Month: November 2018

Why do you use a stepper motor?

Easy to use: 34%
Inexpensive: 17%
Simple operations:16%
No need for tuning: 12%
Other: 21%

Key Points: Ease-of-Use, Simple Operations and Low Cost
According to the survey of stepper motors users, many favor stepper motors for their “ease-of-use,” “simple operations”, and “low cost stepper motor” derived from the structure and system configuration. It makes sense that many users find such positive aspects in stepper motors, thanks to the simple structure and system configuration. However, some readers may be skeptical about the actual performance of the motor in terms of its accuracy and torque. It is not easy to fully grasp the whole idea unless there are comparison examples against other control motors such as servo motors. By knowing the characteristics and taking on different approaches based on required operations, stepper motors certainly can reduce the cost of the equipment. The characteristics and technical information of stepper motors are explained below:

Impressive “Stopping Accuracy.” Moves Quickly in the “Low / Mid-Speed Range”
Stepper motors have remarkable stopping accuracy, and accurate control with open-loop is possible. For example, when using the RK II Series for positioning of a rotating table, its stopping accuracy is within ±0.05° (with no load). Because stopping position errors do not accumulate between steps, high accuracy positioning is possible. The structure of the stepper motor, which requires no encoder, allows for the simple drive system and low cost.

Step Modes

Stepper drivers for sale online come in a variety of step modes, which influence the rotation speed, size, and action of the drive shaft. A complete set of steps does not necessarily imply a full rotation of the drive shaft. In some instances, multiple complete step sequences are required to achieve a quarter of a turn of the drive shaft. The following sections discuss several different step modes and their characteristics.

Wave Step Mode

Wave step mode is found on a number of older stepper driver models. Wave step mode involves passing energy to one phase of the motor at a time. This creates fluctuating torque, which is unstable. It is also considerably less efficient than other step modes, because energy is lost in between energizing each phase. Because of the energy loss, less torque is produced than with other step modes.

Full Step Mode

With a full step mode driver, both phases of the motor are energized simultaneously and continuously. This produces full torque for all phases of the motor at all times. Full step motors are very stable, and one complete turn of the drive shaft is equal to one complete step sequence. For example, if there are 50 steps in the step sequence, 50 steps equals one 360-degree rotation of the drive shaft.

Half Step Mode

Half step mode is an alternating energy circuit. First, one coil phase is energized, swiftly followed by both coil phases being energized. This technique doubles the resolution of the step sequence. It creates varying degrees of torque because the single coil phase energy produces half the torque of the double coil phase energy. High end models decrease the torque differential by increasing the amount of current that passes through the single coil phase. This enables the single coil to increase torque output. Half step mode is very stable and doubles the possible resolution and number of step positions. A 50-step sequence step motor capable of operating in half step mode to allow for greater resolution has 100 step positions.

Micro Step Mode

Micro stepping mode is only available on high end cheap stepper motor drive models, and, as such, are more expensive than other kinds. Micro stepping energizes each coil to a set fraction, allowing fractional steps to occur. This creates very high resolution and precise stepping. Micro stepping creates a very smooth step motion, and some models can move at 1/256th of a step.

For design engineers in the process of selecting components for an application, the motor or gearmotor can be one of the more difficult components to source. There are so many types of motors and gearmotors all screaming for your attention and promising to be the “most efficient”, the “highest quality” or boasting about “high precision”. Groschopp uses 4 simple steps to lead designers to the best motor/gearmotor choice for a particular application. There are several key design parameters that should be considered when selecting a motor or gearmotor for a motion control application.

Table 1: Gear stepper motor Selection Process – steps to complete to ensure a gearmotor properly matches the application

As the motor or stepper motor planetary gearbox selection process begins, the designer must gather the relevant technical and commercial requirements. This first step is often overlooked, but it is a critical component in the design process. The gathered design inputs information will then be used in the selection process and will dictate the ideal motor for the application. Failure to gather the proper inputs can lead the designer down an untended path. For this reason, it is helpful to use the Application Checklist (Table 2) when developing the motor specification. These parameters, along with some project specific requirements, will be helpful when navigating the selection process.

Table 2: Application Checklist – use this checklist to help formulate the specific requirements to ensure the gearmotor vendor has the critical information necessary to achieve the best match between the gearmotor and the application

Next, the designer must consider what type of motor technology best suits the intended application. Using the design inputs, the Motors Quick Reference Guide (Table 3) can be used as a selection matrix in the first step of the decision process. This reference guide details four common motor types and provides general information to consider when selecting each stepper motor spur gear. Because each application has its own unique characteristics, it is important to determine which of the parameters (e.g. horsepower, efficiency, life, starting torque or noise ratings) are most important to the application under consideration. During the motor selection process, by looking at the required speed and torque of the application, it should become evident to the designer if the motor chosen requires a gearbox to meet the necessary requirements. If a gearmotor is necessary for the application, another level of complexity will be added and several additional criteria need to be evaluated.

Microstepping can improve stepper motor system performance in a number of applications, and it can reduce system cost and complexity relative to half- and full-step driving techniques. In addition, microstepping can help solve noise and resonance problems all while increasing step accuracy and resolution.

A stepper motor system’s natural frequency is determined by holding torque, rotor and load inertia, and the number of full-steps per revolution. When stepper motor system damping is low you risk generating noise or losing steps when the stepper motor operates at or near the resonant frequency depending on damping, total inertia, and the type of stepper motor. These issues can happen at or near integer multiples and fractions of the natural frequency. Typically the frequencies closest to the natural frequency cause the most problems.

The principal source of these resonances is that the stator flux moves in a discontinuous way when you use a non-microstepping stepper motor driver — forty-five or ninety degrees at a time — causing a pulsing energy flow to the rotor, and these pulsations excite the resonance. Using half-steps rather than full-steps reduces the excitation energy to roughly twenty-nine percent of the full-step energy. If we microstep the motor in 1/32-full-step mode only point one percent of the full-step energy remains. You can reduce the excitation energy to a low enough level that all resonances are completely eliminated by microstepping.

However, this is only true of an ideal stepper motor. In practical applications there are additional sources that excite system resonances. Regardless, microstepping improves movement in nearly all applications and, in many cases, microstepping alone will sufficiently reduce noise and vibrations for most applications.

When you run a stepper motor at low frequencies in half- or full-step mode the movement is discontinuous, noise and vibrations are generated, and there will be significant ringing. The frequencies where this occurs are below the stepper motor system’s natural frequency, which is why microstepping offers a safe, simple means of extending noiseless stepping frequencies approaching zero hertz.

You don’t usually need steps smaller than 1/32-full-step — electrical step angles this small are easily absorbed by the stepper motor’s internal friction, meaning the stepping doesn’t generate overshot or ringing. The microstepping positions will deviate from a straight line because of uncompensated sine/cosine profiles.

Step motors are unique among electronic motors in that they move in a series of discrete steps (hence their name) rather than a continuous motion. This is a useful property since it allows steppers to have positional and velocity control that is both accurate and easy and doesn’t even require feedback to maintain (under normal operation). However, one of the primary disadvantages of this style of motor comes as a direct result of this discrete nature and open loop control.

When a stepper takes a single step, it will overshoot its target destination slightly and will oscillate a bit before settling down on target. This is due mostly to the inertia of the rotating mass briefly overpowering the magnetic field of the motor. This isn’t a big deal by itself but when you start chaining multiple steps together to get a larger movement this oscillation occurs at each step taken on the way. If the frequency that the controller is outputting new step commands to the motor matches the natural frequency of the motor then the oscillations will tend to become more severe as they propagate through the motor. Eventually they are so large that they will overpower the magnetic field for long enough on a given step to miss the subsequent step command, and you begin missing steps. Since steppers are typically run in open loop, the controller has no knowledge of these missed steps; The result is the motor will not get to its destination successfully. The effect can become so pronounced that the motor loses torque completely and stops rotating. Depending on the synchronization of the steps, it can even reverse the direction of rotation.

Why are steppers still so popular then? As bad as this issue sounds, there are a few mitigating factors. First, this phenomenon is only problematic at the stepper’s natural frequency. This means that it will only be at work in a certain velocity band. Depending on what speeds you are running your motor at, you may never even notice it at all! Second, the resonance takes time to build up. It’s not like you will hit a bad velocity and immediately lose torque. The oscillations will take a few seconds to get to the troublesome levels. Since this issue only crops up at certain velocities, you can typically accelerate through a bad region and emerge unaffected on the other side. You are only at risk if you are staying in the bad region for extended periods of time. Finally, this effect is greatly reduced by having load on the motor. If the motor has load on it, then the inertia is much greater and the oscillations will be reduced substantially. Notice how in the video, when I apply some pressure with my hand (effectively loading the motor down) it starts operating normally again. You are far less likely to experience ringing on a motor that is loaded than on one that is not. This is good since the vast majority of the time the motor will be loaded (what are you using it for after all?).

There are commercially available dampers, like the MDR Damper Roll and  Nema17 Dampers, that act sort of like a flywheel that can be attached to the rear shaft of a stepper to artificially load it down to combat ringing, if necessary (though I cannot speak to their effectiveness).

Overall, provided you know about the problem it is fairly easy to avoid. The best thing to do when you get a new stepper is to quickly run it through the available velocity range and find where the motor has trouble keeping torque. That is the region you should avoid in your application.

Today we will talk about how to control such a motor together with a simple example, involving a H-bridge electronic circuit and simple scripting. In our implementation we have used a nema 23 bipolar stepper motor, however minor changes in control sequences are required for other types of stepper motors.

To summarize, the electromagnetic coils are located on the stator of the stepper motor, while permanent magnets, equal in pair numbers, are located on the rotor. A more detailed discussion about stepper motors can be found in our dedicated article, but making a long story short, like any DC motor, these motors rotate when the coils are energized however, if the coils are continuously energized in the same way, the movement will stop when opposite magnetic poles are aligned, e.g. S-pole on coil aligned with N-pole on rotor permanent magnet.

Electromagnetic coils found in a bipolar hybrid stepper motor  are arranged as independent windings, each of them corresponding to one phase. Usually such stepper motors have 6 terminals, 5 if the common wires of the two windings are internally connected, and the terminals can be identified by measuring the resistance between terminals using a multimeter.

In short, if resistance of a coil, between its two end terminal has a certain value, the resistance between the common lead and any of its terminals must a value divided by 2. In 5-lead motors, coil terminals can be determined by touching each two wires together. When the rotor shaft becomes harder to turn it means that the two connected wires belong to the same winding.