How to Control Stepper Motors for More Accurate Robotic Movements

Within many of the technologies that drive today’s world, you’ll find stepper motors hard at work. These relatively simple but highly efficient motors are an excellent choice for many different applications, including manufacturing applications that use robotic components.

Stepper motors, however, demand control systems that can provide the degree of precise operational control that creates accurate and precise robotic movements. And when an engineer is designing these control systems, they’ll need to pay careful attention to the design of the controller and the electrical enclosure box used to house it.

In today’s edition of the Polycase TechTalk blog, we’ll dive into the basics of the stepper motor and its applications. Then, we’ll talk about how to control stepper motors for the mechanical and electrical engineers designing and specifying the motor control systems.

A partially disassembled stepper motor. Note the magnetic coils surrounding the gear rotor.

What Is a Stepper Motor?

A stepper motor is a common type of DC-powered brushless motor. To understand how stepper motors work and why we use them, you should first understand a few things about how electric motors work.

The basic purpose of an electrical motor is to convert electrical energy into mechanical energy. To accomplish this, an electrical motor uses various types of magnets that interact with each other and cause the motor’s physical components to move. The movement of the components turns the motor shaft which transfers the mechanical energy out of the motor to power other parts of the system.

There are two commonly used types of electric motors: brushed motors and brushless motors. Each uses a different method of converting electrical energy into mechanical energy:

• A brushed motor uses electrical contacts called brushes that transfer current to an armature wound with metallic coils. Two permanent stator magnets, one positive and one negative, surround the armature and cause the energized armature to spin. When the armature spins, its kinetic energy turns the motor shaft.
• A brushless motor uses multiple stator electromagnets arranged in a ring around a magnetized rotor. The electromagnets can be activated and deactivated in sequence to control the turning of the rotor and position it in precise alignment with each magnet. Here, the spinning of the rotor is what produces the kinetic energy that turns the motor shaft.

You’ll still find brushed motors in use in various applications, such as household appliances. However, brushless motors have supplanted brushed ones in many other areas, including in many of the digital technologies that drive our world.

Thus, it’s important to understand how to control stepper motors and how to build control systems that operate them safely and efficiently. Before we dive into how they work, though, let’s get a better understanding of how stepper motors are used by looking at the advantages stepper motors can offer and the applications they’re commonly used in.

An example of the basic design of a stepper motor. Each of the coils is activated in sequence to produce consistent and predictable movement in the rotor.

Advantages of Stepper Motors

So, why are stepper motors often a strong choice for robotics applications? Here are some of the major advantages of stepper motors that make them ideally suited for robotics:

• Extremely precise control over motor movement and positioning
• Much lower maintenance requirements than brushed motors
• Highly efficient and reliable operation at rated speeds and loads
• Substantially smaller footprint than most brushed motors
• Generates much less heat and electrical noise than brushed motors

Unipolar stepper motors, which energize only one side of the stator coil ring at a time, are also relatively inexpensive which make them popular among electronics hobbyists and professionals alike. Bipolar stepper motors, which also transmit a reverse current to the coil opposite the energized stator coil, are more complex and more expensive. However, their smoother and more precise operation makes them more desirable for industrial applications.

A stepper motor isn’t the right choice for every application. If your project requires continuous motor rotation at very high speeds (typically over 2,000 RPM), the numerous poles in a stepper drive can cause conditions, such as eddy current loss that reduce the motor’s efficiency. Additionally, the wear on the rotor at such high speeds may reduce its lifespan.

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Robotics Applications for Stepper Motors

Stepper motors are useful for a wide variety of applications, particularly in the robotics sector. Because stepper motors excel at precise positioning and reliable repeated motions, they’re ideal for applications that require long sequences of fast, accurate movements. Common robotic applications of stepper motors include:

• CNC machining
• 3D and 2D printing
• Textile manufacturing
• Medical imaging devices
• Hard drive disk storage systems

Each of these applications requires pinpoint control over rotors, lasers, print heads, styluses and other automated robotic components. Stepper motors provide the precision control and reliability engineers need when they design robotic systems. For stepper motors to fulfill their potential in these applications, however, it’s important to have a solid grasp of how to control stepper motors using the various methods of voltage control available.

Stepper Motor Control Basics

Fundamentally, the basic method of controlling a stepper motor is energizing and de-energizing the coils that surround the gear in the correct sequence. Varying the sequence and timing of the coil activations is how engineers customize the operation of a stepper motor to the needs of their applications.

Changes in the flow of current to a stepper motor can produce extremely subtle and precise differences in positioning. The precision of a stepper motor depends on its number of steps—that is, the number of different positions it occupies over the course of a full rotation cycle. A stepper motor with more steps will be more able to create more precise positioning differences. The level of precision a stepper motor can achieve is known as the motor’s resolution.

The major modes of stepper motor control are:

• Wave Drive/Single Phase: One coil is activated at a time in sequence. This is the most basic mode of stepper motor operation and produces the lowest resolution.
• Full Step: Two coils are activated simultaneously in sequence, so the rotor’s polar positions are actually between each coil. This mode is useful for improving the motor’s torque and speed, but it doesn’t increase the motor’s resolution because it will still have the same number of steps.
• Half Step: One coil is activated and then two coils are activated simultaneously in the next step. Thus, the rotor moves a half-step at a time, from a directly aligned position when one coil is active to a split alignment when two coils are active. This mode actually adds extra steps to the motor’s rotation, so it substantially increases resolution.
• Microstep: The coils are activated using a series of sine wave pulses that move the rotor in extremely small steps. This method produces the highest resolution of any listed here, thanks to its ability to subdivide the rotor’s cycle of full steps into up to 256 steps. Microstepping also moves the rotor very smoothly and consistently, which helps reduce vibration, noise and wear and tear on motor components. Due to these benefits, microstepping is the most common activation mode for stepper motors in today’s applications.

It’s also possible to increase a stepper motor’s resolution by increasing its number of coils and poles. Most stepper motors used by professional engineers have a large number of coils that provide around 200 steps before microstepping. Put together, these options give engineers using stepper motors many different options for increasing the precision of the motor’s resolution.

Every stepper motor is controlled via a driver circuit, which is what transmits current to the coils in order to activate them. Since the entire mechanism that controls the stepper motor’s movements begins with the motor controller, it’s vital that stepper motor controls be designed carefully and according to best practices. In the next sections, we’ll take a quick look at the basics you’ll need to know when designing the driver circuit for a stepper motor.

Basics of Stepper Motor Drive Selection

A stepper motor’s driver circuit is typically attached to a microcontroller that provides the motor’s operator with control over the voltage pulses sent to the coils. Many different options are available for stepper drive circuits, including some models designed for out-of-the-box use as a stepper controller. Others use programmable microcontrollers like the Arduino Uno, which can be programmed using its open-source stepper control library.

When choosing a stepper motor driver circuit, start with four basic factors:

1. Voltage: The driver should be rated for a wide variety of voltage levels since you’ll likely need to test out different voltages to find the best option for your stepper motor.
2. Current: The driver should be rated for at least 1.4 times the rated maximum current of the stepper motor.
3. Microstepping: The driver should have multiple step resolution options so you can experiment with different microstepping settings.
4. Maximum Step Pulse: The driver should be rated for a sufficient amount of step pulses to turn the rotor at the speed your application requires.

Other Factors in Stepper Drive Selection

Despite the relatively simple design of stepper motors, specifying the right stepper motor and designing an appropriate control system can be challenging tasks. The four factors we just discussed can be affected by a wide variety of other factors that merit consideration.

Make sure to account for the following when designing a system that incorporates a stepper motor:

• When your application requires specific rotation speeds and torque, it can be challenging to match the right motor and drive even when the above conditions have been considered. Generally, the most accurate method for matching a motor and drive is to use engineering software with appropriate speed-torque curves. Start with a drive that has the desired voltage and current characteristics and then match it to a motor using your speed-torque curve.

• Torque mismatches are a common issue in stepper motor drive design that can cause stalling (from insufficient torque) or vibration and noise (from excessive torque). To get the right amount of torque, start by calculating the load torque of the desired task the motor needs to perform. Once you’ve determined the required load torque, use load and motor torque matching techniques to ensure your motor torque is in the sweet spot.
• Remember, a stepper motor needs full current to produce holding torque, which means a stepper motor can become extremely hot when held in position for an extended time. Make sure to account for this when designing cooling systems.
• Bipolar stepper motors require slightly more complex drive circuits than unipolar stepper motors do because they need to transmit a negative current simultaneously to the opposite coil. If you’re using a bipolar stepper motor, make sure your drive unit is rated to control bipolar motors.
• The stepper motor’s control unit should be protected by electrical enclosures to keep it protected from environmental hazards. Depending on the motor’s operating environment, its junction box wiring can become fouled through dust and debris or damaged by splashing water (among many other possible dangers).

Polycase Enclosures for Stepper Motor Control Units

Polycase electrical enclosures are an excellent choice for engineers designing stepper motor control units. Engineers choose Polycase enclosures when they want UL-rated performance, sleek designs and the highest quality materials for their stepper motor enclosures. Plus, we offer enclosure customization options that make it incredibly fast and easy to modify an enclosure with custom CNC enclosure machining cutouts and digital printing.

The following are some of the most popular electrical enclosures for our industrial customers who use them to safeguard equipment, such as stepper motor controls:

• ZH Series: Rugged hinged polycarbonate enclosures suitable for indoor or outdoor conditions.
• SA Series: Stainless steel outdoor enclosures with performance to match their sleek looks.
• EX Series: Extruded aluminum enclosures with an attractive finish and built-in rails for easy PCB mounting.

Go with a Polycarbonate Enclosure That Is Durable and Affordable

A fully-assembled stepper motor, including its metallic enclosure.

Do you need the perfect electrical enclosure to house your stepper motor and/or motor control unit? Our electrical enclosure professionals are always glad to help you select the model that meets your needs. Call Polycase at 1-800-248-1233 to speak to an enclosure expert or contact us online at any time.

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