Load Disturbance

7.5 Conclusion

Any realistic autotuner should possess some ability to handle load disturbances. This is especially important for slow industrial processes. The reason is fairly obvious: load disturbance can give erroneous results in system identification. Despite the fact that the relay feedback test is more resilient to load changes, the estimates of K_u and ω_u deteriorate exponentially as the magnitude of the load increases. Since external load disturbance generally leads to an asymmetric oscillation, an intuitive approach is to restore a symmetric output response using the biased relay. In this chapter, a simple relationship is derived to find the bias value for the outputbiased relay. The results show that good estimates of K_u and ω_u can be achieved under step-like and non-step-like load changes.