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issue: October 2005 APPLIANCE Magazine Part 2: Motors & Air-Moving Devices

Feature - Motors and Air - Moving Devices
Simplifying Motor Control with Microcontroller Innovations


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Figure 1. The inverter circuit comprises three half bridges divided into high-side (Q1, Q3, Q5) and low-side (Q2, Q4, Q6) devices. The high-side transistors switch according to the PWM control signal and the low-side devices are left “on” during the phase supply to the motor, according to a six-step commutation routine.

Historically, the electronics required to control brushless DC (BLDC) motors were too complex and costly for most appliance applications. That is, until dedicated microcontrollers (MCUs) became available. Such MCUs typically integrate eight-bit computational engines, analog-to-digital converters (ADCs), comparators, counters, timers, and other circuits to commutate and vary the speed of BLDC motors commensurate with the power requirements of load. Although the integrated motor controllers have brought costs within reach for many applications, the number of additional components required for a given project can vary, depending upon which controllers are considered. Several new approaches simplify the implementation of BLDC motor control in appliances.

Depending upon the complexity and expense of control electronics, BLDC motors can be far better candidates than AC induction or universal motors for almost any appliance now that production costs for the motors themselves are competitive. Not only do BLDC motors operate much more efficiently because their speed can be regulated according to load, they are also designed to be smaller, quieter and more durable than AC induction and universal motors with the same horsepower rating. Accordingly, differences among BLDC motor controls strongly influence whether or not those advantages can be realized in more appliance applications.

In the May 2005 issue of APPLIANCE, Dal Ohm, consultant, Drivetech, Inc. [1] contributed a tutorial about BLDC motor commutation and closed-loop speed control, but an extract may prove helpful to appreciate the new approaches. Each BLDC motor incorporates a permanent magnet rotor, which turns as drivers create a rotating field through the fixed phase windings of the stator. The stator flux must be synchronized with the rotor magnetic flux; therefore the rotor position must be known.

Rotor position can be ascertained from the back electro motive force (EMF), which the phase windings generate in opposition to the applied field. A six-step inverter circuit commutates three-phase BLDC motors that have bipolar rotors (See Figure 1). Basically, during each of the six steps for commutation, voltage is applied to two of the three-phase windings. Each of the phase voltage states occurs in 60-degree intervals. In the idle or un-energized phase, the back EMF generated in a winding transitions through a zero crossing. Therefore, the zero crossing events for the three phases collectively pace the phase energizing sequence.

The output devices in the three-phase inverter bridge are driven via pulse width modulation (PWM) and pulses of fixed frequency are produced. Voltage to the motor windings can be effectively raised or lowered by controlling the duration (duty cycle) of the fixed-frequency pulses, thereby varying the motor speed.

Figure 2. An example of a motor MCU incorporating the logic and I/O functions needed to drive multi-phase and single-phase motors in appliances.

“Time-Stamp” for Speed Control

Most controllers in closed-loop systems use at least one dedicated comparator to detect the zero crossing of back EMF signals so that the output driving pulses can be adjusted to properly regulate the motor speed. An alternative approach based on a motor control MCU eliminates the need for that comparator by employing an ADC in conjunction with a timer. In this case, the ADC samples the back EMF voltage, with the timer running in the background.

Once the ADC samples the back EMF zero crossing, the timer count is read, the result is written to a register and, in turn, cues the timers for the output PWM pulses to efficiently regulate the speed of the motor. This “time stamp” approach results in a simpler and more cost-effective system for closed-loop speed control.

Fault Response

Over-current faults can result from different causes in appliances driven by BLDC motors and are sometimes destructive unless safeguards are well planned. Shorted motor windings, shorted motor leads, problems in mechanical drives and linkages, a stuck rotor, breakdowns or misfiring of power devices, and many other problems can arise, some of them permanent, some merely temporary. Whatever the origin of an over-current condition, the motor rotation must be halted. Protection circuits must act quickly; rather than triggering a hard shutdown of the entire system when a fault is detected, it is desirable to realize cycle-by-cycle disabling of the PWM outputs, with normal operation resuming once the fault condition is no longer detected. If the over-current condition persists, a hard shutdown ensues.

Motor control MCUs typically incorporate input elements (such as a comparator) for sensing over-current conditions. In many cases, the current signal is routed to the ADC. This approach has a major drawback due to the time associated with data processing before the outcome can disable the PWM. The resulting data processing latency could delay system shutdown beyond the next switching cycle and catastrophic damage could result.

To avoid the processing delay inherent with an ADC, an over-current comparator is directly coupled to the PWM module, guaranteeing that shutdown truly can occur in a cycle-by-cycle mode. This approach not only improves controller fault response, it also circumvents a vulnerability that is inherent with the conventional approach. Namely, if the controller clock were to stop functioning, there would be no risk of accomplishing a shutdown in response to an over-current fault, as there would be if the system’s ADC were involved.

Sense Scheme Relieves Time Dependency

Appliance controllers almost invariably monitor motor speed by sensing the current through the windings, using sensor and sensorless techniques in conjunction with the ADC. Ordinarily, sampling instances by the ADC are synchronized by the MCU.

In this process, the ADC samples the generated signal(s) and outputs the result to the processor, respectively. The processor will, in turn, “synthesize” the PWM commutation outputs to control motor speed.

However, there is a better approach to sense, process and regulate motor speed, providing for asynchronous operation while minimizing the data processing overhead. In this scheme, an on-chip integrated operational amplifier along with a sample-and-hold module are used to “preserve” the value of the input signal(s) in between the clock cycles, so that the sampling events can be designed to optimize the CPU’s processing cycles and are not required to be precisely synchronized with the incoming signal(s). The implementation is further testimony to the efficient, yet powerful attributes of eight-bit motor control microcontrollers, while continually maintaining motor energy consumption at optimum value.

References

1.Dal Y. Ohm, The Basics of Brushless Motor Drive Design, Appliance Magazine, May 2005.

This information is provided by Peyman Hadizad, vice president of Applications Engineering, and Rex Allison, senior application engineer for Motor Control Processors, Zilog, Inc.

 

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