Load or Force Sensors
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Posts: 46,084
I am trying to measure individual finger and/or full hand grip
strength (Pressure excerted will be 150lbs or less) and then sending
the signal through an A/D converter to get a digital readout. I have
just purchased the BS2 and the 12bit A/D converter.
I would like to know if anyone has had any experience with Load or
Force Sensors. I have been looking at the Flexiforce Sensors from
Tekscan. They seem to be compact enough for my application. I am open
to any and all suggestions.
Thanks
Kay
strength (Pressure excerted will be 150lbs or less) and then sending
the signal through an A/D converter to get a digital readout. I have
just purchased the BS2 and the 12bit A/D converter.
I would like to know if anyone has had any experience with Load or
Force Sensors. I have been looking at the Flexiforce Sensors from
Tekscan. They seem to be compact enough for my application. I am open
to any and all suggestions.
Thanks
Kay
Comments
Load Cells transducers are utilized in weighing system.They needs a special
kind of IC amplifier.AD7730 bridge transducer from Analog Devices
is a complete front end for weigh-scale.I have ordered a one to build load
cell based electronic scale.
Regards
Mohamed Refky
>From: Kayclifton@a...
>Reply-To: basicstamps@egroups.com
>To: basicstamps@egroups.com
>Subject: [noparse][[/noparse]basicstamps] Load or Force Sensors
>Date: Mon, 12 Jun 2000 19:35:12 -0000
>
>I am trying to measure individual finger and/or full hand grip
>strength (Pressure excerted will be 150lbs or less) and then sending
>the signal through an A/D converter to get a digital readout. I have
>just purchased the BS2 and the 12bit A/D converter.
>I would like to know if anyone has had any experience with Load or
>Force Sensors. I have been looking at the Flexiforce Sensors from
>Tekscan. They seem to be compact enough for my application. I am open
>to any and all suggestions.
>Thanks
>Kay
>
>
>
>
>
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sensors. This had to do with their sensors configured in a fine grid to
measure pressure distribuition inside a shoe:
Beau Schwabe wrote:
These sensors are "Smile" ...One reason we developed our own sensor
for the purpose of monitoring pressures inside a prosthetic foot or
a diabetic foot with the onset of "distal neuropathy" (<-loss of
feeling in the extremities).
The FSR's from TEKSCAN will not last long under the SHEER stresses
inside a shoe, and quickly become "de-laminated". Any deep creases
and the sensors become saturated. The software allows you to
"normalize" or "zero-out" the returned values to accommodate for
any sensor bending, but again, when you do this it severely dulls
the response and range of the sensor.
The TEKSCAN sensor is great if you are standing still, but for any
"real" gait analysis where a patient must walk, the sensors poorly
survived our study. After $10K+ we quickly learned our lesson the
hard way.
My own application of this type of sensor a few years ago was under pans of
food that were being visited by animals, as a kind of indication of the
animal's presence. So that was more like "standing still"-no shear stress.
The sensor resistance is temperature dependent and quite nonlinear, and
repeatable only to within a few percent. But you cannot beat it for low
mass.
-- Tracy Allen
Electronically Monitored Ecosystems
http://www.emesystems.com
to totally rule it out as a possibility, since it seems reasonably priced and
pretty straight forward compared to any thing else I've come across. But
since I have no experience with sensors, it's hard to make a wise decision.
You mentioned that:
"The sensor resistance is temperature dependent and quite nonlinear, and
repeatable only to within a few percentage. But you cannot beat it for low
mass."
What is meant by temperature dependent?
When you said nonlinear, would that mean I would get gaps in my readings?
What is considered low mass? Would that describe the 0 to 150 LB range that I
want to measure?
Since my intention is to compress the cell "head on" would this eliminate
shearing problems.
How long or how many repetitions were you able to get from the sensor?
Did you use the independent sensors (the ones that did not have to be
connected to the software, that is the ELF) and if so how did you calibrate
them?
I know I've thrown a lot of questions your way, and any response will be
appreciated.
Thanks
Kay Clifton
Kayclifton@a...
Date: Mon, 12 Jun 2000 19:35:12 -0000
From: Kayclifton@a...
I am trying to measure individual finger and/or full hand grip
strength (Pressure excerted will be 150lbs or less) and then sending
the signal through an A/D converter to get a digital readout. I have
just purchased the BS2 and the 12bit A/D converter.
I would like to know if anyone has had any experience with Load or
Force Sensors. I have been looking at the Flexiforce Sensors from
Tekscan. They seem to be compact enough for my application. I am open
to any and all suggestions. Thanks Kay
<<<
Kay, there are quite a number of ways to do this - depending on degree
of motion you can tolerate from 0-150# and accuracy, repeatability.
One of the simplest things you might consider is to use a potentiometer
mechanically coupled to a squeeze spring [noparse][[/noparse]like a grip exerciser] so
that squeezing the spring makes the potentiometer turn. Read the pot.
with RCTIME
Another possibility with close to zero travel is a piston coupling a
non-compressible fluid to a pressure transducer which has appropriate
electrical output.
Another possibility is an oscillator with a mechanically actuated
capacitor or inductor which varies its frequency: then divide this
down to a lower frequency with flip-flops and measure with PULSIN.
Resistor pads that vary resistance with pressure have been made
- one company is Interlink.
You can also use an encoder wheel coupled to a spring - I've seen this
in bathroom scales. Digital bathroom scales are a cheap source of the
whole system - spring electronics and digital display... [noparse][[/noparse]most seem to
use LVDTs or linear voltage differential transformers as the transducer].
You can modify a digital bathroom scale to be hand squeezable with some
levers and pivots... but that would leave out the STAMP!
Then there are strain gauges on springy material.... and carbon
piles and granules which change resistance when you squeeze...
You don't say anything about speed of response or stability etc. so
I in turn don't know what to say about that... The encoder wheel and
spring give a direct digital output which you'd read with as many
ports as you need for resolution - or turn the signal into a serial
output to one port. With muscle contraction that you imply you are
liable to get quite a bit of "noise" or varying grip tension so you'll
need to deal with that...
Good luck - if any of this is unclear to you ask [noparse][[/noparse]direct if you wish].
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> Thank you for your response on the Tekscan force sensor. Since I don't
want
> to totally rule it out as a possibility, since it seems reasonably priced
and
> pretty straight forward compared to any thing else I've come across. But
> since I have no experience with sensors, it's hard to make a wise
decision.
> You mentioned that:
> "The sensor resistance is temperature dependent and quite nonlinear, and
> repeatable only to within a few percentage. But you cannot beat it for
low
> mass."
> What is meant by temperature dependent?
It means that the resistance of the sensor depends not only on the force
applied to the sensor, it also depends on the temperature by about 2% per
degree C. Say that the resistance at a certain applied force is 10000
ohms. If you see a change in resistance down to 9000 ohms, it might be due
to a 5 degree C change in temperature, or it might be due to a change in
applied force. Does that matter? Not if you are dealing with relatively
large changes in force and relatively small and slow changes in
temperature.
> When you said nonlinear, would that mean I would get gaps in my readings?
No, it just means that the relation between resistance of the sensor and
the applied force is not an exact simple math function. It is
approximately resistance = 1/force. But there is more to it. The sensor
has "wear in", that is, the properties change the first few times you apply
pressure. Then there is a slow drift of the properties, if a load is
applied continuously, that can amount to 5% change. There is "hysteresis",
which means that the resistance is history dependent--from a constant
force, going up and back down will be different from going down and back
up, by a few percent. Again, these considerations are unimportant if the
range of forces and time scale you are looking at are well over the few
percent range and faster than the slow drift. If you need to "push the
envelope" your software may have to compensate in some way, say, to
establish a slowly changing baseline, or to compensate for change
sensitivity with temperature.
> What is considered low mass? Would that describe the 0 to 150 LB range
that I
> want to measure?
I was referring to the sensor element itself. It is very thin, light in
weight and simple and the electronics to read it out can be very simple.
You can purchase the sensors with a wide range of sensitivity to applied
force.
> Since my intention is to compress the cell "head on" would this eliminate
> shearing problems.
> How long or how many repetitions were you able to get from the sensor?
They still work after almost 10 years, but most of that time has been in
storage! Total iterations maybe 1000. Hardly can make any extrapolation
to what you want to do with finger pressure.
> Did you use the independent sensors (the ones that did not have to be
> connected to the software, that is the ELF) and if so how did you
calibrate
> them?
Yes, we purchased the individual sensor "buttons" in bulk. I sandwiched
three of them under three legs of low-profile pans, so that we could
"weight" the objects on the pans. The sensors were in parallel, so that to
first approximation,
total weight = Force1 + Force2 + Force3
= 1/R1 + 1/R2 + 1/R3
And then we calibrated the pan by putting known weights on top. The
software also had to establish a base line and some temperature
compensation. We had to reduce our expectations several times, but the
study did provide useful qualitative and quanititative information at low
cost.
> I am trying to measure individual finger and/or full hand grip
> strength (Pressure excerted will be 150lbs or less) and then sending
> the signal through an A/D converter to get a digital readout. I have
> just purchased the BS2 and the 12bit A/D converter.
If you really want to be quantitative about static forces, forget about the
FSR technology. It is not accurate. Premena and others provided some
alternatives. If you are looking for qualitative information in the 10%
accurate range or with the time course of the grip strength, well it might
work. But keep in mind the issue of shear forces.
> I know I've thrown a lot of questions your way, and any response will be
> appreciated.
> Thanks
> Kay Clifton
> Kayclifton@a...
You're welcome, and good luck!
-- Tracy