Spencer Manley, and Mathias Müller, Rutronik
As car manufacturers strive to find ways to increase energy efficiency, and therefore reduce CO2 emissions, the adoption of low-powered, low-weight electric motors for auxiliary units - which work separately from the main drive motor - is on the increase. The electrification of comfort and safety devices like fans and pumps has also fuelled the demand for standalone electric motors from vehicle manufacturers. Highly integrated microcontrollers are being developed that support a wide range of these requirements. The various types of electric motor available offer different advantages depending on how and where the motor is being used. Here's a quick refresher on the main types.
DC motors – direct control
Brush-type DC motors are one of the simplest types of motors and provide precise control and easy regulation of speed and torque. The motor speed is approximately proportional to the voltage provided to the armature, while the torque produced is proportional to the armature current if the stator field is constant.
This means for uni-directional purposes a simple transistor controlled by pulse width modulation (PWM) is all that is needed, while a half-bridge is suitable for right/left rotation. These motors are typically found in automotive applications with comparatively short duty cycles, like drives for adjusting seats and mirrors, or wash system pumps.
Although inexpensive and generally reliable, brush-type DC motors do require more maintenance and can create electromagnetic interference. The current that controls the motor is provided to the windings on the rotor via brushes (made of carbon), which, because they make contact with the rotating armature, will wear down over time. This produces a build-up of fine, electrically conductive, carbon dust within the motor that can create brush arcing. Also, they are not very well suited to high-torque applications as, when speed increases, brush friction increases and viable torque decreases.
Stepper motors – for incremental precision
Stepper motors are brushless synchronous drives with at least two phases that are controlled by PWM via half-bridges. In other words, these motors convert electric pulses into accurate and discrete 'steps' of mechanical shaft rotation. The amount of rotation produced by a stepper motor directly corresponds to the number of electric pulses it receives, while the motor's speed is proportional to the frequency of the pulses. This means it can only take one step at a time and each step will be of the same size.
Because of the precise control offered by stepper motors, they are often found in automotive applications such as air control valves, headlight adjusters, windscreen wiper controllers, temperature gauges and fuel gauges.
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Figure 2 - Packaging
BLDC motors – highly durable
The more complex but highly durable Brushless DC motors (which are also known as DC commutatorless motors, electronically commutated motors, AC synchronous motors or DC servomotors) are increasingly being found in mechanically driven applications with higher duty cycles, such as power steering, fuel pumps, transmission actuation and engine cooling systems.
They are essentially an inside-out configuration of a brushed DC motor, with a rotor consisting of an array of permanent magnets and a stationary armature that is excited by an electronic commutation controller to produce torque. These motors are known as 'synchronous' because the magnetic fields generated by the stator and the rotor revolve at the same frequency.
While mechanically simple, BLDC motors require sophisticated control electronics and regulated power supplies. They are more complex to control as the controller must produce several (usually three) alternating voltages whose frequency and voltage can be adjusted and these always need to be correctly in phase to ensure the motor runs quietly and smoothly. Hall-effect sensors may be embedded in the stator to indicate the relative positions of the stator and the rotor to the controller, so that it can power the windings in the correct sequence and at the correct time. There are also sensorless varieties which utilise back EMF to regulate the motor.
This more complicated setup, however, confers many advantages. The lack of brushes and physical commutator means they do not suffer the mechanical limitations such as friction, electrical losses or worn out parts that are seen in brushed types, and means they can offer higher speeds, torque, efficiency and life expectancy. They also provide better heat dissipation and greater power density in a smaller size and lower weight than brushed motors. As they are virtually maintenance free, they are also very suitable for areas likely to be contaminated with oil, grease and dirt as they can be safely sealed away.
Partly because of the cost erosion of the more complex controllers, BLDC motors are increasingly becoming the electric motor of choice in automotive applications, ousting brushed types in all but the most niche areas.
Advanced BLDC controller with embedded microcontroller
The STSPIN32F0 Advanced BLDC controller is perfect for those looking to benefit from the precise motion control and stable operation offered by BLDC motors. As one of ST’s System-in-Package solutions, it combines a three-phase BLDC electric motor driver and an STM32 Cortex-M0 microcontroller (MCU), all housed in a tiny 7 x 7mm package.
Consisting of an analog and a digital IC, the STSPIN32F0 includes a three-phase gate driver with 600 mA current sink and source, operational amplifiers, as well as a comparator. It can accept an operating voltage as low as 8V or as high as 45V, and can output 3.3V when using a DC/DC buck converter, and 12V when using an LDO linear regulator.
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Figure 3 – Block Diagram
The embedded ARMv7 MCU means there is no longer any need for an external processor, reducing design complications, costs and power needs. Its flexibility means engineers can quickly use options like vector control to increase precision (also known as Field-Oriented Control or FOC) without the need for any additional components.
The MCU has 4KB of embedded SRAM which is accessed at CPU clock speed and has 32KB of non-volatile Flash memory for running programs and storing data. The unit also offers read and write protection modes for the Flash memory to protect against malicious hackers trying to take control of the system. The MCU has 16 GPIO ports and is compatible with popular interfaces such as I2C, SPI, and USART.
The STSPIN32F0 is a robust piece of kit, with an unusually wide operational temperature range starting at -40C and rising to +125C. To help protect against damage from over-heating it includes an embedded overtemperature shut-down protection that automatically restarts when temperatures come back into the safe range.
The analog IC's integrated comparator protects against over currents, and the threshold where this cuts out can be set by the MCU for special use scenarios. The power supplies also come with an under voltage lock-out protection to prevent low voltages harming components on battery operated devices.