UAV Basics - Composition - Motor Drive Control

Model aircraft motor drive control

Motor drive control is to control the rotation or stop of the motor, as well as the speed of rotation. The motor drive control part is also called electronic speed controller, referred to as ESC, English electronic speed controller (ESC). The motors used by the ESC are different, and there are brushless ESCs and brushed ESCs.

The permanent magnet of the brushed motor is fixed, the coil is wound on the rotor, and the direction of the magnetic field is changed through the contact of the brush with the commutator to keep the rotor rotating continuously. Brushless motor, as the name implies, this motor has no brushes and commutators. Its rotor is a permanent magnet, while the coil is fixed and directly connected to an external power supply. The problem is, how to change the direction of the coil magnetic field Woolen cloth? In fact, an electronic governor is needed outside the brushless motor. This governor is a motor drive. By changing the direction of the current inside the fixed coil, it ensures that the force between it and the permanent magnet is mutually exclusive, and continuous rotation can be achieved. 

The brushed motor can work without an ESC, and it can work by directly supplying electricity to the motor, but it cannot control the speed of the motor. The brushless motor must have an ESC, otherwise it cannot be rotated. The direct current must be converted into three-phase alternating current by the brushless ESC, and then it can be output to the brushless motor to rotate.

Generally, the duty cycle of PWM is used to control the speed of the motor.

Crazepony Motor Drive

The operation of the brushless motor is relatively troublesome, and the brushed motor is the kind of motor on the four-wheel drive we played when we were young. It can turn violently when it is connected to electricity, and it can turn violently when it is connected in reverse. That’s it. Simple.

Crazepony uses a brushed hollow cup motor, so the control of the motor belongs to the brushed DC motor control, which is much simpler than the brushless ESC. Crazepony uses a brushed hollow cup high-speed motor with a speed of about 3W rpm. To drive the brushed motor, it is very simple, only need to increase the driving ability of the signal, then the brushed motor can be driven.

So what components are selected to provide such characteristics? Crazepony's motor driver IC selection has experienced the failure of three-stage transistors and medium power transistors, and finally the field effect transistor (ie MOSFET) SI2302 is selected.

Since the author chose the aircraft entirely out of a strong hobby, I didn't even know the physical structure difference between a brushed motor and a brushless motor at first. What is an ESC? stupidly can not tell……

At the beginning, the triode was used as the motor driver, and a very classic common-emitter circuit was used. "Should the triode work in the on-off state?" I drew the PCB board driven by the triode, and found that the motor turned slower and slower, and it was boring at all. "Maybe it's because the triode can't carry a large current. Well, then I'll replace it with a medium power tube. The maximum current of the collector is 6A, right?" It is conceivable that the result is not acceptable.

First of all, understand why the triode is not a desirable solution as a simple motor drive: * As an ancient semiconductor pioneer, the triode appeared as an amplifier device. It has concentrated characteristics in the linear region, and saturation and cut-off are both It is an extreme working state, and as a motor drive, we can only choose its two extreme working modes. * When a triode is used as a drive tube for a large current load, the influence of its own tube voltage drop on the load has to be considered, which is very serious. The self-dissipation is getting bigger and bigger, the motor and the tube are connected in series, the battery voltage is only 3.7V, the motor can only turn slower and slower

There is also a transistor in the transistor family that is complementary to the characteristics of the triode, and all characteristics are concentrated in the switching state of the transistor, the field effect transistor, that is, the MOSFET. When the usual FET is fully turned on, the source-drain resistance is in the mΩ level, that is, its own dissipation is very small. It's perfect to use it as a drive tube. Finally, a field transistor (SI2302) with a SOT23 package and a turn-on voltage Vgs<4v was selected, and the result showed a good driving performance.

Each FET is connected to a large resistor to pull down, the purpose is to prevent the motor from turning violently due to the unstable PWM signal when the microcontroller does not take over the control of the motor. A pull-down resistor is connected to ensure that the input signal of the field tube is either high or low, and there is no uncertain third state. Then the motor has only two states, either turning or not turning. The main control output is a PWM waveform, which is used to control the closing and conduction of the field effect tube, thereby controlling the rotation speed of the motor. This is the principle of the crazepony motor drive. It's that simple.
Brushless ESC (pronounced as tiao)

In the article "Motor and Propeller", we mentioned that the big four axes are basically brushless motors, and the brushless motor control must be used with brushless ESCs.

The input of the brushless ESC is DC, usually directly connected to the model aircraft battery. The output is 3 wires, driving a brushless motor. In addition, the brushless ESC has three signal lines, which generally input PWM signals to control the speed of the motor. For model aircraft, especially the quadcopter, due to its particularity, a special model aircraft ESC is required.

So why do you need a special ESC on a quadcopter, and what's so special about it? The quadcopter has four paddles, which are opposite each other in a crisscross structure. The rotation of the paddle is divided into forward rotation and reverse rotation, which can offset the spin problem caused by the rotation of a single blade. The diameter of each paddle is small, and the centrifugal force when the four paddles rotate is dispersed. Unlike the propeller of a straight plane, there is only one inertial centrifugal force that can generate concentrated centrifugal force to form a gyroscopic nature, so that the fuselage is not easy to roll over quickly. Therefore, the update frequency of the motor control signal used by the model aircraft is usually very low, while the update frequency of the control signal used by the model aircraft quadcopter is very high.

In order to respond quickly to the drift caused by the attitude change, the four axes need a high-response ESC. The update speed of the conventional PPM ESC is only about 50Hz, which cannot meet the speed required for this control, and the PPM ESC has a built-in MCU PID stable speed control can provide smooth speed change characteristics for conventional aircraft models. It is not suitable for use on four axes. What four axes need is a rapid response to motor speed changes. Using a high-speed dedicated ESC and IIC bus interface to transmit control signals, the speed of the motor can be changed hundreds of thousands of times per second, and the attitude can be kept stable at all times during four-axis flight. Even if it is suddenly impacted by external forces, it is still safe and sound.

For quadcopters, we generally use an inductionless brushless DC motor, and the ESC uses the induced electromotive force of the third phase to measure the rotor position. For the corresponding car model and ship model, because the motor needs to be started, stopped, and reversed frequently, and the weight of the entire power system is not very particular, so the sensor brushless motor is used, and the ESC uses the Hall effect device on the motor. to detect the rotor position.

The picture below shows the sensory brushless DC motor and the corresponding ESC used on the car model/ship model, the Quicrun series 3650 model from Hobbywing.



BLHeli is an open source ESC hardware and software project. The perfection and popularity of BLHeli ESCs came in 2015, following the development of the traverse machine market. BLHeli ESC is a milestone in the development of ESC. In the field of 10-30A ESC, it broke the isolated and closed source pattern of several large ESC manufacturers. Now many old ESC brands have launched ESCs that support BLHeli firmware.

Since the hardware and software of BLHeli are open source, there are many ESCs based on BLHeli on the market. The most famous ones are FVT Littlebee ESC, etc., of which Littlebee is the codename of one of the BLHeli firmware versions, commonly known as Littlebee ESC.

BLHeli ESCs can be upgraded through the BLHeliSuite host computer for firmware version upgrade, parameter modification, etc., which is very convenient. The following is the interface of the host computer.