If your application demands a reliable, time-tested, low cost motor, then brushed DC motor technology may be what you’re looking for. The key here is simplicity. A brushed motor is designed to run off of straight-line DC voltage and can even be connected directly to a properly sized battery. When a DC voltage is applied across the terminals of a brushed motor, a potential difference is achieved and current is induced into the windings on the rotor. The brushes allow this current to flow through a rotating mechanical switch called a commutator. The rotor windings act as electromagnets and while powered form two poles that terminate at the commutator segments. This entire assembly is known as an armature.

Fig. 1 – Coreless DC motors can be manufactured in very compact sizes
While rotating, the commutator allows the direction of the current to reverse two times per cycle. This permits the current to flow through the armature and the poles of the electromagnets attracting and repelling the permanent magnets that encompass the motor’s inner housing. As the energized windings of the armature pass the permanent magnets, the polarity of the energized windings reverses at the commutator. This process is called mechanical commutation and is found only in brushed motors.

During the instant of switching polarity, inertia keeps the rotor going in the proper direction and allows the motor to continue turning. The result is power in its mechanical form measured in watts. Mechanical power is the product of torque multiplied by the rotational distance per unit time (or speed). Torque is the force vector component that rotates a load about an axis and is inversely proportional to speed.

We can see that there is a price to pay for how much power a motor can deliver. The amount of current that flows through the windings directly affects to the torque the motor can produce. Adjusting the supply voltage will force a proportional change in the motor’s speed so the output shaft’s angular velocity (speed) will have to be sacrificed as torque demands increase. There are also other factors that come into play, such as losses. For example, static friction is defined as the friction torque a motor must overcome in order for the shaft to begin turning. Then there are brush contact losses caused by the friction of the brushes upon the commutator. Also, copper losses in the form of heat, sometimes referred to as I2R losses, plays a role.

Although, when torque and speed are measured empirically, the resulting graph may not be perfectly linear in all cases. However, we can see that both torque and speed are inversely proportional and that a linear relationship exists. Because of this, feedback may not even be necessary in all cases. Feedback is usually provided by an encoder, tachometer, or resolver. It tells the servo system where the motor is and what speed the shaft is turning. Taking all this into account, we can establish that a properly designed closed loop servo system will have a predictable response to a controlled input. And thanks to this directly linear relationship, a servo can easily compensate for any unwanted changes introduced into the system.

Coreless Brushed DC Motors

Fig. 2 – Brushless DC technology is ideal for applications requiring high speed, quiet operation, low EMI and longevity.
The answer to some of the problems with iron core technology was addressed in the 1940s by Dr. Fritz Faulhaber with the invention of the coreless DC micro-motor. This design opened up a whole new multitude of possibilities for space constrained applications requiring high precision. These motors have a self-supporting, progressive, skew-wound, ironless rotor coil that has demonstrated incredible efficiency when compared to iron core motors. For the first time, DC motors did not require the use of iron laminations in the armature. Thanks to this construction, the rotor is extremely light, yielding a low moment of inertia. In effect, this allowed for faster acceleration resulting in a much smaller mechanical time constant. Another benefit to coreless DC motors is that they can be manufactured in very compact sizes. That is why they excel in space constrained applications. The rotor also rotates smoothly without cogging and the coreless DC motor’s windings have very low inductance. All of these characteristics help reduce brush wear and prevent electro-erosion thus increasing the motor’s lifespan.

Unfortunately, with no iron laminations, coreless motors are somewhat prone to overheating. In some instances, a heat sink can be used to alleviate this problem. Also, cost would have to be factored into most applications as the high precision and repeatability of coreless DC motors comes at a bit of a price. These motors are designed for specific applications and would not be the best choice to use in most consumer products. (See Figure 1)

The most common applications are large OEMs in industries requiring very high precision, primarily medical, aerospace, military, robotics, and automation. Some example products are aesthetic lasers, diabetic insulin pumps, collision avoidance scanners, and unmanned aerial vehicle applications. These applications have demanding micro-positioning needs, dimensional constraints, and, sometimes, vacuum compatibility needs. Coreless DC motors seem to excel in situations where reliability, precision, longevity, and repeatability are of the upmost importance.

Going for Longevity with Brushless Technology

Fig. 3 – Linear DC servomotors allow great user flexibility.
If an application requires high speed, quiet operation, low EMI, and longevity, then brushless DC technology (BLDC) might be what you are looking for. There are many advantages to brushless motor technology, and speed is one of them. Higher speeds are achievable because there are no mechanical limitations being imposed by the brushes and commutator.

Another advantage is the elimination of the current arcing/electro-erosion problem commonly experienced with brushed motors. BLDC motors also possess higher efficiency and generate lower EMI, which is excellent when used in RF applications. They also possess superior thermal characteristics over brushed motors since the windings are on the stator. The stator is connected to the case; thus, the heat dissipation is much more efficient. As a result, the maintenance on a brushless motor is virtually non-existent. (See Figure 2)

Unfortunately, the higher cost of construction puts BLDC technology out of reach for many applications. You can easily spend twice as much on a brushless system and lose the simplicity of a brushed motor. Don’t forget to save room for the control/drive electronics, too. You’ll need to mount it somewhere it if it isn’t integrated in the motor. The motor also can’t be mounted too far away from the drive as long cable runs tend to introduce noise into the system. To compensate, the phase leads can be twisted and shielded from the sensitive feedback leads to reduce noise.

As with brushed motors, brushless must overcome starting friction as well. Again, this is the sum of torque losses not depending upon speed. Dynamic friction is dependent upon speed. In fact, dynamic torque friction is the only thing defining torque losses proportional to speed for BLDC. A function of speed (for example in metric units of mNm/rpm), dynamic friction is due to the viscous friction of the ball bearings, as well as to the eddy currents in the stator originated by the rotating magnetic field of the magnet. Overall, you can expect the speed-torque curve to demonstrate excellent linearity for BLDC technology.

Linear Motion with Actuators

The term linear actuator normally refers to a stepper or brushless motor with a leadscrew attached to its shaft. Sometimes a nut and gearbox would be included to form a compact package designed to deliver precise linear motion. At the time of development, this was a clever way to convert rotational to linear motion. Brushless motors work well when smooth operation coupled with low EMI is desired. Add high resolution feedback and accurate positioning can be achieved. Even utilizing a stepper motor can deliver the advantages of instantaneous starting and stopping.

But this arrangement is not without its disadvantages. The conversion from rotary to linear motion is complex one. The problem was that the mechanical losses in the form of friction were too much to bear for situations where every bit of power was critical. This is especially true for aerospace applications. This reduced the efficiency since the linear actuator is not a direct drive mechanism and torque is a vector component of force.

This has led to a growing demand in the area of direct drive motion and a need for small size linear motors. In response to this, new linear servomotors have been developed in the motion control industry. With the absence of a leadscrew, ball-screw, nut, and friction, this direct drive unit could apply a purely linear force. The innovative structure of these motors allows great usage flexibility, tailored to satisfy the market demand. The self-supporting coil windings together with a high precision sliding cylinder rod, filled with permanent magnets, provide the motor with a particularly high performance-to volume ratio. (See Figure 3) Specially developed calculation software enables easy setting of the control parameters, displaying specifications, data and graphs of the various profiles. This actuator’s structure boosts flexibility. It exhibits no residual static force and its output force is linear with current input, so it is suitable for micropositioning. If nano-positioning is required by an application, then a Piezo motor might be the best choice.

How does one prepare for a project that incorporates linear motion? Well, step one is to define the speed profile for the application at hand. Start by defining the speeds characteristics of load movements: What is maximum speed? How should mass be accelerated? What length of movement must the mass traverse? How long is the application’s rest time? If movement parameters are not clearly defined, a triangular or trapezoidal profile is recommended. These two values indicate which motors are suitable for the application.

Another option for linear servomotor selection is calculation software, which enables control parameter setup, specification display, and charts of various profiles. Some motion software also allows plug-and-play configuration of controllers to optimally run the motor. Many times if feedback is necessary, then a linear encoder, such as a glass encoder, may be considered.

This article was written by George Hunt, Application Engineer, MICROMO, Clearwater, FL. For more information, Click Here " target="_blank" rel="noopener noreferrer">http://info.hotims.com/49747-165.


Medical Design Briefs Magazine

This article first appeared in the July, 2014 issue of Medical Design Briefs Magazine.

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