It would help if you post or URL for the laod cell datasheet.
Basically 80% of the project can be done by placing the delsig on schematic, right click,
and you pick "find example project" for A/D, and import it into your build or use standalone.
For starters you need an Instrumentation Amplifier, PSOC can do that,
but at less precision than is avaiable as a dedicated IA chip. Plus the load
cell, if excited by > PSOC Vdda, implies the IA cannot be internal to PSOC,
due to its CM input range limitation on inputs.
I have attached some info that may be of help. Further info here on IA design -
I strongly recommend you set an error/precision/accuracy goal, then do an end
to end analysis of the errors to see if the PSOC IA solution will suffice.
Load Cell Reference Material.zip 447.7 K
Thanks for the help Dana!This is my big problem because i'm beginner and i'm not able to select the right components.For this reason i'm looking for a ready avaiable project refer to this document http://www.cypress.com/?docID=34330 ,Figure 5. Integrated Weigh scale solution based on a Cypress PSoC 3 device .
Hi BRAKA, do you have any thing done with weight scale??? Because I'm having the same problem as you. If you did any can you kindly lend me your project? I will credit that it is yours. Or at least lend me a hand with the weight scale.
Edgardo from Buenos Aires-Argentina
My guess is BRAKA is no longer monitoring this thread from a year ago. Regards, Dana.
we have a requirement to design a weighing system to weigh a variable Load in the range 0-20 Kgs as accurately as possible , which will be placed above a fixed weight of about 1000 Kgs.
Can any one suggest a possible solution?
Hopefully these comments are at least little bit useful for you.
Your question contains so few details that any suggestion may go horribly far from the answer you are looking for...
E.g you should take a different approach depending on how many measurement systems/products will be produced.
Anyway, here goes. I would suggest you would start to narrow down the solution by first understanding the mechanical aspects of the construction and minimum accuracy requirement you will need to meet. And as always, you need to know your feasible price range for the end solution.
Load cell = piece of metal with strain-gauge attached to the side of it.
Load cell + AFE (analog front end) + MCU
First: You need to understand how much mechanical strain the piece of metal (load cell) will have to endure. Not just static load but also dynamical impacts etc. and how much over the nominal value the metal should hold static weight before starting to yield (safe requirements).
--> now you can look for load cells or know the cross-section are of your metal piece and can figure out what kind of strain-gauge is good for you.
(If you are designing a product: read about strain-gauges components. HBM has very good material available about their products as well as a lot of general info. If you are not able to always calibrate or be able to just measure changes to "empty scale", you should read also about how strain-gauges drift over the years.)
Second (a): Select suitable load-cell. If you find product with large gauge factor you need less from your AFE. (make sure mechanical requirement are met.)
Second(b) "building a product": Now you should know mechanical details about your load cell metal piece in order to select suitable strain gauge. (material, cross-section area for Tensile Strength calculation(UTS and YTS) --> then you can calculate strain with max. load (full scale) --> select strain gauge.
Now, your load cell material should be known not the yield under any possible condition and strain gauge component is selected to endure max. stretching of the metal under full load.
Okey, now you can calculate what kind of excitation voltage you will get with your input voltage to the bridge measurement ( at least half bridge would be advisable) and with the load measurement accuracy you are looking for.
TI has quite good AFEs for this purpose. I would start there. Select top of the line AFE component, some 24 to 32 bits model.
check that your full scale reading is in under max. input voltage value of AFE. if not, adjust.
Calculate how many AFE bits is your accuracy requirement is (converted to excitation volts). Make sure your requirement is well over the AFEs probable noise level and that your AFE is more accurate than your accuracy requirement (4 times more accurate is good, 10 times is golden).
That's it, more or less. Connection to e.g. PSOC is quite simple.
btw. If the "about 1000kg" load is always present unchanged and you are able to calibrate the measurement circuit, you could calibrate the static load off by adjusting the measurement bridge circuits balance(if adjustable or self designed). Then, when starting near zero kg load (meaning near zero excitation voltage), you could perhaps supply larger input voltage to bridge circuit and get larger excitation voltage with "full 20 kg load" that would be still under AFE´s max input range . Well, there where a lot of "if"s in the last chapter...
I just noted how long it has been since you posted the question......;)
well, perhaps somebody will get some useful info out of this answer.