Mechanically actuated automotive systems, such as power steering and fuel and water pumps, are being replaced with systems that utilize electric motor technology. Consequently, the presence of semiconductors in automobiles is increasing geometrically, in part due to automotive systems developers' desire for electronic motor control to address consumer requirements for safer, more efficient cars.
One contingent of the synchronous motor group, brushless direct current (BLDC) motors, are ideal for automotive applications that continuously rotate, such as pumps, cooling fans, and stepper motors. BLDCs equip positioning systems with start and stop functions where high reliability is required.
Furthermore, the electronic control that BLDC motors offer is critical to statutory vehicle requirements, including energy-savings, reduced environmental and emissions impact, and the creation of safer vehicles. BLDC motors are also useful for variable-speed applications where space is tight, as in fuel pump control and electronic/electric power steering. In these types of applications, electronic control is essential because of the need for fault diagnostics and wide temperature and voltage operational ranges.
The embedded processor is a critical tool for automotive system designers as they address the increasing needs and demands of today's driver. The increased use of electronic controls enables automotive system designers to meet these needs, while meeting their own requirements to develop low-cost, low-noise, high-accuracy systems with faster time-to-market.
A multitude of embedded-processor solutions is available to automotive design engineers. One single-chip architectural platform that is ideal for BLDC motor control is the 16-bit digital signal controller (DSC). This type of platform delivers the control of a microcontroller, along with the computation and throughput capabilities of a digital signal processor (DSP). In this way, DSCs are excellent at executing the complex, high-speed mathematical functions required by many automotive electronics systems.
DSCs, such as Microchip's dsPIC, offer a seamless migration path and pin-for-pin compatibility, which enable the re-use of hardware and software building blocks. This combination of a 16-bit MCU with DSP capabilities enhances the performance of automotive electronic systems, while lowering system costs and enabling designers to get products to market faster.
The DSC combines MCU control and DSP number crunching functions.
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Overview of BLDC Motors
The BLDC motor does not operate directly off a DC voltage source, nor does it use brushes for commutation. Instead, the BLDC motor contains a rotor with permanent magnets and a stator with windings, and performs commutation electronically.
Commutation is the act of changing the motor phase currents at the appropriate times to produce rotational torque. Commutation is performed mechanically in brush motors, but must be performed electronically in BLDC motors.
Stacked steel laminations on the stator equip the BLDC motor with windings placed in slots, which are cut axially along the inner periphery. While the stator, in general, is similar to that of an induction motor, the motor windings can be configured in a non-distributed format. Each winding is constructed of numerous small coils that are placed in the slots and interconnect to form the larger winding. Each winding is distributed over the stator periphery to form an even number of poles. Stator windings can be either trapezoidal or sinusoidal, with each generating different types of back electromotive force (EMF). The phase current also has trapezoidal or sinusoidal variations.
All rotors have permanent magnets of some type, and can vary from two to eight pole pairs. The proper magnetic material with which to create the rotor is selected based upon the required magnetic-field density. Ferrite magnets have traditionally been used to make permanent magnets. However, rare earth alloy magnets are becoming more popular, as they generally have a higher magnetic density per volume and enable the rotor to compress further for the same torque. Alloy magnets improve the size-to-weight ratio and deliver higher torque than a motor of the same size that is comprised of ferrite magnets. A method to detect the position of the rotor magnets is required for BLDC motors.