Reducing Electromagnetic Interference in Motion Systems
- Thursday, 01 April 2010
EMI (Electromagnetic Interference) comes in different forms, frequencies, and levels. Electronic equipment has different categories for both emissions and susceptibility. Under normal circumstances, elimination is not possible so reduction to acceptable levels is what can be reasonably achieved. Coupling factors, rise time, emissions, and susceptibility are phrases that must be understood with confidence.
Understanding the coupling methods of EMI is necessary in order to reduce emissions and susceptibility. Some of the techniques for shielding are effective for both, but there are issues that must be handled differently. On the susceptibility side, to make a system more immune, the reception path of the EMI or noise needs understanding.
Most of the power devices today are switch mode. PWM (Pulse Width Modulation) techniques are used in more and more devices at lower power levels to achieve greater efficiency. Concerns of radiated and received noise are falling into lower and lower categories. Techniques once thought only necessary for high-power electronics are now migrating into low-level power supplies and regulators. Linear devices, for all their inherent good traits of EMI, are just too inefficient.
Magnetically coupled noise is a phenomenon that has sufficient power to turn data lines (optocouplers) on. The process requires an emitter and receiver, so you must shield both the source and reception devices. You should also attenuate the source whenever possible. Attenuation can be achieved by inductance or PWM conditioning (slowing the rise-time) in most applications. This can occur on two planes of reference, common mode or differential mode. Magnetic inducted noise follows the path of least inductance. This noise will be limited by reducing the inductance of the intended path to less than the inductance of the unintended path. You must maintain that the area that a conductor encircles is proportional to the inductance. If there is an opportunity for a signal to couple into a circuit that is closer in proximity (less inductance through less area), you may induce a voltage onto that circuit.
Capacitive induced noise is noise that does not have power, but can wreak havoc with high-speed data lines, analog, or highimpedance inputs. Typically, this is one of the easier noise coupling mechanisms to resolve. Issues begin when a capacitive induced signal gets into an amplifier, where it can have the power to radiate very high frequencies that the system is not prepared for. The identification of this is that it is usually high frequency (100 MHz or higher) and has a net zero DC voltage value when observing the signal through an oscilloscope.
Radiated or RFI noise requires a transmission source and reception antenna that is at least λ/20 long (wavelength/20). You must also be ½λ away from the source minimum. It is the least likely source of trouble in most systems. At 100 MHz, λ/20 is 1.54 meters since λ is 30 meters.
With an understanding of the coupling factors, you can assess what your problem is, make a qualified judgment as to its cause, and decide what the mechanism of coupling is. Armed with this knowledge, you can use proven methods to reduce the cause and affect, whether it is to meet an EMC (Electro-Magnetic Compliance) standard, or if you simply want to reduce spurious failures of an electronic circuit. Servo amplifiers, power supplies, and electronics with clock frequencies all must work uninterrupted from outside interference, as well as not interfere with themselves. Sensitive analog circuitry must be immune from the interference of outside radiated signals and still be able to perform their function.
With all noise immunity measures, things start with a good grounding and shielding scheme. The first rule in grounding is safety first. No safety ground can be compromised in the interest of noise immunity. There are times when it appears to contradict a good grounding scheme to have multiple earth grounds in a system, but if the safety code requires it, learn to work with it. The ground should be a non-current- carrying conductor. Only in failure would there be current. It is a reference. Phases, neutral, and shields can all be current-carrying at times, but the ground should be at reference.