Application Analysis of Oscilloscope in Brushless DC Motor Industry

In recent years, brushless motors have been widely used in high-precision control industries such as medical, industrial control, consumer electronics and automotive electronics. The performance of brushless motors largely depends on the motor driver. How do engineers use the fast and convenient help in the development stage? , Really analyze the drive signal? This article mainly introduces the typical test and analysis of the ZDS4054Plus data mining oscilloscope to the motor driver.

As a "newcomer" in the motor industry, brushless motors are well-deserved up-and-coming stars, rushing into high-precision control industries such as medical, industrial control, consumer electronics and automotive electronics with wild waves. Is "brushless" the future motor The development trend of the industry? This article takes a look at those things about brushless motors in the form of this article!

One. Introduction
With the development of power electronics and the emergence of new permanent magnet materials, the brushless DC motor has been rapidly developed. The brushless DC motor realizes the commutation of the motor through electronic devices, replacing the traditional mechanical brushes and inverters. It is composed of a motor main body and a driver, and is a typical mechatronic product. The stator windings of the motor are mostly made into a three-phase symmetrical star connection, which is very similar to a three-phase asynchronous motor. Magnetized permanent magnets are glued to the rotor of the motor. In order to detect the polarity of the motor rotor, a position sensor is installed in the motor. The driver is composed of power electronic devices and integrated circuits. Its function is to receive the start, stop, and brake signals of the motor to control the start, stop and brake of the motor; to receive the position sensor signal and the forward and reverse signals to control the reverse The on and off of each power tube of the variable bridge produces continuous torque; it accepts speed commands and speed feedback signals to control and adjust the speed; provides protection and display. Brushless motors are widely used in medical, industrial control, consumer electronics, power tools, electric vehicles and other fields due to their low noise, long life, high speed, small size, good dynamic performance, large output torque, and simple design.

2. The working principle of brushless motor
First, look at the block diagram of the brushless motor driver, as follows:

As can be seen from the above figure, the MCU output six PWMs through the configuration register is only a control signal, and its voltage is only 5V, which cannot directly drive the motor. Instead, the motor runs by controlling the switch of the power tube. The drive circuit is generally composed of multiple MOSFETs. Drive axle and motor drive axle power tube are formed. The commutation of the brushless motor is based on the detection of the rotor position. The inductive driving method uses the Hall sensor to detect the rotor position, and the non-inductive driving method detects and calculates the current during the rotation of the brushless motor. , Voltage and other parameters change, estimate the rotor position, and then carry out commutation.

commutation principle

A Hall sensor is installed inside the brushless motor, which can give an output signal of 1 or 0 according to the distribution of different magnetic field directions at different positions of the rotor. The three sensors are evenly installed, which occurs 6 times at an electrical angle of 360 degrees. Flip the level, each time the difference is 60 degrees in electrical angle, and the position of the rotor is measured according to the signal encoding of the three sensors. This is the commonly used inductive driving method. In addition, the non-inductive drive method is to estimate the rotor position by detecting and calculating the current, voltage and other parameter changes during the rotation of the brushless motor, and then perform commutation.

The working principle of the drive circuit

Simplified diagram of drive circuit

In the figure, Q1 to Q6 are power FETs. When the AB phase is required to be turned on, only the Q1 and Q4 tubes need to be turned on, while the other tubes are kept off. At this time, the current flow path is: positive pole→Q1→coil A→winding B→Q4→negative pole. The gate of the MCU to Q1 is a PWM signal, and the gate to Q4 is a normally open signal, so you can control the effective voltage of the driving motor by controlling the duty cycle of the PWM signal at the input of Q1. The same goes for the other five-step guide switch. The measured waveforms of each phase are as follows:

The measured effect of the voltage waveform of each phase

Three, the actual measurement application and analysis of ZDS4054Plus
Regarding the PWM signal analysis of the brushless motor driver mentioned above, what new test experience does the ZDS4054Plus oscilloscope have?

512Mpts large data storage
Regarding the driving voltage of the brushless motor, when the engineer observes the PWM signal, if there is an abnormality in the signal, it is difficult to trigger through the trigger method. It is necessary to analyze the signal in the envelope through the zoom mode under a large time base, (in the zoom window Observe the details of the waveform). The frequency of the PWM signal is more than tens of K, and a higher sampling rate is required. At the same time, the PWM signal is also accompanied by carrier signals such as current and encoder. It requires multiple channels to be observed separately. From the waveform time, sampling rate, and more From the three aspects of the channel, large storage is required. Figure 1 shows the PWM drive signal of the brushless motor. When the storage depth is set to 350M, the 7S waveform is captured, and the sampling rate is still as high as 50M Sa/s, ensuring that the waveform is not distorted. From the formula: storage depth=waveform time*sampling rate, ZDS4000 series oscilloscopes are equipped with a standard storage depth of 512Mpts to ensure that they can capture long-term waveforms while maintaining high sampling rates.

Dual ZOOM mode + smart label
As shown in the figure above, how to analyze the PWM drive signal or abnormal signal after capturing a long-time waveform? In addition, in industrial servo applications, under different working conditions, when switching different loads, the drive waveform changes or abnormal signals corresponding to different times, the entire load is switched to a stable process, which takes a long time, and it also needs to be stored in the large storage. In depth, view the waveform details. In view of the above situation, the ZDS4000 series oscilloscope supports dual ZOOM zoom mode while ensuring a large storage depth. You can set the coefficients for the two zoom windows separately, and cooperate with the intelligent annotation function to be interested in any place. Signals are labeled. The figure shows the PWM drive signal. The waveform in the main time base is enlarged in two ZOOM windows. ZOOM1 is the PWM period signal, and ZOOM2 is the oscillation waveform of a certain peak of the PWM. With the guarantee of the large storage depth, the sampling rate is 50M Sa /s, to ensure the authenticity of the waveform details. At the same time, with the intelligent marking function, such as making a mark on the main time base, you can quickly find the marked point on ZOOM1 and ZOOM2, and you can see the marked point in ZOOM1-the third peak of PWM, which can be viewed in ZOOM2 The oscillation situation and amplitude of the spike.

Four, summary
ZDS4000 series data mining oscilloscope, with 512M deep storage, dual ZOOM mode, template trigger, FIR hardware filtering and intelligent calibration functions, can quickly and truly locate and analyze the abnormal waveform of the brushless motor driver, which is the waveform of the brushless motor industry Debugging provides the perfect solution!