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Proper shielding and grounding are essential for good servo design. Choosing the appropriate cable and devoting effort to the grounding of the encoder in the planning stage will save a considerable amount of time and expense in troubleshooting and correcting signal noise problems once the design is complete.
A cable can transfer noise by acting as an antenna, or it can pick up noise that is radiated from other equipment. The effects of signal interference can be mitigated by proper wiring and shielding, which can reflect it from the signal carrying wires and conduct the interference to ground.
MicroE encoders are equipped with double-shielded cables with twisted differential pairs. The inner shield is connected to circuit ground, while both the connector housing and sensor are connected to the outer shield. A path to ground for electrical noise is provided when the encoder is connected to ground, either at the connector or the sensor.
Figure 1. Cross-section of double-shielded cable
The twisted pairs leverage differential signaling where one line carries one signal and the other its complement. The difference measured between the two lines is the measured signal. The differential signal propagated in twisted pairs will be less susceptible to noise and is more robust to cross talk.
An efficient grounding system is one that helps to reduce electromagnetic interference effects from disturbing the encoder signal outputs. A multipoint ground operates best at high frequencies, and a single-point grounding scheme works better at low frequencies.
The ground connection would ideally be made at the sensor head when designing a ground system for MicroE sensors. However, the bracket on which the sensor is mounted is often attached to moving parts. There is no guarantee of a good ground contact on moving parts and so, it may be best to make the ground connection at the connector in these cases.
The connections of the outer inner and outer shields of a MicroE encoder can be seen in Figure 2. In the first image, the encoder is properly grounded and will provide the best protection against signal noise interference.
For units with 2x5 micro-connectors, the outer shield is terminated at the hex sleeve.
When more than one point is used for ground connections, ground loops can occur and cause these points to be at different ground potentials. This difference of potentials can cause noise, due to induced electromagnetic interference and AC voltage. Ground loops can occur if the connector shell and sensor are each grounded separately.
It is not likely that the the sensor mounting bracket and the electrical chassis ground at the receiving end will be at the same electrical potential. A single ground connection is recommended for most applications.
Two different ground symbols have been used in the following figures to emphasize this point. In systems where the expected noise sources will be relatively low in frequency, it is best to ground at only one point – either the connector shell or the sensor – but not both.
In some medical equipment applications where high frequency noise sources exist, multiple ground points are desirable. Using a 0.1 µF capacitor between the ground and the connector shell can be highly effective in such cases.
In this instance, the capacitor will act as an open circuit for low frequency noise (or a DC potential between grounds) and a short circuit for higher frequencies. In applications where high frequency interference is common, this technique is highly recommended.
Henry W. Ott, “Noise Reduction Techniques In Electronic Systems,” 1988
Howard Johnson, “High-Speed Signal Propagation: Advanced Black Magic,” 2003
This information has been sourced, reviewed and adapted from materials provided by MicroE.
For more information on this source, please visit MicroE.