Scan Time issue

[ADVANCED-Joe]

Hi all,

I've recently been trying to make home-made profiler with a transducer and a laser.

The setup is pretty simple, the laser slides on a rail which is attached to the magnet of the transducer. The transducer gives me the position (x) of the laser and the laser gives me the distance to an irregular surface .

Combined together, if i take the distance given by the laser at every 1mm, I can create the profile of the irregular surface.

My issue is the PLC scan time which is about 20ms to 50ms. Since the scan time too long, the system has time to go through multiple positions before the scan is done which makes it impossible to get a distance value at every 1mm.

Does anyone have experienced something like this before?
I'm looking into buying a data logging card, but i don't know is this is my best option.

 

[James Mcquade]

welcome to the forum.

what is the brand name and model of the laser, transducer, and plc?
how are you moving the transducer along the slide?

a few more details about the setup of your system is needed.

james

 

[ADVANCED-Joe]

Laser : Keyence IL-100

Transducer : MTS Temposonics R-Series, RP S 1200U D60 1 A01

Both sensor give 4-20ma signal

Both of them connected to Flex/IO 1794-AENT in an Isolated analog input 1794-1F41

PLC is an Allen-Bradley 1756

For now the laser is on a rail and is moved by hand. The transducer magnet is clamped with the laser so when I move the laser, the magnet follow. It’s a very primitive setup and will be upgrade later to automate the laser motion.

 

[AustralIan]

Beckhoff has some modules that will buffer the readings for you into your normal scantime. The ethercat network is good at synchronising time between modules should you want the exact same timestamp on your two inputs. Taking beckhoff part number EL3702 for example, the -10 to +10V version. This module can transmit 100 samples per scan, up to 100k samples per second. 1ms or smaller cycle times are typical for beckhoff and other Codesys controllers. There are many other Codesys controllers that can use these ethercat terminals as well, like Siemens IOT2000.

 

[AustralIan]

Looking at the data sheets, your fastest update rate of your two sensors is 1ms. Is that enough for you?

I can't think of an AB remote IO solution that will give you 1ms update, perhaps you will need to wire to local IO. Even then 1ms can be tough.

 

[ADVANCED-Joe]

Yeah 1ms is enough, for my application I would need max 3ms.

 

[OkiePC]

Don't forget about the latency of the analog input itself. You may be able to configure the input point or card for reduced latency, but don't forget that it will still likely be greater than the PLC scan time and a bigger influence on the success of your project.