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Force Platform
Mounting
Introduction
AMTI biomechanics force platforms may be used either
free-standing or mounted in place with appropriate mounting hardware. When deciding what your mounting
requirements are the following things should be considered
Free standing platforms may work well for activities with
relatively low frequency dynamic content, low impulse loading and low side
loads such as balance and gait activities.
Yet even for these activities care must be used to place the
platform on a flat surface so that it does not rock or shake when loads are
applied. If the platform rocks
against a hard flooring surface, noise artifacts may be introduced into the
data. Carpeted floors will generally
work reasonably well as the compliance of the carpet distributes the load
evenly between the platform and the floor.
On hard surface floors, such as asphalt tile flooring, it is best to
shim the platform under the mounting pads until no rocking is evident. This can be done using paper or scotch
tape shims. Alternatively a thin
layer of compliant material, such as 1/8th inch closed cell
foam, can be placed under the platform to distribute the loads.
AMTI biomechanics force platforms are designed to
measure forces and moments applied to their top surface and as a direct
result these instruments are sensitive to accelerations as well. Building vibration and ground vibration
can be transmitted to the platform and can result in unwanted signal
noise. Heavy concrete construction or
ground laid concrete slabs provide the best flooring situations.
For the highest accuracy measurements the force platform
should be mounted in a manner that minimizes vibration of the entire force
platform system including the base structure. This is best accomplished by
bolting the force platform to a flat plate that is bonded to a solid
structural foundation. AMTI mounting plates are available for each
force platform model. This document discusses methods for mounting the
platform using AMTI mounting fixtures or by designing your own.
Site Design: Raised,
walkway, or recessed pit
For balance applications AMTI
force platforms may be mounted directly on the floor where the top surface
of the platform is raised several inches from the floor. As the balance activity is passive and
the entire range of motion is contained within the platforms top surface
this arrangement is often a satisfactory solution for these applications.
Gait and athletic performance
studies require that the platform top surface be level with the surrounding
floor providing a lead-in area of several stride lengths and a walk-out of
similar length. The exact size of
the performance area  depends on the activity to be
studied.
Pit mounting is a good alternative
for ground floor, installations. Pit
mounting involves creating (or pouring) a recessed pit in the concrete
floor. The pit depth must be
somewhat greater then the sum of the platform height and the mounting rail
height (typically about 4.5 inches, please check your platform and mounting
rail specifications before designing).
A concrete footing or pylon should be poured under the pit to
provide a rigid inertial foundation between 6 and 18 inches in thickness. Mounting rails are again epoxied in place on the footing surface. The leveling screws in the mounting rails
are used to adjust the height so that the platform surface is flush with
the floor. The primary drawback with
pit mounting is the limited flexibility for platform placement.
Many gait labs have successfully
employed computer room floors such as the Tate ConCore
systems to construct the activity stage.
This type of flooring utilizes a bolted stringer system and
removable floor panels which can provide extensibility and flexibility for
multiple platform gait labs. Design
considerations include:
Force platforms must be flush with the floor surface.
The flooring must not touch the top surface of the plate (2mm
separation).
Stringers must be arranged for desired platform arrangements.
Each plate location requires epoxy mounted rails for platform bolt
down.
The greatest flexibility can be
achieved with AMTI’s air bearing mounting system. This system can provide a flat mounting
surface several meters square. The
system consists of a gravity leveled epoxy pond on a concrete floor which
provides a very flat, level surface.
A magnetic stainless sheet is epoxy bonded to the leveled pond using
a vacuum bagging technique. Special
mounting platforms equipped with magnetic hold downs and air bearings which
may be activated to move the platforms allow you to move the platforms to
any position on the mounting stage.
These mounting stages must be manufactured in place and require
considerable expertise. AMTI will be
pleased to quote the cost of materials and labor to install such a system
at your site. Please contact our
sales offices for additional information.
Fabricated walkways or runways
provide yet another alternative for platform activity stages. Such an installation can be designed to
accommodate several platform locations by use of alternative sets of
mounting rails epoxied to the concrete
floor. The walkway can be fabricated
in sections to provide enclosure around the platforms. Again it is important to remember that
the walkway cannot contact the platform top surface.
Flat, rigid and level
AMTI’s recommended mounting system involves
the use of precession machined steel rails epoxied
in place on an underlying concrete floor structure. The steel rails are leveled and height
adjusted using leveling screws prior to the epoxy bonding process. Once the rails are properly positioned
and leveled, epoxy grout is used to make a permanent bond to the floor. The resulting composite structure of
concrete, epoxy and ground flat steel provides an ideal mounting
configuration for the platforms. In
general the platforms should be bolted to surfaces flat over the area of
the force platform to better than 0.05 mm (0.002”). If you are considering other mounting
arrangements please contact AMTI before finalizing your design. There are many possible mounting
configurations which appear satisfactory at first glance but are not
sufficiently flat, rigid or level to achieve optimum platform performance.
Building vibration
Force platforms behave like accelerometers and produce
outputs from the floor vibrations.
Ground floor installations are usually the most stable and the least
likely to vibrate. If upper floor installation is unavoidable it is
best to locate the force platforms over support beams, near support columns
or near supporting walls if possible. Also try to avoid locating near
large plant equipment such as heating and ventilating systems which can
cause high levels of floor vibration.
AMTI has designed custom vibration isolation systems and our
engineers will be pleased to review your specific needs for gait laboratory
design.
Platform layout
considerations
Most gait laboratories use several force platforms to
capture the reaction forces for the full gait cycle. Two and four platform installations are
the most common. Platform layout and
spacing are largely dependent on the stride length of the subjects to be
tested. Children clearly require
closer platform spacing then adults.
Different platform orientations can be easily accommodated
in software. AMTI’s NetForce software allows you to enter the platform
angle in degrees of rotation (relative to a designated platform). A rotational transformation is applied to
the data to produce outputs that are consistent with the biological
coordinate system across the several platforms
The connectors on
AMTI’s platforms are located on the y axis (usually associated with the
posterior sense in physiological coordinates). The connectors prohibit tight mounting if
both platforms are oriented the same direction (see figures 1 and 3 below). However, by rotating one platform 180
degrees the connector interference will be eliminated. A 180 degree rotation angle included in
the platform setup of the NetForce software will
correct the data from the second platform so that is consistent with the
first.
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The staggered layout shown to the right,
requires at least 5 cm of platform separation unless the 7615R right angle connector is
used on the second platform to prevent interference.
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Fig 1 - Staggered
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The reverse staggered layout, shown to the right, allows
the platforms to be mounted without connector interference. With this layout the data from the
second platform will be sign inverted on the x and y channels. AMTI’s NetForce
software will automatically correct for the platform orientation by
entering an orientation angle of 180 degrees in the platform setup screen. For non NetForce
users our 7615RP connector can be used to correct the polarity of the
inverted signals.
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Fig 2 - Reverse staggered
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The inline layout shown to the right,
requires at least 5 cm of platform separation unless the 7615R right angle connector is
used on the second platform to prevent interference.
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Fig 3 - Inline
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The reverse inline layout, shown to the right, allows the
platforms to be mounted without connector interference. With this layout the data from the
second platform will be sign inverted on the x and y channels. AMTI’s NetForce
software will automatically correct for the platform orientation by
entering an orientation angle of 180 degrees in the platform setup screen.
For non NetForce users our 7615RP connector can
be used to correct the polarity of the inverted signals.
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Fig 4 - Reverse inline
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The side-by-side layout shown to the right can be
accomplished with AMTI’s special tandem mounting plate which allows
access to the mounting bolts on the adjacent platforms.
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Fig 5 - Side by side
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Architectural
and engineering support
AMTI technical support staff will be pleased to assist you
with your force platform installation design. Please contact us by email or phone at:
Planning check list
The following is a brief checklist to use in planning your
installation.
Make the pit
deep enough to accommodate the force platform/mounting plate combination
that you will be using. (See table 1 below for dimensions)
Make the pit
long and wide enough to adequately configure the force platform(s) for
their intended use.
If the pit is
for a single platform, it may be wise to make it large enough to add
additional platforms in the future. (This is especially important in new
construction where the cost for constructing a larger pit during the
initial construction is small compared to enlarging the pit in the future)
Allow about 15
cm (6”) of free space between the connector end of the platform and the
side of the pit to allow access to the connector.
Allow for
about 15 cm of space on the side of the force platform(s) to fit your hand
and a wrench to tighten the mounting fasteners.
Allow for
electrical conduit to be run under the floor from the pit to the computers
area. This is for the force platform cable(s) The
minimum recommended conduit diameter is 5+ cm (2”) (inside diameter).
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Force Platform
Model Number
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Force Platform size in mm
(width X length X height)
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Mounting Rail* Size in mm
(width X length X height)
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OR6-6
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464 X 508 X 82.5
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101 X 508 X 25.4
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OR6-7
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464 X 508 X 82.5
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101 X 508 X 25.4
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BP400600
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400 X 600 X 82.5
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101 X 600 X 25.4
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BP400800
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400 X 800 X 82.5
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101 X 800 X 25.4
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BP600900
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600 X 900 X 102
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101 X 900 X 25.4
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BP900900
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900 X 900 X 102
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101 X 900 X 25.4
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BP6001200
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600 X 1200 X 102
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101 X 1200 X 25.4
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BP12001200
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1200 X 1200 X 102
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101 X 1200 X 25.4
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*Each platform requires 2 mounting rails, one on each
side.
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