Analysis of the technical principles of electric bicycle controllers

The development of modern electric bicycle technology has made the controller far beyond the traditional single drive control function, and has become the energy management and control center of electric bicycles. This is the core and key to ensure the safe driving, comfortable riding, high power performance, economy and energy saving of electric bicycles. It samples, compares and analyzes various working status information and converts them into a series of control or protection instructions, automatically monitors the motor and control circuit to enable the electric bicycle to operate safely and reliably. The performance and quality of the motor control system will determine the power performance, driving performance and safety performance of the electric bicycle.

Electric bicycles The products of controllers are changing with each passing day, and the technology is advancing by leaps and bounds, but the level of cognition of people in the electric bicycle industry is uneven. In order to popularize the knowledge of electric bicycle controllers and improve the technological level of this category, here we will introduce the basic control principles to the intelligent system composed of single-chip microcomputer (MCU) control.

To illustrate the principle, we equate the main control circuit of the electric bicycle motor (brushless or brushless) to a simple motor control circuit as shown in Figure 1-1:

According to the working characteristics of the motor, the speed of the motor can be changed by changing the armature voltage. Since the battery voltage of the electric bicycle is basically fixed, if you want to adjust the speed, you must find a way to regulate and change the armature voltage applied to the motor. At the same time, when the electric bicycle encounters actual situations such as stalling and uphill during use, its working current will be very large. From the equivalent circuit, it can be seen that although there is only an internal resistance of about 0.4Ω, if calculated according to the power supply voltage of 48V, its current value I=U/R=48V÷0.4Ω=120A. With such a large current, its hazards can be imagined: it is easy to burn the winding of the motor itself and the main circuit wire of the whole vehicle. At the same time, it is easy to cause fatal problems such as over-discharge of the battery and damage to the battery.

How to solve the problem of regulating the motor speed under battery-powered conditions? How to control the current? This is the topic introduced in this chapter, and we will also explain the basic principles of the electric bicycle control system.

We know that the motor is an inductive load, and the inductor is an energy storage element. When the inductor coil is suddenly powered off, it will generate a pump-up voltage, that is, the back electromotive force, so it can be regarded as a variable voltage source. In this way, we can equate the main control circuit of the permanent magnet motor used in the electric bicycle to the circuit shown in Figure 1-2 below for analysis:

Then, based on the above equivalent circuit, we can find out its calculation method. If voltage U is applied to the motor, its voltage equation is: U=E+IR+L Formula (1-1)

The generated back electromotive force is: E=KE formula (1-2)

The electromagnetic torque generated is: T=KTI formula (1-3)

Where KE is the back EMF constant, KT is the torque constant, and n is the motor speed.

The physical meaning of these formulas: Back electromotive force E can be regarded as a variable voltage source, which is proportional to the motor speed; electromagnetic torque T is also proportional to armature current I; armature internal resistance R (usually very small) has a certain current limiting effect when the motor starts and stalls; armature inductance L has an impact on dynamic performance. When the armature is powered intermittently, it plays a smoothing and filtering role on current pulsation, and the up and down fluctuations of current are very small, and the fluctuation of torque is also very small. As shown in Figure 1-3 below.

Based on this characteristic, under the condition that the battery voltage is basically constant, the intermittent power supply method is used to change the average value of the motor power supply voltage to control the motor speed and current. Recognizing and mastering this law is essentially the key concept PWM (Pulse Width Modulation, PWM for short) pulse width modulation control mechanism we will talk about in the following chapters, that is: within the required time, the DC voltage is modulated into a series of voltage pulses of equal amplitude and unequal width to achieve the purpose of controlling frequency, voltage and current. The electric bicycle controller uses the PWM circuit to control the output power of the motor. The high-frequency switching power device MOS tube is used to realize the switch modulation. It is used as a switch to perform high-frequency chopping intermittent power supply, thereby effectively solving the controllability of the motor speed and current.

This control method can be easily implemented through a single-chip microcomputer. Therefore, whether it is a brushed motor or a brushless DC motor, its control system can realize PWM control, thus providing an opportunity for digital control of the motor.