Post by thread:[EE] resistive force sensing

Question was brought up, and perhaps someone has an answer. The problem to solve is to be able to repeatable and reliably measure a weight of a range of 35lbs to 150lbs using some sort of flat & flexible surface. I've used load cells before but they are too bulky and expensive. Ideally he wants this to be portable and flexible so some sort of resistive network maybe? The range can also be divided up...say 35 to 65, 65 to 95, etc...one model does not need to cover the entire range. Also it does not need to be accurate to the pound..can be +/- 2 to 4 pounds. The micro would measure the output of the sensor, and do a lookup table or something based on what is coming in. Its more of a go/no go type application. Any thoughts?

> > The problem to solve is to be able to repeatable and reliably measure a weight of a range of 35lbs to 150lbs using some sort of flat & flexible surface. I've used load cells before but they are too bulky and expensive. Ideally he wants this to be portable and flexible so some sort of resistive network maybe? The range can also be divided up...say 35 to 65, 65 to 95, etc...one model does not need to cover the entire range. Also it does not need to be accurate to the pound..can be +/- 2 to 4 pounds. The micro would measure the output of the sensor, and do a lookup table or something based on what is coming in. Its more of a go/no go type application. > Can you measure the flex of the surface from the bottom? With a laser or even IR you I would think you could get into that +/- 2 pounds depending on the "flexible" surface. Most of the robot hobby sites have write ups on distance measuring with pics and other microcontrollers.

> The problem to solve is to be able to repeatable and reliably measure a > weight of a range of 35lbs to 150lbs using some sort of flat & flexible > surface. I've used load cells before but they are too bulky and expensive. > Ideally he wants this to be portable and flexible so some sort of > resistive network maybe? The range can also be divided up...say 35 to 65, > 65 to 95, etc...one model does not need to cover the entire range. Also it > does not need to be accurate to the pound..can be +/- 2 to 4 pounds. The > micro would measure the output of the sensor, and do a lookup table or > something based on what is coming in. Its more of a go/no go type > application. Many years ago one of my customers did some research on using fiber optics to measure weight. You arrange a piece of fiber so that it is 'bent' when the weight is applied, then measure the change in transmissivity. I don't think they ever commercialized the idea, but it might work. -- Bob Ammerman RAm Systems

> Date: Mon, 23 Mar 2009 06:17:40 -0700 > From: .....micro_eng2KILLspam@spam@yahoo.com > Subject: [EE] resistive force sensing > To: piclistKILLspammit.edu > > > Question was brought up, and perhaps someone has an answer. > > The problem to solve is to be able to repeatable and reliably measure a weight of a range of 35lbs to 150lbs using some sort of flat & flexible surface. I've used load cells before but they are too bulky and expensive. Ideally he wants this to be portable and flexible so some sort of resistive network maybe? The range can also be divided up...say 35 to 65, 65 to 95, etc...one model does not need to cover the entire range. Also it does not need to be accurate to the pound..can be +/- 2 to 4 pounds. The micro would measure the output of the sensor, and do a lookup table or something based on what is coming in. Its more of a go/no go type application. > > Any thoughts? > Contact Sensor Products, ask for Jason Blume. http://www.sensorprod.com/index.php 973.884.1755 (USA, Eastern Standard Time) Rob

An anchored beam with a glue on strain gauge sensor. The sensors are very low cost and can be read with one of the charge balancing circuits to at least your requirements. http://www.bytecraft.com/Low_Cost%2C_Low_Speed_A_D_conversion_for_Embedded_Systems Use Ri as the sensor in the above example. Tie the open end to Vcc. It will be slow a few ms to read but very low cost The output is linear, with two point calibration. Regards, -- Walter Banks Byte Craft Limited http://www.bytecraft.com alan smith wrote: > Question was brought up, and perhaps someone has an answer. > > The problem to solve is to be able to repeatable and reliably measure a weight of a range of 35lbs to 150lbs using some sort of flat & flexible surface. I've used load cells before but they are too bulky and expensive. Ideally he wants this to be portable and flexible so some sort of resistive network maybe? The range can also be divided up...say 35 to 65, 65 to 95, etc...one model does not need to cover the entire range. Also it does not need to be accurate to the pound..can be +/- 2 to 4 pounds. The micro would measure the output of the sensor, and do a lookup table or something based on what is coming in. Its more of a go/no go type application. > > Any thoughts? > > > -

>Many years ago one of my customers did some research on using fiber >optics to measure weight. You arrange a piece of fiber so that it >is 'bent' when the weight is applied, then measure the change in >transmissivity. I don't think they ever commercialized the idea, >but it might work. I believe the Netherlands railways system uses a similar idea to know how far down a block of track a train has travelled. I believe the fibre is attached to the rail somehow, and the weight of the train bends the track enough that the stresses in the fibre are sufficient for a TDM pulse to signify how far away the train is with sufficient accuracy to be a useful measuring tool.

Thanks for the suggestions....I will follow up in the next week and see if either might be a solution to this. --- On Mon, 3/23/09, Walter Banks <.....walterKILLspam.....bytecraft.com> wrote: {Quote hidden}> From: Walter Banks <EraseMEwalterspam_OUTTakeThisOuTbytecraft.com> > Subject: Re: [EE] resistive force sensing > To: "Microcontroller discussion list - Public." <piclistspam_OUTmit.edu> > Date: Monday, March 23, 2009, 9:21 AM > An anchored beam with a glue on strain gauge sensor. The > sensors are very > low cost and can be read with one of the charge balancing > circuits > to at least your requirements. > > www.bytecraft.com/Low_Cost%2C_Low_Speed_A_D_conversion_for_Embedded_Systems > > Use Ri as the sensor in the above example. Tie the open end > to Vcc. It will be slow a few ms to > read but very low cost > > The output is linear, with two point calibration. > > Regards, > > -- > Walter Banks > Byte Craft Limited > http://www.bytecraft.com > > > > > alan smith wrote: > > > Question was brought up, and perhaps someone has an > answer. > > > > The problem to solve is to be able to repeatable and > reliably measure a weight of a range of 35lbs to 150lbs > using some sort of flat & flexible surface. I've > used load cells before but they are too bulky and expensive. > Ideally he wants this to be portable and flexible so some > sort of resistive network maybe? The range can also be > divided up...say 35 to 65, 65 to 95, etc...one model does > not need to cover the entire range. Also it does not need > to be accurate to the pound..can be +/- 2 to 4 pounds. The > micro would measure the output of the sensor, and do a > lookup table or something based on what is coming in. Its > more of a go/no go type application. > > > > Any thoughts? > > > > > > --

Someone already posted the FlexiForce sensor link, but if you want to get one in a hurry and also look at some of the work already being done for you (eval circuit, sample code): http://www.parallax.com/Store/Sensors/PressureFlexRPM/tabid/177/CategoryID/52/List/0/Level/a/ProductID/384/Default.aspx?SortField=ProductName%2cProductName Hope that helps. -marc On Mon, Mar 23, 2009 at 9:17 AM, alan smith <@spam@micro_eng2KILLspamyahoo.com> wrote: {Quote hidden}> > Question was brought up, and perhaps someone has an answer. > > The problem to solve is to be able to repeatable and reliably measure a > weight of a range of 35lbs to 150lbs using some sort of flat & flexible > surface. I've used load cells before but they are too bulky and expensive. > Ideally he wants this to be portable and flexible so some sort of resistive > network maybe? The range can also be divided up...say 35 to 65, 65 to 95, > etc...one model does not need to cover the entire range. Also it does not > need to be accurate to the pound..can be +/- 2 to 4 pounds. The micro would > measure the output of the sensor, and do a lookup table or something based > on what is coming in. Its more of a go/no go type application. > > Any thoughts? > > > > -

> I believe the Netherlands railways system uses a similar idea to know > how far down a block of track a train has travelled. I believe the fibre is > attached to the rail somehow, and the weight of the train bends the track > enough that the stresses in the fibre are sufficient for a TDM pulse to > signify how far away the train is with sufficient accuracy to be a useful > measuring tool. I saw a TV item on that (on a Beyond 2000 science program many years ago). ISTR the method was to compare time travelled, as flexing causes more bouncing off the walls of the cable. One use of the system was for automatic and emergency stopping at a terminus. Pistons pushed a brake rail towards the running rail, clamping the wheel flanges

A variation on a theme - the piezo cable that is used in traffic counting or time lapse speed checking - the rubber looking cable that is layed across the road. Maybe measure the voltage/frequency returned by the piezo - I beleive these cables (with unit) can differentiate between trucks/axles/cars/bikes etc. Colin -- cdb, KILLspamcolinKILLspambtech-online.co.uk on 24/03/2009 Web presence: http://www.btech-online.co.uk Hosted by: http://www.1and1.co.uk/?k_id=7988359

{Quote hidden}> From: RemoveMEpicTakeThisOuTnope9.com > To: spamBeGonepiclistspamBeGonemit.edu > Subject: Re: [EE] resistive force sensing > Date: Mon, 23 Mar 2009 14:00:46 -0600 > > If no one has mentioned it yet... > check out > www.sparkfun.com/commerce/product_info.php?products_id=8685 > > They have 3 varieties of force sensors > MA > -- Those are the Tekscan single point sensors, also available from Parallax, Jameco and a few other sources. The Parallax web page was already referenced. Rob

> Question was brought up, and perhaps someone has an answer. > > The problem to solve is to be able to repeatable and reliably > measure a weight of a range of 35lbs to 150lbs using some sort of > flat & flexible surface. I've used load cells before but they are > too bulky and expensive. Ideally he wants this to be portable and > flexible so some sort of resistive network maybe? The range can > also be divided up...say 35 to 65, 65 to 95, etc...one model does > not need to cover the entire range. Also it does not need to be > accurate to the pound..can be +/- 2 to 4 pounds. The micro would > measure the output of the sensor, and do a lookup table or something > based on what is coming in. Its more of a go/no go type application. > > Any thoughts? > Apply the force to a metal strip that will flex with respect to another conductor. measure the change in capacitance. There are a gazillion ways to measure capacitance. There are ICs that measure capacitance. www.discovercircuits.com/S/s-capacitance.htm http://www.analog.com/en/analog-to-digital-converters/capacitance-to-digital-converters/ad7150/products/product.html

cdb wrote: > A variation on a theme - the piezo cable that is used in traffic > counting or time lapse speed checking - the rubber looking cable that > is layed across the road. Maybe measure the voltage/frequency returned > by the piezo - I beleive these cables (with unit) can differentiate > between trucks/axles/cars/bikes etc. > I'm pretty sure that the ones they stick over the road actually work on air pressure, IE they are a hose rather than an expensive piezo bit. I believe that they do use piezo for speed cameras that are buried in the road.

:: I'm pretty sure that the ones they stick over the road actually :: work on :: air pressure, IE they are a hose rather than an expensive piezo :: bit. :: I believe that they do use piezo for speed cameras that are buried :: in :: the road. Maybe there are two types of sensors. I've never taken one apart, just recall an article in the UK about them along with an offer of 1000' drum of the stuff for GBP25.00 - this was back in the early 90's so don't all rush to find some. Here is a link to a modern day supplier - they've got some interesting stuff to play around with, including pressure sensing. http://www.meas-spec.com/product/t_product.aspx?id=2476 Colin -- cdb, TakeThisOuTcolinEraseMEspam_OUTbtech-online.co.uk on 24/03/2009 Web presence: http://www.btech-online.co.uk Hosted by: http://www.1and1.co.uk/?k_id=7988359

> > A variation on a theme - the piezo cable that is used in traffic > > counting or time lapse speed checking - the rubber looking cable that > > is layed across the road. Maybe measure the voltage/frequency returned > > by the piezo - I beleive these cables (with unit) can differentiate > > between trucks/axles/cars/bikes etc. > > > I'm pretty sure that the ones they stick over the road actually work on > air pressure, IE they are a hose rather than an expensive piezo bit. > I believe that they do use piezo for speed cameras that are buried in > the road. All the ones I've seem work on air pressure. Simple, cheap, works, what more do you want? Road sensors are current loops, the presence of enough steel over them alters the current flow, unless you ride a motorcycle. (As a bonus, you can ride between or beside them and not trip the sensor...) Tony

Jake Anderson wrote: > I'm pretty sure that the ones they stick over the road actually work on > air pressure, IE they are a hose rather than an expensive piezo bit. The ones I have seen work that way. There may be different techniques for sensing the air pressure change, but the part that goes on the road is just a rubber hose. > I believe that they do use piezo for speed cameras that are buried in > the road. The permanent in-road sensors I've seen have all been large loops of wire, about a meter or two accross (they tend to be elongated in the direction of travel). ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000.

Tony Smith wrote: {Quote hidden}>>> A variation on a theme - the piezo cable that is used in traffic >>> counting or time lapse speed checking - the rubber looking cable that >>> is layed across the road. Maybe measure the voltage/frequency returned >>> by the piezo - I beleive these cables (with unit) can differentiate >>> between trucks/axles/cars/bikes etc. >>> >>> >> I'm pretty sure that the ones they stick over the road actually work on >> air pressure, IE they are a hose rather than an expensive piezo bit. >> I believe that they do use piezo for speed cameras that are buried in >> the road. >> > > > All the ones I've seem work on air pressure. Simple, cheap, works, what > more do you want? > > Road sensors are current loops, the presence of enough steel over them > alters the current flow, unless you ride a motorcycle. (As a bonus, you can > ride between or beside them and not trip the sensor...) > > Tony The circuits I have seen, and documentation I've read on the vehicle sensors have used the the loops as inductors in an oscillator. The vehicle changes the inductance and thus the frequency. The change in frequency is smaller for cycles and inside the chosen 'ignore' band of the sensor.

Coming in on this thread late, but around here last year they placed temporary "sensors" on the roads everywhere. there were approx 4"x4" flat (maybe 1/2" tall??) units stuck down to the center of the lanes with what looked like big pieces of rubber adhesive tape. After a week or so, they came around and removed them all, leaving the square outline in each piece of "tape" where they were cut out. I googled looking for these devices but found nothing; I wonder if anybody has any idea what they were and how they worked.....I'm sure they were self contained devices with a power source and a uC to log vehicle traffic..... Mark Skeels Engineer Competition Electronics, Inc. TEL: 815-874-8001 FAX: 815-874-8181 http://www.competitionelectronics.com Olin Lathrop wrote: {Quote hidden}> Jake Anderson wrote: > >> I'm pretty sure that the ones they stick over the road actually work on >> air pressure, IE they are a hose rather than an expensive piezo bit. >> > > The ones I have seen work that way. There may be different techniques for > sensing the air pressure change, but the part that goes on the road is just > a rubber hose. > > >> I believe that they do use piezo for speed cameras that are buried in >> the road. >> > > The permanent in-road sensors I've seen have all been large loops of wire, > about a meter or two accross (they tend to be elongated in the direction of > travel). > > > ******************************************************************** > Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products > (978) 742-9014. Gold level PIC consultants since 2000. >