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.

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.

Take the case of a packaging equipment manufacturer with a machine based on a modular design that could be adapted to different boxes depending on the end customer. Let us consider that the primary function of this machine is filling, folding and sealing boxes. Some customers need desiccant bags put in the box before sealing, so a separate modular section will be added to the machine for this purpose.

The desiccant bags come on a reel with a registration hole punched in between each bag. The bags must be separated by cutting at, or near, the registration hole, and then placed one in a box. Typically, air cylinders are used as the feed-to-length and cutter actuators, and a PLC controls the motion. The PLC takes its input from a photocell used to detect the registration hole. Relays and solenoid valves on the output of the PLC control the air which in turn drives the cylinders.

This arrangement would work fine, but to be able to handle different sized desiccant bags the machine needs to be completely reworked. The desiccant bag feeders’ limiting factor is throughput. In addition to this, the air cylinders are the only things that need compressed air on the entire machine. Compressed air processes are known to be noisy, dirty, and they require regular maintenance.

IMS’s AC line driven stepper system would offer a vastly superior stepper solution. A pinch roller driven by an MDrive34AC Plus2 Motion Control stepper motor/driver and control system would replace the feed mechanism. The pneumatic cutter would be replaced with an electric cutter and actuated by a relay controlled by one of eight configurable I/O resident on the MDrive34AC Plus2 . The MDrive34AC Plus2 would be programmed to move at a constant velocity until the registration hole is detected. Once detected, the driver would advance a fixed number of microsteps from that position, then stop and fire the cutter. The desiccant bag cutter module would now work with any size bag without any adjustments or reprogramming required.

This stepper approach would result in a superior system. Precision would be improved and scheduled maintenance reduced to sharpening the cutter blade. Adaptability to different desiccant bags would go from very difficult to completely automatic and transparent. The stepper solution is smaller and requires considerably less wiring and panel space. Throughput would increase due to the vast velocity capability of the AC line driven system. The machine no longer would require compressed air. The stepper system solution would work more efficiently with fewer parts and offers an integrated motion solution in one small package.