A turn for the better
Slip rings with brush contacts are a popular and well-proven technology. In motors and generators, a brush provides a sliding contact to carry electrical signals to the commutator or slip ring on the rotating part of the machine. Industrial machinery generally requires a 24 VDC power supply for the controller, I/O modules and sensors, and in some cases an additional 400 VAC supply. Often, servo drives are provided with 560 to 750 volts of DC power through the slip ring, rather than the 400 volts from the DC bus.
Industrial Ethernet with slip ring technology
Communication has historically been provided using traditional bus systems such as CAN, PROFIBUS, DeviceNet or other technology from this generation. These systems generally operate at 125 kbit/s (CAN and DeviceNet) and up to a maximum of 500 kbit/s (DeviceNet), 1 Mbit/s (CAN) or 12 Mbit/s (PROFIBUS). One of the challenges is ensuring reliable contacts, which is relatively easy to manage with the low speed of 10 to 100 rpm at which the carousel rotates. Another is meeting EMC requirements.
Today's machines rely on industrial Ethernet solutions to provide communication at 100 Mbit/s. When you put this next to a DC bus carrying 560 or 750 volts, electromagnetic interference can produce errors within the data frames. The protocol detects this with a CRC, and the faulty frame is retransmitted. Motion control applications will allow a single retransmission without negatively affecting the machine. If electromagnetic interference causes a second invalid frame, however, the machine will automatically shut down and production will stop.
Transmission method affects immunity
Passing industrial Ethernet communication through a slip ring is especially susceptible to interference since each individual wire in the cable must be transmitted via a separate brush track. Twisted pair cables provide protection against alternating magnetic fields and electrostatic interference. Every time you open and untwist the cable or add a connection, the system becomes more vulnerable. Yet a system's immunity to interference is determined not only by the physical components, but also by the communication protocol being used. The main differentiation made here is between single frame and summation frame methods. As defined in the standard Ethernet protocol, single frame methods can offer up to fifty times greater immunity to electromagnetic interference than summation frame methods.
With a summation frame method, there is also an exponential increase in susceptibility to interference as you add nodes to the network. The basic rule of thumb is that the summation frame method is more susceptible to interference than the single frame method by a factor of 1.25 times the number of nodes. For a machine with 5 nodes, this equates to a factor of 6.25, whereas a machine with 40 nodes is 50 times more susceptible. One of the advantages of the single frame method is that the frames are shorter and only contain data for a single node. If a frame is compromised, the machine as a whole is not directly affected. With the summation frame method, however, a single disturbance means the loss of an entire network cycle and the data for all of the nodes in the network.
The errors produced by EMC interference are sporadic in nature. As a result, the possibility of electromagnetic interference is often overlooked at first, with investigations primarily centered around defective hardware or an error in the software.
Engineers who have dealt with EMC errors know how difficult it is to identify the source. Poorly assembled contacts, improper shielding or grounding or even minor wiring modifications made in the field may be to blame. Testing equipment can provide clues, but not an exact location. Part of the problem is often that the error cannot be reproduced in a clean development lab, and the investigation must be performed on-site at the end customer's location. Not only does the resulting production downtime come at a high price, the long drawn out search for an error can mean cashing in reputation capital as well. With or without a slip ring – the selection of a bus system plays a decisive role in the robustness of a machine or system.