This paper introduces the special chip μPD78F0712 motor control as the core of E-BIKE with brushless DC motor controller design, focuses primarily on the PWM control circuit and power switching devices of choice, drive circuit, protection circuit and software design.
Keywords: μPD78F0712; brushless DC motor; controller; electric bicycle

Introduction

Electric bicycle is a safe, economical and clean, green transport, not only in energy, the environment has its unique advantages and competitiveness, and can more easily control the use of modern technology to achieve its goal of mechatronics, which has broad prospects for development.

Speed ​​accuracy is not required in many of the applications, brushless DC Utility Vehicle Manufacturer (BLDC), mainly through the open-loop speed control mode PWM modulation to achieve. Turn-on logic signal applied to the PWM signal given by external PWM duty cycle to achieve the speed of change, the hardware implementation easier, lower cost. The current speed of electric bicycles on the market they use this approach.

To improve the E-BIKE (bike) with a brushless DC motor control performance, this paper presents a dedicated motor using NEC's core control chip μPD78F0712 control program, and the motor speed and current closed-loop control, purpose is to improve the dynamic and static characteristics, to optimize the control performance.

μPD78F0712

μPD78F0712 for the NEC's 8-bit motor control dedicated driver chip, which is characterized as follows: Minimum instruction execution time of up to 0.1ms; external clock frequency of 20MHz; high-speed on-chip crystal oscillator (8MHz); 32 8-bit general purpose registers; on-chip multiplication / division device; 16K ROM, 768 Bytes RAM; self-programming function; on-chip watchdog timer; 10 inverter control circuit; 4-channel 10-bit A / D conversion; No. 15 IO ports; real-time output port.

Brushless DC motor control system

ΜPD78F0712 brushless DC motor based on the control system block diagram shown in Figure 1.

   
Figure 1 Block diagram of control system
Diagram of the role of each function block is as follows: speed bicycle handle a given analog circuit has been given speed; drive circuit to the MCU power MOSFET drive signal into the drive signal to control the conduction state of the power tubes; DC power circuit to exchange transformation, control of the motor current; current feedback is used to detect the motor current; position feedback is used to detect the rotor position; no sensor circuit according to the motor back-EMF to determine the rotor position. The following describes the various parts of the specific circuits and functions.

Position detection circuit

Brushless DC motor using three spatial difference of 120o of the Hall device for position detection. The output waveform shown in Figure 2.

   
Figure 2 Hall device output signal
The three filtered signals processed by the MCU external interrupt input pin of the three processing, get the correct rotor position information. Determined by the MCU according to the commutation control logic for motor commutation, obtain a continuous output torque, allowing the motor to the correct continuous operation. Under this three-way signals at the same time the location information can be obtained through the differential calculation of the actual motor speed, in order to ensure system stability, response speed filtering. Location of the sampling circuit shown in Figure 3.

   
Location of the sampling circuit in Figure 3
Voltage detection circuit

The role of the voltage detection circuit is real-time detection of the output voltage of the battery to prevent battery during operation due to over-discharge damage. The output voltage through the resistor divider and the control chip A / D converter circuit to convert the program to achieve the control voltage protection.

Current detection circuit

Current detection circuit for detecting the operation process of the flow through the mobility scooter windings and the power circuit of the current instantaneous value and the current sampling for the current closed-loop control systems, and overcurrent protection. To reduce system cost, brushless DC motor according to the pairwise power control method works in the next three-phase drive circuit and systems to bridge between the string into a 5-ohm power resistors, the two terminal voltage after amplification for current sampling.

Power circuit

Power circuit shown in Figure 4, the DC supply voltage of 48V, three-phase bridge inverter circuit, work is twenty-two power, that is, in each work cycle for every 60 degrees relative to a power tube, each power MOSFET is 120 degrees, the same bridge arm do not need to add two MOSFET dead time.

   
Figure 4 power circuits
Drive circuit

Driver circuit generally includes two forms: the use of special driver chip or discrete devices constitute a drive circuit. The system used in the latter program, you can ensure the reliability of the premise of reducing system cost. By the discrete components of one phase leg of the driver circuit shown in Figure 5.

   
Figure 5 drive circuit
Software control strategy

The system uses the speed and current double closed loop digital control, which is a debugging feature is convenient, easy to control the policy change. The speed loop PI control strategy using the traditional, current loop with a back-EMF compensation using PI control scheme. Software design is modular control scheme, given the speed and position from the sampling rate, the closed-loop regulation and other functions are controlled by the MCU software. System software flow shown in Figure 6.

   
Figure 6 Control block diagram
The control of motor speed by software based on the actual closed-loop voltage and current is calculated to change the PWM output pulse duty cycle. PWM drive pulse frequency 20KHz, the maximum reduction of the system of electromagnetic noise. Which the calculation of the current loop cycle is 100ms, the speed loop calculation period 500ms. The control principle structure shown in Figure 7.

   
Figure 7 Control Principle structure
Prototype testing and analysis

According to system requirements, new day as a test prototype elegance No. 6, the use of brushless DC motor of the indicators are as follows:

Rated voltage: 48V; rated power: 240W;
Rated torque: 6N.m; Rated speed: 300rpm;
Motor efficiency: ≥ 80%; rated current: 7A.

Other system control circuit to ensure the circumstances remain unchanged after the replacement control board prototype shown in Figure 8.

   
Figure 8 test prototype
According to the performance of electric bicycles are required to complete the functional tests, system performance has also been greatly improved.

· 120o/60o motor switching
By changing the commutation logic controller to complete the switch for both motor control.

· Brake power.

· Electric auxiliary brake and reverse charging
Software can control the braking force by the size and anti-charge current.

· Cruise control.

· Speed ​​and current closed loop
Closed-loop control to improve tracking accuracy and speed the process of reducing the torque ripple operation.

· Stall protection
In the case of stall protective role of the power circuit.

· Undervoltage protection
Battery to prevent over-discharge protection.

· Other auxiliary functions.

After the system test proved that with the current market, E-BIKE, this system has achieved all the basic control functions, and on this basis, an increase of speed and current closed-loop control to improve the system's dynamic response.