【Introduction】We all know that it is important to see the road clearly when driving a car. This is even more stringent when implementing technologies such as autonomous driving. The self-driving car’s “eyes” are called light detection and ranging (LiDAR) technology, which provides an accurate picture of the car’s surroundings. Lidar uses light sources and sensors to detect objects.
The field of view (FOV) of a lidar system determines the width of the image that the lidar can capture, so this field of view is important for autonomous driving decision-making algorithms. There are many ways to expand the FOV, one of which is to use mechanical scanning, using motors to help achieve a 360-degree FOV. Brushless DC (BLDC) motors can achieve this goal, and are highly efficient and low-noise, making them popular.
Figure 1 shows a lidar module, and Figure 2 shows a lidar-equipped self-driving car.
Figure 1: Lidar Module
Figure 2: Self-driving cars with lidar
Self-Driving Cars Equipped with Mechanically Scanning LiDAR
A typical LiDAR module consists of a light source and a sensor, enabling a 120-degree FOV. Unfortunately, a 120-degree field of view is far from enough for an autonomous driving environment. To achieve 360-degree omnidirectional FOV, a mechanical scanning lidar consisting of light sources and sensors placed on a rotating platform is required to transmit information about the car’s surroundings to the car’s advanced driver assistance system. The rotating stage rotates at a speed that allows a seamless 360-degree scan of the light source and sensor, providing accurate feedback of image position. Given the relatively light weight of the platform, a motor of about 40W can be used.
BLDC motors are ideal for these rotating stages, and BLDC motor commutation can be achieved using Hall-effect sensors. Figure 3 shows a block diagram of a mechanically scanned lidar.
Figure 3: Mechanical scanning lidar block diagram
How to Drive a BLDC Motor
There are several ways to drive a BLDC motor of about 40W. You can use microcontrollers (MCUs) with trapezoidal commutation algorithms with integrated three-phase BLDC gate drivers and external metal-oxide-semiconductor field-effect transistors (MOSFETs). If your design requires smaller board size and lower power motors, you might consider combining a three-phase gate driver with integrated MOSFETs (like the DRV8316) and an MCU with a commutation algorithm and three for position feedback used with the Hall effect sensor. The DRV8316 integrates MOSFETs with high-side and low-side combined on-state resistance (RDS(on)) of 95mΩ and provides comprehensive fault coverage for diagnostic purposes.
BLDC motors can also be driven using a motor driver with integrated control algorithms such as the MCT8316Z, which integrates sensor trapezoidal control and eliminates the need for motor control software development. The MCT8316Z integrates a MOSFET that adds RDS(on) on the high and low sides of the MOSFET, which is 95mΩ as in the DRV8316. An external housekeeping MCU can communicate with the MCT8316Z to set configuration parameters and provide simple rotation commands such as speed and direction.
The MCT8316Z features a DC/DC buck converter to supply current to a housekeeping MCU or other circuitry on the board, a configuration that helps save board space for mechanically scanned lidar motor drivers. Both the DRV8316 and MCT8316Z operate from 4.5V to 35V, making them ideal for automotive 12V battery applications. With 8A peak motor winding current support, these drivers can achieve up to 70W of motor power from a 24V supply, enough to drive a LiDAR system’s rotating platform.
The MCT8316Z offers a Serial Peripheral Interface (SPI) and is highly configurable, although system designers may choose to use the hardware interface option instead of SPI to configure common settings. Internal drive registers that can be read via SPI provide detailed troubleshooting results that can help in the event of a problem with the motors in the rotating platform. Figure 4 shows a simplified usage example of the MCT8316Z.
Figure 4: Example of MCT8316Z Motor Control
Self-driving cars equipped with mechanically scanning lidars are an exciting development in the automotive world. The convenience brought by self-driving cars enhances the ride experience and makes you crave interstate road trips in such a vehicle.