Process control using amplitude control loop

1. A method comprising:

receiving a reference signal comprising an adjustable-amplitude sine wave;

receiving a voltage feedback signal from a voltage control loop comprising a point-of- regulation sine wave, wherein the voltage control loop comprises a first response time;

comparing the reference signal to the voltage feedback signal to generate an error value;

controlling the point-of-regulation sine wave based on a proportional-integral-derivative input and a current feedback signal, wherein the proportional-integral-derivative input is based on the error value; and

adjusting the reference signal based on an output of an amplitude control loop, wherein the amplitude control loop comprises a second response time that is at least one order of magnitude slower than the first response time of the voltage control loop.

2. The method of claim 1 , wherein the proportional-integral-derivative input comprises a control function including a proportional component, an integral component, and a derivative component.

3. The method of claim 1 , wherein the output of the amplitude control loop comprises a control function comprising a proportional component and an integral component.

4. The method of claim 3 , wherein the control function further comprises a derivative component.

5. The method of claim 1 , wherein the first response time is less than 10 ms and the second response time is greater than 100 ms.

6. The method of claim 1 , wherein the point-of-regulation sine wave is controlled by an inverter, a converter, or an output filter.

7. The method of claim 1 , wherein the point-of-regulation sine wave comprises a polyphase power output.

8. The method of claim 1 , comprising:

comparing the adjusted reference signal to a new voltage feedback signal to generate a new error value;

controlling the point-of-regulation sine wave based on a new proportional-integral- derivative input and the current feedback signal, wherein the new proportional-integral- derivative input is based on the new error value; and

adjusting the adjusted reference signal based on a new output of the amplitude control loop.

9. The method of claim 1 , comprising:

receiving the current feedback signal from a current control loop, wherein the current feedback signal indicates an output current at a point-of-regulation, and the current control loop comprises a third response time that is the same order of magnitude as the first response time.

10. A system comprising:

a proportional-integral-derivative control block, comprising:

a first error amplifier configured to receive a reference signal and a voltage feedback signal and to output a first error value;

a process configured to control a point-of-regulation sine wave based on a proportional-integral-derivative control function and a current feedback signal, wherein the proportional-integral-derivative control function is based on the first error value; and

a voltage control loop configured to provide the point-of-regulation sine wave to the first error amplifier from the process, wherein the proportional-integral-derivative control block comprises a first response time; and

a reference signal control block, comprising:

a second error amplifier configured to receive an amplitude of the point-of- regulation sine wave and a direct current reference voltage and to output a second error value;

a proportional-integral control function generator configured to generate a proportional-integral control function based on the second error value;

a reference amplitude controller configured to control the reference signal received at the first error amplifier based on the proportional-integral control function;

and

an amplitude control loop configured to provide the proportional-integral control function to the reference amplitude controller, wherein the reference signal control block comprises a second response time greater than the first response time..

11. The system of claim 10 , further comprising:

a complex programmable logic device, a field-programmable gate array, or a microcontroller configured to generate a digital sine wave; and

a digital-to-analog controller configured to generate the reference signal based on the digital sine wave and a reference amplitude voltage generated by the reference amplitude controller.

12. The system of claim 10 , wherein the reference signal control block further comprises an amplitude detector configured to detect the amplitude of the point-of-regulation sine wave.

13. The system of claim 10 , wherein the process comprises an inverter, a converter, or an output filter.

14. The system of claim 10 , wherein the first response time is less than 10 ms, and the second response time is greater than 100 ms.

15. The system of claim 10 , wherein the point-of-regulation sine wave comprises a polyphase power output.

16. A method comprising:

receiving a reference signal comprising an adjustable-amplitude sine wave;

receiving a voltage feedback signal comprising an output voltage of a process;

comparing the reference signal to the voltage feedback signal to generate a first error value;

controlling the output voltage of the process based on:

a first proportional control signal that is proportional to the first error value;

a first integral control signal based on an integral of the first error value over time, wherein the first integral control signal comprises a first integral gain value;

a derivative control signal based on a rate of change over time of the first error value; and

a current feedback signal;

comparing an amplitude of the output voltage of the process to a constant direct current reference voltage to generate a second error value; and

controlling the adjustable-amplitude sine wave based on:

a second integral control signal based on the integral of the second error value over time, wherein the second integral control signal comprises a second integral gain value that is smaller than the first integral gain value.

17. The method of claim 16 , wherein controlling the adjustable-amplitude sine wave is further based on:

a second proportional control signal that is proportional to the second error value.

18. The method of claim 16 , wherein the first integral gain value controls a first response time of controlling the output voltage, wherein the second integral gain value controls a second response time of controlling the adjustable-amplitude sine wave, and wherein the second response time is at least an order of magnitude slower than the first response time.

19. The method of claim 18 , comprising receiving the current feedback signal from a current control loop, wherein the current feedback signal indicates an output current of the process, and the current control loop comprises a third response time that is the same order of magnitude as the first response time.

20. The method of claim 16 , wherein the output voltage comprises a polyphase voltage output.