Choosing the right drive strategy for stepper motor applications

Stepper motors are a popular choice for many motorized systems, particularly because they are easy to drive. However, knowing more about the different stepper motor technologies and how they are driven can be a significant benefit. With an understanding of bipolar and unipolar driving, it is easier for designers to implement a truly optimized solution.

Clémence Muron, Application Engineer at Portescap, explains the differences between bipolar and unipolar drives for stepper motors and examines their advantages and disadvantages in specific implementations.

For motor-driven applications that require precise and predictable positioning in a compact, low-cost package, stepper motors have been a popular choice since their introduction in the 1960s. In recent years, advances in both the motors themselves and drive electronics have given stepper motors significant performance advantages in demanding applications ranging from machine tools to medical devices.

The simplicity of the drive technology for modern stepper motors is one of the main reasons for their continued popularity. Nevertheless, design decisions must be made with the control concept, which can have a major impact on the success of the application. In particular, the designer is faced with the question of whether he should opt for unipolar or bipolar control of the motors.

While it is true that many stepper motor applications today are implemented with a bipolar drive. However, unipolar motors are still available on the market and in certain circumstances the use of unipolar motors also makes sense. Designers who understand the differences between these technologies—some beneficial and some disadvantageous—are able to select the most appropriate technology for a given application.

The stepper motor is a type of brushless DC motor with a large number of poles. This technology is generally operated in open loop without a feedback sensor, which means that the current is usually applied to the phases without knowing the rotor position. The rotor moves to align with the stator magnetic flux, then the current can be fed to the next phase.

As mentioned earlier, there are two ways to power the coil: the bipolar way and the unipolar way. A unipolar motor requires at least two coils per phase and by switching the transistors in the drive electronics the current only flows in one direction for each coil. The drive electronics can be easily implemented since there is only one transistor per coil. When the transistor is closed, the coil is energized. To turn the motor, the transistors are closed and opened one after the other.

In contrast, a bipolar motor requires at least one coil per phase and current can flow in either direction through the coils. In the drive electronics, a motor with the same number of phases requires twice the number of transistors.

In terms of wiring, a unipolar motor can have 6 or 8 leads, while a bipolar motor typically has 4 or 8 leads. For the 8-wire versions, the motor can be configured to be either unipolar or bipolar depending on how these wires are connected. For the bipolar, the coils can be mounted in series or in parallel, which affects the electrical properties of the coils that affect voltage and resistance. The 8-wire design gives designers more flexibility to meet the specific performance needs of the application.

Advantages and disadvantages

There are a number of different control techniques to consider, but let’s look at voltage control and current control first. With voltage control, controlling unipolar motors is very simple and requires only four transistors. In contrast, a bipolar regulator for the voltage drive requires eight transistors configured in two H-bridges. For voltage control, the unipolar control strategy offers a far simpler and less expensive solution.

However, there are also performance aspects to consider. In voltage drive, the bipolar motor offers more torque, while a unipolar motor has less torque at low speed, mainly due to Joule losses. However, in a certain case, the unipolar motor can sometimes offer higher torque than the bipolar motor at high speed because the current in the coil can flow faster.

Generally, when torque is important, the bipolar motor can produce 40% more torque than a unipolar drive, or in other words, for the same energy loss through the Joule effect, the bipolar motor will perform better than a unipolar drive.

But what about the power control? As electronics have become cheaper, pulse width modulation (PWM) has become the most popular stepper motor controller, varying the duty cycle at a fixed frequency to adjust the voltage or current to the desired target value. In current drive, the PWM controls the current in each phase.

With a bipolar motor, modern drive electronics make it relatively easy to implement PWM current control, opening up the possibility of microstepping the motor. Running a unipolar motor in current mode requires much more complex electronics at the same time lower motor power.

So, in summary, we have seen that given the benefits of PWM control and the reduction in the price of electronics, there is a growing trend towards using current-drive bipolar stepper motors. For voltage drive, the unipolar drive can still be a viable option when simplicity and/or cost are important considerations, or when higher torque at higher speeds is required.

As always, it pays to consult a knowledgeable supplier such as Portescap from the earliest stages of a project to enable the designer to find the most suitable solution for a particular application.

About Portescap

Portescap offers the broadest miniature and specialty motor products in the industry, encompassing coreless brush DC, brushless DC, stepper can stack, gearheads, digital linear actuators, and disc magnet technologies. Portescap products have been serving diverse motion control needs in wide spectrum of medical and industrial applications, lifescience, instrumentation, automation, aerospace and commercial applications, for more than 70 years.

Portescap has manufacturing centers in the United States and India, and utilizes a global product development network with research and development centers in the United States, China, India and Switzerland.

For more information, visit www.portescap.com