kuusama@etana.tut.fi (Kuusama Juha) (07/09/89)
I'd like to build a small (pocket-size) weight scale, with a range of about 1 to 20 kilograms and accuracy about 1%. If I get voltage, I can surely handle the rest. But what to use as the converter? The catch: I prefer the converter not to cost more than $20. -- Juha Kuusama, kuusama@korppi.tut.fi ( ...!mcvax!tut!korppi!kuusama)
jayer@hpcvia.CV.HP.COM (Jay R Reinhart) (07/11/89)
/ hpcvia:sci.electronics / kuusama@etana.tut.fi (Kuusama Juha) / 1:58 am Jul 9, 1989 / >I'd like to build a small (pocket-size) weight scale, with a range of >about 1 to 20 kilograms and accuracy about 1%. If I get voltage, I can >surely handle the rest. But what to use as the converter? > >The catch: I prefer the converter not to cost more than $20. >-- >Juha Kuusama, kuusama@korppi.tut.fi ( ...!mcvax!tut!korppi!kuusama) >---------- There is a company by the name of OMEGA ENGINEERING, INC. which sells strain guages which are used in converting weight into voltage using a bridge circuit. One address for them is ONE OMEGA DRIVE BOX 4047 STAMFORD CT 06907-0047. If your not in a hurry send me your address and I can have them send you a full catalog of their devices. Hope this helps------------------Jay Reinhart
barry@hprmokg.HP.COM (Barry Fowler) (07/11/89)
My best guess would be to use a strain gauge. I don't know if it would fit in your budget constraints, however. The principle of operation would be to measure the amount of deflection in a lever with the gauge and convert the output of the gauge to an analog (and possibly later, digital) measurement. Again, your cost constraints will be the most difficult problem, especially if you want a digital readout (A/D converter, clock circuit, display costs). 'Any other answers out there?
brianr@tekig5.PEN.TEK.COM (Brian Rhodefer) (07/11/89)
Again the sad refrain, "the automagic e-mail system isn't". Three attempts at private mail have bounced, so here are some blue-sky guesses for everyone's amusement: 1) National Semiconductor used to sell pressure transducing modules. connect one to a small oil-filled hydraulic cylinder, and load the cylinder's piston with your test weight. Force->pressure->volts. 2) Tape a strain gauge to a metal rod, and use the rod to suspend the unknown weight. I don't remember much about the care and feeding of strain gauges, other than their signal outputs are fairly weak, and that they have fairly bothersome temperature and aging drifts. 3) Build a torsion balance: Suspend the test weight by a cord wrapped around a wheel, attached to a shaft. Support the shaft by bearings, and connect a torsion coil spring to it. Anchor the other end of the coil spring to the same support the bearings use. Add an optical or magnetic rotational encoder wheel to the shaft, and count the number of pulses produced when the weight is loaded onto the end of the cord. 4) Buy a run-of-the-mill spring scale. Rig an LED/Phototransistor module up so as to get a thrill whenever the indicator is just past its "rest" position. Attatch the object to be weighed to the scale's hook. Shock the resultant system into mechanical oscillation with an external force, and measure the time intervals between the pairs of pulses from the phototransistor, which will give the period of the mechanical oscillator composed of the spring and the test mass, and will be a function of the test mass (provided the damping coefficient isn't too unreasonably large). 5) Build yourself a capacitor by sandwiching a squishable dielectric between two rigid conductors (probably metal plates). The capacitance ought to vary as a function of how tightly the dielectric is squoze, which, if your test mass is placed atop the pile, will be a function of how heavy it is. By incorporating this capacitor into an oscillator, you can measure weight as a function of frequency. If you use two layers of the squishable dielectric, with foil in the middle, you can AC ground both the "outside" metal plates, to confine the capacitance changes to those induced by the weight. I think this last one's a winner. Use a Bose (emitter-coupled astable) oscillator; as RC oscillators go, they're bulletproof, and unaffected by temperature and supply voltage variations. Best of all, if you're trying for computer-readable input, time-related phenomena are easier to interface than voltage-related ones. Suggestively yours, Brian Rhodefer
jonnyg@umd5.umd.edu (Jon Greenblatt) (07/12/89)
In article <10040010@hprmokg.HP.COM> barry@hprmokg.HP.COM (Barry Fowler) writes: >My best guess would be to use a strain gauge. I don't know if it would >fit in your budget constraints, however. The least expensive solution that comes to my mind would be a transformer with a moving core. The core is connected to a spring of the apropriate weight. The rest of the system will consist of a regulated AC power supply and an AC meter. JonnyG.
ken@cs.rochester.edu (Ken Yap) (07/12/89)
| The least expensive solution that comes to my mind would be a |transformer with a moving core. The core is connected to a spring of the |apropriate weight. The rest of the system will consist of a regulated AC power |supply and an AC meter. One of the more imaginative solutions I have seen is a project in Elektor where they used a loudspeaker. The perimeter of the cone was cut away. A platform was then glued onto the cone. A photoelectric sensor was attached. The circuit was an op-amp which pumped enough current through the coil to clear the photoelectric sensor (feedback loop). The current was then a measure of the weight on the platform. A DVM chip and display completed the project. Unfortunately this balance only did up to 2 kg. Maybe you can get a monster loudspeaker from a rock group and increase the range. :-)
erk@americ.UUCP (Erick Parsons) (07/13/89)
>From: kuusama@etana.tut.fi (Kuusama Juha) Message-ID: <7903@etana.tut.fi> >I'd like to build a small (pocket-size) weight scale, with a range of >about 1 to 20 kilograms and accuracy about 1%. If I get voltage, I can >surely handle the rest. But what to use as the converter? >The catch: I prefer the converter not to cost more than $20. ^^^^^^^^^^^^^^^^^^^^^^^^^ Good Luck :-) Its been a while since I've been in the Scale business but I will try to give you a somewhat intelligent answer. The only type of transducer that I've seen are strain gauge transducers. Basically a Wheatstone bridge affair. The x-ducers are quite expensive due to the fact that almost all are made to conform to state and federal specifications which are .1 % error. Yes that's 1 tenth of 1 percent or 1 pound in 1000 allowable error. They come in differant flavors for differant applications ie. 25 LB 50 LB 100 LB all the way up to 1000 LB total capacity. Prices vary and I think that they were about 250.00 up. These units would put out about 5 or 10 microvolts / graduation of rated capacity and thus require some pretty sensetive equipment to use fully. They are excited by about 12 - 15 VDC highly regulated and sensed supply. most are linearized mechanically though the meathod lately is to linearize them via uP lookup tables which could possibly have driven the price down to a more reasonable level )but I doubt it :-( If you are interested in buying one your nearby scale shop should be able to set you up. Who knows... the prices may be within reason by now. BTW all of the instruments that I've seen use a dual slope conversion which is rather slow but extremely accurate. ~ 2500.00 Your real best bet would be to buy a ready made scale such as an O Haus diet scale or such. There are a million of them on the market lately but I wouldn't bet my life on the accuracy or linearization of such beasts, 1 % though doesn't sound too unreasonable. -- ------------------ // -----------Cut-Here---------------------------------- Erick Parsons // Words for the wise: *If it works don't fix it.* Sacramento Ca // mail to: ...pacbell!sactoh0!americ!erk ------------- \\// --------------------------------------------------------
john@stiatl.UUCP (John DeArmond) (07/13/89)
In article <5081@umd5.umd.edu> jonnyg@umd5.umd.edu (Jon Greenblatt) writes: >In article <10040010@hprmokg.HP.COM> barry@hprmokg.HP.COM (Barry Fowler) writes: >>My best guess would be to use a strain gauge. I don't know if it would >>fit in your budget constraints, however. > > The least expensive solution that comes to my mind would be a >transformer with a moving core. The core is connected to a spring of the >apropriate weight. The rest of the system will consist of a regulated AC power >supply and an AC meter. > > JonnyG. I don't remember the original purpose of the load cell but if high precision is not needed, one of the absolute cheapest cells is a piece of conductive foam rubber sandwiched between 2 pieces of printed circuit board. More weight == more conductivity. You can thermally compensate the device by building 2 identical cells and fixing the plates of one against outside force (epoxy does well here) and then putting them in a wheatstone bridge arrangement. John -- John De Armond, WD4OQC | Manual? ... What manual ?!? Sales Technologies, Inc. Atlanta, GA | This is Unix, My son, You ...!gatech!stiatl!john **I am the NRA** | just GOTTA Know!!!
hcooper@esunix.UUCP (Harrison Cooper) (07/13/89)
Depending on how much weight it is you want to measure (can't remember from the original posting), why not use a force sensing resistor. I have a business card from a company that actually includes on the card a force sensing resistor. By attaching a ohm meter and pressing on the card, one can see the resitance vary. This can then be used to generate a voltage swing, run thru an A/D and digital counter (see Intersils Data Book page 7-66 for details). Harrison Cooper N7KST ** I have no affiliation with Intersil or its affiliates - but they do make some good stuff to play with **
jad@dayton.UUCP (J. Deters) (07/13/89)
A 'el cheapo' method I saw in a Radio-Electronics column once was:
Use the conductive foam that your ICs come packaged in as the main
ingredient in a copper sandwich. A picture will help:
-------- <-copper disk (penny, PC board, whatever)
~~~~~~~~
~~~~~~~~ <-conductive foam
~~~~~~~~
-------- <-copper disk
Your wires are connected to the copper disk. When compressed, the
resistance of the conductive foam is reduced. You could easily
experiment to find values.
-j
--
"Captain's log: Stardate 2734.3. 'I am nailed to the hull.'"rkarlqu@hpscdc.HP.COM (Rick Karlquist) (07/17/89)
> I'd like to build a small (pocket-size) weight scale, with a range of > about 1 to 20 kilograms and accuracy about 1%. If I get voltage, I can > surely handle the rest. But what to use as the converter? > > The catch: I prefer the converter not to cost more than $20. > -- > Juha Kuusama, kuusama@korppi.tut.fi ( ...!mcvax!tut!korppi!kuusama) Interlink Electronics, Santa Barbara, CA makes "Force-sensing resistors" They are much cheaper than strain gages and also don't require exotic instrumentation amplifiers either. Should be ideal for your application. Their phone number is (805)-965-5155. Rick Karlquist rkarlqu@hpscdc.hp.com
frankb@hpsad.HP.COM (Frank Ball) (08/15/89)
*One of the more imaginative solutions I have seen is a project in *Elektor where they used a loudspeaker. The perimeter of the cone was *cut away. A platform was then glued onto the cone. A photoelectric *sensor was attached. The circuit was an op-amp which pumped enough *current through the coil to clear the photoelectric sensor (feedback *loop). The current was then a measure of the weight on the platform. *A DVM chip and display completed the project. Unfortunately this *balance only did up to 2 kg. Maybe you can get a monster loudspeaker *from a rock group and increase the range. :-) In school I had to build an electronic scale for a project. I suspended the weight from a solinoid. There was a light and a photo transistor to sense the position of the solinoid. The photo transistor was part of a feedback loop with an op amp and drive transistor that varied the current in the solinoid to hold the weight at the same height every time. A volt meter was attached to the solinoid to measure the voltage across it. A lookup table was emperically derived to convert from voltage to weight. It wasn't linear, but it worked and it cost <$20. Most of the class used strain gauges. I don't know where they got them, they aren't cheap. A friend had an interesting scale: The weight was placed on the end of a horizontal bar about 3" long and 1/2" square. The bar was firmly attached to a base at the other end. A phonogragh cartrige was mounted under the bar, near the middle, with the stylus just touching the bar. The output of the cartridge was connected to a FET input op amp and measured the deflection of the bar when the weight was put onto it. I don't remember the electronics side of it, I guess he must have integrated the output to get a voltage proportional to the weight. Frank Ball 2LR-O frankb@hpsad.HP.COM Hewlett Packard (707) 794-4168 1212 Valley House Drive fax: (707) 794-4452 Rohnert Park CA 94928-4999 I'm the NRA.