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 & 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 toady'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. 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.
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.
Good Wiring and Shielding Practices
- Find the Noise
- Disconnect all unnecessary I/O.
- Connect I/O one at a time until problem reoccurs.
- Shut down any other equipment that may be emitting noise.
Rules of Wiring
- Power Supply and Motor wiring should be shielded twisted pairs, and run separately from signal-carrying wires.
- A minimum of one twist per inch is recommended.
- Motor wiring should be shielded twisted pairs using 20 gauge, or for distances of more than 5 feet, 18 gauge or better.
- Power ground return should be as short as possible to established ground.
- Power supply wiring should be shielded twisted pairs of 18 gauge for less than 4 amps DC and 16 gauge for more than 4 amps DC.
Rules of Shielding
- The shield must be tied to zero-signal reference potential. It is necessary that the signal be earthed or grounded, for the shield to become earthed or grounded. Earthing or grounding the shield is not effective if the signal is not earthed or grounded.
- Do not assume that Earth ground is a true Earth ground. Depending on the distance from the main power cabinet, it may be necessary to sink a ground rod at the critical location.
- The shield must be connected so that shield currents drain to signal-earth connections.
- The number of separate shields required in a system is equal to the number of independent signals being processed plus one for each power entrance.
- The shield should be tied to a single point to prevent ground loops.
- A second shield can be used over the primary shield, however the second shield is tied to ground at both ends.