Resistive Noise of Slip Ring: All You Need to Know

by | Aug 19, 2021

Slip rings are widely used in several applications. These applications of slip rings include transmission of power and data between rotating and stationary parts of motors, generators, or rotating gears. Under ideal circumstances, they will provide an excellent transmission line and do not affect the transmitted data. But they often cause some interference or drift in the transmitted signal. This can often be referred to as resistance noise

What is resistive noise?

Noise is an undesirable phenomenon of resistors, while it is essential for some applications—for example, high-gain amplifiers, load amplifiers, and low-level signals. Resistor noise is usually defined as the microvolt noise per volt applied at a bandwidth of 1 MHz. Thermal noise is the primary noise source of resistors and depends on three variables: the resistor’s resistance, temperature, and bandwidth.

Why resistive noise is important

 In the 1950s, when they developed measurement platforms or inertial units (IMUs), the signal quality of electrical sliding contacts became the focus of attention. These platforms or inertial measurement units (IMUs) were miniaturized To control missiles and aircraft. A quadrature velocity gyroscope and three quadrature accelerometers measure angular velocity and linear acceleration, respectively.

 These three axes require a set of sliding contacts (slip rings) to transmit signals from sensors and motors that drive gyroscopes and electronic devices. These slip rings must be tiny and reliable, but in essence, they must transmit signals without significant electrical noise.

It solves this problem by plating a gold ring on the rotating shaft, and the sliding contact is made of a precious metal brush on a fixed bracket. Although alternative technology has replaced the gyroscope sensor, the signal on or from the turntable is necessary for many other applications.

 The first slip ring is used to process shallow-level analog signals. Although the rate of change is not a problem, the signal amplitude can only be a few millivolts, and the measurement bandwidth is usually below 10 kHz. The millivolt level will seriously degrade the signal quality. Although the requirements have changed during the transition from analog data transmission to digital data transmission, signal noise is still significant when evaluating the characteristics of electrical sliding contacts.

Where does the slip ring noise come from?

Let us see what causes the signal noise in the electrical sliding contacts. There are three primary sources of slip ring noise. The first and most apparent is the change in resistance when the electrical contact slides. Since current flows through this changing contact resistance, this change can cause ohmic voltage interference or signal noise.

 These changes in sliding contact resistance depend on many vital parameters like surface and structural physical properties. Other parameters include changes in the film, sliding speed, surface roughness, vibration, or contamination from wear particles or external sources of pollution. The resistive noise occurs at the minimal level, I.e., the area where the two elements of the electrical contact (ring and brush) have an interface. When using slip rings, the contact noise can be a massive contributor to the noise floor when transmitting low-level analog signals.

Also, do not take the low-frequency contact noise in electrical sliding contacts for granted. The  wear debris and the Insulating films from accelerated contact wear can be crucial sources of signal degradation. Majorly in slip rings that are deficient in their matching to the application. Plating thickness, contact materials, and contact design are the primary factors that cause the low contact noise of sliding electrical contacts.

How to eliminate electric noise of slip ring?

Regardless of the signal type, it would be best to control the slip ring noise generated by the traffic channel. And the noise is usually measured by the signal-to-noise ratio.

Let’s talk about how to get rid of the influence of noise when transmitting slip ring signals.

  1. The contact resistance of gold is essentially the lowest. However, gold is costly. The primary use is electroplating; that is, a layer of wear-resistant gold is plated on the contact surface of the precious metal. In this way, we can save costs and significantly reduce electrical noise and contact resistance.
  2. The design is the key to further reducing electronic noise. The slip ring should not oscillate within the wide range of rotational friction between the contact and the ring, so it sets a reasonably high standard for radial alignment.
  3. Lubrication

Some other characteristics that determine the noise produced by a slip ring are;

  1. The service life of the slip rings
  2. The control level of electrical noise from the slip ring.

Conclusion

Noise or fluctuations in the contact resistance between the rotating ring and the fixed brush as a rotation function are an inevitable feature of all slip ring designs.

If you are a user or manufacturer of slip rings, you should understand the impact of resistance noise on your products and the importance of testing for resistive noise. Excessive resistance noise may result in the degradation of signal equality, fluctuations in power transmission, and certain types of noise may also indicate impending slip ring failure.