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This page encompasses several sections that may be useful when considering new transducers or gait equipment.

Facility Design
Biomechanics Discussion
Teachers' Resources
Terminology
Things to Consider

Things to Consider

Force Transducers

• Does a standard product meet your needs?

• What are your space limitations / requirements?

• Do you need any external attachments mounted to the transducer?

• How large are the forces you will measure?

• Will the transducer be in an extreme environment?

• What are your monetary limitations?

External Amplifiers

• Do you need digital or analog signals?

  • Motion capture requires analog outputs

  • Data acquisition may use digital outputs

• Do you need adjustable gains?

Cables

• How far will the force plate be from the computer?

• What type of inputs does your A/D card accept?

• Will the cables be in an extreme environment?

Mounting

• Does the environment dictate how the transducer must be mounted?

• Do you need multiple configurations for your force plates?

• Bertec always has custom mounting solutions available.

Facility Design

Bertec is currently manufacturing custom analysis equipment for a gait lab at the Ohio State University, Columbus, OH and The Children’s Hospital, Denver, Colorado.  We are working with these facilities to design their gait labs to meet their needs.

Below is just one of many possible layouts for a gait lab.  This lab uses two offset force plates sunk into the walkway.  As anyone experienced in gait analysis knows, there are many possible positions for the force plates. If you are planning a new gait lab, we recommend you ask the opinions of experienced clinicians or contact us. 

If space is a constraint in your facility or you wish to perform analysis on runs or sprinters, we suggest you install an instrumented treadmill in place of the force plates.  With an instrumented treadmill, your patients will be able to walk at their normal speed and in their normal manner.  You will not have to be concerned about aligning their steps with the force plates.  As the subject is essentially not moving in relation to their environment (including the motion capture cameras), you can record data for an indefinite time span.

When considering your space requirements you must first consider your patients and your cameras.  The average person will not reach a steady pace until 1.5 gait cycles, so when using force plates the walkway must be long enough to encompass the volume of space you wish to record plus the length of 1.5 gait cycles before and after that recorded space.  This will ensure that your patient will maintain a steady pace through the recorded space.  A walkway 7 to 8 meters long will work on average.  But, you must consider that the length of gait cycles changes as the patient’s velocity changes.  Your walkway can be relatively short if you test slow moving elderly, but it needs to be considerably longer for testing college track sprinters (possibly 30m on either side of the recorded area).

In addition to the walkway length, you also must include the cameras positions.  They must be positioned far enough from the desired recorded space to capture the entire area.  If you are conducting a whole body motion analysis over a full step cycle, then each camera must cover a volume of 2m by 2m.  If you are doing only lower body analyses, then the cameras may be positioned closer to the walkway.  For three-dimensional analysis using passive or active markers, each marker should be covered by at least two cameras.

An instrumented treadmill will save you lab space as a walkway is eliminated and the cameras can be brought in closer.  Bertec’s treadmill is approximately 2m long, which saves 6m in the overall length of your lab.

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Biomechanics Discussion

For extensive information about biomechanics, gait, and force measurement devices, we recommend
Biomch-L.  This website is an email based discussion forum where topics range from the proper gait lab setup to human and animal motion.

Teachers’ Resources

Our balance research engineer, Kimberly Edginton, gives Expert Electronic Educations presentations at the Columbus, Ohio Center of Science and Industry (COSI).  If you are a teacher interested in balance from a biomechanical engineering or sports injury standpoint, please take a look at a web page designed for the presentation.

Terminology

• Analog output – an output that represents data by measurement of a continuous physical variable, e.g. voltage.

• Crosstalk – An electromagnetic interference between circuits in which the signals become confused and cross over each other.

• Digital output – a digitized output in a numeric form readily used by a computer.

• Fixed gain – the gain is set to a particular amplifier 1, 2, 5, 10, 20, 50, or 100.

• Force transducer – any force measuring device in any shape, may be one load cell or many.

• Fx, Fy, Fz – applied force in the x, y, or z direction (see graphic below).

• Gain – output signal amplification.

• Hysteresis – the difference between load cell output readings for the same applied load.  One reading is obtained by increasing the load from the minimum load; the other by decreasing the load from the maximum load.

• Load cell – the individual load-measuring component of a transducer.

• Load range / load capacity – the load(s) for which the force plate is calibrated.  Bertec’s standard force plates are 1000lb, 2000lb, or 4000lb vertical load capacity.

• Mounting plate – a specialized aluminum plate to which the force plate is attached.  This ensures accurate readings.

• Mx, My, Mz – applied moment in the x, y, or z direction (see graphic below).

• Natural frequency – the vibration inherent in the force plate measured in Hertz.  The higher the natural frequency, the better.

• Origin – the top surface of the center of the plate (x=0, y=0).

• Overload capacity – the maximum load capacity plus 50%.

• Piezoelectric – quartz crystal through which an electric current is run.  As a load is applied, the crystal is compressed and the electrical resistance is increased.  Not good for use in test involving static loading.

• Resolution – the load capacity divided by the potential output.

• Sensitivity – the ratio of the change between output and input.

• Strain gage – a very thin wire mounted to an elastic element (commonly a beam or pylon).  An electric current is run through the wire, and as a load is applied to the element the resistance increases.

• Variable gain – the gain is selectable from 1, 2, 5, 10, 20, 50, or 100

Standard force plate coordinate system: the origin is on the top surface, and at the center of the plate. Positive y-direction is opposite to the connector end; x-axis is to the left when looking in the y-axis; and the z-axis is defined downwards by the right hand rule.
 

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