3rld world country step-force monitoring device (pedometer)

[deeohgoh]

hey guys! i am a final year student in biomedical engineering and my masters project involves a lot of electronics that i am not familiar with.

i have to build a cheap/strong step/force monitor device for thirld world amputation patients. the problem is that i am more of a mechanics person so the electronics are kind of worrying me! i took this project because it winteresting and (maybe too much) challenging.


the pedometer would have to measure number of steps per day + force applied on the crutch evolution through time. these would need to be stored in some kind of memory device for about 2 months.

so far I bought some accelerometers, ADXL330
and i have done the stress analysis and will order some strain gages to measure crutch strain while walking.
i will also have to buy a PIC (which one would you recommend?) and next years student will work on the memory device and output ports.

I now have to start testing the accelerometer and strain gages on amputation patiets to see what outputs they give me, so that the PIC can be programmed. you have any idea on what kind of testing circuits I would have to build for it?

after I know what signals they give me, I will have help of a technician to programme the PIC. what PIC would you advise me to buy? and how will the programming go? i know it is going to be a long iterative process.

finally, when i have the pic programmed, which is the better way of putting everything together? the "pedometer" would have to be hidden in the crutch for better diagnostical results.

i am very sorry to bother you but i love the fact that this project will actually make an impact in some peoples lives and the fact that i know nothing on electronics wasnt strong enough to make me chose another one.
i will love some help and for more information jt let me know and i wil provide you with it.


ps. any help on strain gages (for small displacements) would also be ry much appreciated

peace out

 

[Ubergeek63]

a load cell would be better...

hey guys! i am a final year student in biomedical engineering and my masters project involves a lot of electronics that i am not familiar with.

i have to build a cheap/strong step/force monitor device for thirld world amputation patients. the problem is that i am more of a mechanics person so the electronics are kind of worrying me! i took this project because it winteresting and (maybe too much) challenging.

Accelerometers will not help since they only measure speed and you have no way to reference that to anything. If you put the crutch down real slow you will get no output, but the weight of the user will still be there.

As to micro I would never recommend a PIC for a low cost volume app, they just are not economical in that setting.

If you want to put a "shock absorber" in the crutch you could put in a linear compression spring and measure the displacement easily. As to load cells I would have to ask my sources about that as I am not a mechy.

I know that pressure transducers that go for $65 in digikey are around $20 at 20K pcs. Based on that you would be looking at $20-$40 depending on the load cell range you needed from the same company.

 

[ronsimpson]

I am working on a similar project in that I need to know if a person is walking verses sitting and how fast they are walking. The last several times I built a home made accelerometer that involved a spring and weight. The finished product has a street price if $5 so I cannot use a $65 part. Do you want a picture of my very cheep sensor?

In my hand I have a vibration/motion sensor accelerometer that looks like a silver tube 0.1inch diameter by .5 inch long. I also have a very sensitive version that is 1 inch long. It is made in China for $.10 in high volume. My problem is I do not know who makes it. I am looking for the manufacturer now.

 

[Ubergeek63]

I am working on a similar project in that I need to know if a person is walking verses sitting and how fast they are walking. The last several times I built a home made accelerometer that involved a spring and weight. The finished product has a street price if $5 so I cannot use a $65 part. Do you want a picture of my very cheep sensor?

In my hand I have a vibration/motion sensor accelerometer that looks like a silver tube 0.1inch diameter by .5 inch long. I also have a very sensitive version that is 1 inch long. It is made in China for $.10 in high volume. My problem is I do not know who makes it. I am looking for the manufacturer now.

That would be a tilt sensor, not an accelerometer. The initial post mentions the force applied to a crutch, significantly more complicated than a simple pedometer.

 

[ronsimpson]

What I am using is not a tilt sensor. It will not respond to turning it upside down.
If it was in the foot of a crutch it will respond to the shock of striking the floor.

I thought about using a piezo cell in the foot of the crutch. It will respond every time the pressure changes on the crutch.

Msiusa.com Piezo Film Vibra Tab Depending on how you use it, it will respond to vibration, strain, movement and change in pressure. Msiusa has many different kinds of sensors. Pressure, force, The Vibra Tab is $0.30 at low volume.
https://www.electro-tech-online.com/custompdfs/2008/04/LDTOSensor.pdf
https://www.electro-tech-online.com/custompdfs/2008/04/PiezoInfo.pdf

FlexiForce Sensor 0volts=0 pounds, 4.2 volts =100 pounds.

 

[Ubergeek63]

What I am using is not a tilt sensor. It will not respond to turning it upside down.
If it was in the foot of a crutch it will respond to the shock of striking the floor.

I thought about using a piezo cell in the foot of the crutch. It will respond every time the pressure changes on the crutch.

Msiusa.com Piezo Film Vibra Tab Depending on how you use it, it will respond to vibration, strain, movement and change in pressure. Msiusa has many different kinds of sensors. Pressure, force, The Vibra Tab is $0.30 at low volume.
https://www.electro-tech-online.com/custompdfs/2008/04/LDTOSensor-1.pdf
https://www.electro-tech-online.com/custompdfs/2008/04/PiezoInfo-1.pdf

FlexiForce Sensor 0volts=0 pounds, 4.2 volts =100 pounds.

Nice... I was actually responding to your "silver tube"

How do you get 4.2V=100lb out of that app? All I see is an open circuit voltage and a total charge based on deflection.

BTW the prices I listed were for MS MPS300 series pressure transducers... I would not expect this app needed 5000PSI, the numbers were more for comparison.

 

[ronsimpson]

I see I was not very clear again.
There is a sensor “FlexiForce Sensor” that produces a voltage. 0 to 4.2 volts with 0 to 100 pounds.

The silver tube responds to shock and vibration with 0 ohms. No vibration = open.

The piezo produces a voltage on change of shape.

I think the idea is to record how much, how hard, and how often a person uses the crutches. The FlexiForce Sensor can record with some accuracy how much weight is applied to the crutch. The piezo sensor can get a very crude idea of pressure. The vibration sensor can only tell if the crutch is in use. A tilt sensor might also tell if the device is in use (moving). I believe the idea is to inform the doctor that the person actually walked for 2 hours a day. This is a big problem in rehab!