load cell output?

[neels]

hi friends
can anybody please tell me that what is the kind of output that a load cell gives and how can i interface it with microcontroller 89c51.
the weigh scale is from 0gms - 25 gms with resolution of 0.01 .which load cell to use for this?

waiting for reply
please help
neelam

 

[williB]

a while ago i googled load cell and found somone that made their own from a brake cylinder..
he then connected the brake cylinder to a pressure gauge..
if you took the output from the cylinder and fed it to a analog pressure sensor , that might work..
EDIT:: or digital pressure sensor..
i am not sure i fully understand your problem ?? the scale you are using IS a load cell..

 

[Beefer3]

neels
try www.omegadyne.com

 

[instruite]

Hi,
Load cell is a transducer which converts pressure (weight) into electrical signal. It basicaaly consists of strain guages and signal conditioning circuit
The range of the output will depend on the kind of laod cell
but standard Industrial output standards are
0-5V DC
1-5V DC
0-10V DC
4-20mA DC

the first two types of output can directly be connected to AD port of microcontroller

third type will need voltage divider
and last type will require current to voltage converter circuit

 

[askcq]

hi

How to calibrate the voltage output from load cell as approximate weight

need some idea

 

[ericgibbs]

hi

How to calibrate the voltage output from load cell as approximate weight

need some idea

hi,
What is the excitation voltage of the load cell, also what is the mV/Vext and the maximum weight specified for the load cell.?

 

[Oznog]

The source voltage is actually not important as long as it's not high enough to cause heating of the resistances inside.

OK, a load cell is a differential Whetstone resistance bridge. There are two output voltages, and only the difference between them matters. The signal is VERY small, only millivolts for full scale. The signal is ratiometric to Vdd, and voltage references for the ADC should NOT be used- Vdd must be the reference.
In short, for a number of reasons, the PIC's (and I assume 89c51) ADC is generally unsuitable unless your precision and offset requirements are very loose.

There are two types of load cells- the 4 wire type has a pair for Vdd/GND (doesn't matter which one you connect to which) connection and 2 output wires. The 3 wire type has two measuring resistances inside and you must add external pullup (or pulldown) resistors which MUST be high precision, stable resistances if you want to avoid offset errors or even saturation of the scale's ADC. You can tie the common wire to Vdd or GND, it doesn't matter, just use the pull resistors to the opposite rail.

It's difficult to build a true differential amp with good accuracy out of discrete components because it's VERY sensitive to offset errors. Best thing in the world is a differential ADC like the LTC2480, which is cheap and you can get free samples.

The load cells instruite refers to are complete scale units with an amplifier built-in. No load cell sensor itself has an output like that, unfortunately. I'm assuming you just have a sensor you took out of a scale or something?
https://www.electro-tech-online.com/members/instruite.html

 

[ericgibbs]

The source voltage is actually not important as long as it's not high enough to cause heating of the resistances inside.

hi Oznog,
The excitation voltage is important if you intend to use the L/C as the manufacturer intended during its factory calibration.

Typically the L/C output is specified at X mV/Vexcitation.

Regards

 

[askcq]

Hi

Iam a begineer in using load cell to industrial automation.
Could you please share me some materials for knowing in depth working of loadcell
I have a basic doubt, loadcells are by default Active.Iam I right.
Are there passive like loadcells? ( loadcell trigger without power)

 

[Oznog]

MOST loadcell cells themselves ARE passive. It's two resistances- one fixed, one variable. That's IT. An LC may be surrounded with a module which converts the tiny resistance change into a large voltage- but that's probably what you're going to be building.

 

[Oznog]

hi Oznog,
The excitation voltage is important if you intend to use the L/C as the manufacturer intended during its factory calibration.

Typically the L/C output is specified at X mV/Vexcitation.

Regards

But the voltage makes VERY little difference in the response, as long as I^2*R heating doesn't become significant. Using too much or too little voltage will still give the same ratiometric output. Even if there IS heating, it's not likely to be a functionally significant change in output. They keep the temp coefficient low and the fact that the resistances are nearly equal means the get nearly identical I^2*R heating, and are in such close proximity they probably won't see a significant temp difference even if significant I^2*R heating does occur.

Most LC sensors are not factory calibrated and trimmed. Since they require surrounding hardware anyways, the exact calibration of scale is up to the OEM or end user in the installed product. The zero point, for example, is not the LC mfg's problem at all, and that's less precise that the delta-R per kg. The OEM's gonna add an unknown table weight and whatever else during installation. And the zero-load point changes when you turn the LC upside down, since the bending arm is pulled a different way by gravity. So the mfg's tare point doesn't even matter all that much.

 

[askcq]

all loadcells need exciting voltage for operation