Feature Article

Figure 1. The phase currents in an open-loop driver create a rotating magnetic field. The rotor-side magnet follows the generated magnetic field more or less like a paper clip on a piece of paper follows the movement of a magnet underneath.

Hybrid stepper motors
Many medical technology and laboratory applications require precise positioning or speed control capabilities and long-life maintenance-free operation. Hybrid stepper motors bring several advantages to these types of applications. Because they have no parts that can wear out, the motors have an extended operating life. Their structure—typically 200 natural steps per revolution—is well suited for precise positioning movements. Additional benefits such as high torque at low speeds—up to 7 N•m—as well as mechanical stiffness against variable loads make this technology an ideal candidate for direct-drive systems in applications at speeds up to 1500 rpm. Peristaltic pumps and positioning systems are two application examples. Despite the obvious benefits in these and other applications, hybrid stepper motor technology is not easy to set up. The large number of steps and intrinsic mechanical stiffness require rapid and precise electrical commutation (i.e., the number of steps demands high commutating frequencies and narrow angles compared with other motor technologies).

For these reasons, hybrid stepper motors traditionally are used in open-loop systems. This control mode works well in many cases, but it also limits its field of interest for the user. The drawbacks are well known: internal operating temperature, operating noise, vibration and possible step losses if the dimensions are off. These problems have caused engineers to turn to brushless dc (BLDC) motors with gearboxes instead of direct-drive stepper motors. A closed-loop system that draws on recent innovations in embedded electronics offers an alternative to these conventional drive solutions.

Open- and closed-loop motor control
Stepper and BLDC motors can be used in an open- or closed-loop configuration. Although open-loop drivers for BLDC motors are rare, open-loop configurations have been, and continue to be, the standard driver type for stepper motors. The difference between an open- and closed-loop driver is the pattern, and, more importantly, the application of phase current to the motor. To clarify, let’s examine the difference between these two driver types.

Open-loop driver
An open-loop driver imposes a specific current pattern on the motor phase to create a rotating magnetic field. The motor follows the magnetic field much in the same way that a paper clip on a piece of paper follows the movement of a magnet underneath the paper. If the paper clip were to encounter an obstruction, however, the magnetic attraction would be interrupted. This can also happen to a stepper motor, a condition that is commonly known as stall or step loss. This is a severe fault condition since the expected movement no longer can be guaranteed, and it is particularly serious in medical and laboratory applications. Just consider the consequences if a dosing device or pump ceases to work as expected. Figure 1 shows the working principle of a stepper motor.

Preventing lost steps in an open-loop system gains in complexity when one considers applications that combine high accelerations and/or driving loads with high mass/inertia. Continuing our analogy: a paper clip is extremely lightweight, but what if it is attached to a load that we will be pulling using only the paper clip and the magnet? When the acceleration increases, one must be careful not to move the magnet too quickly or it will lose its hold on the paper clip.

To avoid step losses, caused either by torque fluctuations or high accelerations, stepper motors that run in open loop are always over-dimensioned. A good engineering practice used in most applications is to specify a stepper motor capable of pulling a load that is 2× the expected maximum load (see Figure 2). In addition to building in two-factor security, this practice ensures full functionality for the life of the application. On the other hand, an over-dimensioned system is not cost-efficient and is prone to heightened heating, vibration and noise levels.

Closed-loop driver

Figure 2. In open-loop mode, stepper motor design must take into account the expected maximum load and acceleration. Two-factor security is recommended.

To understand the advantages of a closed-loop driver compared with an open-loop driver, let’s revisit our friend the paper clip. In the open-loop example, we came to the conclusion that it is necessary to use a very strong magnet—one that is at least twice as strong as what is called for by the application—to overcome obstacles that might stop the paper clip from moving in concert with the magnet.

The process would be much easier if we used transparent rather than opaque paper. If we were able to see the magnet and paper clip simultaneously, then we could slow down as we approach an obstacle. We could also make the paper clip creep around obstacles since we know where the magnet is positioned with respect to the paper clip. At this point, you might be asking yourself what transparent paper and a paper clip have to do with closed-loop motor control. Well, the answer is a great deal! The key in closed-loop motor control is knowing at all times where the rotor is in relation to the magnetic field that is being generated. This can be achieved using a position sensor, which can be compared with a sheet of transparent paper.

The transparent paper tells us the relative positions of the magnet and paper clip. In a similar way, a position sensor gives the relative position between the phase current and rotor. For a BLDC motor, a low-resolution position sensor is sufficient. For a closed-loop stepper motor driver, however, the position encoder must be very precise because of the high number of steps, which can exceed 200.

By knowing the motor’s position, we can slow it down as the load increases and, in fact, take complete control of its acceleration. If the motor is not accelerating in the way that we want it to, we can simply slow it down and wait for the rotor to catch up, similar to the way in which we would adjust the movement of the magnet, if we are monitoring it through the transparent paper.

The use of a position sensor and closed-loop motor driver renders two-factor security margins unnecessary. With closed-loop control, the current level can be automatically modulated and adapted to the load. The end result is a much smaller stepper motor that can pull the same load as the aforementioned open-loop system or achieve significantly higher acceleration rates. This is an undeniable advantage in medical or laboratory applications, where motor size usually accounts for a large part of the finished device.

Position encoders
There are many types of position sensors or encoders, but the most commonly used for motion control are optical or magnetic Hall effect sensors.

The simplest type of optical sensor is the incremental disc encoder. Light from a photodiode shines through slits in a rotating disc, and a light-sensitive element on the other side of the disc captures its rotation. This type of encoder has very high precision and achieves resolution up to 20,000 increments/revolution. It is, however, sensitive to condensation and dust and can only be used in well-controlled environments.

Figure 3. In a closed-loop stepper motor system, there is no risk of step loss. Compared with open-loop motion control, a smaller motor can be used or more load can be pulled with a motor of the same size.

A more robust and cheaper alternative to the optical encoder is a magnetic Hall sensor, as shown in Figure 3. It is based on Hall effect elements stacked in an integrated circuit. A simple magnet is all that is necessary on the motor shaft. This encoder design is much more robust in terms of withstanding dust, condensation and other impurities. It does not have the resolution of an optical encoder, but a magnetic encoder powered by LoadSense technology, developed by Sonceboz SA, is sufficiently precise to run a stepper motor delivering 200 steps/revolution in a fully closed loop. The encoder can be combined with integrated driver electronics or it can be used separately to replace expensive optical encoders, for example. Compared with an optical encoder, the advantages of a LoadSense encoder are its small size and integration with the motor, resulting in an extremely compact and robust product.

Vector control
The dc motor has been a workhorse for position and speed control applications for many years, and it still is in some applications. Its popularity can be attributed to ease of use, design engineers’ familiarity with the technology and the relative lack of competitive alternatives. The medical device and laboratory industries, however, require motion control systems that are maintenance free for the product’s entire life cycle, which can exceed 20,000 hours. This cannot be achieved easily with a dc motor because of the replaceable commutators and brushes.

The introduction of fast microprocessors such as DSPs and inexpensive high-performance semiconductors during the last 20 years enabled the development of new generations of motion control systems. Speed and position control applications that previously required the use of dc motors can now be designed with BLDC or stepper motors, thanks to the introduction of vector control theory in the early 1980s.

In a dc motor, the phase angle between the phase current and the induced voltage is fixed mechanically by the commutator and the brushes. A stepper motor, obviously, has no mechanical commutators and this angle has to be controlled by electronic hardware and the control loop. This means that the current vector, amplitude and angle are commanded by the control algorithm. The term vector control comes from this principle (Figure 4).

Figure 4. The vector control of a stepper motor involves the transformation from a stator fixed coordinate (Ua and Uß) system to a rotating coordinate system (Uq and Ud).

If the current vector is not properly controlled in a closed-loop driver, less-than-optimal performance and an intolerance to disturbances in the load torque will result. By using vector control, stepper motors can replace dc motors in demanding applications, thus eliminating the disadvantages of dc motors such as periodic maintenance and the inability to operate in corrosive environments. Furthermore, dc motors cannot be used in controlled or cleanroom environments, which are often required by medical applications.

There are many ways to control the current vector of a closed-loop driver. The most commonly used method is the Clarke/Park transformation. This transformation only involves a change in the coordinate system, from a stator fixed coordinate (aß) system to a rotating coordinate system (dq). The key to simplification is to fasten the dq-coordinate system to the rotor. This operation will transform all sinusoidal phase currents into a representation where the currents are constant. Since everything in the new dq representation is direct current, we are back to a simple control structure that has been used so successfully with dc motors.

LoadSense technology
Eliminating dc motors in medical applications has undeniable advantages when it comes to durability and operating life. With recent advances in closed-loop control, engineers now have options and can choose between a BLDC or stepper motor. BLDC motors are highly efficient but have a rather low torque/size ratio. Consequently, stepper motors are increasingly being considered for use in applications that require precise speed or positioning performance. Stepper motors, furthermore, are capable of handling light to heavy loads. Whereas BLDC motors need a gear mechanism in most applications, stepper motors can be designed as direct-drive systems without a gearbox in many cases. This is extremely beneficial for lifetime and durability.

State-of-the-art stepper motor design is a combination of well-established techniques such as closed-loop vector control and high-precision Hall encoders. Eliminating traditional open-loop control in stepper motors represents a breakthrough in terms of heat reduction, motor size, quiet operation, reduced vibration and increased motor efficiency. Closed-loop vector control also eliminates any risk of stalling or lost steps: the motor simply cannot lose steps since the position of the rotor is known at all times.

Figure 5. A high-resolution stepper motor with integrated LoadSense technology incorporates a closed-loop vector control driver. It is suited for direct-drive speed and position control applications.

An innovative example of the closed-loop driver is the fully integrated LoadSense driver from Sonceboz (Figure 5). The “smart” driver permanently adapts the phase current in response to changes in the load torque. This fully automatic adaptation is achieved by the elegant integration of a precise Hall sensor encoder directly on the electronic printed circuit board. A standard magnet on the motor shaft ensures continuous position information. Moreover, the LoadSense driver comes with a range of customisable communication interfaces such as CAN, PWM, Clock & Direction, RS-485/RS-232 and I2C. The LoadSense driver is suited for both position and speed control applications.

Conclusion
Advances in electronics control have stirred a debate about open- vs closed-loop control. Because closed-loop control improves the overall efficiency and performance of stepper motors, the choice between dc and stepper motors tends to favour the latter. Closed-loop systems, such as LoadSense technology, have clear benefits in medical and laboratory applications: step-loss-free operation, stall detection, lower operating temperatures, more compact dimensions and less power consumption (annual savings up to 200 kWh), resulting in a longer lifetime. Closed-loop technology also permits condition monitoring and predictive maintenance, augmenting the traditional analytical and feedback capabilities of stepper motors. An array of applications from positioning or dosing functions in diagnostic products to homecare devices will benefit.

Jan Persson
is Electronics R&D Manager and
Jacques Antille*
is Product Manager at Sonceboz SA,
Rue Rosselet-Challandes 5,
CH-2605 Sonceboz, Switzerland
tel. +41 32 488 1111
e-mail: jacques.antille@sonceboz.com
www.sonceboz.com

*to whom all correspondence should be
addressed