AMTI's multiaxis force platforms are used by hundreds of gait and biomechanics laboratories around the world. Excellent performance characteristics and durable designs coupled with AMTI's commitment to provide responsive and comprehensive customer support makes these force platforms the logical choice for gait laboratory and clinical installations.

 

   AMTI's force platforms are the culmination of many years of research and development and they represent the state of the art in multi-axis force sensing technology. Each force platform provides six outputs corresponding to the three orthogonal forces and the three orthogonal moments acting on the platforms top surface.

 

General force platform operation

    Multi-component force platforms measure the forces and moments acting on the top surface of the platform. In three-dimensional space forces and moments can be broken down into three components acting along the three coordinate system axes. Conventionally the axes of the coordinate system are called x, y and z and the associated force and moment components are called Fx, Fy, Fz and Mx, My and Mz.

 

Platform coordinate system

     AMTI force platforms use a right hand coordinate system with the positive z axis oriented downward.  The positive y axis is oriented away from the connector and the positive x axis is oriented to the left when facing in a positive y direction.  The coordinate system axes are illustrated in the figure below. 

 

 

 

 

 

Additional force platform information

     For additional information about designing a gait laboratory please see our mounting instructions pages and our system components pages.  In the sections below many helpful tips are provided to help you select the right force platforms for your research or clinical application.

 

Selecting a force platform

    Selecting a force platform begins with deciding what size working surface or top plate will best suit your application. AMTI's OR6 series force platforms, which are dimensioned at approximately 18 inches by 20 inches, are well sized for most gait laboratory and balance installations. With these dimensions multiple force platforms can be staggered in a traditional walkway arrangement which can be spaced to accommodate the stride of a wide range of children and adults. If your size requirements differ from our OR6 series then check out the BP series of platforms. AMTI manufactures the BP series in a range of sizes where the model number, such as BP600900, indicates the length and width of the platform (in this case 600 mm by 900 mm).


    Once having chosen the size of the force platform then the next consideration is the capacity of the platform. Most of AMTI's force platforms are available in 1000, 2000 and 4000 lb. capacities (Fz capacity). The capacity must be chosen to accommodate the highest load that you plan to apply to the platform. The force platform may be damaged if the rated capacity is exceeded. Please check the specification tables to make sure that the horizontal capacities are sufficient for your application as well.


    It should be noted there is a trade off between the capacity of the force platform and the resolution or sensitivity of the system. Doubling the capacity will effectively halve the resolution. It is recommended that you consult the available literature to determine the optimum capacity and sensitivity for your application. AMTI's sales and engineering staff will be pleased to help you finalize your platform selection decisions based on our many years of experience. Please use the information request form in Sales page to contact us for more information.

 

Selecting a force platform for gait

    The majority of AMTI's force platforms are suitable for gait. The OR6 series provide a standard size of approximately 18 by 20 inches while the BP series provide a range of available sizes. Gait research has demonstrated that the 99 percent of the energy in the gait cycle is spectrally constrained to less then 30 Hz. A platform natural frequency of between five and ten times the highest frequency of interest will adequately protect your measurements from error associated with platform ringing. All of AMTI's gait platforms provide ample margin from a natural frequency perspective.


    Selecting the best capacity for your platform will depend on whether you intend to perform other non-gait work in your laboratory. If your platform is being used strictly for human gait and balance work the 1000 lb capacity platforms will provide the best resolution. Gait research has shown that the maximum Fz loads can be in the order of 140% of body weight. Thus with a robust 300 lb individual the peak gait load could be in the order of 420 lb. This is comfortably less then the rated capacity of the 1000 lb platforms. If you are planning on conducting athletic performance testing then selecting a higher capacity platform is a good idea.

 

Suggested force platforms for gait

OR6-7: Standard for gait labs
BP-400600: 400 by 600 mm work surface
AccuGait: Extreme portability

 

Selecting a force platform for balance

   Static balance applications are unique in that they are very passive. The vertical loads seldom exceed body weight and the horizontal loads are very small. In order to obtain the highest COP resolution it is best to choose the lowest capacity (and hence the highest sensitivity) platform available. Another consideration is that most balance applications will require only a minimum top surface area (sufficient to accommodate the desired stance) so the OR6 series and smaller BP series platforms are indicated.


   AMTI's AccuSway platform is a perfect choice for portable balance-only applications. This platform is ideal because of its low weight and simple RS-232 serial interconnection with a lap top computer. Because the platform is optimized for balance it provides a very accurate measure of COP.

 

Suggested force platforms for Balance

OR6-7: Standard for gait and balance
BP-400600: 400 by 600 mm work surface
AccuSway: Extreme portability
AccuGait: Portable dual purpose

 

Selecting a force platform for sports

   Sports performance applications are more demanding then normal gait and balance applications. Applications such as analysis of take-off and landing in jumping produce large impulse forces which are potentially several times body weight. The impulse rise times are relatively short leading to higher frequency content in the signal. Both of these attributes indicate the choice of our higher capacity platforms (2000 and 4000 lb) which also have higher natural frequencies.



   Both the OR6 and BP series instruments are available in capacities suitable for sports performance applications. Special high frequency platforms (designated by the HF suffix) are also available in both series. The high frequency platform designs use a composite design to minimize the top surface mass and thereby increase the natural frequency. These platforms are useful when the highest possible natural frequency is desired.

    The BP series platforms provide a wide range of size options. These are ideal choices when larger target areas are needed as in the case of many sports performance applications. The PB series platforms are available with 400 x 600 mm, 400 x 800 mm, 600 x 600 mm, 600 x 900 mm 600 x 1200 mm, 900 x 900 mm and 1200 x 1200 mm top surfaces. Several of these platforms are also available with the high frequency design.

 

Suggested platforms for sports

 

Selecting a force platform for portability

    AMTI's AccuSway and AccuGait platforms are ideal portable balance and gait solutions. The AccuSway and AccuGait platforms employ state of the art hall effect sensing technology to provide the highest possible measurement accuracy, repeatability and reliability for portable use. AccuSway is optimized for balance measurements and AccuGait is optimized for gait measurements. Both platforms provide a convenient RS-232 output which may be connected to the serial port of a PC for your data acquisition needs. No additional signal conditioning or data acquisition equipment is required for operation.


   AMTI's NetForce software is recommended to acquire data from the AccuGait platform for gait studies while AMTI's Balance Clinic software is the ideal companion for AccuSway balance studies. Both the AccuSway and AccuGait platforms provide acquisition speeds of up to 200 data sets per second and can be connected in a two platform arrangement to a single RS-232 port.

 

Suggested force platforms for portability

 

Special force platform applications

    AMTI has designed and fabricated platforms for many special environments and applications. Our OR6-GT is a glass top platform with an approximately 1 inch thick glass plate with a 14 inch square clear viewing area. Our BP-400600NC platform is a non conductive design ideal for use with magnetic motion tracking technologies. AMTI's OR6-WP platform is a water proof design which may be completely immersed and is often used in aquatic sports studies.

 
    If you have a particular need which is not satisfied by one of our standard platform designs please contact us using our convenient custom platform questionnaire.

 

Suggested force platforms for special applications