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Controlling Brushless DC Motors to Tackle New Challenges

The advent of simplified field-oriented motor-control algorithms, that can be hosted on an affordable embedded controller, is an important factor in the success of brushless DC (BLDC) motors, which are now preferred over ordinary brushed DC and line-powered AC motors in an increasing variety of scenarios. Their applications span industrial actuators and machine tools, robotics, computer peripherals, medical equipment such as respirators and analyzers, automotive drives, blowers, and pumps, and domestic appliances, to name a few.

The advantages of BLDC motors extend beyond merely enhancing reliability and relieving the audible noise and electrical interference associated with carbon commutating brushes. Whereas the brush motor is primarily voltage-controlled, the BLDC’s reliance on electronic commutation brings opportunities to manage rotor position, speed, and acceleration, as well as the motor’s output torque, efficiency, and other parameters with greater precision, to meet specific application requirements.

BLDC Control Strategies

Control of a BLDC motor is dependent on knowing the rotor position. This information enables the controller to coordinate powering the rotor coils in relation to the magnetic field to ensure the motor delivers the requested response: This may be to maintain speed, accelerate, decelerate, change direction, reduce or increase torque, emergency stop, or other response depending on the application and operating conditions.

The rotor position can be detected directly using a sensor or encoder positioned on the rotor shaft. Various types of encoders are available, broadly divided into relative- and absolute-position types. There are also various types of sensing techniques, such as magnetic-coil resolvers, or Hall-effect, optical or capacitive sensors. Any of these types may be suitable for a given use case, depending on requirements such as resolution, ruggedness, or cost.

Sensorless control is a practicable alternative that leverages the computing power of today's microcontrollers to calculate the rotor position using measurements of the back electromagnetic field (EMF) in each of the rotor windings. Eliminating the need for an encoder can save bill of materials costs and simplify assembly, as well as increasing reliability. In conjunction with field-oriented control (FOC), which resolves the rotor current into direct (d) and quadrature (q) vectors as slow-changing DC values that simplify control challenges, sensorless detection of the rotor position is highly effective in scenarios such as domestic appliances and automotive window, mirror, or seat controls, where ultimate accuracy is less critical than cost and reliability.

On the other hand, sensorless control is less effective at low rotor speeds, when only a small back EMF is generated.

Controller and Power-Module Selection

To control the flow of current in the BLDC rotor phases, the microcontroller first translates the application commands into a pulse width modulation (PWM) switching signal for each coil. These signals are input to gate drivers that ultimately control the power switches—usually metal-oxide semiconductor field-effect transistor (MOSFETs)—that deliver current to the rotor coils. Very small motors that have low current demand can be managed using fully integrated motor-control MCUs that contain built-in gate drivers and small power MOSFETs. On the other hand, larger, higher-power motors require dedicated external drivers and MOSFETs.

Figure 1

MOSFETs in an H-bridge configuration can be controlled to reverse the current flow through the motor coils, allowing bidirectional rotation.

MOSFETs in an H-bridge configuration can be controlled to reverse the current flow through the motor coils, allowing bidirectional rotation.

The power MOSFETs are most commonly connected as an H-bridge, or full-bridge, configuration between the motor and a bipolar power supply, as shown in Figure 1. The upper and lower MOSFETs are controlled as pairs located across diagonal corners: That is, transistor 1 is paired with transistor 2, and transistor 3 with transistor 4. This enables the coils to be energized in one direction or the other, to drive the motor in either forward or reverse direction. In this configuration, the motor may not be connected to ground, which typically requires the MOSFET drivers to be electrically isolated from the microcontroller using a pulse transformer or optocoupler.

To choose the most suitable MOSFETs for building the H-bridge, the designer must consider factors such as the required voltage and current ratings, switching speed, and switching and conduction losses. The gate drivers, in turn, must be capable of quickly charging and discharging the MOSFETs’ gate capacitance to ensure crisp switching up to the maximum frequency required by the application.

A wide variety of microcontrollers and dedicated motor controllers are available in the market, positioned for BLDC-control applications. One example is the Cypress Semiconductor PSoC 3 series of Programmable System on Chip ICs. As Figure 2 shows, the PSoC 3 architecture provides a comprehensive set of functional blocks needed for BLDC motor control.

Figure 2

Click here for larger image 
The PSoC 3 architecture is richly featured for BLDC control, with multiple PWM blocks as well as monitoring and communication features. (Source: Cypress)

The PSoC 3 architecture is richly featured for BLDC control, with multiple PWM blocks as well as monitoring and communication features. (Source: Cypress)

Building a motor controller with a PSoC 3 device gives developers access to versatile on-chip resources that can enhance flexibility and integration. Up to four PWMs on-chip enable a single PSoC 3 to control four motors simultaneously, or as many as eight motors on a multiplex basis. Built-in current monitoring allows the system to detect rotational resistance and respond appropriately, and to detect short circuits or burnouts. There is also provision for pulse detection that simplifies monitoring of rotor position and speed and allows position memory and presets.

Integrated Power Module

PSoC 3 contains most of the functional elements needed to control a BLDC motor, except the H-bridge and driver. To implement the driver, a device like the Microchip MCP8024 3-phase BLDC Power Module shown in figure 3 provides a convenient solution that can replace discrete circuitry to interface the PWM signals generated by the PSoC 3 to the MOSFET H-bridge.

Figure 3

Click here for larger image 
The MPC8024 is a highly integrated power module designed to control the gates of external MOSFETs that control delivery of power to a BLDC motor. (Source: Microchip Technology)

The MPC8024 is a highly integrated power module designed to control the gates of external MOSFETs that control delivery of power to a BLDC motor. (Source: Microchip Technology)

The MCP8024 integrates essential features such as three half-bridge drivers rated up to 12V and 0.5A, with shoot-through protection and independent input control for high-side and low-side MOSFETs, and a buck converter for powering a companion microcontroller. There are also three operational amplifiers for phase-current monitoring and rotor-position detection, an over-current comparator, two-level translators, and 5V and 12V 20mA LDO regulators. Further built-in protection features include under- and over-voltage lockout, short-circuit protection, and thermal shutdown. This extensive functionality is integrated into a compact 40-lead 5mm x 5mm QFN or 48-lead 7mm x 7mm TQFP.

Conclusion

The BLDC motor has quickly become the preferred motor type for applications ranging from cost-conscious mass-market opportunities such as consumer products, automotive drives and actuators, and domestic appliances to high-end industrial and medical equipment. Valued for their high reliability, versatility, low audible and electrical noise, and ease of use they can be controlled with or without rotor-position sensors, using a lightweight field-oriented control strategy that can be hosted on a low-cost microcontroller or programmable system on chip (SoC). When combined with a suitable power module and power switches, a PSoC 3 controller can manage multiple motors simultaneously and integrates circuitry for advanced motor-management and monitoring functions.

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