Encoders on Devicenet - experience ?

Have anyone experience with encoders on Devicenet?

We are starting up a project where we have to measure product length, and it is done with an encoder. Is it possible to use encoder on Devicenet with a good result?

We will have around 5 IO nodes on Devicenet and use a CompactLogix PLC.

I am not looking for encoder type, but only experience with use of encoders on Devicenet?

I have experience with absolute encoders on DeviceNet. For slow-moving, precise applications they are terrific.

I've also used encoder/counter modules like the 1734-IJ/IK or -VHSC on DeviceNet. Again, they're terrific for what they do.

What you're not going to want to try is to close a position or velocity feedback loop using a networked encoder or encoder module.

Once you measure the product length, what do you have to do next ? Can you cite how many pulses per revolution you intend to get from the encoder, and how precise you measurements need to be ?

I have used Peperl + Fuchs absolute Device Net encoders on the Sydney Harbour Bridge into Omron CS1 PLCs with great results. The application is slow moving cranes where position is not absolutely critical. The encoders are being used on the crane long travel up the bridge to lift hydraulically operated arms at the cross braces and to detect distance travelled down by the work platforms that hang off the sides of the cranes. The cranes are used for painting the bridge, lifting up Sydney's famous NYE fire works, lifting other gear and as work platforms for repair and maintenance works.

My favorite absolute encoder application on DeviceNet is not quite as cool as yours, Bob, but it's also Big and Heavy.

A dam on the Columbia River uses an absolute transducer on DeviceNet to measure the depth and velocity (though it's measured in millimeters per minute) of an "RSW"; a Removable Submersible Weir, which floats or sinks to facilitate the flow of infant salmon fry over the dam's spillway instead of through the dam's turbines. The previous 4-20 mA method did not have enough resolution to give them a decent velocity measurement (the counts were 0 per minute or 1 per minute) so the RSW didn't respond fast enough once it got really heavy and began sinking. The digital interface gives them about 60x better resolution.

I'll mention the most common pitfall, too: if you use an older controller platform like SLC-500 you will not have 32-bit math readily available to you for position calculations. Most absolute encoders will have a maximum output value that is greater than the 32767 you can get from a 16-bit signed integer. You won't have that problem with CompactLogix (or with most Omron controllers that support DNet, right Bob ?) but it's worth mentioning.

True Ken. The thing I liked about the Pepperl + Fuchs DM58 is that it is programmable from within the Device Net configuration software. We programmed these devices to represent true millimeters rather than heaps of numbers that had to be scalled/calculated. The Omron and AB software both have this ability I believe. Kept the numbers down too. The internal scaling was set up to match the circumferance of the steel rope wheel where the encoder is mounted to the shaft by way of a flexible coupling.

The design for the job presented quite a challenge for both the mechanical engineer and myself. At the end of the day it works very well. I like these kinds of challenges and "toys". Great to work with and fun and frustration to work out which way to go.

The weir job would have been interesting also Ken. Keeps the grey matter working sorting these things out.

Hi Ken and Bob

Thanks for the answers.

The machinery is not high speed; it is winding product at a speed of 10 m / min.
We are measuring length of the product and when the length = setpoint, we stop the winding and start the saw to cut the product. After the cut we start winding again to measure the next etc. etc.

The length of the product has to be in +/- 1 mm, so it has to be a precise measuring we are making. I was thinking of an encoder with 3 - 4 pulses per mm.

I will take a look at the Peperl + Fuchs encoder to see about it is better then the one there is on the list. It is a feature that is programmable from within the Device Net configuration software.

Jera.
We use AB absolute encoders on devicenet and they are accurate and reliable. Technically I do not think that they are actually "absolute" at all but that they are incremental encoders with enough built in intelligence to give an absolute position.
Andybr

I prefer a factor of 10 to my required resolution. If I were needing +-1 mm, I'd look for something in the 20 pulses/mm.