timk5000

Connecting Sensors to External devices

5 posts in this topic

We are looking to build a monitoring station to monitor several machines in our plant.  
It will have a power supply, PLC and a HMI. Is there a preferred way to connect a PLC to sensors in several machines.

In some cases we would have inputs connected to auxiliary contacts just to determine if a machine is running
We will have 24 vdc sensors in other machines. Is it best to power the sensors from the Monitoring Station using the same power as the PLC or should it come from the machine.

  Thanks

 

Share this post


Link to post
Share on other sites

Ask three Automation Experts the question you just asked and you'll get at least nine different answers.

A lot will depend on your brands of PLC and HMI involved.  Can you give some idea what brand(s) are involved/

Are all your machines PLC automated?

Have you considered a SCADA solution like Ignition, Wonderware or OSISOFT PI  for monitoring?

Share this post


Link to post
Share on other sites

My opinion is that it is it best to power the sensors from the monitoring station end using a single large DC power supply.  

My experience is that using the power from a remote device to power local sensors is very likely to produce ground loops, an electrical artifact due to the fact that earth ground potential between different areas is not the same and current flows when connected between points at different potentials.   And devices eventually reference earth ground.   

When a central power supply is used with isolated field transmitters like analog 4-20mA 2-wire loop powered transmitters, ground loops are rare.    Powering device with the remote device power supply produces ground loop, common mode problems devices like,
- unexplainable offsets that do not appear when a battery powered analog source is substituted for the field device,
- analog signal levels that drive off-scale
- formerly functional, working signals that either suddenly offset or drive offscale when an additional signal is added to a multi-input module, or
- in extreme cases, analog input burn-out due to excessive common mode voltage.  

The reason for the development of 2-wire loop powered devices was to
- use a common DC power supply at the monitoring end where all the signals end up
- use the 2 signal wires to provide operating DC power to the field sensors
- eliminate ground loops   

 

1 person likes this

Share this post


Link to post
Share on other sites

Loop power exists because one cannot trust electricians to power field devices. 

Share this post


Link to post
Share on other sites

LOL, that could be one of the reasons...

are those machines automated with some sort of industrial controller (PLC, DCS, Robot ....).

if so what are the network options? for anything inside plant today, one would try to go Ethernet. plenty of existing installations use legacy bus type networks. 

The idea is that each controller would take care of local automation and if needed communicate with other units.

 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now