Punt for critical damping

lcapy/expr.py

@@ -3018,14 +3018,15 @@ def poles(self, aslist=False, damping=None, pairs=False):
 
         """
 
-        if self._ratfun is None:
+        ratfun = self._ratfun
+        if ratfun is None:
             # Handle expressions such as A(s) * (1 - exp(-s * T)) / B(s).
             return self.D.roots(aslist, pairs)
 
         try:
             poles = self._poles
         except AttributeError:
-            poles = self._ratfun.poles(damping=damping)
+            poles = ratfun.poles(damping=damping)
             self._poles = poles
 
         polesdict = {}

lcapy/inverse_laplace.py

@@ -144,6 +144,10 @@ def ratfun(self, expr, s, t, **kwargs):
 
         uresult = 0
 
+        if damping == 'critical':
+            # This can be gnarly so punt for now.
+            damping = None
+
         QP = [Root(p, 1, damping) for p in P]
 
         for m in range(len(R)):

lcapy/quadraticroot.py

@@ -122,8 +122,9 @@ def from_expr(cls, expr, damping=None):
                 dexpr = -dexpr
                 scale /= I
         elif damping == 'critical':
-            # This puts constraints on variables in dexpr.
-            dexpr = 0
+            # This puts constraints on variables in dexpr.  For example,
+            # if have sqrt(a - b) then a = b.
+            pass
         elif damping == 'under':
             if not scale.is_imaginary:
                 dexpr = -dexpr
@@ -134,6 +135,9 @@ def from_expr(cls, expr, damping=None):
         if dexpr == 0:
             return None
 
+        if damping == 'critical':
+            dexpr = 0
+
         return cls(offset, scale, dexpr, damping)
 
     def is_conjugate_pair(self, d):

lcapy/ratfun.py

@@ -836,13 +836,15 @@ def as_QRPO(self, damping=None, method=None):
         var = self.var
 
         sexpr = Ratfun(expr, var)
-        poles = sexpr.poles(damping=damping)
 
         if damping == 'critical':
             # Critical damping puts constraints on variables.  If
             # these variables do not occur in numerator of expr
             # perhaps the following code will work.
-            raise ValueError('Critical damping not supported')
+            warn('Critical damping not supported')
+            damping = None
+
+        poles = sexpr.poles(damping=damping)
 
         if len(poles) == 0:
             return Q, [], [], [], delay, undef