Motion System Design

DC motor selection for medical, robotic, automation, and and unmanned aerial vehicle (UAV) applications: Part 2 of 2

Industry demand has spurred the use of small precision motors and linear actuators. Choose wisely.

If enough voltage is applied across the terminals of a dc motor, the output shaft will spin at a rate proportional to that applied voltage. This makes for predictable operation, useful in a myriad of applications. Covered in this article:

  • Brushless micromotors
  • Designing with steppers
  • Electronics and actuator designs

In the last installment of this article series, we discussed the background of dc motors, brushed types, and ironcore versus coreless types. Here we cover other dc motor varieties and their use in applications.

Driving brushless micromotors

Figure 1

Unlike brushed varieties, brushless dc motors cannot be operated directly off of straight-line dc voltage. Remember, brushless motors use electronic commutation — so again, no brushes make physical contact with the commutator. A permanent magnet rotor initiates motion by chasing a revolving magnetic field induced by the current in the stator windings. Creating this motion is done with electronics and is usually an on/off signal called pulse width modulation or PWM.

Normally supplied by a comparator, the PWM signal is voltage, generated from a sinusoidal command signal and a saw-tooth carrier or chopper frequency. The PWM signal is either on or off and delivered at a duty cycle governed by chopping frequency. This signal is high when the command is greater than the carrier (chopper or switching frequency.)

With lower chopping frequency, current has more time to build amplitude. In this case, the motor continuously accelerates and decelerates, with an accompanying increase in current density. Such harsh changes in amplitude can result in more ripple in the output as well as shortened motor life — so it is important that switching frequency is sufficiently high.

Six semiconductor switches (which send amplified current through the correct, corresponding phase) are what control discrete on/off states. When current is reversed by the semiconductor switches, stator windings are used more efficiently because more than one winding is energized.

Micropositioning with mini PMDC steppers

The drive requires feedback to turn the phases on and off at just the right time, and maintain a commutation angle at about the ideal 90°. For this reason, brushless motors usually require a closed-loop (servo) system to operate properly. In many cases, digital Hall effects are employed to provide the required feedback and commutate BLDC motors. For smoother operation, sometimes a sinusoidal commutation (or linear Hall effects) can be used instead.

If precise positioning with the benefits of brushless technology are requirements in an application, then permanent-magnet dc stepper motors may be most suitable. A PMDC stepper is a synchronous motor with a magnet rotor and electromagnet stator. The rotor usually has 12 pole pairs, as does the stator. Normal stepper motors are two phase — though one, three, and even five-phase motors are available.

Steppers are electronically commutated and are also brushless, so exhibit the benefits of those designs. Relatively immune to wear by mechanical commutation, steppers are an excellent choice for positioning applications where response to starting, stopping, and reversing is critical. Very wide speed ranges are possible, because speed is proportional to input frequency, normally supplied by a frequency generator or drive. PMDC stepper motors also maintain a small bit of torque, even when the coils are unenergized. Called detent torque, this is due to magnets interacting with the steel stator. As a result, the rotor holds its position even without power delivered to the motor. This attribute is useful in aerospace applications, where power is limited.

Figure 2

In most instances, feedback is not a necessity. Therefore, open-loop operation is quite common in PMDC stepper applications: Position can easily be determined just by keeping track of input step pulses. This keeps project costs down and makes for a lower-profile, compact design. It is perfectly acceptable to put an encoder on a stepper, but not without potential consequences.

Due to inertial mismatches inherent to PMDC stepper motors, they can sometimes overshoot a target step and even oscillate while settling into position; specifically, the rotor shifts from a de-energized detent state to an energized alignment state. This resonant behavior can appear to be intermittent if a low-resolution encoder is used.

Because stepper motors are commutated electronically, drive electronics must be employed. Sometimes these electronics are also put to higher use. How? One powerful feature of steppers is their ability to half-step and even microstep. Microstepping is a driving method where current is continuously varied in the windings and full steps are divided into many smaller discrete steps. It can be a powerful feature, but requires electronics designed with that capability. The degree of microstepping possible is governed by the motor's angular accuracy, which determines whether a setup can ¼ step, ⅛ step, and even 1/16 step. Error is noncumulative and usually around 3 to 5% of a full step. (Like many other specifications, this is highly dependent on parts and construction quality.)

Figure 3

Sizing PMDC stepper motors can be tricky, but understanding how torque develops with a stepper makes it easier. A motor's pull-in curve, also called the start/stop region, indicates the maximum frequency at which a loaded stepper can start and stop instantly without losing synchronism.

Linear motion with actuators

A stepper must also be accelerated or decelerated (ramped) into and out of the pull-in curve region; it cannot be instantaneously started and stopped, because this curve represents the maximum frequency at which a motor can operate before desynchronizing. Any inertial mismatch can change the torque-speed curve significantly, so a 90 to 100% safety margin is recommended when sizing steppers. This can be an issue in precision applications.

For example, say we saddle a battery-driven stepper motor inside a vibrating stuffed animal toy with a load. As the motor accelerates, the windings heat up. This causes the temperature coefficient of copper to increase, in turn raising winding resistance and changing the way the stepper responds. For such a basic application, a slight decrease in performance may be acceptable. However, pick-and-place robots, optics controls, or surgical/medical applications cannot tolerate unexpected performance variations. For these situations, another system design may be more suitable. It's recommended that designers consult manufacturers for assistance in designing these tightly parameterized applications.

The term linear actuator normally refers to a stepper or brushless motor with a leadscrew attached to its shaft. Sometimes a nut and gearbox are also 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 required. Adding high-resolution feedback makes for accurate positioning as well. Even using a stepper motor can deliver the advantages of instantaneous starting and stopping.

Figure 4

This arrangement has disadvantages: The conversion from rotary to linear motion is complex. Mechanical losses in the form of friction are too much to bear for situations where every bit of power is critical. This is nearly always the case in aerospace applications. Linear actuators are not direct-drive mechanisms, and torque is a vector component of force (Torque = F × × r) so efficiency loss is unavoidable. This has led to growing demand for direct-drive motion and miniature linear motors.

In response, some new linear servomotors in the motion control industry run without run without leadscrews, ballscrews, nuts, and friction. These direct-drive units apply a purely linear force: F = ma. In one innovative motor structure, self-supporting coil windings and a high-precision sliding cylinder rod (filled with permanent magnets) provide a high performance-to-volume ratio. Calculation software enables easy setting of control parameters — displaying specifications, data, and graphs of various profiles.

In some designs, a motion controller controls position of a linear dc servomotor though an RS-232 or CAN interface. Motion manager software allows quick configuration of the controllers to optimally run the motor.

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 nanopositioning is required by an application, then a piezomotor might be the best choice.)

In this installment:

How does one prepare for a project that incorporates linear motion? Step one is to define the speed profile for the application at hand. Start by defining the speed 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.

Figure 5

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. If feedback is necessary, then a linear encoder (such as a glass encoder) may be added.

Part 1 of this two-part series can be found at Call MICROMO at (800) 807-9166.

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