Created Date: June 29,2006

Created by: Bent

If you 're running analog signals with high voltage signals, typically you'll have some noise if you don't have proper grounding and shielding. It can be a rather complex topic, but you generally want to shield and ground your analog signals at the point where you 're measuring the signals, i.e. at the PLC.

If the other end of your analog signals or shields are grounded at local ground of a remotely located sensor, you'll often get common mode noise on your lines. These signals can be quite large. If this is the case, you will likely have reliability problems since the signals can spike due to power line surges, lightning etc, and potentially blow up your sensitive analog input.

Anyway, if your noise signals are not too large, if there is no rectified DC component to them, and you don't need high speed measurements, the simplest thing to do is take advantage of the PLC and simply average your data over a relatively long period of time (long compared with 60Hz signals that is) with an exponential average.

Good luck

Created Date: June 29,2006

Created by: robertkeenan

I have tried to average the readings in the PLC using a technique I found in this forum. It did make a small change but the manager of the factory thought that it had shifted the temperature slightly. The cable is earthed at the PLC end, so it must be the fact that the multicore is mixed in with the high voltage cables. Although as an experiment I wired up one of the probes with about 10m of telephone cable and set the probe in between the supply cables for a 15Kw motor. With no screening I was expecting to see the same results but was suprised to find the cable very steady and only drifting with some air movement. The temperature on sensors from the multicore can start at 10oC go up to 14, down to 8 then back to 10 within a few seconds.

Created Date: July 02,2006

Created by: DenisOZ

I have used many RTD modules in 205 & 06 PLC's as well as terminator systems and have come to the conclusion that all of them are particularly prone to picking up RF - especially from EMF generated by variable speed drives.

I have tried all manner of cable and earthing systems with varying degrees of success.

I have tried to decouple the RTD signals with capacitors, but the spread of EMF intererence seems to defeat this.

This is particularly annoying when retrofitting discrete set point controllers with a PLC based system, only to find that systems that worked fine with discrete controllers gave wildly fluctuating readings when the PLC system is implemented.

Here are some observations:

1. Use good quality screened cable and one 3 wire screened cable per RTD sensor. Using multicore cable with more than one circuit per cable will not work.

2. Even though I suspect EMF corruption of the signals, I find that grounding the shield at the source end works best.

3. Ensure that any VSD's in the vicinity have properly screened cable installations. In Australia this is now mandatory for all new installations anyhow.

4. Route cables away from mains where possible, and cross cable ways perpendicular.This is not always convenient, but does help.

5. Keep runs as short as possible.

6. I find that I have to use averaging for nearly all installations, and this is difficult for native negative temp measurement such as cool stores.

7. These units work great when used in machines such as compressor sets where the runs are very short.

I have given up using any of the RTD modules in many installations in favour of 4-20ma circuits. This does not make a lot of sense when condsidering the fact that the impedence of an RTD circuit is very low, but the analog inputs have the advantage of lower cost per input on larger installations.

This is a pity, because the accuracy and ease of setup of the RTD modules are a real advantage.

I have worked on any number of discrete systems over the years that have been wired with nothing more than household flex run over tens of metres, and I have looked very silly trying to justify my whizz bang PLC system to customers when the system is varying continually.

I have been meaning to set this up as a topic on the forum for some time now - I am glad you raised it Robert.

I wonder what FACTS take on this ongoing issue might be??

Created Date: July 03,2006

Created by: Bent

DenizOZ, did you try ferrite chokes like the cable clamp types, either in combination with or without shunt capacitors to ground?

If the interference is common mode (typical) and high frequency, the ferrite chokes should knock it down. Adding the capacitors will provide a shunt impedance for the choke to work against as well as help normalize the impedance across the lines to prevent the common to differential mode conversion of the interference.

Why do you think that the 4-20 signals work better in these long runs than the RTD type signals? Is it that the magnitude of the signals is larger? The impedance difference?

Your point about the multicore cable is a good one. For a three wire RTD, the two wire pairing in the cable is not configured for three wire RTD and individual shielded three wire cables would work best.

However, before abandoning a cable run that is already in place, I would suggest that RobertKeenan try:

1. Putting a ferrite choke on the cable. (This can be a cable clamp type choke or even just a ferrite toroid that you wind the cable through. If you have shunt capacitors, the details of the specific ferrite material aren't an issue as long as it works at high frequencies.)

2. Shunt active cable terminations to ground through a ceramic capacitor like an 0.1uF X7R.

3. Terminating any unused wires in the cable either directly to ground or to ground through a 10 - 100 ohm resistor. (floating wires can make EMI problems a lot worse. Using a resistor will help suppress high frequency resonances in these long lines, but is usually not needed, especially when you have a ferrite choke.)

Created Date: July 05,2006

Created by: FACTSTech

Robertkeenan, there are several test you can perform to figure out where your problem starts, using channel #1.

1. You must use 3 wires for RTD's

2. place a fixed resistor at the Junction block where you start the multicore cable. If you still show errors here then the capacitance of the cable is affecting your readings. Try moving the test resistor to the module input, remember you need 3 wires.

3. Any time you use a VFD/VSD you must use input reactors when using analogs, the application will depend if you need output reactors also.

RTD signals are very low level signals, excitation current is 200uA, wiring practices are very critical, I typiically do not like junction boxes in the middle of a run, the best configuration is a 3 conductor shielded cable from the RTD to the module, i.e. Belden 8403 or equiv. Depending upon the application the shield should be connnected at the RTD end but, if nothing to connect it to then the next best is to the same grounding point as the PLC ground.

Created Date: August 04,2006

Created by: robertkeenan

Thanks for all the good ideas but unfortunatly the solution turned out to be very simple. Went round all the probes with the electrician to check the wiring. As I said all the multicore screens are earthed at the PLC, but we found one probe at the other end did not have its screen connected to the multicore. Thought this might have been the problem so connected it up. Could not see any real improvement so we thought about changing the probe. When we took the lid of the box for the probe we found that the screen cable to the probe was earthed to the beam of the building. Removed this and fault cured. Even on the PLC's which are connected by different multicores.

Created Date: June 24,2006

Created by: robertkeenan

I have some DL06 PLC's with PT100 probes measuring temperatures in cold stores. The probes are brought back to a junction box and then connected back to the PLC with a 30 core multicore cable. This cable is wire aromoured and connected to the main panel with a brass gland. The cables are twisted in pairs and covered with an overall screen. My problem is that the temperature fluctuates more than normal and can only be caused by electrical noise. I recently had to fit an extra probe and this was wired with a single 3 core screened cable back to the PLC. When the screen is eathed at one end the temperature is rock steady. I have tried earthing the multicore but no matter what I do the temperature fluctuates. Could this be due to the fact that I am using multicore cable although I believe there should be no interfernce in the multicore. Any unused cables have been connected together and earthed. Or is it that no matter how good the screen is, the fact the cable runs along with the high voltage cables for quite a few meters will only cause problems.