Slip Ring Considerations For High Speed Signals

Dditors:gassrini Time:2016-03-30

When signals must pass rotating surfaces such as in security cameras, gun turrets and sensor gimbals, engineers often employ sliding contact slip rings to allow unlimited rotation. The trend is for more digital signals and more high density digital signals. Today's 'vetronics' suite for armored vehicles, for example, increasingly requires higher speed digital signals to support Ethernet communications and better images from sensors. Slip rings must provide the interface of these signals between the vehicle hull and the turret. Sliding contact slip rings have limitations for passing these new signals. What limits and options does this leave the system designer? What are the design trades? What are the cost impacts?

Maintaining signal integrity

Before addressing the trades, let's address a more fundamental question. What are the fundamental challenges to passing high frequency signals through sliding contact slip rings?

1.Impedance matching:
High frequency signals are passed over coaxial cable (typically 50 or 75 Ω) or twisted pair cable (typically 110 Ω). For best fidelity, this impedance level must be maintained over the length of the cable. The cable impedance (measured in ohms, denoted by Ω) is determined by the materials used and by the physical dimensions of the cable. However, a slip ring intrinsically violates both of these constraints: its geometry and materials used are very different from those of cable. Therefore, special care must be exercised in the design and manufacture of a slip ring to be used for high frequencies so as to cause the least disruption to the system impedance level.

2.Signal path:
Signals travel around the rotor in both clockwise and counterclockwise directions. While for low frequency signals this path difference is negligible, at high frequencies it is not. Depending on the position of the rotor relative to the stator, these distances will not be equal, resulting in varying signal distortion as the joint is rotated. For high frequency signals, the rotor diameter (and hence the path lengths) must be kept small. Depending on the mechanical constraints of the system, the rotor may have a section of smaller diameter to accommodate such signals.

3.Crosstalk
At high frequencies, signals are not bound by insulation and electromagnetically travel from conductor to conductor. Thus, the signal on one circuit may interfere with that of a neighboring circuit. Careful design of the slip ring can minimize this interference, bringing it to a low enough level that it will not cause problems in the system.

4.EMI shielding
Often it is necessary to ensure that a signal does not radiate from its cable and/or is not susceptible to external electromagnetic radiation. For this reason, system designers use shielded cable, which has a grounded braid surrounding the conductor. This shielding is interrupted when the signal must pass through a slip ring. Careful design techniques are used to ensure shielding integrity while the signal is passing through the slip ring.

5.Noise
Because a slip ring contains sliding contacts, a certain amount of noise will be introduced as the brush passes over microscopic imperfections in the rotor. The noise can be mitigated by adjusting the brush pressure and by the use of multiple contacts on each ring. 

 

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