# How to measure stress/tension on a rope?

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

1. ### Robert ScottGuest

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

2. ### larweGuest

Study catenary curves. Apply some high school physics and algebra.

larwe, Sep 8, 2005

3. ### John DevereuxGuest

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

John Devereux, Sep 8, 2005
4. ### Hans-Bernhard BroekerGuest

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. ### Tony LimsonGuest

(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. ### Phil AllisonGuest

"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. ### Mitch BerksonGuest

Make a loop in the middle of the string and put an electronic scale or load
cell in there.

Mitch

Mitch Berkson, Sep 8, 2005
8. ### Mark HarrissGuest

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. ### Dave HansenGuest

Umm, no. But thanks for playing.

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

Regards,

-=Dave

Dave Hansen, Sep 8, 2005
10. ### Rufus V. SmithGuest

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

Rufus V. Smith, Sep 8, 2005
11. ### Tony LimsonGuest

(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. ### GuillaumeGuest

Maybe you're trying to design the ultimate way to hang people?

Guillaume, Sep 8, 2005
13. ### Not Really MeGuest

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
characteristics.

Not Really Me, Sep 8, 2005
14. ### Bryan HackneyGuest

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
could.

Not that this applies to this case, but just to be complete.

Bryan Hackney, Sep 8, 2005
15. ### Jim StewartGuest

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. ### martin griffithGuest

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

martin griffith, Sep 8, 2005
17. ### EricGuest

Eric, Sep 8, 2005
18. ### Franc ZabkarGuest

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:
http://www.industrologic.com/cptdd.htm

-- Franc Zabkar

Franc Zabkar, Sep 8, 2005
19. ### Mark BorgersonGuest

Lots of people make load cells designed to measure tension on lines.
One compact version---albeit for higher loads, is shown at:

http://www.atairaerospace.com/das/sensors/

Mark Borgerson

Mark Borgerson, Sep 9, 2005
20. ### Si BallengerGuest

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.

http://tinyurl.com/96srw

Si Ballenger, Sep 9, 2005