Should servomotor cables be shielded, and if so, why?

Contributed by Frances Richards

As with many things in life and engineering, it depends whether you should use shielded servomotor cables. However, in most industrial environments, the answer is a definite yes. Later in this entry, the few instances where unshielded servomotor cables may be appropriate will be discussed. For now, let’s consider some of the specific environmental conditions in which servomotors operate to understand why shielding is so important.

One of the top concerns regarding cable shielding involves electromagnetic interference (EMI). If the servomotor’s operating environment includes sources of EMI such as other motors and machinery, radio frequency (RF) devices, or high voltage cables, shielding can help reduce the impact of EMI and RFI on the signals being transmitted through the servomotor cables. An interesting note to keep in mind is that EMI protection is a two-way street: Shielding the servomotor cable also serves to keep its own signals from interfering with nearby equipment.

Two related concerns involve signal integrity and noise immunity. Let’s take a closer look at each of these. Shielding a servomotor cable can help preserve signal integrity by reducing signal degradation caused by external interference, including EMI. This is critical for servomotor systems in which precise control signals are essential for accurate positioning and motion control, such as a pick-and-place robot arm. Regarding noise immunity, shielded cables can provide better protection than unshielded versions by minimizing the impact of external electrical noise on the signals being transmitted through the cables. This is especially true for industrial environments where electrical noise from machinery and other equipment can affect signal quality and integrity. Note that proper grounding is critical to ensure effective shielding. In fact, improper grounding can make EMI problems even worse by creating ground loops.

Now let’s consider the applications where unshielded servomotor cables may be an appropriate choice. In settings that include few sources of EMI, shielding may not be necessary. One example is a laboratory environment where there are no sources of electrical noise in the immediate vicinity. Another situation involves the length of the cable run. In applications where the servomotor cable has a very short run and is not exposed to EMI sources, there is less need for shielding. A good rule of thumb is that short cable runs are much less susceptible to EMI than longer runs.

Flexibility and ease of installation are other aspects to consider. Shielded cables are often less flexible than unshielded ones because of the additional shielding layer. This can affect their practicality in certain applications where flexibility is critical, such as in robotics and motion control systems. However, some cable suppliers offer highly flexible shielded cables to address this issue. Cables featuring reverse spiral shields offer more flexibility than those with braided shields. That said, copper braided shields feature better EMI protection.

One final consideration is cost. Shielded cables typically cost more than unshielded ones. For applications where the overall budget is a major concern and EMI interference is low, unshielded cables may be a worthwhile option to reduce expenses without compromising signal quality. The best advice? Think about your application’s cable requirements upfront, not as an afterthought.