Brushless Mortor｜Basic

Methods of Sensorless 120° Commutation Driving Startup 2: Startup on Detection of Permanent Magnet Stopped Position

2023.08.30

Points of this article

・The method of detecting the stopped position of the permanent magnet to start the motor avoids the problems of reverse rotation and low-torque startup of the method in which the induced voltage from synchronous operation is detected for startup, and so reduces the time needed for startup.

・To detect the stopped position of the permanent magnet, current is passed in six patterns over short intervals during which the motor does not rotate, and the pattern for which the power supply current is largest (or smallest) is determined.

In the previous article, among the two basic methods used for sensorless starting of a 3-phase full-wave brushless motor, the method in which an induced voltage due to synchronous operation is detected to start the motor was explained. In this article, the method of detecting the stopped position of the permanent magnet to start the motor is explained.

Problems with the Method of Startup by Detecting the Induced Voltage Due to Synchronous Operation

The previous article explained a number of problems with the method of starting a motor by detecting the induced voltage due to synchronous operation. The method explained here, in which the stopped position of the permanent magnet is detected to start the motor, is a method that resolves these problems. For reference, we shall once again state these problems.

＜Problems with the Method of Starting a Motor by Detecting the Induced Voltage Due to Synchronous Operation＞

・A synthetic magnetic field is created regardless of the position of the permanent magnet, so that depending on the motor state, a reverse-direction torque may act, and depending on the stopped position of the permanent magnet, more time may be required for startup.
・Ordinarily, the positional relationship between the permanent magnet and a synthetic magnetic field to obtain a large torque is a difference of 90°, but because the synthetic magnetic field is created regardless of the permanent magnet position, the starting angle is for example 60° or 70°, so that the desired large torque is not obtained.

Methods of Sensorless 120° Commutation Driving Startup 2: Startup on Detection of Permanent Magnet Stopped Position

The following is an explanation of the method of motor startup by detecting the stopped position of the permanent magnet, which resolves the above-described problems.

The first diagram shows the state in which the permanent magnet (rotor) used in the explanation is stopped. The magnet is stopped with the S pole at the 3 o’clock position and the N pole at the 9 o’clock position.

Next, the circuit diagram is a schematic diagram used to explain the principles and operation involved in detection of the permanent magnet position. Using the outputs A1, A2, A3, the coils are energized such that current flows through the coils in the six patterns ① to ⑥. This energizing is for a short time duration, such that the permanent magnet does not rotate.

The energizing waveforms and current waveforms (IVM) for the six patterns are shown in the waveform diagram. A1 to A3 output voltages corresponding to the energization patterns ① to ⑥. The power supply current is of varying sizes for each of these patterns. This is because the current for each pattern differs depending on the position of the permanent magnet; in this method, the difference in currents is used to detect the position of the permanent magnet.

A specific example is explained. In ③, energization results in the S pole at coil 2 and the N pole at coil 3. The permanent magnet S pole opposes coil 2, and the permanent magnet N pole opposes coil 3, so that magnetic polarization of the coils is impeded. Hence the current rise is most gradual, and the current is small.

However, ⑥ is the opposite of ③; energization causes an N pole to appear at coil 2 and an S pole at coil 3. Because the permanent magnet S pole opposes coil 2 and the permanent magnet N pole opposes coil 3, the coil magnetic polarization is promoted. As a result, the current rise is most rapid, and the current is large.

In other words, by determining the energization pattern resulting in the largest or the smallest current, the permanent magnet position can be detected.

This method is explained in somewhat more detail using a circuit block example and operation waveform diagram for a specific driver.

The circuit block for this method 2 is basically the circuit block of method 1 for detecting the induced voltage from synchronous operation to start the motor, to which is added a circuit for generating the six energization patterns and detecting the power supply currents, comparing the currents, and generating an initial driving pattern. Some parts are omitted, but the portions surrounded by blue are the outputs A1 to A3.

(1) The six patterns generated by the position detection pattern generation block are sent to a driving basic waveform synthesis block, and energization is performed by A1 to A3. The detected power supply currents are converted into voltages by a current detection resistor and an amplifier, and based on (2) a sample and hold circuit and (3) a current comparison/maximum value pattern detection circuit, (4) an initial driving pattern generation block generates an initial driving pattern based on the permanent magnet position, and returns the pattern to the driving basic waveform synthesis block, and driving begins.

Short pulses are shown immediately after power-on in the output voltage waveforms A1 to A3 in the operation waveform diagrams. This represents energization of the six patterns for position detection. As explained above, the energization is over a short time, and so in contrast with the other waveforms, the time is very short.

Immediately after power-on, the permanent magnet position is detected, and based on the result, an initial driving pattern is used for driving, so that the problems of reverse rotation and low-torque startup that occurred in the method of startup by detecting an induced voltage are avoided, and the motor can be rotated by a large torque from the very beginning.

Operation after startup basically relies on the same process as in the method of startup by detecting an induced voltage. The timing for switching the driving pattern during initial driving is, similarly to the method of startup by detecting an induced voltage, based on ST_CLK.

However, rotation is begun using a large torque from the start, so that a sufficient induced voltage is obtained using several patterns (in these waveform diagrams, four ST_CLK signals), and steady-state driving is begun using an induced voltage. That is, the problem with the method of startup by detecting an induced voltage, that time is required for startup, is alleviated.

【Download Documents】Overview of Brushed DC Motor Structure, Operating Principle, Drive Method, Characteristics, Features, and Applications

Basic overview of brushed motors, including structure, principle of operation, drive method, and features.

Download

List of articles related to the「Methods of Sensorless 120° Commutation Driving Startup 2: Startup on Detection of Permanent Magnet Stopped Position」

Download Technical Documents

Overview of Brushed DC Motor Structure, Operating Principle, Drive Method, Characteristics, Features, and Applications

Basic overview of brushed motors, including structure, principle of operation, drive method, and features.

Download