What is noise?

“Noise” or “Interference” consists of unwanted electrical signals which superimposes on and masks the desired signal. Designing a control system is challenging enough, but designing a control system that has noise immunity adds a whole other dimension. Ideally, you want the noise-to-signal ratio to be as small as possible. Noise is always present in a system that involves high power and small signal circuitry. The key is to manage the noise so that it does not interfere with the performance of the system at hand.

 

Sources of noise

Sources of noise can be external to the stepper motor system as well as within. The most common external sources are relays and motors. Internally, the relatively high current motor drivers are the source. All bi-polar stepper motor drivers apply a chopping function to the applied voltage of each phase. This chopping enables use of higher voltages than the motor is rated for, achieving higher speeds while keeping the motor from getting too hot. The combination of the chopping and inductance of the motor creates noise on the ground plane. This [ground plane noise] can be introduced into nearby external systems if proper wiring and shielding precautions are not taken. The result can be intermittent failures of the system as a whole.

Components of noise and how to manage them
In order to manage noise it is important to understand its components. Noise [Interference] is categorized into two groups: radiated and conducted. Radiated interference is transmitted by electromagnetic fields and picked up by the antenna effect of other equipment. If it were always possible to isolate susceptible receivers and radiating sources from one another, radiated interference would be more manageable. As distance increases, radiation fields become weaker thus energy becomes dampened along a conduction path. Unfortunately with today’s limited system real-estate, distance isn’t usually an option. Reducing the antenna effect and adding shielding, controls this type of interference. (Improper shielding can cause more problems than no shielding at all. (See Wiring and Noise Shielding Best Practices.)

Conducted interference is that which is introduced into a circuit by either direct or indirect coupling. Both direct and indirect coupling are classified into three specific types: Resistive, Capacitive, and Inductive. These types of coupling are most frequent where common return circuits and power supply grounds exist. Conducted interference can originate from a variety of sources, such as relay and switch contacts, fan motors, power switching or digital devices with short rise and fall times. The effect of conducted interference cannot be eliminated as easily as shielding eliminates the effect of radiated interference. Good wiring practices are necessary to minimize Conducted interference. Give close consideration to connections to and from power supplies. Give particular attention to common grounds. Ultimately, the whole system must be referenced to them. (See Wiring and Noise Shielding Best Practices.)

How to detect noise

The first step in troubleshooting a noise problem is acquiring the right tools for the task. An isolated Oscilloscope is the chosen tool for detecting noise. A battery powered scope [if one is available] achieves the best circuit isolation, however a scope with an isolated ground will still be an effective tool. Also, keep in mind that a Digital scope may mask the noise depending on it’s sample rate and frequency response. Therefore, an Analog Oscilloscope is better than a Digital scope for detecting asynchronous signals of high frequency such as noise. Along with the scope, a wiring diagram and a basic knowledge of the systems operation are the best tools. The next step is to simplify the system. Start by removing power; then disconnect all system components from the Stepper driver that are not absolutely necessary for basic motion. Keep an open mind, even experiment a little by using a jumper wire to introduce noise and simulate the failure mode you are experiencing. Remember there may be more than one noise source.

SEM encourages our customers to ask questions and take advantage of our Application Support Team early in your design. We can review your system and make suggestions on the interfacing and wiring practices. We may suggest other tips that are application specific, but as a starting point refer to Wiring and Noise Shielding Best Practices for the basic rules.