In this video we will discuss best practices for grounding and cable shielding for linear position sensors, electromechanical devices that are used to measure changes in an object’s position from small fractions of an inch to many inches and output a proportional electrical signal. The word “linear” is used to distinguish the object’s motion from rotational motion.
The standard practice for linear position sensor cabling is to use shielded cables. The first decision to make is whether to use cables with individually shielded conductors, shielded twisted pairs, a multiple conductor cable with overall shielding, or some combination of these configurations.
For DC in/DC out analog output linear position sensors such as an LVIT, DC-LVDT, or potentiometer, it is satisfactory to utilize an all-shielded cable. For AC-in/AC-out linear position sensors such as an AC-LVDT or inductive half-bridge, the best practice is to use shielded twisted pair conductors, one pair for the excitation input and another pair for the differentially-connected secondaries’ output. Having individually shielded conductors is rarely required for most linear position sensors.
Another important choice is whether to use braided shielding or foil-wrapped shielding with a drain wire. Braided shielding has better mechanical properties and is adequate for low to medium frequency applications, but is somewhat harder to work with and is typically a bit more expensive. Foil shielding is typically less expensive than braided, is easier to work with, and is often better for high frequency applications, and for use in locations with strong EMI fields that could interfere with the sensor’s output.
Linear position sensor systems often face the issue of ground loops causing unwanted current flow. To eliminate ground loops, maintain one common ground point for the system. It is strongly recommended not to connect the sensor cable shields or drain wires to the sensor's housing or to separate ground points. Connect the shield's other end to the system's master ground as a Faraday shield. For sensor support instrumentation, a single wire should be connected to the system’s master ground from only one ground terminal on the equipment.
Another consideration is cable capacitance, both between individual conductors and to the shield which depends on conductor size and shielding. It is less critical for short cable runs (<30 feet (9 m). Yet for lengthy cable runs, up to 300 feet (90 meters) maximum, especially with AC-operated sensors like AC-LVDTs, cables with a low capacitance cables of around 15 pF/foot (50 pF/meter) are crucial. They minimize undesired phase shifts between sensor input and output and help to maintain system frequency response.
In summary, grounding and shielding for linear position sensor systems have many facets and nuances. For help finding the ideal solution for your industrial position measurement application, contact us now to speak to an experienced applications engineer.
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