The Hall-effect sensor again comes into play to verify direction of rotation. The MCU uses an edge detection technique to verify the motor is turning in the proper direction. For example, correct rotation is indicated by a rising edge on the Hall-effect sensor when a specific stator coil energizes. If the MCU senses a falling edge, the motor is rotating backwards.

A jammed rotor or other problem that keeps the rotor from turning is a motor fault. The Hall sensor is again used to look out for a stalled rotor. As the fan turns, the Hall sensor sends a series of pulses into the MCU. If those pulses stop, the MCU can kill the PWM drive to the coils to prevent overheating. It can also sound a buzzer, alert the operator, and notify the system about the malfunction.

The most common temperature sensor used in these fans is a forward-biased diode. When connected to a constant-current source, its forward voltage drop changes with the ambient temperature. The diode voltage is measured using an on-chip analog-to-digital converter ( ADC) found in many MCUs. However there is another lower-cost alternative.

Assume the maximum target speed of the fan is 4,000 rpm. That implies 67 rps or 15 msec/rev. As mentioned above, the rotor requires four commutation steps to complete one mechanical revolution. In the other words, the PWM driving period is always longer than 3.75 msec. This value is a rather long time for a general 8-bit MCU. Even with an 8-bit MCU running at 4 MHz, 3.75 msec takes 15,000 CPU-bus cycles. That implies the MCU is mostly idle during the PWM period. Instead of using an MCU that has an embedded ADC, there is ample bandwidth to perform emulated ADC functions based on a lower cost MCU that includes only an analog comparator rather than a full ADC.

An emulated ADC function is created by connecting the temperature sensor to the inverting input terminal of the comparator. Then connect the junction of a simple resistor-capacitor (RC) network to the noninverting terminal. When the ADC is not measuring or is in the idle state, the capacitor C is fully discharged. When the measurement begins, voltage across C starts to ramp up. The MCU timer measures the time the capacitor takes to charge. When the voltage of the capacitor equals the voltage of the temperature sensor, the comparator output triggers and the timer count is recorded. Given that the value of R and C are known, the RC charge profile is also known. The voltage across the temperature sensor is read using a simple table lookup from the value of the timer.

There are literally thousands of motor designs in the market. Some are based on high-end processors such as a 32-bit DSP while others use low-end 8-bit processors. Motor designs are specific to the end applications. Obviously, a motor control designed for a washing machine is not usable for the power-steering unit of a car. The choice of MCU comes out of what the motor must produce for speed, torque, size, precision of motion, and so forth.

A simple 8-bit processor is all that is needed to create the dc cooling fan. Given the calculation shown above for the temperature monitor, a general-purpose 8-bit MCU running a 4-MHz bus will provide more than enough processing power to drive a 4,000-rpm fan. It will also need some internal modules, such as a simple timer for PWM timing, an internal clock source to eliminate space and cost for an external crystal, and an analog comparator for temperature measurement.

The designer should also consider the physical size of the MCU. Dc fan electronics are usually hidden under the motor shaft inside the fan. The printedcircuit-board area is limited. Small geometry packages would certainly be a big help. MCUs that do not need external crystals are a big plus in terms of reducing printed-circuit-board area and cost.

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