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re: [SeattleRobotics] SMT toaster oven problems

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  • Robert Dyer
    Peter, Have you seen the Sparkfun tutorials on reflow soldering? Here s their one on paste stenciling. http://www.sparkfun.com/tutorials/58 It talks a lot
    Message 1 of 17 , Oct 24, 2012
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      Peter,

      Have you seen the Sparkfun tutorials on reflow soldering? Here's their one on paste stenciling. http://www.sparkfun.com/tutorials/58
      It talks a lot about solder paste and the aging issues from their perspective.

      I'm currently building a "skillet" version using a hot plate. Instructables has a great article on that. 
      http://www.instructables.com/id/Closing-the-Loop-on-Surface-Mount-Soldering/?ALLSTEPS

      I want to add an IR element that will move over the top on a track so I can use the skillet for only the preheat and the IR for the reflow.

      Robert


      From: "Peter Balch" <peterbalch@...>
      Sent: Wednesday, October 24, 2012 9:50 AM
      To: SeattleRobotics@yahoogroups.com
      Subject: [SeattleRobotics] SMT toaster oven problems


       

      Whe I first tried SMT soldering user a toaster oven it worked wonderfully.

      But now it doesn't. Most of the solder more-or-less melts and the components
      are stuck down but there's a lot of grey goo (i.e. solder paste) left on the
      pcb near to the pads.

      I'm pretty sure I 'm using the same I'm using the same temperature profile -
      it's the same oven and it doesn't have any sort of temperature control. It's
      the same solder paste. So what's going wrong?

      Am I seeing an effect of the solder paste ageing? I've kept it in the fridge
      as one is supposed to but it is now a few months old.

      Peter


    • Tony Mactutis
      Peter, I don t think the solder paste would go bad in a few months if it has been in the fridge. I ve used much older paste than that. I would make sure to
      Message 2 of 17 , Oct 24, 2012
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        Peter, I don't think the solder paste would go bad in a few months if it has been in the fridge.  I've used much older paste than that.

        I would make sure to leave the board in the oven until all the solder is melted.  Don't worry about the profile.  I just turn my oven up all the way (the dial goes up to 500 F) and wait for it to hit temperature.  I then put the board in and wait for the paste to melt.  When it has all melted (maybe 4 to 5 minutes), I give it perhaps 20-30 seconds and then take the board out to cool.  No timing required.

        There may be scattered beads when you are done; just use a soldering iron to clean them up.  All you usually have to do is touch them with the tip. 

        On 10/24/2012 9:50 AM, Peter Balch wrote:
         

        Whe I first tried SMT soldering user a toaster oven it worked wonderfully.

        But now it doesn't. Most of the solder more-or-less melts and the components
        are stuck down but there's a lot of grey goo (i.e. solder paste) left on the
        pcb near to the pads.

        I'm pretty sure I 'm using the same I'm using the same temperature profile -
        it's the same oven and it doesn't have any sort of temperature control. It's
        the same solder paste. So what's going wrong?

        Am I seeing an effect of the solder paste ageing? I've kept it in the fridge
        as one is supposed to but it is now a few months old.

        Peter


      • Randy Carter
        The aging factors in solder paste are the solvent in the flux evaporates and the solder corrodes. The solvent evaporating is slowed by chilling the
        Message 3 of 17 , Oct 25, 2012
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          The aging factors in solder paste are the solvent in the flux evaporates and the solder corrodes. The solvent evaporating is slowed by chilling the container. The corrosion of the solder is going to make it a little more difficult to solder. Requiring a little more heat and time. Also you may be using too much paste. It's difficult when applying it manually but stencils etc. are difficult to use and expensive for just 2 copies of a PCB.

          The process I use is to heat the board to 95C for 30 minutes to chase out any moisture that may have been absorbed by the components. (I don't have the facilities to reseal moisture packs.) Then I crank it all the way up and wait until 220C is reached then I turn off and open the door, letting the oven cool down. If I'm in a hurry I set a small fan to blow into one side of the oven and allow the heat to escape out the other side until I can pick up the boards with my hands.

          I do have a fair amount of solder balls stuck to the board that are easily knocked off with a pick. I also have a fair amount of solder bridges especially on the finer pitched IC's. Solder wick is my friend here.

          ----------------------------------------------------
          "What the detractors and critics of electric vehicles
          have been saying for years, is true. The electric
          vehicle is not for everybody, given the limited range
          it can only meet the needs of 90% of the population."

          Ed Begley Jr.
          ----------------------------------------------------

          ---------- Original Message ----------
          From: Tony Mactutis <tony@...>
          To: SeattleRobotics@yahoogroups.com
          Subject: Re: [SeattleRobotics] SMT toaster oven problems
          Date: Wed, 24 Oct 2012 21:15:17 -0700

          Peter, I don't think the solder paste would go bad in a few months if it has been in the fridge. I've used much older paste than that.

          I would make sure to leave the board in the oven until all the solder is melted. Don't worry about the profile. I just turn my oven up all the way (the dial goes up to 500 F) and wait for it to hit temperature. I then put the board in and wait for the paste to melt. When it has all melted (maybe 4 to 5 minutes), I give it perhaps 20-30 seconds and then take the board out to cool. No timing required.

          There may be scattered beads when you are done; just use a soldering iron to clean them up. All you usually have to do is touch them with the tip.

          On 10/24/2012 9:50 AM, Peter Balch wrote: Whe I first tried SMT soldering user a toaster oven it worked wonderfully.

          But now it doesn't. Most of the solder more-or-less melts and the components
          are stuck down but there's a lot of grey goo (i.e. solder paste) left on the
          pcb near to the pads.

          I'm pretty sure I 'm using the same I'm using the same temperature profile -
          it's the same oven and it doesn't have any sort of temperature control. It's
          the same solder paste. So what's going wrong?

          Am I seeing an effect of the solder paste ageing? I've kept it in the fridge
          as one is supposed to but it is now a few months old.

          Peter
        • Peter Balch
          From: Tony Mactutis ... That s good to know. ... Yes, that s how it used to be. But now there s too much grey goo. Same solder, same oven. And I think the
          Message 4 of 17 , Oct 25, 2012
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            From: "Tony Mactutis"
            > Peter, I don't think the solder paste would go bad in a few months if it
            > has been in the fridge. I've used much older paste than that.

            That's good to know.

            > There may be scattered beads when you are done;

            Yes, that's how it used to be. But now there's too much grey goo.

            Same solder, same oven. And I think the same profile.

            > I would make sure to leave the board in the oven until all the solder is
            > melted. Don't worry about the profile. I just turn my oven up all the
            > way (the dial goes up to 500 F) and wait for it to hit temperature. I
            > then put the board in and wait for the paste to melt. When it has all
            > melted (maybe 4 to 5 minutes), I give it perhaps 20-30 seconds and then
            > take the board out to cool. No timing required.

            That's pretty much what I do except that I _do_ worry about the profile.

            The oven has an aluminium tray that I sit the board on, a bottom element
            below the tray and a top element above the tray. I turn on the lower element
            for a couple of minutes to pre-heat the board. The aluminium tray nicely
            spreads the heat evenly. Then I turn on the upper element as well until the
            solder melts. Then leave it another 20 sec. Then turn off and open the door.

            It used to work just fine.


            From: "Randy Carter"
            > The aging factors in solder paste are the solvent in the flux evaporates
            > and the solder corrodes. The solvent evaporating is slowed by
            > chilling the container.

            It was sold as a 50ml syringe of solder. So it's always completely sealed
            apart from the very tip of the needle. I don't see how it can evaporate or
            oxidise other than _through_ the plastic barrel. Which is unlikely. Right?

            > The corrosion of the solder is going to make it a little more difficult to
            > solder. Requiring a little more heat and time.

            Well, I tried more time. All that happened was that I over-cooked some of
            the chips.

            (I can't turn the temperature up any further.)

            I wonder it I'm not ramping up the heat fast enough or I pre-heat for too
            long. Maybe the flux has all evaporated before I turn the oven up to
            solder-melting temperature. That way, the solder can flow together into a
            single blob.

            > Also you may be using too much paste.

            Could be but I don't think I'm using more that I used to.

            On the pads with too much paste, the solder used to ball-up. Now it doesn't.

            > The process I use is to heat the board to 95C for 30 minutes
            > to chase out any moisture

            I don't think that's a problem with the chips I've been using.

            We just got some accelerometer and RTC chips which came with big warnings
            about "use within 160 hours of opening the package". I haven't tried
            soldering them yet.

            > Then I crank it all the way up and wait until 220C is reached then I turn
            > off and open the door,

            Aha. So it's a quick up and down. Not a long pre-heat that might destroy the
            flux.

            > I do have a fair amount of solder balls stuck to the board
            > that are easily knocked off with a pick.

            Yes, I used to get those. As you say, they're no big deal.

            Peter
          • David Jason
            Hi,    I am a newbie. It might be very basic and typical Question :)   I have a 6 DOF Serial robotic arm manipulator (ED7220C). At the moment it is only
            Message 5 of 17 , Nov 1, 2012
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              Hi,
               
               I am a newbie. It might be very basic and typical Question :)
               
              I have a 6 DOF Serial robotic arm manipulator (ED7220C). At the moment it is only equipped with motor encoders. What I want to do is to let the gripper hold various kinds of objects (fragile objects like egg without breaking it and rigid objects like metallic). One way is to add Force sensors to the Gripper and then implement closed-loop feedback control system. What kind of sensors I need? Any clue (part no)? Would be happy to read your experiences? Secondly, is there more better and easy way of achieving my goal?
               
              Thanks for your help.
               
              Regards,
              DJ
              ____________________________________
               
               
            • Max Cato
              Hi DJ, I m by no means an expert on this, but I thought I d take a stab. For fragile stuff, I d recommend a piezoelectric or capactive sensor. A quick google
              Message 6 of 17 , Nov 1, 2012
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                Hi DJ,

                I'm by no means an expert on this, but I thought I'd take a stab. For fragile stuff, I'd recommend a piezoelectric or capactive sensor. A quick google search turned up this: http://www.trossenrobotics.com/c/robot-force-sensor-fsr.aspx which may or may not be what you're looking for, but could at least be a good starting point. 

                Hope this helps!

                -Sean


                From: David Jason <davidjason441@...>
                To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                Sent: Thursday, November 1, 2012 11:32 AM
                Subject: [SeattleRobotics] Gripping Fragile parts with Robotic Arm

                 
                Hi,
                 
                 I am a newbie. It might be very basic and typical Question :)
                 
                I have a 6 DOF Serial robotic arm manipulator (ED7220C). At the moment it is only equipped with motor encoders. What I want to do is to let the gripper hold various kinds of objects (fragile objects like egg without breaking it and rigid objects like metallic). One way is to add Force sensors to the Gripper and then implement closed-loop feedback control system. What kind of sensors I need? Any clue (part no)? Would be happy to read your experiences? Secondly, is there more better and easy way of achieving my goal?
                 
                Thanks for your help.
                 
                Regards,
                DJ
                ____________________________________
                 
                 


              • Peter Balch
                Hi DJ As no-one else has answered yet, I ll have a go. ... I d say there isn t really a standard part that people use. You ll be pretty much building your own.
                Message 7 of 17 , Nov 6, 2012
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                  Hi DJ

                  As no-one else has answered yet, I'll have a go.

                  > What kind of sensors I need? Any clue (part no)?

                  I'd say there isn't really a standard part that people use. You'll be pretty
                  much building your own.

                  In general, people line the inside of the gripper fingers with something
                  that measures pressure. If it were me, I'd look at the following:

                  QTC. QTC is good stuff but you buy it as little rubber "pills" whose
                  resistance goes down with pressure. But they are _really_ hard to work with.
                  In particular, you don't just get a pair of wires to solder to - you have to
                  somehow put a metal conductor on the surface of the pill. How? Glue it on
                  with conductive glue? I've never found a good answer.
                  http://www.peratech.com/qtcmaterial.php

                  Resistive sensors. These are paper-thin pads that measure pressure. They're
                  reasonably responsive (conductance proportional to pressure) but have
                  problems with noise, non-linearity and drift (and, it seems to me, have
                  strange reactance properties so your amplifier oscillated easily). They're a
                  bit expensive and hard to buy in small quantities.
                  http://www.tekscan.com/
                  Here's an article comparing different makes
                  http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&ved=0CEgQFjAB&url=http%3A%2F%2Fciteseerx.ist.psu.edu%2Fviewdoc%2Fdownload%3Fdoi%3D10.1.1.186.4191%26rep%3Drep1%26type%3Dpdf&ei=3chqUOGeH8mk0QW7oIDAAw&usg=AFQjCNGSTAi7Ud7WzivqRksz9JmMYQtwqw&sig2=dT37Z5O04J-EKeDX_8DSjA

                  Strain gauges: A possibility, if you're building a big robot arm, is to use
                  strain gauges. They're generally expensive devices and not often used by
                  hobbyists but, if you take apart an electronic bathroom scale (from a
                  charity shop?) you'll usually find four small good quality sensors in the
                  form of "pads". Kitchen scales can also be good - they usually contain one
                  beam sensor. You'll have to make your own precision amplifier (which isn't
                  as hard as it sounds).

                  Carbon impregnated foam. This is the really cheap and cheerful alternative
                  often used by hobbyists. Just put a sheet of tinfoil on either side of some
                  conductive foam (the sort electronic components often come packed in. There
                  are various grades of foam - some is spongy some is more brittle. The
                  disadvantage is that it only lasts for a few thousand operations before it
                  starts to deteriorate. And it's not particularly linear. And it has a lot of
                  noise. But for hobby robotics, it will do the job and it's really cheap.
                  https://www.google.co.uk/search?q=conductive+foam+sensor+images

                  There are other sensors you might consider that don't use resistance -
                  capacitative sensors for instance. I suspect they'd give better results than
                  many cheap resistive sensors but that's more of a research project.

                  Peter
                • jamericanfreddy
                  i have done some testing on sensors and capactive works better then piezo and sparkfun has a lots of them, also flex force sensors work great too
                  Message 8 of 17 , Nov 29, 2012
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                    i have done some testing on sensors and capactive works better then piezo and sparkfun has a lots of them,
                    also flex force sensors work great too

                    --- In SeattleRobotics@yahoogroups.com, Max Cato <maxsthekat@...> wrote:
                    >
                    > Hi DJ,
                    >
                    > I'm by no means an expert on this, but I thought I'd take a stab. For fragile stuff, I'd recommend a piezoelectric or capactive sensor. A quick google search turned up this: http://www.trossenrobotics.com/c/robot-force-sensor-fsr.aspx%c3%82%c2%a0which may or may not be what you're looking for, but could at least be a good starting point. 
                    >
                    > Hope this helps!
                    >
                    > -Sean
                    >
                    >
                    >
                    > ________________________________
                    > From: David Jason <davidjason441@...>
                    > To: "SeattleRobotics@yahoogroups.com" <SeattleRobotics@yahoogroups.com>
                    > Sent: Thursday, November 1, 2012 11:32 AM
                    > Subject: [SeattleRobotics] Gripping Fragile parts with Robotic Arm
                    >
                    >
                    >  
                    > Hi,
                    >  
                    >  I am a newbie. It might be very basic and typical Question :)
                    >  
                    > I have a 6 DOF Serial robotic arm manipulator (ED7220C). At the moment it is only equipped with motor encoders. What I want to do is to let the gripper hold various kinds of objects (fragile objects like egg without breaking it and rigid objects like metallic). One way is to add Force sensors to the Gripper and then implement closed-loop feedback control system. What kind of sensors I need? Any clue (part no)? Would be happy to read your experiences? Secondly, is there more better and easy way of achieving my goal?
                    >  
                    > Thanks for your help.
                    >  
                    > Regards,
                    > DJ
                    > ____________________________________
                    >  
                    >  
                    >
                  • Peter Balch
                    ... I can t see any capacitive force sensors there. Which are they? And, yes, piezo sensors tend to respond to rate of change of force whereas for a gripper
                    Message 9 of 17 , Nov 30, 2012
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                      > i have done some testing on sensors and capactive works better then piezo
                      > and sparkfun has a lots of them,

                      I can't see any capacitive force sensors there. Which are they?

                      And, yes, piezo sensors tend to respond to rate of change of force whereas
                      for a gripper one generally wants the static force.

                      Peter
                    • jamericanfreddy
                      it was at work while back when i was doing some testing for a digital scale ,i was in charge of design and test lab,mostly i do testing on sensors they give me
                      Message 10 of 17 , Dec 3, 2012
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                        it was at work while back when i was doing some testing for a digital scale ,i was in charge of design and test lab,mostly i do testing on sensors they give me and i build high precision in-house testers
                        they gave me piezo sensor and capacitive type of force sensors
                        if you go to sensors then capacitance you will see some chips
                        little of hard part is to make sensor ,from what i remember you need 2 copper plates and material in between them
                        sparkfun does have a lot of flex force sensors that might be much easy,i have some havent added to my arm design,i just ordered my metal lathe machine and milling machine to make my arm parts and more

                        --- In SeattleRobotics@yahoogroups.com, "Peter Balch" <peterbalch@...> wrote:
                        >
                        > > i have done some testing on sensors and capactive works better then piezo
                        > > and sparkfun has a lots of them,
                        >
                        > I can't see any capacitive force sensors there. Which are they?
                        >
                        > And, yes, piezo sensors tend to respond to rate of change of force whereas
                        > for a gripper one generally wants the static force.
                        >
                        > Peter
                        >
                      • Peter Balch
                        ... Thanks. I can see some touch sensor chips but not any capacitive force sensors or chips. I know that capacitive force sensors exist but I haven t found an
                        Message 11 of 17 , Dec 3, 2012
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                          > they gave me piezo sensor and capacitive type of force sensors
                          > if you go to [sparkfun ] sensors then capacitance you will see some chips

                          Thanks.

                          I can see some touch sensor chips but not any capacitive force sensors or
                          chips.

                          I know that capacitive force sensors exist but I haven't found an easy
                          supply yet.

                          Peter
                        • jamericanfreddy
                          Here is the page for one type ,they do have others https://www.sparkfun.com/products/7902 Now to make the sensor you need a 2 tiny copper plates and some soft
                          Message 12 of 17 , Dec 4, 2012
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                            Here is the page for one type ,they do have others
                            https://www.sparkfun.com/products/7902

                            Now to make the sensor you need a 2 tiny copper plates and some soft insulator material to go between them
                            Its been a long time since i work on that project,but since i got a full machine shop with every machine need,like a lathe and milling machine
                            Will be working on my hand design some time soon
                            I am thinking thin layer of open cell foam might work,can easy get it at home depot and one side is sticky,i think the copper needs to be facing each other,
                            ALSO there is circuits for capacitance to analog
                            FRED
                            --- In SeattleRobotics@yahoogroups.com, "Peter Balch" <peterbalch@...> wrote:
                            >
                            > > they gave me piezo sensor and capacitive type of force sensors
                            > > if you go to [sparkfun ] sensors then capacitance you will see some chips
                            >
                            > Thanks.
                            >
                            > I can see some touch sensor chips but not any capacitive force sensors or
                            > chips.
                            >
                            > I know that capacitive force sensors exist but I haven't found an easy
                            > supply yet.
                            >
                            > Peter
                            >
                          • jamericanfreddy
                            Do see they are touch sensor chips many circuits on the internet for capacitance to voltage and if you have a really good meter it has the circuit,its great to
                            Message 13 of 17 , Dec 4, 2012
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                              Do see they are touch sensor chips
                              many circuits on the internet for capacitance to voltage
                              and if you have a really good meter it has the circuit,its great to test the capacitance sensor first and then build the circuit around it
                              most like that chip i gave a page too the front end uses that type of circuit and then add a touch on/off circuit

                              --- In SeattleRobotics@yahoogroups.com, "Peter Balch" <peterbalch@...> wrote:
                              >
                              > > they gave me piezo sensor and capacitive type of force sensors
                              > > if you go to [sparkfun ] sensors then capacitance you will see some chips
                              >
                              > Thanks.
                              >
                              > I can see some touch sensor chips but not any capacitive force sensors or
                              > chips.
                              >
                              > I know that capacitive force sensors exist but I haven't found an easy
                              > supply yet.
                              >
                              > Peter
                              >
                            • jamericanfreddy
                              small search found this AD7150 capacitance to analog voltage chip with I2C buss,i think a 555 timer can work too
                              Message 14 of 17 , Dec 4, 2012
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                                small search found this AD7150 capacitance to analog voltage chip with I2C buss,i think a 555 timer can work too

                                --- In SeattleRobotics@yahoogroups.com, "jamericanfreddy" <jamericanfreddy@...> wrote:
                                >
                                > Do see they are touch sensor chips
                                > many circuits on the internet for capacitance to voltage
                                > and if you have a really good meter it has the circuit,its great to test the capacitance sensor first and then build the circuit around it
                                > most like that chip i gave a page too the front end uses that type of circuit and then add a touch on/off circuit
                                >
                                > --- In SeattleRobotics@yahoogroups.com, "Peter Balch" <peterbalch@> wrote:
                                > >
                                > > > they gave me piezo sensor and capacitive type of force sensors
                                > > > if you go to [sparkfun ] sensors then capacitance you will see some chips
                                > >
                                > > Thanks.
                                > >
                                > > I can see some touch sensor chips but not any capacitive force sensors or
                                > > chips.
                                > >
                                > > I know that capacitive force sensors exist but I haven't found an easy
                                > > supply yet.
                                > >
                                > > Peter
                                > >
                                >
                              • Peter Balch
                                ... Excellent. I hadn t spotted that chip. When I saw touch sensor I assumed it was on/off. I haven t got my head around the data sheet yet but it seems to
                                Message 15 of 17 , Dec 6, 2012
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                                  > Here is the page for one type ,they do have others
                                  > https://www.sparkfun.com/products/7902

                                  Excellent. I hadn't spotted that chip. When I saw "touch sensor" I assumed
                                  it was on/off.

                                  I haven't got my head around the data sheet yet but it seems to imply that
                                  maybe one should make a sensor with a capacitance change of less than 1pF
                                  over the force range you're interested in.

                                  For two plates,
                                  Capacitance = permittivity * area / distance.

                                  If we assume permittivity = 10pF/m then, for example, plates with an area of
                                  1 sq cm separated by 1mm give a capacitance of 1pF. (I think I've got all
                                  the zeros right!)

                                  Of course, for a force sensor, one would be looking for a _change_ of 1pF
                                  and I think the chip allows a "parasitic" capacitance of 40pF on top of that
                                  change.

                                  > Now to make the sensor you need a 2 tiny copper plates and
                                  > some soft insulator material to go between them

                                  It's a nice idea to make a home-made sensor. I'd be worried that the foam
                                  would degrade pretty quickly. Open-cell foams often can't take many crush
                                  cycles (although closed-cell neoprene isn't too bad). I wonder whether a
                                  silicone baking sheet between two copper-clad boards would do the job. One
                                  of the boards could have the chip on it to keep the sensor leads short and
                                  stable. I'm generally measuring a max of a hundred kilos but you may be
                                  trying to measure less force.

                                  The chips are not cheap but they could well be worth it if the completed
                                  sensor is accurate. I think I'll buy some.

                                  Peter
                                • jamericanfreddy
                                  I was over 5 years ago when a professor of physics that was the part owner of the company i worked for that ,design that sensor for a scale it measure up to
                                  Message 16 of 17 , Dec 7, 2012
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                                    I was over 5 years ago when a professor of physics that was the part owner of the company i worked for that ,design that sensor for a scale
                                    it measure up to 100 lbs

                                    SO be nice to make that sensor,i know we didnt use that chip
                                    it might used a 555 timer to get capacitance to voltage

                                    I am thinking small a circuit that used in most meters is what we need.

                                    --- In SeattleRobotics@yahoogroups.com, "Peter Balch" <peterbalch@...> wrote:
                                    >
                                    > > Here is the page for one type ,they do have others
                                    > > https://www.sparkfun.com/products/7902
                                    >
                                    > Excellent. I hadn't spotted that chip. When I saw "touch sensor" I assumed
                                    > it was on/off.
                                    >
                                    > I haven't got my head around the data sheet yet but it seems to imply that
                                    > maybe one should make a sensor with a capacitance change of less than 1pF
                                    > over the force range you're interested in.
                                    >
                                    > For two plates,
                                    > Capacitance = permittivity * area / distance.
                                    >
                                    > If we assume permittivity = 10pF/m then, for example, plates with an area of
                                    > 1 sq cm separated by 1mm give a capacitance of 1pF. (I think I've got all
                                    > the zeros right!)
                                    >
                                    > Of course, for a force sensor, one would be looking for a _change_ of 1pF
                                    > and I think the chip allows a "parasitic" capacitance of 40pF on top of that
                                    > change.
                                    >
                                    > > Now to make the sensor you need a 2 tiny copper plates and
                                    > > some soft insulator material to go between them
                                    >
                                    > It's a nice idea to make a home-made sensor. I'd be worried that the foam
                                    > would degrade pretty quickly. Open-cell foams often can't take many crush
                                    > cycles (although closed-cell neoprene isn't too bad). I wonder whether a
                                    > silicone baking sheet between two copper-clad boards would do the job. One
                                    > of the boards could have the chip on it to keep the sensor leads short and
                                    > stable. I'm generally measuring a max of a hundred kilos but you may be
                                    > trying to measure less force.
                                    >
                                    > The chips are not cheap but they could well be worth it if the completed
                                    > sensor is accurate. I think I'll buy some.
                                    >
                                    > Peter
                                    >
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