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.

Lexium MDriveMDrive / MForce

Introduction: Sinking or Sourcing

The terms sinking and sourcing are used in the Lexium MDrive I.O description to describe the configuration of the I/O point with regards to the current flow through the device when active.

  • Sinking: Voltage “sinks” the current to ground.
  • Sourcing: Voltage “sources” the current from a voltage “source.”

Sinking Inputs

Sinking Output

Connection Examples

Sinking input

The sink/source operation of the inputs is determined by the reference potential seen at the input reference input. For sinking input operation, the input reference should be connected to the +VDC side of a +5 to +24 VDC supply.

Sinking input – Switch

Open-Collector (NPN) Sinking Interface

Sourcing input

For sourcing input operation, the input reference should be connected to the return (GND) side of a +5 to +24 VDC supply.

Sourcing Input – Switch

Open-Collector (PNP) Sourcing Interface

Analog input

The Analog input may be interfaced to devices outputting the following signal types and used to control applications such as Speed control, torque control, program interactions.:

  • 0 to +5 VDC
  • 0 to +20 VDC
  • 0 to 30 mA
  • 4 to 20 mA

Dry-contact type power outputs

The outputs of the Lexium MDrive are isolated, dry contact outputs with a voltage range of -24 to +24 VDC and current rating of -100 to +100 mA.

 

Signal (trip) output

The trip output is a high speed, low current output which can be connected as either open-collector or open-emitter. As this output current range max is 5.5 mA, it should not be used with inductive loads, but only as a signal indicator.

Sinking I/O

A sinking device provides a path for the current to ground. Terms used to describe sinking devices include NPN, Open Collector, Normally High, and IEC Negative Logic.

The Motion Control device’s I/O points are programmable as sinking or sourcing, general purpose or dedicated functions with the active state programmable as HIGH or LOW.

Sourcing I/O

A sourcing device provides the power or a positive potential to an I/O point. Sourcing devices ‘push’ the current through the load. Other terms used to describe sourcing devices include PNP, Open Emitter, Normally Low, and IEC Positive Logic.

Note that sourcing outputs are only available on Motion Control devices equipped with Plus2 expanded features.

Motors have windings that are electrically just inductors, and with inductors comes resistance and inductance. Winding resistance and inductance result in an L/R time constant that resists the change in current. It requires five-time constants to reach nominal current. To effectively manipulate the di/dt or the rate of charge, the voltage applied is increased. When traveling at high speeds, there is less time between steps to reach current. The point where the rate of commutation does not allow the driver to reach full current is referred to as Voltage Mode. Ideally, you want to be in Current Mode, which is when the driver is achieving the desired current between steps. Simply stated, a higher voltage will decrease the time it takes to charge the coil and therefore will allow for higher torque at higher speeds.

Regeneration current (Back EMF)

Also, a characteristic of all motors is Back EMF, and though nothing can be done about back EMF, we can give a path of low impedance by supplying enough output capacitance. Back EMF is a source of current that can push the output of a power supply beyond the maximum operating voltage of the driver and as a result could damage the stepper driver over time.

How steppers use power

Bipolar chopping steppers are very current efficient as far as the power supply is concerned. Once the motor has charged one or both windings of the motor, all the power supply has to do is replace losses in the system. The charged winding acts as an energy storage in that the current will re-circulate within the bridge, and in and out of each phase reservoir. While one phase is in the decaying stage of the chopper, the other phase is in the charging stage; this results in a less than expected current draw on the supply.

Stepper motor drivers are designed with the intention that a user’s power supply output will ramp up to greater or equal to the minimum operating voltage. The initial current surge is quite substantial and could damage the driver if the supply is undersized. If a power supply is undersized, upon a current surge, the supply could fall below the operating range of the driver. This could cause the power supply to start oscillating in and out of the voltage range of the driver and result in damaging either the supply, driver or both. There are two types of supplies commonly used, regulated and unregulated, both of which can be switching or linear. All have their advantages and disadvantages.

Unregulated supplies

An unregulated linear supply is less expensive and more resilient to current surges. However, the voltage decreases with increasing current draw. This can cause serious problems if the voltage drops below the working range of the driver. Also of concern is the fluctuations in line voltage. This can cause the unregulated linear supply to be above or below the anticipated voltage.

Regulated Supplies

A regulated supply maintains a stable output voltage, which is good for high-speed performance. They are also not bothered by line fluctuations. However, they are more expensive. Depending on the current regulation, a regulated supply may crowbar or current clamp and lead to an oscillation that as previously stated can lead to damage. Back EMF can cause problems for regulated supplies as well. The current regeneration may be too large for the regulated supply to absorb and may lead to an over voltage condition.

Switching supplies

Switching supplies are typically regulated and require little real-estate, which makes them attractive. However, their output response time is slow, making them ineffective for inductive loads. IMS has designed a series of low-cost miniature non-regulated switchers that can handle the extreme varying load conditions which make them ideal for stepper motor drivers and DC servo motors as well.

Rules of thumb

Base specifications.

  • Type: Unregulated Linear
  • Ripple Voltage: ± 5%

Cabling best practices

  • Use shielded twisted pairs.
  • Do not run power cable along side signal cablings such as communications and I/O.
  • Earth the cable shield at the power supply end of the cable.
DC Power Supplies
Product Voltage Range Current
(per device)
Wire AWG
10-25′ (3 – 7.6 m)
MDrive Plus NEMA 14 +12 to +48 0.75 A 20
MDrive Plus NEMA 17 +12 to +48 2.0 A 20
MForce MicroDrive +12 to +48 3.0 A 18
MDrive Plus NEMA 23 +12 to +75 2.0 A 20
MDrive Plus NEMA 23 (Quad length) +12 to +60 3.5 A 18
MDrive Plus NEMA 34 (85 mm) +12 to +75 4.0 A 18
MForce PowerDrive +12 to +75 4.0 A 18
Lexium MDrive NEMA 17 / LMM +12 to +48 2.0 A 20
Lexium MDrive NEMA 23 +12 to +60 3.5 A 18
Lexium MDrive NEMA 34 +12 to +70 4.0 A 18