Guide to field-oriented control: Step 3

In Part 1 and Part 2, we discussed measuring motor current as well as the most critical piece in the field-oriented control (FOC) process, comparing the measured current to desired values and generating error signals. Now that we have two error signals based on a rotating reference frame (figure 1), we need to complete our four-step process. What do we do with the signals? More importantly, how do we translate our signals back down to the stationary reference frame? We will answer these questions by proceeding to the final steps of FOC.

 

 

Amplify the error signals to create correction voltages. If you've made it this far, you can relax a bit; these last two steps are super easy. Just like with brush DC motor torque control, we turn to our old friend the P-I controller to take the current error signals and amplify them to create the motor voltages. The only difference with this step compared to a brush DC motor is that we now have two P-I controllers: one for the d-axis and one for the q-axis. And since they exist on a rotating reference frame synchronous to the rotor flux axis, both d and q currents are DC VALUES when the motor is operating under steady-state conditions, regardless of speed! Figure 2 shows the two P-I controllers along with recommended tuning coefficient values.

In the final instalment, we will tackle the last step in the field-oriented control (FOC) process.

About the author
Dave Wilson is Senior Industrial Systems Engineer for C2000 Microcontrollers at Texas Instruments.