On Tuesday, in article
<1ji6729m0kad7ap5intj1tosf56phr122j@4ax.com>
nobody@spam.prevent.net "Anton Erasmus" wrote:
>On Mon, 22 May 2006 13:24:04 -0700, Tim Wescott <tim@seemywebsite.com>
>wrote:
>>Anton Erasmus wrote:
>>
....
>My volume is fairly low. Initially prototypes, and then 500 or so per
>year. Rolling my own seems to be the only option.
>
>>Some roll-your-own possibilities:
>>
>>Make a balance with an angle detector. The balance part is obvious, the
>>angle detector could be a potentiometer or an LVDT or a fine encoder.
>
>This looks like it can be quite simple witjout the need for complex or
>time consuming calibration.
....
>>Use a big, compliant spring with a potentiometer or encoder. This is
>>the same theory as a load cell, but it makes for an easier sensor design.
>
>A helix spring with a sliding potentiometer ? What sort of lengths /
>sizes of liding potentiometers are avialable ?
I would suggest searching for Penny and Giles, their conductive plastics
division makes all sorts of sliding pots, meant for machine position
reading. So if the paper tray already ahs some form of spring for the
paper of take up to move it could be attached to that or your mechanism.
--
Paul Carpenter | paul@pcserviceselectronics.co.uk
<http://www.pcserviceselectronics.co.uk/> PC Services
<http://www.gnuh8.org.uk/> GNU H8 & mailing list info
<http://www.badweb.org.uk/> For those web sites you hate
Low cost weight sensor ?
Started by ●May 22, 2006
Reply by ●May 23, 20062006-05-23
Reply by ●May 23, 20062006-05-23
On Tue, 23 May 2006 14:53:37 -0400, Spehro Pefhany <speffSNIP@interlogDOTyou.knowwhat> wrote: [Snipped]>>> >>>Whatever is done, this field might be peppered with patents. >> >>You mean that one will easily infringe some sort of patent ? >> >>I was hoping one would find cheap MEMS based devices or something >>that implements a cheap robust weight sensor. Units with strange >>gauges already mounted seem to be quite expensive. > >Strain gauges are even more expensive.. >:-) One writes strange things, when kids want attention. Regards Anton Erasmus
Reply by ●May 23, 20062006-05-23
On Tue, 23 May 2006 19:58:06 +0200, in comp.arch.embedded Anton Erasmus <nobody@spam.prevent.net> wrote:>On Mon, 22 May 2006 13:24:04 -0700, Tim Wescott <tim@seemywebsite.com> >wrote: > >>Anton Erasmus wrote: >> >>> Hi, >>> >>> I am looking for a low cost weight sensor. I am trying to provide a >>> indication of the amount of paper available. The empty container is >>> approx. 1kg, and approx. 10kg full of paper. It should be able to >>> handle industrial temperature range. A resolution of 1kg would be >>> acceptable. If I can get 100g resolution, it would be great. >>> I would prefer SPI or I2C interface, but analog out would be >>> acceptable - something that can directly interface with a MCU. >>> >>> Regards >>> Anton Erasmus >> >>What's your volumes, and what's your willingness to build your own? As >>Jim Stewart mentioned there are possibilities, but the only commercially >>available weight (or force) sensors that I know of are all stainless >>steel devices meant to go into an industrial setting and last forever >>while being sprayed with salt water and whacked with hammers. That's >>good for the guy who needs one and doesn't want to spend a lot of >>engineering time qualifying the sensor, but not too good for a product. > >My volume is fairly low. Initially prototypes, and then 500 or so per >year. Rolling my own seems to be the only option. > >>Some roll-your-own possibilities: >> >>Make a balance with an angle detector. The balance part is obvious, the >>angle detector could be a potentiometer or an LVDT or a fine encoder. > >This looks like it can be quite simple witjout the need for complex or >time consuming calibration. > >>Make your own load cell with your own LVDT or strain gauge. There will >>be qualification issues, but I suspect that most really inexpensive >>scales out there (like bathroom and postal scales) use this scheme. The >>nice thing is that LVDTs are simple in their 1st-order approximation, >>and a load cell can be as easy as some spring steel that's been laser >>cut in a certain way. Strain gauges can be harder to implement, but >>there may be some possibilities there, too. > >We have used this sort of setup, but for the current product I would >want something in the US$20 range. > >>Use a big, compliant spring with a potentiometer or encoder. This is >>the same theory as a load cell, but it makes for an easier sensor design. > >A helix spring with a sliding potentiometer ? What sort of lengths / >sizes of liding potentiometers are avialable ? > >Regards > Anton ErasmusDigikey have some precision linear pots by Panasonic IIRC, sort of 1% accuracy, quite cheap martin
Reply by ●May 23, 20062006-05-23
On Tue, 23 May 2006 23:45:01 +0200, in comp.arch.embedded martin griffith <mart_in_medina@yahoo.esXXX> wrote:>On Tue, 23 May 2006 19:58:06 +0200, in comp.arch.embedded Anton >Erasmus <nobody@spam.prevent.net> wrote: > >>On Mon, 22 May 2006 13:24:04 -0700, Tim Wescott <tim@seemywebsite.com> >>wrote: >>>>>Make your own load cell with your own LVDT or strain gauge. There will >>>be qualification issues, but I suspect that most really inexpensive >>>scales out there (like bathroom and postal scales) use this scheme. The >>>nice thing is that LVDTs are simple in their 1st-order approximation, >>>and a load cell can be as easy as some spring steel that's been laser >>>cut in a certain way. Strain gauges can be harder to implement, but >>>there may be some possibilities there, too. >> >>We have used this sort of setup, but for the current product I would >>want something in the US$20 range. >> >>>Use a big, compliant spring with a potentiometer or encoder. This is >>>the same theory as a load cell, but it makes for an easier sensor design. >> >>A helix spring with a sliding potentiometer ? What sort of lengths / >>sizes of liding potentiometers are avialable ? >> >>Regards >> Anton Erasmus >Digikey have some precision linear pots by Panasonic IIRC, sort of 1% >accuracy, quite cheap > > >martinhttp://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=10&M=EVA-W7LR04B34 martin
Reply by ●May 24, 20062006-05-24
On Tue, 23 May 2006 20:40:51 GMT, Joerg <notthisjoergsch@removethispacbell.net> wrote:>>??(someone else wrote): >>>Load cells would work, but will be quite expensive. >>>Springs, levers and opto or hall sensors would >>>be another possibility. >> I was hoping to not have to roll my own. > > Just an idea: A couple of years ago I bought fancy bathroom scales. Nice > glass design and four pillars. There must have been four pressure > sensors in the pillars since wires went there and there was neither any > mechanical cantilevering nor did this thing move down at all if I > stepped on it.My apologies if someone else has already described this approach. Some years back I pulled apar... er, "conducted an engineering analysis" on a used ElCheapo bathroom scale. It was a remarkably simple mechanism, consisting of an LED display, a photointerruptor, four springs, a small PC board with a few components, and a slotted encoder wheel with a gear driven by a moving flat bar. (The gear mechanism is similar to the one used in those squeeze-to-spin "sparking wheel" toys popular some time back) The top plate is "suspended" from the bottom by the four springs, and when a weight is placed on top it compresses the top and bottom together forcing the bar to move, rotating the encoder. All the circuit board has to do is count the photointerruptor pulses up and down and reset the counter when no weight is present. F=-Kx. Now, a bathroom scale doesn't have to meet commercial load-cell precision or truck-weight loads, but there maybe something in the concept that the OP can adapt. Another problem, depending on the load range, may be the suspension mechanism. A "sufficiently heavy" weight might require that it be connected to the full load through some kind of weight-reduction gearing or leverage in order to keep the sensor from being pushed past its limits ("crushed" <grin>). If needed, this will add to the cost and complexity. Good luck! Frank McKenney, McKenney Associates Richmond, Virginia / (804) 320-4887 Munged E-mail: frank uscore mckenney ayut minds pring dawt cahm (y'all) -- Anyone who is not genuinely addicted to the search for knowledge is unlikely to have the psychological energy to be a true scholar in any field. But in history this work clearly resembles more that of a detective than that of a scientist -- a search for and judgment of particular evidence rather than a repeatable experiment. The detective side of historical research needs skill, background, and intuition. -- Robert Conquest, "The Dragons of Expectation" --
Reply by ●May 24, 20062006-05-24
On Wed, 24 May 2006 14:39:00 GMT, Frnak McKenney <frnak@far.from.the.madding.crowd.com> wrote:>On Tue, 23 May 2006 20:40:51 GMT, Joerg <notthisjoergsch@removethispacbell.net> wrote: >>>??(someone else wrote): >>>>Load cells would work, but will be quite expensive. >>>>Springs, levers and opto or hall sensors would >>>>be another possibility. >>> I was hoping to not have to roll my own. >> >> Just an idea: A couple of years ago I bought fancy bathroom scales. Nice >> glass design and four pillars. There must have been four pressure >> sensors in the pillars since wires went there and there was neither any >> mechanical cantilevering nor did this thing move down at all if I >> stepped on it. > >My apologies if someone else has already described this approach. > >Some years back I pulled apar... er, "conducted an engineering >analysis" on a used ElCheapo bathroom scale. It was a remarkably >simple mechanism, consisting of an LED display, a photointerruptor, >four springs, a small PC board with a few components, and a slotted >encoder wheel with a gear driven by a moving flat bar. (The gear >mechanism is similar to the one used in those squeeze-to-spin >"sparking wheel" toys popular some time back) > >The top plate is "suspended" from the bottom by the four springs, >and when a weight is placed on top it compresses the top and bottom >together forcing the bar to move, rotating the encoder. All the >circuit board has to do is count the photointerruptor pulses up and >down and reset the counter when no weight is present. F=-Kx. > >Now, a bathroom scale doesn't have to meet commercial load-cell >precision or truck-weight loads, but there maybe something in the >concept that the OP can adapt. > >Another problem, depending on the load range, may be the suspension >mechanism. A "sufficiently heavy" weight might require that it be >connected to the full load through some kind of weight-reduction >gearing or leverage in order to keep the sensor from being pushed >past its limits ("crushed" <grin>). If needed, this will add to the >cost and complexity. >Thanks for all the ideas. It looks like all the weight measuring sensors uses some sort of spring form material which deflects under the weight. One then measures the amount of deflection, from which the weight cab be calculated. The less deflection one can accommodate, the more expensive the sensor. Have anyone used piezo electric material to measure weight ? Would a piezo speaker be suitable ? Regards Anton Erasmus
Reply by ●May 24, 20062006-05-24
Anton Erasmus wrote:> On Wed, 24 May 2006 14:39:00 GMT, Frnak McKenney > <frnak@far.from.the.madding.crowd.com> wrote: > > >>On Tue, 23 May 2006 20:40:51 GMT, Joerg <notthisjoergsch@removethispacbell.net> wrote: >> >>>>??(someone else wrote): >>>> >>>>>Load cells would work, but will be quite expensive. >>>>>Springs, levers and opto or hall sensors would >>>>>be another possibility. >>>> >>>>I was hoping to not have to roll my own. >>> >>>Just an idea: A couple of years ago I bought fancy bathroom scales. Nice >>>glass design and four pillars. There must have been four pressure >>>sensors in the pillars since wires went there and there was neither any >>>mechanical cantilevering nor did this thing move down at all if I >>>stepped on it. >> >>My apologies if someone else has already described this approach. >> >>Some years back I pulled apar... er, "conducted an engineering >>analysis" on a used ElCheapo bathroom scale. It was a remarkably >>simple mechanism, consisting of an LED display, a photointerruptor, >>four springs, a small PC board with a few components, and a slotted >>encoder wheel with a gear driven by a moving flat bar. (The gear >>mechanism is similar to the one used in those squeeze-to-spin >>"sparking wheel" toys popular some time back) >> >>The top plate is "suspended" from the bottom by the four springs, >>and when a weight is placed on top it compresses the top and bottom >>together forcing the bar to move, rotating the encoder. All the >>circuit board has to do is count the photointerruptor pulses up and >>down and reset the counter when no weight is present. F=-Kx. >> >>Now, a bathroom scale doesn't have to meet commercial load-cell >>precision or truck-weight loads, but there maybe something in the >>concept that the OP can adapt. >> >>Another problem, depending on the load range, may be the suspension >>mechanism. A "sufficiently heavy" weight might require that it be >>connected to the full load through some kind of weight-reduction >>gearing or leverage in order to keep the sensor from being pushed >>past its limits ("crushed" <grin>). If needed, this will add to the >>cost and complexity. >> > > > Thanks for all the ideas. It looks like all the weight measuring > sensors uses some sort of spring form material which deflects under > the weight. One then measures the amount of deflection, from which the > weight cab be calculated. The less deflection one can accommodate, the > more expensive the sensor. Have anyone used piezo electric material to > measure weight ? Would a piezo speaker be suitable ? > > Regards > Anton ErasmusJust a goofy thought... There are cheap accelerometers these days (airbags, etc.); what is their precision and resolution? If one secured the target mass to a platform connected to a calibrated solenoid, applied a known force and measured the acceleration, the mass is easily computed... Regards, Michael
Reply by ●May 24, 20062006-05-24
Anton Erasmus wrote:> Thanks for all the ideas. It looks like all the weight measuring > sensors uses some sort of spring form material which deflects under > the weight. One then measures the amount of deflection, from which the > weight cab be calculated. The less deflection one can accommodate, the > more expensive the sensor. Have anyone used piezo electric material to > measure weight ? Would a piezo speaker be suitable ?I had another idea you might find interesting. If the paper is neatly stacked, could you measure its dielectric constant with rf? I know that soil moisture and body fat can be inferred this way. You could avoid moving parts if you could make it work.
Reply by ●May 24, 20062006-05-24
Hello Anton,> ... Have anyone used piezo electric material to > measure weight ? ...Something like that must have been in the bathroom scales I described. Maybe you could just hop into a store, buy one and take a look. Regards, Joerg http://www.analogconsultants.com
Reply by ●May 25, 20062006-05-25
"Anton Erasmus" <nobody@spam.prevent.net> wrote in message news:42g97292vmtg52rbbu4negmk2nev5jaaui@4ax.com...> On Wed, 24 May 2006 14:39:00 GMT, Frnak McKenney > <frnak@far.from.the.madding.crowd.com> wrote: >[snip]> > Thanks for all the ideas. It looks like all the weight measuring > sensors uses some sort of spring form material which deflects under > the weight. One then measures the amount of deflection, from which the > weight cab be calculated. The less deflection one can accommodate, the > more expensive the sensor. Have anyone used piezo electric material to > measure weight ? Would a piezo speaker be suitable ?How about placing a piezo disk under the paper stack with the piezo wired as an electro-mechanical oscillator. The frequency of oscillation will depend on the load mass. Under excessive load the oscillator may not start though. Peter
