INTRODUCTION

A Motor Control Unit (MCU) is an electronic module that interfaces between the pack of batteries and Motor to control the electric vehicle’s speed and acceleration based on the throttle input. The main function of MCU is to transform the battery’s direct current into alternating current using a Voltage Source Inverter (VSI) by applying pulse width modulated (PWM) waveforms to the gates of VSI switches/MOSFETs. PWM pulses are generated according to the position sensors (Hall/Encoder/Resolver) feedback and the ON Time/Duty Cycle of these pulses varies according to the required Speed/Torque command. To achieve efficient closed loop control of Motor drive, Vector Control methods/Field Oriented Control (FOC) are adopted. In Field Oriented Control, torque and flux can be controlled separately which results in a fast and efficient control of Motor drive. In addition, Vector Control methods reduce the harmonic content of the Motor current by using a fixed frequency PWM switching, which also helps to minimize the Motor temperature rise and noise. This Motor control unit (MCU) can fulfill the following functions irrespective of the Motor used.

• Control the Motor torque and speed
• Start/Stop the Motor
• Prevent from electrical faults
• Provide overload protection
• Change the motor rotation direction
• Regenerative Braking

The figure given below shows the basic block diagram description for MCU design. It consists mainly of  Power Supply Section, Sensing Circuits, VSI, CAN Transceiver and Microcontroller etc.

MOTOR CONTROL UNIT BLOCK DIAGRAM DESCRIPTION

Following are the description for each block of MCU.

1. Microcontroller: The main function of microcontroller is to control the electrical energy received from battrey using VSI and sensing signals. While the controller itself gets the main control signal/input from the throttle signal that can be controlled by the vehicle driver. This throttle signal will determine how the duty cycle of PWM pulses vary to obtain the desired speed and torque. To achieve efficient and fast control, FOC control is implemented in the microcontroller.

2. VSI: The main function of VSI is to convert DC to AC by using position feedback from the Motor. Generally, six MOSFETs are used to implement a VSI. However, to increase the current capacity of the MCU, parallel combination of MOSFETs are also used.

3. Phase Current Sensing: To sense the motor phase current, Hall Effect based current sensors are used. Generally two current sensors are utilized to sense the two phase currents and the third phase current is derived from these two.

4. Power Supply: As MCUs have inbuilt sensors, therefore, to properly bias these sensors, power supply is required. Also to provide the supply to microcontroller, motor temperature sensor and position feedback sensors, different level supply is required. To fulfill these requirement a power supply section converts a fixed DC voltage into different level of voltages according to the requirement.

5. Gate Driver: A gate driver circuit is used to amplify the PWM pulses voltage level generated by the microcontroller.

6. CAN Transceiver: The role of the CAN transceiver is basically to drive and detect data to and from the  CAN bus. It converts the single ended logic used by the controller for the differential signal transmitted over the CAN bus.