Electronic – Frame grounding, data & power – EMI

communicationemcgroundloopspowershielding

I'm looking into building some large robotics with (ofcourse) a frame, (shielded) data/communication cables – EtherCAT, and power lines.

Now different 'experts', websites and people say all different kinds of things in order to prevent noise on the data lines, to prevent ground loops and to use the frame as ground.

What we have figured out now is to use the frame as 'ground' for the data cable shielding. Connect both ends to the frame and connect the frame to battery ground on one point only as close as possible to the battery.

How does this not create ground-loops in the shielding? Or have we figured it out wrong? Shouldn't the electrical components' ground plate be connected to the frame on multiple points? Or should they best be electrically isolated from the frame? And if the latter is true, how about capacitances between the frame and ground plates of these components? What if we have to thermally cool these components on the frame?

What is true and what is false? Because we can't see the wood for the trees any longer.
Tips & tricks to keep in mind?

I added a diagram as requested with maybe some clarification. The green 'frame' is electrically connected (green lines represent the connection) and the purple boxes are motor controllers creating some noise when powering and moving the motors represented by the yellow circles. The data cables are shielded EtherCAT cables.

enter image description here

Best Answer

I would highly recommend that you read ETG1600: Guideline for Planning, Assembling and Commissioning of EtherCAT Networks , a public white sheet that covers the suggestions for system design and covers grounding considerations.

In EtherCAT the signal processing chain is digitized and goes through an EtherCAT slave controller or equivalent on every single device.

enter image description here

EtherCAT Network Topology courtesy of Beckhoff

Therefore, The Daisy chain topology of EtherCAT is not actually a physical daisy chain and does not resemble a traditional bus interface of E.G RS485 or CAN.

enter image description here

Signal processing chain inside the slave device, note that up to 4 ports are allowed allowing for a star topology, but typically 2 are seen on devices, the ethernet frame processing occurs in the "EtherCAT processing unit" between the input port and any subsequent network segments. Courtesy of Beckhoff

Furthermore, EtherCAT relies on specific physical layer requirements for isolation and grounding. 100 Mbit Ethernet over CAT5/CAT5e should be isolated to 1KV via transformer. As long as your frame voltage does not exceed 1KV between two nodes, you should not have any impact from frame coupling the devices.

In fact, the typical recommended scheme specifically recommends against connecting frame shield to ethernet shield in either case. Although this will not necessarily be enforced by spec.

enter image description here

Ethernet Grounding recomendations for EtherCAT slave device developers Beckhoff ET1100 Datasheet

EtherCAT does not address power grounding requirements, this is left to sector/industry/product specific requirements. Furthermore, EtherCAT does not specify a novel physical layer, but relies on existing specifications for physical layer (e.g. fiber, gigabit, ethernet)

Ultimately it is up to the system designer and in some cases an industry consortium to decide what is the grounding scheme for their systems. For example

If isolation and/or long distances (1KM-20KM) are required - a fiber physical layer is completely isolated.

Ultimately, if the typical isolation grounding scheme of ethernet devices, in particular the tendency to couple frame ground to ethernet shield, in case of shielded cables, may present a challenge if you are also using frame-coupled DC distribution systems.

Conflict of Interest Disclosure: I have some affiliation with ETG

Related Topic