1. This forum section is a read-only archive which contains old newsgroup posts. If you wish to post a query, please do so in one of our main forum sections (here). This way you will get a faster, better response from the members on Motherboard Point.

How to measure stress/tension on a rope?

Discussion in 'Embedded' started by Robert Scott, Sep 8, 2005.

  1. Robert Scott

    Robert Scott Guest

    Without breaking the string, you can insert an in-line tension sensor
    composed of three pulleys. The middle pulley is supported by a force
    sensor. By knowing the angle of deflection of the string and the
    force on the sensor, you can calculate the tension in the string.

    -Robert Scott
    Ypsilanti, Michigan
    Robert Scott, Sep 8, 2005
    1. Advertisements

  2. Robert Scott

    larwe Guest

    Study catenary curves. Apply some high school physics and algebra.
    larwe, Sep 8, 2005
    1. Advertisements

  3. I think you need to say if you are allowed to cut the rope in order to
    insert a transducer (load cell etc).

    Also the question appears a bit homework-like! So if it isn't, perhaps
    you should tell us a bit more about the application so that people are
    not put off replying.
    John Devereux, Sep 8, 2005
  4. Without access to the string, it can't be done in any way that could
    justifiably be called "electronically". You're talking about doing
    some serious physics here. Like: shoot a lot of x-ray intensity at it
    and have an expert interpret the diffraction pattern for you to
    determine the lattice length of the string, from that (assuming you at
    least know the material) the deformation and from that, in turn, the
    tension. Or shoot acoustic energy at it over a wide spectrum and try
    to find its resonance frequency.
    Hans-Bernhard Broeker, Sep 8, 2005
  5. Robert Scott

    Tony Limson Guest

    (a) Imagine an 5-100kg (we do not know exact weight) object is hanged with a
    piece of string/rope/wire and swings randomly.
    (b) We do not have access to either end of this string.
    (c) How can we measure, electronically,em the stress/tension on the string?
    Tony Limson, Sep 8, 2005
  6. Robert Scott

    Phil Allison Guest

    "Hans-Bernhard Broeker"

    ** That is the key.

    You need to know about the string/wire/rope ( ie mass per unit length ) to
    relate resonant frequency to tension.

    Then just pluck the string and measure the frequency of vibration - many
    ways to do that.

    ........... Phil
    Phil Allison, Sep 8, 2005
  7. Make a loop in the middle of the string and put an electronic scale or load
    cell in there.

    Mitch Berkson, Sep 8, 2005
  8. Robert Scott

    Mark Harriss Guest

    You have described a "pendulum" : oscillation frequency is
    proportional to pendulum length and weight on the end.

    If you know the time for one oscillation, the gravitational
    acceleration, length of the rope, you should be able to
    solve for the mass of the pendulum (weight + rope).

    Once you have the mass at the end and the velocity of it you
    should be able to calculate the force exerted on the rope
    through centrifugal force.

    Sounds like a lot of mucking around.
    Mark Harriss, Sep 8, 2005
  9. Robert Scott

    Dave Hansen Guest

    Umm, no. But thanks for playing.

    Hint: where is the "weight" in 2*pi*sqrt(L/g)?


    Dave Hansen, Sep 8, 2005
  10. Pendulum, yes.

    But length only, not weight. Therefore, probably no help here.

    Perhaps if you knew the length and weight of the string, you
    could calculate the center of mass by the period of the swing,
    then from that the mass of the object.

    But my last Physics class was about 30 years ago...

    Rufus V. Smith, Sep 8, 2005
  11. Robert Scott

    Tony Limson Guest

    (a) Imagine an 5-100kg (we do not know exact weight) object is hanged

    No, this is not homework, and I'm not a student.

    Imagine a parachute. How can you measure stress/tension on a parachute line?
    You can not access the either end of the string/line. (One end connectected
    to parachute, the other is connected to carabiner.)
    Tony Limson, Sep 8, 2005
  12. Robert Scott

    Guillaume Guest

    Maybe you're trying to design the ultimate way to hang people? :D
    Guillaume, Sep 8, 2005
  13. Can we contact the string/line at all? Do we know what it is made of?

    If so we could potentially measure stretch over a small sample if we can
    access it before the load is applied and can previously know the stretch
    Not Really Me, Sep 8, 2005
  14. Unless the rope is infinitely rigid (wonderful first year physics assumption),
    the period and the swing itself will vary with the mass, because the rope will
    stretch with the angular acceleration. I don't want to do the math, even if I

    Not that this applies to this case, but just to be complete.
    Bryan Hackney, Sep 8, 2005
  15. Robert Scott

    Jim Stewart Guest

    I'm sure Gary Peek would know. He's an
    occasional poster on this group and has
    used our controllers to measure tension
    on parachute shrouds.
    Jim Stewart, Sep 8, 2005
  16. I was videoing something similar on a winsurf rig. It was to measure
    the downhaul tension of the sail during various manouvers, to help the
    designer compensate for the change in geometry and los of power. It
    was done by a guy doing his PhD in sports technology. ( I had to fix
    the data recorder.)

    basically the downhaul (http://www.murrays.com/archive/69.pdf) was
    modifeid with a calibrated strain gauge feeding a data recorder.

    I would suggest modifying the carabina by epoxying a SG on it. But I
    dont know enough to know if this is possible with parachute technology

    martin griffith, Sep 8, 2005
  17. Robert Scott

    Eric Guest

    Eric, Sep 8, 2005
  18. Robert Scott

    Franc Zabkar Guest

    Would it be acceptable to fit a sensor to the harness and assume the
    load is equally distributed amongst the risers?

    Otherwise, is this what you are looking for?

    Collecting Parachute Test Drop Data:

    -- Franc Zabkar

    Please remove one 'i' from my address when replying by email.
    Franc Zabkar, Sep 8, 2005
  19. Lots of people make load cells designed to measure tension on lines.
    One compact version---albeit for higher loads, is shown at:


    Mark Borgerson
    Mark Borgerson, Sep 9, 2005
  20. Robert Scott

    Si Ballenger Guest

    You probably could make a simple gizmo to measure the strain
    using inexpensive force sensors like below (search
    www.digikey.com for force sensors). You would just slip the
    parachute line in the gizmo sideways and when the string is
    pulled tight the gizmo flexes pinching the sensor in preportion
    to the pull on the line.

    Si Ballenger, Sep 9, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.