Force sensor

[hazzer123]

Part of a project i have, is to electronically detect when an asthma inhaler valve is fully compressed. Force exerted on the valve is created using a stepper motor connected to a ballscrew, which is in turn connected to an arm which pushes the top of the canister.

I can not seem to find a good suitable force sensor for the job. Has anyone any ideas where to get one from, or even maybe an idea for a new method of detection?

[sdk32285]

Hi
Is it strictly a function of force? or is it also a function of position (limit switches).

[Admin]

is to electronically detect when an asthma inhaler valve is fully compressed

It sounds like you want a limit switch, not a force sensor.

But if what you want is a force sensor . . . what forces are you expecting? What accuracy do you need? What is your budget?

check the ads on this page:
http://www.societyofrobots.com/sensors_forcetorque.shtml

Ive bought force/torque sensors from:
http://www.transducertechniques.com/
(and one other I cant remember at the moment)

You might also want to try:
http://www.sensotec.com/torque.asp

[robotvisionary]

Go to To get a swith I used for the construction of one of my robots.

Heres the way to get it working for your robot

http://www.gorobotics.net/Articles/Robots/How-to-Build-a-Simple-Robot-%11-Beetle-Robot/6/

Sorry about the website here is the correction.

Go tohttp://www.radioshack.com/product/index.jsp?productId=2049718&cpo get a swith I used for the construction of one of my robots.

Heres the way to get it working for your robot

http://www.gorobotics.net/Articles/Robots/How-to-Build-a-Simple-Robot-%11-Beetle-Robot/6/

[hazzer123]

Hi

i don't think its a limit switch i need, since the machine is measuring unknowns. Its goal is to measure how far the certain canister needs to be compressed before its internal spring is compressed. The machine is for qualtiy control, testing a sample of a batch of canisters.

Force is probably going to be around 200N max
Accuracy - not much needed.
Budget - around $200

Thanks

[Admin]

Force is probably going to be around 200N max
Accuracy - not much needed.
Budget - around $200

Oh thats easy then! Call up a few companies and ask what their cheapest item is that can handle those specs.

How do you plan to measure the sensor value?

I had to develop an amp circuit and ran that to a microcontroller ADC. My microcontroller then uploaded data by serial to hyperterminal to process in excel. You can also use LABview.

potentially useful for you:
http://www.societyofrobots.com/programming_data_logging.shtml
http://www.societyofrobots.com/sensors_interpret.shtml

[hazzer123]

Yeah thats what i'm planning to do - the ADC on the MCU.

But there are different types of force sensor, which would you recommend? Like are the peizoelectric ones good? or will it be strain gauge based?

Also, any good (preferably UK based) companies?

Thanks

[Admin]

I just remembered that the other force sensor I used is from Cooper Instruments.

But there are different types of force sensor, which would you recommend?

Your application is ultra simple, so Id just go with 'your cheapest load cell.' They will ask what your application is and choose for you, anyways.

[hazzer123]

I just rang a company called Omega and they pointed me to their cheapest load cell - http://www.omega.co.uk/ppt/pptsc.asp?ref=LCM302&Nav=pref03

This is nearly $400. Cooper instruments doesnt seem much cheaper... Any ideas for more sources of load cells?

Also, how would i be able to use this with a microcontroller? Are the voltage outputs large enough? Will an op-amp circuit raise the voltage enough?  The rating on the site is 2mV/V. What does this unit mean?

[Soeren]

Hi,

[...] Its goal is to measure how far[...]

So, now you want to measure distance ??   

If you could make a loose drawing or two of what you are planning, I think something much easier might pop up.

[Admin]

This is nearly $400. Cooper instruments doesnt seem much cheaper... Any ideas for more sources of load cells?

Keep looking, Im confident you can find one for under $200. If not, you should consider making your own. Since you do not require high precision and only one axis, it shouldnt be too hard.

Also, how would i be able to use this with a microcontroller? Are the voltage outputs large enough? Will an op-amp circuit raise the voltage enough?

I use an op-amp fed into my microcontroller ADC.

The rating on the site is 2mV/V. What does this unit mean?

That means the maximum output voltage for 1V supply power will be 2mV. So lets say you supply your load cell with 10V, it wont output more than 20mV. Using this knowledge you can calculate how much gain your op-amp needs for the microcontroller ADC to get a good reading.

You will also need to calibrate your sensor - plot weight vs microcontroller ADC output and develop an equation using excel. If you are lucky, your sensor will be linear (mine are), and it wont un-calibrate itself over time.

Oh, almost forget. Since your sensor will have a repetitive load, spec the sensor for 2x that expected load. This will prevent fatigue failure of your sensor.

[hazzer123]

hah great! I found a really cheap load cell at digikey. And also it wont need any amplifier since its output range is 0.5-4.5V.

http://dkc3.digikey.com/PDF/T072/P2119.pdf
http://www.meas-spec.com/myMeas/download/pdf/english/sensors/LoadCellFC2122.pdf

Do you think its the right one?

[Admin]

Wow only $55! Looks ok to me.

[sdk32285]

hah great! I found a really cheap load cell at digikey. And also it wont need any amplifier since its output range is 0.5-4.5V.

http://dkc3.digikey.com/PDF/T072/P2119.pdf
http://www.meas-spec.com/myMeas/download/pdf/english/sensors/LoadCellFC2122.pdf

Do you think its the right one?

Hi
I was actually looking at those last week.
Make sure to let us know how they work if you get one.
Thanks

[hazzer123]

OK I'm getting them ordered and i will give you a bit of feedback when they arrive.

So, now you want to measure distance ??   Wink

The distance measuring has been taken care of, since i use a stepper motor to drive the canister compressing mechanism. The number of steps is proportional to the distance pushed on the canister.