issue: October 2005 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices
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Feature - Motors and Air-Moving Devices
Microcontroller-Based Sensorless Control for a Brushless DC Motor
Fig. 1. Back EMF zero-crossing detection circuitry embedded inside the MCU
During the last decade, continuing improvements in power semiconductors and controller ICs, as well as in permanent magnet brushless motor production, have made it possible to manufacture reliable, cost-effective solutions for the appliance industry. A patented, direct-back electro motive force
(EMF) sensing method to improve the performance of the drive system will be discussed.
A dedicated, sensorless BLDC microcontroller (MCU), implementing the patented back EMF zero-crossing detection circuit as one of its peripherals, has been developed. The MCU’s imbedded programmable memory and other precision analog and digital peripherals provide advantages such as lower overall system cost, reduced board space and superior system performance and reliability.
Innovative software tools help designers fine-tune most of the motor control parameters in a simulated real application environment, and shorten time from feasibility study to industrialized solution.
Direct Back EMF Sensing for Sensorless BLDC Drives
Generally, brushless DC (BLDC) motors are driven by a three-phase inverter with a six-step commutation. The conducting interval for each phase is 120 electrical degrees. Only two phases conduct current at any time, leaving the third phase floating. In order to produce maximum torque, the inverter should be commutated every 60 degrees so that the current is in phase with the back EMF. The commutation timing is determined by the rotor position, which can be determined every 60 degrees by detecting zero crossing of back EMF on the floating coil of the motor.
To measure the back EMF on the floating phase, the conventional method requires monitoring the phase terminal and the motor neutral point. The zero-crossing of the back EMF can be obtained by comparing the terminal voltage to the neutral point. In most cases, the motor neutral point is not available. The most-commonly used method is building a virtual neutral point that will, in theory, be at the same potential as the neutral point of the wye-wound motor. Voltage dividers and low-pass filters are necessary to process the signals. The first problem with the scheme is a low signal/noise ratio at low speeds, since the back EMF amplitude is directly proportional to the motor speed. The second problem is that the low pass filter at high speeds introduces too much phase delay. Consequently, the conventional back EMF sensing method is only good for a narrow speed range.
If the proper pulse width modulation (PWM) strategy is selected, the back EMF voltage referred to ground can be extracted directly from the motor terminal voltage without the use of dividers and filters. In the proposed scheme, the PWM signal is applied on the high-side switches only. The low-side switches are only switched to provide commutation. The back EMF signal on the floating phase is detected during the PWM off time.
First, assume that at a particular step, phase A and B are conducting current and phase C is floating. The upper switch of phase A is controlled by the PWM and lower switch of phase B is on during the entire step. When the upper switch of phase A is turned on, the current is flowing through the switch to winding A and B. When the upper transistor of the half bridge is turned off, the current freewheels through the diode paralleled with the bottom switch of phase A. During this freewheeling period, the terminal voltage Vc is detected as Phase C back EMF when there is no current in phase C.
From the circuit, it is easy to see:
Where Vc is the terminal voltage of the floating phase C, ec is the phase back EMF and Vn is the neutral voltage of the motor.
From phase A, if the forward voltage drop of the diode is ignored, we have
From phase B, if the voltage drop on the switch (MOSFET) is ignored, we have
Adding (1) and (2), we get
Assuming it is a balanced three-phase system, considering fundamental frequency only, we know
From (3) and (4),
So, the terminal voltage Vc,
From the above equations, it can be seen that during the off time of the PWM, which is the current freewheeling period, the terminal voltage of the floating phase is proportional to the back EMF voltage without superimposed switching noise. It is also important to note that this terminal voltage is directly referred to the ground instead of the floating neutral point, thus eliminating the common mode noise problem caused by the neutral point in the conventional method. By this direct back EMF sensing scheme, precise back EMF zero crossing can be detected without low pass filtering and voltage dividing.
The proposed back EMF sensing scheme is implemented in a hardware macro cell inside an MCU. The MCU integrates the analog detection circuit and other motor control peripherals with a standard micro core, reducing the total system cost. Figure 1 shows the hardware implementation for the back EMF zero crossing detection.
The back EMF signals go through a multiplexer, and the controller selects the input to be sensed according to the motor commutation stages. The selected signal is compared to a fixed voltage reference, which is usually close to zero. During the off time, the back EMF is compared with reference voltage. The rising edge of the PWM, at the beginning of the PWM “on” time, will latch the comparator output to capture the zero crossing information. The system block diagram of the described sensorless BLDC driver is illustrated in Figure 2. Only three resistors for current limiting purpose are needed for back EMF sensing.
The commutation algorithm used is the standard BLDC control algorithm. The commutation will happen 30 electrical degrees after the back EMF zero crossing. Due to the programmability of the MCU, the system has much flexibility to adjust the commutation angles.
Software Tools Shorten Development
Software development also counts for a major part in project costs and cycle time. Appliance manufacturers usually have a good knowledge of the system, not necessarily in the motor drive. If the semiconductor manufacturer can provide a development tool which includes the hardware platform and software platform to run the motor, the developer is left only to optimize some of the libraries or parameters for their application based on the development tool, thus minimizing the development struggle and cycle.
A PC software tool was developed to help designers quickly tune their system and set-up software library parameters. The software interfaces with a hardware platform, which includes all the required components to drive most of the small horsepower brushless motors (front end rectifier, switching power supply, microcontroller, and IGBT inverter bridge). The MCU is linked to the PC with a proprietary protocol ICC (in-circuit-communication). This protocol is used for several purposes:
• In-situ programming
• In-circuit debugging, thanks to the embedded
hardware debug module
• Demonstration and tests as it allows real-time
data while the motor is running
The developers can directly use the development tool to run their motors. From the main parameters entered on the PC, the tool is able to select the appropriate library in a binary file format. Once the parameters have been selected, they can be flashed into the MCU. This allows the turning of almost any motor within minutes. After fine tuning the parameters, source code files in C language can be exported. From this point, the developer can start the application-specific development without worrying about acquiring an in-depth knowledge of the MCU’s dedicated peripheral’s bits, bytes and operating modes to run the motor.
Through the use of a dedicated MCU, including most of the required analog and digital circuitry, component count can be drastically reduced for a brushless motor drive. Also, with the help of the described user-friendly software tool, set-up of a brushless motor drive is greatly simplified, thus leading to a shortened design cycle time. Both of these elements help to minimize the brushless motor complexity paradigm, and are well received by designers in the appliance industry.
This information is provided by Jianwen Shao, from STMicroelectronics Power Systems Applications Lab, and Vincent Onde, from STMicroelectronics Microcontroller Application Lab
1. D.Erdman, U.S. Patent No.4654566, Control system, method of operating an electronically commutated motor,and laundering apparatus,March 1987.
2. J.M. Charreton, J.M. Bourgeois, U.S. Patent 5859520, Control of a Brushless Motor, January 1999.
3.J.Shao, D.Nolan, and T.Hopkins, A Novel Direct Back EMF Detection for Sensorless Brushless DC
(BLDC) Motor Drives, Applied Power Electronic
Conference (APEC 2002), pp 33-38.
4.STMicroelectronics Datasheet ST7FMC Rev 2.0, 2005.