The slip ring module was an off-season engineering exercise. While discussing the difference between the co-axial and distributed swerve modules, the limitation of rotation of the distributed swerve module was discussed. The question was asked if there was any device that could be used to allow the module to rotate freely.
For those that don’t know, a distributed swerve module has the motor integrated into the swerve wheel module and therefore the CIM motor rotates around the steering axis. This means the CIM motor power leads rotate around the steering axis as well and limit the rotation that the swerve module can rotate before the CIM power leads twist too much and do bad things. This is due to the winding of the power leads around the steering axis as the motor and wheel assembly rotates.
Our sponsor Genesis Automation, uses Mercotac rotary electrical unions on their automated machinery. These are liquid mercury filled devices that have no mechanical wear and allow an electrical connection through a rotating device. These are not rated for the amperage that a CIM motor can draw and mercury filled devices are no longer FIRST legal.
In researching the slip ring we asked the question about how much brush contact area we need to power a CIM. This led to the question about how much brush contact area the CIM has internal to the motor. So we disassembled a CIM motor and looked at the brushes. This led to the idea of using the CIM brush assembly in a slip ring module. We determined that using two CIM brush assemblies in the slip ring would double the brush surface area that the CIM uses. This led to the design of a slip ring module from available materials.
After building a prototype we wired it in series with a CIM on one of our practice bots drivetrains. We then pushed a heavy box around our practice field at a low motor speed using the power of that one CIM alone. This simulated a high current load on the brush/slip ring. The main concern was fusing the brush to the slip ring under such high current loading. Another concern was sending too much heat into the slip ring and melting the solder joint between the slip ring and power lead. The polycarbonate housing was selected to be an electrical insulator as well as allow us to see if there was arcing between the brush and slip ring.
Since the slip ring was not being rotated in our practice bot like it would in a swerve, we were able to concentrate the current flow at one position on the slip ring. After pushing the box around the field we disassembled the slip ring unit and found no noticeable wear or deposits. This proved the CIM slip ring is a viable solution to use on a distributed swerve module. This was a perfect engineering exercise and a great learning experience for our team. We knew this was a topic discussed in the past so we thought we should share our experience on Chief Delphi.