fmgst@unix.cis.pitt.edu (Filip M Gieszczykiewicz) (07/12/90)
Greetings. I recently got an elecronic digital bathroom scale. Being the curious type, I would like to know how it works. When I took the cover off, and stepped on it, I could hear a "click" (from an on-off switch), and then a high pitched "whine" (from an oscilator of some type, I presume) Now the questions: 1) It does not use a load cell. It, reather, has a series of coils with a movable "plunger" that moves depending on the weight of something. (I presume it's cheaper) How does it work? 2) I read in Consumer Digest of a advertising scheme that some companies used in similar scales. Seems some claimed that THEIR scales could be used on carpets. But in the fine-print stated otherwise, WHY? Mine seems to work on a carpet but the directions specify a hard, level surface, why? 3) There is an adjustment pot for the end-user (on the fron panel) and another one on the pc board that's not accessible so easily, what is it for? Might as well post to the net, I'm sure others want to know... Take care and have fun (in that order :-) -- _______________________________________________________________________________ "The Force will be with you, always." It _is_ with me and has been for 10 years Filip Gieszczykiewicz "A man without a dream is like a fish without water." FMGST@PITTVMS or fmgst@unix.cis.pitt.edu "My ideas. ALL MINE!!"
charles@hpsdl216.sdd.hp.com (Charles Keith) (07/14/90)
In article <25815@unix.cis.pitt.edu>, fmgst@unix.cis.pitt.edu (Filip M Gieszczykiewicz) writes: questions: > > of coils with a movable "plunger" that moves depending > on the weight of something. (I presume it's cheaper) > > How does it work? > > 3) There is an adjustment pot for the end-user (on the > fron panel) and another one on the pc board that's not > accessible so easily, what is it for? > Thousands will tell you that this scale probably uses an lvdt (linear variable differential transformer) for sensing the position of the core. The springs in the scale make the cores position proportional to weight. I am following up because I have an interesting story from many years ago when these scales first came out. A friend of mine told me about another friend who really liked to drink beer, so much so that he kept a keg in his refrigerator at all times. But he never knew when the darn thing was going to run out (keeping track of how much he drunk was out of the question). I suggested that he get one of these scales, move the display to a convenient place, move the "on" microswitch to a convenient place, and adjust the zero offset pot (the "user accessible" pot mentioned above) and the scale would read out directly in pints remaining. Alternatively, the second pot ("span" or "calibration") could be used to make it read out glasses of any reasonable size. Simply push a button, get beer status. I was told that the "Beer-O-Meter" was a smashing success, a true triumph of modern technology, and a winner at any party. It was accurate down to a couple of glasses remaining. Cheers! ->Charles(have you hugged your Beer-O-Meter today?) Keith -- Charles Keith Hewlett-Packard Co. UUCP: {hplabs|ucsd|hpfcla}!hp-sdd!charles 16399 W. Bernardo Dr. Internet: charles%hp-sdd@hplabs.HP.COM San Diego, CA 92127 Disclaimer: I am not a qualified spokesperson.
bill@videovax.tv.tek.com (William K. McFadden) (07/17/90)
In article <25815@unix.cis.pitt.edu> fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) writes: > 2) I read in Consumer Digest of a advertising scheme that > some companies used in similar scales. Seems some > claimed that THEIR scales could be used on carpets. But > in the fine-print stated otherwise, WHY? Mine seems to > work on a carpet but the directions specify a hard, level > surface, why? It seems that since the carpet has a springy pad underneath, the scale will read too low. This is because the scale will only measure the compression of its internal spring, and some of that compression gets used up by the carpet. Now, if the scale had pointy feet on the bottom, it would have less carpet to compress, hence giving a more accurate reading. > 3) There is an adjustment pot for the end-user (on the > fron panel) and another one on the pc board that's not > accessible so easily, what is it for? The user pot is most likely the zero offset adjustment. This is how you set it to read zero when no one is on the scale. The internal pot probably adjusts the gain (the number you multiply by to get the weight). If it has enough range, you can probably calibrate it for either pounds or kilograms. My mom once had an electronic scale that was exactly the same as a mechanical one, except the dial had been replaced by an optical interrupter wheel that fed a digital counter. Talk about cheap! When it broke I told her to get a mechanical scale. It was equivalent to her electronic one and a whole lot cheaper! -- Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077 bill@videovax.tv.tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill Phone: (503) 627-6920 "The biggest difference between developing a missle component and a toy is the 'cost constraint.'" -- John Anderson, Engineer, TI
svoboda@motcid.UUCP (David Svoboda) (07/18/90)
bill@videovax.tv.tek.com (William K. McFadden): > > It seems that since the carpet has a springy pad underneath, the scale will > read too low. This is because the scale will only measure the compression of > its internal spring, and some of that compression gets used up by the carpet. > Now, if the scale had pointy feet on the bottom, it would have less carpet to > compress, hence giving a more accurate reading. > Wait a second. Think about this for a moment. What EXACTLY do you mean by "compression get used up by the carpet"? We are talking about forces here. As long as the bottom of the scale is not accelerating wrt the earth, the *force* imparted on the scale by the mass of the person is constant, regardless of what surface it rests upon. To be more specific, the force imparted is m(kg) * g(m/sec^2) = f (Newtons) eqn (1) Now, the compression upon a compressed spring is f = kx eqn (2), where f is in Newtons, x is compression in meters, and k is the spring constant in (Newtons/meter). For a stressed member, an an identical stress force is felt by each point or region on that member. That's why a rope with a thin spot breaks there; the thin spot feels the same stress as the rest of the rope, and the stress is not "used up" by any other portion of the rope. Likewise the region of stressed material, including the spring and carpet, feels a constant force throughout. Now, since the compression of the spring is proportional to the *force* on it (2), the reading on the scale will be correct regardless of the surface it rests on, since the reading is proportional to the spring compression. I do not submit a reason for the scale not to work on carpet; it has me puzzled. Dave Svoboda, Motorola CID, RTSG, Arlington Heights, IL uucp => {uunet|mcdchg|gatech|att}!motcid!svoboda internet => motcid!svoboda@chg.mcd.mot.com Health is merely the slowest possible rate at which one can die.
larry@kitty.UUCP (Larry Lippman) (07/18/90)
In article <4145@ash31.UUCP>, svoboda@motcid.UUCP (David Svoboda) writes: > > It seems that since the carpet has a springy pad underneath, the scale will > > read too low. This is because the scale will only measure compression of > > its internal spring, and some of that compression gets used up by carpet. > > Wait a second. Think about this for a moment. What EXACTLY do you mean by > "compression get used up by the carpet"? We are talking about forces here. > As long as the bottom of the scale is not accelerating wrt the earth, the > *force* imparted on the scale by the mass of the person is constant, > regardless of what surface it rests upon. > ... > I do not submit a reason for the scale not to work on carpet; it has me > puzzled. I can't remain silent any longer. The reason why some scales are inaccurate when used on a floor with carpet has nothing to do with "compression gets used up by carpet", as stated by the author of the first article. The author of the second article explained a rather basic point of physics to further illustrate why the explanation of the first author is incorrect, but still did not shed light on this age old mystery. Most household scales are made of rather flimsy metal and/or plastic. When such a flimsy scale is placed on a spongy surface, the scale housing will temporarily deform due to unequal stresses on the case of the scale. This temporary deformation and unequal stress will most likely cause an erroneous weight reading due to spurious stress and/or deformation imparted upon the spring mechanism and/or LVDT or load cell in the case of an electronic scale. Take a good look at a household scale sometime, place it on a carpet, and shift your weight around while watching a changing reading. Try the same thing on a hard, level floor. The reason for inaccurate readings on carpet should then be obvious. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" {boulder||decvax||rutgers||watmath}!acsu.buffalo.edu!kitty!larry VOICE: 716/688-1231 || FAX: 716/741-9635 {utzoo||uunet}!/ \aerion!larry
erk@americ.UUCP (Erick Parsons) (07/25/90)
>In article <5907@videovax.tv.tek.com> bill@videovax.tv.tek.com (William K. McFadden) writes: >In article <25815@unix.cis.pitt.edu> fmgst@unix.cis.pitt.edu (Filip Gieszczykiewicz) writes: >> 2) I read in Consumer Digest of a advertising scheme that >> some companies used in similar scales. Seems some >> claimed that THEIR scales could be used on carpets. But >> in the fine-print stated otherwise, WHY? Mine seems to >> work on a carpet but the directions specify a hard, level >> surface, why? >It seems that since the carpet has a springy pad underneath, the scale will >read too low. This is because the scale will only measure the compression of >its internal spring, and some of that compression gets used up by the carpet. >Now, if the scale had pointy feet on the bottom, it would have less carpet to >compress, hence giving a more accurate reading. More than likely it is the way all scales are designed or at least the majority of them anyway. Scales usually have 4 pivot points on them to collect weight info from the platform that you stand on. These pivot points are usually reducing in nature (similiar to the way a lever makes it easier to lift a boulder- but backwards :-). The pivot points are at each corner of the platform and are in themselves miniture scales. The trick in weighing is to sum the weight on the four corners and produce a consistant weight to (in the case of an electronic scale) a load cell. If the scale is not level there will be more force applied to 2 of the pivot points. In manufacturing bathroom scales the ratios of the individual pivot points cannot be cheaply calibrated to each other so there will almost always be a differant weight reading were the weight concentrated on any individual corner. When a scale is placed in an environment where it is possible for the level to change depending on weight distribution on the platform there will always be a differance in readings due to the mismatched corner calibrations. The only way to overcome this is to make sure that the scale is perfectly level at all times and insuring that weight distribution on the four corners is consistant. Commercial scales are overseen be the munincipalities that be (Weights & Measures) and are adjustable at the individual corners. The corners MUST be within .1% tolerance of the wieght applied on it. The test wieght is usually about 25% of the full scale rating of the platform. Those that don't will be repaired OR put out of service until the repairs have been performed. -- -------------------------------------------------//------------------------- (ames att sun)!pacbell! ----> sactoh0!pacengr!americ!erk Multitasking, ucbvax!ucdavis!csusac! --/ // Never leave uunet!msac! -/ Erick Parsons \Sacramento Ca Home Without it --------------------------------------------\X/----------------------------- Please Don't Wait...