Without slip rings, it would be impossible to operate many styles of continuously rotating cranes. It's the slip rings that pass current and communications between the cab, boom, or platform on mobile cranes, firefighting equipment, or other heavy-duty gear.
It's not uncommon for even relatively simple vehicles to have multiple slip rings. Each electronic gage, solenoid, or other electrical component in the rotating section gets handled by a dedicated slip ring.
All slip rings within a vehicle will be installed as a single stack. The more complex a vehicle, the more slip rings in the stack. Slip rings are generally not easy to install. It's a challenge to design in enough space for these stacks, which can measure up to two and a half feet high. The installer must carefully avoid damaging the assembly and ensure installation at the exact center of rotation.
Insatiable feature creep can ultimately force designers into a larger chassis. In addition, slip rings demand a degree of precision that can lead to costly repairs and downtime if a ring fails. Often the failure results from either overvoltage, overcurrent, or the leakage of hydraulic swivels on the same center of rotation. Frequently slip rings are part of the hydraulic swivel assembly.
Controlled-area network (CAN) technology now lets manufacturers install fewer slip rings. Moreover, these networks bring opportunities to expand machine capabilities often while reducing costs.
A host of slip-ring variables can affect information transmission. These include ring and brush-frequency response (bandwidth), assembly impedance, and crosstalk.
Furthermore, the higher the stack, the greater the challenge of lining up rings properly due to wobble. And taller stacks are more difficult to service. Murphy's Law seems to dictate that the ring that fails is the one most difficult to access.
The development of more-sophisticated machines has brought new design approaches. One big change has been better engine communications. Vehicles now incorporate the J1939 protocol for CAN systems. Off Road Tier II regulations require that an engine having electronic control must employ SAE J1939 for diagnostics. Often when machine manufacturers think CAN “costs too much,” they are looking only at the CAN module cost.
Nevertheless, CAN promises to dominate off-road equipment monitoring and control as EPA regulations work their way down to smaller engines. CAN systems let multiple microprocessor units talk with each other over the same twisted pair of wires. For example, CAN systems have been employed to coordinate engine, transmission, and ABS and send the information across the swivel assembly for use on the other side of the slip rings.
Before discussing the changes brought about by CAN, here's a brief overview of the slip-ring assembly. Ring assembly provides one or more circuit paths. Each ring is electronically conductive and provides a circuit path over the full 360° of rotation of the ring assembly.
Brushes make the electrical contact between rotating and stationary parts of the assembly. The brushes usually have a bifurcated design and ride on the outside portion of the ring to compensate for dead spots, which can impede contact. These dead spots can occur from arcing, dirt, and insects.
Leads connect the rings and brushes to the harnesses leading to the circuit. Connectors link to the slip ring and assembly wiring.
Slip-ring installation is rather easy and not a major issue in manufacturing. When slip rings arrive at the machine builder, each stack is usually ready to bolt in place regardless of how many rings are involved. The complexity comes in with the volume of wiring harnesses leading up to the rings.
In most equipment converted to CAN, just two microprocessor-based modules sit in the chassis and connect by a twisted pair of wires. Such systems have significantly less connection hardware than would otherwise be the case. The two modules generally have a peer-topeer relationship (though sometimes the modules are set up as master/slave), sharing information and acting on commands from each other.
The CAN protocol provides a robust signal immune to crosstalk, unaffected by frequency response of the rings and brushes, or affected by other issues inherent with slip rings. CAN communicates over a twisted-wire pair using just one wire to transmit the signal and the other providing a reference. Alternatively, it can use fiber-optic conductors.
CAN's serial, asynchronous, multimaster protocol traffics signals from the electronic-control modules positioned throughout the vehicle to monitor and manage vehicle components. The SAE J1939 protocol sets the data rate at a robust 250 kbytes/sec offering, among other qualities, constant system self-diagnosis. This capability is generally built into the CAN protocol.
The protocol is such that nodes on the bus often transmit information only if requested. Some diesel engines have over 250 measurements but engine controllers only transmit a few except when requested. This reduces unnecessary traffic on the bus.
There is a growing body of evidence illustrating how CAN simplifies designs. One crane manufacturer went from 55 slip rings on an older model down to 13. Two of the rings handle bus traffic controlling all functions in the vehicle including the engine. The remaining rings carry system power and ground across the point of rotation. Some systems dedicate a second set of datatransmission rings for engine diagnostics communication when the engine is located where access is difficult.
Use of a bus can also cut component wiring by up to 40%. There are obvious benefits in assembly time and easier access within the chassis. An ancillary benefit is that the laptop becomes a convenient tool for various functions. Specially designed programs can let technicians run quality assurance checks on components at various stages of assembly. Rather than crawling around the vehicle with a continuity tester, an assembler can often plug in the laptop through the RS-232 connector to assess the system and isolate any problems.
Once the vehicle is in the field, these same programs significantly reduce downtime by quickly isolating problems to sensors, actuators, wiring, control modules, or software. This approach greatly reduces the chance of misdiagnosisand the possibility of replacing the wrong part. If the issue involves a software revision, the technician can often make the fix on the spot.
CAN may also boost vehicle versatility. The ability to make modifications through software rather than by swapping out hard-wired circuitry minimizes the work of adding features. It can be tough to add new functions when doing so means another slip ring. In contrast, upgrades to CAN systems may involve simply writing a few lines of code.
CAN modules usually have numerous spare I/O contacts for new functions, components, and other upgrades. At worst, owners could easily add a module to handle the load. All in all, manufacturers can add features at a small incremental cost that may be quite valuable to the buyer of the machine.
HED Inc. ,