How do I read a load cell just with this data?

Question

I have a load cell (capability = 23 kg or 50 lb.) Its output is 2 mV/V. I have read that that means 2 mV per each excitation volt.

The power connection of this load cell is +15V-GND, so 0.002 * 15 = 30 mV. I guess this is the maximum output that the load cell can achieve. For the maximum weight (23 kg) I will get 30 mV, or for the maximum force (23 * 9.807 = 225.56 Newtons.)

I will get 0.03V. Measuring the output I can read LC- = 7.4 V and LC+ = 7.4 V for no force applied (actually, there are more decimals, due to the offset) (experimental power supply is 14.8V.) How do I know if it is tension or compression? For example, 0.3 can be the result of 7.7 - 7.4, but it can be the result of 4.5 - 4.2 too. What am I missing here? I can read the differential output with a differential op-amp, but how can I relate that voltage measure to a real good force (N) value?

Additionally, measuring the offset gives me weird values:

• LC+ = 7.4013
• LC- = 7.4030
• Difference = 0.0017

According to Force = (23* Vmeasured/0.03) *9.807 the result is 12.78 Newtons (way too much for offset, isn't it?)

Can I calculate force in Newtons just by multiplying by 9.807 in the previous calculations even if the force is not vertical (gravity) (being horizontal)?

Answer 1

The rated full scale is 23 kgf, which is non-standard but more accessible to the general population way of writing 225 N. kgf is often written as kg, when the writer either assumes it will be used for weigh scales, or ignorantly, not knowing or caring that mass is not weight or force.

At full scale, the output will be 2 mV per volt of excitation voltage, or 30 mV for 15 V excitation.

You're reading an offset of 1.7 mV in 30 mV, or 5.6%. That doesn't sound unreasonable, unless it's a highly specified device. What does the data sheet claim for unloaded offset?

For compression, you'll read 30 mV with one polarity, for tension you'll read the other polarity. You can either read the data sheet carefully to see whether they have specified which way round the leads are, or you can give it a pull and observe the meter. Better still, do both.

The load cell is physically a Wheatstone Bridge inside, with four nominally equal resistors. The outputs will always be approximately mid-way between the excitation terminals. There is no way that you could get outputs around 4 V with 0 and 15 V excitation, except with a broken load cell. You may be able to use the fact that the output will always be approximately mid-rail to simplify your measurement electronics, you do not need a differential amplifier with a wide input voltage common mode range.

The load cell responds to force. A force of 225 N can also be written in a non-standard way as 22.9436 kgf, the magnitude of the force exerted on a mass of 22.9436 kg in a standard gravitational field strength of 9.80665 m/s2.

Your local gravity will be within 1% of this, the variation due to crust composition and the earth's rotation. There will additionally be a reduction of gravity with altitude. As your local gravity varies, the mass required to generate a weight of 225 N will also vary slightly from 23 kg.

A force can act in any direction. The load cell can be placed in any orientation, and will respond to a force along its measurement axis, whether vertical, horizontal, or some other direction.