Jump to content
OMRON Forums

Properly Ground a MS-3 or MS-4.


Recommended Posts

The attached white paper discusses Microscan MS-3 / MS-4 grounding scenarios for large, medium and mild voltage discharge scenarios typically encountered in industrial settings, and provides best grounding practice to combat each scenario.

  

Common Ground System (MS-3 / MS-4)

 

This is a cut from the schematic of the MS-3. It shows that signal and chassis are tied together within the unit. This is also the same for the MS-4.

User-added image
 

Note: The Wiring Diagrams show that the communication lines are not going through the IC332. These lines route straight through the IC-332 and have no components attached within. Thus this white paper address either case whether using or not using IC-332 in system.

 

Typical Damage caused by grounding disruption

The machine that the product is mounted to receives a large discharge

User-added image
This picture shows how insufficient Earth Grounding on the machine in question leads to damaged communication equipment. Charge is dumped onto the machine surface, then radiates out and skims the surface towards all lower potential areas. Once there is enough potential at the scanner, voltages begin to rise on the power commons looking for earth. Once this rises above part thresholds, damage begins.

Common grounding can be an issue if the system that the scanner is mounted to receives a large discharge. If the grounding in the machine is not capable of routing all of the current such that the voltage stays at a low potential, then voltage builds up and finds alternate paths to Earth. Because Microscan’s MS-3 / MS-4 chassis is connected to the system machine that receives the discharge, the voltage will enter into the ground, shield (earth) and communication I/O then follow it to earth.

Not only is the Ohmic value of the Earth Drain important but the surface area acts as a component as well. Heavy braided wire is better than solid wire for taking discharge events. Discharge prefers to travel along the surface of conductors not within. In this case either system can measure the same resistance but not be equivalent. Routing is also important, there may be a negligible Ohmic difference when measured but once struck with a large charge, the charge will move along HF low impedance paths. The 1 ground wire may be too inductive to handle this instantaneous surge and thus the charge will start entering into the scanners chassis. Charge moves along multiple paths, not just one. Using the best grounding system on the machine will work as a Current/Voltage divider. If the current can’t reach earth from the machine quickly enough it builds in the entire system as voltage. There will be varied voltage seen at different points in the machine and equipment depending on the nature of the mounting and surface characteristics. The point is that there will likely always be some component at the scanner but it takes steps to get this component down to below damage levels.

 

Three levels of undesired ground potential.

Typical to  mild discharges:


If the discharges are typical human body models, then the first line of defense to protect low voltage devices is to provide a lower impedance path to earth. This lower impedance path allows a faster current drain. This will prevent the discharge from rising up in voltage and exceeding parts tolerance levels within the Microscan units. There is still likely be a small current path back down the products but it will be reduced down to a noise level rather that high voltage power path.
 
Typical ground noise with good Earth potential
User-added image
Mild  machine  chassis discharges:
Mild discharges can come from moving parts building surface static until they build enough potential energy to jump a gap. Some of the typical ways of attenuating these discharges are ferrites and transorbs on I/O lines to minimize the voltage seen by the communication chips. Ferrites usually affect incoming transients on the I/O lines from reaching the scanner. Transorbs help both directions by clamping the voltage to safety limits and converting the surge into heat before they break down the more sensitive parts. These are attenuators for issues that are not severe. Once the situation gets into the sever category these will no longer provide protection.
 

Severe machine chassis  discharges:

The only way to stop the severe discharge from getting into the I/O and communication lines is to isolate the scanner from the machine. This does not isolate the products from earth but merely breaks the path from machine to the scanner that was providing sufficient voltage to induce failure. The voltage will still rise on the surface of the machine but it cannot enter into the scanner.
 
The picture below gives a graphical representation of electrically isolating the scanner from the machine that has the potential in it.
  
Isolated from machine to protect low voltage devices
User-added image

Recommendations and Best Practice:
Characterize the type of Grounding electrical events which are likely to occur in your environment.
 
Ensure the machine has a very solid grounding path. An Ohm meter is only a partial verification. If the ground potentials are high current low voltage in nature, then Ohm measuring is much more of a determiner of the quality of ground. If the events come from static high potential discharge, then an Ohm meter cannot correctly measure the quality of the ground system.
 
 
Choose a grounding strategy similar to the above examples which is most suited for the level of noise/static in the environment.

Operating System

Any

Link to comment
Share on other sites

  • Replies 0
  • Created
  • Last Reply

Top Posters In This Topic

Popular Days

Top Posters In This Topic

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share


×
×
  • Create New...