balanced and no signal will be present on those terminals. When a load is applied to the sensing
element small mechanical strains will subtly change the resistance of the
bridge arms and the bridge will become unbalanced. When this occurs a very small electrical
signal will be observed across the bridge. The output signals from the strain gage
bridges must be amplified in order to produce signals of sufficient strength to be useful. Typically a well designed transducer (or
force platform) will need an amplifier gain of between 1000 and 4000 to
produce a usable output signal. Once the raw output signals have been
amplified they may be digitized using an analog to digital converter (often
written A/D or ADC). The process of
converting the analog signals into digital values is often accomplished on an
A/D card mounted in a PC. The digitized signals represent values which
are proportional to the loads applied to transducer but they need to be
properly scaled to produce values in engineering units such as pounds for
force or inch-pounds for moment. Data
acquisition software running on the PC is used to gather the data from the
A/D card and to apply the proper scale factors before storage and display of
the final results. Typical Analog Installation The analog system shown in the figure below
shows the various components used to carryout the process described
above. AMTI’s MiniAmp amplifiers are used
to provide the excitation voltage source for the platforms and to amplify the
platforms output signals. Each
platform requires one MiniAmp. The
platforms and amplifiers are interconnected via the 7615 cable which is a
100% shielded cable using twisted signal and excitation pairs to provide the
highest possible noise immunity. The amplified signals are transmitted from
the amplifiers to the ADI-32
interconnection box via the 5405E amplifier cables. Each amplifier requires one 5405E
cable. The ADI-32 can accept up to
four amplifier cables and has six spare inputs which can be used to accept
inputs from other user devices. The ADI-32 collects the signals from the
amplifiers and in turn transmits them to a DT3002 card mounted in a PC via a
100 pin ribbon cable. If the user
chooses to use their own data acquisition system the ADI-32 box also included
a standard DB-37 connector for user configured interconnection. AMTI’s NetForce software can be used
for data acquisition. The NetForce software is
designed to seamlessly interface with up to four amplifiers (and thus four
platforms) connected as described above.
NetForce is an excellent turnkey acquisition system, ideal for gait,
balance and other ergonomic studies. |
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AMTI’s DigiAmp is a single instrument solution
which simplifies the interconnection cabling and provides the highest
accuracy signal conditioning and noise immunity. The DigiAmp instrument provides
strain gage bridge excitation, signal amplification with gains of 1000, 2000,
4000 and 8000, and analog to digital conversion (16 bit). Unlike the analog installation discussed in
the above section the DigiAmp interfaces with the PC via a simple robust Ethernet
cable. The DigiAmp provides many
convenience features which are particularly useful if you are designing a
flexible laboratory to be used for children and adults and plan to perform
balance, gait and human performance testing.
The DigiAmp’s software selectable amplifier gains and excitation
voltages make it possible to easily adjust for low body weight subjects on
the fly without changing hardware settings.
Everything is controlled from the computer control panel. Software selectable filter settings let you
change from a low frequency setting for balance to a high setting for gait
with a few mouse clicks. |
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