Post by thread:PIC Scale -

Hi to all. I was wondering if anyone out there has attempted to make an electronic scale using PIC's. I have a few ideas for the PIC side of things but 'What to use for a SENSOR??' I was thinking of strain guages but over here in sunny South Africa, these are either hard to come by or very costly. I'm considering a LVDT (Linear Variable Differential Transformer) option but I wan't to keep mechanics to a minimum. I was wondering if any of you out there have some idea's that you would like to share on the topic. Thanks. P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for their help, my programmer is now up and running! Jacques Vrey Iscor Profile Products Newcastle Tel:27-(0)3431-48759 spam_OUTjvreyTakeThisOuTit.new.iscorltd.co.za

Gday Jacques, I have seen a long time ago a design based on a printed cct board track. It consisted of a fine track that traced a s pattern up and down a "finger" that was used as a strain gauge. It worked on the principle that the resistance varied with the amount of bend on the finger. The track went up and down the finger about 10 times. regards Lee McLaren Tasmania Australia Jacques Vrey wrote: {Quote hidden}> > Hi to all. > > I was wondering if anyone out there has attempted to make an > electronic scale using PIC's. I have a few ideas for the PIC side > of things but 'What to use for a SENSOR??' I was thinking of > strain guages but over here in sunny South Africa, these are either > hard to come by or very costly. I'm considering a LVDT (Linear > Variable Differential Transformer) option but I wan't to keep > mechanics to a minimum. I was wondering if any of you out there have > some idea's that you would like to share on the topic. > > Thanks. > > P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for > their help, my programmer is now up and running! > > Jacques Vrey > Iscor Profile Products Newcastle > Tel:27-(0)3431-48759 > .....jvreyKILLspam@spam@it.new.iscorltd.co.za

{Quote hidden}> Bob Lunn > 08/25/97 07:22 PM > > > > I was wondering if anyone out there has attempted > > to make an electronic scale using PIC's. > > I have been building electronic scales using > PIC's for a dozen years. All of these scales > have been built using strain gauge sensors. > > There are a few points you should clarify: > > - what is the maximum capacity of the scale. > - what is the dead load of the scale. > - how many graduations are required. > - how is the load to be applied. > - what is the nature of the load. > > Please don't underestimate the difficulty of > building an accurate and robust load sensor! > > ___Bob > In answer, The following: - 5Kg - no more than 500g(probably less) - 1 (0-5Kg) - vertical mass measurement - anything Exceptional accuracy is not critical (Glorified Kitchen Scale) Jacques Vrey Iscor Profile Products Newcastle Tel:27-(0)3431-48759 jvreyKILLspamit.new.iscorltd.co.za

>Hi to all. > >I was wondering if anyone out there has attempted to make an >electronic scale using PIC's. I have a few ideas for the PIC side >of things but 'What to use for a SENSOR??' I was thinking of >strain guages but over here in sunny South Africa, these are either >hard to come by or very costly. I'm considering a LVDT (Linear >Variable Differential Transformer) option but I wan't to keep >mechanics to a minimum. I was wondering if any of you out there have >some idea's that you would like to share on the topic. I'm working a lot with scales using PIC's. The best way, and the only way I'm working, is with strain-gauge load cells. You have to pay special atention on the amplification, or use no amplification at all if you use some special A/D. Regards, Octavio ====================================================== Octavio Nogueira - e-mail: .....nogueiraKILLspam.....mandic.com.br http://www.geocities.com/~oct_nogueira "ProPic" Production PIC Programmer Windows under US$20 ======================================================

At 08:07 AM 8/25/97 -0300, you wrote: >>Hi to all. >> >>I was wondering if anyone out there has attempted to make an >>electronic scale using PIC's. I have a few ideas for the PIC side >>of things but 'What to use for a SENSOR??' I was thinking of >>strain guages but over here in sunny South Africa, these are either >>hard to come by or very costly. I'm considering a LVDT (Linear >>Variable Differential Transformer) option but I wan't to keep >>mechanics to a minimum. I was wondering if any of you out there have >>some idea's that you would like to share on the topic. You may want to look at the new AD7730 chips. They are specifically designed for use with strain gage load cells. They interface to a PIC via an SPI bus. These chip are really a specialized DSP with a programmable gain instrumentation amplifier front end. They have lots of capability for filtering, tare, and self calibration and test. They are also about $12 in small quantities. As you can tell, I like the chip. But, beware, there is a steep learning curve to use them. Its the price you pay for all the functionality. Russ

On Monday, August 25, 1997 2:46 AM, pic microcontroller discussion list On Behalf Of Jacques Vrey wrote: {Quote hidden}> Hi to all. > > I was wondering if anyone out there has attempted to make an > electronic scale using PIC's. I have a few ideas for the PIC side > of things but 'What to use for a SENSOR??' I was thinking of > strain guages but over here in sunny South Africa, these are either > hard to come by or very costly. I'm considering a LVDT (Linear > Variable Differential Transformer) option but I wan't to keep > mechanics to a minimum. I was wondering if any of you out there have > some idea's that you would like to share on the topic. > > Thanks. > > P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for > their help, my programmer is now up and running! > > Jacques Vrey > Iscor Profile Products Newcastle > Tel:27-(0)3431-48759 > EraseMEjvreyspam_OUTTakeThisOuTit.new.iscorltd.co.za > > Hello Jacques, Years ago we did a talking scale that used a Pic. The technology was an optical up/down counter that was spun by a spring mounted mechanism. The unit measured in pounds, kiligrams, ounces and stone ans spoke in all of the radixes. The trick was to keep the count going while the multiplies and divides for radix conversion was being accomplished and displayed in real time. Therefore, not strain guage at all and it was "inexpensive". Tom

On 25 Aug 97 at 15:20, Glenn Johansson wrote: > >I'm working a lot with scales using PIC's. The best way, and the only way > >I'm working, is with strain-gauge load cells. You have to pay special > >atention on the amplification, or use no amplification at all if you use > >some special A/D. > > How much do those cells cost? What about springs and some simple > motion control > sensor? I wouldn't like to predict accuracy / long term repeatability - this is how the early digital bathroom scales worked - springs and a slotted wheel / counter system - and they seemed to give a different result every time you stepped on them. (within seconds - not days, guys <g>) If you wanted to try the idea, you could get a cheap set of spring kitchen scales and a mouse, hook the thing up so one mouse axis spun when you put a weight on scales and do the quadrature trick, or you could even just read it using the standard serial output from mouse. MikeS <mikesmith_ozspam_OUTrelaymail.net> === For PICLIST help (including "unsubscribe" instructions), === send an e-mail containing the single phrase "help piclist" === to: @spam@listservKILLspammitvma.mit.edu

On 25 Aug 97 at 22:45, Robert Lunn wrote: {Quote hidden}> Jacques Vrey wrote: > > > In answer, The following: > > > > - 5Kg > > - no more than 500g(probably less) > > - 1 (0-5Kg) > > - vertical mass measurement > > - anything > > > > Exceptional accuracy is not critical (Glorified Kitchen Scale) > > Jacques, If I'm reading you right you want to > weigh 5000 grammes by 1 gramme. > > That's 5000 grads! > > This is not a glorified kitchen scale. > > Can we maybe reduce this by an order of magnitude? I'd be inclined to reduce it to PIC A/D proportions - probably the 14000, since the 16c7x is only 8bits resolution, much too coarse to be any use, unless you stuffed around a lot with ranging. This would give the 5000 graduations easily - in fact, data sheet indicates it could go up to 65536 ... The harder part would be the analog circuit - I seem to recall they are somewhat prone to drift with temperature. A scale that could measure down to fractions of a gram would be somewhat useful - counting things - measuring correct ratio of sulphur / charcoal / potassium chlorate etc (no, I don't make this in 5k lots!) <g> MikeS <KILLspammikesmith_ozKILLspamrelaymail.net> === For PICLIST help (including "unsubscribe" instructions), === send an e-mail containing the single phrase "help piclist" === to: RemoveMElistservTakeThisOuTmitvma.mit.edu

The 5kg load cells costs me about US$60 and I don't thing the spring/motion sensor could have the same precision as load cells, but if you don't need precision, it's just ok, but then you have mor mechanics to deal with. Regards, Octavio P.S. if anyone interested in buing load cells in large quantity, send me private e-mail ====================================================== Octavio Nogueira - e-mail: spamBeGonenogueiraspamBeGonemandic.com.br http://www.geocities.com/~oct_nogueira "ProPic" Production PIC Programmer Windows under US$20 ====================================================== ---- De: Glenn Johansson <TakeThisOuTglennEraseMEspam_OUTWRITEME.COM> Para: RemoveMEPICLISTTakeThisOuTMITVMA.MIT.EDU Data: Segunda-feira, Agosto 25, 1997 11:07 Assunto: Re: PIC Scale - >>I'm working a lot with scales using PIC's. The best way, and the only way >>I'm working, is with strain-gauge load cells. You have to pay special >>atention on the amplification, or use no amplification at all if you use >>some special A/D. > >How much do those cells cost? What about springs and some simple motion control > sensor? > >Regards, >Glenn >

Hi Most scales use a device called a load cell which is like a Z shaped chunk of metal that gives an output proportional to load. These use a strain gauge principal. Try the South Africa branch of Maplin Electronics. They should get you strain gauges or try RS Components. Remember to use at least two sensors. It gives the best readings. The CPR practice doll I was using during a heartstart course used a large spring and a slotted opto switch with a very fine scale that went through it vertically. This gave amazing accuracy and was cheap and easy. As below ___________Surface____________ /// - /// /// - /// /// - /// /// - /// /// - /// /// - _____ /// /// |-|_____| ///Spring /// | /// /// | /// /// | /// /// | /// /// | /// /// | /// /// |Post /// _____________________|_________ _____ |-|_____| Represents slotted opto switch As pressure is put on spring, a pulse train is output. The post limits maximum weight and should come to half total height. Tim At 08:46 25/08/97 +0200, you wrote: {Quote hidden}>Hi to all. > >I was wondering if anyone out there has attempted to make an >electronic scale using PIC's. I have a few ideas for the PIC side >of things but 'What to use for a SENSOR??' I was thinking of >strain guages but over here in sunny South Africa, these are either >hard to come by or very costly. I'm considering a LVDT (Linear >Variable Differential Transformer) option but I wan't to keep >mechanics to a minimum. I was wondering if any of you out there have >some idea's that you would like to share on the topic. > >Thanks. > >P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for >their help, my programmer is now up and running! > >Jacques Vrey >Iscor Profile Products Newcastle >Tel:27-(0)3431-48759 >jvreyEraseME.....it.new.iscorltd.co.za > > ------------------------------------------------------------------ Personal Web Pages: http://web.ukonline.co.uk/members/tim.kerby/ Email: EraseMEtim.kerbyukonline.co.uk ------------------------------------------------------------------

I've been thinking about a capacitive sensor. You might use a couple of conductive parallel plates, with the mass changing the spacing. I'd probably use the capacitor in an oscillator, and then count pulses with a PIC. Then you'd have to correct the non-linearities, either in software or in the oscillator. (Maybe a 1/n transfer-function V-F converter?) It appears the resolution would be a lot better at one end than the other. I imagine it would be pretty drifty with temperature. Mechanically, it could be simple and rugged. Then again, all the good scales I've ever seen were based on either load cells or quartz crystals. later, newell

On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira <RemoveMEnogueiraEraseMEEraseMEMANDIC.COM.BR> writes: >The 5kg load cells costs me about US$60 and I don't thing the >spring/motion >sensor >could have the same precision as load cells, but if you don't need >precision, >it's just ok, but then you have mor mechanics to deal with. > Wow! I'm surprised they're that much, though I never tried to buy one. I have, however, bought some pressure transducers from Motorola. As I recall, they were pretty cheap (I think under $10 US) and included conditioning circuitry so something like 0 to 5 volts came out. So... maybe make a scale by having a piston and cylinder drive a pressure transducer? There's gotta be an easier way! Harold

part 0 946 bytes -----Original Message----- From: Harold Hallikainen [SMTP:RemoveMEharoldhallikainenspam_OUTKILLspamJUNO.COM] Sent: Monday, August 25, 1997 4:04 PM To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU Subject: Re: PIC Scale - On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira <EraseMEnogueiraspamspamBeGoneMANDIC.COM.BR> writes: >The 5kg load cells costs me about US$60 and I don't thing the >spring/motion >sensor >could have the same precision as load cells, but if you don't need >precision, >it's just ok, but then you have mor mechanics to deal with. > Wow! I'm surprised they're that much, though I never tried to buy one. I have, however, bought some pressure transducers from Motorola. As I recall, they were pretty cheap (I think under $10 US) and included conditioning circuitry so something like 0 to 5 volts came out. So... maybe make a scale by having a piston and cylinder drive a pressure transducer? There's gotta be an easier way! Harold

> I have seen electronic scales use current to electromagnetically counter > gravity, with the equilbrium point being determined by light that was > obstructed by a blade on the moving part. The current servos until the > light is dimmed to about half. Over narrow ranges the > response was quite linear. But I would think that force transducers > would be a more rugged approach. Interesting notion. How about a mechanical balance with a weight that slides along a lever under software control via either stepper or motor+quad encoder? Should be simple enough to program, and the mechanics shouldn't be too bad either.

On Mon, 25 Aug 1997 16:03:57 EDT Harold Hallikainen <RemoveMEharoldhallikainenKILLspamJUNO.COM> writes: >On Mon, 25 Aug 1997 13:24:55 -0300 Octavio Nogueira ><nogueiraSTOPspamspam_OUTMANDIC.COM.BR> writes: >The 5kg load cells costs me about >US$60 and I don't thing the >>spring/motion >>sensor >>could have the same precision as load cells, but if you don't need >>precision, >>it's just ok, but then you have mor mechanics to deal with. >> > > Wow! I'm surprised they're that much, though I never tried to >buy one. I have, however, bought some pressure transducers from >Motorola. As I recall, they were pretty cheap (I think under $10 US) >and included conditioning circuitry so something like 0 to 5 volts >came out. So... maybe make a scale by having a piston and cylinder >drive a pressure transducer? There's gotta be an easier way! A piston and cylinder would be hard to make as well as suffer from leakage and friction. Something like a bellows or air bag holding the load platform up might be better for a low-capacity scale. It would be very resistant to overloading, since those sensors can withstand something like 10x rated full-scale pressure. The pressure in the air system with no load on the platform would need to be positive to keep the sides of the air bag from collapsing.

Bob Lunn 08/26/97 11:18 AM > A scale that could measure down to fractions of > a gram would be somewhat useful - counting things > - measuring correct ratio of sulphur / charcoal / > potassium chlorate etc (no, I don't make this in > 5k lots!) <g> Laboratory scales and commercial counting scales are most certainly useful things. You might care to price these items. This will give you a first order estimate of the difficulty of building one. In considering mechanisms to use for load or mass determination please keep in mind: span drift zero drift hysteresis creep ...just for starters. Jacques requires a mechanism to measure a load from 0 to 5,000 grammes with a resolution of 5 grammes, with a dead load not exceeding 500 grammes. We should still clarify the temperature range over which the scale is to operate, the repeatability of readings, long term drift of readings, need for zero and tare mechanisms, and requirements (if any) for standards certification. These days, the conversion of an electronic signal to a digital value is easily solved for reasonable cost. The greatest problems usually arise in the physical load sensing mechanism. There's no point in having a 16-bit A/D converter if an elastomeric mounting imposes 5% hysteresis, or a knife edge continually binds. Anyway, the most common sensing techniques for electronic load measurement are: metal foil and silicon strain gauges (configured to respond to shear, bending, torsion, compression, or tension) piezoelectric bending beams capacitive measurement of displacement optical measurement of displacement Strain gauges are most commonly used for loads in the range of 50kg to 25,000kg. The most common implementation would be a shear beam, with a nominal accuracy of 2000 to 3000 grads. Physical constraints on the load sensing element to which the strain gauge is bonded makes the use of strain gauges for very small, and very large, loads more difficult. Piezoelectric systems have probably had most success with very large loads requiring modest accuracy, such as gross tonnage measurement of trucks. The difficulty with piezoelectric elements is that they only respond to changes in load. A static load produces no output. They can, however, be used to build very robust (and thus safe) weighing systems. Hence their success with high capacity weighing. Capacitive and optical measurement of displacement are the most common mechanisms for small loads. For a 5kg scale these are certainly the approaches I would be considering. I would not use a strain gauge based load cell for this purpose. Scale design and manufacture is a _very_ competitive industry, and has been so for many decades. There are a large number of patents covering scale design and manufacture. An excellent education in these issues can be drawn from reading patent documents. Given that many of these patents have expired, the techniques they describe are available for use. ___Bob

At 08:46 AM 8/25/97 +0200, you wrote: {Quote hidden}>Hi to all. > >I was wondering if anyone out there has attempted to make an >electronic scale using PIC's. I have a few ideas for the PIC side >of things but 'What to use for a SENSOR??' I was thinking of >strain guages but over here in sunny South Africa, these are either >hard to come by or very costly. I'm considering a LVDT (Linear >Variable Differential Transformer) option but I wan't to keep >mechanics to a minimum. I was wondering if any of you out there have >some idea's that you would like to share on the topic. > >Thanks. > >P.S. Thanks to Mal Goris, Andrew Errington and Don McKenzie for >their help, my programmer is now up and running! > >Jacques Vrey >Iscor Profile Products Newcastle >Tel:27-(0)3431-48759 >spamBeGonejvreySTOPspamEraseMEit.new.iscorltd.co.za > I was at a customer's place one day and he explained to me how his Metz__r scale worked. The principle is pretty slick. A balance is attached to the plate. A sensor ( i forget, magnetic or optical) sensed when the balance was centered. The other end of the balance has a low tech solenoid attached. The circuit adjusts the current through the solenoid until the balance balances, and voila, the current being sent to the solenoid is proportional to the weight. I am sure there are issues, such as temperature compsation, but Metzler (oops) solved it, that scale was super accurate and not cheap. But on that principle, you could develop a reasonably cheap version. I was tipped off to ask him about the darned thing when I noticed that the platter did not apear to deflect at all. Word of warning, I would check and see if they have a patent on the principle. It is too neat not to. Chris Eddy Pittsburgh, PA

> A scale that could measure down to fractions of > a gram would be somewhat useful - counting things > - measuring correct ratio of sulphur / charcoal / > potassium chlorate etc Laboratory scales and commercial counting scales are most certainly useful things. You might care to price these items. This will give you a first order estimate of the difficulty of building one. There's an excellant source of information on electronic (and mechanical balances (from a consumer point of view) at http://www.balances.com. Aside from a wide selection and good prices, their web-page setup is WONDERFULLY cross-connected (a page for a given scale will have buttons "need more capacity?", "need better accuracy?", "check another vendor", etc. Just bought two scales from them, in fact (one for weighing that potassium chlorate.) Had an interesting and informative discussion with the proprietor as well... BillW

On Mon, 25 Aug 1997, Tim Kerby wrote: > Hi > Most scales use a device called a load cell which is like a Z shaped chunk > of metal that gives an output proportional to load. PTM: Most cheap letter scales use counter, spring, coil and a magnet. A coil and a magnet from a bass loudspeaker makes a good scale 0..200g. Of course you can alleways use lewers as they use in the bathroom scales to make this 0..200g -> 0..200kg. Another way is to use this thing that I used as a pressure meter: I had a moving foil and a HEDS1000. I read the the light that reflected from the foil. It was _sensitive_ when you selected an EU-normal condom as a foil -------------------------------------------------------------------------- PTM, KILLspampasi.mustalahtispamBeGoneutu.fi, EraseMEptmustaEraseMEutu.fi, http://www.utu.fi/~ptmusta Lab.ins. (mikrotuki) ATK-keskus/Mat.Luon.Tdk OH1HEK Lab.engineer (PC support) Computer Center OI7234 Mail: Turun Yliopisto / Fysla, Vesilinnantie 5, 20014 Pt 02-3336669, FAX 02-3335632 (Pk 02-2387010, NMT 049-555577) --------------------------------------------------------------------------

{Quote hidden}> Hi > Most scales use a device called a load cell which is like a Z shaped chunk > of metal that gives an output proportional to load. These use a strain > gauge principal. Try the South Africa branch of Maplin Electronics. They > should get you strain gauges or try RS Components. Remember to use at > least two sensors. It gives the best readings. > > The CPR practice doll I was using during a heartstart course used a large > spring and a slotted opto switch with a very fine scale that went through > it vertically. This gave amazing accuracy and was cheap and easy. As below > > ___________Surface____________ > /// - /// > /// - /// > /// - /// > /// - /// > /// - /// > /// - _____ /// > /// |-|_____| ///Spring > /// | /// > /// | /// > /// | /// > /// | /// > /// | /// > /// | /// > /// |Post /// > _____________________|_________ > > > _____ > |-|_____| Represents slotted opto switch > > As pressure is put on spring, a pulse train is output. The post limits > maximum weight and should come to half total height. > > > Tim > Hi there again I checked through the Maplin catalogue and couldn't find anything - I do remember that RS does have something like that in their catalogue - I'll check. Dare I say, I like the Idea from Chris Eddy - using a solenoid to balance the load and then measure the current needed to achieve this. The mechanics shouldn't be too difficult to implement, or the electronics for that matter. Jacques Vrey Iscor Profile Products Newcastle Tel:27-(0)3431-48759 @spam@jvrey@spam@spam_OUTit.new.iscorltd.co.za

I remember an article in Czech electronic magazine describing electromagnetical scale using a big speaker, a tube glued with epoxy to the cone holded a board on which you put the load. Also had some optoelectronical sensor. Then you apply current to the speaker, until the opto is some half way covered and measure the voltage on the speker proportional to the current. This system had a phase shift due to the mechanical part even at low frequencies and dependable on the load, so the feedback part was a little bit tricky. Another drawback - it uses quite a lot of current. I am just not sure, if you can find that big speaker to load it directly with 5 kg. Otherwise it is mechanicly very simple and cheap if you use speaker with little damage, not suitable for HiFi applications. Josef Hanzal >From: John Shreffler <spamBeGonejohnsKILLspamAVENUETECH.COM> >I have seen electronic scales use current to electromagnetically counter >gravity, with the equilbrium point being determined by light that was >obstructed by a blade on the moving part. The current servos until the >light is dimmed to about half. Over narrow ranges the >response was quite linear. But I would think that force transducers >would be a more rugged approach.

Hi Russ, I'm using the AD7714 and I can not find the AD7730, could you tell me where You buy it? Regards, Octavio ====================================================== Octavio Nogueira - e-mail: .....nogueiraspam_OUTmandic.com.br http://www.geocities.com/~oct_nogueira "ProPic" Production PIC Programmer Windows under US$20 ====================================================== ---- De: Russ Steinbach <TakeThisOuTRussell.E.Steinbach.....TakeThisOuTLERC.NASA.GOV> Para: TakeThisOuTPICLISTKILLspamspamMITVMA.MIT.EDU Data: Segunda-feira, Agosto 25, 1997 09:32 Assunto: Re: PIC Scale - {Quote hidden}>At 08:07 AM 8/25/97 -0300, you wrote: >>>Hi to all. >>> >>>I was wondering if anyone out there has attempted to make an >>>electronic scale using PIC's. I have a few ideas for the PIC side >>>of things but 'What to use for a SENSOR??' I was thinking of >>>strain guages but over here in sunny South Africa, these are either >>>hard to come by or very costly. I'm considering a LVDT (Linear >>>Variable Differential Transformer) option but I wan't to keep >>>mechanics to a minimum. I was wondering if any of you out there have >>>some idea's that you would like to share on the topic. > >You may want to look at the new AD7730 chips. They are specifically >designed for use with strain gage load cells. They interface to a PIC via >an SPI bus. > >These chip are really a specialized DSP with a programmable gain >instrumentation amplifier front end. They have lots of capability for >filtering, tare, and self calibration and test. They are also about $12 in >small quantities. > >As you can tell, I like the chip. But, beware, there is a steep learning >curve to use them. Its the price you pay for all the functionality. > > >Russ >