Pressure/Force Sensor-How to get it to work for me

Discussion in 'The Projects Forum' started by NewGuy, May 7, 2006.

  1. NewGuy

    Thread Starter Member

    May 7, 2006
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    I recently started a personal project that has panned out very well for me. The upgrade to this project is a redesign that will have no mechanical moving parts. This is where my problem lays. The moving parts I'm refering to are in the form of an on/off switch. I want to move to a "Ribbon-Type" of "Force Sensor" that I've found online.(FlexiForce) My problem is that my strengths lay on the mechanical end of this project. I'm afraid I'm Electronically challanged. My initial design was basically a battery source with Power and Ground going to the small electrical gadget that was turned on and off with a mechanical switch. The "Force Sensor" I'm wanting to upgrade to is a specialty sensor that can be calibrated to activate my small electrical gadget when it senses a specific force of anywhere between 0-100 pounds. I need to make up a circuit that will make this "Force Sensor" activate my small electrical gadget when it senses a force of 50 pounds or more. If there is less force or no force being exerted on the sensor, the small electrical gadget will remain in the off mode. At present, the dimensions of this project are exactly 1"x1"x1.5". Any electrical circuit that I use will have to be equally as small. Because I'm a new guy to this forum, I'm not really sure how much detail in need to convey here in order to get a good answer. I'm hoping that this short initial posting will solicit some insight on that as well. If need be, I can share a schematic emailed to me by my contact person at the website where I came across this "Force Sensor;" just to give an idea of what I'm looking for. I've been told that the type of circuit I'm looking for is relatively simple but, as I stated above, my strength lay in the mechanics of this project, not electronics. I'm hoping someone out there has the know how and the will. Look forward to hearing from you.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    It's quite a challenge. The sensor you're trying to use is piezoelectric, and only has a significant voltage output while the applied force is changing - i.e., there is no output after the 50 pounds force is resting on the tab. It only responds as the force is changing.

    It might be possible to integrate to sensor's output over time to see if enough change has occured to correspond to 50 pounds force applied. One problem is that the magnitude of the output is dependant on the rate of change, so 50 pounds applied over 10 seconds would look different from 50 pounds applied in 50 milliseconds.

    You might do better with a small airtight cell and a pressure gauge sensor.They do have continuous outputs, and the calibration to trip the circuit at 50 pounds force is no problem. Don't know about your space requirements, though.
     
  3. n9352527

    AAC Fanatic!

    Oct 14, 2005
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    I thought FlexiForce was a 1/R proportional and not a dV/dT pressure sensor? Maybe NewGuy could give us more info or a link to the sensor data?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    According to their web site (the only part I found was A201), the sensor is piezoresistive, not piezoelectric. The conductance appears to be a (nearly) linear function of force. NewGuy, what power supply(s) do you have available? What kind of load (voltage, current, inductive, etc.) do you want to switch?
     
  5. NewGuy

    Thread Starter Member

    May 7, 2006
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    Greetings to all of you that responded to my delema and please forgive me for not replying to you all individually. The small electrical gadget that will be activated by this Force Sensor is powered by a stack of three L1154 Button Cell batteries. It is a very low power set up but, for its intended use it is more than sufficient. With the exception of setting up my initial circuit which included these batteries, an on/off switch, power/ground wires and the small electrical gadget, I'm afraid that cover the extent of my knowledge about electrical circuitry. Even the "Electronees Language" that you all so eloquently use in your replies is generally foreign to me. As far as Electrical Schematics are concerned, they may as well be Egytian Hirogliphs. I have a hard time making any sense of them. You can't imagine the frustration of having knowledgable individuals attempting to help and not even knowing enough about electrical circuitry to give pertinant information or ask the right questions. If you are willing, perhaps you can re-word your responses as if they were directed to a 2 or 3 year old.
    Voltage, Current, Ohms; I have only the slightest knowledge of the meanings of these words. Piezoelectric, Inductive, Piexoresistive; way over my head. You could be cussing me out in another language with these words and I wouldn't know it.
    Please don't give up on me just yet. I consider it a big step forward in solving this delema just by having you all reply to my initial posting. Short of divulging the application of this project, tell me specifically, and in 2 or 3 year old english, what information I can give you that will help you to better understand exactly the kind of circuitry I'm needing.
    The exact sensor I'd be using is not listed in the product line of the "FlexiForce" website. It is a custom sensor specifically designed for one of their established customers. It would most closely resemble the "Trimable Sensor" on the following web page: http://www.tekscan.com/flexiforce/OEM.html.
    The only Schematic I can direct you to with regard to the use of this type of sensor can be found on this web page: http://www.tekscan.com/flexiforce/specs_elf.html
    I have one more Schematic that was hand drawn by myself and the contact person at this website. it differs slightly from the one on their website because I'm the one that drew most of it. The remainder of this Schematic was filled in by the FlexiForce contact person.
    If need be, I can upload it to my personal web-space and give any interested person that web-address to view this hand drawn schematic.
    Thank you all again for you responses. I look forward to hearing from you again.
    I'll check back again tomorrow.
    Also; one of you mentioned another type of pressure sensor in your responses that sounded intriguing. Could you clarify?
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    Does it need to toggle (push-on-push-off), or is it momentary contact (only on while being pushed and off while not being pushed)?

    Do you plan to build this (these) yourself, or have someone else make it (them)? (Can a higher level of skill than yours be applied to assembly?)

    Does it absolutely have to sense pressure (something pushing on it) or can it work on some other principle? Other principles might be body heat, capacitance, or a change in lighting resulting from a finger placed over a small window.
     
  7. NewGuy

    Thread Starter Member

    May 7, 2006
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    Greetings thingmaker3;
    And thanks for your continued interest in my ongoing delema. To answer your first question, presently, the mechanical switch I'm using is one that toggles on when I put pressure on the switch and off when there is no pressure on the switch.
    What I'm wanting to upgrade to will give me the same results except that the switch will no longer be a mechanical one; it will be an electrical one. As you have stated, it will "only be on while being pushed and off while not being pushed." The exception is that this Sensor/Switch would be calibrated to turn On only when it senses a pressure amount of 50 pounds or more; and Off when anything less than 50 pounds is sensed.
    Because this little device is designed in conjunction with a type of physical activity, (outdoor sports) it absolutely has to sense when something is pushing on it. The other principles you've listed would not be applicable for this particular application.
    These little gizmos that I'm eluding to are already being built by me. They are really quite simple to assemble once I've milled the parts. It really doesn't require "A higher level of skill" to assemble them. My delema is that I want to take it to the next level; the "New And Improved" next level. This means doing away with all mechanical/moving parts and using electrical components to turn this gizmo on and off.
    Right now, I lack the electrical circuitry knowhow to figure this out on my own. I suppose if I continue my R&D I will eventially come up with a solution on my own but, who knows how long that will take. Hence, my reason for posting these inquiries on this forum.
    I'm greatful for your input. I know it may sound stupid but even when no truley useful advice is given, just the fact that someone took a little interest in your delema helps a person not to loose heart and give up all together.
    I noticed that your user name is thingmaker. I take that to mean that you've dreamed some dreams and made some things just like me and countless other people. During the time you were building your things, no doubt, you've encountered some roadblocks on the way to acheiving your dreams. Well, that's where I'm at right now with this Electrical On/Off Switch delema.
    I welcome the advice of those who know more about this aspect of building things than I do. Although the advice may not be particularly applicable, the fact that someone has taken just a little time to comment goes a long way. Sometimes our brains already contain the answers were looking for; and it only takes another thingmaker to jarr them loose.
    Thanks very much for your reply. Any other advice, questions or comments will be further appreciated. Hope to hear from you or anyone else that can offer some input. I'll check back again tomorrow.
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Does it matter to you which side of the battery (plus or minus) connects to your electronic switch? If so, which side? Switching the negative terminal will be slightly less expensive, but either can be done.
     
  9. NewGuy

    Thread Starter Member

    May 7, 2006
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    Greetings Ron;
    I'm afraid my lack of knowledge about this type of circuitry keeps me from following your train of thought regarding which side of the battery connects to what; and for what reason. Right now, with my mechanical switch in place, I can connect positive or negative wires to either end of my power source/batteries and the small electrical gizmo will still function as designed.
    I'm not sure if this is anywhere close to the answer you were looking for but, sorry, that's the best I can do.
    I took the liberty of uploading a hand drawn schematic to my personal webspace. (http://www.members.cox.net/tmontano/sensor.jpg) This schematic is the same one I sent to my contact person at the FlexiForce website along with an attatched note requesting his input on the placement of the circuitry in question.
    In his reply, he sent back the same schematic with the circuitry placed on the positive wire on the output side of the sensor just before it connects to my small electrical gizmo. I tried to upload that reply to my webspace but for some reason it wouldn't go through. I'll have to try it again later.
    If you have the time, perhaps you can check it out and give me some input as well. You state that, "It may be slightly less expensive if it connects to the negative terminal." It almost sounds as though you may have a solution to my delema. Or, at least some ideas. Hopefully I'm correct in my assumption. I'd like to get this problem solved so that I can move forward with this project upgrade.
    Thanks very much for your courteous reply. I'll check in again tomorrow to see if maybe I'm a little closer to an answer to this delema.
    Many thanks.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Do you have any idea what the resistance of your sensor is when 50lbs is applied? Since you apparently don't have a datasheet, you will have to measure it. Do you have an ohmmeter? I do think the solution is fairly simple, but knowing the resistance, at least approximately, will help a lot.
     
  11. NewGuy

    Thread Starter Member

    May 7, 2006
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    Hello again Ron.
    I do have an Ohm meter but, right now what I don't have is a sample of the sensor I'm wanting to use. I've been putting off ordering it because I figured at this stage it would do me little good to buy a sensor that, although in theory should work as I had hoped, due to my ingnorance in regard to Electronics in general, I couldn't get it to work for me.
    Ordering one is not a problem. It will just take a few days to get to me. May I ask what part the "Resistance" plays in whatever you have in mind? I've been told by my contact person/sales person at the FlexiForce website that the circuitry for solving this delema is relatively simple for someone that knows about these things.
    I more or less took that with a grain of salt because I know how some salespeople can be sometimes just to make a sale; and because I also know that the word, "Simple" is a relative term. It may be an easy fix for someone with the knowhow but, for someone like myself who doesn't know how, it's not so simple.
    I'll contact him again before ordering. Perhaps they already have that "resistance" information available. Either way, eventually I'm going to have to get hold of a sample in order to make up a working prototype of my upgraded gizmo.
    Thanks again Ron for your interest and help. I'll check back here again tomorrow. Hopfully I'll have some usful info about the "resistance" you asked about.
     
  12. Spoggles

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    Dec 2, 2005
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  13. Ron H

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    Apr 14, 2005
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    I stumbled across the same chart originally, but he says the actual part he will use does not have a datasheet on the Internet.
    Also - you read the chart incorrectly. The resistance at 50lbs is actually about 120k. You were looking at the conductance line and reading its value on the wrong axis.
     
  14. Ron H

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    Apr 14, 2005
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    Resistance is the electrical parameter that changes as a function of force, so you need to measure it. It would be good to have at least 3 data points, say 0 lbs, 50 lbs, and 100 lbs.
     
  15. NewGuy

    Thread Starter Member

    May 7, 2006
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    Hi Ron:

    I contacted the person I've been corresponding with at the FlexiForce website and was able to get some information that may or may not be of some help. I say "May or May Not" because it May or May Not be a little different from the information they have listed on their website. Following is a quote from his reply:


    "As for the resistance level of the sensor when 50lbs is applied, that is hard to give a definitive answer. Our sensors start with an unloaded resistance of around 10-20M Ohm (essentially an open circuit), and when force is applied the resistance decreases. The tricky part is that how much the resistance decreases depends on a lot of things: first- what force rating the sensor is. A “50 lb sensor” as specified by us will “saturate” at 50lbs at around 40K Ohms, but this is given our electronics and test procedures, you could easily manipulate this by using a larger/smaller rated force sensor and different electronics. The typical resistance curve for a 0-100lb sensor is attached. Also, there is a part to part variation associated with our manufacturing process so no 2 sensors will saturate at EXACTLY 40K Ohms, in fact it is more likely the saturation will range from ~24K-56K Ohm depending on what sensor you sample from the batch (+/- 40%). This part-to-part variation, however will be specified by you so if you want it tightened that is possible as well as loosened- most customers leave it at +/-40% to secure the best price and simply leave enough adjustment in their electronics to calibrate this difference out."

    I believe that within the above paragraph are some of the answers to your previous inquiries/posts. This should give you a little food for thought. I wish I could figure out how to upload the graph he emailed me along with his reply. I'm not sure if it's the same chart that you're refering to in you comment to Spoggles.
    It is good to know that there are others showing some interest in this ongoing dilemma of mine.
    Just for the sake of my own understanding, can you tell me what the purpose of having the 3 data points you've mentioned? And, how does that play into the final configuration of any Circuit that might be applied to this project? Can it be explained in laymans terms? Remember, you're dealing with an Electronically-Challanged individual.

    Many thanks for your gracious input. I look forward to reading any further information you might feel compelled to share. I also want to thank Spoggles for his/her interest as well. I'm sorry for not addressing you both individually but, It seems that since both of you have been kind enough to collectively read my posts, then anyone else that might happen to read them will accept my replies as having been written to all of you as well.
    I'll check back again tomorrow. Every day that passes bring me closer to my goal of finding the answer to this circuitry dilemma.


     
     
  16. NewGuy

    Thread Starter Member

    May 7, 2006
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    Hi Ron:

    I contacted the person I've been corresponding with at the FlexiForce website and was able to get some information that may or may not be of some help. I say "May or May Not" because it May or May Not be a little from the information they have listed on their website. Following is a quote from his reply:


    "As for the resistance level of the sensor when 50lbs is applied, that is hard to give a definitive answer. Our sensors start with an unloaded resistance of around 10-20M Ohm (essentially an open circuit), and when force is applied the resistance decreases. The tricky part is that how much the resistance decreases depends on a lot of things: first- what force rating the sensor is. A “50 lb sensor” as specified by us will “saturate” at 50lbs at around 40K Ohms, but this is given our electronics and test procedures, you could easily manipulate this by using a larger/smaller rated force sensor and different electronics. The typical resistance curve for a 0-100lb sensor is attached. Also, there is a part to part variation associated with our manufacturing process so no 2 sensors will saturate at EXACTLY 40K Ohms, in fact it is more likely the saturation will range from ~24K-56K Ohm depending on what sensor you sample from the batch (+/- 40%). This part-to-part variation, however will be specified by you so if you want it tightened that is possible as well as loosened- most customers leave it at +/-40% to secure the best price and simply leave enough adjustment in their electronics to calibrate this difference out."

    I believe that within the above paragraph are some of the answers to your previous inquiries/posts. This should give you a little food for thought. I wish I could figure out how to upload the graph he emailed me along with his reply. I'm not sure if it's the same chart that you're refering to in you comment to Spoggles.
    It is good to know that there are others showing some interest in this ongoing dilemma of mine.
    Just for the sake of my own understanding, can you tell me what the purpose of having the 3 data points you've mentioned? And, how does that play into the final configuration of any Circuit that might be applied to this project? Can it be explained in laymans terms? Remember, you're dealing with an Electronically-Challanged individual.

    Many thanks for your gracious input. I look forward to reading any further information you might feel compelled to share. I also want to thank Spoggles for his/her interest as well. I'm sorry for not addressing you both individually but, It seems that since both of you have been kind enough to collectively read my posts, then anyone else that might happen to read them will accept my replies as having been written to all of you as well.
    I'll check back again tomorrow. Every day that passes bring me closer to my goal of finding the answer to this circuitry dilemma.


     
     
  17. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I just thought that 3 datapoints would give me some sense of the sensitivity of the part. Stand by.
     
  18. thingmaker3

    Retired Moderator

    May 16, 2005
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    NewGuy - I'm about to launch into "geek speak." I'm "thinking out loud" more to contribute to ideas than to give any step-by-step instructions. The best English translation would be "yeah, pretty simple." You'll need to learn how to solder, if you don't already.

    Looks like one of these sensors could be used in a voltage divider, and the divider o/p fed to a comparator. O/P from comparator could drive mosfet switch.

    Variance among sensors could be compensated by using a trim pot as the other half of the divider. Adjustment could be made with (for example) a 45 and 55 pound mass on the sensor.
     
  19. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Thingmaker and I think alike. Below is my first cut at a workable circuit, not knowing the specs on your sensor. You might be able to reduce the value of the pot (R2) once you know the range of resistance that represents 50lbs.
     
  20. NewGuy

    Thread Starter Member

    May 7, 2006
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    Greetings thingmaker3:

    I'm about to launch into a display shear ingnorance by asking; what does all that mean?
    With the exception of those parts where you refer to learning how to solder and "yea,pretty simple," I don't have a clue what you just said. It Sure Sounds promising though.
    (O/P, Mofet Switch, Comparator, Voltage Divider) These are electrical components I've never heard of.
    This item called a Trim Pot; if one is used, can it be used as a type of manual adjustment by the end user of this little electrical gizmo to adjust the sensitivity of the force sensor to a specific force bewteen 0 to 100 pounds?
    I realize you're just putting forth ideas here with this reply and all but, even with my very,very, very limited knowledge about such things, they come across as plausable solutions though, even to someone like me.
    Just for the sake of argument, lets say we used all or most of the electrical components you've just listed. How much space will a circuit consisting of such components take up? Can they be assembled in a "Micro-Circuitry" type of configuration? What would be a ball park price of the individual components you've listed?
    Again, I know and appreciate that you are just thinking out loud and throwing ideas into this conversation.
    The ideas you've put forth are ideas that I never in a million years could have come up with myself. I don't want to come across sounding sarcastic or ungreatful. My reason for asking the questions that I've put forth are two fold. First is the economics of this project and the fact that I'm limited in regard to the amount of space I can take up with this circuitry. the second is that I feel like a ignorant self-serving shmuck if I don't contribute anything to this project forum; especially since I'm the one asking for the help.
    Can you recommend a website with some kind of Electronics Glossary where I might be able to look up the meanings and applications of some of the components you mentioned? I'd like to be just a little bit smarter the next time I log onto this site if I can.
    In the mean time, I hope that nothing I've written in any of my replies comes across in such a way that drives folks away instead of inviting them to contribute; or, continue to contribute.

    Mothers day tomorrow. Obligations and all. I'll try to log on again tomorrow evening.
    Thanks very much for your help thingmaker3. You're help is appreciated.
    There's a saying in the hispanic community that one says to a person like yourself and the other good people that have contributed to the solution of my ongoing dilemma.
    "Dios te lo paga."
     
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