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.

The impact of electrical noise in a system is greatly reduced by following a few basic practices:Signal cabling should be 22 AWG, shielded twisted pairs.

  1. Provide adequate strain relief for all cables.
  2. Minimum cable bend radius = 4 x cable diameter.
  3. Cut the shield at the I/O device end, leave floating.
  4. Connect the shield at the Motion Control Device end earth.
  5. Ensure that all earth connections to the control cabinet are free of paint.
  6. Do not run power cabling along side signal cabling.
  7. Keep grounds as short as possible.
  8. If a secondary shield is used over the primary, tie both ends of the secondary shield to earth.

Design variations between products

Lexium MDrive inputs:

  • +5 to+24 VDC
  • Isolated (optical)
  • Fixed as inputs

Lexium MDrive outputs:

  • +5 to+24 VDC
  • Isolated (optical)
  • Fixed as outputs
  • Dry-contact type

MDrive/MForce I/O

  • +5 to +24 VDC
  • Programmable as input or output
  • No isolation

Sinking I/O

Sinking: 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.

Sinking input

Sinking input
Figure 1: Sinking input

Sinking output

Sinking output
Figure 2: Sinking output

Sourcing I/O

Sourcing :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.

Sourcing input

Sourcing input
Figure 1: Sourcing input

Sourcing output

Sourcing output
Figure 2: Sourcing output

Cabling

The impact of electrical noise in a system is greatly reduced by following a few basic practices: Signal cabling should be 22 AWG, shielded twisted pairs.

  1. Provide adequate strain relief for all cables.
  2. Minimum cable bend radius = 4 x cable diameter.
  3. Cut the shield at the I/O device end, leave floating.
  4. Connect the shield at the Motion Control Device end earth.
  5. Ensure that all earth connections to the control cabinet are free of paint.
  6. Do not run power cabling alongside signal cabling.
  7. Keep grounds as short as possible.
  8. If a secondary shield is used over the primary, tie both ends of the secondary shield to earth.

Cabling best practices

Cabling best practices
Cabling best practices

 

Lexium MDriveMDrive

Single interface

Wiring diagram:  Full duplex (4-wire) only

lnd422single
Figure 1: Single Lexium MDrive full duplex point-to-point configuration

 Notes and checkpoints

  • Ensure the crossover between the interface and Lexium MDrive (Rx—Tx / Tx—Rx)
  • Ensure the crossover between the interface and Lexium MDrive (Rx—Tx / Tx—Rx)
  • Ground is galvanically isolated on the Lexium MDrive
  • If using a pre-made cable, verify wire color to pin location – when in doubt, ring it out

Party mode interface

Party mode is an advanced communications configuration requiring a working knowledge of the ANSI/TIA/EIA-422 and TIA/EIA-485 standards, as well as SEM MDrive products and the MCode programming and control language.

lexium-mdrive-422-multipoint
Figure 2: Lexium MDrive RS-422 multipoint system, bus and stub configuration

Single interface

Wiring diagram:  Full duplex (4-wire) only

Single MDrivePlus/MForce - full duplex connection
Figure 1: Single MDrivePlus/MForce – full duplex point-to-point connection

  Notes and checkpoints

  • Ensure the crossover between the interface and Lexium MDrive (Rx—Tx / Tx—Rx)
  • Ensure the crossover between the interface and Lexium MDrive (Rx—Tx / Tx—Rx)
  • Ground is galvanically isolated on the Lexium MDrive
  • If using a pre-made cable, verify wire color to pin location – when in doubt, ring it out.

Party mode interface

Party mode is an advanced communications configuration requiring a working knowledge of the ANSI/TIA/EIA-422 and TIA/EIA-485 standards, as well as SEM MDrive products and the MCode programming and control language. The diagram below shows a bus and stub type network configuration, MDrive/MForce RD and RL style connectors provide an additional signal contact point for daisy-chain configurations.

MDrive/MForce RS-422 multipoint network
Figure 2: MDrive/MForce RS-422 multipoint network, bus and stub configuration

 

Stepper Motors
Figure 1: Stepper Motors

The capabilities of the different stack length motors are often confusing to people specifying them and designing them into systems.  The best way to think of the capabilities of the different stack lengths of a particular frame size is to think of the trade-offs.  The smaller MDrive stack length has lower torque that persists to a higher speed.  The larger stack length MDrives have the much higher torque that falls off much more rapidly as speed increases.  This is due to substantially higher inductance in the longer stack lengths.  This higher inductance results from the two things that create the inductance in the first place: iron and copper.  Longer stack length motors have more of both.

Iron and copper are also great at producing magnetism and thereby torque.  By their nature, steppers require the interruption and reversal of motor phase current. The higher the inductance, the more time this takes.   The result is that the current in a phase will be switched before it has been able to rise to the nominal value.  Since full current is never reached, the torque output is lower.

This is all very important when gearing is being selected.  There are times when the application needs more torque than a triple stack can provide, so gearing is added. Since the torque falls off with speed is so severe with the triple, some applications that use gearing are done with single stack motors since they can run faster which is required by the gear ratio.

Steppers are different from other motors in that they produce great torque at low speeds, but at higher speeds, that torque starts to fall off quickly (more so in larger stack sizes).  In other motors, like AC or DC motors, the torque is quite constant from low speeds up to some base speed like 1800 RPM or 3600 RPM.  Above that speed, the torque will then start to fall off.  Since they hold their torque to this higher RPM, they have much higher power because power is the product of both speed and torque.

Since there are so many variables affecting MDrive running temperature, it is difficult if not impossible to accurately predict thermal performance in a particular application.  For this reason, the Lexium MDrive and many other servo and stepper drives are designed so that they can produce more power than they can handle from a thermal point of view. If they were designed so they would never overheat, the user with the 5% duty cycle would not benefit from all that time for cooling.  The following are some of the variables can affect thermal performance:

  • Ambient temperature
  • Air flow over the device
  • Heat sinking from the front end-bell to the machine frame
  • Current settings in both run and hold mode. (Rc and Hc)
  • Duty cycle
  • Running speed
  • Load on the shaft
  • hMTechnology settings

When testing an application with Lexium MDrive, the important thing to ensure is that the heat sink temperature never exceeds 85°C and the motor lamination temperature never exceeds 100°C. The preliminary thermal performance graphs below show some of the frame sizes and lengths.  More graphs will be added as they become available.  Note that raising or lowering the ambient temperature raises or lowers the drive temperature by a roughly equivalent amount.

Thermal Performance Graphs

LM57 NEMA23 Single Length

Device: LMDCM571

  • NEMA 23 (57 mm) Single length
  • Speed: 2000 full steps/sec
  • Voltage: 45 VDC
  • Torque ≈ 84 oz-in (59.3 N-cm)
  • Mounting: 6.5” x 6.5” x 1/4” aluminum plate

LM57 NEMA23 Double Length

Device: LMDCM572

  • NEMA 23 (57 mm) Double length
  • Speed: 2000 full steps/sec
  • Voltage: 45 VDC
  • Torque ≈ 95oz-in (67.1 N-cm)
  • Mounting: 6.5” x 6.5” x 1/4” aluminum plate

LM57 NEMA23 Triple Length

Device: LMDCM573

  • NEMA 23 (57 mm) Triple length
  • Speed: 2000 full steps/sec
  • Voltage: 45 VDC
  • Torque ≈ 116oz-in (81.9 N-cm)
  • Mounting: 6.5” x 6.5” x 1/4” aluminum plate