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The electromechanical specifications table is organized into two groups of
columns corresponding to the force and moment channels of the platform. Fx, Fy and Fz
head the force column specifications with a fourth column labeled units.
Similarly the moment column specifications are headed by Mx,
My and Mz with a fourth column labeled units.
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AMTI's platforms are available in several capacities. The specified capacity
corresponds to the Fz capacity. The capacities of
the other force and moment channels are specified in this row of the
electromechanical specifications table. When English units are selected the
capacities for the force channels will be in pounds (lb) and the moment
channels will be in inch-pounds (in-lb). Similarly if Metric units are
selected the force channels will be displayed in Newtons
(N) and then moment channels in Newton-meters (N-m). |
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Each of AMTI's platforms is designed for
maximum sensitivity given the design capacity. In general, for a particular
platform, the sensitivity will very inversely with the capacity. For example
a platform with twice the capacity will have one-half the sensitivity. The
various channels of the platform have different sensitivities,
these are listed in this row of the specifications table. When English units are selected the
sensitivities for the force channels will be displayed in micro volts per
volt per lb (µV/V-volt-lb) and the moment channels will be in micro-volts per
volt per inch-pound (µV/V-in-lb). Similarly if Metric units are selected the
force channels will be displayed in micro volts per volt per The sensitivity values listed in this table
are nominal values. Every platform that AMTI ships is
subjected to a rigorous multi-point, multi-load calibration procedure in our
calibration laboratory. The data from the calibration procedure is analyzed
and a calibration matrix is derived. The calibration matrix is supplied to
the customer on diskette for seamless integration with AMTI's data
acquisition software. Hardcopy is provided for backup and documentation. The sensitivities listed in the table allow
us to determine what the output of the platform will be for a given input
load. They also allow us to “calibrate” our data acquisition
software so that it's outputs appear in engineering
units such as pounds of force or in-pounds of moment. Strain gage instruments produce very
low-level signal outputs and they always require amplification and a source
of excitation voltage to operate (see AMTI's amplifier products for turnkey
solutions). Amplification is specified by a Gain factor. The Gain is simply a
multiplier, which acts on the amplifiers input. The excitation voltage, which
is applied to the transducer effectively, drives the strain gages so they
will produce an output, the output will always be proportional to the
excitation voltage. The equation for calculating the output
voltage (after amplification) is: Vo = S • (V/µV) • G
• Vex • Fa
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Every mechanical system has a preferred
frequency at which it will vibrate when it has been properly excited. A good
day to day example is church bells. Each bell, by design, vibrates at a
different frequency to produce the notes of the musical scale. The mechanical
properties of the bell, such as the mass and the thickness of the walls
define the note or frequency at which the bell will ring. This frequency is
called the natural frequency of the bell. Instruments such as force platforms also
have a natural frequency, although in the case of force platforms the design
emphasis is to minimize the effect rather then to produce a clear musical tone.
This is done by designing platforms with the highest possible natural
frequency to avoid ringing when conducting tests and making measurements. It is desirable for the platform's natural
frequency to be considerably higher then the frequency content of the signal
of interest. For instance the force signature of normal gait has spectral
content (frequency) which primarily lies below 30Hz. Thus to avoid artifacts
caused by platform ringing the platform should have a natural frequency 5 to
10 times that of the gait signal (150-300Hz). During static balance the
spectral content of postural adjustments is under
3Hz, so in this case a considerably lower natural frequency would be
satisfactory. |
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The product specification screen allows you
to view the electrical and mechanical specifications of AMTI's multi-axis
force platforms. On the left hand side of the screen are three list boxes
which can be used to control the display. The list boxes are titled Select units, Select product,
and Select capacity. The Select units list box allows you to choose your preference of
English or Metric units. The Select product list box lists all of our
standard platforms by general category and the Select capacity list box
presents the standard capacities for the selected platform. Once you have selected a platform and
capacity the electrical and mechanical specifications will be displayed in
the tables on the right side of the screen. If you have a question about the
meaning of a table entry click on the cell in question
to display a detailed explanation. On the menu on the left of the screen is an
entry labeled Output calculator. Once you have
selected a platform you may use the Output calculator to review the
performance of the selected platform with our several amplifier products. The
calculator will allow you to select gain and excitation voltage and calculate
the full scale range and the maximum resolution for the selected platform
given the particular amplifier settings. |