Slip rings and transmission of power between rotating components - Page 1
Forum section: electronics / projects

author: profdc9
author_rank: Frequent Contributor
timestamp: June 01, 2020, 06:18:06 pm
content: I have been constructing a persistence display. For this display, a PCB is twirled at high speeds, about 1800 rpm. In an earlier thread,https://www.eevblog.com/forum/projects/smt-pcbs-subjected-to-torsion-and-high-g-forces/msg3076319/#msg3076319I asked about the PCB that was shaking itself to pieces. I remade it and I am glad to report that now, after adding a 1/4 inch thick aluminum back plate, adding cutouts for the brush tabs, and moving the microcontroller onto the opposite side from the tabs, it is no longer popping the chips off the board. So, good times.But now I am having another problem that perhaps the hive mind could help with. I have a diagram of it which I have attached to this message. My original idea for transmitting power to the PCB is to use the scheme where there are two tabs at the edge of the PCB where brushes attach. These brushes would be strips of metal, and there is a commutator ring which these metal strips brush against. The commutator ring is split into positive and negative halves. The two brushes are connected to a full-wave bridge rectifier. Each rotation, the polarity reverses and back. When the polarity reverses, the change is detected by a comparator and triggers the timer on the microcontroller. This is how I orignally intended to detect the position of the PCB so that I know how to time the LEDs. I chose this scheme rather than using a hall effect sensor because I thought it would not require any proprietary hall effect sensor chip that might be hard to get in the future.In retrospect, this is a terrible idea, mostly because the contact with the ring is made at a large radius where the speed of the contact is very high. Instead it should be done at the axle. Instead, I was thinking of building the slip ring on the axle, like any sane person would, and then using the carbon brushes from a power tool to contact the axle. Are there any electromechanical wizards who have experience building slip rings? I was thinking of perhaps trying to use the slip ring from an alternator. Perhaps there's a method of detecting the axle position that uses a standard Hall effect sensor or a standard photocell interruptor type scheme? I would like this device to not just be a one-off if possible.

author: Benta
author_rank: Super Contributor
timestamp: June 01, 2020, 07:11:37 pm
content: The first thing that came into my mind:Why not an electromagnetic coupling? I mean, the thing is rotating anyway, so it's a slam-dunk.Permanent magnets (not moving) placed under the rotor, and PCB etched inductors on the rotating carrier disc. How much voltage and current do you need?It could even be improved with planar ferrite cores on the rotating carrier disc.

author: profdc9
author_rank: Frequent Contributor
timestamp: June 01, 2020, 08:51:50 pm
content: I had considered that, but I am not sure that the inductance of a PCB coil could be enough to provide a significant voltage. Also, the coils would add a lot of area to the PCB which would increase the moment of the PCB that has to be turned. I do not think it could be placed around circuitry unless the circuitry itself did not make any loops that would effectively act as shorted turns and decrease the effective area of flux captured by the coil.I was considering creating a half-bridge to generate an alternating field at 10s to 100s of kHz in a second stator coil so to that the coil on the PCB would only require a few turns, however, I have basically created an induction cooker then and would probably cook the electronics with eddy currents. I was looking into resonance power transfer at 13.56 MHz but I don't think those could handle the kind of load (5-7 VDC, 250-500 mA) that is needed.So I am not really sure how to make a wireless power transfer scheme work.Another idea I had which I had more hope for and maybe someone could provide feedback on is to modify a motor itself to transfer power. If the armature coils were tapped to provide power, that would be a possible viable wireless power transfer method which would be between the field and armature windings I have studied a couple of induction motors however and I do not know yet how to pass wires through the bearing up the shaft. I suppose if a hole was drilled through the center of the shaft, wires could be passed down to the armature coils that way. Every motor I have studied has the bearings go up right to the motor shaft and so there is no room to pass a wire through to route power.