Overview

The frequency-response identification techniques of Chapters 7 and 8 were limited to the consideration of single-input/single-output (SISO) systems (e.g., p/ ? _lat). Of course, aircraft systems and subsystems generally consist of several inputs and outputs, and generally more than one input is present during the test maneuvers. For example, whereas a typical flight-test record can be characterized as a lateral stick frequency sweep, there will be secondary inputs in the remaining axes as supplied by the pilot and/or SCAS to maintain a roughly constant reference condition (Sec. 5.9). The response will be influenced by both the primary and secondary inputs. So we need techniques that are applicable to the identification of multi-input/multi-output (MIMO) systems from flight-test records with multiple channels of excitation.

In this chapter, we will see that the starting point for MIMO system identification is the single-input/single-output (SISO) frequency-response identification method of Chapter 7. The SISO analysis is first conducted on the pair-wise combinations of inputs and outputs. Additional procedures are then necessary to condition, or correct, these frequency responses to take into account the effects of multiple partially correlated inputs. These procedures are referred to as multi-input identification techniques. The results of the multi-input identification are the conditioned frequency responses and partial coherence shown in the system-identification flowchart (Fig. 2.1). Bendat and Piersol106 provide an excellent derivation and physical insight into this conditioning process and the relationship of the conditional spectral quantities to the SISO or ordinary equivalents. Twisdale and...