[Engineering Essentials]
Mechatronics Means Motors
Movement requires motors, and motors are now including smarter controllers to deliver better efficiency and performance.
The three-phase motor control also works differently from the polled H-bridge example. In a three-phase motor, only two of the coils in the rotor are powered at any one time. The power control unit has three pairs of transistors instead of the two pairs that the H-bridge has.
Timing is also key to proper operation of a motor control program. Delays are necessary at various points in algorithms to allow for motor inductance and other effects that prevent proper sensing. For example, in a three-phase BLDC application, there will be a feedback pulse when a winding is commutated at the start of open phase voltage transition.
More robust motor control algorithms need to account for a number of different factors. Likewise, control algorithms can predict the results of the sensing system and compare the predictions with the actual results. Additional optimizations can be performed when this information is available.
Finally, startup can be an issue. In three-phase BLDC motors, the rotor can be found in one of six logical positions. The position is initially unknown because there is no back EMF to detect. The trick is to assume a position, start the motor, and change the results once the motor is moving.
REFER TO THE APP NOTE Motor control mavens can tackle motor control chores, but mere mortals need a starting point. This can be true when motor control is only part of the application being placed on a microcontroller. This is where application notes, reference designs, and development kits come in.
Development kits like Luminary Micro’s RDK BLDC often include a motor and matching power circuitry (Fig. 4). Luminary Micro has a three-phase version as well. These kits are great for learning about motor control. The circuit diagrams make a good starting point, especially if the end motor characteristics are close to the motor included with the kit.
Luminary Micro’s kit has a 32-bit microcontroller based on ARM’s Cortex M3. The latest 100-MHz incarnation uses 130-nm technology with a 4-µA hibernate mode. It has Ethernet and USB with a multiply accumulate (MAC) and physical layers (PHYs) as well as a CAN 2.0b MAC. Control-area network (CAN) PHYs are always separate chips because of the variation used in different industries.
These communication interfaces represent one way that motor control is changing. Network interfaces are becoming more common in replacing simple serial interfaces.
The Renesas SH7286 Starter Kit highlights another way vendors are delivering motor control solutions (Fig. 5). The starter kit is a typical development module that plugs into a power board. Renesas will loan reference design boards and provide design services to help match power support to motors.
The 100-MHz, 32-bit SH7286 also has USB, CAN, and Ethernet support. Based on Renesas’ superscaler 200 MIPS SH-2A core, it is designed for dual motors. A typical high-performance motor control overhead is 30% or about 60 MIPS/motor. This leaves a significant chunk of time available for other services, even with two motors. It has a six-channel PWM module with automatic shutdown with one fault input pin per motor. Faults can be noted by edge transition or pulse duration.
The platform can handle ac induction motors and universal motors. It also can handle three-phase BLDC motors using highend, field-orientation control algorithms, which are designed to deliver high efficiency as well as low-noise motor control.
The module is designed for development purposes. The processor and the power elements are often built onto a single custom board. Several vendors offer modules that are used in their development kits, which are also available for deployment. For example, the Texas Instruments controlCard is a dual-inline memory module (DIMM) with a C2000 digital signal controller. It must be paired with a board that has power transistors. These host boards are found in development kits as what must be designed to address actual deployment.
Motor control algorithms can be quite complex. But once implemented, they can be simple to control. This is where the other half of mechatronics control comes into play, yet it tends to be more application-dependent. For instance, servo control is used for robotic arms while drive motors handle rolling for many robots.
These movements require precision control. Precise feedback control is also needed for balancing robots that only use a pair of wheels. The Segway PT (personal transporter) also uses this balancing inverted pendulum technique.
Motivated by hybrid cars, regenerative braking is more complicated because the motor acts as a generator but also because the system must account for the power sink. Batteries aren’t created equal, and most have a limit as to how fast they can be charged, making the circuitry and control more complex.
Motor control algorithms are well supported, though regenerative braking is not. Finding a development kit that supports it will be a bit more difficult than finding one to control the same motor. Especially challenging is ac induction motor regenerative braking because the variable frequency control is key to generating current at lower speeds.
Regenerative braking is just one aspect of braking. Dynamic braking can be used to quickly stop or reverse a motor for ac motors by feeding the stator windings with dc current. It isn’t possible to recoup power as in regenerative braking, but braking can be smoother and more power-efficient under microprocessor control.
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