# Ziegler-Nichols Method
**Goal :** Choosing kI,kP,kD appropriately

Ziegler-Nichols is an iterative, online method to choose kI,kP,kD values.


where Ki=Kp/Ti
      Kd=Kp*Td

**Procedure:**
1. Start with Kp=small; Ki=Kd=0
2. Increase Kp until neutral stability
4. Record critical/ultimate gain Ku="Kp at neutral stability"
5. Look up Kp,Ki,Kd values 
6. Compute Ki,Kd

**Advantages:**
* Does not require expert pre-requisite knowledge about control theory
* Does not require a model/simulation, differential equations etc.
* Often produces good initial Kp,Ki,Kd

**Disadvantages:**
* Not mathematically robust/rigorous
* Requires a stable system to begin with; if not reaching neutral stability is not possible and the moment the system is perturbed it would blow up
* If a model/simulation does not exist, then real experiments would be costly
* Requires a system that can be driven unstable with increas in proportional gain