Feedforward control: Advanced control and maximum precision for laser diode drivers

Precision is essential in the world of laser technology. Whether in medical technology, research or industrial applications - the precise and fast control of laser diodes is essential to ensure stable performance, efficiency and safety. Meerstetter Engineering’s laser diode drivers set new standards: The innovative feature of feedforward control for PID controllers results in a significant improvement in the speed of current control - without the need to purchase expensive drivers capable of picosecond control.

Feedforward control makes it possible to approach the target current in control cycle 1 and achieve it almost instantaneously. Unlike conventional PID control, which is based on iterative adjustments, feedforward control uses a predictive approach, using a previously characterized model of the load. This combination of speed and precision makes the PID controller with feedforward control particularly suitable for demanding applications with high dynamic requirements - a decisive advantage for Meerstetter Engineering's customers, who are leaders in the high-tech industry.

With feedforward control, the laser diode drivers from Meerstetter Engineering are characterized by efficiency, reliability and user-friendliness and open new possibilities for highly dynamic applications.


    

Basics of control technology: Feedforward control vs. PID control

A PID controller is purely reactive: it continuously measures the deviation between the actual value and the target value, calculates a correction based on this deviation and adjusts the output iteratively. This results in delays, particularly if the target value is changed abruptly.
Feedforward control complements this approach with predictive control:

  • Model based: It uses a mathematical model of the load (e.g. current-voltage curve) to directly calculate and instantly set the required output value.
  • Speed: As the initial value is not measured iteratively, the target value is reached much faster.
  • Combinability: In many applications, feedforward control is combined with PID. Feedforward control quickly brings the system close to the setpoint, while PID compensates for small deviations.

Feedforward control with PID

Mathematical basics

Feedforward control is based on the load characterization, which can be described and parameterized by a function: Vout = ƒ (Itarget)

Thereby Vout is the output voltage and Itarget is the target current. The ƒ function is parameterized by a load characterization. The following models can be used for different load types:

Linear model
Linear Model Linear loads such as resistors

Non-linear model
Non-linear Model Shockley equation with series resistance; ideal for more complex loads such as laser diodes

Rise time and fall time in feedforward control

Feedforward control can significantly shorten the rise and fall time of a laser diode driver. Further, it may also:

  • Reducing the time it takes for set point changes to propagate to the output.
  • Keeping overshoot and undershoot minimal.
Definition:
  • Rise time is the time required for a system to rise from a low initial value (e.g. 10 % of the target value) to a higher value (e.g. 90 % of the target value).
  • Fall time is the time required to fall from a high value (e.g. 90 %) to a low value (e.g. 10 %).

Technical implementation

The practical applications of feedforward control require a careful configuration:

Load characterization:

The load is measured in the desired operating current range using an automatic characterization process. The resulting parameters can be stored in the control unit by the user with just two mouse clicks.

Parameter optimization:
  • Gain: Scales the output value; typically set to 100 %, but can be reduced for highly inductive loads to avoid overshoot.
  • Slope Limit: Limits the steepness of the change in the output signal. This is usually needed to prevent overshoot.
  • PID Option: This option allows a pure I-controller to be used, which can be useful in combination with feedforward control.
  • PID Freeze: The PID controller runs by default when the output is enabled, which may cause overshoots during steep feedforward control slopes. To prevent this, the PID can be frozen or reset during feedforward control. Available Options are: Off (PID always runs), On (PID disabled during feedforward control), and Reset (PID frozen during feedforward control and reset afterwards).
  • PID Kp, Ti and Td: Feedforward PID parameters work like regular PID but shouldn't be copied directly. They are typically less aggressive, starting near the diode's forward voltage and balancing only dynamic deviations (e.g., temperature).
Integration with PID

While the feedforward control takes over the main control as mentioned above, the PID controller compensates for any remaining deviations.

Practical challenges

The introduction of feedforward control also presents challenges:

  • Operating conditions: Changes in temperature, input voltage or load characterization can affect the accuracy of the prediction. A new characterisation is required in such cases.
  • Wiring-Inductance: Long or unsuitable cables can affect response time and lead to overshoot. Optimized wiring is essential.

How to set up feedforward control

Watch the short video below to see how easy it is to set up feedforward control with our free of charge PC software:



Examples and use cases

The combination of feedforward control and PID provides an achievement of the target value that is up to 10 times faster than PID control alone. This is particularly beneficial in pulsed laser applications or use cases where a very fast rise time (<100 µs) is required. For example material processing or spectroscopy.

You aren't sure yet if feedforward control is the right choice for your project? Let's talk about it - book a free consultation with one of our experts!


Conclusion

The feature of feedforward control from Meerstetter Engineering complements conventional control technology by combining speed and precision in one model. In combination with PID controllers, feedforward control delivers outstanding results, especially in highly dynamic applications. With a view to future developments, it will play a key role in many industrial and technological areas.


Result of feedforward control

Take the next step

Bring the benefits of feedforward control to your projects and experience new opportunities of control technology. Buy your licence for the new feature or book a free consultation with one of our specialists to learn more about your options with feedforward control.




For more information on feedforward control for PID controllers, also visit the news article available on our website.