# ncfmargin

Calculate normalized coprime stability margin of plant-controller feedback loop

## Description

`[`

returns the normalized coprime stability margin of the multivariable feedback loop
consisting of a controller `marg`

,`freq`

] = ncfmargin(`P`

,`C`

)`C`

in negative feedback with a plant
`P`

:

The normalized coprime robust stability margin (also called the gap metric stability margin) is an indication of robustness to unstructured perturbations. Values greater than 0.3 generally indicate good robustness margins.

`[`

specifies the sign of the feedback connection assumed for the margin calculation. By
default, `marg`

,`freq`

] = ncfmargin(`P`

,`C`

,`sign`

)`sign = -1`

. Set `sign = +1`

for
positive-feedback interconnection.

`[`

calculates the normalized coprime factor metric with the specified relative accuracy.`marg`

,`freq`

] = ncfmargin(___,`tol`

)

## Examples

## Input Arguments

## Output Arguments

## More About

## Tips

While

`ncfmargin`

assumes a negative-feedback loop, the`ncfsyn`

command designs a controller for a positive-feedback loop. Therefore, to compute the margin using a controller designed with`ncfsyn`

, use`[marg,freq] = ncfmargin(P,C,+1)`

.

## Algorithms

The computation of the normalized coprime stability margin is as described in Chapter 16 of [1].

## References

[1] Zhou, K., Doyle, J.C.,
*Essentials of Robust Control*. London, UK: Pearson, 1997.

## See Also

`ncfsyn`

| `robstab`

| `diskmargin`

| `gapmetric`

| `wcdiskmargin`

**Introduced before R2006a**