Reading a Load Cell Value with Picaxe

[hartparr]

I have a load cell and one of THESE cool signal amplifiers from Omega.

Picaxe can read a ADC value between 0 and 255 right? I want to be able to measure 0 to 900 Kilograms on the load cell by the whole Kg.

How do I go about accomplishing this if (what I understand anyway) 255 is my max increment?

Thanks in advance.

 

[eclectic]

Have a look at Manual 2

Readadc10

1024 units / increments.

e

 

[lbenson]

>1024 units / increments.

But with unit accuracy?

 

[mega]

That's an expensive amplifier, I've found this chap has a good handle on them.

http://www.steeman.be/?p=192

 

[hartparr]

Also just had a thought about this load cell. Will the values be linear so I can always just do a little math to convert the reading to Kg?

 

[sghioto]

Generally a good load cell should be linear within 1%. Do you have the spec sheet or a link to the cell? I would use the readadc10 as suggested and just convert the ADC reading directly to Kg. That would make the scale from 0 to 1023 Kg.

Steve G.

 

[Dippy]

I see there are 3 versions of that (apparently) cool and totally awesome signal amplifier.

Which version do you have?

What output do you get under the range of loads you require?
If the signal is not 0 to 5V (or 0 to a nice Volt) then extra fiddling may be required.

Linearity? I'll bow to Steve's experience. The keen projecteer would actually experiment with known masses and determine a linearity curve.

What is your level of electronics?

 

[MFB]

Too expensive!

That’s sound advice about using readadc10 and direct scaling to 1024 kg but there is no way you should spend that much money on an amplifier module, unless you are operating a remote low impedance load cell in a electrically noisy environment. The expensive features of the module relate to compensating for the resistance of, and rejecting noise pick-up in, the connecting wires between the load cell and amplifier.

Depending on the impedance of the load cell and the operating environment, you should be able to obtain 0.1% accuracy using a single-chip instrumentation amplifier costing only a few £UK. This device should be mounted near the load cell and the amplified signal sent to the ADC over a screened cable.

 

[hartparr]

Specs

The Amp is an Omega DMD-466-DC so I can run it on a car battery. I don't actually have it in hand yet, its just on order, so I assume I'll be able to send it back if you guys suggest a more ecinomical option. This is only my second electronics project, so I was at the mercy of the Omega guys. Sounds like they were running up the ticket.

The Load cell is a LCM101-1K "S" beam load cell also from Omega.
Excitation: 10 Vdc, 15 Vdc max
Output: 3 mV/V ±.0075 mV/V
Linearity: ±0.03% FSO

So what exactly would you guys suggest for the Amp that is a lower cost option?

Also, what kind of tachometer interfaces well with Picaxe?

 

[MFB]

I have often used the Maxim MAX4194 for amplifying signals from pressure sensor bridges and other types of low level sensor. However, this choice was mainly determined by the need to operate from a single 3-volt supply. I suggest you make your instrumentation amp selection based on the application notes that are available on the Analog Devices and Maxim web sites. These include information about the best way to screen the wires between the load cell and the amplifier, but even this may not be necessary if you can mount the amplifier near the bridge.

 

[hartparr]

What about the tachometer?

 

[MFB]

You really need to supply more information about your tacho requirements. For example, what rpm range, resolution and the sensor environment. There is a choice of sensors (opto, magnetic etc) and PICAXE input options. The count command may be better for higher rpm, whilst pulsin could be used for lower rpm. You could even use an F-V chip and feed its output to an ADC channel.