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Boost automotive electric motor control with DSCs

Posted: 16 Apr 2007 ?? ?Print Version ?Bookmark and Share

Keywords:DSC control processor? embedded processor? embedded DSC architecture?

By Willie Fitzgerald
Microchip Technology

Mechanically actuated automotive systems, such as power steering and fuel and water pumps, are being replaced with systems that utilize electric motor technology. Consequently, the presence of semiconductors in automobiles is increasing geometrically, in part due to automotive systems developers' desire for electronic motor control to address consumer requirements for safer, more efficient cars.

Brushless direct current (BLDC) motors, one contingent of the synchronous motor group, are designed for automotive applications that continuously rotate, such as pumps, cooling fans, and stepper motors. BLDCs equip positioning systems with start and stop functions where high reliability is required.

Furthermore, the electronic control that BLDC motors offer is critical to statutory vehicle requirements, including energy-savings, reduced environmental and emissions impact, and the creation of safer vehicles. BLDC motors are also useful for variable-speed applications where space is tight, as in fuel pump control and electronic/electric power steering. In these types of applications, electronic control is essential because of the need for fault diagnostics and wide temperature and voltage operational ranges.

The embedded processor is a critical tool for automotive system designers as they address the increasing needs and demands of today's driver. The increased use of electronic controls enables automotive system designers to meet these needs, while meeting their own requirements to develop low-cost, low-noise, high-accuracy systems with faster time-to-market.

A multitude of embedded-processor solutions is available to automotive design engineers. One single-chip architectural platform that is ideal for BLDC motor control is the 16-bit digital signal controller (DSC). This type of platform delivers the control of a microcontroller, along with the computation and throughput capabilities of a DSP. In this way, DSCs are excellent at executing the complex, high-speed mathematical functions required by many automotive electronics systems.

DSCs, such as Microchip's dsPIC, offer a seamless migration path and pin-for-pin compatibility, which enable the re-use of hardware and software building blocks. This combination of a 16bit MCU with DSP capabilities enhances the performance of automotive electronic systems, while lowering system costs and enabling designers to get products to market faster.

BLDC motor overview
The BLDC motor does not operate directly off a DC voltage source, nor does it use brushes for commutation. Instead, the BLDC motor contains a rotor with permanent magnets and a stator with windings, and performs commutation electronically.

Commutation is the act of changing the motor phase currents at the appropriate times to produce rotational torque. Commutation is performed mechanically in brush motors, but must be performed electronically in BLDC motors.

Stacked steel laminations on the stator equip the BLDC motor with windings placed in slots, which are cut axially along the inner periphery. While the stator, in general, is similar to that of an induction motor, the motor windings can be configured in a non-distributed format. Each winding is constructed of numerous small coils that are placed in the slots and interconnect to form the larger winding. Each winding is distributed over the stator periphery to form an even number of poles. Stator windings can be either trapezoidal or sinusoidal, with each generating different types of back electromotive force (EMF). The phase current also has trapezoidal or sinusoidal variations.

The DSC combines MCU control and DSP number crunching functions.

All rotors have permanent magnets of some type, and can vary from two to eight pole pairs. The proper magnetic material with which to create the rotor is selected based upon the required magnetic-field density. Ferrite magnets have traditionally been used to make permanent magnets. However, rare earth alloy magnets are becoming more popular, as they generally have a higher magnetic density per volume and enable the rotor to compress further for the same torque. Alloy magnets improve the size-to-weight ratio and deliver higher torque than a motor of the same size that is comprised of ferrite magnets. A method to detect the position of the rotor magnets is required for BLDC motors.

BLDC motors are popular because they are fast, noiseless, efficient, and exhibit a longer operating life. BLDC motors are also popular due to their compact size, controllability, high efficiency, low EMI and high-reliability. Their compact size is a direct result of technological advances in magnets noted earlier that deliver efficiency improvements.

Additionally, the ratio of torque delivered in BLDC motors relative to motor size is higher than in non-BLDC motors, making BLDC motors an excellent match for space and weight-sensitive applications.

BLDC motors can be designed into systems that are sensor-based or sensorless. The implementation of sensorless BLDC motor systems eliminates the cost of Hall Effect or optical sensors and their supporting electronics. The sensorless operation is also desirable if the rotor is operating while immersed in fluid such as fuel, oil, or water. In sensorless control, back EMF (BEMF) zero crossing is used for commutation.

How DSCs control BLDC apps
Achieving cost and performance targets while maintaining flexibility continues to challenge embedded systems designers. DSCs offer lower system costs and are excellent for real-time control applications that require high robustness and increased reliability. Additional system benefits derived from DSCs include:

  • Reliable watchdog timer that operates from its own internal oscillator, independent of the system clock.

  • On-chip clock monitor that forces a chip reset when it detects a system clock failure.

  • The on-chip oscillator eliminates the need for an external crystal, saves critical board space, and reduces system costs.

  • An intelligent on-chip power-on-reset (POR) circuitry eliminates the need for external reset circuitry, which, when coupled with brownout protection, resets the chip in the event of a power glitch. This results in a more robust system at a lower cost.

  • Advanced analog peripherals including a high-resolution analog/digital converter (ADC) operating at 1.1MSps with the ability to support simultaneous sampling and holds of up to eight inputs, which improves system throughput.

  • Enhanced motor control peripheral with up to eight channels of motor control pulse width modulation (PWM) that are either center-aligned or edge-aligned, plus an on-chip quadrature encoder interface (QEI), improving system performance and reducing software overhead.

A preferred implementation to determine the position and speed of a sensorless BLDC is back EMF (BEMF) "zero-crossing." The figure below illustrates the voltage changes of the three wires to the BLDC motor. The motor's BEMF waveform is a function of position and speed, as determined by resistor dividers and operational amplifiers. This system detects instances when the BEMF of an inactive phase is zero.

By using the motor leads of a BLDC motor, the "zero-crossing" of the BEMF signal appears in sectors 0 through 5. Each sector corresponds to a 60-degree portion of the electrical cycle. There is a 30-degree offset between the BEMF zero-crossings.

The BEMF zero-crossing detection system is suitable for a wide range of motors. To facilitate ease of design, a design engineer can use a Y and connected 3-phase motor theory in a design. Since the zero-cross BEMF technique searches for an instance where the rise and fall of signals cross a threshold voltage, this method is insensitive to motor manufacturing tolerance variations. The zero-cross BEMF technique also works with voltage or current-control circuits.

Caveats
However, a primary disadvantage to the zero-cross BEMF method is that the motor must generate sufficient BEMF by moving at a minimum rate. A secondary disadvantage is that abrupt changes in the motor load can cause the BEMF loop to go out of lock. But the software algorithm of a DSC often corrects such a lock condition.

The figure below illustrates a hardware example of a sensorless BLDC system. In this figure, a six-channel PWM register from Microchip Technology's dsPIC30F2010 DSC drives the BLDC through a 3-phase inverter. The PWM portion of the DSC generates multiple synchronized outputs. The PWM module has six PWM input/output (I/O) pins with three duty-cycle generators. The PWM counter provides up to 16-bit resolution and the system developer can perform "on-the- fly" frequency changes. The six 16 kHz PWM output channels drive four input channels for simultaneously-sampled 10-bit ADCs of bus current, bus voltage, demand pot, phase voltage samples (synchronized to the PWM module), and three timers. The six-channel PWM register drives the BLDC motor.

In this hardware block diagram of a sensorless BLDC motor control circuit, a 10-bit ADC monitors the back EMF of the BLDC motor. The ADC inputs AN3, AN4, and AN5 simultaneously sample the three legs of the BLDC motor and test for bus voltage.

A 10-bit ADC monitors the back EMF of the BLDC motor. The ADC inputs AN12, AN13 and AN14, simultaneously sample the three legs of the BLDC motor and testing for bus voltage. The 10-bit ADC also measures the "zero-crossing" voltage (VDC) for BEMF testing.

Additionally, a current-feedback circuit that uses an amplifier/comparator network connects to a PWM fault protection pin, FLTA. If a PWM fault is detected, the motor shuts down. The output of the sample and hold is the input into the converter, which generates the result. The analog reference voltages are software-selectable to either the device supply voltage (AVDD/AVSS) or the voltage level on the pin (VREF+/VREF-).

The BEMF zero-crossing routine is implemented and motor speed is controlled through software. A 30-degree phase advance extends the motor operating speed range. The high execution speed of the dsPIC DSC architecture provides the computational power required without degrading motor control performance.

A typical DSC architecture possesses a central processing unit (CPU) and peripheral characteristics that render it suitable for use in a number of automotive BLDC applications. Specifically some of the beneficial features of Microchip's 16-bit dsPIC DSCs for motor control applications include:
? Dual, 40-bit wide accumulators;
? Single-cycle, 16 x 16 multiply accumulate (MAC) operation;
? 40-stage barrel shifter;
? Dual-operand fetches;
? Saturation and rounding modes; and
? DO and REPEAT loops

The dsPIC DSCs also offer flexible interrupts, a watchdog timer, and real-time emulation. The above features address the electronic-control challenges associated with BLDC-based applications.

A significant feature of 16-bit DSCs is extensive mathematical processing capability. True DSCs (such as the dsPIC30F and dsPIC33F) include two 40-bit accumulators, which store the results of two independent, 16-bit x 16-bit multiplication operations. DSCs such as these execute most instructions in one cycle.

Many performance-oriented signal-processing algorithms involve the calculation of a running "sum-of-products." Special instructions such as multiply-and-accumulate (MAC) provide the ability to multiply two 16-bit numbers, add the result to the accumulator, and pre-fetch a pair of data values from RAM?all in a single instruction cycle. With two accumulators, it is also possible to "write back" the data in one accumulator while the other performs computations at the same time.

In addition, unlike standard MCUs, DSCs have the ability to support fractional arithmetic calculations when interpreting data in fractional form, rather than assuming the data to be in integer form.

Flexible interrupt structure
DSC architectures provide extremely high flexibility with regards to interrupt structure. Usually, DSCs support a large number of individually selectable and prioritizable interrupt sources, a highly desirable attribute for any application that involves multiple sensors and actuators. In this case, interrupt latency is highly deterministic, which makes life easier for system developers.

Run-time self programming (RTSP)
Many automotive applications require the storage of constants to calibrate data obtained from sensors, variations between transducers, and pre-measured offsets. Many DSC devices utilize program flash memory and flash-based data EEPROM, which reliably and efficiently store and access constants. Flexible and secure flash memory is available to system developers in many DSCs.

In-circuit serial programming (ICSP)
With ICSP technology, flash DSCs enable the easy upgrade of application firmware in the field. In addition, ICSP allows the same controller to be used for many different automotive subsystems and operating environments. These functional enhancements can be delivered immediately at minimum cost.

High-resolution ADC
It is important to select a high-speed, high-resolution, on-chip ADC to measure small, rapid changes in the application. One of the most significant factors to consider in selecting an appropriate DSC is its on-chip ADC's ability to measure different samples simultaneously. This allows the measurement of motor voltages and currents to occur in-phase to avoid errors in the control loop.

A high-speed analog-to-digital conversion rate is beneficial for many reasons. First, speed minimizes sample latency, which increases closed-loop performance. Secondly, high-speed conversion allows sampling of multiple channels with high throughput for all channels. Additionally, a high-speed conversion rate, coupled with the DSP capabilities of the DSC core, allows oversampling and filtering of noisy motor feedback signals.

Pulse width modulation
DSCs support the automatic generation of PWM signals of specified waveforms and polarities. Some DSCs incorporate extensive on-chip peripheral support for these advanced types of PWM-based algorithms, which simplifies code development and increases overall system flexibility in several ways.

First, multiple PWM generators provide complementary outputs and automatic dead time insertion for sinusoidal commutation of the BLDC motor. The PWM module also furnishes override control to implement six-step commutation. Many different inverter schemes are possible in this case, including synchronous rectification, which controls current flow to ensure maximum inverter efficiency.

Likewise, PWM-based algorithms can utilize fault pins for latched or automatic over-current protection. In addition, the ADC synchronizes with the PWM for correct shunt-resistor current measurement.

Quadrature encoder interface
Accurate and rapid measurement of vehicle speed and position, as well as the speed and position of the mechanical components therein, are essential for effective electronic control of many aspects of automotive operation. Quadrature encoders facilitate these measurements and, therefore, enable closed-loop control in many motor control applications.

Controller area network
With the continuing evolution of microcontrollers toward including more functionality and speed, it is essential for vehicle electronic control modules to communicate with each other efficiently and reliably. The CAN bus is expected to be the leading standard for automotive-networked nodes over the next five to seven years.

The CPU of a DSC supports a powerful suite of DSP instructions and flexible addressing modes, thus enabling fast and accurate arithmetic and logical computations. Many DSC architectures are particularly well suited for control actions, such as:

  • Periodic service interrupts: Provide periodic samples of vehicle speed and steering angle to calculate the brake pressure required for the anti-lock brake system to function properly.

  • Data capture from multiple sensors and control inputs: Simultaneously measures vehicle speed, acceleration, relative body/wheel motion and steering angle, in order to determine the damping level of an active-suspension control system.

  • Transmittal of data and control pulses to actuators: Enables a variable duty-cycle PWM signal to open and close a fuel injector for the appropriate periods.

  • Data sharing with other controller modules in a distributed system: Networks various subsystems so that they periodically transmit status data to a diagnostics module or the user display panel.

Summary
With the continued advancement of automotive electronics, opportunities for electronically controlled BLDC motors are expanding in demanding applications such as fluid pump control and electronic power steering, where efficiency and reliability are key concerns. The DSC is an ideal embedded control processor option for these types of emerging BLDC-based applications.

It is important that systems developers invest in a DSC architecture that is well suited to address the challenges of their particular applications. The single-chip DSC architecture must have software-compatibility, robust peripherals, fast interrupt-handling capability and the ability to perform high compute-intensive operations in space conscious applications. Today's system designers have many DSCs to choose from to create high-performance automotive electronics systems, while minimizing system costs and speeding time-to-market. Coupled with the hardware are the extensive arrays of development tools, applications libraries, development boards and reference designs that are available from most major DSC providers. All of these tools enable system development engineers to implement their designs in an efficient, timely manner.

References:
Sensorless BLDC motor control using dsPIC30F2010, Stan D'Souza, Microchip Technology Inc., AN992, Microchip Technology.
Variable speed brushless DC motor, Microchip Technology Inc., Microchip Online Motor Control Design Center.

About the author
Willie Fitzgerald
is an electrical engineer in the Automotive Products Group at Microchip Technology.




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