The output of a photodiode is used as the feedback signal of a laser intensity control loop. The controller currently implements PI control, and the laser together with the optical path forms the plant transfer function.
When a step change is applied to the system, the photodiode output shows the response in the attached waveform. The rising edge exhibits a moderate overshoot, which is expected due to loop dynamics. However, at the falling edge there is a sharp positive spike just before the signal drops to the lower level.
The spike is very narrow compared to the rest of the transient response and appears consistently during the falling transition.
What mechanisms in a laser intensity control loop could produce a positive overshoot spike at the falling edge of the photodiode response?

 

hi s27,
Do you have a circuit diagram of the PI control loop you could post?
E

 

hi s27,
Do you have a circuit diagram of the PI control loop you could post?
E

I have shared the circuit diagram of the simulation I performed, where the component values are approximately the same as those used in the actual system. (for simulation i have taken a RC as a dummy plant)
The PI controller has been tested and was working fine on one system. However, this type of observation was seen on a second system. It would be helpful to understand what attributes of the plant or the circuit could lead to this kind of behavior.

 

The "wild" guess is that the response of the amplitude control feedback loop is slower than the response of the amplitude control input, so that there is an attempt tp correct the amplitude drop faster than the actual amplitude drop.

 

The "wild" guess is that the response of the amplitude control feedback loop is slower than the response of the amplitude control input, so that there is an attempt tp correct the amplitude drop faster than the actual amplitude drop.

The issue was resolved when the optical gain of the transfer function was reduced.

 

THAT seems like it removed the condition that I tried to describe by a simpler method.