What is the best of connecting a load cell to a plc

Hi guy
I have been asked if we can connect a loadcell to a plc and control an output ,
What they won't to do in the ladder is to use the input value ( load cell) if that equals the setpoint then switch output,

Not to sure about the connections , I am using a micrologix 1200 not to sure what cards I will need , and help would be great thanks stu

Can you specify which load cell amplifier you are using ?

You will need an analog input card for your PLC. Plus, most load cells take a specified supply voltage (usually 10V) and output a millivolt level signal that is a ratio to the input voltage. For example, if the ratio is 3 mV/V and you have a 10V excitation, then the output will range from 0-30mV (empty to fully loaded). This signal is too small to be accurately read by a typical PLC input, so you need a signal conditioner to amplify the voltage.

Omega has a good selection:
http://www.omega.com/toc_asp/subsectionSC.asp?subsection=E04&book=Pressure

There are also devices that will amplify the signal and provide a fieldbus output rather than analog. I've used one from Red Lion (with Devicenet interface) with good results.
http://www.redlion.net/Products/DigitalandAnalog/DigitalPanelMeters/StrainGage.html

this is early stages of this project,so what ever you can suggest.
thanks stu

I've used FMS in the past e.g.

stu said:

Hi guy
I have been asked if we can connect a loadcell to a plc and control an output ,

Click to expand...

Yes, but it may not be the best option.

What they won't to do in the ladder is to use the input value ( load cell) if that equals the setpoint then switch output,

Click to expand...

Why? What is the purpose?

We deal with press manufacturers and metal forming all the time. It is important to have a load cell and amplifier that are fast relative to how fast the force increases. Some things are compliant and the force increases relatively slowly and a PLC will do. Other things are not compliant and the force rises so quickly that the load cell amplifier and/or PLC isn't fast enough.

I use load cell amplifiers from PR Electronics with a 4-20mA Output.

stu said:

Hi guy
I have been asked if we can connect a loadcell to a plc and control an output ,
What they won't to do in the ladder is to use the input value ( load cell) if that equals the setpoint then switch output,

Not to sure about the connections , I am using a micrologix 1200 not to sure what cards I will need , and help would be great thanks stu

Click to expand...

Yes you can, you naturally need amplifier as pointed out. But what is this process? is it just slow weighting system or does it need to control force fast?

stu said:

Hi guy
I have been asked if we can connect a loadcell to a plc and control an output ,
What they won't to do in the ladder is to use the input value ( load cell) if that equals the setpoint then switch output,

Not to sure about the connections , I am using a micrologix 1200 not to sure what cards I will need , and help would be great thanks stu

Click to expand...

check out Hardy instruments they make loadcell modules that plug straight into micrologix
http://www.hardyinst.com/

it converts straight to actual weight you just configure it for unit of measure and you can trigger outputs by comparing the real weight with the preset

the only thing i am not sure of is if they offer a compatible card for micrologix 1200.

the only thing i am not sure of is if they offer a compatible card for micrologix 1200.

Click to expand...

Good question. If they had one it would save you a lot time on the programming side.

One good thing about Allen Bradley Micrologic is the program can be carried over to another Micrologic processor. The bad thing is you will spend the money on a new Micrologic processor.

I am using a micrologix 1200 not to sure what cards I will need , and help would be great thanks stu

Click to expand...

Stu,

It is not widely known or used, but if you have a 1762 Thermocouple/mV Input card, then you can use it to get a load cell millivolt signal directly into your MicroLogix 1200. I have done this and it did work.

This method is not recommended for precision weighing, but if you need a "close" (fairly accurate) weight, then it can be made to work. The reason is that the Allen Bradley Thermocouple/mV cards have an option where you can select (instead of a TC type), the milivolt input options of 0-to-100 millivolts or 0-to-50 millivolts. The 0-50 mV range can be used for many load cells.

It helps if you have a 10 volt load cell with the 3mv/V ratio. Then your max output will be 3mV x 10V = 30 mV, which is getting close to the 50 mV range of the 1762-IT4 and 1762-IT6 Thermocouple/mV Input modules. If you nudge your load cell excitation voltage up to 12 volts, then you can get even closer - 36 mV. All you need to do then is in your PLC program, scale your actual maximum 30 or 36 mV input to represent the maximum weight that you will have on the load cell.

PS: Use shieled cables for both the excitation voltage to the load cell, and for the signal wires from load cell to the PLC input module. Also make sure the load cell is mounted on a stable platform with minimum vibration. Still you may have to use filtering methods to reduce the background signal noise.

kwires said:

check out Hardy instruments they make loadcell modules that plug straight into micrologix
http://www.hardyinst.com/

it converts straight to actual weight you just configure it for unit of measure and you can trigger outputs by comparing the real weight with the preset

the only thing i am not sure of is if they offer a compatible card for micrologix 1200.

Click to expand...

Just be careful, the hardy module is designed for actual weigh-scale load cells, and not what we in industry commonly call load cells. If you use a weigh-scale amplifier with a typical industrial load cell, the signal level will be much too high. You can use an external resistive divider bridge to bring the signal level down though.

rdrast said:

Just be careful, the hardy module is designed for actual weigh-scale load cells, and not what we in industry commonly call load cells.

Click to expand...

RD, I did not know that. How can I tell the difference between the two types of load cells?

rdrast said:

Just be careful, the hardy module is designed for actual weigh-scale load cells, and not what we in industry commonly call load cells. If you use a weigh-scale amplifier with a typical industrial load cell, the signal level will be much too high. You can use an external resistive divider bridge to bring the signal level down though.

Click to expand...

I have used the Hardy module a couple of times with 2 single cell scales controling fill valves on a filler, but i was under the impression you could also use them with industrial load cells and you wouldn't need the amplifier. Maybe i mis-read the literature?

Lancie1 said:

RD, I did not know that. How can I tell the difference between the two types of load cells?

Click to expand...

kwires said:

I have used the Hardy module a couple of times with 2 single cell scales controling fill valves on a filler, but i was under the impression you could also use them with industrial load cells and you wouldn't need the amplifier. Maybe i mis-read the literature?

Click to expand...

A typical shaft tension load cell, like those made by Cleveland Machine Controls cartridge cell (and similar designs) has a nominal output of approximately 350mV at full rated load with a 5VDC excitation supply. These type of cells also generally have a turndown ratio of between 8:1 and 40:1, though can be useful at higher ratios if care is taken.

A weigh scale load cell on the other hand, has a typical output of about 2mV/VDC excitation, so with 5VDC excitation would have a full scale output of 10mV at full rated load. The turndown ratio on a weigh scale load cell is usually on the order of 100:1 minimum, but can easily go to >1000:1 on decent systems.