FIELD ORIENTED CONTROL MOTOR DRIVER
There are two subsystems for the project, the driver portion and the position sensing. Each of which have their own PCB so that the user has more flexibility for mounting the position sensor. The whole system consists of the following:
•A Microcontroller Chip to schedule and time tasks, facilitate communication between subsystems, measure current, and perform the necessary computations needed for field oriented control.
•A Pre-Gate Driver to generate a high and low PWM signal to the power MOSFETs. It controls dead time to prevent current shoot-through when switching the MOSFETs on or off. The pre-gate driver is responsible for current and voltage protection as well as fault detection for the motor.
•Six Power MOSFETs for switching the motor terminals from input to output. Each current phase has a high side and a low side MOSFET to prevent current shoot-through. The pre-gate driver controls switching.
•An Absolute Angle Encoder System to read the rotor position and send the information to the microcontroller chip. A diametric magnet is mounted onto the back of the rotating motor shaft and the absolute encoder is mounted onto the stationary back plate of the motor.
Below, is a high level diagram of the motor driver system including the type and direction of the data signal.
Most of the communication methods chosen were selected because that was what the part required however how we communicated with the user’s microcontroller was a design decision. SPI was chosen to send the position and speed information since it is a fast communication protocol, most people are familiar with it, it’s found on most off-the shelve microcontrollers and SPI was already being used to communicate with the encoder, making the system less complicated.
The connection between the motor and the encoder is electromagnetic. There is a diametric magnet on the back of the rotor and the encoder reads the change in the magnetic field as the motor rotates.
After component selction and determining the connection requirement came the schematic and ironing out the smaller details.
Next was the PCB layout. While doing the layout, the requirements for each component were carefully considered. For example the pre-driver required a heat sink pad underneath it and that there weren't any components beneath it on the other side of the board. Ease of use was also considered while doing the PCB layout and was addressed by clearly labeling all connections to the board.
The designs were sent out to be manufactured to PCBway. They were chosen due to good peer reviews and how quickly the finished boards would be sent to use (in a week!).
After receiving the PCB and digikey orders, the project entered the assembly stage. All the component were hand soldered with an fine tipped conical (ETA size) soldering iron. The most challenging part with soldering were the integrated circuits (ICs). Too much solder was initially used which caused the bridging (pins were short circuited). Multiple methods were used to attempt to remove the solder including using a soldering pump, a wick and flux. The latter had the best results. This process involved applying flux to the bridged areas and then using a very clean soldering iron quickly pressing on the problem area. After soldering the pcb the flux residue was careful cleaned off using isopropyl alcohol. The result was a very clean, professional looking build, as shown below.
Connectors were also assembled by hand using a specialized crimping tool.