1. Technical Field of the Invention
The present invention relates generally to sensing motor back EMF signal zero crossing events.
2. Description of Related Art
In the course of operating and controlling brushless DC (BLDC) type motors it is often necessary to monitor the voltage at the motor terminals in order to determine the instant in time that this voltage changes polarity with respect to the circuit common. This event is called the zero crossing. When employing modern control circuitry, this task is often accomplished by connecting these signals to the Analog to Digital Converter (ADC) input pins of a micro processor. The problem which is faced in connecting these signals to the micro processor is that the voltage level of these signals is often quite high, commonly near 350 Volts, while the highest voltage level that can be safely connected to the micro processor is normally in the range of 3 to 5 Volts. It is necessary to protect the control circuitry from damage due to the high voltage while preserving the integrity of the signal so that it can be used for control sensing purposes.
A method which has been used previously to address this problem is to couple the motor terminal signal to the sensing circuit through a resistive attenuator circuit. An example of such an attenuator circuit is shown in FIG. 1. The resistive attenuator circuit is, in this example, a common resistive divider. One resistive divider is provided for each sensed phase (PHA, PHB and PHC) of the three-phase motor. In a typical case, if the terminal voltage may be as high as 350 Volts and the maximum allowable voltage at the micro processor is only 5 Volts, then the attenuation factor of the circuit must be 70:1 or greater.
The foregoing attenuation factor poses a significant limitation because much of the signal strength is lost in the attenuator circuit. At high motor speeds, the BEMF signal present at the terminals is proportionately high so that, even with high attenuation, sufficient signal strength is presented to the control circuitry so that a valid zero crossing detection can be obtained. However, at relatively low motor speeds, the signal which is left after attenuation will not have sufficient strength to obtain a reliable zero crossing detection.
If the motor phase signal can be safely coupled to the control circuitry without attenuation then sensing of the BEMF signal can be obtained at considerably lower speeds. This increases the operational speed range of a sensorless BLDC motor drive system and is very desirable.
Another technique which has been previously used to couple the terminal voltage to the sensing circuitry in a safe manner is to employ a diode clamping circuit. An example of such a circuit is shown in FIG. 2. The circuit includes a current limiting resistor in series with a Zener diode. One such circuit is provided for each sensed phase (PHA, PHB and PHC) of the three-phase motor. In an alternative implementation, two diodes could be used for connecting to safe voltage reference levels. This circuit does not reduce the base signal level as does the resistive attenuator so operation at relatively low speeds is preserved. Since the sensing circuitry (in this case the micro processor) is only looking for the time of the zero crossing, it is not a concern that the signal is “clamped” to levels no higher than the zener breakdown voltage (normally about 5 Volts) and no more negative than the forward voltage of the diode (about 0.7 Volt).
The major problem encountered with the circuit of FIG. 2 is the very high power dissipation encountered in the current limiting resistor. Many micro processors require that the equivalent source resistance of circuits which connect to their ADC inputs should be no higher than 10K ohms in order to get good results. If a value of 10K ohms is used, and the terminal voltage goes to 350 Volts, then the instantaneous power dissipation of the resistor is 11.9 Watts. This power dissipation level is very high and it is not practical to design a circuit in this manner. For motors operating at significantly lower voltages, however, this circuit can be economical and efficient.
A need exists in the art which addresses the deficiencies of the prior art circuits of FIGS. 1 and 2.