Pressure sensor design

Thread Starter

samjesse

Joined Sep 14, 2008
212
Hello

I need help in designing a piezo based devise which is portable and can be mounted on a bicycle paddle so that when pushed against by the user foot, can read up to 1000N.
So I was reading about sensor stiffness and the like but comments from someone with previous experience would be much appreciated.
I am thinking that it needs to be wide so as to collect the user push force from whole foot push area (considering a shoe without full flat sole).
Once I get the voltage reading, I can write a code to convert to force in N. How about the actual body where the sensor will be? I am a bit overwhelmed and thankful for any pointers or suggestions.
 

Reloadron

Joined Jan 15, 2015
5,104
There are a few sensors to consider, the simple solution would likely be a strain gauge. The first problem is you have a pedal and wish to measure the force exerted on the pedal. Unfortunately the pedal is rotating which gets to not just measuring the force but getting the force output signal to a stationary signal conditioner. You will need a rotary slip ring connector of sorts or a wireless torque transducer with the latter being an expensive device. Either way, it will not be easy or inexpensive. Hopefully another member has a suggestion I am overlooking.

Using a strain gauge this is a good place to start. I say a good place to start because Micro Measurements, now a division of Vishay had quite a bit knowledge on their website which applies to a wide range of strain gauges. You want to start with applications and products. The strain gauge becomes part of a bridge, a Google of Strain gauge bridge circuits will get you started with the bridge. Once you have a bridge you need signal conditioning which is build or buy. Here is an example of a strain gauge amplifier which has a data out rather than an analog voltage proportional to the strain.This is a good read on Bridge Amplifier Systems from Texas Instruments.

Using a strain gauge is a one time affair as once it is mounted it is permanent. This will not give you portability but a single strain gauge would be placed on each system and the conditioning and data recording moved. Using a rotary torque transducer is much, much more permanent.

Again, hopefully another member will have more thoughts and less expensive solutions.

Ron
 

joeyd999

Joined Jun 6, 2011
4,326
I need help in designing a piezo based devise which is portable and can be mounted on a bicycle paddle so that when pushed against by the user foot, can read up to 1000N.
I assume you are trying to compute power expended by the rider? Wouldn't a torque sensor be more appropriate (Nxm)? Add RPM measurement, then you have Power = Torque * RPM (units adjusted, of course).
 
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wayneh

Joined Sep 9, 2010
16,229
Why not reverse engineer the many commercial products that have already been designed and reviewed for this application? You should have a thorough knowledge of what's already been tried before setting off to design something new.
 

Thread Starter

samjesse

Joined Sep 14, 2008
212
A strain gauge needs a power supply, wouldn't a piezo type sensor work without a battery since it produces its own voltage in response to the applied pressure?, The paddle is stationary and pushes against a shock absorber and does not rotate sorry for misleading you as originally described, it is a home build thing which has a shock absorber that a paddle pushes against, the stroke distance and forces is variable and I need to measure the force exerted on the paddle while pressing.
 
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wayneh

Joined Sep 9, 2010
16,229
A strain gauge needs a power supply, wouldn't a piezo type sensor work without a battery since it produces its own voltage in response to the applied pressure?, The paddle is stationary and pushes against a shock absorber and does not rotate sorry for misleading you as originally described, it is a home build thing.
No, the piezo won't be useful without a battery to run the circuit that receives and conditions the signal. It makes only tiny amounts of power. In theory you could harvest this and once enough is collected, run a circuit off it, but I suspect that is not really your goal.

I think any design will be challenged to ensure all the force applied to the pedal is focused onto the sensor uniformly so that the reading is meaningful. My limited reading on it is that there are several solutions out there with varying pros and cons. That tells me it's a difficult problem without a single accepted solution.

Have you seen the great book, Bicycling Science? I think you'd find it worthwhile for the chapter on this topic.
 

wayneh

Joined Sep 9, 2010
16,229
Uniform and tangential to the rotation. This is why I suggested a torque sensor.
There are commercial torque sensors for this purpose, but they can only measure the resultant force on the crank. There are also lateral (ie. wasted) forces applied by the rider, potentially in every direction. The sensor strategy you need depends on the questions you're asking.

Some reading on the topic.
http://www.mdpi.com/1424-8220/14/12/22921
Measuring Pedal Forces - Springer
 
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Reloadron

Joined Jan 15, 2015
5,104
Piezo sensors are popular for detecting shock and vibration, such as engine knock in an automotive engine or excessive vibration in a jet engine as well as other shock and vibration applications. They are not force sensors in that they do not maintain an output after an initial shock or force is applied. You can read about the sensors and needed signal conditioning here but a piezo sensor is not what you want.

If you place a strain gauge on a pedal shaft you can build or buy a bridge circuit to place around it. A strain gauge Wheatstone bridge. Then build a strain gauge amplifier or buy one. A bicycle uses a crank arm with a pedal on one end and generally a sprocket on the other end. Well I call it a crank arm. If I lock the sprocket and the sprocket is 18 inches from center sprocket to the pedal shaft center (Sorry I am in the US so you get inches) and I hang a 2 Lb weight on my pedal shaft the applied force on the sprocket shaft will be the Distance * Weight of 36 LbIn or 3.0 LbFt of force. So if I know the distance from sprocket center to the pedal shaft and I hang a known weight out there I can do a dead weight calibration of the force and calibrate my strain gauge. This setup would give you the force exerted on the pedal.

You want 1,000 Newtons full scale so that would be about 224.8 pounds of force. So use the distance as I mentioned and hang a weight using a strain gauge on the shaft at midpoint.

Another option is a rotary torque sensor which we won't get any further into because for a home brew project they are expensive. I mentioned their use earlier.

Ron
 
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