Hi I am doing a project were I would like to measure the weight of an object of up to 2Kg with a reasonable level of accuracy. The aim is to keep it as low cost as possible, strain gauges are a bit too expensive for what I am looking for. Is there anything else? I was thinking that I could use those piezo disks by measuring the internal resistance like suggested in this tutorial, is that for dynamic measurements though? Any pointers would be really appreciated.

 

Hi I am doing a project were I would like to measure the weight of an object of up to 2Kg with a reasonable level of accuracy. The aim is to keep it as low cost as possible, strain gauges are a bit too expensive for what I am looking for. Is there anything else? I was thinking that I could use those piezo disks by measuring the internal resistance like suggested in this tutorial, is that for dynamic measurements though? Any pointers would be really appreciated.

You could hand-wind your own LVDT and drive it with a 555 timer.

 

What do you consider too expensive? Here is a load cell for under $10:

http://www.kr4.us/load-cell-10kg-straight-bar-tal220.html

Combine with a load cell amplifier for $10 and you're set:

http://www.kr4.us/sparkfun-load-cell-amplifier-hx711.html

 

There are load cells on ebay starting at 99c up to as high as $3.00!
Max.

 

Piezo disk are not at all well suited for your intended application. They are designed more along the lines of shock and vibration sensors. If you want to measure weight a strain gauge is the viable solution and they are very inexpensive. I have bought them on Amazon for under $10.00 USD and strain gauge amplifiers can also be had very inexpensive as was pointed out. Anyway a piezo will not get you where you want to go. A piezo crystal is not designed to be a variable resistive device. Piezoelectric technology is not what you want for a scale.

Ron

 

a strain gauge is the viable solution and they are very inexpensive.

Sounds like something I should build for my wife's car. That way she can see just how much whip-lash she gives me when she drives. Hard on the gas. Just when you're recovering from the sudden acceleration she totally lets off the gas and your head flies forward. And just when you've about got your head pulled back onto your shoulder she hammers the gas again. She's the same with the brakes too. That's why in the van pool she rides in - nobody wants her to drive. Then there's the drifting out of lane and the jerk to correct. About every 30 seconds.

Load cells. Good advice. I'd think with the piezo's they measure change in force. If a force is applied at a steady rate, I don't think they'd register anything. What I mean is - if you apply 50 grams of pressure to one of the pads, the act of applying the pressure will register. But once the force is stabilized (50 static grams of pressure) then the piezo probably won't generate a signal.

And I'm surprised to hear they have a resistance. I thought with quartz crystals, if you strike them they can generate a high voltage. It's the compressive concussion that is the mechanical force that makes them generate a signal. Set a 50 pound weight on one and leave it for 2 seconds and I bet it's not going to generate any voltage or current. Pick up the weight and you should see a response.

Piezo's are used as speakers. When a voltage is applied they distort. When the voltage is taken away they relax. When the voltage swings in the opposite direction (current I mean) they distort in the opposite direction. Hence, they make excellent speakers for the higher frequencies. No need for cross-overs. They respond well to a resonant frequency and respond fairly well to harmonic frequencies down to a point. But below a point they just move too fast to make any useful sound such as a midrange or a base carrier.

 

Here you go

Piezoelectric load cells work on the same principle of deformation as the strain gauge load cells, but a voltage output is generated by the basic piezoelectric material - proportional to the deformation of load cell. Useful for dynamic/frequent measurements of force. Most applications for piezo-based load cells are in the dynamic loading conditions, where strain gauge load cells can fail with high dynamic loading cycles. It must be remembered that the piezoelectric effect is dynamic, that is, the electrical output of a gauge is an impulse function and is not static. The voltage output is only useful when the strain is changing and does not measure static values. However, depending on conditioning system used, "quasi static" operation can be done. Using a so-called "Charge amplifier " with "Long" time constant allow accurate measurement lasting many hours for large loads to many minutes for small loads. Another advantage of Piezoelectric load cell, conditioned with a Charge amplifier, is the wide measuring range that can be achieved. Users can choose a load cell with a range of hundred of kN and use it for measuring few N of forces with the same Signal/Noise ratio, again this is possible only with the use of a "Charge amplifier" conditioning.

 

Hi I am doing a project were I would like to measure the weight of an object of up to 2Kg with a reasonable level of accuracy. The aim is to keep it as low cost as possible, strain gauges are a bit too expensive for what I am looking for. Is there anything else? I was thinking that I could use those piezo disks by measuring the internal resistance like suggested in this tutorial, is that for dynamic measurements though? Any pointers would be really appreciated.

Cheap sensor is one requirement but what device are you using to convert the sensors output to a human-readable value?

Are you using a volt meter, ohm meter, frequency counter, comparator, micro controller (timer, ADC, comparator, ...), ...

 

Sounds like something I should build for my wife's car. That way she can see just how much whip-lash she gives me when she drives. Hard on the gas. Just when you're recovering from the sudden acceleration she totally lets off the gas and your head flies forward. And just when you've about got your head pulled back onto your shoulder she hammers the gas again. She's the same with the brakes too. That's why in the van pool she rides in - nobody wants her to drive. Then there's the drifting out of lane and the jerk to correct. About every 30 seconds.

That was seriously funny. Tony, I think what you need is an accelerometer:

An accelerometer is an electromechanical device used to measure acceleration forces. Such forces may be static, like the continuous force of gravity or, as is the case with many mobile devices, dynamic to sense movement or vibrations. Acceleration is the measurement of the change in velocity, or speed divided by time.

A three axis accelerometer with a simple recording device would be fine.

Movement - Accelerometers

Ron

 

I will be using an ADC. I was looking into the LVDT idea and quite like the idea of measuring the gap and calculating the force from the known spring value (do you know how big of a problem creep would be?). What do you think the absolute cheapest way to measure the gap electronically would be? I was thinking along the lines of making some kind of potentiometer or using the disks and seeing if some kind of sonar would be possible using the arduino that I already have.

 

Better go which chinese Load Cell and HX711 with a PIC16F877A.

 

I was thinking that I could use those piezo disks by measuring the internal resistance like suggested in this tutorial, is that for dynamic measurements though?

Piezoelectric sensors DO NOT change their internal resistance (which is usually extremely high, often hundreds or even thousands of megohms) in response to stress; rather, they produce a charge. This charge can be read out from a charge amplifier, which converts it to a voltage, or directly from the sensor if it is connected to an amplifier or buffer having an extremely high input impedance. They are useful ONLY for dynamic stresses.

This Wikipedia article has a lot of good information. Be wary of electronics "tutorials" on the Internet; too many of them are written by people who don't know what they're talking about.

 

I will be using an ADC. I was looking into the LVDT idea and quite like the idea of measuring the gap and calculating the force from the known spring value (do you know how big of a problem creep would be?). What do you think the absolute cheapest way to measure the gap electronically would be? I was thinking along the lines of making some kind of potentiometer or using the disks and seeing if some kind of sonar would be possible using the arduino that I already have.

Creep will be a function of the spring construction, and effects load cells as well.

I posted this link years back, if it helps:

http://www.cs.rochester.edu/~nelson/courses/csc_160/linear_systems/lab_lvdt.html

 

I will be using an ADC. I was looking into the LVDT idea and quite like the idea of measuring the gap and calculating the force from the known spring value (do you know how big of a problem creep would be?). What do you think the absolute cheapest way to measure the gap electronically would be? I was thinking along the lines of making some kind of potentiometer or using the disks and seeing if some kind of sonar would be possible using the arduino that I already have.

OK, you have a few considerations here. You need to start defining a few things. You want to measure a maximum of a 2.0 Kg weight. You only mention a "reasonable level of accuracy". You need to consider what reasonable is? You will have things like the uncertainty, the resolution and the repeatability in your measurement plane. Next you mention using an ADC but do not mention a specific ADC?

You need to sit down with a nice clean sheet of white paper, a sharp pencil and a large eraser and start laying out what you want to do, starting with the sensor. While a LVDT can be used to measure travel as a spring is compressed designs like this more favor measuring travel than in a scale or weight measuring design. Not to say a LVDT won't work, just to say there are much better ways to go about this. A good A/D converter can be anywhere from inexpensive to very expensive depending on the bit count. How accurate do you want? What resolution do you want? What repeatability do you want? All of that starts with the chosen sensor.

Ron

 

Weighing Machine which I made recently. It is very accurate.

I used HX711 24 Bit ADC Based Load Cell sensor module, PIC16F877A and 5 Kg Load Cell with Mechanical setup.

Here is the video

http://www107.zippyshare.com/v/94DL5xwk/file.html
http://www107.zippyshare.com/v/hhTjOBlH/file.html

Password is pa55pa55what

Download and extract the video file and view it in VLC Player.

If you are interested then I can make a Weighing Scale for you using HX711 and any PIC or AVR or even 8051 microcontroller but I will charge a small amount of 100 USD for it. I will give you circuit and C Code with firmware.

 

Here is the video

http://www107.zippyshare.com/v/94DL5xwk/file.html
http://www107.zippyshare.com/v/hhTjOBlH/file.html

Password is pa55pa55what

Download and extract the video file and view it in VLC Player.

If you are interested then I can make a Weighing Scale for you using HX711 and any PIC or AVR or even 8051 microcontroller but I will charge a small amount of 100 USD for it. I will give you circuit and C Code with firmware.

YouTube.com doesn't work in India?

 

Youtube works in India but file size is big and sometimes when I upload video to youtube then youtube changes it resolution and resolution will be very poor and grainy. Hence the video file.

If zippyshare.com doesn't work then I will upload to other server and provide links.

 

Youtube works in India but file size is big and sometimes when I upload video to youtube then youtube changes it resolution and resolution will be very poor and grainy. Hence the video file.

If zippyshare.com doesn't work then I will upload to other server and provide links.

If a LONG video is required to demonstrate such a simple concept, I'll let someone else watch and repot back here.

 

It is not long video. Camera resolution is high and hence video file size is big.

It shows the working of weighing scale which I made. First I measure with a calibrated weighing scale and then with my weighing scale to show the weight readings which are accurate upto microgram.

I have Malwarebytes Anti-malware and hence no problem downloading files from zippyshare.com.

You can just use pop-up blocker when downloading from zippyshare.com

 

hi moo,
Have you seen this 5kg L/C from Amazon, I have one for testing.
https://www.amazon.co.uk/dp/B00TK0ULH8/ref=pe_3187911_185740111_TE_item

E