[Engineering Essentials]
Mechatronics Means Motors
Movement requires motors, and motors are now including smarter controllers to deliver better efficiency and performance.
Simple electric motors turn when you apply power. They have been used this way since their inception. Yet today, a microcontroller may be sitting between the motor and the switch, and motors can be found everywhere from white goods to hybrid vehicles.
Electric motors are key to almost all robotic work as well, with a couple hydraulic motors included for good measure. Most electric motors utilize magnetics for motive power. But at the low end of the spectrum, it is possible to use piezoelectric crystals to construct tiny rotational and linear motors like Newscale Technologies’ Squiggle Motor (Fig. 1).
The plethora of options means that motors are more than just boxes with shafts sticking out. Issues such as power, torque, starting torque, efficiency, speed, speed control, and linearity come into play.
Coverage of this area can fill a library. Entire books have been written about single types of motors, and most EE curriculums have at least one motor course in the mix. Few computer science majors run into motors, and it is often rare for embedded developers to need a closer look at motors.
Still, motors are becoming closer cousins to embedded electronics because of the increase in motor control requirements in designs. That on/off switch is often replaced by a microcontroller pulling multiple duties as a capacitive touch input device, network node, and motor control. So for those developers, here’s a whirlwind tour of motors and motor control.
TYPES OF MOTORS Electric motors are typically divided into three types: ac, dc, and piezoelectric. Universal motors can run on ac or dc current. Developers typically want to use a motor rather than design one. Before they can choose a motor for their application, though, they still need to understand the fundamentals (see “Motor Terms”).
The most common motors are continuous rotational motors with a stator and rotor. The rotor contains an electromagnet or a permanent magnet. Electromagnets often require power that is usually provided through a commutator that is on the shaft of the rotor. Commutators are a source of wear, and the brushes employed in some motors, especially large motors, are often items that can be easily replaced.
Selecting a motor or motor technology can be a challenge. Requirements such as speed, torque, heat dissipation, power requirements, durability, precision, accuracy, and size need to be addressed. Motors often are chosen with high overcapacity because the characteristics of the operating environment are unknown or highly variable, or the developer doesn’t know how to translate the system requirements into motor specifications. Sometimes limitations such as cost or the power supply are sufficiently high that more powerful motors can be chosen.
Yet these choices can lead to less efficient solutions. Likewise, some motors are chosen to match simple motor control requirements. A more complex motor control solution may be more efficient and less costly in the long run, so it pays to know the alternatives. This is especially true for small to midrange motors where non-dedicated microprocessor control is an option. In this case, the microprocessor in question may control the motor, but it has sufficient headroom to handle other aspects of a product such as the user interface plus system monitoring and control.
As the workhorses in the home and industry, ac motors revel in fixed locations accessible to the ac power distribution system that is readily available in most industrial countries. Heavy-duty ac motors aren’t the only alternatives, though, with small and mobile solutions being used in a variety of applications.
One of the most common ac motors, the induction motor, uses an electromagnet in the rotor but doesn’t require a commutator because the stator induces the current in the rotor. Essentially, the motor is a transformer. The rotor has closed coils that typically have low resistance for handling large current.
Even though dc motors can address heavy-duty applications, they also are the motor of choice as size comes down. These types of motors fall into two categories: brushed and brushless. Brushless motors use permanent magnets in the rotor.
Brushed dc motors have an armature that must be powered. Separately excited or brushed dc motors have independently powered rotor and stator windings. A shunt configuration is simply a separately excited motor with the rotor and stator connected in parallel. A series configuration connects the rotor and stator windings in series. Some motors can switch between shunt and series configurations for more efficient startup.
The windings for a brushed dc motor can be wound around an iron core. Eliminating the core allows a lighter rotor that is desirable for rapid acceleration. These kinds of motors are often found in high-speed servos.
Much of the activity in the motor arena has focused on brushless dc (BLDC) motors because of the need for more active control for efficient operation. Low-cost microcontrollers make these motors very practical. Also, improvements in the computational power and performance of the processors enable developers to combine motor control functions with the rest of the application instead of having to dedicate a chip to a motor.
Universal motors run off ac or dc current, though they are typically used with an ac source. Meanwhile, solenoids are common devices that are really linear motors. They can be found in automotive applications where an open/close operation is needed.
Linear motors have other applications as well. Some require precise movements, as in plotters. Magnetic levitation trains use linear motors for traction often combined with levitation support. In this instance, the stator is the track, but normally only a portion of the track will be active at one time.
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