The AN1299 is a single shunt 3 phase current reconstruction algorithm for a sensorless FOC of a PMSM. A large number of motor control applications are consistently and continuously looking for methods to improve efficiency while reducing system cost. These are the two main factors that are driving the efforts to improve existing motor control techniques, such as trapezoidal control, scalar control and Field-Oriented Control (FOC). FOC has become more popular in recent years due to the fact that the cost required to implement this technique is no longer a constraint. The available technology and manufacturing process now make it possible to implement this control technique in a 16-bit fixed-point machine such as the dsPIC® Digital Signal Controller (DSC).
Efficiency is another reason that has allowed FOC to gain ground over scalar and trapezoidal control techniques on low-cost and mid-cost applications. It is also well suited in applications in which hard requirements are low noise, low torque ripple and good torque control over a vast speed range. Field-oriented control can be implemented using position sensors such as encoders, resolvers or Hall sensors. However, not all motor control applications require such granularity given by a resolver or encoder; and, in many cases, they do not require control at zero speed. These applications are a perfect target for using sensorless techniques in which the motor position can be estimated using the information provided by the currents flowing through the motor coils.
There are two popular approaches to this sensing technique: the dual-shunt resistor and the single-shunt resistor. The dual-shunt resistor technique utilizes the information contained in the current flowing through two motor coils in order to estimate the motor position.
The single-shunt resistor technique utilizes only the information contained in the current flowing through the DC bus to reconstruct the three-phase currents, and then estimate motor position. In this application note, the single-shunt approach is discussed. For information on the dual-shunt resistor approach, please refer to the application note, AN1078 “Sensorless Field Oriented Control of PMSM Motors”.
This application note illustrates the advantages, limitations and constraints of the single-shunt algorithm. The single-shunt algorithm method is able to recreate the current flowing through the motor phases using a single-shunt resistor to sense the current flowing through the DC bus. In order to obtain the information contained in the DC bus current, Space Vector Modulation is used. SVM creates a series of sampling time windows that allows the observation of the current flowing through the motor phases.
These time windows are classified and grouped in the shunt resistor truth table. This truth table shows the relationship between the information present at the shunt resistor versus the state of the electronic switches. However, it is not possible to obtain the desired information from the DC bus current in certain SVM areas. This limitation is overcome by modifying the SVM switching patterns. Modifying these patterns makes it possible to extract the desired information from the single-shunt resistor in every SVM operating state.
These practical results demonstrate that the singleshunt resistor technique provides information accurate enough to meet the requirements of Field-Oriented Control. It is possible to obtain the motor information such as position and torque based on the reconstructed information extracted from the current flowing through the DC bus.