All-electric vehicles that function without an internal combustion engine need safe, cost-effective, and high-capacity energy storage systems. Efficient software algorithms, powerful microcontrollers and highly efficient electric motors make possible maximum use of available energy, and a high level of integration enables leaner and cheaper motor control systems. New generations of highly integrated MCUs developed specifically for the hybrid and electric vehicles include timer structures, which generate motor control signals, along with various I/O ports and interfaces.
Before detailing how these new MCUs operate, let us first conduct a quick overview of how HV and EV motors function.
The figure above shows the broad classification of HV and EV vehicles. A central element of hybrid and electric vehicles is the electric motor in the powertrain, which is deployed in combination with a conventional internal combustion engine in hybrid cars or as an independent source of power for electric vehicles. Selecting the motor requires careful analysis of the dimensions, weight, reliability, robustness, required torque, and total efficiency.
There are two basic types of applicable motors. Asynchronous motors are robust and reasonably priced in part because they do not require magnets made from rare earth elements. Their properties are easily controlled with software algorithms, and maintenance is not required. These motors are slightly less efficient than the other type, synchronous motors, and have lower torque at start-up. Disadvantages also include slightly lower efficiency, around 90%, and greater weight.
The other choice, permanent magnet synchronous motors (PMSMs) feature high torque coupled with compact dimensions and high efficiency approximating 94%. Synchronous motors cost more because they are built with expensive materials from rare earth elements, required for the permanent magnets. Brushless versions of both motor types mean that brush loss is not an issue. PMSMs offer a better dimensions/torque ratio and higher efficiency and are currently the first choice for use in the powertrains of electric and hybrid electric vehicles.Control
As mentioned previously, brushless motors are available for both motor types. While such motors require more effort for commutation, they also enable safe and efficient control—a fundamental pre-requisite for use in powertrains. The challenge is to find the perfect balance of motor, power electronics, control unit (microcontroller), and control software.
The algorithms used must be adapted to the respective motor and application so that the electronic controller commutates the motor optimally at all times. Failure to adapt these correctly may lead to undesired effects such as irregular running and excessive noise, which together have a negative impact on the degree of efficiency that can be achieved. Motor control combines various control algorithms depending on the application.
Sensor-based rotor position detection can be conducted with various sensor systems. In general, detection of the rotor position is essential for precise motor control. As a key component, the rotor position sensor has significant influence on the performance and efficiency of the motor system.
Hall position sensors are based on the Hall Effect, in which a voltage is induced by changing the magnetic field around a current-carrying conductor. With the help of a magnetic ring attached to the rotor and a sensor unit affixed to the stator, the Hall Effect sensor is a cheap and easy means of detecting angles. The greater the number of magnetic poles and Hall elements, the higher the resolution and accuracy—as well as susceptibility to magnetic interference.
One frequently used sensor is the incremental encoder. This is available in a wide range of designs, featuring both mechanical and optical scanning to determine the current angular position. To measure an angle, an incremental encoder must be based on a zero or reference position.
For the microcontroller, actual angle determination only involves detecting the direction of rotation and counting the pulses emitted. The angular rate can be calculated by simply measuring the intervals between two pulses. The insensitivity to magnetic interference is beneficial; in contrast, any mechanical friction losses and susceptibility to dirt in the case of optical systems are disadvantageous.Resolver
The resolver is a very robust sensor that is often used in the automotive industry. The resolver is not at risk from magnetic interference and dirt, or subject to friction losses during angle detection. It consists of a rotor, which is permanently attached to the motor shaft (motor rotor), and a ring-shaped stator, which is permanently attached to the motor housing. The stator consists of at least one excitation coil and two sensor coils. Higher resolution can be achieved by increasing the number of pole pairs.
The figure below shows a resolver. The excitation coil is fed with an analog sinusoidal signal. The analog signal is transmitted to the two sensor coils, set at 90° to each other, via the magnetic coupling (induction). Evaluation of the analog sinusoidal and cosinusoidal signals returned by the resolver requires a resolver-to-digital converter (RDC), which is used to determine the angular position and rate from the analog data.
Schematic and mechanical structure of a resolver (source: Tyco Electronics Corporation, MCR605)
Resolvers may not be superior to the competing technologies in terms of performance and precision, but they are more robust and offer better protection against dirt and extreme temperature conditions. They are also able to detect the motor’s absolute position at any time, even when it is not moving. Incremental encoders and hall sensors cannot perform that function.